In spite of the quite remarkable progress achieved in incompressible flows in the past two decades, turbulence and chemistry are still one of the open problems in compressible flows. Most of the development in incompressible flows has resulted in the successful implementation of DNS and LES to realistic case studies and to the creation of a large data base of both industrial applications and experiments. In stark contrast, the physics of reactive compressible turbulent flows is very poorly understood, even though applications abound. Supersonic combustion (SC)suffers problems with
fuel and air mixing, ignition, and flame stabilization due to flow residence timescales being of same order as chemical kinetics timescales. Creating regions of lower speed flow and recirculation through modifications in geometry such as addition of cavities, or using ramp injectors, has been found to mitigate the characteristic time disparity, but lead to significant pressures losses and difficulties in thermal management. Consequently, exploration of new methods to enhance ignition and flame stabilization is critical. Plasma assisted ignition and combustion could be a promising way to
initiate and maintain combustion of ultra-lean mixtures and/or at high fluxes, to anchor the flame and to reduce pollutants. With this background, the purpose of this work is:1 to clarify the fundamental physics of SC when compressibility effects are significant identifying key mechanisms for flame anchoring and combustion stability and 2
the direct coupling effect between plasma and flame;3 to develop a detailed model for plasma assisted combustion and a turbulence/chemistry SGS model accounting for compressibility;4 to translate these models into numerical subroutines
with the aim to make available for the CFD users a full validated numerical code for SC. Progress in this field will not only improve understanding of physics but also performance and emissions of future engines and transportation systems.
The understanding of physics in supersonic flow is thus mandatory for future very disruptive applications for new generation of combustion devices that could replace conventional combustors in commercial aircraft engines. This technology is therefore of great interest and could be strategic from a scientific, technologic and economic point of view. Innovations and potentialities may be divided into three: formal objectives of the research proposed, technological fall-out, and knowledge/social.
Among the first is a set of deliverables containing subroutines, analytical models and code(s) enabling an aerospace engineer to start realistic sizing of a scramjet engine (`design criteria').
The second may be even more important: technological fall-out in regenerative hydrocarbon fuel reforming, applicable to hydrogen production for fuel cells (the technology to produce hydrogen from hydrocarbon for scramjet is the same of terrestrial applications); compact reformer heat exchangers, with surface/volume ratios of order 1000 ft2/ft3, applicable also to all process industry (steel, glass, ceramic, where heat must be recovered); novel combustion chambers for commercial aeroengines, with better flame anchoring even in fast airstreams, capable of lower NOx production consequent to more strained flames; and others.
Besides these technology areas, the third type of results is knowledge: hypersonic propulsion is specialized knowledge, with a myriad implications shedding light on gas dynamic science (compressible turbulence theory, vorticity creation, non-equilibrium chemical kinetics to mention a few). Working in hypersonics is tantamount to enlarge ones vision when looking the scenery from high ground, as it encompasses subsonic fluid-dynamics as a subset. Thus, by doing research in this difficult area, the human potential of researchers is developed faster and put to the test better than in more conventional fields.