Combustion stabilization based on a center flame strut in a liquid kerosene fueled supersonic combustor

A newly designed strut is proposed in this paper for fuel injection and flame holding in a liquid-kerosene-fueled supersonic combustor.The thickness of the strut is 8mm and the front blockage is about 8%.The characteristic of this strut is that extra oxygen can be injected through a set of orifices at the back of the strut,which can change the local flow field structure and ER(Equivalence Ratio).Based on the above mentioned strut,a stable local flame is generated at the back of the strut and the main combustion can be organized around this local fire.Numerical simulation is conducted to compare the local flow field distribution at the back of the strut with/without extra oxygen injection.Experiments are conducted to test the combustion characteristics based on this fuel injection and flame holding strategy.The temperature distribution which can reflect the local flame characteristic has been measured in the experiments conducted under cold incoming supersonic air flow condition.In addition,the overall combustion performance in a full-scale supersonic combustor has been evaluated in the experiments conducted under hot incoming supersonic air flow condition.Results show that this strut strategy is very promising since it can organize stable supersonic combustion at the center of the combustor without any cavity or rearward facing step.Besides that,even with the 8mm thick strut,the combustion can be stable in a wide range of ER from 0.25-1 by using liquid room-temperature kerosene.     

Numerical investigation on implication of innovative hydrogen strut in comparison with multi strut injector on performance and combustion characteristics in a scramjet combustor

Detailed numerical analysis on a cavity-based scramjet combustor is carried out by introducing an innovatively shaped strut and multi strut with backward-facing step to generate intense vorticity, which helps in efficient mixing of fuel and oxidizer. In this study, the flow dynamics with finite volume approach on commercial software Ansys-Fluent 20.0 to solve the compressible two-dimensional fluid flow with RANS equation by considering the density-based solver with SST k- ω turbulent model. The species transport model with volumetric reaction and finite rate/eddy dissipation turbulence chemistry interaction is adopted to study the combustion phenomena. Numerically calculated results are validated with its corresponding experimental results by comparing pressure distribution along the length of the combustor, distribution of H₂ mole fraction for different axial locations of combustors, and it is found that the interaction of the shear shock layer enhances the mixing rate by intensifying turbulence. It is found that the multi strut improves the mixing and combustion efficiency compared with that of the single strut owing to the formation of a significant separation layer, resulting in multiple shocks, vortices, and a larger recirculation zone.     

Numerical investigation on implication of dual cavity on combustion characteristics in strut based scramjet combustor

The present study investigates the implication of a dual cavity on combustion and mixing characteristics of strut based combustor numerically. A scramjet combustor with and without cavity are considered to evaluate the influence of cavity on combustion characteristics. Moreover, the effect of spacing between the cavities on combustion performance is also discussed. A separate study is carried out to understand the interplay of the Mach number and the spacing between the cavities on combustion characteristics. Our study reveals that the introduction of a dual cavity in scramjet combustor enhances the mixing and improves combustion performance. Further, it is found that the mixing length is lesser and the penetration height is more when a dual cavity is employed in a combustor. It can also be observed that the size of the recirculation region is found to be larger for the combustor with dual cavity among all configurations at the spacing S₂ = 12.7 mm. Further, the combustion efficiency and fuel penetration height are found to be maximum at Mach 2.5 with a spacing S₂ = 12.7 mm.     

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.     

Effect of variation of inlet boundary conditions on the combustion flow-field of a typical double cavity scramjet combustor

The present research work deals with the numerical simulation of double cavity scramjet combustor by using two equation standard k-$\epsilon$ turbulence model and finite-rate/eddy-dissipation reaction models which is again coupled with Reynolds-Averaged Navier-Stokes (RANS) equations to investigate the influence of variation of inlet boundary condition of air as well as H₂ fuel on the combustion flow-field of scramjet engine subsequently. At the same time, the validations of the current computational approach have been completed against a standard experimental data which is available in the literature. An acceptable similarity is observed between present numerical approach with the experimentally attained schlieren photograph and the pressure distribution curve. In the present work, 8 different cases are studied. Among them, first four cases are investigated for the variation of inlet boundary condition of air and the remaining four cases are studied for the variation of inlet boundary condition of H₂ fuel. The obtained results show that the formation of high-pressure region around the cavities for case 3 and case 4 actually helps to push the greater amount of air to the cavity region where it is mixed with adequate amount of H₂ for proper and stable combustion whereas for case 6, it is observed that most of the combustion phenomena closely fitted into a small space of the combustor and mainly occurs near the cavity region.     

Investigation of flow characteristics in supersonic combustion ramjet combustor toward improvement of combustion efficiency

Supersonic combustion ramjet (scramjet) is a variant of ramjet in which the combustion takes place at supersonic velocity. The flow physics inside the scramjet combustor is quite complex due to the fact that the mixing and completion of the combustion take place in a short time, which is of the order of milliseconds. This study focuses on flow characteristics within the combustion chamber of the scramjet engine that is designed to improve energy efficiency by enhancing combustion efficiency. The effect on combustion performance and thereby the energy efficiency on using strut‐based flame stabilizer is evaluated at different positions. Reynolds averaged Navier‐Stokes equations are solved with the Shear Stress Transport k‐ω turbulence model. Single strut configuration is used to validate with the experimental data. Single strut is then compared with three‐strut configuration. In the three‐strut configuration, the location of the primary strut is kept constant, and the secondary struts are relocated in x and y directions. Combustion performance was evaluated for the cases of flow from primary strut only and through three struts. It was found that the placement of secondary strut in a three‐strut configuration plays a vital role in improving energy efficiency. A maximum of 33.86% improvement in combustion efficiency was observed in comparison to the single strut combustor. A reduction in unburned fuel was observed, making the system more energy efficient. If the struts are not placed optimally, the combustion performance of the combustor was observed to be lower than that of a single‐strut configuration. The shock reflection and expansion fans within the primary combustion zone and the secondary strut region enhance the combustion efficiency. The wall static pressure was observed to increase with the addition of secondary struts. For certain strut configurations, flow separation was seen on the combustor walls. If the secondary strut was placed close to the primary strut, combustion efficiency was found to enhance. It was seen that combustion efficiency was also enhanced for the cases of reacting flow from primary strut only. It could also help to increase fuel efficiency, as additional fuel is not supplied to the secondary strut, making the overall system energy efficient. As the secondary strut is introduced, total pressure loss also increases. It could also be noted that if the combustor length was increased, there could be further increased in combustion efficiency.     

Multi-factor impact mechanism on combustion efficiency of a hydrogen-fueled micro-cylindrical combustor

As it is important to achieve higher combustion efficiency for applications of micro-cylindrical combustor, the multi-factor impact mechanism on the combustion efficiency of a hydrogen-fuelled micro-cylindrical combustor is investigated in this work. Firstly, six factors such as hydrogen/air equivalence ratio, inlet velocity, inlet temperature, wall thermal conductivity, wall emissivity and convective heat transfer coefficient of outer wall and five levels of each factor are determined. Orthogonal design table L₂₅(5⁶) is introduced to arrange cases. Secondly, grey relational analysis is adopted to investigate the effects of the six factors on combustion efficiency. Finally, the results of grey relational analysis are validated by analysis of variance. Based on grey relational analysis and analysis of variance, it is determined that the impact ranking from the largest to the smallest is hydrogen/air equivalence ratio, inlet velocity and inlet temperature, followed by the other three factors. The impact of wall thermal conductivity, convective heat transfer coefficient of outer wall and wall emissivity is considered to be equal due to their difference of impact on combustion efficiency is very small. This work provides us significant reference for optimizing combustion efficiency of a hydrogen-fuelled micro-cylindrical combustor.     

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 investigation on implications of four strut injectors on combustion characteristics of a Doubly-Dual cavity-based scramjet combustor

The present study numerically investigates the implications of four strut injector on combustion characteristics of a hydrogen fueled doubly-dual cavity based scramjet combustor. Large Eddy Simulation (LES) is adopted to reveal the combustion characteristics of the doubly-dual cavity based scramjet combustor. Firstly, the fluid flow characteristics of a three strut injector are compared with four strut injector to highlight the influence of 4-strut injector. Thereafter, a separate study is carried out to investigate the influence of Mach number on the performance of the combustor. Our study reveals that the four strut injector arrangement greatly affects the shock wave interaction like, shock-shock and the shock-shear layer which leads to enhance the formation of vortices and recirculation region as compared to the three strut injector arrangement. In addition, introduction of 4-strut injectors found to result in better mixing and complete combustion as evidenced by the higher static temperature and the lesser amount of oxygen mass fraction at the outlet of the combustor.Also, the flame blow out condition is seen at the trailingedge of a strut and found to be minimum when 4 strut arrangement is use dowing to maximum value of OH mass fraction. Further, the higher air-fuel mixing rate is observed at Mach 2.5 of the free stream of their inlet which aids to stabilize the flame leading to complete combustion.     

Investigation on hydrogen-fueled combustion characteristics and thermal performance in a micro heat-recirculation combustor inserted with block

Numerical investigation on the premixed H₂/air combustion in a micro heat-recirculation combustor inserted with/without block is conducted. Effects of block setting, heat-recirculation, and flow rate on combustion characteristics and thermal performance are depicted and analyzed. The results demonstrate that the block enhances the flame stability and preheating effect, which also reduces the heat loss via exhaust gas, while it shortens reactants residence time. The combustor setting with a transverse block gains a better thermal performance than that inserted with a longitudinal block. With the increase of transverse block height, the high-temperature zone is broadened and radiation is improved. However, the block with a height of 10 mm separates the fluid field and weakens the effects of heat recirculation, leading to a lower outer wall temperature. Furthermore, the appropriate block insertion method and height contribute to the significant improvement of heat transfer, radiant efficiency and further optimization of micro power generator.     

The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine

The port-injection-type hydrogen engine is advantaged in that hydrogen gas is injected into the intake pipe through a low-pressure fuel injector, and the mixing period with air is sufficient to produce uniform mixing, improving the thermal efficiency. A drawback is that the flame backfires in the intake manifold, reducing the engine output because the amount of intake air is reduced, owing to the large volume of hydrogen. Here, the backfire mechanism as a part of the development of full-load output capability is investigated, and a 2.4-liter reciprocating gasoline engine is modified to a hydrogen engine with a hydrogen supply system. To secure the stability and output performance of the hydrogen engine, the excess air ratio was controlled with a universal engine control unit.The torque, excess air ratio, hydrogen fuel, and intake air flow rate changes in time were compared under low- and high-engine speed conditions with a wide-open throttle. The excess air ratio depends on the change in the fuel amount when the throttle is completely opened, and excess air ratio increase leads to fuel/air-mixture dilution by the surplus air in the cylinder. As the engine speed increases, the maximum torque decreases because the excess air ratio continues to increase due to the occurrence of the backfire. The exhaust gas temperature also increases, except at an engine speed of 6000 rpm. Furthermore, the increase in exhaust gas temperature affects the backfire occurrence. At 2000 rpm, under low-speed and wide-open throttle conditions, backfire first occurs in the No. 4 cylinder because the mixture is heated by the relatively high port temperature. In contrast, at 6000 rpm, under high-speed and wide-open throttle conditions, the backfire starts at the No. 2 cylinder first because of a higher exhaust gas temperature, resulting in a lower excess air ratio in cylinders 2 and 3, located at the center of the engine.     

Effects of different channels on performance enhancement of a nozzle hydrogen-fueled micro combustor for micro-thermophotovoltaic applications

Under the background of the international energy crisis, it is urgent to develop a micro-thermophotovoltaic system using hydrogen as combustion energy. In order to optimize the micro-combustor in the system, the nozzle micro-combustors with five different channels are designed. All nozzle micro-combustors are numerically studied by using the mechanism of 9 components and 19 elementary reactions within the ANSYS Fluent 20.0, and their advantages and disadvantages in thermal performance, flame performance and chemical reaction are compared. It is concluded that the nozzle micro-combustor with constriction-expansion channel has the best performance among the five micro-combustors because of its reasonable segmented structure. Then, a new type of nozzle micro-combustor with segmented channel is designed, and the numerical study of segmented micro-burners and non-segmented micro-combustors with different inlet velocities and hydrogen/air equivalence ratios shows that the thermal performance of segmented micro-combustors is much better than that of non-segmented micro-burners. Therefore, compared with non-segmented nozzle micro-combustors, segmented nozzle micro-combustors have better application potential in micro-thermophotovoltaic applications.     

Numerical investigation of combustion characteristics for hydrogen mixed fuel in a can-type model of the gas turbine combustor

Numerical simulations are performed to analyze the combustion characteristics of propane fuel mixed with different amounts of hydrogen in a can-type combustor. The volume fraction of the hydrogen fuel varies from 0% to 100% in the fuel mixture. The results indicate that the hydrogen enrichment of the fuel significantly affects the flow structure, mixture fraction, and combustion characteristics. An increase in the volume fraction of hydrogen significantly affects the mean mixture fraction distribution, promotes combustion, and increases the flame temperature and the width of the flammable range within the combustor. Therefore, the degree of temperature uniformity at the outlet of the combustor increases with hydrogen enrichment, corresponding to an increase of 49.64% in the uniformity factor. The hydrogen enriched fuel can also reduce the emissions of CO and CO₂, owing to the reduced amount of carbonaceous fuel.     

Numerical investigations on thermal performance and flame stability of hydrogen-fueled micro tube combustor with injector for thermophotovoltaic applications

In order to design a micro tube combustor with good thermal performance and flame stability for thermophotovoltaic applications, in this work, thermal performance and flame stability of hydrogen-fueled micro tube combustors with backward facing step and with injector are compared. It is found that the decrease of diameter ratio d₂/d₁ leading to expansion of the symmetrical recirculation zone, which is helpful for fluid and heat circulation, and higher flame locations, which is not helpful for flame stabilization. Furthermore, effects of diameter ratio d₂/d₁ on thermal performance and flame stability are analyzed and discussed. Results suggest that when the diameter ratio d₂/d₁ is decreased from 0.9 to 0.8, positive effects of injector on thermal performance are enhanced and flame stability is improved under lower hydrogen/air equivalence ratio. Finally, the applications conditions of the micro tube combustor with injector are achieved. This work will provides us significant reference for designing micro tube combustor with injector.     

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.     

Experimental study on combustion characteristics of compost using New‐type combustor

The aim of the present study is to develop the biomass furnace combustor, which can effectively use the compost as a fuel. Here, the compost that is made from pig's waste and has the calorific value of 2000 kcal/kg is employed here. Emphasis is placed on the optimum conditions of fuel and air flow rates and moisture content of the compost and the corresponding combustion gas components and combustion gas temperature in the combustor. It is found from the study that (i) except 40% of the compost's moisture content, the self‐combustion of compost as the fuel takes place, (ii) the combustion gas concentrations are affected by gas temperature, and (iii) the optimum value of the air‐to‐fuel ratio is obtained based on the gas temperature and the concentration of combustion gases. Copyright © 2016 John Wiley & Sons, Ltd.     

燃气轮机双燃料燃烧室温度场优化研究

为了优化双燃料燃烧室温度场分布,针对某型逆流环管双燃料燃烧室,设计了3种不同的火焰筒配气结构。利用ANSYS FLUENT软件,选用Realizable k-epsilon湍流模型及Finate Rate Chemistry and Eddy-disspation燃烧模型对燃烧室额定工况下的温度场及速度场进行了数值模拟。研究表明:对比液体燃料,由于气体燃料扩散较快,燃烧室在使用气体燃料时高温区分布周向收缩并沿火焰筒轴向后移。对于本型燃烧室,适当增大主燃孔孔径并在火焰筒轴向方向偏后布置,可以有效解决双燃料燃烧室使用气体燃料时高温区后移的问题,对气/液两种燃料条件下的温度场组织更为有利。     

Role of the backward-facing steps at two struts on mixing and combustion characteristics in a typical strut-based scramjet with hydrogen fuel

Scramjet is one of the most promising propulsion systems for the new generation supersonic/hypersonic air-breathing flight vehicles. To achieve sufficiently high combustion efficiency and relatively low total pressure loss, developing a new strut is an attractive approach. In this study, we focus on the backward-facing steps and investigate the geometry parameters of the backward-facing steps of the two struts. In this work, Reynolds-averaged Navier-Stokes equation coupled with the one-step H₂-air reaction finite-rate/eddy-dissipation model is adopted to simulate all the two-strut cases, then this code is validated by the available experimental data. Next, we investigate the effects of different lengths and heights of the backward-facing steps on the two-strut based scramjet performance. For cold flow, enhancing the length of the backward-facing step can promote partly the mixing process with few extra total pressure losses. Further, the mixing process is weakened with increasing the height of the backward-facing steps to certain content, meanwhile, the total pressure loss is increased. In terms of reacting flow, increasing the length enhances the combustion efficiency, whereas the total pressure loss relatively decreases and thereafter increases. Enhancing the height of the backward-facing steps makes the combustion efficiency relatively decreases and thereafter increases, moreover, the total pressure loss increases as a result.     

几何结构对贫油直喷燃烧室流场特性影响的研究

基于现有单点贫油直喷燃烧室,采用数值模拟方法研究了头部几何角度对燃烧室流场特性的影响。分别对比了冷态与燃烧态条件下不同头部几何角对燃烧室轴向速度分布、燃烧效率、总压损失等特性的影响。研究表明:在所研究的几何角度范围内,冷态下头部几何角度对回流区的长度影响很小,对回流区内轴向速度分布具有较大影响。燃烧态下随着头部几何角度的增加,回流区轴向尺寸逐渐增加,中轴线上轴向速度值逐渐降低。燃烧室的出口平均温度、燃烧效率、总压恢复系数基本保持不变,60°结构产生的NO_x生成量最低,30°结构产生的NO_x生成量最高。