Wall heat transfer and flame structure transitions in stagnating spray flames

Abstract

Wall-stagnating spray flames are of importance in a number of engineering applications, including accidental leakage of liquid fuel from pressurized fuel lines as well as in direct-injected compression and spark-ignition engines. The wall heat flux generated by such flames is an important consideration in the design and analysis of affected components. To identify and analyze the key parametric dependencies of the flame structure and wall heat flux, we carry out one-dimensional simulations of wall-stagnating spray flames employing an Eulerian-Eulerian formulation and a realistic 54-species chemical mechanism for n-dodecane/air combustion. Conjugate heat transfer is accounted for by considering a finite-thickness wall. We find that the configuration permits three distinct flame structures, namely wall-stabilized, detached, and injector-stabilized flames. Since the near-wall flame structure determines the wall heat flux, the parameter space is explored to identify points of flame structure transition. We show that the Stokes number and the liquid mass loading play a key role in controlling the transition in the flame structure and its non-linear coupling to the wall heat flux, the former parameter by controlling the droplet evaporation time and the latter by affecting the amount of fuel able to penetrate the flame. The wall boundary temperature has a direct effect on the wall heat flux, and we show that it exhibits hysteresis in flame structure transition, with an associated change in wall heat flux of up to 30 percent.