Researchers Joe Oefelein and Guilhem Lacaze (both in Sandia’s Reacting Flow Research Dept.) won the American Institute of Aeronautics and Astronautics’ (AIAA’s) best paper award for their work on scramjet engine simulations. The paper, “A Priori Analysis of Flamelet-Based Modeling for a Dual-Mode Scramjet Combustor,” was a collaboration with Jesse Quinlan and James McDaniel (Univ. of Virginia) and Tomasz Drozda (NASA Langley Research Center).
AIAA is the largest aerospace professional society in the world, serving a diverse range of more than 30,000 individual members from 88 countries and 95 corporate members. AIAA’s mission is to inspire and advance the future of aerospace for the benefit of humanity. Their award was presented by the AIAA High-Speed Air-Breathing Propulsion Technical Committee for accomplishment in the arts, sciences, and technology of air-breathing propulsion systems.
Their paper presents a detailed analysis of combustion regimes in a scramjet, an engine that operates at super- to hypersonic speed and will be used in the future for military, point-to-point transport, and access-to-space applications. “The results presented in the paper are an excellent example of how collaborative teams across institutions can combine their expertise to provide new knowledge supporting the development of predictive combustion models for these systems,” Oefelein said.
The transition of a scramjet engine from dual-mode operation to scram-mode operation occurs in flight as a vehicle accelerates along its flight trajectory. During this transition, the combustion changes from primarily subsonic to largely supersonic. Understanding and predicting the physics of this transition is important to maintaining robust engine operation. Experimental investigations of dual-mode transition are typically limited by the inability of current hypersonic test facilities to vary the flow Mach number in real time through the transition process.
In their research, they modeled JP-7 fuel combustion chemistry using a 22 species, 18-step chemical reaction mechanism. They compared simulation results to experimentally obtained, time-averaged, wall-pressure measurements to validate their simulation solutions. Their analysis of the flowpath’s dual-mode operation showed regions of predominately nonpremixed, high-Damköhler number*, combustion. Regions of premixed combustion were also present, but associated with only a small fraction of the flow’s total heat-release. Their results also suggest that for scram-mode flowpath operation, the primary injector flames exhibit mixed combustion modes, in which significant heat release was found in regions of both nonpremixed and premixed conditions and at both moderate (Da = 1) and high (Da >>1) Damköhler numbers. Detailed analysis of the premixed combustion data suggest thin and broken reaction zones.
The research team found the effects of compressibility and heat losses have a significant effect on combustion—suggesting that a suitable flamelet manifold’s defining parameters should be pressure and enthalpy. Their analysis of the temperature and pressure at theoretical flamelet boundaries further supported the necessity of including pressure and enthalpy as manifold dimensions and suggested that the standard practice of using a single set of flamelet boundary conditions is an approximation.
This work fits into the philosophy of Sandia’s Combustion Research Facility where simulations complement experiments and bring key insights to improve real engines. “Because of the extreme velocities, experiments are rare and limited. That’s why we simulate those systems, to better understand how to optimize them,” Lacaze said. “To perform those simulations, we need to use models that accurately represent the flame, and our paper shows which approach is the most relevant and why.”
The study will help define the best simulation techniques needed to optimize future scramjets. Improved numerical accuracy at lower cost should help designers explore the key design attributes required for supersonic engine breakthroughs. The work has also helped establish new funding for Sandia through an award from the Defense Advanced Research Projects Agency (DARPA) involving uncertainty quantification of scramjet combustion.