Recent analytical advances in understanding the performance continuum (the thermodynamic spectrum) for air-breathing engines based on fundamental second-law considerations have clarified scramjet and ramjet operation, performance, and characteristics. Second-law based analysis is extended specifically in this work to clarify and describe the performance characteristics for dual-mode scramjet operation in the mid-speed range of flight Mach 4 to 7. This is done by a fundamental investigation of the complex but predictable interplay between heat release and irreversibilities in such an engine; results demonstrate the flow and performance character of the dual mode regime and of dual mode transition behavior. Both analytical and computational (multi-dimensional CFD) studies of sample dual-mode flow-fields are performed in order to demonstrate the second-law capability and performance and operability issues. The impact of the dual-mode regime is found to be characterized by decreasing overall irreversibility with increasing heat release, within the operability limits of the system.

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Thermodynamic Analysis of Dual-Mode Scramjet Engine Operation and Performance

A numerical investigation of an experimental dual-mode scramjet configuration is performed. Both experimental and numerical results indicate significant upstream interaction for this case. Several computational cases are examined: these include the use of jet-to-jet symmetry and entire half-duct modeling. Grid convergence, turbulence modeling, and wall temperature effects are studied in terms of wall pressure predictions and flow-field characteristics. Wall pressure comparisons between CFD and experiment show fair agreement for the jet-to-jet case. However, further computations of the entire half-duct show the development of a large sidewall separation zone extending much further upstream than the separation zone at the duct centerline. This sidewall separation is the dominant feature in the CFD-generated flowfield but is not evident in the experimental data, resulting in a unfavorable comparison between CFD and experimental data. Current work aimed at resolving this issue and at further understanding asymmetric flow-structures in dual-mode flow-fields is discussed.

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Three-Dimensional Effects in Modeling of Dual-Mode Scramjets

Asymmetric Flow-Structures in Dual Mode Scramjet Combustor With Significant Upstream Interaction

Supersonic mixing and supersonic and dual-mode combustion data bases that have been generated at the University of Virginia's Aerospace Research Laboratory are described. The unique features of the two wind tunnels and the laser-based diagnostics for nonintrusive measurements of gasdynamic properties in these tunnels are discussed. A research program whose objective it is to collect a high fidelity data base in the combustor dual-mode regime using a wide array of diagnostic techniques is starting in support of several hypersonic airbreathing propulsion systems.

Combustor data bases for hypersonic airbreathing propulsion systems

Results from numerical simulations of the three dimensional flow in the intake-isolator of a dual mode scramjet are presented. The FANS calculations have utilised the SST k -ω turbulence model. The effect of cowl length and cowl convergence angle on the inlet mass capture ratio, flow distortion, shock strength and pressure rise are studied in detail. Three cowl lengths and four or five cowl convergence angles for each cowl length are considered. The predicted values of the dimensionless wall static pressure and inlet mass capture ratio are compared with experimental data reported in the literature. The numerical predictions are shown to agree well with the experimental data. In addition, details of the flow field such as shocks, expansion fans and shock boundary interaction are also captured accurately. Inlet unstart is also demonstrated for one case.

Numerical simulations of the flow through the inlet and isolator of a Mach 4 dual mode scramjet

This paper presents an accurate and reliable numerical method for simulating compressible free shear layers in a scramjet engine. A special attention is focused upon the proper modeling effort of the turbulence and temperature fluctuation effect in the Reynolds-averaged simulations. As the specific turbulence model, a two-equation k — e model is adopted instead of the widely-used algebraic model. To take the temperature fluctuation effect into account, a two-equation type transport model is solved and the modified Arrhenius type reaction rate equation is used. Numerical simulation results for four types of flows typically found in a scramjet engine are shown, and detailed discussion is given about the prediction capability of our numerical code and the critical problems concerning free shear layers in a scramjet engine such as the effect of flow compressibility on the free shear layer growth rate, key factors to affect the flame holding and the effect of turbulence temperature fluctuation on the combustion process.

Yuichi Matsuo

Papers

Parallel numerical simulation of compressible free shear layers in a scramjet engine