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.     

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.     

Steady and unsteady flow characteristics of dual cavity in strut injection scramjet combustor

This study aims to understand the flow performance of a dual wall-mounted cavities in a strut-injector mounted scramjet combustor for steady-state and transient reacting conditions. Conventionally, two-dimensional Reynolds Averaged Navier-Stokes approach is adopted to compute the steady flow, whereas the current research employs Delayed Detached Eddy Simulation for predicting the unsteady flow characteristics as well. The calculated flow patterns, density, pressure, and temperature fields of dual cavities are compared with shadowgraph and wall pressure measurements from DLR experiments. The dual cavities position substantiates to explore the interplay between wave propagation and shear layer mixing characteristics. Employing a dual cavities arrangement accelerates toward the complete combustion relative to the baseline model. The combustion zone widens in the lateral direction as the dual cavities shift the shock train downstream of the strut injector owing to intense shock shear layer interactions. These cavities' existence significantly modifies the dominating frequencies and affects the strength of the diverging section's coherent flow structures.     

Numerical study on the interaction characterization of rotating detonation wave and turbine rotor blades

Rotating detonation as a kind of pressure gain combustion is expected to greatly improve efficiency when applied to gas turbine engines. In this paper, the operation of rotating detonation combustor and turbine rotor blade was studied. Firstly, the analysis of the interaction between detonation wave and turbine blade shows that the compression of gas by detonation wave and reflected wave will lead to a sharp increase in the temperature at the wall of blade. When the detonation wave propagates, the oscillation amplitudes of pressure and temperature at the turbine inlet are 70% and 75% respectively, and the detonation oblique shock will change the flow trajectory of the air flow, resulting in the flow direction deviating from the incident angle. Then the comparison between detonation and deflagration shows that the total pressure of detonation is higher and will have greater work potential. The torque generated by the blades under detonation has the characteristics of high-frequency oscillation, which may be detrimental to the operation of the engine.     

Numerical and experimental study on contact face and shock wave motion in the receiving tube of gas wave refrigerator

The contact face and shock wave motion in an open ends receiving tube of gas waverefrigerator are investigated numerically and experimentally.The results show that,velocity of the contact face rises rapidly as gas is injected into the receiving tube,anddrops sharply after a steady propagation.However,velocity of the shock wave in thetube is almost linear.With increasing of inlet pressure,velocity of the shock waveand steady velocity of contact face also increase.In addition, time and distance ofcontact face propagation in the receiving tube become longer.     

Numerical study on supersonic combustion with cavity-based fuel injection

The present study describes the numerical investigations concerning the combustion enhancement when a cavity is used for the hydrogen fuel injection through a transverse slot nozzle into a supersonic hot air stream. The cavity is of interest because recirculation flow in cavity would provide a stable flame holding while enhancing the rate of mixing or combustion. Several inclined cavities with various aft wall angle, offset ratio and length are evaluated for reactive flow characteristics. The cavity effect is discussed from a viewpoint of total pressure loss and combustion efficiency. The combustor with cavity is found to enhance mixing and combustion while increasing the pressure loss, compared with the case without cavity. But it is noted that there exists an appropriate length of cavity regarding the combustion efficiency and total pressure loss.     

A study of the weak shock wave propagating over a porous wall/cavity system

The present computational study addresses the attenuation of the shock wave propagating in a duct, using a porous wall/cavity system. In the present study, a weak shock wave propagating over the porous wall/cavity system is investigated with computational fluid dynamics. A total variation diminishing scheme is employed to solve the unsteady, two-dimensional, compressible, Navier-Stokes equations. The Mach number of an initial shock wave is changed in the range from 1.02 to 1.12. Several different types of porous wall/cavity systems are tested to investigate the passive control effects. The results show that wall pressure strongly fluctuates due to diffraction and reflection processes of the shock waves behind the incident shock wave. From the results, it is understood that for effective alleviation of tunnel impulse waves, the length of the perforated region should be sufficiently long.     

Numerical analysis of hydrogen fueled scramjet combustor with innovative designs of strut injector

In this research paper, passive method has been considered to improve the mixing and combustion efficiency of scramjet combustor. Modified and innovative strut injector designs have been developed from the reference of DLR scramjet. Strut is the mechanical component which is placed in the flow stream of supersonic airflow and used to inject the fuel. The newly introduced strut injector models are named as rocket and double arrow shape strut injectors. Numerical analysis of computational domains with innovative strut injectors has been carried out by considering the ANSYS – FLUENT 15.0, which is a numerical simulation tool. Shear stress transport model (SST k−ω model) has been considered for turbulence modeling under high speed flows. From the numerical results, it is observed that the compressed air entered into the combustion chamber is at higher pressure than the basic DLR scramjet model and it reduces the ignition delay. It is also found that both the rocket and double arrow strut models increases the boundary layer separation and vortex formation and therefore it enhances the better mixing of fuel and air and finally it improves the combustion efficiency.     

Effect of variation of hydrogen injection pressure and inlet air temperature on the flow-field of a typical double cavity scramjet combustor

In the present research work, computational simulation of the double cavity scramjet combustor have been performed by using the two-dimensional compressible Reynolds-Averaged Navier–Stokes (RANS) equations coupled with two equation standard k–ɛ turbulence model as well as the finite-rate/eddy-dissipation reaction model. All the simulations are carried out using ANSYS 14-FLUENT code. Additionally, the computational results of the present double cavity scramjet combustor have been compared with experimental results for validation purpose which is taken from the literature. The computational outcomes are in satisfactory agreement with the experimentally obtained shadowgraph image and pressure variation curve. However, due to numerical calculation, the pressure variation curve obtained computationally is under-predicted in 5 locations. Further, analyses have been carried out to investigate the effect of variation of hydrogen injection pressure as well as the variation of air inlet temperature on the flow-field characteristics of scramjet engine keeping the Mach number constant. The obtained results show that the increase in hydrogen injection pressure is followed by the generation of larger vortex structure near the cavity regions which in turn helps to carry the injectant and also enhance the air/fuel mixing whereas the increase in the inlet temperature of air is characterised by the shifting of incident oblique shock in the downstream of the H₂ injection location. Again for T₀ = 1500 K, the combustion phenomena remains limited to the cavity region and spreads very little towards the downstream of the combustor.     

Implication of diamond shaped dual strut on combustion characteristics in a cavity-based scramjet combustor

The present study deals with the implication of the novel diamond-shaped dual strut with a backward-facing step on the combustion characteristics of a cavity-based scramjet combustor. The dual strut with diamond shape is considered for the investigation since it triggers the flow separation leading to the disturbances in the flow especially at the top and bottom wall thus serving the flame holding purpose. Firstly, the combustion characteristics of a combustor with a single strut are compared and evaluated with a dual strut based combustor to portray the effect of a dual strut. A separate study is carried out to investigate the influence of spacing in a dual strut on the performance of the scramjet combustor. Our study reveals that the dual strut with a cavity greatly affects the formation of vortices, separation region, and recirculation region which is evident from an increase in the mixing and combustion efficiency. It can be observed that the formation of vorticity and the recirculation region is found to be larger due to the strong and multiple reflections of a shock in the case of the diamond-shaped dual strut with a backward-facing step injection as compared to the single diamond-shaped strut. Further, it is observed that the spacing(D) in the dual strut also affects the mixing and combustion performance. It is found that the value of mixing and combustion efficiency decreases with an increase in spacing independent of Mach number due to the presence of larger separation regions. It can also be observed that the length of the recirculation region occupies entire the cavity making flame stable when the spacing is the least. This is desirable as far as engine performance is concerned.     

Numerical study of shock waves propagating through right angled multiple elbows

Introduction The elbow is one of the important elements used in pipe lines, and it is very important to clarify the propagating phenomena of shock waves through the elbow for engineering applications. Although some investigations of the propagating shock in the single and double elbows have been carried out[1?The working gas is air. The numbers of the grid in the computational domain are 251·(a) Type 4-1 (b) Type 4-2 Fig.3 Pressure distributions on each wall the merging with the 2nd shock…     

Characteristics of Spherical Shock Wave and Circular Pulse Jet Generated by Discharge of Propagating Shock Wave at Open End of Tube

When the shock wave propagating in the straight circular tube reaches at the open end, the impulsive wave is generated by the emission of a shock wave from an open end, and unsteady pulse jet is formed near the open end behind the impulsive wave under the specific condition. The pulse jet transits to spherical shock wave with the increase in the strength of shock wave. The strength is dependent on the Mach number of shock wave, which attenuates by propagation distance from the open end. In this study, the mechanism of generating the unsteady pulse jet, the characteristics of the pressure distribution in the flow field and the emission of shock wave from straight circular tube which has the infinite flange at open end are analyzed numerically by the TVD method. Strength of spherical shock wave, relation of shock wave Mach number, distance decay of spherical shock wave and directional characteristics are clarified.     

Investigation of counter-rotating shock wave phenomenon and instability mechanisms of rotating detonation engine with hollow combustor and Laval nozzle

The counter-rotating shock wave phenomenon and instability mechanisms in the hollow rotating detonation combustor with a Laval nozzle are investigated. The in-house solver BYRFoam based on the OpenFOAM platform and the detailed chemical reaction kinetic mechanism are used. The coupling of the detonation wave and the shock wave is revealed, with the continuous presence of the counter-rotating shock wave in the combustor that propagates in the opposite direction to the detonation wave and oscillates in intensity. The intensities of both the detonation wave and the counter-rotating shock wave are found to be constantly oscillating, and this instability is referred to as the collision-oscillation instability. It is caused by the complex interaction between the detonation wave, the fresh gas and the counter-rotating shock wave. The velocity difference between the detonation wave and the counter-rotating shock wave is found to lead to the migration of the collision point, which in turn leads to the peak periodic undulation phenomenon of the pressure curve, that is, the wave packet. This instability is called the counter-rotating instability, which co-exists with the collision-oscillation instability for a long time. The results of this study are compared with the experimental data, and the reasons for the oscillation of the experimental pressure signal are explained.     

Effect of duct geometry on shock wave discharge

This paper describes computational work to understand the unsteady flow-field of a shock wave discharging froman exit of a duct and impinging upon a flat plate.A flat plate is located downstream, and normal to the axis of theduct.The distance between the exit of the duct and fiat plate is changed.In the present study,two different ductgeometries(i.e.,square and cross section)are simulated to investigate the effect of duct geometry on theun-steady flows of a shock wave.In computation,the total variation diminishing(TVD)scheme is employed tosolve three-dimensional,unsteady,compressible,Euler equations.Computations are performed over the range ofshock Mach number from 1.05 to 1.75.Computational results can predict the three-dimensional dynamic behav-iour of the shock wave impinging upon the flat plate.The results obtained show that the pressure increase gener-ated on the plate by the shock impingement depends on the duct geometry and the distance between the duct exitand plate,as well as the shock Mach number.It is also found that for the duct with cross-section,the unsteadyloads acting on the flat plate are less,compared with the square duct.     

Characteristics of the oblique detonation flow field induced by a complex wave structure

In this paper, the initiation characteristics of the oblique detonation flow field induced by single- and double-wedge surfaces of finite length in a confined space are investigated. Numerical simulations with a detailed H₂/air reaction and theoretical shock polar analyses are combined to study the influence mechanism of a complex wave system structure on the characteristics of the oblique detonation. The effects of expansion waves on the oblique detonation waves (ODWs) and their flow field characteristics for different equivalent ratios and geometric sizes are analyzed in single-wedge and double-wedge structures with the same inflow parameters. The results show that the length of the induced ODW is shorter in the double-wedge structure than in the single-wedge structure. For the single-wedge structure, the strength of the expansion wave increases, the wall temperature drops, and the characteristic length of the induction zone decreases with increasing deflection angle of the second wedge. If the strength of the expansion wave is sufficiently large, the ODW is initiated. For the double-wedge structure, the ODWs interact and form a complex wave system structure, consisting of a Mach stem, two reflected detonation waves and slip lines. The length and the temperature before and after the Mach stem decrease with an increase in the strength of the expansion waves. The effects of the expansion waves on the flow field of the ODW are relatively small at a large equivalent ratio and significantly larger at a small equivalent ratio.     

Effects of injection strategies on thermal performance of a novel micro planar combustor fueled by hydrogen

As a high and uniform wall temperature is desired for thermophotovoltaic applications, a novel micro planar combustor with multi inlets and outlets is designed in this work. Effects of injection strategies on thermal performance of the novel micro planar combustor fueled by hydrogen are analyzed and discussed. It is found that there are two straight flames under Injection Strategy A (Coflow mode), while there are two curved flames under Injection Strategy B and C (Counterflow mode), which is negative to the flame stabilization. In addition, the flame interactions under Injection Strategy B and C are much stronger than that under Injection Strategy A which benefits flame stabilization. Moreover, favorable and unfavorable effects of flame interactions under Injection Strategy B and C on flame stabilization are analyzed. Furthermore, the effects of inlet velocity, hydrogen/air equivalence ratio and solid wall materials on the thermal performance of the micro planar combustor are numerically examined under different injection strategies, the application conditions of the injection strategies are determined. This work will provide us significant suggestions for micro-thermophotovoltaic applications.     

Unsteadiness of Shock Wave／Boundary Layer Interaction in Supersonic Cascade

ＵｎｓｔｅａｄｉｎｅｓｓｏｆＳｈｏｃｋＷａｖｅ／ＢｏｕｎｄａｒｙＬａｙｅｒＩｎｔｅｒａｃｔｉｏｎｉｎＳｕｐｅｒｓｏｎｉｃＣａｓｃａｄｅＵｎｓｔｅａｄｉｎｅｓｓｏｆＳｈｏｃｋＷａｖｅ／ＢｏｕｎｄａｒｙＬａｙｅｒＩｎｔｅｒａｃｔｉｏｎｉｎＳｕｐｅｒｓｏｎ...     

受限通道中三维激波与侧壁面作用的数值模拟研究

为了了解超燃冲压发动机中三维斜激波与壁面的相交情况,以冷态模拟为基础,对两种不同形状的斜劈结构产生的斜激波进行数值模拟。当斜劈展向与气流方向成45.0°时,激波和膨胀波结构呈现三维特性,并且激波在两侧壁面处的反射现象不同。以激波面上游为视角,当激波与壁面相交成锐角时会产生马赫反射;而激波与壁面相交成钝角则会以叠加膨胀波的形式减弱激波,并且使激波面的法线方向与壁面的法线方向垂直。由于两侧壁面的影响,随着高度的增加激波强度减弱,同时气体经过斜激波后偏转角度也减小。对于构型Ⅱ,有斜劈的区域产生三维斜激波会延伸到无斜劈区域,此时下壁面与斜激波相交成钝角,同样会以叠加膨胀波的形式减弱斜激波。     

Flow control effect on unsteadiness of shock wave induced separation

In usual cases of significant pressure gradients and strong shocks, the front shock takes a fixed location along the wall, at which separation starts. Usually the rear shock is responding to vortex sheding by its deflection angle. In consequence main shock and rear shocks are moving whilst front shock is stable. The goal of the measurements presented here is to find out how the k-foot behaves during shock oscillations in the case when front shock is not fixed by the pressure gradient. Unsteady shock behaviour is also investigated when air jet vortex generators （AJVG） are used. Counteraction of the separation is directly related to the influence on unsteady processes in the shock wave induced separation.