On the behavior of a coupled transient flamelet/pdf transport equation model for nonpremixed turbulent flames

Abstract

This work introduces a new methodology for the modeling of turbulent flames, based on the combination of two frequently used models: the Eulerian composition probability density function (PDF) transport equation model and the transient laminar flamelet model (TLFM). The current new coupled TLFM=PDF model aims to combine the advantages of both models while avoiding their disadvantages. The flame chosen in this work for the validation was experimentally studied by Barlow and Frank (Effects of Turbulence on Species Mass Fractions in Methane=Air Flames, Proc. Combust. Inst., 27, 1087-1095 and shows visible local extinction at around an axial distance of 30 jet radii and reignition farther downstream. Therefore, this configuration is a good test case to study the capability of numerical models for flames with local extinction and reignition regions. The Eulerian composition PDF transport equation model with a four-step reduced chemistry and the coupled TLFM=PDF model are compared and validated against experiments. INTRODUCTIONCombustion modeling has been a necessary challenge during the past few decades, driven by the importance of combustion processes in many industrial applications. There are several issues that must be handled by the model to describe the physics of turbulent combustion: the correct prediction of the turbulent flow, the turbulence and chemistry interaction, the regime of applicability, the unsteady effects that are relevant to the local extinction and reignition phenomena, the detailed chemistry information, and, beyond these, reasonable computational costs. All these issues are still in progress in turbulent combustion research, which in turn can be classified into three categories, namely, nonpremixed combustion, premixed combustion, and partially premixed combustion. In turbulent nonpremixed combustion, the fuel and the oxidizer are separated and combustion takes place simultaneously due to the turbulent mixing process (Bilger and Beck, 1989). In contrast, for premixed combustion, the fuel and the oxidizer are completely mixed before entering the combustion zone; thus, the rate of chemical reaction is controlled by a flame front that propagates from the hot combustion zone into the cold nonreacted region (Bray and Libby, 1994). Last, partially premixed combustion can be observed in nonpremixed flames when fuel and oxidizer have mixed without burning. Partial premixing may result from the frozen flow mixing before ignition or from local quenching and can occur in recirculation zones of burners with diffusion flames. In applications the fuel can be mixed with a primary air before entering the combustion zone because partial premixing helps to stabilize the flames and reduce pollutant emission.This work concentrates on the modeling of high-Reynolds-number nonpremixed pilot jet flames where the flame stabilization of the main jet is controlled by the premixed pilot flames. The hot gases of the pilot force the flame to extinguish at high jet velocities. Thus, local extinction occurs due to highly s...

Conditional moment closure modelling of soot formation in turbulent, non-premixed methane and propane flames

“…Both achieve the goal of the inclusion of finite-rate chemistry into turbulent flow calculations, the former model doing so via the solution of a multidimensional transport equation of species and vector quantities, with solutions obtained by the implementation of a Monte Carlo technique. Although displaying very promising results in similar applications [9,10], the method is computationally expensive, especially when the number of chemical species is large. At present, the deterministic CMC method provides a less restrictive approach to a wide range of practical applications, and can be easily integrated with a complex chemical kinetic scheme without requiring significant computer run times.…”