Design method for length reduction of a streamline-traced hypersonic inlet

Abstract

A scramjet propulsion system is the only promising design candidate for sustained hypersonic flight. A scramjet engine offers advantages over other air-breathing engines due to its lack of moving components and simple structure. These are achieved through the dual function of a scramjet inlet, which not only ingests but also compresses freestream air. A scramjet inlet is located at the front end of the propulsion system and is responsible for one of the key processes in the thermodynamic cycle by providing the necessary flow to the combustion chamber through an appropriate compression process. Therefore, the primary objective of an inlet design is to develop an aerodynamic surface that efficiently compresses the incoming air to enhance combustion, while also considering geometric constraints. A streamline-tracing technique is one of the methods used in aerodynamic design, generating the surface of the inlet by tracing streamlines of a known flow field with isentropic compression characteristics such as a Busemann flow field. A REST (Rectangular-to-Elliptical Shape Transition) type inlet is a representative example of streamline-traced inlets. A REST-type inlet, generated by integrating the streamline-tracing method with the shape transition, features a rectangular capture shape and an elliptical throat shape, exhibiting the isentropic compression characteristics of the Busemann flow field. By employing a notch process, unnecessary portions for compression are eliminated, resulting in reduced internal drag and improved starting characteristics. Streamline-traced inlets deliver the air with higher total pressure to the combustion chamber compared to wedge-type inlets, which rely on multiple oblique shock waves. However, a drawback of streamline-traced inlets is that the flow requires a longer compression length to undergo the isentropic compression process. Considering the short residence time of supersonic flow in the combustion chamber and the characteristics of the propulsion system that require an additional boosting device, the length of the inlet becomes a critical factor in both propulsion and aircraft design. This study presents a design method aimed at reducing the length of a scramjet inlet operating at Mach 4 to 6. The REST-type inlet was employed as a baseline, and the effect of each design feature was analyzed through computational fluid dynamics. Typically, hypersonic inlet designs have adopted the maximum operating Mach number as the design point in order to minimize flow spillage, resulting in an increase in the inlet length with higher opearating Mach number. However, in this study, the streamline-tracing method was employed with the Busemann flow field of the median operating Mach number, rather than the maximum operating Mach number. This approach reduced the length of the inlet by approximately 24%. To maximize the utilization of the reduced compression length, an additional circular arc was introduced along the outermost streamline of the Busemann flow field in the upper part of the capture shape. This modification resulted in a spatially efficient inlet shape while preserving the isentropic compression characteristics of the Busemann flow. As a result, the compression ratio was increased by 31% with a similar total pressure recovery. The inlet was notched for maximum operating Mach number to minimize air spillage, and the range of operating Mach number and angle of attack was extended. Viscous effects were compensated by an appropriate truncation angle in order to preserve the exact circular throat shape for efficient manufacturing. The length-reduced inlet showed a wide operating range and high compression performance.