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.     

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.     

柴油机孔式喷油嘴内空穴流动的模拟分析

利用混合多相流体模型加空穴模型的方法,模拟了柴油机孔式喷油嘴稳定喷射时嘴内的空穴流动现象,分析了空穴在喷油嘴内形成机理及其分布情况。基于这一模型进一步分析了喷射压力、背压和喷孔长径比、喷孔入口圆角比、非轴对称喷孔等几何结构参数对喷孔内空穴分布的影响。通过模拟计算可知,提高喷射压差和减小喷孔入口圆角半径都可以提高空穴强度,同时也发现提高喷孔长径比可以使空穴在喷孔出口截面上分布更为均匀。从燃油空穴雾化理论的角度出发,空穴强度的提高以及在出口截面上的均匀分布都有利于燃油的破碎雾化。     

一种新型配气机构的数值模拟

本以一汽ＣＡ４８８发动机配气机构为研究对象，对配气机构建立了一种新型的多动量动力学模型。在此模型中，考虑了气门间隙液压调节器等因素，从而获得了与实测数据基本吻合的动力学数值计算结果。     

煤粉复合旋流火焰燃烧特性研究（2）：数值模拟

本文数值模拟了煤粉旋流火焰燃烧过程，燃烧数值计算包括理论物理模型建立，数值方法两个大部分，计算模型处理了气相湍流与燃烧、气固两相流动、煤颗粒燃烧过程和辐射传热等物理化学过程，以ｋ－ε模型模拟湍流流动；ＰＤＦ法模拟气相扩散火焰燃烧；颗粒运动计算颗粒运动少颗粒湍流浓度方程模拟颗粒湍流扩散；通量法计算火焰辐射传热，煤粉颗粒复杂燃烧模型计算了颗粒尺寸、形状变化和颗粒孔隙内部燃烧、表面平度对整个颗粒的燃烧过程影响。计算获得了气相速度分布场、气相ｋ和ε分布场、气相温度场、气相组份场和颗粒浓度场及运动过程，揭示了煤粉复合旋流燃烧特性。     

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.     

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.     

汽油机缸内直喷混合过程三维瞬态数值模拟

以KIVA程序作为三维瞬态模拟软件,对缸内直喷汽油机在4000r/min和6000r/min工况下缸内油气混合过程进行了数值模拟。结果表明：在4000r/min和6000r/min下缸内混合气的速度和浓度分布相似,火花塞附近的混合气当量比在1.2~1.5范围;在接近点火时刻,4000r/min的混合气分层效果较6000r/min的更为明显,有利于分层稀燃。计算结果可为直喷式汽油机的设计和性能改善提供一定的理论依据。     

Investigation of small-scale unintended releases of hydrogen: Buoyancy effects

Measurements were performed in small-scale hydrogen leaks to characterize the dimensional properties and flow characteristics of the resulting ignitable hydrogen cloud. The data are intended to provide a technological basis for determining hazardous length scales associated with the formation of ignitable mixtures due to unintended releases. In contrast to a previous study where momentum-dominated releases were considered, the present study focuses on smaller-scale releases at lower flow rates where buoyancy becomes important. A turbulent jet flow is selected as representative of releases in which the leak geometry is circular. Laser-based Rayleigh scattering is used to characterize the hydrogen concentration field downstream of the leak. Particle Image Velocimetry (PIV) is also used to characterize the flow velocity. Time-averaged mean and fluctuating hydrogen concentration statistics are presented and compared with results in momentum-dominated flows to elucidate the effects of buoyancy on the H₂ dispersion process. Over the range of Froude numbers investigated (Fr = 268, 152 and 99), increasing effects of buoyancy are seen as the Fr is reduced and at downstream locations where the influence of buoyancy increases relative to jet momentum. The primary effect of buoyancy is to increase the centerline decay rate of the time-averaged H₂ mass fraction relative to momentum-dominated flows. Acceleration due to buoyancy also results in a slower decay of the time-averaged axial velocity component along the centerline. Radial profiles of the time-averaged H₂ mass fraction also collapse onto the same curves as results in momentum-dominated flows when plotted against the same similarity/scaling variables. While buoyancy is found to have a negligible effect on centerline velocity fluctuations, the maximum H₂ mass fraction fluctuation intensity increases by 70 percent in the buoyant regime and the peak value shifts from the mixing region to the jet centerline. The database presented should provide a good test for the validation of CFD models being developed to predict unintended hydrogen releases under conditions where buoyancy is important.     

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.     

Three-dimensional simulation of a rotating detonation engine in ammonia/hydrogen mixtures and oxygen-enriched air

Rotating detonation using ammonia as fuel may be a potential carbon free combustion technology for gas turbine. The detonation wave structure and flow field of a rotating detonation annular combustor are investigated by three-dimensional simulation with detailed chemistry of ammonia/hydrogen-air. The detonation properties, propagation mode, combustor performance and emission characteristics are studied by varying the equivalence ratios and hydrogen concentrations. Both the increases of the combustor pressure and the hydrogen concentration promote the chemical reaction rate of the ammonia burn and the detonation wave velocity gradually increases with increasing hydrogen proportion based on one-dimensional simulation. A stable single-rotating waves resulting in ammonia/hydrogen combustor are observed for a wide range of equivalent ratios only when the hydrogen concentration is at least 0.2. The steady run of the single rotating detonation had an optimal cycle efficiency when the hydrogen concentration is increased to a critical value of 0.3. NOx emissions are more dependent on equivalent ratios than hydrogen concentration in equivalence ratios ranging from 0.70 to 1.40.     

Numerical simulation of a tubular solar reactor for methane cracking

This study addresses the solar thermal cracking of methane for the co-production of hydrogen and carbon black as a medium to avoid CO₂ emissions from natural gas combustion processes. The objective of this work is to numerically simulate the transport processes of momentum heat and mass in an indirect heating solar reactor, which is fed with an argon-methane mixture. The reactor is composed of a cubic cavity receiver, which absorbs concentrated solar irradiation through a quartz window and a graphite reaction tube is settled vertically inside this cavity. A series of numerical experiments were carried out in order to gain a better understanding of the interaction between the several transport phenomena taking place. The simulations showed that, in general, when the temperature of the reaction chamber is higher than 2000 K, the methane conversion is practically 100%. To validate our simulation results we compared them with available experimental data obtaining good agreement. Moreover, our results clearly evidence that most of the reaction takes place at the bottom of the reactor, which is the zone with the highest temperature profiles. Therefore, we propose modifications in the reactor design to increase conversion. The results of this work can thus serve to improve design and control of solar reactors for light hydrocarbons.