Michael Cornwell

Application of Flameless Combustion in Gas Turbine Engines

The current study examines the adaptability of flameless combustion to gas turbine engines. This mode of combustion has been used in industrial furnaces as a means of producing uniform combustion temperature with low emissions. Industrial applications use multiple staged burners, or heavy exterior recirculation lines unsuited for aero-engines. This study is to determine the viability of a flameless burner design for aero-engines. Variations of inlet temperature, pressure drop, and combustor geometry were tested to determine the limits of flameless combustion within constraints acceptable to current and future aircraft engines. Important aspects of flameless combustion include uniform temperature distribution, low emissions, and decoupling between heat transfer, fluid mechanisms, and acoustics. To determine these relationships, emissions of CO, NOX, O2, CO2, and UHC were taken. Multiple thermocouples at the combustor exit were placed to determine exit temperature uniformity, while a microphone was used to collect acoustical measurements. Data was taken starting at equivalence ratio of 0.7 and decreasing to lean-blow-out (LBO) to determine flammability limits using the flameless burner.

Dynamics and Control of a High Frequency Fuel Valve and its Application to Active Combustion Control

An active combustion control valve capable of high-frequency large-amplitude fuel modulations is presented. This valve utilizes a hydraulic piston-cylinder structure. Mean fuel flow rate is controlled by valve opening via a step motor; and fuel modulation is achieved via the extension and contraction of a Terfenol-D rod. Experiments and low-order modeling are performed to optimize valve performance. Finite compressibility of fuel within the valve is considered based on fuel modulus. A fuel with smaller modulus favors larger fuel modulations. Fuel modulations could be more than 40% of mean flow at 700 Hz. An adaptive controller is developed to track and regulate mean flow. Phase-shift fuel control achieves pressure attenuation up to 27 dB in an unstable atmospheric swirling combustor.

Characteristics and Control of an Unstable Liquid-Fueled Multi-Swirling Combustor

This paper reports our initial results on active control of combustion instability in a lean direct fuel injection combustor featured with multiple air swirls and distributed fuel injection. Fuel modulation is achieved by “pushing” fuel out of the valve cavity using a magnetostrictive rod that extends or contracts with unsteady currents going through its surrounding coil. To follow the flow commands and quickly reject exogenous disturbances on mean fuel flow rate, an LQG pulse-width modulation controller based on system identification models is developed. As a starting point, we tried a phase shift controller whose optimal control phase is directly obtained from system identification models. Pressure attenuation up to 20 dB is achieved within 150 ms using pressure feedback that achieves more pressure attenuation than optical fiber feedback. It is found that, during unstable combustions, pressure has a higher signal noise ratio than the optical fiber output mainly because the thermoacoustic system has a high gain around its resonant frequencies. Time-varying phase relationship exists between heat release and pressure. Partial blockage of the combustor exit considerably reduces combustion oscillation intensity and postpones the occurrence of unstable combustion to a higher equivalence ratio. This may be because the exit blockage increases the acoustic impedance at the combustor exit and modifies the reacting flow field. With higher preheat temperature, strong combustion oscillations may occur at a lower equivalence ratio. Pressure pulsations may exhibit hysterisis with equivalence ratio, especially at lower preheat temperature.Copyright