1.1 Intake design objectives

The hypersonic intake decelerates and compresses the freestream airflow as it flows into the combustion chamber. In doing this, (a) the intake flow must start and achieve the required decrease in Mach number with a high thermodynamic efficiency; (b) the compressed airflow, flowing into the combustor, should be uniform and stable; (c) the intake should operate efficiently and stably over the entire flight envelope bounded by flight Mach number, altitude and angle of attack; (d) the intake structure should be as light as possible; (e) drag of external surfaces and aerodynamic heating loads should be minimal; (f) for useful engine operation, the intake duct shape and flight conditions must be such that the intake airflow is predictable, properly conditioned (uniform in some sense) and aligned with the combustor walls as it flows into the combustor. These requirements for intake design are often contradictory, making it difficult to attain optimum individual operating conditions simultaneously. For example, in a fixed-geometry intake, flow starting and the need for substantial Mach number reduction pose a very serious geometric flowpath contraction contradiction that leads to an onerous design task.

Thermodynamic cycle calculations of high Mach number airbreathing engines, such as scramjets, have shown that the engines should have air intakes that contract and compress the flow by factors of 6–10 and 10–20 respectively and that this contraction and compression should be done with minimal loss of total pressure. Aside from high contraction and compression, the attainment of efficient intake performance is critically dependent on the freestream Mach number and the lateral and stream-wise contours of the intake surface, both being factors in determining the character of the flow in the intake and its performance.

Since all these design concerns are intake shape-dependent, it is most convenient and reasonable to start an iterative type intake design procedure with the selection

The Busemann Air Intake for Hypersonic Speeds DOI: http://dx.doi.org/10.5772/intechopen.82736

of an intake shape that is known to produce a geometrically simple, compressive flow. Flow on a plane inclined wedge, Prandtl-Meyer flow and flow over a circular cone, as well as their combinations, have been used extensively as starting points for supersonic intakes because their aerodynamic characteristics are simple and easily predictable analytically. These "textbook" flows are usually adaptable to physical variations in geometry where shape change may be required for optimised performance over a range of Mach numbers and to ensure intake flow startability. In selecting such simple and easily predictable flows and their streamlines, for intake applications, we search for geometric streamlines that join a uniform and parallel freestream entry flow to an equally uniform and parallel exit flow. For most intakes the exit flow direction should be the same as that of the freestream. The flow compression in the intake should be longitudinally distributed so as to be isentropic at the high Mach number, upstream end of the intake. Minimal shock losses are obtained when compression through shock waves occurs at the lower Mach number, downstream end. So as to minimize viscous losses, all surfaces should contribute usefully to the compression task by individually supporting positive pressure gradients. The desirable qualities should not deteriorate significantly at off-design conditions of flight Mach number, altitude or angle of attack.
