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Measurements of the Heat Transfer Coefficient Distribution on Flat and Ribbed Surfaces with Interrupted Heating Using Thermochromic Liquid Crystals

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Master's thesis

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Measurements of the heat transfer coefficient distribution on flat and ribbed surfaces with interrupted heating are presented. One application of this type of thermal boundary condition is the cooling of surface mounted modules in electronic packaging. Heating occurs at the modules, with no heating between the modules. The heat transfer measurements in the present study are made using the heated-coating method with a thermochromic liquid crystal TLC. The coating is vacuum-deposited gold on a plastic sheet, which is mounted on a Styrofoam surface. The gold film is electrically heated to produce the surface heat flux. The surface temperature is measured by capturing color images of the TLC, using the hue technique. These surface temperature plots are transformed into heat transfer coefficient distributions on the surface of the models. Interrupted heating on the flat surface is obtained by heating strips perpendicular to the freestream flow, with insulated spaces between the strips. The ribbed surface is heated on the top of the ribs only, with insulated unheated cavities in-between. The results show that, as expected, the local heat transfer coefficient is higher on the heated strips of the flat surface interrupted heating case, than would be present at the same locations with a uniform heat flux. The interruption of the thermal boundary layer at each unheated section allows it to reset, thereby causing the higher heat transfer on the following heated section. The ribbed model shows a slight increase in heat transfer on the ribs above that of the flat plate interrupted heating model. The cavities appear to introduce mixing that causes this change. The increase in h due to the ribs is small, however, when compared to the effect of the interrupted heating.

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  • Thermodynamics
  • Electrical and Electronic Equipment

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