The flow and heat transfer characteristics of mist/air cooling in the rotating ribbed two-pass rectangular channel are numerically investigated using the CFD software ANSYS-CFX. In this article, a comparison in heat transfer performance between the mist/air cooling and the air-only cooling is performed. Additionally, the effect of the initial mist diameter, temperature, velocity and the channel rotation speed on the mist/air cooling performance is analysed. The results show that the droplet flow distance and Nusselt number of the mist/air cooling increase as the initial mist temperature decreases. In addition, as the initial mist diameter decreases, the diameter of mist on the whole channel decreases, resulting in the higher heat transfer, whilst the mist concentration also decreases, leading to the lower heat transfer. Therefore, there is an optimal initial mist diameter which makes the heat transfer performance best. Nevertheless, the droplet movement and heat transfer performance of mist/air cooling are nearly insensitive to the initial mist velocity. It is also noted that the Coriolis force increases with the channel rotation speed, causing the flow deflection becomes more obvious. Consequently, as the channel rotation speed increases, in the first passage the averaged Nusselt number on the trailing wall increases, and that on the leading wall decreases, while the trend in the second passage is reversed.
This paper describes the development of a complete methodology for the aeroservoelastic modelling of horizontal axis wind turbines at the conceptual design stage. The methodology is based on the implementation of unsteady aerodynamic modelling, advanced description of the control system and nonlinear finite element calculations in the Samcef Wind Turbines design package. The aerodynamic modelling is carried out by means of fast techniques, such as the blade element method and the unsteady vortex lattice method, including a free wake model. The complete model also includes a description of a doubly fed induction generator and its control system for variable speed operation. The Samcef Wind Turbines software features a nonlinear finite element solver with multi-body dynamics capability. The full methodology is used to perform complete aeroservoelastic simulations of a realistic 2 MW wind turbine model. The interaction between the three components of the approach is carefully analysed and presented here.
Savonius turbines have been the subject of various wind energy projects due to their good starting characteristics, easy installation, and independency of wind direction. However, the Savonius rotor suffers from low aerodynamic performance, which is mainly due to the adverse torque of the returning blade. A recently introduced design suggests using pivoted blades for the rotor to eliminate the negative torque of the returning blade. In this study, the aerodynamic performance of the newly proposed turbine has been investigated experimentally and numerically. The experimental measurements are performed in a subsonic open-jet type wind tunnel facility. The numerical simulations are performed using ANSYS-Fluent commercial software, by making use of the multiple reference frame model. The effects of the number of blades (3-, 4-, and 6-bladed) on the torque and power coefficients are examined in details, at several Reynolds numbers. Results show that the new rotor has no negative torque in one complete revolution and that the 3-bladed rotor has the best aerodynamic performance, in a manner that, it reaches a maximum power coefficient of 0.21 at TSR = 0.5. Although increasing the number of blades decreases the output torque oscillations, it also decreases the average power coefficient of the rotor. Results show that Reynolds number does not have a significant effect on the average power coefficient of the rotor in the studied range of 7.7 x 104 ≤ Re ≤ 1.2 x 105.
Internationally, there has been a move by nations to decarbonise their electricity systems in an effort to tackle rising territorial emissions. No consensus has been fully reached on best approach, which has led to significant divergence in energy policy between countries and a consequential lack of long-term clarity. Additionally, recent UK policy failures, in terms of stimulating greater energy efficiency and encouraging energy innovation, highlight the huge challenge involved in developing and achieving a low carbon future. Steps to decarbonise electricity whilst also providing a secure and affordable supply, can lead to varying life-cycle environmental consequences. A UK research consortium developed three pathways to explore this move to a more electric low carbon future out to 2050. These pathways have been previously evaluated in terms of their life-cycle energy and environmental performance within a wider sustainability framework. Over the course of the project, greater understanding of the generation technologies and the functionality of the overall system under the different regimes were gained. Here, the environmental consequences of the most recent version of the pathways are presented on a life-cycle basis from ‘cradle-to-gate.’ Thus, the environmental impact of technological trends in UK energy policy and their effect on the pathways are explored through a series of sensitivity analyses. The three UK energy futures incorporating ‘disruptive’ technological options were examined based on the phase out of coal use in favour of gas-fired power, ranging penetration levels of carbon capture and storage, and the allocation and fuel type used for combined heat and power. Recommendations are proposed to help frame future energy policy choices in order to limit the environmental consequences of future electricity systems.
A volute is the only circumferential asymmetric component in a centrifugal compressor, and thus, it should account for the circumferential asymmetry of the flow in a vane diffuser. This study performs a transient numerical analysis to investigate the effect of a volute on the flow in the vane diffuser of a centrifugal compressor under three operating conditions (near-stall, middle, and high mass flow). We compare numerical and experimental performance of the compressor, including polytropic efficiency, total pressure ratio, and unsteady pressure on a diffuser vane. The numerical scheme is proven valid owing to the fact that the numerical and experimental results considerably agree well with each other. Under middle and high mass flow conditions, the time-averaged static pressure recovery and the total pressure loss coefficients for all the diffuser passages indicate that the performance of the passages near and upstream of the volute tongue is affected negatively by the volute, whereas that of the passages downstream of the volute tongue is less affected. Under near-stall condition, the performance of all the passages is disturbed, and the diffuser passage marked as DP 3 demonstrates the worst performance. Investigation on the time-averaged aerodynamic forces, loading, and pressure on the vanes yields results that are consistent with those of the investigation on the performance of the passages. The harmonics with 0.5fb and fb, which are included in the unsteady loading and pressure on the pressure and suction sides of the vanes, are dominant, where fb is the impeller main and splitter blades passing frequency. Their amplitude values increase as mass flow deviates from the middle mass flow condition. Under middle and high mass flow conditions, the harmonic with 0.5fb is affected more negatively because of the larger amplitude on the vanes near and upstream of the volute tongue than those downstream, whereas the harmonic with fb is less affected by the volute. Under the near-stall condition, the transient vorticity fields along with the harmonics of 0.5fb and fb are investigated to evaluate the performance of the diffuser passages. DP 3, which is located at approximately 90° downstream of the volute tongue, suffers the strongest flow deterioration and is inferred to stall first. Further researches for designing more matching diffuser/volute combination will be performed by referring this study.
Theoretical analysis shows that the temperature drop of a pre-swirl system directly relates to the pre-swirl effectiveness of the pre-swirl nozzle. Besides, increasing the discharge coefficient and reducing the actual flow angle are the main ways to increase the pre-swirl effectiveness. A new design of pre-swirl nozzle called vane-shaped hole nozzle was introduced and analyzed in this paper. By keeping the throat area, the radial location and the pre-swirl angle (15°), numerical comparisons were carried out between the vane-shaped hole nozzle and other three typical nozzles, widely used in industry: simple drilled nozzle, aerodynamic nozzle and cascade vane nozzle. In order to involve the mixing and rotating effects in the pre-swirl cavity downstream of the nozzles, the rotating pre-swirl cavity and receiver hole were included in the computational models. Numerical results show that aerodynamic nozzle could alleviate the rapid acceleration and deflection at nozzle inlet, which results in 13.7% higher discharge coefficient than the simple drilled nozzle. The discharge coefficient of the cascade vane nozzle can be as high as 0.97 due to the aerodynamical design; however, the pre-swirl effectiveness is only 0.86 because of a large deviation angle (2.4°). For the vane-shaped hole nozzle, the vane height/pitch ratio could be flexibly adjusted to an appropriate value, which makes its performance better than that of the traditional ones. Higher vane height/pitch ratio and close to zero trailing edge radius lead to a small deviation angle (0.56°). Consequently, the pre-swirl effectiveness of the vane-shaped hole nozzle is about 8% higher than that of the cascade vane nozzle, though the discharge coefficient is about 5% lower. Additionally, the volume of the solid block in the vane-shaped hole nozzle is larger than that in the cascade vane nozzle, which could reduce the difficulty in manufacturing but increase the total weight.
To better understand the characteristics of tip leakage flow and interpret the correlation between flow instability and tip leakage flow, the flow in the tip region of a centrifugal impeller is investigated by using the Reynolds averaged Navier–Stokes solver technique. With the decrease of mass flow rate, both the tip leakage vortex trajectory and the mainflow/tip leakage flow interface are shifted towards upstream. The mainflow/tip leakage flow interface finally reaches the leading edge of main blade at the near-stall condition. A prediction model is proposed to track the tip leakage vortex trajectory. The blade loading at blade tip and the averaged streamwise velocity of main flow within tip clearance height are adopted to determine the tip leakage vortex trajectory in the proposed model. The coefficient k in Chen’s model is found to be not a constant. Actually, it is correlated with h/b (the ratio of blade tip clearance height to blade tip thickness), because h/b will significantly influence the flow structure across the tip clearance. The effectiveness of the proposed prediction model is further demonstrated by tracking the tip leakage vortex trajectories in another three centrifugal impellers characterized with different h/b (s).
Parametric studies of recirculating casing treatment were experimentally performed in a subsonic axial flow compressor. The recirculating casing treatment was parameterized with injector throat height, injection position, and circumferential coverage percentage. Eighteen recirculating casing treatments were tested to study the effects on compressor stability and on the compressor overall performance at three blade speeds. The profiles of recirculating casing treatment were optimized to minimize the losses generated by air recirculation. In the experiment, the stalling mass flow rate, total pressure ratio, and adiabatic efficiency of the compressor were measured to study the steady-state effects on the compressor performance of recirculating casing treatments, and static pressure disturbances on the casing wall were monitored to study the influence on stall dynamics. Results indicate that both the compressor stability and overall performance can be improved through recirculating casing treatment with appropriate geometrical parameters for all the test speeds. The influence on stall margin of one geometric parameter often depends on the choice of others, i.e. the interaction effects exist. In general, the recirculating casing treatment with a moderate injector throat and a large circumferential coverage is the optimal choice to enhance compressor stability. The injector of recirculating casing treatment should be placed upstream of the blade tip leading edge and the injector throat height should be lower than four times the rotor tip gap for the benefits of compressor efficiency. At 71% speed, the blade tip loading is decreased through recirculating casing treatment at the operating condition of near peak efficiency and increased near stall. Moreover, the outlet absolute flow angle is reduced in the tip region and enhanced at lower blade spans for both operating conditions. The stall inceptions are not changed with the implementation of recirculating casing treatment for all the test speeds, but the stall patterns are altered at 33% and 53% speeds, i.e. the stall with two cells is detected in the recirculating casing treatment compared with the solid casing with only one stall cell.
A cylindrical burner accommodating stoichiometric fuel–air mixture combustion via multiple pairs of opposing jets and a cross-flow provided heat intensification and duplication of the stagnation impact for extending the firing limits and maximizing the power density. Six pairs of circumferentially opposing stoichiometric mixture jets sustained bulk injection velocities as high as 21.8 m/s and were associated with NOx emissions of 22 ppm, while emissions of 10 ppm were recorded upon reaching a lean limit equivalence ratio of 0.59. A stoichiometric mixture jet issuing perpendicular to the opposing jets at a momentum flux ratio of 0.3 increased the turbulence production rates to the extent that increased the maximum bulk injection velocity to 28.3 m/s and reduced the NOx emissions to 17 ppm. Since the recirculation zones between the two stagnation centers got compressed by increasing the momentum flux ratio to 0.8, the corresponding residence time reduction decreased the NOx emissions to 12 ppm. As the cross-flow mixture was made fuel–lean, dilution of the stoichiometric mixture by the fuel–lean mixture combustion products made it possible to get NOx emissions of single digit ppm. Emissions of 9 ppm resulted from using the cross-flow fuel–lean mixture jet due to compromising the flame stability limit extension and the temperature reduction in the post flame region. Such emissions, in turn, decreased to 4 ppm as the momentum flux ratio increased to 1.7 at which the stoichiometric mixture flames shrank into their ports. A minimum NOx emission index of 0.27 g/kg fuel was thus obtained at a volumetric heat release of 50.4 MW/m3. The momentum flux ratio corresponding to merging the two stagnation zones was correlated with Reynolds and Froude numbers, the jets’ separation as well as the density and viscosity values pertaining to the lean and stoichiometric mixtures’ flame temperatures.
The purpose of this work is to understand the properties of the injection flow through slots opening surfaces with steady and unsteady simulations. The feasibility of evaluating slot effectiveness by steady results is demonstrated. Transient features of injection flow are detailed investigated. Numerical investigations are carried out in a 1.5 axial transonic compressor stage at a specified rotating speed with seven kinds of slot-type casing treatments. Comparisons between steady/unsteady results show that differences of overall performance and injection mass flow rate are dependent on simulation methods, rather than slot configurations. Thus, correlation analysis by steady results of seven slot configurations is considered valid and reveals strong linear correlation between injection mass flow and stall margin improvements/efficiency drops. Therefore, it is practical to evaluate the effectiveness of a specific slot configuration in this compressor with steady results by calculating injection mass flow rate. Afterwards, unsteady simulations are performed with a specific configuration of arc-curve skewed slots. It is clarified that the dividing locations between suction/injection regions moves along the axial direction based on the relative rotor/slots location. Exchanging flow through slots opening surfaces displays periodic variations over time. The variation cycle for one single slot equals blade passing period T. For summation of mass flow through all slots, the cycle equals to T divided by slots number in one passage. The net flow rate through all opening surfaces is always less than zero during a blading passing period, i.e. injection mass flow rate is larger than suction flow all the time.
Partial surge is characterized by axisymmetric low-frequency disturbance localized in the hub region and with the potential risk of causing full compressor instability in a transonic axial flow compressor. To find out what determines the low frequency of partial surge, most typical vibro-acoustic frequencies in the test compressor system are first examined, and the observed frequency of partial surge is found to be the closest to the Helmholtz frequency estimated with Greitzer’s duct-compressor-plenum model. Experiments are then conducted to observe the frequency of partial surge during the evolution of instability in the same compressor system with varied lengths of compressor inlet duct and inlet duct in front of the settling chamber and at the same rotor speed. The result also suggests that the dominant frequency of partial surge can be determined by the Helmholtz frequency of the system. When the length of compressor inlet duct is increased beyond a critical range, a different type of system oscillation is also observed in the form of a disturbance at higher frequency. The occurrence of this disturbance is discussed in this article with an extended theory of system oscillation.
This paper presents an experimental and numerical study of the flow in a 1:1 scale, automotive turbocharger centrifugal compressor. Particle image velocimetry measurements have been carried out in the vaneless diffuser at 50% of the design speed. The challenges involved in taking optical measurements in the current small-scale compressor rig are discussed. The overall stage performance and the measured diffuser flow are compared with the results of steady-state computational fluid dynamics calculations. A good agreement between the computational fluid dynamics and the experimental results demonstrates that the numerical methods are capable of predicting the main flow features within the compressor. The synthesis of measured and predicted data is used to explain the sources of the flow and performance variations across the compressor map, and the differences in loss production between small and large compressors are highlighted.
The laminar-turbulent transition process in the boundary layer is of significant practical interest because the behavior of this boundary layer largely determines the overall efficiency of a low pressure turbine. This article presents complementary experimental and computational studies of the boundary layer development on an ultra-high-lift low pressure turbine airfoil under periodically unsteady incoming flow conditions. Particular emphasis is placed on the influence of the periodic wake on the laminar-turbulent transition process on the blade suction surface. The measurements were distinctive in that a closely spaced array of hot-film sensors allowed a very detailed examination of the suction surface boundary layer behavior. Measurements were made in a low-speed linear cascade facility at a freestream turbulence intensity level of 1.5%, a reduced frequency of 1.28, a flow coefficient of 0.70, and Reynolds numbers of 50,000 and 100,000, based on the cascade inlet velocity and the airfoil axial chord length. Experimental data were supplemented with numerical predictions from a commercially available Computational Fluid Dynamics code. The wake had a significant influence on the boundary layer of the ultra-high-lift low pressure turbine blade. Both the wake’s high turbulence and the negative jet behavior of the wake dominated the interaction between the unsteady wake and the separated boundary layer on the suction surface of the ultra-high-lift low pressure turbine airfoil. The upstream unsteady wake segments convecting through the blade passage behaved as a negative jet, with the highest turbulence occurring above the suction surface around the wake center. Transition of the unsteady boundary layer on the blade suction surface was initiated by the wake turbulence. The incoming wakes promoted transition onset upstream, which led to a periodic suppression of the separation bubble. The loss reduction was a compromise between the positive effect of the separation reduction and the negative effect of the larger turbulent-wetted area after reattachment due to the earlier boundary layer transition caused by the unsteady wakes. It appeared that the successful application of ultra-high-lift low pressure turbine blades required additional loss reduction mechanisms other than "simple" wake-blade interaction.
Several emerging electrical energy storage technologies make use of packed-bed reservoirs to store thermal energy for subsequent conversion back to electricity. The present paper describes analysis and optimisation of such reservoirs under transient and steady-state cyclic operation. The focus is on thermodynamic issues, but a simple costing model is also included in order to determine the influence of cost factors on the main design parameters. A major part of the paper is devoted to segmentation (or layering) of the packed beds, which has previously been proposed as a means of simultaneously attaining high storage efficiency and full utilisation of the reservoirs. As illustrative examples, three different reservoirs are modelled, corresponding to the hot and cold thermal stores of a pumped thermal energy storage system, and a larger thermal store suitable for integration with adiabatic compressed air energy storage.
Numerical results of frequency-dependent rotordynamic force coefficients and leakage flow rates are presented and compared with three types of labyrinth gas seals, which include a tooth-on-stator (TOS) labyrinth seal, a tooth-on-rotor (TOR) labyrinth seal, and an interlocking-tooth (INT) labyrinth seal. These three labyrinth seals represent the typical labyrinth seal designs used in rotating machinery as shaft seals to limit leakage and ensure a robust rotordynamic design. The three labyrinth seals have the identical rotor diameter, sealing clearance, tooth number, and profile. Using a proposed transient computational fluid dynamics (CFD) method based on the multi-frequency elliptical orbit whirling model, transient CFD solutions were conducted at rotational speed of 7000 r/min, inlet pressure of 6.9 bar, ambient out pressure, and three inlet preswirl ratios up to 1.0. The accuracy and availability of the present transient CFD method were demonstrated with the experiment data of frequency-dependent rotordynamic coefficients of the TOS labyrinth seal with different rotational speeds and inlet preswirl ratios. The rotordynamic coefficients were generally well-predicted by the present numerical method, with direct stiffness and direct damping modestly under predicted (~67% and ~34%, respectively). Numerical results compared leakage flow rates and rotordynamic coefficients for three labyrinth seals, while paying special attention to the cross-coupling stiffness, effective damping, crossover frequency, and swirl velocity development along the flow direction in seal. The INT and TOS labyrinth seals consistently leaks less than the TOR labyrinth seal across all preswirl ratios, respectively, by factors of 27% and 5%. The TOS labyrinth seal is relatively most stable, followed by the INT labyrinth seal and then the TOR labyrinth seal. The stability of labyrinth seals can be improved by reducing the swirl velocity at the seal entrance (installing anti-swirl devices) and in the seal cavity (applying "textured" or "pocket" stator surfaces).
Fan blades of high bypass ratio gas turbine engines are subject to substantial aerodynamic and centrifugal loads, producing the well-known phenomenon of blade untwist. Accurate fan blade shape prediction is very crucial for high-performance aero-engine. In order to investigate the effects of static aeroelastic deflections on aerodynamic performances, a time domain two-way fluid–structure interaction method was applied to simulate the deflection of NASA Rotor 67 fan blades under different operating conditions. This paper pays attention to the deviation of fan profile and aerodynamic performance due to the varying aerodynamic loads, especially for off-design conditions. The results show that the static aeroelastic deflection has a 1.8% impact on the total pressure ratio under near stall condition, and a 1.7% impact on the choke mass flow rate, relative to the cold configuration. Thus, the blade design in industrial practice should adopt a two-way fluid–structure interaction method to consider the influence of static aeroelastic problems on aerodynamic performance.
In order to meet the needs in engineering, an existing low specific speed centrifugal pump may have to be run at a high rotating speed. Unfortunately, experiments on transient performance of such a kind of centrifugal pump in starting and stopping periods at a high speed have been unavoidable so far. In this paper, the transient hydraulic performance of a low specific speed centrifugal pump with an open impeller during starting and stopping periods is measured with an updated test rig. The correlations of rotating speed, flow rate, head, static pressures at the pump inlet and outlet, and shaft power with time are obtained at different discharge valve openings. The results show that the flow rate rises slowly at the beginning of starting period, and suddenly drops at the end of stopping process. There is an obvious impact phenomenon in local static pressure curves at the pump inlet and outlet during starting period. With the augment of discharge valve opening, the required time for flow rate to reach a steady value is gradually prolonged, and the impact phenomenon in local static pressures at the pump inlet is postponed and the impact difference is also decreased step by step. The impact phenomenon in shaft powers is also very prominent, especially at small discharge valve openings. At the beginning of stopping process, the flow rate declining delay phenomenon becomes weak with increasing discharge valve opening, and the time for flow to stop fully is also extended gradually.
Unstable or flat head-flow curves can cause problems in parallel operations or in flat systems. Despite the considerable efforts that have been devoted to the study of head-flow curve instability in single-stage centrifugal pumps with volute casing, the cause of such phenomenon is not sufficiently understood. In this study, we investigated the variation of hydraulic losses based on the relationship between velocity distribution and entropy generation fields. Steady-state and unsteady simulations were obtained for a pump with an impeller outlet diameter of 174 mm, and the unsteady results are more coincided with the experiments. Results showed that the losses mainly focused on the blade suction surface and volute tongue, as well as in the region of the volute discharge at high flow rates. The entropy generation rate of the pump casing at partial flow rates changed slightly with a decrease in flow rate, whereas the energy losses in the impeller increased steeply when the flow rate dropped to 35 m3/h (the design flow rate was 60 m3/h). The losses in the impeller were mainly concentrated on the region near the inlet and outlet and were lower near the impeller inlet than near the impeller outlet, where a counter-rotating vortex was developed near the blade trailing edge. The vortex caused a drastic increase in the entropy generation rate on the pressure surface and in the flow passage. Such increase was the main cause of the head-flow characteristic instability.
The over-tip leakage represents a third of the loss encountered in a typical high-pressure turbine stage. In this paper, numerical investigations are carried out to study the effects of different tip designs on the aerodynamic performance and cooling requirements. A parametric design tool is used to conduct an automatic optimisation of the blade tip. The parameterisation allows overhangs to be added to the tip of the aerofoil to form a winglet, and in addition, a recessed cavity can be applied to produce a squealer tip. The squealer rim may also be opened near the leading-edge and the trailing-edge of the aerofoil. Flow computations are performed by an in-house 3D high fidelity computational fluid dynamic solver for predicting the performance of the component. The solver has been validated with experimental data. Following a preliminary design of experiment, a meta-model is built and an automatic, multi-objective optimisation is carried out to reduce the loss introduced by the over-tip leakage and minimise the heat load on the blade. Three novel designs from the Pareto front have been further analysed. They show a significant improvement over a reference closed squealer in terms of the aerodynamic performance and the heat load. The flow mechanisms providing these benefits are explained in detail.
The rapid increase of renewable energies (e.g. wind and solar energies) requires hydroturbines (e.g. large-scale Francis turbines) to be operated at part load more frequently in order to improve the stability and flexibility of the power supply system. A device named as guide plate is currently being introduced into Francis turbines in order to shrink the size of the unit for the cost reduction. However, the effect of the guide plate on the instability of Francis turbines (especially at part load) is still an open question. This paper aims to elucidate the influence of the guide plate on the instability (e.g. in terms of large pressure fluctuation and its propagation) of the prototype Three Gorges turbines and its generation mechanisms. Computational simulations of the prototype Francis turbine have been performed and validated using on-site measurement. The results reveal that the addition of the guide plate induces an extremely low-frequency and high-amplitude pressure fluctuation at part load (e.g. guide vane opening 16°). This low-frequency pressure fluctuation initially generates in the draft tube, and then further propagates upstream (e.g. runner, guide vanes), acting as the dominant frequency. Our data analysis shows that the above pressure fluctuation is generated by the interactions between the vortex in the spiral casing induced by the guide plate and the swirling vortex rope in the draft tube.
The article discusses various types of expanders used in the organic Rankine cycle systems to turn thermal energy into electrical energy. In addition to basic information on scroll, screw, vane, piston, and turbine expanders, the paper also describes the principles of their operation as well as major advantages and disadvantages. The following characteristics of expanders have been developed on the basis of the most recent scientific publications and own experimental research conducted at the IFFM PASci, in Gdansk. The analysis of various expansion devices available on the market, including those that are still in the development phase, revealed a wide variety of problems related to their operation. The design differences between them have a significant impact on their efficiency and reliability when operating with different working fluids. The article is an authoritative compendium of research-based information about designs and performance of various types of expansion devices. It may be useful for people who want to apply such devices in different types of cogeneration systems.
The screw compressor has a promising application in the process industry due to its appropriate pressure and flow rate. In this paper, a mathematical model of a two-stage oil-free screw compressor is developed to simulate the working process under various operating conditions. The results show that the interstage pressure of the two-stage oil-free screw compressor is built automatically depending on the equal mass flow rate of each stage. It increases with the suction pressure, discharge pressure, and clearance, but decreases with rotating speed. Although the power consumption of the 2nd stage decreases with the suction pressure, the power consumption of the whole compressor always increases with the suction pressure, discharge pressure, rotating speed, and clearance. The volumetric efficiency increases with the suction pressure and rotating speed, but decreases with the discharge pressure and clearance for each stage. The indicated efficiency increases with the rotating speed but decreases with the clearance. A maximum indicated efficiency is found. The volumetric and indicated efficiency of the 2nd stage is more sensitive to the operating condition than that of the 1st stage.
Compact self-recirculating injection that bleeds air from the casing downstream of a rotor blade row and injects the air as a wall jet upstream of the same rotor blade row is experimentally studied after the elaborated design of its structure. The bleed ports, injection ports, and recirculating channels are circumferentially discrete and occupy only 38% of the circumference. Separate tip air injection and outlet bleed air are simultaneously selected for comparison with the self-recirculating injection. Results show that the compact self-recirculating injection can improve the most stall margin by 6.12% among all the three cases on the premise of no efficiency penalty and can also enhance the efficiency (maximum of 1%) for only 0.47% of the total injected momentum ratio recirculated near stall. The details of the flow field are obtained using a multihole probe, a time-resolved Kiel probe, and pressure transducers. The detailed comparative analysis of the characteristic flow in terms of tip leakage flow, blade load, rotor wake feature, and blockage indicates that the self-recirculating injection can postpone the occurrence of stalling in the proposed compressor through a coupling influencing mechanism. One mechanism is to weaken the self-induced unsteadiness of tip leakage flow and to delay the forward movement of the interface between the tip leakage flow and the main flow. The other mechanism is to unload the blade tip and to recover the rotor wake. All these responses can lead to improved stall margin in the self-recirculating injection. This study may be helpful to guide the design of self-recirculating injection in actual application.
Optimization of a centrifugal pump with double volute was performed using surrogate modeling and a multi-objective genetic algorithm to minimize the radial thrust at off-design conditions with minimal loss in hydraulic efficiency. Both steady and unsteady numerical analyses were conducted using three-dimensional Reynolds-averaged Navier–Stokes equations with a shear stress transport turbulent model. The three objective functions are hydraulic efficiency at the design flow rate, and radial thrust coefficient at 70% and 120% of the design flow rate. In addition, three geometric parameters related to the surface curve and the locations of both ends of the rib structure were selected as the design variables for optimization. The Latin hypercube sampling method was used to choose the design points in the design space. The three objective functions were approximated using response surface approximation models. Pareto-optimal solutions representing the trade-off between the objective functions were obtained using a genetic algorithm. Representative optimal designs show that the optimized double volutes significantly reduce both the radial force and the amplitude of the radial force fluctuation with small loss in hydraulic efficiency.
This paper presents a numerical study of a linear compressor cascade to investigate the effective end wall profiling rules for highly loaded axial compressors. The first step in the research applies a correlation analysis for the different flow field parameters by a data mining over 600 profiling samples to quantify how variations of loss, secondary flow and passage vortex interact with each other under the influence of a profiled end wall. The result identifies the dominant role of corner separation for control of total pressure loss, providing a principle that only in the flow field with serious corner separation does the does the profiled end wall change total pressure loss, secondary flow and passage vortex in the same direction. Then in the second step, a multiobjective optimization of a profiled end wall is performed to reduce loss at design point and near stall point. The development of effective end wall profiling rules is based on the manner of secondary flow control rather than the geometry features of the end wall. Using the optimum end wall cases from the Pareto front, a quantitative tool for analyzing secondary flow control is employed. The driving force induced by a profiled end wall on different regions of end wall flow are subjected to a detailed analysis and identified for their positive/negative influences in relieving corner separation, from which the effective profiling rules are further confirmed. It is found that the profiling rules on a cascade show distinct differences at design point and near-stall point, thus loss control of different operating points is generally independent.
Due to the centrifugal effect of the radial impeller, side channel pumps are a kind of regenerative pumps that provide high head at low flow rate. The geometry of the impeller affects flow patterns and energy conversion directly, greatly influencing the performance of side channel pumps. To investigate the effect of blade profile for suction side on the performance of a side channel pump, three different base angles of 10°, 20°, and 30°, respectively, on the blade suction side were discussed and analyzed both with numerical and experimental methods. The hydraulic performance, exchange mass flow, and velocity vectors were discussed in detail. The numerical work was validated by the comparison between the simulated result and tested result. The results show that the hydraulic performance of the impeller with 30° angle is the best one of the three impellers, especially for head performance. The evaluation method based on exchanged mass flow also confirms that the performance of the side channel pump can be improved by increasing the angle on the suction side of the blade. In addition, the radial vortex on the impeller flow passage has negative effect on the performance of the side channel pump. However, the axial vortex among the impeller and side channel directly affects the energy conversion and has a beneficial effect on the performance of the pump. The results could be used to modify the design models and conclude the effect of blade shapes on the performance of a side channel pump.
Three-dimensional numerical simulation for a prototype pump-turbine has been performed to investigate the transient characteristics and flow behaviors in the startup process. The rotational speed, which varies with runner torque and inertial of the rotating systems, is obtained by hydraulic-force coupling method. To simulate the guide vane motions, dynamic overset mesh technique is applied with good maintenance of mesh quality. Comparisons with the existing experimental results verify the validity of the current method in predicting the startup transient characteristics of a pump-turbine with good accuracy. During the startup transient, the guide vanes open according to a prescribed sequence, which brings in increasing flow rate to the unit. The runner accelerates due to growing water power energy exerted on the runner blades. When approaching the speed no-load condition, a ring-shaped flow enlacing the entire vaneless space, that we term "water-ring," is found to block the through-flow along the circumference, and the pump-turbine features a prominent partial pump flow state in the runner, which equalizes the torque to no load. Thus, the runner rotational speed stops increasing and stabilizes toward rated speed. The prediction shows that the pressure fluctuation amplitude of the speed no-load can be 3.8 to 8 times that of the full load, presenting evidently stronger instability at the speed no-load.
In this study, the aerodynamic performance of helical Savonius rotors with 30° and 45° twist angles have been investigated experimentally and numerically and compared with the conventional rotor with zero twist. This comparison has been performed at two Reynolds number of 1.50 x 105 and 1.84 x 105 (corresponding to wind velocities of 7.3 and 9 m/s) on Savonius type rotors with an aspect ratio of 1.0. Numerical simulations have been performed using the ANSYS-Fluent commercial software using the sliding-mesh method for considering the rotation of the turbine. The experimental measurements, however, are performed at an open-circuit wind tunnel facility. Results show that the maximum power coefficient of the helical rotors are less than the conventional rotor, in a manner that this coefficient decreases from 0.12 to 0.11, when the conventional rotor is replaced with the helical rotor having 45° twist angle. However, the helical rotors have a more uniform time variations of the torque coefficient. The maximum power coefficient for all the rotors has occurred at the tip speed ratio of 0.7. The good agreement observed between the numerical and experimental results revealed the suitability of the employed numerical scheme for predicting the aerodynamic performance of Savonius-type wind turbines.
Flow-induced vibration and noise widely exist in turbomachines and the connecting piping systems. The combined numerical method of computational fluid dynamics, computational structural dynamics, and computational vibroacoustics is employed to investigate the vibration and noise induced by the unsteady flow in the centrifugal compressor and pipes. Computational results indicate that the strongest pressure fluctuation is located on the clearance between the impeller and the stationary units due to the periodic rotor–stator interaction. However, the intense vibration occurs on the inlet and outlet pipes of the centrifugal compressor. Moreover, the computational result of the normal active acoustic intensity shows that the primary vibro-noise source is located on the outlet pipe. The computational result also clarifies that the thickness of the outlet pipe is one of the key parameters affecting the acoustic power output of the centrifugal compressor.
The cooling effectiveness of air-cooled steam condenser units is impacted by the performance of the large diameter axial flow cooling fans, which ultimately affects the overall efficiency of the power plant. Because of the large diameters of these fans, performance tests are carried out at test facilities with smaller, standardized diameters and measuring equipment. The performance of the large scale fans can be predicted based on the small scale test results using the similarity laws and scale-up formulae. This article details the results of small scale experimental tests and numerical simulations that were performed on a pair of 1.25 m diameter axial flow fans. Full scale, 10.360 m, diameter simulations of the same axial flow fans were subsequently performed and compared with the experimental results that were scaled up using the fan scaling laws.
A gearless one-motor concept for contra-rotating fans is presented in this article. The rotors are mounted to an electric motor using only one shaft. The coupling between both rotors is realised by utilising the conservation of angular momentum. The contra-rotating fans has a diameter of 200 mm at a design speed of 2100 min–1 for the first stage and 1200 min–1 for the second stage. It has been designed and investigated through a series of experiments by the Institute of Air Handling and Refrigeration in Dresden. The performance map and 2D particle image velocimetry measurements have been conducted. Numerical models for 3D quasi-steady state and transient simulations have been implemented and carried out by the Institute of Mechanics and Fluid Dynamics. The results show a good agreement between the quasi-steady, the transient simulations and the experiment. However, when close to stall, the time-resolved simulations show a superior performance compared with steady-state computations.
In this paper, the one-dimensional design and the three-dimensional numerical analysis through computational fluid dynamics simulations of a small size fan are carried out. The fan in consideration provides the airflow used for controlling the surface thermal gradient of ceramic tiles between the dryer and the digital printing stage. An experimental campaign performed on a backward-curved centrifugal fan prototype with an optimized air blowing device demonstrates that fan performances (efficiency and air flow rate) meet the eco-design demands and the air velocity field at the blowing device outlet is suitable to obtain the expected heat exchange. The new cooling system can also reduce acoustic emissions up to 3 dB(A) with respect to the existing one.
Common blade design techniques are based on the assumption of the airflow laying on cylindrical surfaces. This behaviour is proper only for free-vortex flow, whereas radial fluid migration along the span is always present in case of controlled vortex design blades. The paper presents a design procedure to increase aeraulic efficiency of fan rotors originally designed using a controlled vortex criterion, based on the assumption that a blade section positioning taking into account the actual airflow direction could be beneficial for rotor aeraulic performance. The proposed procedure employs a three-dimensional aerofoil positioning and blade forward sweep. The procedure is applied to a rotor-only tube-axial fan featuring a 0.44 hub-to-tip ratio, an almost constant swirl velocity distribution at the rotor outlet and a quite low blade Reynolds number. Rotor prototypes deriving from step-by-step blade modifications are experimentally tested on an ISO 5801 standard test rig. Results show the importance of considering radial fluid migration for highly loaded rotors.
An extensive experimental and numerical database provides detailed information on some transition and noise mechanisms encountered in the low-Reynolds-number flows of low-speed axial fans for the first time. Two different similarly instrumented mock-ups built from the same industrial controlled-diffusion airfoil have allowed the first consistent comparison of wall pressure and near-field velocity statistics on the same geometry with and without rotation in perfect similitude of Mach and Reynolds numbers. The experimental and numerical results on the stationary airfoil constitute the largest unique aeroacoustic data set for airfoil trailing-edge noise characterization including installation effects. A similar experimental aeroacoustic database has been built on the rotating controlled-diffusion blade in the Michigan State University-Automotive Fan Research and Development (MSU-AFRD) test facility for different fan configurations, rotational speeds and flow rates. This yields a unique test-bed for fan code validation. The comparisons between the stationary and rotating airfoils suggest that the wall pressure statistics are hardly influenced by rotation in the trailing-edge region, and that the differences in the velocity statistics in the near-wake are a more energetic wake with smaller velocity deficits and diffusion, and a far-less uniform inviscid region in the rotating case.
The multistage axial compressor is one of the critical components of aero-engines and plays a key role in their performance, reliability, and economy. Tip clearance has a significant impact on the performance and stability of multistage axial compressors. Due to blade and disk deformations, tip clearance will vary significantly in different operating conditions. Thus, tip clearance should be accurately estimated when evaluating compressor performance. This paper proposes a new model to predict changes of tip clearance of multistage axial compressors in different operating conditions. A first-principles approach is used to estimate the change of tip clearance caused by thermal and mechanical deformation. The span-wise temperature distribution across each stage of the multistage compressors is considered by the proposed model in this paper. The model was validated by General Electric Company (GE) E3 engine experimental results. Using the model, the performance of an 11-stage axial compressor is simulated. The results show that accounting for tip clearance variations has a 0.5% impact on the calculated mass flow rate and a 1% impact on the calculated efficiency. Thus, variations of tip clearance at different operating conditions cannot be ignored and the proposed new model is useful to accurately predict the performance of multistage axial compressor.
Cyclones are well known for their simple structure and stable performance. However, cyclones’ separation efficiency is not high enough, especially for particles smaller than 10 µm. The conventional cyclone is improved by adding rotor blades to the cyclone; this new configuration is known as a dynamic cyclone. To explore dynamic cyclone further, the separation efficiency and flow field in this dynamic cyclone were experimentally and numerically investigated. A homogeneous computational fluid dynamics (CFD) model, which included a turbulence model based on the RNG k– model and the Reynolds stress model (RSM), was developed to analyse the flow field in the cyclone. Then, the discrete phase model (DPM) based on the Eulerian–Lagrangian method was used to predict the separation efficiencies and particle trajectories. The performance of the cyclone was numerically investigated in detail in terms of the tangential velocity, separation efficiency and total pressure. The effects of the inlet velocity, rotational speeds and the configuration of the blades on the dynamic cyclone were also analysed. The model was verified through experimentation, and the data were obtained from an electrical low pressure impactor (ELPI), digital microtonometer and Pitot tube. The results indicate that the separation efficiency and total pressure drop of dynamic cyclones increase significantly as the rotational speed and inlet gas velocity increase. The number of blades and the inclination angle of the blades have strong impacts on the separation efficiency and total pressure drop of the apparatus. In addition, the simulation predictions demonstrate that the tangential velocity distributions are dominated by the rotational speeds of the blades and inlet velocities of the gas. The simulation of the tangential velocity distributions can more clearly explain the increase in the separation efficiency.
Wet steam flow in steam turbines leads to degraded efficiency and blade erosion in the turbine stages. The Baumann rule, which has been used to predict wetness losses, is increasingly being questioned. More recently, the non-equilibrium condensation model is being increasingly applied to analyse wet steam flow. However, most of the influences caused by wetness losses on the aerodynamics of a wet steam turbine are excluded when this approach is used. Therefore, the efficiency of a wet steam turbine calculated by the non-equilibrium approach does not match the experimentally obtained values. To improve the accuracy of evaluating a wet steam turbine as well as the wetness losses, a quantitative evaluation program of wetness losses has been developed based on the calculation results of wet steam flow with non-equilibrium condensation using the FORTRAN language. Three-dimensional (3D) simulation of the wet steam flow with non-equilibrium condensation in turbine stages is first conducted. Then the 3D results are circumferentially averaged in the meridian plane, which are subsequently used to quantitatively evaluate the wetness losses. The wetness losses are divided into five categories: thermodynamic loss, droplet drag loss, braking loss, capturing loss and centrifuge loss. The wetness losses in the low pressure (LP) cylinder of a fossil steam turbine are calculated. The results show that the thermodynamic loss is mainly generated in the nucleation stage and the last stage of the turbine where non-equilibrium condensation occurs. The droplet drag loss is small in all wet steam stages. The braking loss is the most important component of the wetness losses, except in the nucleation stage. The capturing and centrifuge losses are moderate in the wet steam stages. The total wetness losses in the LP cylinder account for 3.65% of the total output power. This is less than the 5.14% losses predicted by the Baumann rule.
The spray comparative tests on diesel/biodiesel–ethanol blends revealed that the spray tip penetration increases by a range from 4.4 to 21.5%, while the spray cone angle decreases by a range between 33.2 and 50.0% upon switching from diesel to biodiesel. Using biodiesel has an impact on the spray angle that is stronger than that on the penetration length. It was found that in order to minimize the relative reduction in the spray angle upon using a 50% petroleum diesel/50% biodiesel blend (B50), injectors of larger spray angles should be used, while no significant changes were found by adding ethanol to the diesel/biodiesel blends. The emission tests on a single cylinder naturally aspirated direct injection diesel engine showed that although the brake-specific fuel consumption (BSFC) increased upon switching from diesel to biodiesel, the unburnt hydrocarbons (HC) and carbon monoxide (CO) emissions decreased. Upon blending ethanol with the diesel/biodiesel mixture, the HC emissions decreased by a relative percent as high as 26.6%, while the percentage of decrease in CO reached 10.8% by increasing the injection pressure from 55 to 95 MPa. Adding biodiesel to diesel increased the nitrogen oxide (NOx) emissions due to the increased oxygen availability. Upon approaching the full load, the correspondingly reduced ignition delay resulted in earlier combustion and higher peak temperatures where an average turbulent kinetic energy of about 320 m2/s2 has been predicted. The NOx emissions were effectively reduced upon adding ethanol to the diesel/biodiesel blend due to the higher latent heat of evaporation of ethanol. Combining the retardation in the injection timing from –25 to –5° crank angle with the exhaust gas recirculation of 15% effectively reduced the NOx emissions to be below 2.6 g/kW.h.
The objective of the paper was to recognize the possible effect of a low concentration of chemical impurity (NaCl) present in the inlet steam (2.27 ppb) in the assessment of the wet steam energy loss occurring in a 1000 MW nuclear low pressure steam turbine. The presented analysis suggests that the used binary nucleation model, accounting for the electrolytic nature of the droplets, predicts a reduced wet steam energy loss of order –9 %, in comparison with the unary pure steam droplet nucleation model. This minor difference, considering the uncertainties of the existing measurements and the incomplete knowledge of the real droplet nucleation conditions in the steam of the turbine (primarily in relation to the necessary input data of chemical impurities), suggests that unary pure steam droplet nucleation (Becker–Döring) is an acceptable approximation at the current state of knowledge.
The present study concentrates on the numerical investigation of pressure side film cooling in a linear nozzle vane cascade typical of a high-pressure turbine in gas turbine engines. The cooling scheme features a pressure side cutback and two rows of cooling holes located upstream of the cutback. The main goal is to evaluate the applicability of a simple numerical method, i.e. the steady incompressible Reynolds-averaged Navier Stokes, in such a complex industrial application. The simulations are performed according to an adiabatic and conjugate approach. Two values of the coolant-to-mainstream mass flow ratio (MFR = 1% and 2.8%) are simulated at exit Mach number of M 2 is = 0.2. The computed flow/temperature fields in the cooled regions of the vane pressure side are presented and compared to available measurements of: holes and cutback exit velocity and discharge behavior; boundary layer along traverses at strategic axial locations, adiabatic film cooling effectiveness. In addition, distributions of overall film cooling effectiveness and heat transfer coefficients are reported for the conjugate cases. Both adiabatic and conjugate techniques provide reasonable predictions of three-dimensional aerodynamic and thermal features of the investigated cooled vane. The conjugate heat transfer is much more complicated than one-dimensional conduction within the vane material.
This paper investigates the influence of boundary layer skew on flow structure, total pressure loss, and flow control technique numerically on a high-loaded axial-flow compressor cascade. We have developed two new models respectively about loss evaluation and end wall flow control mechanism for more specific analysis. The result shows that boundary layer skew weakens the secondary flow and delays the generation of passage vortex when incidence approaches 0°. This results in a reduction of total pressure loss mainly (89.4%) due to relieved corner separation. However, as incidence exceeds a certain value (+7°), severe corner separation or even earlier corner stall can be induced by inlet boundary layer skew. Optimization procedure for profiled end wall at inflow condition of +7° incidence is further carried out to investigate the impact of boundary layer skew on flow control technique. The result shows that boundary layer skew should be counted in the optimization design of profiled end wall because of its significant influence on the development of end wall flow. The optimum profiled end walls for cases with and without boundary layer skew show great difference in the manner of end wall flow control. According to the improvement of cascades’ performance, end wall profiling seems more efficient in reducing loss when influenced by the boundary layer skew.
Centrifugal compressor blade trimming can be used for the purpose of changing the performance characteristics of an impeller or allowing a single impeller design to be used for a range of operating conditions. There are different methods of impeller blade trimming that may be employed to change the impeller flow rate, the pressure ratio, or both. In this study, computational fluid dynamics is used to model the effects of two different methods of blade trimming on a single centrifugal compressor design. Impeller performance characteristics and analysis of the flow field are presented for a series of trims. Trimming the passage area from inlet to outlet along the meridional length reduced the flow rate of the impeller and narrowed the effective operating range. The head coefficient and efficiency relative to the choked flow coefficient remained unchanged as the passage area is reduced; however, the flow rate is reduced by a greater amount than the inlet area is reduced. Trimming the impeller blades by shifting the shroud profile in the axial direction caused the head coefficient to be reduced while maintaining a constant flow coefficient. This method of trimming is limited by choking in the radial portion of the passage, and the head coefficient for the impeller studied was able to be reduced by up to 15% before further trimming limited the choked flow rate.
The drive towards lower emissions in aerospace engines promotes more efficient and physically smaller engines. One way to decrease the size of the axial turbine is to minimize the distance between successive stator and rotor rows. This can usually have either a positive or negative influence on the turbine performance. The reasons for this behaviour are not currently fully understood. In this study, a novel approach is taken to find new insights into this design question by analysing the influence of different design parameters on the turbine efficiency behaviour. Several different turbines are analysed using the literature. For the first time, the performed analysis reveals the design parameters, which correlate with the different efficiency curve shapes. The correlating parameters are the stator–rotor axial clearance, stator pitch to axial chord ratio, turning velocity Mach number and rotor aspect ratio. The mechanisms behind the found correlations are further analysed to connect the physical phenomena with the design parameters.
The paper presents a multi-objective optimization of circumferential casing grooves geometries for the NASA Rotor 37 transonic compressor. The depth normalized by the tip clearance and the width normalized by the tip chord are selected as the design variables. The stall margin and peak efficiency are used as the objective functions. The Latin Hypercube Sampling technique was used to select the sample points in the design space. Based on the numerical results of the sample points, the radial basis function network model of the artificial neural network was constructed. The NSGA-II multi-objective evolutionary algorithm is then employed to search for Pareto-optimal solutions. The leave-one-out cross validation method was also used to evaluate the precision of the radial basis function network model. The results of the optimization show the present method can be effectively used for the design of circumferential casing grooves to take account of the stall margin and efficiency. From the Pareto-optimal solutions, two groove configurations are selected and the internal flow fields are compared with the smooth casing. The effect mechanism of the circumferential casing grooves on the performance of the transonic compressor is discussed by the analysis of the flow in the blade tip region.
In the recent times, there has been a proliferation of a producer gas-engine-based power generation systems, particularly in market segments such as small enterprises and village electrification in countries such as India. The electrical load for such application is largely of inductive type, the major ones being water pumps, appliances and industrial loads. As the demand side load varies, the gas-engine generator is expected to match the demand within the shortest possible time and without large fluctuations in speed or frequency. Unlike a natural gas-fuelled engine, where fuel gas is available at the required pressure and flow rate at all times, the producer gas-engine is coupled to a biomass gasifier, and the fuel gas, i.e. producer gas, is generated on demand. Therefore, the gasifier and gas-engine form a coupled system with one mutually driving the other. The study of the coupled or integrated system becomes intriguing, particularly during transient conditions. But at the same time, the study of the coupled system is complicated, and therefore, to simplify the study, an attempt is made in this paper to model the gas-engine generator part along with the gas-fuelling system but without considering the upstream effects of the gasifier system. The behavior of the engine during steady-state and transient conditions is predicted by assuming producer gas to be available at the required pressure upstream of the engine. The results are validated against a few experimental results, and an attempt is made to explain the mismatch in a scientific manner. The need for an integrated model of the gasifier and gas-engine system has been brought out.
Pulverized coal co-firing with opposing premixed methane/air flames in a cross-flow arrangement was found to extend the coal burning limits and reduce its NOx emissions by creating a high temperature NOx-reducing zone between the resulting multiple flame envelopes. By using a primary air stream, the stability limit reached a peak value of 25.1 m/s while the exhaust NOx mole fraction got values as low as 1.2 x 10–4 due to HCN reaction with NOx generated from both the coal primary flame envelope and the premixed flames. The NOx exhaust emissions were reduced to values below 60 ppm as the gas/coal heat input ratio increased to 1.5, where a nonunity Lewis number predicted the peak NOx concentrations across the annular region surrounding the flame. The effectiveness of NOx reduction increased by employing triple flames in place of the primary air since each NOx peak generated from methane combustion was followed by a region of NOx reduction upon consuming a significant amount of the HCN released from coal. Extending the coal flame stability limit to 32.3 m/s contributed to NOx reduction via decreasing the residence time for NOx formation across the combustor exit section. Decreasing the triple flame mixture fraction gradient and keeping a higher overall equivalence ratio reduced the NOx exhaust concentrations to 50 ppm and increased the percent of heat transfer by radiation to reach a peak of 48.8%. Decreasing the particle size to 135 µm and increasing the number of opposing jets to 16 pronounced favorable aerodynamic effects by minimizing the HC and NOx emissions, respectively, to values below 0.16% and 65 ppm. Increasing the opposing jets’ diameter to separation reduced the flame length by 26.7%, while maximizing the staging height kept the NOx emissions below 40 ppm at a strain rate of 2050 s–1.
Fluid flow through curved ducts is essentially characterised by the secondary flow effects due to duct curvature and cross-sectional flow geometry. Such flows produce vortex structures making the fluid behaviour vastly different than those in straight ducts while intrinsically promoting forced convection through fluid mixing. Examining the unique features of secondary flow and wall heat transfer, this paper presents a numerical simulation on the fluid flow through curved elliptical ducts, including circular geometry. The study develops and validates a novel numerical model based on three-dimensional vortex structures (helicity) and a curvilinear mesh system to overcome previous modelling limitations. Considering several duct aspect ratios, flow rates and wall heat fluxes, computations are performed to obtain the flow patterns and thermal characteristics. Parametric influences on flow features and forced convection are described through physical interpretation. The onset of vortices due to secondary flow instability is carefully examined in relation to the duct aspect ratio and flow rate. Appraising their merits, two techniques are developed for accurate detection of secondary flow instability and integrated into the computational process, which was not previously feasible. An approach based on the Second Law irreversibility is evaluated for thermal optimisation of fluid flow through curved elliptical ducts.
Water flow and silt movement in a double-suction centrifugal pump were simulated using an Euler–Lagrange multiphase flow model. Blade erosion rates were predicted using a particle erosion model and the influence of inlet and outlet shapes on silt abrasion was analyzed. The results show: the inlet relative velocity is larger on the suction side than on the pressure side; the blade inlet and outlet are severely silt abraded and the average erosion rate is always larger on the suction side than on the pressure side; the inlet relative velocity and the impact angle are two important influencing factors, and can be controlled by changing the inlet and outlet shapes to reduce erosion rate and increase pump efficiency. In this simulation, two effective means of reducing erosion rates are decreasing the hydraulic loss and increasing pump head and pump efficiency.
The flow coastdown transient analysis is crucial for the design and safety analysis of a nuclear reactor because, in such an event, the core cooling depends upon flywheel inertia of reactor coolant pumps. In this paper, the results of real time flow coastdown transient initiated by power failure to the reactor coolant pumps on the CHASHMA Nuclear Power Plant Unit-2 (CHASNUPP-2) are presented. To examine the effect of thermal-hydraulics parameters, a mathematical model of core thermal-hydraulics for the flow of coastdown transient has been developed. The resulting equations are solved analytically for a pressurized water reactor. The accuracy of the model depends on the core time constant , which has been computed to be 9.28 s for CHASNUPP-2, the calculation methodology is described in the paper. The results are compared with the transient data, good agreement is found between the model prediction and the flow coastdown transient data. It is inferred that the coastdown flow rate is effective in removing the stored heat in the core.
This paper discusses the flow structure in typical rotor–stator systems with ingress and egress. Measurements of concentration, velocity and pressure were made using a rotating-disc rig which experimentally simulated hot gas ingestion into the wheel-space of an axial turbine stage. Externally-induced ingress through rim seals was generated from the non-axisymmetric pressures produced by the flow over the vanes and blades in the external annulus. Measurements were conducted using several single- and double-seal geometries and for a range of sealing flow rates and rotational speeds. The concentration measurements showed that the amount of ingress, which increased with decreasing sealing flow rate, depended on the seal geometry. The swirl velocity in the fluid core increased with decreasing sealing-flow rate, but outside the outer region in the wheel-space, it was largely unaffected by the seal geometry or by the amount of ingress. The radial distribution of static pressure, calculated from the measured swirl velocity in the core, was in good agreement with the pressures measured on the stator. The data for the double seals demonstrated that the ingested gas was predominately confined to the region between the seals near the periphery of the wheel-space; in the inner wheel-space, the effectiveness is shown to be significantly higher. The results are of direct relevance to the engine designer who uses complex rim seals often designed through computational fluid dynamics. The designer needs to know how much sealing air is required to prevent ingress, what is the effect of ingress on metal temperature and stresses, and how these factors are governed by the flow structure in the wheel-space.
This paper describes results obtained from an experimental facility, which models ingress through the rim seal into the upstream wheel-space of an axial-turbine stage. The experimental rig included 32 nozzle guide vanes and 41 symmetrical turbine blades, and the paper presents measurements of (the sealing effectiveness) for single- and double-clearance seals for both over-speed (where the blades rotate faster than at the design point) and under-speed conditions. The design flow coefficient was CF = 0.538, and tests were conducted for 0 < CF < 0.9, which is larger than the range experienced in engines. The measured values of were correlated by the ‘effectiveness equations’ for rotationally-induced (RI) and externally-induced (EI) ingress. The correlated effectiveness curves were used to determine
This paper presents the experimental investigation of the stall inception of a state-of-the-art transonic compressor that resembles a typical front stage of a commercial jet engine. The compressor was designed and manufactured in cooperation with Rolls-Royce Deutschland and assembled in the Darmstadt Transonic Compressor test facility at Technische Universität Darmstadt, Germany. Utilising the 1.5-stage design it is possible to investigate stall inception for different rotor inflow conditions. The aerodynamic features prior and during stall inception are investigated by means of unsteady pressure probes in the casing above the rotor. All stall inception events have a spike-type stall character. However, in some cases the spikes are preceded by modal-type activities. In addition to the unsteady wall pressure, speed lines are presented. The combination of the data shows a clear effect of measurement position and averaging process on the shapes of the speed lines but did not allow to predict the type of stall inception.
The measurement of the sound emission of axial fans is standardized by internationally accepted guidelines. While it is known that the sound emission of fans is significantly influenced by the inflow conditions, the standards address this issue barely. A detailed definition of the inflow conditions for standardized acoustic measurements of axial fans does not exist. In this investigation, it is shown that disturbed inflow conditions exist even when the test rig meets the requirements of the current standards. Flow visualization and RANS simulation reveal unexpected vortex-like flow structures at the intake and inside the duct originating in the large anechoic chamber. These large-scale eddies in the inflow are also recognized by flush mounted pressure transducers on the blade surface of the impeller. It turns out that a hemispherical inflow control device may be suitable to get closer to the state of test rig independent acoustic measurements of axial fans.
The use of the physical linear characteristic of the modified equation of fluid motion allowed the authors to develop a novel numerical algorithm to solve advanced flow problems with reduced computational effort. The applied algorithm simplifies the velocity iteration procedure during the solver execution. The goal of the current paper is to describe an in-depth study of this numerical implementation of the modified equation of fluid motion for incompressible flow. The application of the developed solver is discussed for rotor–stator interaction in a radial pump. The results are evaluated by the numerical results from the Navier–Stokes solver and measurement data. The comparisons indicate that the developed solver, requires about 60% the computation time of the Navier–Stokes solver, and it produces physically reasonable results validated by measurement data.
The article describes investigations on the three-dimensional (3D) flowfield development near the endwall of a linear compressor cascade which is caused by specific part gap and endwall concepts of adjustable stator vanes. Their beneficial or harmful characteristics, previously measured with outlet loss and flow turning distributions, are investigated inside the passage at different stagger angles to analyse the origin and the interaction of the geometry-induced vortex system. Qualitative blade-to-blade measurements were conducted with particle image velocimetry in several spanwise positions as well as oil flow visualisation on the blade surfaces and the endwall. Improved three-dimensional (3D) numerical Reynolds-averaged Navier–Stokes (RANS) calculations with Reynolds-stress turbulence models were carried out and enhance the experimental findings. Results indicate extensive interactions between secondary flow and leakage flow through a penny gap depending on the aerodynamic loading. The part clearance vortex development and its impact to the blade boundary layers downstream the passage is visualised and enhances the understanding of the geometry effect. Also, the impact of the endwall concepts without radial clearances to the endwall boundary layer is shown and explains their beneficial characteristics compared to a reference geometry.
This study focuses on numerical simulations of a small automotive turbocharger compressor stage. Two medium speed characteristics were reproduced from nominal operating points to surge and were compared with experimental measurements. The aim of this study is to analyze the flow unsteadiness occurring near the surge line. The complete geometry of the impeller is meshed; hence, no spatio-temporal hypothesis is done during simulations. The main flow patterns are investigated to identify structures that might be responsible of surge inception. Results show that both impeller and diffuser are affected by stall. Blade channels are affected by a complete shroud recirculation extending from upstream of the impeller inlet to the diffuser inlet. Three radial recirculation zones are detected on the vaneless diffuser walls, strongly influenced by the two-pike asymmetric pressure field induced by the volute tongue. Its influence is observed at the inlet of the compressor, increasing the inter-blade flow unsteadiness.
This paper presents detailed flow-field measurements for a compressor cascade equipped with synthetic jet actuators for active flow control. The synthetic jets are mounted on the cascade sidewall and the suction side surface of the blade to reduce the total pressure loss caused by strong secondary flow structures developing in the passage. There are certain articles reporting that synthetic jets are well suited for flow control applications even in axial compressors and cascades. Most of them are focused on the parameter variation to optimize the efficiency of the control approach, still very little is known on the interaction of the synthetic jet actuators with the flow field. Detailed x-wire and pressure measurements were conducted to understand how synthetic jets influence the flow field and what causes the significant loss reduction in the cascade wake. It seems that the added momentum is not the key parameter for flow control with synthetic jets. In fact, the high mixing and the unsteadiness of the jets seem to amplify existing velocity fluctuations in the flow field. These increased fluctuations result in a shift of the shear layer between the flow separation and the surrounding flow, and thus in a dethrottling of the compressor cascade. Together with the increased mixing, loss reductions of approximately 10% can be reached using synthetic jet actuators.