Progress in probability density function methods for turbulent reacting flows

Flame characteristics and a plausible flameholding mechanism in a supersonic combustor, with hydrogen injection upstream of a cavity flameholder, were investigated in the present study. Instantaneous OH radical distribution of the combustion flowfield was obtained using OH planar laser-induced fluorescence. According to the similarity of experimental observations with different cavities, a typical L/D = 7 cavity was chosen, and its supersonic combustion flowfield with hydrogen injection was calculated by large-eddy simulation. The results showed that the cavity shear layer plays a very important role in the flameholding process. An approximately steady flame existed in the cavity shear layer and hot combustion products were transported into the injection jet by the vortex interaction ofthejet-with-cavity shear layer. Flame then spread gradually following the counter-rotating vortex induced by the jet until the whole injection jet was ignited. Combustion products, which generated from the cavity shear layer and the jet, were convected into the cavity by the unsteady motion of the cavity shear layer and transported with the recirculation flow to the cavity front wall. These hot products and their intermittent combustion then heated up the cavity, and the fuel that entered into the cavity shear layer was preheated. Thus, the flameholding cycle was formed.

Flame Characteristics in Supersonic Combustor with Hydrogen Injection Upstream of Cavity Flameholder

A comprehensive numerical analysis has been carried out for both non-reacting and reacting flows in a scramjet engine combustor with and without a cavity. Transverse injection of hydrogen is considered over a broad range of injection pressure. The corresponding equivalence ratio of the overall fuel/air mixture ranges from 0.167 to 0.50. The work features detailed resolution of the flow and flame dynamics in the combustor, which was not typically available in most of the previous studies. In particular, the oscillatory flow characteristics are captured at a scale sufficient to identify the underlying physical mechanisms. Much of the flow unsteadiness is related not only to the cavity, but also to the intrinsic unsteadiness in the flowfield. The interactions between the unsteady flow and flame evolution may cause a large excursion of flow oscillation. The roles of the cavity, injection pressure, and heat release in determining the flow dynamics are examined systematically.

http://www.yang.gatech.edu/publications/Journal/Combustion%20Institute%20(Choi,%202005).pdf

Combustion oscillations in a scramjet engine combustor with transverse fuel injection

An implicit multigrid method for turbulent combustion

In this study, large eddy simulations (LES) of supersonic combustion using a compressible flamelet/progress variable (CFPV) approach are performed for the HIFiRE scramjet at Mach 8 flight conditions. The LES results show good agreement with the ground test experimental measurements. The combustion model is based on an efficient flamelet approach, where compressibility corrections are devised based on assumed functional forms of important thermo-chemical quantities. Specifically, the source term of the progress variable is rescaled with the local density and temperature in the LES, leading to improved predictions relative to existing flamelet models. A modified equilibrium wall-model, capable of predicting the viscous heating, is used in the viscous near-wall region. Temperature near the wall increases significantly due to viscous heating, enhancing the reaction rate and heat-release. This is shown to be a crucial step for accurately predicting the pressure rise in the combustor.

/pdf/large-eddy-simulations-of-the-hifire-scramjet-using-a-3fhkruw2l6.pdf

Large eddy simulations of the HIFiRE scramjet using a compressible flamelet/progress variable approach

Scalar and joint scalar-velocity-frequency Monte Carlo PDF simulation of supersonic combustion

Comparison of Eulerian and Lagrangian Monte Carlo PDF methods for turbulent diffusion flames

An implicit multigrid method is applied for solving the Navier-Stokes, turbulence, species and variance transport equations. As to account for turbulence chemistry interaction assumed probability density functions (pdf) are used. The pdf’s assumed are a Gaussian distribution for temperature and a multivariate ,&pdf model for species mass fractions. For turbulence closure a twoequation q-w low-Reynolds-number turbulence model is employed. Strongly non-linear source terms in species, turbulence, and variance conservation equations usually keep multigrid methods from converging. It is shown that a simple freezing of coarse grid source terms formed by spatial derivatives and restriction damping in regions of high chemical activity may remedy this problem. As to avoid stiffness problems associated with chemically reacting flows, time integration is performed by an implicit LU-SGS algorithm. Two finite-rate chemistry test cases with methane and hydrogen combustion at supersonic speed are investigated and a strong reduction in required CPU time is demonstrated.

Multigrid' approach for turbulent combustion using assumed probability density functions

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Scalar and joint scalar-velocity-frequency Monte Carlo PDF simulation of supersonic combustion

Comparison of Eulerian and Lagrangian Monte Carlo PDF methods for turbulent diffusion flames

Multigrid' approach for turbulent combustion using assumed probability density functions