Development of particle image velocimetry for plasma induced flow measurements
There has been an increased interest in the use of flow control in aerodynamics and combustion to improve efficiency and reduce emissions. Plasma flow control is one way by using active flow control affect these desired changes. Spark plasma actuators have capabilities of inducing heat and momentum to the flow field. The flow field generated by this plasma induces complex pressure and temperature gradients that lead to the development of complex flow structures. The experiment described in this research is particularly difficult due to its small scale, and the dynamic range of velocities that are induced by the flow field. This flow field is yet to be quantified by previous research. The flow field generated by spark plasma has not been quantified to date. The development of a set-up to capture and process preliminary findings of the flow field generated by a spark plasma under quiescent conditions is imperative to understanding the capabilities of this plasma actuator. A 2-D PIV system is used to analyze the flow field from a spark plasma actuator. An Nd:YAG laser is pulsed at a frequency of 10kHz to illuminate the test section. A Photron FASTCAM SA-Z camera is operated at 20,000 fps to take double frame images of the flow field. The flow field is generated between two electrodes connected to a DC power supply containing a high voltage transformer rated up to 45kV. The test section enclosed in a steel chamber with fused quartz windows for optical access is seeded with aluminum dioxide (Al2O3) particles with a manufacture specified nominal diameter of 0.3?m. PRANA (PIV Research and ANAlysis), a PIV processing software is used to process the captured images of the flow field. A multipass, method with deform is used to process instantaneous velocity fields and an ensemble correlation with deform is used to process the average velocity field. Robust phase correlation is used in each of the methods to correlate image pairs. Velocity field interpolation is performed using a bicubic interpolation method and image interpolation back onto the rectilinear grid is done using a windowed-sinc filter with a blackman window. 64 x 64 pixel interrogation windows are used to analyze the instantaneous flow field and 48 x 48 pixel interrogation windows are used to analyze the ensemble correlated flow field. Vector post processing is performed using a median filter for the universal outlier detection method, and a Gaussian smoothing function is applied on the velocity field in the initial passes. Three different phase averaging techniques are examined to determine the best method to assess twenty sets of instantaneous data taken under similar conditions. Ensemble correlation, ensemble averaging and correlation of ensemble images are the three methods tested. A final determination of the best method is left for future work, but for the purposes of this experiment, ensemble correlation was used to present phase averaged images as prior art found this to be the best way to increase signal to noise ratio (SNR) among the three. Ensemble averaging conserves the most information contained in the instantaneous flow field. The repeatability of the flow field is assessed and the turbulent nature of the flow field is revealed. Voltage measurements show that there is varying standard deviation in the voltage during the spark. Analysis of the flow field shows 70-90% deviations in magnitude of velocity. Preliminary results show flow concentrated in the center of the electrode gap at initial times, followed by an outward flow toward the surrounding gas. The effect of electrode gap on the flow field generated is also briefly studied, with larger gaps showing higher magnitudes of velocity and faster decay rates. The system is capable of measuring flow fields generated by the spark plasma and a characterization of the process is possible in the future using the some improvements to the set-up.
Bane, Purdue University.
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