Characteristics of Combustion and Heat Transfer of Excess Enthalpy Flames Stabilized in a Stagnation Flow. 2nd Report, Heat Flux at High Flow Rate and Effects of Lewis Number.

Abstract

Characteristics of an excess enthalpy flame stabilized in an axisymmetric flow impinging on the heat receiver wall (the stagnation surface) in a combustion chamber, and heat fluxes utilized from flames under the conditions of high flow rates near the extinction limits were experimentally examined. The effects of Lewis number of the deficient reactant on the characteristics of combustion were also discussed. (1) The excess enthalpy flames are stabilized near the heat receiver wall due to the effect of heat recirculation for the lean methane-air flames (Le≒1.0) and the lean propane-air flames (Le>1.0), and due to the effects of heat recirculation and the Lewis number for the lean hydrogen-air flames (Le<1.0), even in cases in which a large amount of heat is utilized from the flames. Thus, the heat flux utilized from excess enthalpy flames of methane, propane and hydrogen through the heat receiver wall increases rapidly with an increase in the stagnation velocity gradient. (2) When the flame is established outside the thermal boundary layer formed on the heat receiver wall, heat flux obtained from the flames through the heat receiver wall can be considered as the Convective heat transfer in the stagnation flow in which only the burned gas of high temperature flows. In contrast when the flame is stabilized inside the thermal boundary layer formed in the stagnation flow in which only the burned gas of high temperature flows, the heat flux transferring through the heat receiver wall increases rapidly with an increase in the stagnation velocity gradient, and the characteristics of convective heat transfer to the heat receiver wall are improved. The characteristics of heat transfer to the heat receiver wall are not dependent on the thickness of the thermal boundary layer, but are significantly affected by the flame location.