Examination and visualisation of heat transfer processes during evaporation in capillary porous structures

This paper summarises the progress and the preliminary results of an investigation into the heat transfer process in wire mesh wicks. Particular emphasis is placed on the boiling and wicking limits in capillary porous structures made up of one or more layers of wire mesh. The two limits are shown to be interrelated: vapour bubbles trapped in the structure of a mesh wick block the wick and reduce the effective cross-sectional area, thus reducing the capillary flow. The presence of local dryout due to trapped bubbles leads to a decrease in the heat transfer coefficient. The experimental facility allowed measurement of the fluid and heater surface temperature and recording of the heat transfer process by using a high-speed video. Different wicks (mesh 50, mesh 100, mesh 150, mesh 200) with a thickness of one, three and five layers have been tested in submerged pool boiling and in wicking mode with distilled water as a working fluid. The parameters investigated included mesh size, number of layers and submergence of wick. Results confirm that the use of multiple layers of mesh wick increases the maximum attainable heat flux (qDHF) in wicking mode, but results in a lower heat transfer coefficient when compared with a bare surface or a single mesh. A linear relationship between wicking height h and qDHF is suggested by the results to date. Visualisation results provide an insight into the heat transfer mechanisms in mesh wicks during steady state conditions and as dryout occurs.