2025 ASEE Annual Conference & Exposition

Pressure and temperature are the most commonly measured properties. They are often used to evaluate and control the performance of engineering systems. For applications involving fluid flow, such as aircraft gas turbine engines, one must distinguish between the static and stagnation values of pressure and temperature. For pressure, it is widely known that the measurement of static versus stagnation values can be easily implemented through the direction of the pressure port with respect to the flow. In contrast, the temperature measured by physical probes, such as thermocouples, is generally taken to be the stagnation value due to the assumption of the flow stagnating on the probe. This assumption is not entirely correct, especially in aerospace applications where the flow is velocities are usually far from negligible.
This paper describes a new experiment to be given in a second-year aerospace engineering course in experimental methods at Polytechnique Montréal to quantify and attenuate the effect of airflow velocity on temperature measurements. The setup consists of a reservoir connected to a source of 100 psig compressed air, with a small nozzle at one of its extremities capable of producing a jet up to Mach 0.8 in velocity. The jet velocity is controlled through a valve in the high-pressure feed line upstream of the reservoir. The reservoir contains a thermocouple probe and its volume is chosen such that the internal flow velocity is small enough so that the temperature measured by the internal thermocouple is the stagnation value. On the outside, downstream of the nozzle, is a system that rapidly swaps a thermocouple probe with a pitot-static tube positioning each at the center of the jet, the former measuring the jet temperature and the latter the jet Mach number.
The goal of the experiment is to first have the students take the temperature and pressure values at different jet velocities and to plot the ratio between the jet temperature measured by the external probe over that obtained with the internal probe against the Mach number. This plot should also include a horizontal line with a temperature ratio value of 1 and a line showing the variation is static over stagnation pressure versus Mach number for adiabatic flows. If the external temperature probe were insensitive to flow velocity, then the test data would fall on the horizontal line. The students will find that there is a deviation between the external and internal (stagnation/reference) temperature measurements, and this deviation grows with Mach number making the jet temperature measurement lie in the region between the stagnation and static temperatures.
Subsequently, the students are asked to repeat the experiment with a special Kiel head adapter placed over the external thermocouple probe that should reduce the previously observed deviation. The students will be asked to explain how this adapter does so.