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1.
Nilanjan Chakraborty Henrik Hesse Epaminondas Mastorakos 《Flow, Turbulence and Combustion》2010,84(1):125-166
The influences of fuel Lewis number Le F on localised forced ignition of inhomogeneous mixtures are analysed using three-dimensional compressible Direct Numerical Simulations (DNS) of turbulent mixing layers for Le F = 0.8, 1.0 and 1.2 and a range of different root-mean-square turbulent velocity fluctuation u′ values. For all Le F cases a tribrachial flame has been observed in case of successful ignition. However, the lean premixed branch tends to merge with the diffusion flame on the stoichiometric mixture fraction isosurface at later stages of the flame evolution. It has been observed that the maximum values of temperature and reaction rate increase with decreasing Le F during the period of external energy addition. Moreover, Le F is found to have a significant effect on the behaviours of mean temperature and fuel reaction rate magnitude conditional on mixture fraction values. It is also found that reaction rate and mixture fraction gradient magnitude \(\vert \nabla \xi \vert \) are negatively correlated at the most reactive region for all values of Le F explored. The probability of finding high values of \(\vert \nabla \xi \vert \) increases with increasing Le F . For a given value of u′, the extent of burning decreases with increasing Le F . A moderate increase in u′ gives rise to an increase in the extent of burning for Le F = 0.8 and 1.0, which starts to decrease with further increases in u′. For Le F = 1.2, the extent of burning decreases monotonically with increasing u′. The extent of edge flame propagation on the stoichiometric mixture fraction ξ = ξ st isosurface is characterised by the probability of finding burned gas on this isosurface, which decreases with increasing u′ and Le F . It has been found that it is easier to obtain self-sustained combustion following localised forced ignition in case of inhomogeneous mixtures than that in the case of homogeneous mixtures with the same energy input, energy deposition duration when the ignition centre is placed at the stoichiometric mixture. The difficultly to sustain combustion unaided by external energy addition in homogeneous mixture is particularly prevalent in the case of Le F = 1.2. 相似文献
2.
Papapostolou Vassilios Turquand d’Auzay Charles Chakraborty Nilanjan 《Flow, Turbulence and Combustion》2021,107(2):367-403
Flow, Turbulence and Combustion - The minimum ignition energy (MIE) requirements for ensuring successful thermal runaway and self-sustained flame propagation have been analysed for forced ignition... 相似文献
3.
Direct Numerical Simulations of Localised Forced Ignition in Turbulent Mixing Layers: The Effects of Mixture Fraction and Its Gradient 总被引:2,自引:0,他引:2
The effects of mixture fraction value ξ and the magnitude of its gradient |∇ξ| at the ignitor location on the localised forced ignition of turbulent mixing layers under decaying turbulence is studied
based on three-dimensional compressible Direct Numerical Simulations (DNS) with simplified chemistry. The localised ignition
is accounted for by a spatial Gaussian power distribution in the energy transport equation, which deposits energy over a prescribed
period of time. In successful ignitions, it is observed that the flame shows a tribrachial structure. The reaction rate is
found to be greater in the fuel rich side than in stoichiometric and fuel-lean mixtures. Placing the ignitor at a fuel-lean
region may initiate ignition, but extinction may eventually occur if the diffusion of heat from the hot gas kernel overcomes
the heat release due to combustion. It is demonstrated that ignition in the fuel lean region may fail for an energy input
for which self-sustained combustion has been achieved in the cases of igniting at stoichiometric and fuel-rich locations.
It is also found that the fuel reaction rate magnitude is negatively correlated with density-weighted scalar dissipation rate
in the most reactive region. An increase in the initial mixture fraction gradient at the ignition centre for the ignitor placed
at stoichiometric mixture decreases the spreading of the burned region along the stoichiometric mixture fraction isosurface.
By contrast, the mass of the burned region increases with an increase in the initial mixture fraction gradient at the ignition
location, as for a given ignition kernel size the thinner mixing layer includes more fuel-rich mixture, which eventually makes
the overall burning rate greater than that compared to a thicker mixing layer where relatively a smaller amount of fuel-rich
mixture is engulfed within the hot gas kernel.
Submitted as a full-length article to Flow Turbulence and Combustion. 相似文献
4.
The head on quenching of statistically planar turbulent premixed flames by an isothermal inert wall has been analysed using three-dimensional Direct Numerical Simulation (DNS) data for different values of global Lewis number Le(0.8, 1.0 and 1.2) and turbulent Reynolds number Ret. The statistics of head on quenching have been analysed in terms of the wall Peclet number Pe (i.e. distance of the flame from the wall normalised by the Zel’dovich flame thickness) and the normalised wall heat flux Φ. It has been found that the maximum (minimum) value of Φ(Pe) for the turbulent Le=0.8 cases are greater (smaller) than the corresponding laminar value, whereas both Pe and Φ in turbulent cases remain comparable to the corresponding laminar values for Le=1.0 and 1.2. Detailed physical explanations are provided for the observed Le dependences of Pe and Φ. The existing closure of mean reaction rate \(\overline {\dot {\omega }}\) using the scalar dissipation rate (SDR) in the near wall region has been assessed based on a-priori analysis of DNS data and modifications to the existing closures of mean reaction rate and SDR have been suggested to account for the wall effects in such a manner that the modified closures perform well both near to and away from the wall. 相似文献
5.
The influences of differential diffusion rates of heat and mass on the transport of the variances of Favre fluctuations of reaction progress variable and non-dimensional temperature have been studied using three-dimensional simplified chemistry based Direct Numerical Simulation (DNS) data of statistically planar turbulent premixed flames with global Lewis number ranging from Le?= 0.34 to 1.2. The Lewis number effects on the statistical behaviours of the various terms of the transport equations of variances of Favre fluctuations of reaction progress variable and non-dimensional temperature have been analysed in the context of Reynolds Averaged Navier Stokes (RANS) simulations. It has been found that the turbulent fluxes of the progress variable and temperature variances exhibit counter-gradient transport for the flames with Lewis number significantly smaller than unity whereas the extent of this counter-gradient transport is found to decrease with increasing Lewis number. The Lewis number is also shown to have significant influences on the magnitudes of the chemical reaction and scalar dissipation rate contributions to the scalar variance transport. The modelling of the unclosed terms in the scalar variance equations for the non-unity Lewis number flames have been discussed in detail. The performances of the existing models for the unclosed terms are assessed based on a-priori analysis of DNS data. Based on the present analysis, new models for the unclosed terms of the active scalar variance transport equations are proposed, whenever necessary, which are shown to satisfactorily capture the behaviours of unclosed terms for all the flames considered in this study. 相似文献
6.
The bubbling dynamic behavior and acoustic radiation characteristics originating from the combustion of pyrotechnic mixtures of KClO 4/Mg-50 %Al alloy, and with different charge density such as 1500 kg/m 3, 1600 kg/m 3, 1700 kg/m 3 and 1800 kg/m 3, were studied by theoretical calculation and experiment. The experimental and numerical correlation results were in a good agreement with each other. Then, the bubbling dynamic behaviors were compared, the results revealed that with the increase of the charge density of each 100 kg/ m 3, the volume of the bubbles, the volume change acceleration and the sound pressure level average increased by 1.78 times, 2.28 times and 1.15 times respectively. 相似文献
7.
燃料空气炸药(FAE)装置爆炸场的研究 总被引:4,自引:0,他引:4
燃料空气炸药(FAE)装置在一次点火后,液体燃料在中心装药爆轰驱动下,抛撒在空气中形成燃料空气云,用高速摄影记录了该燃料空气云的形成过程。为达到所要求的气云形状,在对高速摄影照片充分分析折基础上,本文对FAE装置的结构设计提出了改进意见。燃料空气云在二次点火后实现爆轰,本文测量了沿地面主力学线的压力时间曲线,用高精度、高分辨率的TVD格式与瞬时爆轰模型时燃料空气云爆炸场进行了初步的数值模拟,得到了 相似文献
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Effects of Temperature-Dependent Viscosity on Forced Convection Inside a Porous Medium 总被引:1,自引:0,他引:1
Considering the exponential viscosity–temperature relation, effect of temperature-dependent viscosity on forced convection
of a liquid through a porous medium, bounded by isoflux parallel plates, is investigated numerically based on the general
model of momentum transfer. Local effects of viscosity variation on the distribution of velocity and temperature are analyzed.
Moreover, global aspects of the problem are investigated where corrections are proposed for total pressure drop and the fully
developed Nusselt number, in the form of out/in viscosity ratio. Results are obtained over a wide range of permeabilities
from clear (of solid material) fluid to very low permeability, where for constant properties one expects a nearly slug flow. 相似文献
10.
Transport in Porous Media - This paper numerically investigates the heat transfer performance of thermally developing non-Darcy forced convection in a fluid-saturated porous medium tube under... 相似文献
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A set of numerical experiments has been conducted to study the effect of a precursor fluid layer on the motion of two phase system in a channel. This system is characterized by coupled Cahn-Hillard and Navier-Stokes system together with slip boundary conditions. The solution of the governing equation involves first the solution of Cahn-Hillard equation with semi-implicit and Mixed finite element discritization with a convex splitting scheme. The Navier-Stokes equations are then solved with a P2-P0 mixed finite element method. Three cases have been investigated; in the first the effect of different wettability scenarios with no precursor layer has been investigated. In the second scenario, the effect of the precursor layer for different wettability conditions is investigated. In the third case, the effect of the thickness of the precursor layer is investigated. It is found that, wettability conditions have considerable effect on the flow of the considered two-phase system. Furthermore the existence of the precursor layer has additional influence on the breakthrough of the phases. 相似文献
13.
含热传导的冲击动力学有限元程序的研究与应用 总被引:2,自引:0,他引:2
从有限变形的基本框架出发 ,建立了含热传导的冲击动力学基本控制方程和合适的初边条件。应用变分原理和伽辽金方法得出了控制方程相应的有限元列式 ,并探讨了数值计算中的几个关键算法 ,主要包括构型转换、旋转张量的算法、本构算法以及计算流程和程序框图等。利用自行研制的程序 ,对长脉冲激光辐照下靶目标的变形和破坏、冲击压缩变形及变形局部化等问题进行了数值模拟和研究 ,所得结果与实验和理论分析相吻合。 相似文献
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Effects of Synthetic jets on a D-Shaped Cylinder wake at a Subcritical Reynolds Number 总被引:1,自引:0,他引:1
Effects of synthetic jets on the wake of a D-shaped cylinder is investigated experimentally at a Reynolds number ReH= 47,000, based on incoming free-stream velocity and the cylinder height (H). The synthetic jets are introduced immediately from the upper and lower trailing edges of the cylinder. The upper and lower synthetic jets are operated in an in-phase or anti-phase mode, and at a momentum ratio Cμ= 1.0% and perturbation frequency StA= 0.11 ?0.37. The cylinder wake with perturbation is examined in detail and compared with that without, based on smoke-wire flow visualization, pressure transducer and hotwire rake measurements, and data analyses of spectra, tempo-spatial cross-correlation and proper orthogonal decomposition (POD). Large-scale vortical structures in the cylinder wake are significantly modified by the synthetic jets perturbations, exhibiting symmetric or asymmetric patterns, depending on the perturbation frequency and phase relationship of the synthetic jets. These observations are internally correlated with the drag force variations. 相似文献
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使用低耗散的Roe格式,数值模拟了Reynolds数(Re)对大攻角细长旋成体绕流滚转角效应的影响.模型头部加了几何小扰动块以引发流场的不对称.在较大的Re数(Re=10 5)下,本文的计算结果与实验是相符的,此时细长体的滚转会导致双稳态、双周期现象,即侧向力随滚转角呈现类似方波形式的双周期变化,方波中侧向力基本保持不变的状态对应于流场的正则态,且两个正则态的侧向力方向相反,方波中侧向力基本保持不变的状态对应于流场的正则态,且两个正则态的侧向力方向相反;而在较小的Re数(Re=4 000)下,如果扰动足够大,细长体的滚转将导致不同的双稳态现象,此时两个正则态的侧向力方向相同,而在较小扰动下双稳态现象不再出现;Re数更小时(Re=1 000),即使在较大的扰动下,双稳态现象也不再出现,侧向力随滚动角仍是连续变化的.本文的计算结果表明,Re数越小,流场对头部扰动的感受性越弱. 相似文献
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Effects of Viscous Dissipation and Flow Work on Forced Convection in a Channel Filled by a Saturated Porous Medium 总被引:1,自引:3,他引:1
Fully developed forced convection in a parallel plate channel filled by a saturated porous medium, with walls held either at uniform temperature or at uniform heat flux, with the effects of viscous dissipation and flow work included, is treated analytically. The Brinkman model is employed. The analysis leads to expressions for the Nusselt number, as a function of the Darcy number and Brinkman number. 相似文献
20.
This paper presents results of the numerical simulation of periodically unsteady flows with focus on turbomachinery applications.
The unsteady CFD solver used for the simulations is based on the Reynolds averaged Navier–Stokes equations. The numerical
scheme applies an extended version of the Spalart–Allmaras one-equation turbulence model coupled with a transition correlation.
The first example of validation consists of boundary layer flow with separation bubble on a flat plate, both under steady
and periodically unsteady main flow conditions. The investigation includes a variation of the major parameters Strouhal number,
amplitude, and Reynolds number. The second, more complex test case consists of the flow through a cascade of turbine blades
which is influenced by wakes periodically passing over the cascade. The computations were carried out for two different blade
loadings. The results of the numerical simulations are discussed and compared with experimental data in detail. Special emphasis
is given to the investigation of boundary layers with regard to transition, separation and reattachment under the influence
of main flow unsteadiness.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献