Investigation into using liquid crystal thermography for measuring heat transfer coefficients and wall temperature profiles at inlets and underdeveloped regions

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.In this paper the wall surface temperature distributions at the inlet regions of a tube-in-tube counter flow heat exchanger were investigated by making use of Liquid Crystal Thermography (LCT). With flow not being fully developed at the inlet region it is difficult to predict the heat transfer coefficient and the appropriate wall temperature on the inner tube under different flow conditions. In this study water was considered in the annulus in the turbulent flow with annular Reynolds numbers of 1 000, 5 000 and 10 000. Local annular heat transfer coefficients were determined from the measured wall temperature profiles and the derived local annular bulk fluid temperatures based on the measured coupled local heat transfer rate in the inner tube. It was found that Liquid Crystal Thermography allowed for the measurement of the average wall temperature which could vary significantly in the inlet region. It was also found that the local heat transfer coefficients varied within a large extent. For the particular heat exchanger under investigation here which had annular diameters of 36 mm and 19.5 mm, heat transfer coefficients reached a maximum from 150 mm onwards after the inlet.