Thermal imaging of ducted hypersonic flows

Abstract

The development of scramjet technology has presented the need for methods to analyse the temperature of the hypersonic flow experienced internally by the scramjet. A new technique has been proposed that incorporates an infrared window in the wall of a duct, a thin black film is placed between the flow and the window. This allows an infrared sensor to measure the surface temperature by reading the radiation emitted from the underside of the film, through the window.A semi-infinite analysis was performed on the thin film, assuming the material was either a graphite coating, or a Krylon™ matte black paint coating. It was found that the graphite film would have to be below 14μm, and the Krylon™ film below 4.5μm for the temperature on the underside of the film to experience a third of the change seen by the surface. Magnesium fluoride was chosen for the IR window due to its transmissivity and material properties. Bench tests were performed by applying heat to one side of the film, and measuring the cooling of either side with IR sensors. This showed a trend for temperature decrease but with a temperature difference of 35°C with the Krylon™ film. The graphite film showed similar trends with a larger temperature difference between surfaces, which is thought to be due to the calibration process only being performed on the paint.An experiment was carried out in the X2 expansion tube using both the painted film and graphite film, and two flow conditions; a Venus atmospheric entry condition and an air condition. Analysis with Schultz and Jones' (1973) exact thin film theory showed an increase in temperature was expected for the graphite film underside, but was unlikely with the painted film. The painted film showed an oscillating response followed by a 40°C, and 20°C temperature change in the Venus and air conditions, respectively. The graphite film shows a temperature change of 325°C and 150°C for the Venus and air conditions, respectively. It was concluded that the Krylon™ film is likely due to errors because of the trend, whilst the graphite film is too transmissive at its current thickness of 5-6μm.The new method of infrared thermography to measure ducted hypersonic flows remains invalidated, which was the main aim of this study. A greater understanding has been gained, and in future development it is recommended that a thicker graphite film is used, or other carbon materials researched, and that the calibration process is undertaken in the exact conditions of which the experiment will be performed.ii AcknowledgementsThe completion of this thesis has been a large undertaking and would not have been possible without the assistance and expertise of a number of people. I would like to acknowledge the following people:Prof Richard Morgan; His wealth of knowledge and experience often provided a simple solution to a complex task, and despite a large number of responsibilities he was always able to make time to discuss this thesis.All of the post-graduate students within the X-labs. Everyone has helped me in one way o...