Passive mitigation of combustion noise and thermo-acoustic instability using porous inert media at elevated pressure

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University of Alabama Libraries

Combustion instabilities have presented major problems in high-pressure, turbulent combustion systems for nearly a century, beginning with rocket propulsion systems. To enhance combustion efficiencies, other engines, such as gas turbines for power generation, operate at high pressures and reactant flow rates that are only small relative to those of rocket engine operation. The majority of power generation systems today extract energy from such efficient combustion processes. Recently, gas turbine engines, both power generation and propulsion platforms, are operated under very lean conditions to reduce flame temperatures and thus, emissions of the primary smog forming constituent, NOx. Extinction, flashback, blowoff , and autoignition pose challenges when operating at lean-premixed conditions. Flame stability at such lean conditions is problematic; thus, a swirled flow method is used to anchor and stabilize these flames. Intense turbulence, resulting from the pressure drop across flow swirlers, drives fluctuations in pressure and heat release rate. The feedback between pressure oscillations and heat release fluctuations in the reaction zone often drives resonant instabilities that propagate through the flow and surrounding structures. Such self-excited instabilities influence high rates of heat release in the reaction zone, which is located near the point of injection. Vibrations and high temperatures lead to the fatigue of injection components, instrumentation, and downstream turbine blades. A novel passive combustion noise control technique is experimentally investigated in the present study. The approach involves the mating of a porous inert material (PIM) with the inlet of a swirl-stabilized, lean-premixed combustor. The foam insert reduces turbulent intensities within the inner and outer recirculation zones of a common swirl-stabilized burner, thus reducing the amplitude of combustion driven instabilities. Experiments are conducted at high pressures, with high reactant flow rates and equivalence ratios. Results show that the ceramic foam insert is effective at mitigating combustion instabilities, suppressing combustion noise, and potentially, acoustic damping. The total sound pressure level for many of the cases investigated is reduced by 10 dB and greater. Furthermore, the approach can easily be retrofitted to commercial, industrial, and propulsion gas turbine combustion systems.

Electronic Thesis or Dissertation
Mechanical engineering, Aerospace engineering