Theses and Dissertations from UMD
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Item SCRAMJET COMBUSTOR MODE TRANSITION BY CONTROLLING FUEL INJECTION DISTRIBUTION(2022) Kanapathipillai, Mithuun; Yu, Kenneth H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dual-mode scramjets are able to expand the operable Mach number range of the simplescramjet through manipulation of a thermal throat. Using the thermal throat, the scramjet can operate in either thermally-choked mode or supersonic combustion mode. The transition between these two events is still not very well understood. Past research has shown natural combustor mode transition to be highly unstable and characterized by frequent mode hopping. Long timescales associated with combustor mode transition also increase the potential for combustion dynamic events to occur. Due to the high level of hysteresis present in these events, designing a way to precisely controlling mode transition timing proves to be an ongoing challenge. The present research seeks to use a distributed fuel injection method to control and better understand combustor mode transition behavior. This study was performed using a laboratory-scale, direct-connect scramjet combustor. The facility simulated Mach 5 flight conditions using vitiation to match the enthalpy conditions necessary and to recreate typical isolator and combustor flowfields characteristic of the dual-mode scramjet. A supersonic nozzle was employed to achieve an isolator inlet Mach number of 2.0. For the reacting flow tests, gaseous hydrogen was injected through one to four injectors using a distributed fuel injection scheme while keeping a global equivalence ratio of 0.52 constant. Various imaging diagnostics and wall pressure measurements were used to better study the relationship between combustor behavior and the number of fuel injectors. The findings revealed that combustor mode operation has a significant effect on combustor performance, as indicated by the pressure rise, axial heat release distributions, and local flowpath Mach number for these cases. The deduced heat release for the single injection case showed that most of the heat release occurs near the cavity flame-holder leading to a relatively large pressure jump causing a premature transition to thermal choking. In the case of distributed injection, heat release occurs more evenly across the expanding portion of the combustor, which prevents early transition to thermal choking. Active control of the combustor mode transition event is demonstrated through the use of a fuel injection distribution and scheduling system as well as fast-response solenoid valves. In the active control cases, the global equivalence ratio was maintained at 0.52, but using the active control system the combustor was able to bidirectionally switch between stable, thermally-choked mode and stable, scramjet mode. Furthermore, actively controlled mode transition occurs at much faster timescales than what was observed for natural mode transition. This allows for the potential to actively control on demand combustor mode transition in a real world dual-mode ramjet-scramjet combustor through appropriately scheduled fuel injection distribution.Item Modeling the integration of thermoelectrics in anode exhaust combustors for waste heat recovery in fuel cell systems(2011) Maghdouri Moghaddam, Anita; Jackson, Gregory Scott; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recently developed small-scale hydrocarbon-fueled fuel cell systems for portable power under 1 kW have overall system efficiencies typically no higher than 30-35%. This study explores the possibility of using of thermoelectric waste heat recovery in anode exhaust combustors to improve the fuel cell system efficiencies by as much as 4-5% points and further to reduce required battery power during system start-up. Two models were used to explore this. The first model simulated an integrated SOFC system with a simplified catalytic combustor model with TEs integrated between the combustor and air preheating channels for waste heat recovery. This model provided the basis for assessing how much additional power can achieve during SOFC operation as a function of fuel cell operating conditions. Results for the SOFC system indicate that while the TEs may recover as much as 4% of the total fuel energy into the system, their benefit is reduced in part because they reduce the waste heat transferred back to the incoming air stream and thereby lower the SOFC operating temperatures and operating efficiencies. A second model transient model of a TE-integrated catalytic combustor explored the performance of the TEs during transient start-up of the combustor. This model incorporated more detailed catalytic combustion chemistry and enhanced cooling air fin heat transfer to show the dynamic heating of the integrated combustor. This detailed model provided a basis for exploring combustor designs and showed the importance of adequate reactant preheating when burning exhaust from a reformer during start-up for the TEs to produce significant power to reduce the size of system batteries for start-up.