Fabrication of Wire Mesh Heat Exchangers for Waste Heat Recovery Using Wire-Arc Spraying
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R. Rezaey, S. Salavati, L. Pershin, T. Coyle, S. Chandra, and J. Mostaghimi (Submitted July 12, 2013; in revised form January 7, 2014) Waste heat can be recovered from hot combustion gases using water-cooled heat exchangers. Adding fins to the external surfaces of the water pipes inserted into the hot gases increases their surface area and enhances heat transfer, increasing the efficiency of heat recovery. A method of increasing the heat transfer surface area has been developed using a twin wire-arc thermal spray system to generate a dense, high-strength coating that bonds wire mesh to the outside surfaces of stainless steel pipes through which water passes. At the optimum spray distance of 150 mm, the oxide content, coating porosity, and the adhesion strength of the coating were measured to be 7%, 2%, and 24 MPa, respectively. Experiments were done in which heat exchangers were placed inside a high-temperature oven with temperature varying from 300 to 900 °C. Several different heat exchanger designs were tested to estimate the total heat transfer in each case. The efficiency of heat transfer was found to depend strongly on the quality of the bond between the wire meshes and pipes and the size of openings in the wire mesh.
Keywords
adhesive strength, high-temperature application, process optimization, thermal conductivity, wirearc process
1. Introduction Heat exchangers (HEX) are designed to transfer heat between two fluids that are at different temperatures while keeping them separate from each other. Compact HEX are widely used due to their large heat transfer surface area to volume ratio, which is called area density (Ref 13). This large surface area is usually obtained by attaching fins to the wall separating the two fluids, with the fluid with the lower heat transfer coefficient flowing on the finned side of the wall. Investigators have been studying the performance of metal foams and wire meshes for several years as candidates to replace fins. One technique has been to use thermal spray processes to create walls around porous structures and form an enclosure through which fluid can flow, where heat is conducted to the fluid along the struts of the porous media. Salimi Jazi et al. (Ref 4) and Tsolas et al. (Ref 5) used a wire-arc spraying process and deposited an Inconel 625 skin on copper and nickel foams to contain air flowing inside the metallic foam. They studied the effectiveness of metal foams as HEX by investigating the forced convection heat transfer rate for R. Rezaey, S. Salavati, L. Pershin, T. Coyle, S. Chandra, and J. Mostaghimi, Centre for Advanced Coating Technologies (CACT), Toronto, ON, Canada. Contact e-mail: rrezaey@mie. utoronto.ca.
Journal of Thermal Spray Technology
various flow rates. Recently, Assaad et al. (Ref 6) fabricated wire mesh HEX using a pulsed gas dynamic spraying technique. They deposited metallic powders on the outer surface of metal wire mesh wafers and created compact heat exchangers. Porous structures such as metal foam and wire meshes can also act as fins to enhance
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