A Method for the calibration of fast response radiation heat transfer gauges

An experimental method and apparatus was developed to determine the optical properties of fast response radiation heat transfer gauges. In particular, this thesis has concentrated on the optical calibration of a range of thin film radiation heat transfer gauges. These gauges are currently being used at the University of Queensland to perform studies of radiant heat transfer during superorbital re-entry missions, such as aerocapture at Mars or Titan, or Lunar return. A better understanding of the mechanisms of radiation heat transfer at superorbital velocities could potentially lead to large mass or cost savings during these missions. A literature review was performed on experimental and theoretical heat transfer studies, hypersonic testing facilities, heat transfer gauges and relevant calibration methods in order to determine an appropriate methodology to calibrate radiant heat transfer gauges. Based on this review a suitable calibration method was chosen whereby the thermal properties of the gauges would first be determined via an oven calibration procedure, and then the optical properties would be studied by illuminating the gauges with a light source of known spectral output and intensity. A review of possible light sources was conducted, and although welding arcs were not a well calibrated or stable light source, due to time and budget limitations in this thesis, they were chosen for their intensity and strong UV output as the light source in the experiments of this thesis. A testing unit consisting of a “black box” with an aperture and a mechanical shutter mounted onto the front of it was constructed. The gauges were mounted inside of the box, behind the aperture. When the arc was struck, the mechanical shutter was opened, the gauges were illuminated with the high intensity light created by the welder, and the temperature response of the gauges was recorded. Heat transfer rates were then determined based on the temperature response of the gauges. Additional shielding was added to the front of the testing unit to protect the gauges and the shutter from welding splatter from the arc. The effect of painting the gauges was investigated by comparing the relative responses of painted and unpainted gauges. Due to fluctuations in the arc intensity between tests, it was impossible to make accurate comparisons between the absolute results between tests. This relative comparison however illustrated that the unpainted gauges absorb 46 ± 9% of the radiation that the painted gauges absorb. This result agreed well with the theoretical prediction of 41%, not only confirming this result, but also demonstrating that the calibration method developed in this thesis could indeed provide accurate, useful results. An investigation into the consistency of the painted gauge results showed that individual gauges consistently absorb the same amount of radiation relative to the other gauges. This illustrated that the optical properties of the gauges can varies between individual gauges. This is most likely due to the way in which the gauges were painted, which would lead to slightly different optical properties for each gauge. This analysis also demonstrated that the calibration method developed in this thesis can indeed produce consistent results between tests, and could easily be used to calibrate individual heat transfer gauges, given a pre-calibrated gauge or a well-calibrated, even light source. An analysis of the error involved illustrated that considerable uncertainty arises from normalizing the data in order to make relative comparisons between tests. This highlighted the need for a stable, well-calibrated light source for future experiments, which would allow for more accurate absolute comparisons to be made between tests.