Transient liquid crystal thermography using a time varying surface heat flux

Heat transfer measurements are required in a wide range of fields, for example to validate new cool -ing concepts in turbomachinery, to assess the performances of heat exchangers, and to provide data for numerical simulations. Thereby transient methods are often applied for local heat transfer resolution. A particular challenge is posed by complex flows, where the determination of the heat transfer coefficients with the commonly applied transient heater mesh method can prove difficult, for instance in cases in which the flow can take different paths leading to mixing flows at different tem peratures, and a difficult determination of the reference temperature. One way to address these complex systems is the transient heater foil method, in which the experiment is driven by a constant heat flux generated at the surface under study, instead of a temperature variation in the flow. However, the accuracy of the measurement remains an open issue compared to the heater mesh method. Here we show a modification of the heater foil method, which uses a linearly increasing surface heat flux to improve the measurement accuracy, especially in the low heat transfer regions. The new method is validated by measuring the heat trans-fer of a single circular jet perpendicularly impinging on a flat plate, and by comparing the results to a correlation available in the literature. Results show good agreement with the literature, while providing considerable accuracy improvement with respect to the heater foil method with constant heat flux. The heater foil method presented here, reaches similar uncertainty values as the state of the art versions of the heater mesh method in low heat transfer regions, while providing better accuracy in the high heat transfer regions. Additionally, it allows for an easier implementation for certain problems, provided that optical access is guaranteed and the surface curvature allows for the addition of the heater foil. (c) 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

Automated summary

Heat transfer measurements are essential in various fields, and transient methods are often used for local heat transfer resolution. However, complex flows can pose challenges for determining heat transfer coefficients. The transient heater foil method addresses this issue by generating a constant heat flux on the surface under study.