Flow Immediately behind a Step in a Simulated Supersonic Combustor

Results are presented from a series of nonburning flow tests in which extensive instream and wall pressure and fluid sample measurements were made to characterize the flow immediately behind a rearward facing step in a simulated supersonic combustor. Pitot pressure data were used to determine simplified flowfield models for the cases of flow both with and without a simulated precombustion shock and simulated fuel injection. The effects of the shock on the flowfield and fuel-air distribution in the main stream and recirculation zone behind the step, as well as the implications for the placement of ignition devices and piloting, are discussed. N supersonic combustion ramjet (scramjets), certain combustor configurations employ in their design a rear- ward facing step downstream of the fuel injector. This sudden step increase in combustor area has been beneficial to com- bustor operation in several ways. Among these are: 1) the step helps to isolate or limit the interaction of the combustion process with the inlet flowfield of the engine; 2) the step limits the combustor pressure rise by spreading the pressure gradient from the combustion shock along the wall, limiting excessive local heat transfer due to shock impingement; and 3) the step serves as a flame-holding region. The purpose of the present work is to improve the understanding of the flow pat- terns and fuel distribution in the mainstream and in the recir- culation zone behind the step, so that fuel injectors and/or ignition aids can be located more effectively to improve the supersonic combustion of hydrocarbon fuels or fuel blends. Generally, in supersonic combustion, an oblique shock will be present at the combustor entrance. Waltrup and Billig developed a semi-empirical pseudo-one-dimensional analysis that defined the pressure field and attendant separation zone in the precombustion interaction region of constant area com- bustors.!"3 Waltrup and Cameron presented wall shear and boundary-layer measurements that completed the depiction of the instream and wall flow structure for a shock-separate d flow of this type.4 These works also demonstrated the feasibility of producing the shock structure and shock/boun- dary-layer interactions analogous to those present at the com- bustor entrance in a simple nonreacting system, where throt- tling rather than combustion is the cause of compression. This technique permits detailed measurement of the flowfield to be made in a much less hostile environment where in- strumentation will not be destroyed by the high temperatures (~4000°R-5000°R) encountered in flows with combustion. Although exact duplication of local flow divergence caused by spatial and time variations in heat release cannot be achieved with throttling, extensive comparison of the throttle test data and combustor data showed that sufficient similarity of the two flows was obtained for studying the effects of the corn-