Testing of Ultra Fast Response, Durable Co-axial Thermocouples for High Enthalpy Impulse Facilities

Fast response heat flux gauges relying on the semi-infinite heat conduction principle have commonly been used to study heat flux in impulse test facilities such as expansion tubes and reflected shock tunnels. For studying the very harsh environments experienced at the stagnation point of entry vehicles, generally metallic gauges such as thermocouples are required to survive the heat loads which may be above ten megawatts per metre squared at peak heating in flight and can be upwards of a hundred megawatts per metre squared in heavily scaled impulse facility testing, which can be challenging for even the toughest heat flux gauges. This paper reports on the design and testing of a new, extremely durable, fast response co-axial thermocouple which was designed at the University of Southern Queensland and has been subjected to over a hundred experiments in the very harsh conditions experienced over a small stagnation point heat flux probe in a free piston driven expansion tube at the University of Queensland. No degradation in the performance of the thermocouple was seen over the more than a hundred experiments and it was shown to maintain its ability to respond to changes in the flow in a matter of microseconds over the whole campaign. Little to no maintenance was required between experiments, even though the thermocouple’s surface temperature had often risen by hundreds of Kelvin by the end of the post-experiment flow. As previous co-axial thermocouples which have been used in these expansion tubes in the past have only survived several experiments before their performance begins to degrade, this is a very positive result.