The evaporator of a heat exchanger is made with a porous, capillary, structure. In the past researchers [7] noticed that the heat flux limits of a bi-porous capillary structure is much greater than that of a mono-porous capillary structure and will be the focus of this work. There are three distinct stages in the heat transfer process in a bi-porous wick. Each of the stages is explored in turn. In the first stage, heat is transferred from the wall across the saturated wick by pure conduction to the evaporating front located on the top of the bi-porous wick. When the boiling limit is reached, bubbles begin to nucleate and the second stage begins. The boiling becomes more and more intensive as the heat flux is increased until all of the liquid from big pores is evaporated, and only small pores remain wetted with liquid. The point reached here is called the capillary limit, which is basically the limit at which the capillary forces are still sufficient to provide the liquid for evaporation into the big pores. The modelling of the different thermal physical processes determining heat transfer within each of the three stages for a bi-porous heat wick are modelled and significant improvement in achievable heat flux is observed. Comparison with experiment is found to be reasonable. Optimal selection of the bi-porous wick characteristics is shown to yield very high heat fluxes.
