High Bandwidth Stagnation Temperature Measurements in a Mach 6 Gun Tunnel Flow

Temperature is an important parameter in most high speed flow experiments, but it is sometimes a difficult parameter to measure, particularly in short-duration facilities. Stagnation temperature measurements have been obtained using transient thin film heat flux probes in a Mach 6 carbon dioxide flow produced by the Oxford University Gun Tunnel. The probes were operated over a range of surface temperatures so that the flow stagnation temperature could be identified independently of the convective heat transfer coefficient of the probes. The time-averaged measurements indicate a significant drop in stagnation temperature with time and this implies that significant cooling of the test gas occurred within the barrel during the compression process and/or during the flow discharge process. During the last 12ms of flow, the time-averaged stagnation temperature indicated by the probe was 610±10K for the present operating conditions. During the same 12ms flow period, the probe measurements also indicate stagnation temperature fluctuations of about 2.3K (RMS) for frequencies between 1 and 25kHz. Based on pitot pressure fluctuation measurements at essentially the same location within the nozzle, it is concluded that the measured temperature fluctuations are primarily due to fluctuations in entropy. Entropy fluctuations within the Mach 6 flow probably arise because of the turbulent heat transfer to the barrel.