Experimental investigation on starting process of supersonic single-stage air ejector

This paper deals with experimental study of flow field of starting process in two-dimensional, single-stage supersonic ejector on different air total pressure. Schlieren pictures of flow field were taken, static pressure distribu-tions on side wall were measured. The obtained results show that, on critical pressure, the starting main shock waves in ejector oscillated back and forth between the second throat and the middle section of the mixing chamber, it causes the pressure in the second half of the mixing chamber acutely fluctuated .When the working pressure of the active flow is higher than the critical starting pressure, ejector starts normally and the inner flow-field of the mixing chamber keeps stable and the shock waves in the second throat have a certain degree of oscillation . After ejector starts, the operating pressure of the active flow may be lower than the starting pressure .     

Numerical analysis of Chevron nozzle effects on performance of the supersonic ejector-diffuser system

The supersonic nozzle is the most important device of an ejector-diffuser system.The best operation condition and optimal structure of supersonic nozzle are hardly known due to the complicated turbulent mixing,compressibility effects and even flow unsteadiness which are generated around the nozzle extent.In the present study,the primary stream nozzle was redesigned using convergent nozzle to activate the shear actions between the primary and secondary streams,by means of longitudinal vortices generated between the Chevron lobes.Exactly same geometrical model of ejector-diffuser system was created to validate the results of experimental data.The operation characteristics of the ejector system were compared between Chevron nozzle and conventional convergent nozzle for the primary stream.A CFD method has been applied to simulate the supersonic flows and shock waves inside the ejector.It is observed that the flow structure and shock system were changed and primary numerical analysis results show that the Chevron nozzle achieve a positive effect on the supersonic ejector-diffuser system performance.The ejector with Chevron nozzle can entrain more secondary stream with less primary stream mass flow rate.     

Transonic instability in entrance part of mixing chamber of high-speed ejector

Introduction The research of flow structure in the entrance part of the mixing chamber of two-dimensional supersonic ejector[1,2] shows, how this structure depends both on stagnation pressure ratio of streams p01/p02[3] and on back pressure ratio pb/p02 [4]. It was found out that the structure of shock waves is not stable, but it oscillates less or more. For the high back pressure ratio a terminal shock wave is in the mixing chamber and due to this shock wave the mixing processes change quali…     

LES study on flow features of the supersonic mixing layer affected by shock waves

The supersonic mixing layer in a model scramjet has been simulated using large eddy simulation (LES) with the consideration of complex shock waves. A comparison with experimental data demonstrates the accuracy and applicability of the employed numerical models and computational methods based on the OpenFOAM solver. The flow features are examined not only from the visualization of flow field, but also from a fundamental point of fluid motion with a focus on evaluating the impacts of shock waves on the turbulent mixing layer. Four basic types of interaction between the shock waves and the mixing layer are discussed according to the flow features and the field development. The mixing properties are analyzed from the evolution of the mixing layer thickness with emphasis on mixing efficiency and total pressure recovery. Results show that three typical developing regions can be observed for the supersonic mixing layer in the scramjet. Shock waves make contributions to the fuel-air mixing due to the amplification of turbulence and the gain of vorticity. The increase in total pressure losses is unavoidable despite the increased mixing efficiency in the presence of shock waves. Additionally, a significant decrease of the convective Mach number is caused by the expansion-fan/shock-wave pattern at the injector exit, leading to a reduction in compressibility effects, which is an important aspect of the physics for the development of the mixing layer in the model scramjet.     

Structural optimization and experimental investigation of supersonic ejectors for boosting low pressure natural gas

The supersonic ejector was introduced into boosting the production of low pressure natural gas wells. The energy of high pressure gas wells, which was usually wasted through choke valves, was used as its power supply to boost the low gas production. The operating performance of natural gas ejectors was determined not only by the operating parameters but also by the structural parameters. This study focused on the structural optimization and operating performance of natural gas ejectors. The optimal structural parameters were obtained by numerical simulation when the maximum pressure ratio was obtained, and the numerical results were validated by experimental investigation. The numerical results showed that the optimal diameter ratio of mixing tube to primary nozzle throat was 1.6, the optimal length to diameter ratio of mixing tube was 4.0 and the optimal inclination angle of mixing chamber was 28°. The entrainment ratios and pressure ratios from the numerical simulation agreed well with the field experimental data, with the maximum value of pressure ratio up to 60%. The operating performance of the supersonic ejector was also investigated by the field experiment, and the results showed that the induced gas flowrate and entrainment ratio showed nonlinear characteristics with peak values when the motive pressure ranged from 8 MPa to 13 MPa. These experimental results have proved the optimized structural parameters of the supersonic ejector. The investigation will help to the further application in boosting natural gas production of supersonic ejector.     

Numerical investigation of wavy wall strut fuel injector for hydrogen fueled scramjet combustor

Mixing and combustion of a fuel with supersonic airstream in a scramjet combustor is a complex phenomenon because of very less resident time of the air in the combustion chamber. Mixing of fuel and air at supersonic speed and the subsequent combustion are greatly affected by the disturbance of the flow field in the form of shock waves, vortices and recirculation regions. In this research paper, the same concept has been considered by introducing an innovative strut fuel injector for the development of more shock waves and streamline vortices. The basic or standard computational domain of the scramjet combustor is considered from the reference of DLR experimental scramjet. The basic scramjet model consists of the wedge-shaped strut fuel injector. In this research, the strut injector has been re-designed such a way that to generate more oblique shock waves. Numerical analysis of the scramjet internal flow field has been performed with basic and innovative strut by solving the Reynolds-averaged Navier-Stokes equations with the help of computational fluid dynamics tool defined as ANSYS-FLUENT 16.0. The internal flow field of scramjet combustor with basic and innovative strut fuel injectors has been visualized from the analysis of pressure, temperature and velocity along with the analysis of flow structure, shock waves, and streamlines vortices. From the analysis of numerical results, it is identified that multiple numbers of oblique shock waves are being generated from the leading curved edge of the newly introduced strut. Both the pressure and temperature of airstream at the entrance of the combustion chamber are higher in the case of the wavy wall strut and it reduces the ignition delay time as compared to the basic strut model.     

Modelling and optimization of two-phase ejectors for cooling systems

A one-dimensional compressible flow model, which is based on the control volume approach, has been formulated to model and optimize one and two-phase ejectors in steady-state operation with particular reference to their deployment in a jet cooling system. The working fluid can be both single-component (NH₃) and two-component (NH₃–H₂O). The developed model takes into account the duct effectiveness, wall friction, momentum loss, ejector geometry, shock waves as well as the acceleration of the induced flow in the conical part of the mixing section. Neither the usual assumption of mixing at constant pressure over the mixing chamber cross section nor that of a constant mixing chamber cross section were made. A comparison with other computation methods as well as with available experimental data from the literature is presented. The performance is significantly influenced by the ejector geometry.     

Experimental and Numerical Analysis on the Internal Flow of Supersonic Ejector Under Different Working Modes

Supersonic ejectors involve very complex phenomena such as interaction between supersonic and subsonic flows, shock trains, instabilities, which strongly influences the performance of supersonic ejector. In this study, the static pressure distribution along the ejector wall and Mach number distribution along the axis are used to investigate the internal flow field of supersonic ejector. Results indicate that when the back pressure is much less than the critical back pressure, there are two series of shock trains, and the change of the back pressure will not affect the flow field before the effective area section, so the entrainment ratio would remain constant. The second shock train moves further upstream and is combined with the first shock train to form a single shock train as the back pressure rises. When the back pressure is greater than the critical back pressure, the position of the shock train, the static pressure at its upstream and the entrainment ratio, will be affected. The “effective area section” in the mixing tube is obtained. The effective area section position moves downstream with the increase of the primary flow pressure, while it moves upstream with the increase of the secondary flow pressure. The entrainment ratio shows inversely proportional relationship with the effective section position. Besides, the first shock train length increases with the increase of primary flow pressure or secondary flow pressure. The critical back pressure represents direct proportional relationship to the first shock train length.     

Effect of oblique shock wave induced from a wavy-wall combustor surface for a splitter plate scramjet combustor

The shorter residence period of supersonic air in a scramjet combustor makes mixing and combustion challenging. Mixing augmentation occurs at the fuel-supersonic air interface. Multiple interactions between shock waves and the shear layer may significantly affect this inter surface. In this research, an attempt has been made to analyze how multiple oblique shock waves interact with the shear layer. The primary splitter plate combustor bottom wall is modified with a wavy-wall surface to ensure the development of multiple oblique shock waves. The internal flow field with and without a wavy wall surface has been analyzed by solving the two-dimensional Reynolds averaged Navier-Stokes equations and SST k-ω turbulence model. The reaction between ethylene fuel and the air is modeled with a global one-step reaction mechanism with finite rate eddy dissipation turbulence chemistry interaction. The flow disturbances with the wavy-wall surface have been evaluated by analyzing the numerical results like the flow structure, pressure, velocity, reaction rate, vortices, turbulence intensity, and interactions among shock wave, shear mixing, and boundary layers. The oblique shock waves induced from the wavy-wall surface significantly impact the mixing of fuel and air and successful reaction mechanism from the visualization of flow structure and concern results.     

A novel thermally driven rotor-vane/pressure-exchange ejector refrigeration system with environmental benefits and energy efficiency

The latest results of an ongoing coordinated experimental and computational program on the design and performance of a novel supersonic rotor-vane/pressure-exchange ejector for thermally driven ejector refrigeration systems are presented. For the supersonic rotor-vane/pressure-exchange ejector, careful management of the entropy rise through the oblique shocks and boundary layers is required for obtaining an advance in ejector performance. Since the invention of this new ejector is quite recent, understanding its aerodynamics, with the consequent optimization of performance, is in the formative stage. This paper shows how the supersonic aerodynamics is managed to provide the desirable flow induction characteristics through computational study and, in parallel, experimental results including flow visualization showing actual behavior with different-shaped rotor vanes. The importance of the existence of the tail part with a long expansion ramp, the sharp leading edge such as knife-edge, the proper height of leading edges, for the overall shape of rotor vane, were observed. Also the larger spin-angle rotor vane produces better flow induction and mixing between primary flow and secondary flow.     

Numerical characterization of 3D nonreacting supersonic cavity combustor with inlet Mach number variation

A numerical investigation of a cavity-based supersonic combustor with non-reacting upstream hydrogen fuel injection is conducted to study the effects of inlet Mach number (Ma) on flow structure and fuel-air mixing. Three different freestream Mach number cases (1.5, 2.5 and 3.5) are investigated at a constant fuel flow rate, injected at the sonic condition by considering governing equations for compressible, turbulent flow using Shear Stress Transport (SST) k-ω model. The complex flow structure is investigated by identifying various flow features namely, upstream three-dimensional bow shock, compression waves, Mach reflection, vortex in the separated boundary layer and horseshoe vortices at the downstream of the injection port. Besides this, the flow physics involved in these complex flow features are unravelled. Moreover, the performance of the combustor is characterized quantitatively in terms of mixing efficiency, total pressure loss and coefficient of pressure. However, the mixing efficiency and total pressure loss for the operating condition of Ma = 1.5, exhibits better performance than that of the other Mach number cases (2.5, 3.5) due to decrease in inclination angle of reattachment shock from 47.6° to 29.9°. The present numerical investigation also demonstrates that the three-dimensional simulation is essential in the characterization of fuel-air mixing in supersonic cavity-based combustors.     

二维流动模型的喷射器性能分析研究

采用二维轴对称流动模型，计算分析了吸入通道内回流现象、喷射器“恒能力”现象与静压力在轴线上分布情况之间的关系；探讨了工作压力对喷射器性能的影响。结果表明：持续降低出口压力会在混合室内形成激波，喷射因数保持不变；工作压力过高会在混合室内产生壅塞，反而降低喷射因数；吸入压力过低会在喷射器吸入通道内产生回流现象，影响喷射式制冷系统运行的安全性。     

基于三维数值模拟的蒸汽喷射器结构优化

蒸汽喷射器可以有效利用余热资源,是一种节能环保的流体机械。采用计算流体力学(CFD)软件对蒸汽喷射器进行三维数值模拟,研究了喷嘴出口马赫数、混合室长度、等截面积混合段长度和扩压室长度对喷射器引射比和临界压力的影响。模拟结果表明:混合室长度存在一个最佳值,且最佳混合室长度与喷嘴出口马赫数有关;等截面积混合段长度和扩压室长度对引射比的影响不大,但是会影响喷射器的临界压力。研究结果对于蒸汽喷射器的设计和结构优化有一定的意义。     

Effects of cavity-induced mixing enhancement under oblique shock wave interference: Numerical study

In this research, the effects of oblique shock on the mixing characteristics in a supersonic combustor equipped with a cavity is numerically investigated. To reveal the flow structure of the supersonic flow field under oblique shock wave interference, three-dimensional steady RANS equations and SST k-ω turbulence model are adopted. The current work focuses on comparing the interaction effects between oblique shock wave and bow shock wave, which are formed by fuel jet on fuel mixing under different conditions. The numerical analysis demonstrates that an optimal angle exists for the mixing efficiency of the ramp. The optimal angle diminishes as the jet-to-crossflow pressure ratio increases. The oblique shock wave in a certain range is conducive to enhance the penetration depth of ethylene. The smaller angle of the ramp does not cause large stagnation pressure losses.     

Numerical study on supersonic combustion of hydrogen and its mixture with Ethylene and methane with strut injection

In this paper, supersonic combustion and flow field of hydrogen and its mixture with ethylene and methane from strut injections in a Mach 2 supersonic flow are studied numerically. The fuel mixture of hydrogen, methane and ethylene represents the major products of pyrolysis of hydrocarbon fuels with large molecules such as kerosene as it acts as coolant and flows through cooling channels and absorbs heat. Detached Eddy Simulation with a reduced kinetic mechanism and steady flamelet model are applied to simulate turbulent combustion. The calculated temperature profiles of hydrogen are compared to the experimental results of DLR supersonic combustor for validation of the present numerical method. The supersonic combustion flows associated with shock waves, turbulent vortices and flame structures are studied. With addition of methane and ethylene, the flame zone moves further downstream of the strut and the maximum flow temperature at chamber exit decreases by 200 K. With analysis of total temperature ratios, it is found that combustion efficiency for hydrogen combustion is 0.91 and it decreases to 0.78 for the fuel mixture. The calculation of ignition delay time and flame speed reveals that fuel mixture of hydrogen and hydrocarbons has considerably larger delay time and smaller flame speed, that contributes to the weakened flame zone and lower combustion efficiency.     

Application of profiled ejector in chemical lasers

Recently, the use of profiled ejectors based on constant rate of momentum change [I.W. Eames, Applied Thermal Engineering 22 (2002) 121] along the mixing chamber has been proposed for enhancing the recovery ratio across an ejector stage by minimizing shock losses for application in ejector based refrigeration system. Such ejectors can achieve pressure recovery ratio in excess of 150, thus making the system a compact one. Chemical lasers in general and chemical oxygen-iodine laser (COIL) in particular fall in the high power lasers category and find numerous applications in defense and industry. However, these lasers have not been exploited fully because these require pressure recovery systems for their operation and as such the practical systems are extremely voluminous and bulky. The profiled ejectors find direct applications in these lasers and thus can make the system extremely compact. The conventional supersonic COIL systems operate at a typical stagnation pressure of nearly 20 torr and a cavity static pressure of approximately 3 torr, which are amongst the lowest in the class of chemical lasers. Thus, a low-pressure operation of the laser system demands a high capacity vacuum system. Alternatively, efficient ejector based pressure recovery system has been utilized for achieving direct atmospheric exhaust of the lasing medium. However, a minimum of two-stage conventional supersonic ejectors need to be employed for the operation of the laser system. Multiple stages of the ejector are essential on account of the stagnation pressure loss occurring across a normal shock at the exit of the mixing chamber in each ejector stage. The present study presents a general treatment on the design of a profiled ejector for the case of dissimilar motive and suction fluids that are typical of these lasers. Also, determinations for the increase in recovery ratio for various conditions of entrainment ratio over the conventional ejectors have also been presented. Finally, a computational study using McCormack’s method for Euler system of equations has been carried out to numerically validate the analytical studies for a peripheral air ejector system suitable for a 500 W class COIL employing a flow rate of 3 gm/s with an entrainment ratio of 0.025. It has been concluded that a single-stage profiled ejector is sufficient to achieve atmospheric pressure recovery even in the low-pressure systems.     

Performance prediction of steam ejector using computational fluid dynamics: Part 1. Validation of the CFD results

The aim of this study is to investigate the use of CFD in predicting performance of a steam ejector used in refrigeration applications. This study is reported in a series of two papers. In this part, the CFD results were validated with the experimental values. The effects of operating conditions and geometries on its performance were investigated. The CFD's results were found to agree well with actual values obtained from the experimental steam jet refrigerator. The CFD was found to be not only a sufficient tool in predicting ejector performance it also provide a better understanding in the flow and mixing processes within the ejector. Phenomena on choke flow, mixing behavior, jet core effect and the presence of oblique shock will later be discussed in Part 2.     

The effect of secondary flows on the starting pressure for a second-throat supersonic ejector

The effect of the secondary flow on the starting pressure of a second-throat supersonic ejector has been investigated by adapting the height of the secondary flow inlet. The obtained results show that an optimum value of the secondary inlet height exists, and the starting pressure of the ejector becomes a minimum at that condition. Based on the results of the pressure measurements, a qualitative analysis has been made to clarify the flow behavior and the physical meaning of the performance diagram. It appears that the choking phenomenon of the secondary flow plays an important role in the starting process of the ejector. When the secondary inlet height is relatively small, the choked secondary flow and the supersonic primary flow could be employed to protect the static pressure in the suction chamber from being disturbed by the back pressure effect at a certain primary stagnation pressure, which is lower than the starting pressure for the case of the zero-secondary flow. However, as the secondary inlet height increases and exceeds a critical value, the static pressure in the suction chamber rapidly increases, and the starting pressure of the ejector increases accordingly.     

Navier—Stokes Computations of the Supersonic Ejector—Diffuser System with a Second Throat

The supersonic ejector-diffuser system with a second throat was simulated using CFD.An explicit finite volume scheme was applied to solve two-dimensional Navier-Stokes equations with standard κ-εturbulence model.The vacuum performance of the supersonic ejector-diffuser system was investigated by changing the ejector throat area ration and the operating pressure ratio.Two convergent-divergent nozzles with design Mach number of 2.11 and 3.41 were selected to give the supersonic operation of the ejector-diffuser system.The presence of a second throat strongly affected the shock wave structure inside the mixing tube as well as the spreading of the under-expanded jet discharging from the primary nozzle.There were optimum values of the operating pressure ratio and ejector throat area ratio for the vacuum performance of the system to maximize.