Effect of flame-induced geometrical straining on turbulence levels in explosions and common burner configurations

Abstract

The modification of turbulence ahead of the flame front in a point-ignited explosion developing in a homogeneous turbulent flow is investigated. The spherical flame front may be freely expanding or confined within a vessel. In the former, geometrical considerations predict that the total strain on a fluid element just before its engulfment by the flame is independent of the initial position of the element. This results in a uniform change of the turbulence. For example, the turbulence intensity generally increases by about 50%. In the case of confined explosions, the distortion of fluid elements leads to increasingly larger turbulence amplifications, starting from a value approximately equal to that of the unconfined case to up to several hundred per cent at the final stages of combustion. Rapid Distortion Theory is used for the analysis. The modified energy spectra are also evaluated. Available experimental data lend qualitative support to the results. The range of validity of this analysis is discussed. The independent variable is cast in terms of a pressure ratio and straining factors, thus permitting the application of the data to a wide range of configurations without the need to delve into the complicated mathematics each time. The conclusions are expected to be moderated but not negated by taking into account the process of nonlinear turbulence decay. Finally, the implications on turbulent flame velocity measurements in various configurations are briefly explored. © 1992.