Design and Calibration of Seven Hole Probes for Flow Measurement

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Crawford, James
fluid mechanics , flow measurement , pressure probe , hot flow , gas turbine
The calibration and use of seven hole pressure probes for hot flow measurement was studied extensively, and guidelines were developed for the calibration and use of these probes. The influence of tip shape, Reynolds number, calibration grid density, and curve fit were studied and reported. Calibration was done using the well established polynomial curve fit method of Gallington. An improvement to this method was proposed that improved the uniformity and magnitude of measurement error. A hemispherical tip was found to be less sensitive to manufacturing defects, and less sensitive to changes in tip Reynolds number than a conical tip. The response of the probes was found to be Reynolds number independent over a tip Reynolds number of 6000 for the entire calibrated range. For flows with an angle of attack less than approximately 20°, the response of the probe was found to be independent above Re = 3000. A minimum calibration grid density of 5° was recommended. Error in the measurement of high angle flows was found to increase significantly when the calibration grid was sparser than this. The response of the probe was found to contain features that were not properly represented by third order polynomial terms, and it was found that it was necessary to include fourth order terms in the polynomial curve fit. The uniformity of calibration error was found to improve significantly when the high angle sectors were calibrated using a small number of additional points from adjacent sectors. The calibration data sorting algorithm was modified to include a calibration point in a given sector if that sector’s port read the highest pressure, or if that port read within a specified tolerance (“overlap pressure”) of the highest pressure. An overlap pressure of 15-20% of the calibration flow dynamic pressure was found to decrease the maximum calibration errors by 10-15%.
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