Numerical investigation on the mechanism of the effects of plate vibration on mixing and combustion of transverse hydrogen injection

This study numerically investigates the effects of plate vibration and deformation on the combustion performance, the shock wave structure, the mixing characteristics and the flame structure for transverse hydrogen injection. The finite-rate method is used to simulate combustion. Results show that plate vibration causes the combustion performance to oscillate both temporally and spatially, while plate deformation can only change the static characteristics of the flow. Plate vibration and deformation increase the intensity and number of shock wave reflections in different ways. In addition, both plate vibration and deformation increase the momentum flux ratio and the jet penetration depth, which enhances mixing. Finally, plate vibration widens the flame and moves it upward to a greater extent than plate deformation.     

Characteristics and mixing enhancement of a self-throttling system in a supersonic flow with transverse injections

A three-dimensional self-throttling system is proposed in a scramjet combustor with transverse fuel jet, and investigated by Reynolds-averaged Navier-Stokes (RANS) simulations with the k-ω SST turbulence model. Numerical validation has been carried out against experiment and LES results. The effects of the jet-to-cross-flow momentum flux ratio and the throttling angle on mixing performance, fuel jet penetration depth and total pressure losses are all addressed. Through the proposed throttling system, the higher pressure upstream of the transverse fuel injection can drive part of the low momentum mainstream air into the downstream lower pressure region. The flow structures and the interactions between the shock waves and boundary layer are significantly changed to improve the mixing performance. The enhancement of mixing efficiency in the self-throttling system is closely related to the magnitude of the jet to crossflow momentum flux ratio, and a smaller throttling angle is found to further improve the mixing. On the other hand, the self-throttling system has a good performance in reducing the total pressure losses.     

Effect of jet schemes of the double-nozzle strut injector on mixing efficiency of air and hydrogen for a scramjet combustor

This work presents the numerical analysis of the DLR scramjet combustor for different jet schemes of the double-nozzle injector, namely the various injection directions, injection angles, and nozzle spacings. After comparing various jet schemes, it is found that the optimal jet scheme for the double-nozzle strut is to set the angle of 60° for the inward injection direction and the nozzle spacing of 3 mm. Furthermore, the mixing efficiency of the optimal jet scheme is investigated at different Mach numbers. The current research focuses on the mixing mechanism of air and hydrogen by analyzing the flow structures in the strut's wake region. It is observed that the double-nozzle configuration increases the number of vortexes behind the strut and creates a recirculation zone between the two jet streams. The mixing efficiency of the scramjet combustor improves significantly with an increase in the injection angle, but the spacing and direction of the double-nozzle have little effect on the mixing efficiency. It is found that the additional total pressure loss generated by the double-nozzle configuration can be negligible. In addition, the results show that the mixing efficiency of the optimal jet scheme for the double-nozzle is improved more significantly at low Mach numbers (e.g., Ma = 2 and 3).     

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.     

Supersonic combustion of hydrogen using an improved strut injection scheme

Numerical investigation of mixing is performed at Mach 2.0 model Scramjet combustor employing parallel strut injection schemes for fuel. In the present investigation, basic strut injector is modified in such a way to produce additional vortices in streamwise direction and improve fuel-air mixing. Air is injected at Mach 2.0 at the combustor inlet and fuel is injected at sonic speed from the blunt end of the strut. The flow field involving high-speed turbulent mixing and heat addition was modeled by three-dimensional Reynolds averaged Navier-Stokes equations. A realizable k-ε model was chosen to close the turbulence problem with the default model constants. Non-premixed combustion of hydrogen and air is modeled using the mixture fraction β-pdf framework. Turbulence-chemistry interactions are handled by a strained flamelet model. Comparisons of numerical results with experimental results have demonstrated the accuracy and applicability of computational grid and a numerical scheme for hot and cold flow solutions. The shock-shear layer interaction present within the combustor increases the local turbulent intensity and has a positive effect on mixing. The mixing efficiency obtained with improved strut injector is compared with the basic strut. Improved strut injection scheme showed a mixing efficiency of >95% with a 45% reduction in length. Further combustion efficiency is calculated in the streamwise direction and plot follows the similar trend as the mixing efficiency. The proposed modification of strut geometry showed improved mixing and combustion performance.     

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.     

Analysis of combustion instability of hydrogen fueled scramjet combustor on high-speed OH-PLIF measurements and dynamic mode decomposition

This paper investigated the combustion instability of spanwise positions in a hydrogen fueled scramjet combustor with a cavity flame holder. High-speed OH-PLIF technique was performed on a direct-connect supersonic combustion facility, and dynamic mode decomposition (DMD) as postprocessing. Combustion instability was investigated by characterizing the dominant frequencies and growth factors. By changing the equivalence ratio of hydrogen, the peak frequencies of scramjet mode and ramjet mode were obtained. Scramjet mode tended to have small oscillation at 150–200 Hz reflected by negative growth factors due to the stable flame structure. At ram-to-scram transition, oscillations at 80–120 Hz were remarkably enhanced due to the positive growth factors. In ramjet mode, the large differences of frequency characteristics in spanwise positions were observed. The dominant DMD modes near the cavity wall appeared to have negative growth factors leading to a stable flame with small oscillations. Besides, the characteristics of frequency-shift were affected by the positions of injector.     

Combustion modes of hydrogen jet combustion in a cavity-based supersonic combustor

Optical diagnosis-based combustion experiments were conducted to investigate the characteristics of cavity assisted hydrogen jet combustion in a supersonic flow with a total pressure of 1.6 MPa, a total temperature of 1486 K, and a Mach number of 2.52, simulating flight Mach 6 conditions. A supersonic combustor with a constant cross-sectional area was employed with several cavity configurations, fueling schemes and equivalence ratios. It was found that stable combustion could not be obtained without a cavity, indicating that pure jet-wake stabilized combustion could not be achieved and the cavity acted as a flameholder. Three combustion modes were observed for the cavity assisted hydrogen jet combustion: cavity assisted jet-wake stabilized combustion, cavity shear-layer stabilized combustion, and combined cavity shear-layer/recirculation stabilized combustion. The cavity assisted jet-wake stabilized combustion was observed to be the most unstable mode, accompanied by intermittent blowoff under the present conditions, while the combined cavity shear-layer/recirculation stabilized combustion mode seemed to be the most robust one.     

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.     

Parametric effect on the flow and mixing properties of transverse gaseous injection flow fields with streamwise slot: A numerical study

Mixing process between the injectant and air in supersonic crossflow depends on the injector configuration and the jet-to-jet spacing heavily. In the current study, the three-dimensional Reynolds-averaged Navier–Stokes (RANS) equations coupled with the two equation SST k-ω turbulence model were employed to simulate the mixing process induced by an array of three spanwise-aligned small-scale rectangular portholes, and the influences of the jet-to-jet spacing, the jet-to-crossflow pressure ratio and the aspect ratio of the injector on the flow field properties were evaluated. Two quantitative objectives were considered in this article, namely the fuel penetration depth and the mixing efficiency. The obtained results show that the flow field induced by the array of three spanwise-aligned small-scale rectangular portholes is a multiobjective design optimization problem, and the large aspect ratio is beneficial for the mixing enhancement in supersonic crossflow. However, it is not beneficial for the flame holding. The interaction between the adjacent injectors has a great impact on the fuel penetration depth in the far-field, especially for the larger jet-to-crossflow pressure ratio, and this is due to its wider fuel plume.     

Effects of hydrogen addition on methane catalytic combustion in a microtube

The objective of this paper is to study hydrogen-assisted catalytic combustion of hydrocarbon on a microscale experimentally. In the experiment, neither methane nor ethane can be ignited by itself, but hydrogen can be ignited and burn steadily in this tube. It is found that there is no significant difference between hydrogen added to the hydrocarbon and hydrogen alone as fuel without the platinum thermocouple, but the temperature will increase and the efficiency of methane combustion will increase considerably when the platinum thermocouple was put into the microtube. Methane can burn steadily without adding hydrogen after ignited by hydrogen. It can be concluded that the addition of hydrogen to hydrocarbon is favorable to ignition and the platinum thermocouple catalyzes the hydrocarbon combustion. The experiment result showed that the added hydrogen acts as an assistant for ignition and expands the range for methane steady burn. After igniting, methane can burn steadily alone at catalytic condition. This is useful for optimization microcombustion fuel.     

Effects of second hydrogen direct injection proportion and injection timing on combustion and emission characteristics of hydrogen/n-butanol combined injection engine

Hydrogen and n-butanol are superior alternative fuels for SI engines, which show high potential in improving the combustion and emission characteristics of internal combustion engines. However, both still have disadvantages when applied individually. N-butanol fuel has poor evaporative atomization properties and high latent heat of vaporization. Burning n-butanol fuel alone can lead to incomplete combustion and lower temperature in the cylinder. Hydrogen is not easily stored and transported, and the engine is prone to backfire or detonation only using hydrogen. Therefore, this paper investigates the effects of hydrogen direct injection strategies on the combustion and emission characteristics of n-butanol/hydrogen dual-fuel engines based on n-butanol port injection/split hydrogen direct injection mode and the synergistic optimization of their characteristics. The energy of hydrogen is 20% of the total energy of the fuel in the cylinder. The experimental results show that a balance between dynamics and emission characteristics can be found using split hydrogen direct injection. Compared with the second hydrogen injection proportion (IP2) = 0, the split hydrogen direct injection can promote the formation of a stable flame kernel, shorten the flame development period and rapid combustion period, and reduce the cyclic variation. When the IP2 is 25%, 50% and 75%, the engine torque increases by 0.14%, 1.50% and 3.00% and the maximum in-cylinder pressure increases by 1.9%, 2.3% and 0.6% respectively. Compared with IP2 = 100%, HC emissions are reduced by 7.8%, 15.4% and 24.7% and NOx emissions are reduced by 16.4%, 13.8% and 7.9% respectively, when the IP2 is 25%, 50% and 75%. As second hydrogen injection timing (IT2) is advanced, CA0-10 and CA10-90 show a decreasing and then increasing trend. The maximum in-cylinder pressure rises and falls, and the engine torque gradually decreases. The CO emissions show a trend of decreasing and remaining constant. However, the trends of HC emissions and NOx emissions with IT2 are not consistent at different IP2. Considering the engine's dynamics and emission characteristics, the first hydrogen injection proportion (IP1) = 25% plus first hydrogen injection timing (IT1) = 240°CA BTDC combined with IP2 = 75% plus IT2 = 105°CA BTDC is the superior split hydrogen direct injection strategy.     

Characteristics of the hydrogen jet combustion through multiport injector arrays in a scramjet combustor

Large eddy simulation of the hydrogen jet combustion in a cavity-stabilized scramjet combustor with three parallel injectors is performed in this study, the emphasis of which is placed on the turbulent flame regime as well as the overall performance analysis. This combustor operates in a scramjet mode with a global equivalence ratio of 0.124, as the chemical heat released is not enough to form thermal chocking. The code framework utilizes an adaptive central-upwind weighted essentially non-oscillatory scheme with a low numerical dissipation to accurately capture turbulent structures in the flowfields, and an assumed probability density function approach to close the terms of the production rate of species. Turbulent fluctuations in the incoming boundary layer are initiated and sustained by a multi-wall recycling/rescaling technique, augmenting the mixing degree of the jet and crossflow. The numerical results show that the large scale vortices between the adjacent jet wakes interfere with each other in the downstream, resulting in a portion of the premixed flame. However, the turbulent diffusion combustion still dominates the whole combustor, occurring in a widespread range of Mach number. And the violent chemical reaction favours a high-temperature environment with a proper scalar dissipation rate. The diameter of multiple jets is smaller in comparison to that of the single injection, so that its penetration height is a little lower under the same spout pressure. Altogether, the parallel injection strategy is beneficial to improve the overall combustor performance, and will not lead to excessive total pressure loss.     

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.     

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.     

Diagnosis and control of abnormal combustion of hydrogen internal combustion engine based on the hydrogen injection parameters

In order to improve the limitation of evaluating the abnormal combustion problem of hydrogen internal combustion engine by single index, the abnormal combustion risk coefficient is proposed and defined based on AHP(Analytic Hierarchy Process)-entropy method. The abnormal combustion risk of PFI hydrogen internal combustion engine is comprehensively evaluated from multiple indexes such as the uniformity coefficient of the mixture, the temperature of the hot area, the maximum temperature rise rate, the residual amount of hydrogen in the intake port and the cylinder temperature at the end of the exhaust. The influence of hydrogen injection parameters on abnormal combustion was explored. The results show that the temperature and the maximum temperature rise rate in the hot area decrease first and then increase with the increase of hydrogen injection angle and hydrogen injection flow rate. Although large hydrogen injection angle and hydrogen injection flow rate can reduce the cylinder temperature at the end of exhaust, they will increase the residual hydrogen amount in the intake port. Appropriate hydrogen injection angle and hydrogen injection flow scheme can ensure that all parameters are at a better level, so that the risk coefficient of abnormal combustion decreases by 2.1%–5.5%, and the possibility of abnormal combustion is reduced.     

Effects of split injection proportion and the second injection timings on the combustion and emissions of a dual fuel SI engine with split hydrogen direct injection

In this paper, a new kind of injection mode, split hydrogen direct injection, was presented for a dual fuel SI engine. Six different first injection proportions (IP1) and five different second injection timings were applied at the condition of excess air ratio of 1, first injection timing of 300°CA BTDC, low speed, low load conditions and the Minimum spark advance for Best Torque (MBT) on a dual fuel SI engine with hydrogen direct injection (HDI) plus port fuel injection (PFI). The result showed that, split hydrogen direct injection can achieve a higher brake thermal efficiency and lower emissions compared with single HDI. In comparison with single HDI, the split hydrogen direct injection can form a controlled stratified condition of hydrogen which could make the combustion more complete and faster. By adding an early spray to form a more homogeneous mixture, the split hydrogen direct injection not only can help to form a flame kernel to make the combustion stable, but also can speed up the combustion rate through the whole combustion process, which can improve the brake thermal efficiency. By split hydrogen direct injection, the torque reaches the highest when the first injection proportion is 33%, which improves by 1.13% on average than that of single HDI. With the delay of second injection timing, the torque increases first and then decreases. With the increase of first injection proportion, the best second injection timing is advanced. Furthermore, by forming a more homogeneous mixture, the split hydrogen direct injection can reduce the quenching distance to reduce the HC emission and reduce the maximum temperature to reduce the NO_X. The split hydrogen direct injection can reduce the HC emission by 35.8%, the NO_X emissions by 7.3% than that of single HDI.     

Effect of a revolved wedge strut induced mixing enhancement for a hydrogen fueled scramjet combustor

The mixing concept of fuel and air is the burning issue for hypersonic vehicles (scramjet) due to the less resident time of supersonic air in the combustion chamber. So far, significant research has been done for mixing enhancement and introduced different technologies; still, there is a lack of research for mixing improvement. Shock wave and shear mixing layer are the main parameters for investigating mixing criteria at supersonic speed. In this research, an innovative fuel injection strut has been designed to develop mixing enhancement by elevating multiple interactions between the shock wave and shear mixing layer. This new strut has been designed with the reference of the DLR scramjet combustor. From the reference of a wedge-shaped strut, a revolved (wedge shape – circular 3D) wedge strut has been modeled with the same fuel injection base points. This new strut's performance has been analyzed for mixing enhancement by visualizing the development of shock wave, shear-mixing layer, and their interactions. Three-dimensional numerical analysis has been carried out by solving the Reynolds-Averaged and Navier-Stokes (RANS) equations. A comparison of results has been made for the basic wedge and new strut and identified the increase in multiple interactions of the shock wave and shear layer, which leads to an increase in mixing enhancement. For the new strut, complete mixing has been achieved within a distance of 0.180 m with an average increase in mixing efficiency of 9% and increased pressure losses of 12%.     

Effects of hydrogen addition on the combustion characteristics of diesel fuel jets under ultra-high injection pressures

Many applications use hydrogen addition and high-pressure fuel injection technology to improve combustion performance. In this study, spray atomization and combustion characteristics of a diesel fuel jet, under the injection pressure of 350 MPa, injecting into a constant volume combustion vessel filled with air-hydrogen mixture at the diesel engine relevant condition are investigated by simulation method. A simplified mechanism of the n-heptane (C₇H₁₆) oxidation chemistry mechanism consisting of 26 reactions and 25 species integrated with the Kéromnès-2013 hydrogen combustion mechanism and EDC combustion model are utilized to predict the diesel fuel spray auto-ignition and combustion. The ambient gas is the mixture of air and hydrogen range in volume fraction from 0% to 10%. The ambient temperature and pressure is set to 1000 K and 3.5 MPa, respectively. The results indicate that as the hydrogen volume fraction is 2%, the minimum overall droplet SMD (Sauter Mean Diameter) is approximately 0.95 μm, which is obviously smaller than that of the case with the conventional high injection pressure. In cases that H₂ v/v% larger than 4%, the maximum gaseous temperature increased significantly up to 2700 K. There are two peaks in the temperature growth rate curves as the hydrogen fraction of 8% and 10%. The high temperature at the outer edge of the spray is clearly seen due to its high value when the hydrogen fraction is larger than 4%. The hot reaction layer is the main location of CO formation. The H, OH radicals are formed at the edge of the spray where the temperature is high. The hydrogen species obviously promotes the oxidation and combustion of diesel fuel.