Distinguishing characteristics of thermally-and mechanically-generated backpressure

Although a large number of studies has been carried out on the backpressure phenomenon and unstart in scramjet models, including the development of pseudo-shock location control algorithms, the distinction between mechanical and thermal causes of backpressure and unstart has not received enough attention. Moreover, a large proportion of the aforementioned studies have focused on artificially blocking the flow downstream of the combustor. In this study it is the objective to provide answers to certain questions pertaining to the backpressure phenomenon in the scramjet engine: How does the backpressure generated from mechanical means, such as those from high fuel injection static pressure or mechanical flaps downstream of the scramjet combustor, differ from that generated by thermal means in the combustor? How do the characteristics of the flow and combustion fields differ? What are the distinguishing dynamics of the shocks – pseudo-shocks or shock trains-in both manners of generating backpressure? What specific general characteristics – in terms of the aerodynamics and reaction field-can be used to differentiate between the two sources of backpressure? How is the interaction of the two causal effects manifested? Answers to the foregoing and similar questions will significantly improve our understanding of the backpressure phenomenon in scramjet engines. To the knowledge of the author, these issues have not received adequate attention in the literature despite the significance. In the present work, backpressures in scramjet engines have been generated using three different approaches: the use of high fuel injection static pressure in a mixing condition, mechanical flap at combustor exit in a non-mixing, non-reacting condition, and backpressure due exclusively to thermal choking from combustion. Significantly different characteristics have been observed which are summarized in this paper.