Combined Methodology for Conceptual Design of 2D External Compression Inlet and Dual Serpentine Diffuser

Jet engines in fighter aircraft require a certain amount of air to reach the Aerodynamic Interface Plane (AIP) at a given flight condition. In the conceptual and preliminary design stages of the aircraft, the inlet size and performance need to be estimated for the integration of the propulsion system and overall aircraft performance evaluation. There is plenty of research published individually on inlet and diffuser, however, a combined design methodology for supersonic inlet and subsonic diffuser for given design conditions (i.e. design Mach, altitude, and engine required mass flow rate) is rarely found in the literature. This study presents the complete design methodology of a two-dimensional supersonic external compression inlet and dual serpentine subsonic diffuser for the initial design stages of the aircraft. Analytical sizing of the two-dimensional external compression inlet was done at Mach 1.8 and an altitude of 11,000 m. The transverse and stream-wise pressure gradients in the diffusing duct were controlled using centerline and area distribution equations obtained through mathematical modeling of the diffuser. A total of nine configurations were considered for the subsonic diffusion of the flow through numerical optimization and the best configuration in terms of pressure recovery, flow distortion, and swirl angle was determined by CFD analysis. The centerline with modest turning at the entrance and exit, and the area distribution with sharp diffusion at the entrance and slow diffusion at the exit gave a maximum pressure recovery of 95.51% and minimum flow swirl of 2.48° with an acceptable distortion coefficient (DC60) of 0.54. Results proclaim the diffuser performance as a complex function of its shape and prone to any slight change in the geometric variables.