The plasma synthetic jet is a novel active flow control method because of advantages such as fast response, high frequency and non-moving parts, and it has received more attention recently, especially regarding its application to high-speed flow control. In this paper, the experimental characterization of the plasma synthetic jet actuator is investigated. The actuator consists of a copper anode, a tungsten cathode and a ceramic shell, and with these three parts a cavity can be formed inside the actuator. A pulsed-DC power supply was adopted to generate the arc plasma between the electrodes, through which the gas inside was heated and expanded from the orifice. Discharge parameters such as voltage and current were recorded, respectively, by voltage and current probes. The schlieren system was used for flow visualization, and jet velocities with different discharge parameters were measured. The schlieren images showed that the strength of plasma jets in a series of pulses varies from each other. Through velocity measurement, it is found that at a fixed frequency, the jet velocity hardly increases when the discharge voltage ranges from 16 kV to 20 kV. However, with the discharge voltage fixed, the jet velocity suddenly decreases when the pulse frequency rises above 500 Hz, whereas at other testing frequencies no such decrease was observed. The maximum jet velocity measured in the experiment was up to 110 m/s, which is believed to be effective for high-speed flow control.
The present work develops a stall warning approach with application to a stall enhancement system. This technique is proposed on the basis of the periodicity of pressure signals sampled by the dynamic pressure sensor which is located on the casing wall at the rotor leading edge. A parameter called Rc, defined to evaluate this periodicity, is calculated by pressure signals within 1 blade pitches in the current one and previous one shaft period. Statistical estimates are implemented on Rc within statistical windows which move with sampling. The probabilities of Rc less than a threshold Rcth are calculated and used as a criterion for stall warning. Experiments are carried out on a low speed compressor with a stall precursor-suppressed (SPS) casing treatment, an efficient stall enhancement system. Results show that the probability increases significantly when the stall boundary is approached. At the same work point, the value of probability will decrease when the SPS casing treatment is turned on. This approach successfully generates a stall warning signal and then triggers the SPS casing treatment when the stall margin needs to be extended.
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