1.2 Simple planar and axial flows

The air intake is one of the three essential components of supersonic and hypersonic airbreathing engines. It conditions the approaching freestream airflow for the combustor and the nozzle, compressing the airflow for best performance of the downstream components. For best overall engine performance, it must do so with minimal losses. The intake's performance can be thought of in terms of what the intake does and how well it is doing it-the capability and the efficiency. Capability can be quantified by such variables as the ratio of entry-to-exit Mach number, ratio of flow areas (contraction) or ratio of pressures (compression). Efficiency can be measured by the total pressure recovery or the entropy rise from entry to exit. These considerations of "what" and "how well" are governed by the First and Second Laws of Thermodynamics. Both capability and efficiency are highly dependent on the geometry of the intake surface. This raises additional design concerns about the intake's length and weight as well as exit flow inclination and uniformity, and the startability of the intake flow. These factors have a major effect on the

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

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

intake's performance and on its design.

the character of the flow in the intake and its performance.

1.1 Intake design objectives

The scramjet and its three components.

Hypersonic Vehicles - Past, Present and Future Developments

Figure 1.

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Scramjet engine thermodynamic cycle calculations and combustor performance place a requirement on the hypersonic air intake to reduce the freestream Mach number by a factor of about three and to do so with a total pressure recovery of at least 0.5. These design targets can be met by employing combinations of simple inviscid flows that are assembled to form the intake flowpath. The simple flows can be based on either planarly symmetric (planar) or axially symmetric (axial) supersonic "text-book" flows. In planar flows, flow properties are the same in parallel geometric planes. In axial flows, flow properties are invariant in planes around a common axis. Because of planar or axial symmetry, the number of independent spatial geometric variables, needed to specify the flow, is reduced from three to two—a great simplification for design and analysis. These simple flows also possess radial symmetry in that there is no variation of flow properties along flat planes (planar flow) or cones (conical Taylor-Maccoll flow). Use of simple flows with flat plate and conical symmetry allows the number of spatial variables that are required to specify and describe the flow, to be further reduced by one, so that only one independent geometric variable remains—a further simplification for intake design and analysis. Examples of such commonly used simple planar flows are the flow behind a flat oblique shock and Prandtl-Meyer flow. Commonly used simple axial flows are the flow over a cone and the Busemann flow. Simple flows and their combinations do not carry shocks that are curved in the flow plane; this keeps the intake flows irrotational and uniform. An important part of intake design consists of combining and connecting the simple flows to yield the desired intake performance. The other part consists of using selected streamline sheets of these flows to form desired flowpath shapes—a technique called wavecatching.

The focus in this paper is on the use of axial, internal flow elements (basic flows) [1–3], rather than planar flow elements, to construct intake flow paths. The axisymmetric intake attains most of its compression by flow convergence rather than flow turning or shock deflection. The converging flow is isentropic, it is similar to sink flow and it causes a Mach number decrease which leads directly to weak terminal shocks waves and efficient intakes with high capability. The internal converging flowfield is the most important feature of an axial flow intake.

Problems of viscous losses and flow starting are eased by use of wavecatcher technology, providing leading edge truncation and sweep and by the fact that high adverse pressure gradients occur in the inviscid core flow rather than in the wall surface boundary layers. Hypersonic intakes that utilize axisymmetric compressive basic flows with specified entrance and exit shapes have received attention because of their high performance (capability and efficiency) and analytical simplicity [4–24].

P-M flow, may have to be used where variable geometry demands the use of planar flow. The resulting flow turning, away from the flight direction, has to be compensated by lossy oblique shock deflections. On the other hand, flow convergence, such as occurs in sink flow, is an effective mechanism because it is isentropic and involves no flow turning. Deflection occurs through an oblique shock; it is nonisentropic and it should be used only when there is no other possibility of orienting the flow. It should not be used to reduce the Mach number. A practical flow, such as Busemann [1], incorporates all three of these aerodynamic mechanisms as they

The three modes of compression are illustrated in the Prandtl-Meyer intake, the Oswatitch intake and the Busemann intake (Figure 2). The Prandtl-Meyer intake obtains performance by isentropic turning through the compression fan, followed by deflection through the oblique shock; there is no convergence. The Oswatitch intake has flow divergence and turning followed by deflection through a shock. The Busemann intake has turning and convergence followed by shock deflection. Three intake models were designed to reduce the Mach number from 8.33 to 4.8 with a static pressure ratio of 26.8. All three intakes were tested in a gun tunnel [11] at Mach 8.33 and it was found that, for the same amount of contraction, the inviscid total pressure recoveries of the Busemann, Oswatitch and Prandtl-Meyer intakes were 0.983, 0.763 and 0.763. Experimental total pressure recoveries were 0.484, 0.485 and 0.240. The reason for the differences stems from the fact that the surface area and consequently the viscous losses, were greatest for the Prandtl-Meyer intake. Sidewalls, needed to contain the planar Prandtl-Meyer flow, did not preserve the intake's efficiency but contributed to the surface area and viscous losses. The lack of an extensive leading edge and attendant viscous flow contributed to the efficiency of the Oswatitch intake. These results illustrate the superiority of axial over planar basic flows where it is the Mach number reduction, achieved by

convergence, that leads to the high performance of the Busemann intake.

interactively contribute to intake performance.

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

Figure 2.

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Schematics of three intakes tested in a gun tunnel at Mach 8.33 [11].

A preferred geometry for a scramjet combustor is a duct with a circular cross section because of its superior ability to withstand both heat and pressure loads. Frictional losses are also at a minimum for such a duct since a cylinder has the smallest surface area for a given cross-sectional area. This leads to a cylindrical (axially symmetric) geometry as being desirable also for the intake that is attached to the front of the combustor duct. The same circular exit geometry for the intake is demanded by a gas turbine engine, in this case because the axial compressor face is circular. Towards these ends, it is pertinent to study an axisymmetric flow and it is entirely fortuitous that axisymmetric, conical, Taylor-Maccoll flow provides a streamtube shape [1, 2] that satisfies the above intake design requirements, both geometric/structural as well as aerodynamic [3]. In recognition of Adolph Busemann's pioneering work [1] on such streamtube shapes, they are called Busemann flows and Busemann intakes. References [1–18, 24] all concern Busemann flow.
