Stable Combustion of a High-Velocity Gas in a Heated Boundary Layer

Abstract

Summary It is generally recognized that stable combustion processes in heated boundary layers may be achieved by either of two concep­ tual mechanisms. In one mechanism it is pictured that the heat transfer to the wall quenches the propagating flame at a certain distance from the surface. The equality between the flow velocity and the normal burning velocity at this quenching distance deter­ mines the position of the propagating flame. In the second mechanism it is conceived that the hot surface provides a con­ tinuous source of ignition in much the same manner that the hot recirculation zone of a bluff body flame holder provides continuous ignition to the gas flowing around it. In this case it is the charac­ teristic time during which the gas must be heated that determines the position of the flame. All experimental work reported to date has been concerned with conditions where the first picture has apparently been applicable. In the present paper, experiment and analysis are given that show under what conditions the continuous ignition mechanism provides the appropriate model and also how the two models are related. To differentiate the two mechanisms an experiment was set up to study flame stabilization in high-velocity boundary layers over a wall heated in the form of a step function. With a turbulent boundary layer and a wall temperature above 1,700°F.f the char­ acteristic time was found to be a systematic and reproducible variable. These observations led to the conclusion that a con­ tinuous ignition mechanism governs stabilization in heated turbu­ lent boundary layers. A rational explanation is made for the transition from the low-speed mechanism known to be applicable in unheated turbulent boundary layers and heated laminar bound­ ary layers to the ignition mechanism applicable in heated tur­ bulent boundary layers. As a further verification of the continuous ignition mechanism an apparent ignition energy was found. The logarithm of the heat added at the lower stability limit was found to be a linear function of the reciprocal of the limiting wall temperature. The activation energy derived from this Arrhenius type of relation agreed reason­ ably well with the estimated value for the fuel used.