Pressure effects on nonpremixed strained flames

This article deals with the effect of pressure on the structure and consumption rate of nonpremixed strained flames. An analysis based on the fast chemistry limit indicates that the flame thickness is inversely proportional to the square root of pressure and that the flame structure may be described in terms of a similarity variable that scales like the product of pressure and the strain rate to the power 1/2. This scaling rule also applies to flames submitted to a time-variable strain rate provided that the frequencies characterizing these changes are low compared to the mean strain rate. It is also confirmed that reactants consumption rates per unit flame surface vary like the square root of pressure and that this rule holds for time-variable strain rates of arbitrary nature. Complex chemistry calculations carried out over a broad range of operating pressures indicate that the pressure dependences deduced analytically are remarkably accurate and can be used for a broad range of strain rates, excluding values in the near vicinity of extinction conditions, where finite rate chemistry effects become important and influence the flame response to pressure. Thus, it appears that the pressure exponent characterizing the heat release rate in nonpremixed strained flames is essentially constant and equal to 1/2. This exponent is independent of finite rate chemistry effects, except when conditions are close to extinction.