Pitot pressure measurement in high enthalpy expansion tubes

Abstract

This report has investigated the use of a new bar gauge design in order to replace previous iterations of bar gauges and pitot pressure probes for use within UQ’s expansion tube facilities. Preliminary hammer calibration testing on different arrangements of the bar gauge showed that the response remained almost constant regardless of shielding, no shielding or different clamping arrangements, varying by only a maximum of 14.59%. It was also concluded that the ratio of strain gauge output to hammer input remained almost constant for the entire test time, and that a calibration factor taken at the 50µs surrounding the peak of the signal will yield the most accurate result. It was determined that due to this reason, deconvolution was not necessary because the response could be characterised using a constant value, known as the calibration factor. Final hammer calibration testing was conducted on the final arrangement in order to characterise the response of the new bar gauge, and determine a calibration factor such that a strain signal from an expansion tube test (in volts) can be converted into a useful pitot pressure reading. The average calibration factor of 12 tests was used in this investigation, with a value of 0.0001772714219 V/kPa and uncertainty of ±1.8174% using a 95% Confidence interval. The results from the three expansion tube tests show that the pitot pressure could be predicted to within ±15.82% on average by comparing to a theoretical model known as PITOT. Reasons on the possible sources of error and inaccuracy have been discussed extensively in Section 5.5. The newly designed bar gauge has achieved an average useful test time of 264.5 µs, a response/rise time of 7µs, with an overall uncertainty of ±3.484% in the measurement of pitot pressure for flows in the order of 100kPa during the useful test time. The new bar gauge is completely reusable, and easily serviceable. There are many possible improvements and recommendations for future research and development into pitot pressure measurement of high enthalpy expansion tubes.