Ph.D. (Engg): ON HIGH-SPEED CURVED COMPRESSION RAMP AIR INTAKES

A scramjet, i.e., a supersonic combustion ramjet, is an air-breathing engine that enables sustained atmospheric flight in the hypersonic regime. It consists broadly of four key components: the air intake, isolator, combustor, and nozzle. The air intake and the isolator collectively comprise the compression system, which captures and conditions the freestream flow to suit the operational requirements of the combustor positioned downstream. The general set of attributes sought in a high-speed air intake is: low structural weight; low drag; low aero-thermal loads; started flow with high thermodynamic efficiency and high compression ratio; operational robustness to back-pressure fluctuations arising from the combustor; and stable engine operation over a wide flight envelope. The intake flow characteristics and performance are primarily governed by its geometric shape. High-speed air intakes with a curved compression ramp (CCR) are a class of rectangular intakes wherein the compression ramp comprises a curved surface followed by a planar surface tangential to the trailing end of the curved surface. A CCR intake compresses the flow through a combination of a curved ramp shock wave and a series of compression waves. These intake geometries can provide improvements in compression ratio, pressure recovery, and allow for a shorter length intake section in comparison to conventional multi-step intake geometries.
The present effort consists of the development of a novel analytical framework to model the CCR intake flow and estimate the intake performance parameters at design and off-design operating conditions, and wind tunnel experiments with a model CCR air intake at Mach 6 to obtain a detailed understanding of the flow dynamics. The analytical framework builds on the principles of mass conservation and the compressible flow theory to model the inviscid flow structure in the intake without any empiricism. A modified Kantrowitz criterion is proposed to examine the ability of a given fixed-geometry CCR intake to spontaneously self-start at the design Mach number. The framework provides a simple, fast, and low-cost tool to develop an effective first-cut design of a self-starting hypersonic CCR air intake for any specified set of operating conditions and performance parameters of interest, such as the startability, compression efficiency, and compression ratio. Inviscid flow numerical experiments of intake starting were carried out to preliminarily verify the starting characteristics predicted by the analytical model.
The analytical framework was then employed to identify a suitable geometric design point for the experimental CCR air intake model. A self-starting intake test model was designed following the strong shock design principle, and built for experimentation in the Roddam Narasimha Hypersonic Wind Tunnel at Mach 6 freestream flow conditions. Time-resolved pressure measurements and high-speed schlieren flow visualization were conducted to understand in detail the flow features internal and external to the intake model, including the dynamics of shock-shock and shock-boundary layer interactions at the cowl and inside the isolator. Performance assessment at design operating conditions involved evaluating the intake’s ability to spontaneously self-start, in addition to examining pressure recovery and compression ratio of the started intake. At off-design operating conditions, the intake dynamics were studied by experimentally varying two parameters: intake back-pressure and angle-of-attack (𝛼). In order to mimic combustor-induced isolator back-pressure variations, a sliding plate was introduced at the isolator exit; the motion of the sliding plate varies the isolator exit area (blockage) and thereby changes the back-pressure. Experimental results showed that the intake auto-reverts to the started state on realizing suitable pressure values at the isolator exit. Coherent flow oscillations with certain characteristic frequencies were observed in the isolator section during the intermediate state of operation (between started and unstarted states). The angle-of-attack (AoA) studies, in the 𝛼 range of -70 to 20.70, show that the relatively gradual distribution of adverse pressure along the compression ramp mitigates the risk of large-scale boundary layer separation, even at very large AoAs. The model intake was found to satisfy shock-on-lip condition and operate in the started state between 𝛼 = 00 and 𝛼 = 100, and supersonic flow was sustained in the isolator section up to 𝛼 = 20.70. Overall, the experimental results were found to validate predictions made by the analytical model. The experiments also allowed for a careful examination of flow during various stages of intake operation, including intake unstart and restart, and quantification of operational margins (in terms of pressure) for the model intake. In addition to aiding performance assessment, these results can also form the basis for the design of a practical early-warning system for preventing engine unstart.
In summary, this work offers a clear experimental demonstration of the advantages offered by CCR air intakes, and an analytical framework that serves as a good starting point for a design exercise. In practical terms, this work exhibits the promise held by CCR air intake in providing a wide flight envelope for an air-breathing hypersonic flight vehicle.

Speaker : Sushmitha Janakiram

Research Supervisor : Duvvuri Subrahmanyam