Fatigue of composite materials: damage mechanisms and fatigue-life diagrams

Abstract

An experimental study of the influence of laminar burning velocity on the structure and propagation of duct-confined premixed turbulent flames has been carried out. Propane, acetylene and hydrogen were used as fuels to vary the laminar burning velocity in the range from 20 to 280 cm/s. These experiments fully verify the three region model (region 1: $u^{\prime}\delta _{\text{L}}$; region 2: $u^{\prime}$ $\approx $ 2$S_{\text{L}},\eta $ $\approx $ $\delta _{\text{L}}$ to $\eta \gg \delta _{\text{L}}$; region 3: $u^{\prime}>2S_{\text{L}},\eta <\delta _{\text{L}}$) of turbulent flames proposed earlier by Ballal & Lefebvre. Since a large increase in the laminar burning velocity has a stabilizing influence it is possible to suppress the 'instability' of region 1 and the 'eddy entrainment' of region 3. The 'turbulent diffusion' mechanism then becomes solely dominant, and the flame shows a 'jet-like' behaviour. For such a flame (i) both the burning velocity and flame turbulence intensity are independent of scale, (ii) the equations developed by Karlovitz and Ballal for regions of stable combustion accurately predict all the experimental data on turbulent burning velocity and flame turbulence, respectively, and (iii) the laminar burning velocity remains an important parameter of flame propagation even at very high turbulence intensity. Finally the important role of shear-generated turbulence and the ability of the flame either to dampen or to generate additional turbulence has been fully confirmed.