Thermal spraying of high-capillarity porous metal coatings for heat transfer applications

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A heat pipe is a device that is widely used for cooling electronic systems. It consists of a hollow copper tube with a porous metal layer on its inner surface that is filled with a volatile liquid and sealed. The liquid evaporates when the tube is heated at one end, carrying away a large amount of heat, and the vapour diffuses to the other end of the tube that is at a lower temperature, where it condenses. The liquid is drawn back through the porous wick by capillary forces towards the hot, dry end of the tube, setting up a circulation loop. The porous wick in a heat pipe is usually made by sintering a metal powder. We propose to make porous metal layers by thermal spraying, a process in which a spray of molten metal droplets is directed onto a surface to be coated. The droplets coalesce with each other when they land, freeze, and form a solid layer. We will spray a mixture of two different powders, and then remove one phase from the solid coating by dissolving or burning it, leaving a porous layer behind. The objective of this proposal is to develop and test structured, porous copper and aluminium surfaces for use in heat pipe and boiling heat transfer applications. In the project we will develop methods of fabricating porous coatings with structures such as channels in them to increase their permeability. We will measure the rate of capillary rise of liquids in these coatings and develop analytic models to simulate the motion of liquid in them. We will also measure the rate at which liquid evaporates from their surface when they are heated. Tests will be done to observe nucleate boiling on these surfaces to determine how much the boiling heat flux is enhanced. The surfaces found to maximize heat transfer will be used to make heat pipes for cooling of electronic systems and the cooling heat flux measured using laboratory test rigs. The heat pipes will also be used for cooling of LED light arrays. It is expected that the project will lead to a new method of fabricating heat pipes that will be much faster than existing techniques and allow porous layers to be applied on much larger areas than currently possible.