On the modelling of turbulent reacting flows in furnaces and combustion chambers

Abstract

A general computer programme was developed to calculate the local flow properties in turbulent reactive and non reactive flows with recirculation; these calculations were obtained by solving the appropriate conservation equations in finite difference form with the corresponding boundary conditions. The calculation procedure employs a two equation turbulence model, and embodies various combustion models appropriate to diffusion, premixed and arbitrary fuelled flames. The phenomenon of unmixedness caused by turbulent fluctuations, which lead to a situation where the instantaneous value of fuel and/or oxidant concentrations and, therefore, their corresponding chemical reaction rates vanish, is investigated. The combustion models considered here are characterized by, for instance, instant reaction with clipped Gaussian probability distribution of concentration, which corresponds to random variation of fuel concentration with time, finite reaction rate with an eddy break up formulation, and finite reaction rate with a second order closure which accounts for temperature and concentration fluctuations. The radiative heat flux, which appears in the energy conservation equation, is obtained using a coupled four flux representation and integrating the radiation intensity distribution over a solid angle of 2π. The validity of the computational procedure incorporating the proposed turbulence, combustion and radiation models was assessed by comparisons with the experimental data in reacting and non reacting flows, and indicated satisfactory agreement. The obtained agreement assesses the validity of the physical assumptions of the models and supports the use of such procedure for furnace design purposes.