Thermal spray fabrication of porous heat exchangers and heat pipes

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The Centre for Advanced Coating Technology (CACT) at the University of Toronto has used thermal spray technology to developing high efficiency heat exchangers and heat pipes. Heat exchanger efficiency can be improved by bonding porous wire meshes, perforated sheets, or metal foams to the surface of heat exchanger tubes, increasing the area for heat transfer. CACT has developed a bonding technique, using a wire-arc spray coating method, to bond metal meshes, foams and perforated sheets to tubes. We will use high-efficiency heat exchangers for a variety of applications, including waste heat recovery, gas heaters and industrial ovens.***Heat pipes are devices that are widely used for rapid transfer of heat from one location to another. A conventional heat pipe consists of a hollow copper tube that has a porous layer on its inner surface, usually made by sintering a layer of copper powder, which acts as a wick that is saturated with a volatile liquid. Sintered wicks are fragile and limit the size and shape of the heat pipe. We have formed porous wicks by spraying copper or aluminum powder using a flame spray system onto a copper substrate and decreasing particle velocity and temperature sufficiently so that the particles fuse only partially with each other. The abilty to spray both the skin and porous wick will revolutionize the manufacture of heat pipes and will allow them to be made very thin, with curved surfaces, and covering very large areas. ***This proposal is for an experimental study of heat transfer and fluid flow in porous media used for heat exchangers and heat pipes. The long term objective of this project is to develop compact heat exchangers and heat pipes using thermal spray technology. This will greatly reduce manufacturing costs, allow new designs and shapes for heat transfer equipment, and promote energy efficiency and waste heat recovery. ***The short term objectives of the study are:***1) Measure convective heat transfer through metal foams, perforated sheets and wire meshes.***2) Measure convective heat transfer in laser sintered metal channels with complex internal structures. ***3) Develop models for flow through a variety of porous media.***4) Use the models to optimize design of heat exchangers and scale them for a range of applications.***5) Develop fabrication techniques for making different shapes and sizes of heat exchangers.***6) Observe capillary driven fluid flow through thermally sprayed copper or aluminum wicks.***7) Measure heat transfer due to boiling of liquids impregnated in porous layers made by thermal spray techniques.***8) Develop models for heat transfer in thermally sprayed metal heat pipes and use them to optimize designs.**