Numerical Simulation on a Reacting Flows of Coaxial Ramjet Dump Combustor with Liquid Fuel Injection

Abstract

Due to the high density and heating value, liquid fuel is attractive for ramjet propulsion system. Liquid fuel requires vaporization and time to mix with incoming air before ignition; insufficient evaporation and mixing results in low combustion efficiency and instability. So the numerical studies are conducted to investigate the spray and combustion characteristics of a liquid-fueled dump type ramjet combustor. The governing equations are solved by means of a finite-volume, preconditioned LDFSS scheme treating chemical reacting flow over a wide range of Mach number. The liquid phase is treated by solving Lagrangian equations of motion and transport for the life histories of a statistically significant sample of individual droplets. A dump type ramjet combustor is considered for a numerical simulation. Flow separation in the diffuser section affects the velocity profile and fuel distributions at the combustor entrance. With the high speed air stream and high temperature in the mixing zone, almost fuel droplets are sufficiently vaporized and burned in the combustor. I. Introduction ump-type combustor has been commonly used in air-breathing liquid-fueled engines, such as ramjet engines, scramjet engines and gas-turbine engines. Ramjet propulsion system requires a solid rocket booster to reach the ramjet operating velocity. In the Integrated Rocket Ramjet (IRR) system, a rocket booster and ramjet combustor operate as an integrated system and utilize a common combustion chamber. During the transition sequence of IRR, supersonic inflow has to be decelerated to subsonic. Because of the high speed incoming air, the flame holder is required for the flame stabilization like a backward step in a coaxial dump combustor. After the inlet port cover open, the supersonic air is introduced to the combustor, so the fuel spreading is limited to certain region of the combustor and complex physical processes are accompanied such as liquid fuel injection, mixing, ignition, flame propagation and so on. To obtain a stable combustion in the combustor, liquid fuel vaporization and mixing, ignition, and maintenance of the subsonic combustion are very important things. Unsteady pressure oscillation in the combustor leads to the oscillating terminal shock and shock/boundary layer interaction at the inlet modifies the air mass flow rate into the combustor. Because the inlet shock may be expelled from the inlet due to the pressure rise in the combustor, coupled consideration of the inlet and the combustor is strongly required. The present study is to explore the influence of liquid spray-combustion flow of a coaxial dump type ramjet combustor. To this end, a two-dimensional computational program for analyzing the spray-combustion flow within the main combustor and attached inlet diffuser section of a ramjet engine is developed. Unsteady transition sequence of the IRR combustor is numerically simulated using the developed RANS code employing preconditioning method, with k-e/k-ω blended SST model, finite-rate chemistry model and dual-time stepping method with parallel computing. The liquid phase is solved by spray modeling based on a dilute spray assumption and fully atomized spray comprised of spherical droplets. The computational code developed herein have validated against variety of flow problems. Unsteady sequence is simulated from the inlet port cover open to a steady operation.