COMBUSTION SCALING LAWS FOR INLET-FUELED SCRAMJETS

Schloegel, Fabrice (2013). COMBUSTION SCALING LAWS FOR INLET-FUELED SCRAMJETS PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland. doi:10.14264/uql.2014.124

       
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Author Schloegel, Fabrice
Thesis Title COMBUSTION SCALING LAWS FOR INLET-FUELED SCRAMJETS
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
DOI 10.14264/uql.2014.124
Publication date 2013
Thesis type PhD Thesis
Supervisor Russell Boyce
Tim McIntyre
Total pages 460
Language eng
Subjects 090107 Hypersonic Propulsion and Hypersonic Aerodynamics
0901 Aerospace Engineering
Formatted abstract
For the class of high Mach number scramjet known as the inlet-fueled radical farming scramjet, the manner in which the ignition and heat release regions scale with pressure (that is, altitude) for different flight Mach numbers, has been studied by means of shock tunnel experiments and high fidelity numerical computations. The generic two-dimensional Busemann-like scramjet configuration of Odam [1] has been re-designed and lengthened, so that the variation of combustion length scales with altitudes, can be examined. A set of experimental conditions were examined over a range of total pressures varying from 5MPα to 40MPα and total enthalpies from 3MJ/kg to 6MJ/kg, corresponding to an equivalent flight Mach number ranging from Mα_flight=8 to Mα_flight=11 and a flight dynamic pressure ranging from q_flight=58kPα to q_flight=194kPα. All tests conducted during the experimental campaign were run fuel-off, fuel into nitrogen and fuel into air in order to dissociate the effects of injection and how they modiy the flow structure within the engine, from the effects of combustion heat release and pressure rise. The experimental results presented here have been generated from surface pressure measurements and Schlieren diagnostic. For all test conditions, a careful numerical analysis of the scramjet inflow was first produced using computation of the facility nozzle outflow, enabling the non-uniformity present within the freestream flow to be accounted for. This computed nozzle outflow was then used as the inflow boundary of the scramjet CFD simulations. This methodology allowed for greater accuracy and better understanding of the complex and coupled phenomena of supersonic flows and supersonic combustion. The comparison between the experimental results and their numerical reconstruction provided insight into the behaviour of scramjet across a broad range of flight conditions and demonstrated the capabilities of CFD at capturing the trends of supersonic combustion in inlet-fueled scramjets. Building on the coupled experimental / numerical study, a detailed numerical study of the combustion process was performed over a much broader range of flight conditions. This study included and extended the conditions achievable in the shock tunnel, and removed all the flow complexities that make shock tunnel experiments different from flight. The analysis uncovered regimes where different combustion scaling laws apply, depending on both the altitude and the equivalent flight Mach number. It was found that at an equivalent flight Mach number of Mα_flight=7.96, the combustion process in the scramjet engine behaves as the radical farming scramjet. There was limited chemical activity in the inlet, with radicals being formed only at the end of the intake ramps in the small regions of flow separations. Combustion was not enabled in the first “radical farm”, or shock-induced hot structure, in the combustion chamber, due to low reaction rates at that flight Mach number. Further convection and mixing of air and fuel is required, and ignition occurs at one subsequent hot structure, depending on the pressure (dynamic pressure, altitude). The pressure sensitivity of the ignition and reaction phases of the combustion process was systematically extracted from the results. It is demonstrated that for this flight Mach number and range of dynamic pressures q, that the ignition length correlation parameters qn∙L is constant for a value of the exponent n such that 1.1 ≤ n ≤ 1.2. In other words, the scaling parameter required to conserve the ignition length relative to the geometric scale of the combustor in the event of changes to the geometric scale is qnL=const where 1.1 ≤ n ≤ 1.2. The fact that n is close to unity highlights the strong dependency of ignition on two-body reactions. On the other hand, the reaction length was found to scale as qnL with n ≃ 1.5. During the length required for heat release reactions, both two-body reactions and three-body heat release reactions are significant (n is between 1 and 2). As the total flow enthalpy is increased, for conditions corresponding to equivalent flight Mach number of flight=9.17, flight=10.30 and flight=11.33, it was found that the inlet plays a major role in radical production. The chemical activity in the inlet is high, with intense production of H radicals as early as the injection location. For an equivalent flight Mach number of flight=9.17, at flight dynamic pressure in the range of 47kPαqflight ≤ 144kPα, the ignition length correlation parameter can be written q1.1L=const. For flight dynamic pressure in the range of 167kPαqflight ≤ 240kPα, the ignition location is anchored at the second hot structure in the combustor, after which the reaction length scales as qn∙L with n ≃ 1.7. For the two higher Mach numbers, the local pressure and temperature combined with the high level of radicals generated in the inlet permit combustion to anchor to the first hot structure at all altitudes considered. The reaction length scaling parameter qnL is constant for a value of 1.7 ≤ n ≤ 1.8 at flight=10.30, and 1.8 ≤ n ≤ 1.9 at flight=11.33, showing that the reaction process was strongly governed by three-body reactions. This study combined the use of experiments and CFD computations to systematically investigate the combustion behaviour in the inlet-fueled radical farming scramjet. It contributes to the science of scramjets by demonstrating that the ignition lengths and reaction lengths can be successfully scaled applying the adequate correlation parameter, depending on the fight Mach number and the flight dynamic pressure (that is, altitude). 
Keyword Scramjet
Hypersonic
Supersonic combustion
Combustion scaling
Airbreathing propulsion

 
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Created: Thu, 05 Jun 2014, 15:36:31 EST by Mr Fabrice Schloegel on behalf of Scholarly Communication and Digitisation Service