Design and Performance Investigation of Supersonic Air-Intakes

Abstract

Supersonic air-intake is an essential component of a ramjet engine. Typically, a ramjet powered missile that operate over a range of Mach numbers and maneuverability conditions uses a fixed geometry intake to ensure cost-efficiency. Such an intake system at off-design conditions would experience adverse flow conditions related to shock-shock interactions, shock wave/boundary-layer interactions (SWBLIs), and many other flow phenomena affecting its performance parameters. Moreover, the flow is likely to “unstart because of SWBLI”. Therefore, fixed geometry intakes must be designed to maintain desirable performance characteristics in the complete operating regime. The thesis focuses on the preliminary design and performance evaluation of a supersonic intake system for a “four-intake symmetric cross or X configuration missile”, to be operating in the freestream Mach number range of about 1.8-2.5, an angle-of-attack range of 0° ≤ AoA ≤ 10° and over a low altitude condition of 5-10 Km, corresponding to a unit Reynolds number of approximately 1.4 -4.0×107 m-1. More specifically, the objective is to provide an in-depth understanding of the effects of internal geometric factors, upstream flow and boundary-layer bleed system parameters that should be considered for the design of a ramjet air-intake. The performance has been evaluated in terms of total pressure recovery (TPR), captured and bleed mass flow ratio (MFR, BMFR), sustainable backpressure ratio (or peak TPR) and distortion index (DI). The highlights of the results could be summarized as follows: In the first phase of the study, a theoretical one-dimensional (1D) optimization tool has been developed for the design of rectangular mixed-compression intakes. The tool makes use of various oblique and normal shock relations, Oswatitsch criterion for planar shocks and available experimental correlations to fix the geometric dimensions of different parts, and aims at maximizing the total pressure recovery. Secondly, a computational fluid dynamic (CFD) model has been proposed for the prediction of intake flow and performance characteristics. The numerical methodology has been verified using established techniques for scientific computing, that involves grid independence study and the estimation of grid convergence index (GCI). Moreover, the model is systematically validated against three experimental test cases/data available in the literature, where the possibility of “unstart because of SWBLI” arises. In the third phase of the work, the effects of several internal geometric factors such as the multi-ramp compression structure, internal contraction ratio, subsonic diffuser internal ducting as well as the sidewall configurations, and several upstream flow conditions have been studied independently to clarify their impact on the compression performance. The analysis shows that, at Mach 2.2, the mixed-compression intake with two-ramp and three-ramp compression leads to a critical TPR of about 0.83 and 0.84, respectively, with a corresponding supercritical MFR of 0.98 and 0.967, respectively. Though the three-ramp configuration shows a higher TPR at the design condition, it leads to a rapid drop in the critical MFR and TPR at the off-design operations compared to that of the two-ramp intake. This is because of the higher losses across the bow shock that forms because of “unstart because of SWBLI” at low off-design Mach numbers and angles-of-attack conditions, as well as the losses across the separation region at the cowl at high off-design Mach numbers. Hence, a supersonic compression structure with lower degree of external compression is useful to achieve enhanced off-design performance. Next, a passive bleed system has been designed to improve the overall intake performance by diverting a fraction of the low energy boundary-layer (BL) at the throat. The analysis shows that with the use of bleed, the exit flowfield uniformity as well as the critical and peak TPR improves considerably at all operating conditions. This is because of the reduction in the flow separation as well as the total pressure loss across the SWBLIs (called as boundary-layer factor), and the stabilization of terminal shock at the bleed entrance (called as stabilization factor). In the last phase of the study, the optimized isolated intake is integrated to an ogive nosed forebody as a four-intake symmetric cross or X configuration missile, and a combined external-internal flow analysis has been carried out to evaluate the performance of the installed intakes. The data confirms that the considered forebody length ahead of the intakes, as well as the diverter height and its position downstream of the ramp lip are sufficient to maintain a good performance over the desired off-design operating conditions without significant flow separation or spillage ahead of the intakes.