Enhancing Total Temperature Measurement Accuracy: Calibration Procedures and Novel Two-Wire Probes

Abstract

Total temperature measurement with thermocouple probes suffers from errors due to heat transfer effects. Two dominant sources of errors are convection and conduction between the thermocouple, the support, and the flow. These effects can be treated in two different categories: the velocity error, created by convection from the internal flow velocity in the probe shield, and the conduction error, involving heat transfer through the wire to the shield and the probe stem due to temperature differences between each component. This article presents a robust approach to experimentally assess and reduce both errors. An open jet test stand at the Purdue Experimental Turbine Aerothermal Laboratory is used to evaluate the effects of the velocity error at various Mach numbers. Infrared (IR) thermography measurements have been conducted to assess temperature gradients on the probe support during this calibration. With this information, it is possible to correct for conduction errors during the calibration and obtain a recovery factor that is solely dependent on upstream velocity. Recovery factor of 0.94±0.05 for Mach 0.2 and 0.98±0.005 for Mach 0.9 is obtained. Progress has been made on a two-wire thermocouple approach to address conduction errors in the testing scenario, where IR thermography is not possible. The readings from two wires with different length-to-diameter ratios are used with a novel linear correction method to correct for the flow total temperature. Advances in the design and manufacturing of the probe are presented, facilitating faster design iterations and implementation, and providing full control over the calibration procedures. This method can correct conduction errors within 0.2 K for Mach 0.6.