Influence of Thermocouple Thermal Inertia in Impingement Heat Transfer Experiments Using Transient Techniques

A typical transient heat transfer experiment for an impingement cooling configuration requires usually a temperature step in the flow. A moving isotherm of the liquid crystal surface coating is then monitored using a video camera. However, for small impingement configurations, when only a small number of holes is used, the massflow rates are relatively low, leading to very small plenum velocities. Therefore, a step change in the power settings does not cause a temperature step in the mainstream flow. Many researchers approximated the real hot gas temperature evolution by a number of ideal temperature steps (Duhamel’s superposition principle). However, thermocouple acquisition measurements are influenced by the size of the thermocouple due to thermal inertia and therefore direct evaluation of the heat transfer coefficient with these data may be in error. This paper suggests that the hot gas temperature which drives the transient experiment and is measured in the plenum can be corrected according to the time constant of the thermocouples. Several experiments were carried out in order to evaluate the time response of fine thermocouples with exposed junction. For the impingement configuration, a single row of five inline impingement holes is used at in a narrow passage configuration over a range of Reynolds numbers (15000-55000). The liquid crystal signal is evaluated with three different hot gas temperature approaches: (1) Perfect temperature step, (2) Duhamel’s principle in the acquired temperature data and (3) Correction of hot gas temperature for thermal inertia prior to Duhamel’s principle. The experimental data are analyzed by means of various post-processing procedures and aim to clarify and quantify the effect of thermocouple thermal inertia on the final results.