Oxygen Enrichment in a Hydrogen Fuelled Scramjet

Sarah Razzaqi (2011). Oxygen Enrichment in a Hydrogen Fuelled Scramjet PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland.

       
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Author Sarah Razzaqi
Thesis Title Oxygen Enrichment in a Hydrogen Fuelled Scramjet
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
Publication date 2011-06
Thesis type PhD Thesis
Supervisor Prof. Michael Smart
Prof. David Mee
Total pages 248
Total colour pages 21
Total black and white pages 227
Subjects 09 Engineering
Abstract/Summary Scramjets are expected to be an integral part of future launch vehicles designed to be reliable, efficient alternatives to current all-rocket powered launchers. An access-to-space scramjet vehicle must operate over a wide range of Mach numbers, including in the hypervelocity regime i.e. Mach numbers in excess of 10. As flight speed increases, it becomes more difficult for scramjet engines to produce sufficient thrust to overcome the vehicle drag. There are two primary reasons for this. First, increasing flight speed leads to an increase in the length required to achieve adequate fuel/air mixing. Also, at high Mach numbers the specific kinetic energy of the captured air is comparable to or greater than the chemical energy released from the combustion of hydrogen fuel. Therefore, it is especially necessary to efficiently convert chemical energy to thrust at hypervelocity conditions. Oxygen enrichment is a thrust augmentation technique proposed for use in scramjet engines to increase the net thrust of a scramjet vehicle at hypervelocity conditions, where oxygen enrichment involves the premixing of oxygen with fuel prior to injection into the engine. This technique addresses the mixing length problems encountered at hypervelocity by providing a head start to the mixing process. In addition, the combustion of fuel at greater than stoichiometric proportions is possible. This work investigates the ways in which oxygen enrichment affects scramjet engine performance at hypervelocity conditions. A set of exploratory experiments was conducted in the T4 Stalker tube at The University of Queensland at two altitude conditions and with varying levels of fuel/oxygen injection. The standard altitude condition represents flight at a Mach number of 12.3 and a dynamic pressure of 40.2 kPa. The high altitude condition represents flight at a Mach number of 11.4 and a dynamic pressure of 9.8 kPa. The experiments were conducted in a geometrically simple, rectangular combustor, which is followed by a short thrust nozzle. A 2D intake compresses the test flow to the desired conditions at the entrance of the combustor. The tests are thus of a semi-direct-connect nature. The combustor inflow is at a Mach number of 3.8 and a total enthalpy of ~7.5 MJ/kg for both altitude conditions. This corresponds approximately to the expected combustor entry conditions of a representative engine operating at the equivalent flight conditions. Fuel/oxygen are injected from a slot in the base of a centrally mounted fuel injector strut. Wall static pressures are measured along the combustor and in the short thrust nozzle. A quasi-one-dimensional cycle analysis tool is used to model the combustor flow and provide estimates of combustion efficiency. The results with fuel injection without enrichment reveal that combustion at the standard altitude case is limited by the amount of mixing which can occur, whereas combustion at the high altitude case is limited by the rate of chemical reactions. However, at both conditions, ignition is limited by the rate of mixing. Approximately half the fuel is able to adequately mix and burn at the standard altitude condition. The same amount is able to mix at the high altitude condition, but at most 25% is able to burn due to the reaction limited nature of the flow. Fuel injection with oxygen enrichment has several notable effects. For one, ignition length is reduced by an amount approximately proportional to the level of enrichment. When enough oxidiser to burn 15% of the fuel is premixed, the ignition length is reduced to ~ 60% of that with no enrichment. For mixing limited conditions the combustion efficiency is also increased by an amount roughly equal to the level of enrichment. Specific thrust increases with enrichment, however, specific impulse generally decreases. At the standard altitude condition the greatest impacts of enrichment are seen immediately upon ignition and in the first half of the duct. Shortening the combustor improves the performance of enriched scramjets at the standard altitude condition. At the high altitude condition oxygen enrichment mitigates the effects of slow chemical reaction rates allowing, at best, a near doubling of combustion efficiency.
Keyword scramjet, oxygen enrichment, supersonic combustion, thrust augmentation, hypersonics, airbreathing propulsion
Additional Notes Colour pages: pdf page number: 39,40,67,69,73,76,86,92,100,101,117-120,122,235,244-248 document page number: 11,12,39,41,45,48,58,64,72,73,89-92,94,207,216-220 Landscape pages: pdf page number: 72,211-217 document page number: 44,183-189

 
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Created: Thu, 02 Jun 2011, 14:49:59 EST by Ms Sarah Razzaqi on behalf of Library - Information Access Service