Thermocouple Thermal Inertia Effects on Impingement Heat Transfer Experiments Using the Transient Liquid Crystal Technique

The transient liquid crystal technique is widely used for impingement heat transfer experiments. Additionally, due to the difficulty of producing pure temperature steps in the flow, many authors assumed the fluid temperature evolution as a series of step changes using Duhamel’s superposition theorem. However, for small impingement configurations where the jets are fed from the same plenum chamber, and hence flow velocities are relatively small, thermal inertia of commercial thermocouples causes a delay, lagging from the real plenum temperature history. This paper investigates thermal inertia characteristics of thermocouples and their effect on the calculation of impingement heat transfer coefficient. Several thermocouples with exposed junction and different wire diameter were considered over a range of plenum flow conditions typically found in impingement heat transfer experiments. The effect of thermocouple time constant on the evaluation of the heat transfer rate was investigated in a narrow channel consisting of five inline impingement jets. The results indicated a significant effect of thermocouple response on the stagnation point region heat transfer, while lower local heat transfer rates are negligibly affected as liquid crystal signals appear later in time and the driving gas temperature history has a smaller influence on the evaluated data.