Turbulence effects on enclosed flames

Abstract

A major factor limiting the development of turbulent flame theory is the dearth of experimental data. Almost all reported measurements of turbulent burning velocity have tended to emphasize the influence of turbulence intensity, but the equally important aspect of turbulence scale (eddy size) has been comparatively neglected. The present work attempts to remedy this situation by providing experimental data on the initiation and development of enclosed, premixed turbulent flames. The test rig is similar to that employed by Lefebvre and Reid[1]. By suitable variation in flow velocity and the geometry of the turbulence-promoting grid it is possible to create at various distances downstream of this grid regions in which turbulence intensity is sensibly constant but where turbulence scale varies over a fairly wide range. Thus measurements of flame speed and other flow parameters can be made under conditions where the effects of turbulence scale and intensity are completely separated. Many such measurements have been made and the results obtained are discussed in relation to current theories on turbulent flame structure and propagation. In particular it is shown that turbulent flame speed increases with scale under conditions of weak turbulence and decreases with increase in scale under conditions of strong turbulence. These two regions are separated by a so-called “transition region,” which occurs when the turbulence intensity is about twice the laminar flame speed. In this transition region turbulent flame speed is independent of both scale and laminar flame speed.