1.3 Intake flow processes and inward/outward flows

The reduction of Mach number, in the various basic flows, is accomplished by one or more fluid mechanical mechanism: (a) compressive flow turning; (b) flow convergence with area contraction and compression in a converging passage and (c) flow deflection through an oblique shock. Flow turning and contraction are isentropic processes leading to no loss in efficiency. Flow deflection through an oblique shock entails an entropy increase—a loss in intake efficiency. If shocks are needed to deflect or re-direct the flow then they should be as weak as possible, occurring at the lowest possible Mach number (e.g., Busemann shock). Planar flow turning by Prandtl-Meyer-type flow requires much turning to accomplish a significant Mach number reduction, so that, after P-M turning, strong shocks are required to re-direct the flow back to the freestream direction for the combustor. On the other hand, isentropic Mach number reduction by area contraction leads to a rapid streamwise Mach number reduction when the flow is axial. In such flows, Busemann flow being typical, there is comparatively little flow turning towards the center line, the compression being accomplished by area contraction and, as a result, there is no need for much deflection (re-turning) by a shock at the exit. Also, since there is considerable Mach number reduction in the converging flow, the terminal shock faces a reduced Mach number. This weaker terminal shock minimizes efficiency losses. The axial flow intakes derive their high efficiency from the axial convergence, being only little degraded by flow deflection through the terminal shock. The axial Busemann intakes have been mistakenly labeled as "inward turning" even when part of their converging flow is turning outward, away from the axis. We suggest dropping the "inward turning inlet" terminology in favor of "axial flow intake" or "converging flow intake," because their fundamental and characterizing distinction is axial convergence. It is precisely the lack of much "inward turning" that leads to the high performance of Busemann intakes. It would be better to use the flow-related and meaningful concepts of turning, convergence and deflection to characterize intake flow types in general. Isentropic turning, as in
