Use of thermochromatic liquid crystals in the study of jet impingement cooling: sensitivity of transient heating methods

Abstract

In the cooling of surfaces jet impingement arrays have been found to provide effective surface heat transfer. Considerable work has been done in identifying the optimal jet array geometry, including jet diameter, spacing and relative distance to the surface to be cooled. Most all of these studies rely on surface averaged heat transfer results. However, there are applications where the local distribution of the impingement heat transfer is important. The magnitude of the local variations may cause serious problems in terms of surface temperature gradients. Thermochromic liquid crystals provide a means to directly measure the surface temperature which can be used to study the local heat transfer coefficient distribution. Both steady state and transient methods have been identified. The steady state method is a direct application of Newton's Law of Cooling. The transient method establishes a step change in the surface boundary condition and solves the conduction problem in the surface substrate. This method can have advantages of lower experimental uncertainty. However, there are practical issues of time response that need to be addressed to determine actual local heat transfer coefficient. This paper addresses the issues associated with the transient method and provides results of impingement cooling. Of primary concern is the transient response and how that is related to the actual instantaneous convective condition at the surface. Results show a nonsteady convective coefficient which must be corrected based on the experimental design parameters.