Xianggeng Wei

Numerical Study of a Boundary Layer Bleedfor a Rocket-Based Combined-Cycle Inlet in Ejector Mode

Abstract Fully integrated numerical simulations were performed for a ready-made central strut-based rocket-based combined-cycle (RBCC) engine operating in ejector mode, and the applicability of using a boundary layer bleed in the RBCC inlet designed for supersonic speeds was investigated in detail. The operational mechanism of the boundary layer bleed and its effects on the RBCC inlet and the engine under different off-design conditions in ejector mode were determined. The boundary layer bleed played different roles in the RBCC inlet for different flight regimes. When the RBCC engine took off, some air was entrained into the inlet through the bleed block, thereby inducing significant flow separation and a low-speed vortex, which deteriorated the inner flow and reduced the entraining air mass flow rate: thus, the total pressure loss increased and extra drag was exerted on the inlet. In the low subsonic regime, the bleed block had almost no impact on the RBCC engine and its inlet. However, as the RBCC engine accelerated into a high subsonic flight regime, the boundary layer bleed had a clearly positive effect, comprehensively improving the performance of the RBCC inlet. A boundary layer bleed operation strategy for the RBCC inlet in ejector mode was also developed in this study.

Numerical Investigation of Effects of Boundary Layer Bleed on a RBCC inlet in Ejector Mode

Nomenclature H∞ = flight height Ma∞ = Mach number of the incoming flow Mastart = starting Mach number Maout = Mach number of the outlet of RBCC inlet σ = total pressure recovery coefficient of RBCC inlet φ = entraining ratio of secondary flow (air) mrocket = mass flow rate of rocket engine mbleed = mass flow rate of bleed block Dinlet = drag of RBCC inlet FRBCC = overall inner thrust of RBCC engine

Numerical Investigation of Cowl Lip Adjustments for a Rocket-Based Combined-Cycle Inlet in Takeoff Regime

Abstract Numerical integration simulations were performed on a ready-made central strut-based rocket-based combined-cycle (RBCC) engine operating in the ejector mode during the takeoff regime. The effective principles of various cowl lip positions and shapes on the inlet operation and the overall performance of the entire engine were investigated in detail. Under the static condition, reverse cowl lip rotation in a certain range was found to contribute comprehensive improvement to the RBCC inlet and the entire engine. However, the reverse rotation of the cowl lip contributed very little enhancement of the RBCC inlet under the low subsonic flight regime and induced extremely negative impacts in the high subsonic flight regime, especially in terms of a significant increase in the drag of the inlet. Changes to the cowl lip shape provided little improvement to the overall performance of the RBCC engine, merely shifting the location of the leeward area inside the RBCC inlet, as well as the flow separation and eddy, but not relieving or eliminating those phenomena. The results of this study indicate that proper cowl lip rotation offers an efficient variable geometry scheme for a RBCC inlet in the takeoff regime.

Large Eddy Simulation of Effects of Primary Rocket jet on Low Frequency Combustion Instability in a RBCC Combustor

This paper reports on effects of primary rocket jet parameters, which are among the most probable sources of combustion instability in a Rocket-Based Combined-Cycle (RBCC) engine, on combustion characteristics of a RBCC combustor numerically. Compressible Large Eddy Simulation (LES) with kerosene sprayed and vaporized is performed on an experimental RBCC combustor. Coupling with a reduced three-step chemical kinetics of kerosene, the LES is used to investigate the large amplitude and low frequency longitudinal oscillations in the engine. LES results are compared with experimental observations in pressure oscillation amplitude and frequency in the low components, all show good agreement. Influence of the primary rocket jet on pressure oscillations of the combustor is analyzed and the relation of its high speed jet oscillation characteristics with that of the combustor is recognized. Results reveal that the unsteady high temperature jet comes out of the rocket, which is always rich in fuel, has a significant influence on the vaporization and combustion features of the fuel downstream of the secondary fuel struts, and consequently on combustion features of the combustor. LES solver is validated with experimental data for a model scramjet located in the Institute for Chemical Propulsion of the German Aerospace Centre (DLR) and shows good predictions.

LES of Turbulent Reacting Flow in a Rocket Based Combined Cycle Combustor

Large Eddy Simulation (LES) has been used to examine the turbulent reacting flow in a model Rocket Based Combined Cycle (RBCC) combustor with a focus on the underlying flow-mixing-combustion physics in the combustor with supersonic fuel-rich rocket jet. For the purpose of validation for the numerical methods and combustion mechanisms, a laboratory scramjet is used as the test-computational configuration. The LES results are compared with shadow-pictures and measurements of velocity and temperature at different cross-sections. In addition, a simplified RBCC combustor is designed with a strut rocket in the middle of the flow path. LES of the hydrogen-fueled RBCC combustor is carried out to investigate the nature of mixing and combustion of the rocket’s supersonic exhaust flow and the air stream. The interaction of the rocket jet and the air flow are studied with a particular focus on the supersonic shear layer that forms at the interface between them. Flow features are examined and a particular emphasis is placed upon the role of the strut rocket in the high-speed reacting flow since its fuel-rich hot gas acts as the ignition source and pilot flame, which is conducive to efficient combustion in the engine.

Numerical Investigation of the Shear Layer Growth Influenced by Shock Waves in a Model Scramjet Engine

A numerical study has been conducted to investigate the effect of shock waves on the boundary layer growth and the mixing layer development in a model scramjet where a hydrogen jet is injected at 2.3 Ma into a Mach 2.0 surrouding air flow. The qualitative and quantitative agreements are found between the experimental data and the simulation results validating calculations are reasonable. The boundary layer thickness is carefullyed examined, which offers a renewed schematic of shock-wave/boundary-layer interaction featured with separation and reattachement. The thickness of the mixing layer is also calculated and plotted over the channel length to assess the impact of shock waves on its development. The mixing layer thickness increases wave-likely over the channel length as shock waves travel downstream and are reflected by the channel wall and the mixing layer. The mixing layer thickness decreases a little at the very location where the shock wave interacts with the mixing layer due to compression. Meanwhile, the induced vorticity makes contributions to the development of coherent structures in the mixing layer and leads to an increase of the mixing layer thickness.