Local Heat Transfer Measurements in Micro Geometries Using Liquid Crystal Thermography

A technique is described on the use of un-encapsulated thermochromic liquid crystals (TLC’s) to measure the local heat transfer coefficient in microchannel geometries. Microchannel heat transfer is at the heart of the microchannel heat sink, a recent technology aimed at managing the stringent thermal requirements of today’s high-end electronics. The microencapsulated form of liquid crystals are well established for use in surface temperature mapping. Limited studies however are available on the use of the un-encapsulated form. This form is advantageous as it offers the potential for high spatial resolution which is necessary for micro geometries. The evaluation of this method and its associated difficulties is therefore the motivation for the experimental facility developed and described in the present work. Measurements are made in a closed loop facility combined with a microscopic imaging system and automated data acquisition. Results are presented for a circular tube made of stainless steel with an inner diameter of 1.0668mm. A localized TLC calibration is used to account for non-uniformities in the coating and variation of lighting conditions. Results for single-phase, thermally developing, laminar and turbulent flows using distilled water are presented. The results show that the correlations for conventional size channels are adequate for predicting the heat transfer characteristics of a nominally sized 1 mm channel.