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High Temperature Dynamic Pressure Measurements Using Silicon Carbide Pressure Sensors
Keywords: Silicon Carbide, Pressure Sensors, Dynamic Pressure
An un-cooled, MEMS-based silicon carbide (SiC) static pressure sensor was used for the first time to measure dynamic pressures (delta_p) at temperatures as high as 600 ºC. The sensor was placed directly in the flow stream of the combustor test rig along with an axially co-located benchmark commercial water-cooled PCB piezoceramic dynamic pressure transducer, but recessed away from the hot flow stream. The results showed good agreement with the benchmark, resolving delta_p of 0.5 psi at 585 ºC and as low as 0.1 psi at 265 ºC across a range of thermoacoustic resonance frequencies at corresponding combustor test conditions. The demand for lower emissions (LE) in aircraft gas-turbine engines has resulted in advanced combustor designs that are critically dependent on lean-burning (LB) operations. However, LB/LE combustors are susceptible to thermoacoustic instabilities, which can produce large pressure oscillations within the combustor that have the potential to disrupt compressor flow or cause premature mechanical failures. Engine ground tests have traditionally utilized miniature dynamic pressure sensors (microphones) to measure thermoacoustic instabilities. These sensors were either cooled or located at some stand-off distance from the combustor, which resulted in high background noise, acoustic propagation delays, as well as limiting the frequency range of the measurement. Potentially, optical sensors can give timely responses and cleaner signals [2], but would be more effective if they could resolve the events near the fuel injectors in the combustor front-end. Miniaturized un-cooled SiC pressure sensors are emerging as a viable alternative due to its high temperature operational capability, thus allowing insertion more nearer the combustor front-end [3]. In this work, the initial goal was to evaluate existing static SiC pressure sensor capabilities to measure delta_p in a high temperature environment, with the ultimate goal of developing SiC dynamic pressure sensors that are more suitable for measuring combustor dynamics. A laboratory characterization of the static SiC pressure sensors was performed to extract key performance parameters (i.e., offset voltage, sensitivity, and full-scale output). Next, the sensors were characterized in a dynamic pressure generator capable of delivering an air flow stream at 600 ºC and pressure perturbations up to 1 KHz. Following the laboratory performance characterizations, a selected SiC pressure sensor was inserted into the combustor test rig, extending into the flow stream where temperature reached 585 oC before entering the combustion chamber. The benchmark water-cooled PCB sensor was axially co-located with the SiC pressure sensor, but recessed away from the hot flow stream. The results showed that the dynamic pressure magnitude measured by the SiC pressure sensor was in good agreement with measurement obtained with the axially co-located benchmark PCB sensor. However, it showed that the signal-noise ratio (S/N) progressively decreased with increasing temperature, which was attributed to the well-known characteristic decreasing strain sensitivity with increasing temperature found in semiconductor piezoresistors. Thus the sensor was not able to fully quantify the thermoacoustic instabilities that existed at lower frequencies. This problem could potentially be resolved by applying an actual SiC dynamic pressure sensor having much thinner diaphragm, hence, higher sensitivity.
Robert Okojie,
NASA Glenn Research Center
Cleveland, OH

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