Sung Don Kim

Robust HLLC Riemann solver with weighted average flux scheme for strong shock

Many researchers have reported failures of the approximate Riemann solvers in the presence of strong shock. This is believed to be due to perturbation transfer in the transverse direction of shock waves. We propose a simple and clear method to prevent such problems for the Harten-Lax-van Leer contact (HLLC) scheme. By defining a sensing function in the transverse direction of strong shock, the HLLC flux is switched to the Harten-Lax-van Leer (HLL) flux in that direction locally, and the magnitude of the additional dissipation is automatically determined using the HLL scheme. We combine the HLLC and HLL schemes in a single framework using a switching function. High-order accuracy is achieved using a weighted average flux (WAF) scheme, and a method for v-shear treatment is presented. The modified HLLC scheme is named HLLC-HLL. It is tested against a steady normal shock instability problem and Quirks test problems, and spurious solutions in the strong shock regions are successfully controlled.

Flow Characteristics of Small-Sized Supersonic Inlets

The flow characteristics were investigated for two types of small-sized supersonic inlet models: rectangular and axisymmetric inlets. The twomodels were designed by the samemethod and are of similar size, with a total length of about 160 mm. The flow phenomena that were generated at the stable and unstable operating conditions of both models are visualized and discussed in detail. The main flow feature for stable operation is the existence of a shock train induced by the shock boundary-layer interactions. The results show that the shock-train shape and position are varied along with the upstream and area ratios, which strongly impact the downstream flow and total pressure recovery. For unstable operation, various buzz phenomena are observed for different area ratios. The results indicate that the buzz phenomenon of the small-sized inlet is identical to that of a large inlet, but the small inlet can be easily affected by the presence of a separation bubble. Although the shapes of the inlets are different, the base frequencies of a big buzz for bothmodels are very similar. The role of the separation on the compression surface in the buzz process of the small-sized inlet is shown.

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

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.

Numerical Simulation of a Liquid Fueled Dump Type Ramjet Combustor

Due to the high density and heating value, liquid fuel is attractive for ramjet propulsion system. Liquid fuel requires time to evaporation and mix with incoming air before ignition; insufficient evaporation and mixing result 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 Integrated Rocket Ramjet combustor. The governing equations are solved by means of a finite-volume using time derivative preconditioning method for chemical reacting flow. 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.