Effect of dual micro fuel jets on mixing performance of hydrogen in cavity flameholder at supersonic flow

In this article, numerical simulations were done to study the influence of the various hydrogen injections on the mixing rate in the cavity flameholder of the scramjet. This study tried to present the main effective parameters on the flow feature and distribution of the hydrogen jet within a cavity in supersonic free stream domain. In order to simulate the cavity flameholder with micro air/fuel jets, a three-dimensional model is chosen and computational fluid dynamic approach is used for the simulations. The effect of significant parameters is studied by using the Reynolds-averaged Navier–Stokes equations with Menter's Shear Stress Transport (SST) turbulence model. The effect of horizontal and vertical fuel injection is comprehensively studied. Moreover, the characteristics of the mixing in various free stream velocities (M = 1.2, 2.2 and 3.2) are examined and the effects of micro air jet on the size of ignition domain for preserving flame holder are investigated. Results show that the increase of free stream Mach number significantly enhances the mixing of horizontal fuel injection in the cavity. The obtained results reveal that the injection of micro air jets enhances the mixing rate in low Mach number (M = 1.2). Our findings also show that vertical hydrogen injection considerably increases the mixing zone within the cavity and the mixing rate significantly improves by rising free stream velocity.     

Shape effect of cavity flameholder on mixing zone of hydrogen jet at supersonic flow

Cavity flameholder is known as an efficient technique for providing the ignition zone. In this research, computational fluid dynamic is applied to study the influence of the various shapes of cavity as flameholder on the mixing efficiency inside the scramjet. To evaluate different shapes of cavity flame holder, the Reynolds-averaged Navier–Stokes equations with (SST) turbulence model are solved to reveal the effect of significant parameters. The influence of trapezoidal, circle and rectangular cavity on fuel distribution is expansively analyzed. Moreover, the influence of various Mach numbers (M = 1.2, 2 and 3) on mixing rate and flow feature inside the cavity is examined. The comprehensive parametric studies are also done. Our findings show that the trapezoidal cavity is more efficient than other shapes in the preservation of the ignition zone within the cavity. In addition, the increase of free stream Mach number intensifies the main circulations within cavity and this induces a stable ignition zone within cavity.     

Computational study of the multi hydrogen jets in presence of the upstream step in a Ma=4 supersonic flow

The injection of the hydrogen is the main noteworthy stage for the advance of the supersonic engine. In our computational study, the incidence of the step condition in the upstream of the hydrogen multi-jet is investigated for the augmentation of the fuel distribution in downside of the fuel jets at Mach = 4. To perform our research, a 3-dimensional computational domain is taken to unveil the primary flow organization of the hydrogen jets and its interactions with the freestream for the advance of fuel mixing. This work comprehensively examined the impression of the jet pressure on the mixing value and flow structure. Besides, the three-dimensional outcome of the step on the pattern of the four multi-jets is compared with the single equivalent jet. According to our results, the existence of step improves the fuel mixing efficiency up to 30% close to of early jets. Our findings reveal that increasing the step height from 0.5 to 2 mm enhances the fuel mixing more than 15%.     

Effect of fuel jet arrangement on the mixing rate inside trapezoidal cavity flame holder at supersonic flow

Fuel mixing inside the supersonic combustion chamber is a significant process for development of modern scramjets. In this article, computational fluid dynamic (CFD) approach is applied to investigate the effect of various fuel injections on the mixing rate inside the supersonic combustion chamber. The mixing of hydrogen jets with four different arrangements inside the cavity flame holder is comprehensively studied. In order to examine the effect of multi jets within a cavity flameholder, a three-dimensional model is established and Navier-stocks equations are solved to simulate the flow and mixing zone inside a cavity region. Obtained results show that the injection of hydrogen jet from the bottom of cavity flame holder considerable enhances the ignition zone within the cavity. Moreover, the backward fuel injection is more superior to forward fuel injection since low-pressure vortex could significantly distribute the fuel and enlarge the mixing zone inside the cavity flame holder.     

Effect of hybrid coaxial air and hydrogen jets on fuel mixing at supersonic crossflow

In this research study, a computational method is applied to examine the impacts of coaxial hybrid air and fuel jets on fuel mixing at the supersonic cross-flow of Mach = 4. This study examined the coaxial air and fuel jet effects on main parameters i. e. circulation, mixing efficiency, and fuel penetration. Computational Fluid Dynamic is employed for the modelling of the coaxial jet at cross supersonic flow. Reynolds Average Navier-Stocks equations with SST turbulence model for achieving hydrodynamic feature of the main model. Impacts of air-jet pressure and nozzle configurations on fuel distribution are also presented and the main effective factors for efficient fuel mixing condition are explained. Our results disclosed that injection of coaxial air and fuel jets at supersonic cross airflow significantly improves the fuel penetration and mixing inside the combustion chamber. Flow study analysis shows that the coaxial injector augments the spiral feature of the fuel jet, which surges fuel mixing downstream. Our circulation analysis confirms that circulation strength increases in far away from an injector by the injection of a coaxial air jet.     

Effect of strut angle on performance of hydrogen multi-jets inside the cavity at combustion chamber

Cavity flameholder is known as a promising technique to improve fuel mixing within the combustion chamber. This article studied the influences of the strut angle on the mixing performance of multi jets released inside the cavity flameholder. Finding the optimum jet configuration is done to promote the mixing performance of fuel through chamber when strut is applied in the upstream of the cavity flameholder. The impact of strut angle, fuel jet direction, and free-stream Mach number on the performance of three multi jets inside the chamber is disclosed in our research. For the simulation of our model, turbulent SST model is employed to obtain fuel distribution through the cavity. Our findings indicate that the counter jet is more operative in mixing of the fuel than co-jet since the main circulation is close to counter-jet, and fuel could efficiently distribute by the main circulation.     

The presence of downstream ramp on fuel mixing of the multi micro jets at supersonic cross flow

Development of the fuel injection system in combustion chamber is greatly important for the overall thrust efficiency of the high-speed vehicles. Current article developed a three-dimensional model to discover the reality of downstream ramp on fuel mixing of the multi-jet at Ma>1. FVM is hired to scrutinize the impact of injector types (3-lobe, circular and rectangular shape) on the mixing productivity of downstream ramp in combustion chamber. Besides, the effects of ramp angle on fuel mixing are also analysed. Fuel mixing mechanisms in the selected models are investigated by comparing the Ma contour and mixing zone. Comparisons of the circulation strength downstream of these models confirm that the 3-lobe nozzles is more efficient than other styles. Our comparison indicates that overall mixing productivity of the circular jet is more than other cases.     

Computational investigation of multi-cavity fuel injection on hydrogen mixing at supersonic combustion chamber

Enhancement of the mixing inside the combustor is a significant process for increasing the efficiency of the scramjet. This work applied the computational method for the investigation of the depth of the cavity on the flow feature of the multi hydrogen jet in the supersonic crossflow. The main focus of this research is to evaluate the depth of the cavity on the mixing rate of the hydrogen jets inside the combustion chamber. CFD method with the SST turbulence technique is applied for the simulation of the fluid flow inside the domain. The impact of the depth of the cavity, the pressure of the fuel jet and the number of the jet are comprehensively explained in this study. Our findings show that the rising of the cavity enhances the mixing inside the domain due to more fuel distribution along the spanwise direction. Our results clearly demonstrate that replacing the single jet with 8 equivalent multi jets increases the mixing rate of more than 45% in the vicinity of the jet injection. Attained results revealed that increasing the jet space develops the mixing in far downstream. Obtained results also show that mixing intensifies 15% when jet space of 8 microjets is increased from 4 dj to 10 dj.     

Release and dispersion modeling of cryogenic under-expanded hydrogen jets

In the present work performed within the framework of the SUSANA EC-project, we address the release and dispersion modeling of hydrogen stored at cryogenic temperatures and high pressures. Due to the high storage pressures the resulting jets are under-expanded. Due to the low temperatures the choked conditions can be two-phase. For the release modeling the homogeneous equilibrium model (HEM) was used combined with NIST equation of state for hydrogen. For the dispersion modeling the 3d CFD methodology was used combined with a) a notional nozzle approach to bridge the expansion to atmospheric pressure region that exists near the nozzle, b) the ideal gas assumption for hydrogen and air and c) the standard (buoyancy included) k–ε turbulence model. Predicted release choked mass fluxes are compared against 37 experiments from literature. Predicted steady state hydrogen concentrations along the jet axis are compared against five dispersion experiments from literature as well as the Chen and Rodi correlation and the behavior of the proposed release and dispersion modeling approaches is assessed.     

Numerical investigation of the flammable extent of semi-confined hydrogen and methane jets

The effect of surfaces on the extent of high pressure horizontal unignited jets of hydrogen and methane is studied using computer fluid dynamics simulations performed with FLACS Hydrogen. Results for constant flow rate through a 6.35 mm diameter pressure relief Device (PRD) orifice from 100 barg, 250 barg, 400 barg, 550 barg and 700 barg compressed gas systems are presented for both horizontal hydrogen and methane jets. To quantify the effect of a horizontal surface on the jet, the jet exit is positioned at various heights above the ground ranging from 0.1 m to 10 m. Free jet simulations are performed for comparison purposes. Also, for cross-validation purposes, a number of cases for 100 barg releases were simulated using proprietary models developed for hydrogen within commercial CFD software PHOENICS. It is found that the presence of a surface and its proximity to the jet centreline result in a pronounced increase in the extent of the flammable cloud compared to a free jet.     

Self-similar characteristics of underexpanded,cryogenic hydrogen and methane jets

Underexpanded, cryogenic hydrogen and methane jets were measured using laser Raman scattering diagnostic. The jets were released from 1 mm to 1.25 mm orifices for the stagnation pressure ranges of 2–6 bar and temperature ranges of 37–46 K (hydrogen) and 112–189 K (methane). Raman signals are inherently small, thus a denoising algorithm was developed to substantially reduce the noise hindering the statistical analysis of the data. The time-averaged concentration and temperature data were plotted to show a hyperbolic decay law along the jet centerline and a Gaussian distribution in the radial direction. The concentration fluctuations of the cryogenic jets are similar to those of warm jets, the centerline RMS mass fraction decays similarly to the mean mass fraction, and the highest radial concentration fluctuations appear in the shear layer. Thus, the self-similar characteristics of the cryogenic jets are comparable with room-temperature jets for the present test conditions.     

Validation of two-layer model for underexpanded hydrogen jets

Previous studies have shown that the two-layer model more accurately predicts hydrogen dispersion than the conventional notional nozzle models without significantly increasing the computational expense. However, the model was only validated for predicting the concentration distribution and has not been adequately validated for predicting the velocity distributions. In the present study, particle imaging velocimetry (PIV) was used to measure the velocity field of an underexpanded hydrogen jet released at 10 bar from a 1.5 mm diameter orifice. The two-layer model was the used to calculate the inlet conditions for a two-dimensional axisymmetric CFD model to simulate the hydrogen jet downstream of the Mach disk. The predicted velocity spreading and centerline decay rates agreed well with the PIV measurements. The predicted concentration distribution was consistent with data from previous planar Rayleigh scattering measurements used to verify the concentration distribution predictions in an earlier study. The jet spreading was also simulated using several widely used notional nozzle models combined with the integral plume model for comparison. These results show that the velocity and concentration distributions are both better predicted by the two-layer model than the notional nozzle models to complement previous studies verifying only the predicted concentration profiles. Thus, this study shows that the two-layer model can accurately predict the jet velocity distributions as well as the concentration distributions as verified earlier. Though more validation studies are needed to improve confidence in the model and increase the range of validity, the present work indicates that the two-layer model is a promising tool for fast, accurate predictions of the flow fields of underexpanded hydrogen jets.     

Influence of coaxial air jet on mass diffusion of hydrogen jet injected through lobe-injector at supersonic flow

The technique of fuel injection in the combustion chamber is crucial for increasing the performance of hypersonic vehicles. This study tries to investigate the mechanism of fuel injection and distribution when fuel and air are injected through coaxial lobe injectors. The main attention of this work is to present the mechanism of fuel mixing of transverse jet injected from various lobe injectors. Comparison of coaxial gets (air and fuel jet) with equivalent simple jet (fuel without air jet) is done to achieve an efficient model for the combustion chamber. In this work, finite-volume is used to simulate and study of fuel injection performance of a transverse hydrogen jet in different lobe injectors. 3-D flow visualizations are done to reveal the mechanism of the fuel penetration and streamline pattern for introduced models. Strength of circulation and fuel mixing efficiency are also investigated in the present work for 2-, 3-, and 4-lobe nozzles. Our outcomes indicate that the mixing performance of coaxial air and fuel jet injected through the 3-lobe nozzle is about 25% better than other nozzle types. Our findings confirm that injection of air jet through the core of the lobe nozzle increases fuel mixing up to 200% at the combustion chamber.     

Surface effects on flammable extent of hydrogen and methane jets

Studies on the effect of surfaces on the extent of the flammable cloud of high-pressure horizontal and vertical jets of hydrogen and methane are performed using CFD numerical simulations. For the horizontal jets, two scenarios pertaining to the location of the surface are studied: horizontal surface (the ground), and vertical surface (side wall). For a constant flow rate release, the extent of the flammable cloud is determined as a function of time. Effects of the proximity of the surface on the flammable extent along the axis of the jet and its impact on the maximum extent of the flammable cloud is explored and compared for both hydrogen and methane. The results are also compared to the predictions of the Birch correlations for flammable extents. It is found that the presence of a surface and its proximity to the jet centerline result in a pronounced increase in the extent of the flammable cloud compared to a free jet.     

Numerical simulation of mixing of hydrogen jet at supersonic cross flow in presence of upstream wavy wall

The spreading of hydrogen jet within the combustion chamber is extremely important for the fuel consumption and enactment of scramjet engines. In this article, a numerical method is used to simulate the influence of wavy wall on distribution of the hydrogen cross flow jet in the downstream of the injectors. To examine the main role of wavy surface on the fuel distribution, a 3-D model is selected with an appropriate grid to detect the primary interaction of the hydrogen fuel jet with the deflected supersonic free stream. Code was developed to solve the Navier-stokes equation with energy and species mass transport equations. This study compares the effect of the amplitude of the wavy upstream wall on the main flow structure and hydrogen fuel distribution within the confined channel. The effects of hydrogen jet pressure on the main stream are also studied. Our findings display that the mixing rate of fuel inside the combustor rises about 35% when high amplitude surface wall is applied in the upstream of jet.     

Mixing efficiency of hydrogen multijet through backward-facing steps at supersonic flow

Increasing the fuel mixing performance in the combustor of scramjet substantially improves the overall efficiency of the scramjet engine. In this article, computational fluid dynamic is used to study the impacts of hydrogen jets injection through the backward-facing multi-steps on the fuel distribution and mixing zone at the supersonic air stream of Mach = 4. This study also analyzes the jet flow feature and circulation of jets in different sections of the combustor at downstream of the multi-injectors. Reynolds average Navier-Stocks equations are solved with SST turbulence model for achieving a precise and acceptable solution. The effects of step height on the jet features are also examined. According to circulation evaluation, low jet total pressure (pressure ratio = 0.1) and high step depth (step depth = 1 mm) is the optimum condition for achieving high circulation value. Our investigations show that the mixing efficiency of the hydrogen multijets improves up to 15% when the step height increases from 0.5 mm to 1 mm.     

Analysis of jet flames and unignited jets from unintended releases of hydrogen

A combined experimental and modeling program is being carried out at Sandia National Laboratories to characterize and predict the behavior of unintended hydrogen releases. In the case where the hydrogen leak remains unignited, knowledge of the concentration field and flammability envelope is an issue of importance in determining consequence distances for the safe use of hydrogen. In the case where a high-pressure leak of hydrogen is ignited, a classic turbulent jet flame forms. Knowledge of the flame length and thermal radiation heat flux distribution is important to safety. Depending on the effective diameter of the leak and the tank source pressure, free jet flames can be extensive in length and pose significant radiation and impingement hazard, resulting in consequence distances that are unacceptably large. One possible mitigation strategy to potentially reduce the exposure to jet flames is to incorporate barriers around hydrogen storage equipment. The reasoning is that walls will reduce the extent of unacceptable consequences due to jet releases resulting from accidents involving high-pressure equipment. While reducing the jet extent, the walls may introduce other hazards if not configured properly. The goal of this work is to provide guidance on configuration and placement of these walls to minimize overall hazards using a quantitative risk assessment approach. The program includes detailed CFD calculations of jet flames and unignited jets to predict how hydrogen leaks and jet flames interact with barriers, complemented by an experimental validation program that considers the interaction of jet flames and unignited jets with barriers.     

Experimental and numerical investigation of turbulent jets issuing through a realistic pipeline geometry: Asymmetry effects for air,helium, and hydrogen

Experiments and numerical simulations were conducted to investigate the dispersion of turbulent jets issuing from realistic pipe geometries. The effect of jet densities and Reynolds numbers on vertical buoyant jets were investigated, as they emerged from the side wall of a circular pipe, through a round orifice. Particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) techniques were employed simultaneously to provide time-averaged flow velocity and concentrations fields. Large eddy simulation (LES) was applied to provide further detail with regards to the three-dimensionality of air, helium, and hydrogen jets. These jets were always asymmetric and found to deflect about the vertical axis. The deflection was influenced by buoyancy, where heavier gases deflected more than lighter gases. Significant turbulent mixing was also observed in the near field. The jets from realistic pipe geometries experienced faster velocity decay and asymmetric jet spreading compared to round jets. These findings indicate that conventional round jet assumptions are, to some extent, inadequate to predict gas concentration, entrainment rates and, consequently, the extent of the flammability envelope of realistic gas leaks.     

Effects of geometry and inconstant mass flow rate on temperatures within a pressurized hydrogen cylinder during refueling

Higher refueling rate leads to higher temperature rise within the cylinder. Excessive temperature should be avoided during the refueling progress. In this paper, we studied the effective methods to control the temperature rise by simulations based on the Computational Fluid Dynamics (CFD). Cylinders of different length to diameter ratios and different inlet diameters were simulated. We found that smaller radio of length to diameter can boost for temperature control and temperature distribution. Larger inlet diameter can restrain temperature rise. Comparing the simulation results with constant, increasing and decreasing mass flow rate, the refueling with increasing flow rate obtains the lowest temperature rise.     

Effect of hydrogen jets in supersonic mixing using strut injection schemes

The prevalence of complex phenomena associated with the fuel mixing of a supersonic stream in scramjet combustor is inherently occurred due to the short residence time. An efficient injection mechanism is required to enhance the mixing and improve combustion efficiency. This numerical simulation study aims to reveal the performance of modified strut injection strategies within a Mach 2.0 flow field. Two-dimensional steady and transient Navier-Stokes computations of the DLR scramjet experiment is performed for various strut injection locations. The Reynolds Averaged Navier Stokes equation with the SST k-ε turbulence model is utilized to solve the flow field under steady conditions. The critical parameters examined to investigate steady solutions are wall static pressure, flow Mach number, and total pressure loss across the combustor. The dual injection configuration in the flow considerably reduces the shock waves impact at the downstream of the strut and preserves the magnitude of internal forces acting on combustor walls and total pressure loss. Unsteady Detached Eddy Simulation (DES) results for hydrogen concentration and velocity field are analyzed by applying Dynamic Mode Decomposition (DMD). Multiple injections are observed to alter the frequency and the number of dominant modes.