Statistical Modelling of Turbulent Reactive Flows

Abstract

Turbulent combustion occurs in many engineering applications: spark-ignition engines, gas-turbine combustors, and furnaces, for example. In each of the applications cited the design process is lengthy and expensive. The industries involved are attempting to improve their design procedures by using computer models of turbulent reacting flows.

Since the fundamental governing partial differential equations are known, the direct approach is to solve them numerically. However, because of the wide range of length and time scales involved, this direct approach is computationally impracticable, now and in the foreseeable future. The alternative is to use a statistical approach. Such approaches face a formidable challenge: superimposed on the difficulties of calculating inert, constant-density turbulent flows, are those associated with reaction and heat release. The reaction rates are typically highly nonlinear functions of the composition variables, which are subject to large turbulent fluctuations. Often reaction takes place in laminar flamelets that are thin compared to turbulent scales. Due to heat release the specific volume of the mixture can increase by a factor of ten which, as may be expected, is found to have a large effect on the turbulence. For example, due to heat release, the turbulence energy can increase by an order of magnitude, and new transport process become dominant and can lead to countergradient diffusion.