Development of thermal sprayed tube and mesh heat exchangers for waste heat recovery

Abstract

Development of a lab-scale heat exchanger with increased mesh to tube contact area was accomplished by flattening tubing prior to attaching wire mesh. This novel heat exchanger prototype utilizes thermal sprayed coating to attach wire mesh onto tubing surfaces instead of conventional brazing or welding of extended surfaces. Twin wire arc spray equipment was utilized to deposit stainless steel coating onto stainless steel mesh and tube. Four samples with varied coating thicknesses and spray deposition settings were fabricated to be compared with an unmeshed sample to quantify the heat transfer enhancement of the mesh. A simple heated wind tunnel was designed in tandem with the lab scale prototype. Air propelled by axial fan towards the sample placed inside a rectangular duct, is heated by a propane torch, mimicking the environment at the exhaust of flare or flue gas stacks. Steady state inlet and outlet water temperatures were measured. For heat transferred into the water, meshed and coated samples performed 38% to 217% better than bare tube. Temperature difference trends were in line with previous studies but magnitudes were 1.5 to 4 times greater, demonstrating the heat transfer enhancement from wire mesh, flattened tube, and thicker coatings. A fin model is developed to quantify the heat transfer from mesh wires and integrated into a discretized energy balance to predict outlet temperatures a priori and quantify thermal resistances in each sample. The results of the model indicate that coating thickness has the largest impact on thermal resistance, with values at least 1 or 2 orders of magnitude greater than other thermal resistances in the heat transfer network of the lab scale meshed tubular heat exchangers. The fin temperature distribution also exhibits small lengths of temperature variation along the fin, meaning future designs need not utilize fins longer than 4 mm, showing potential reductions in material, weight and size without impacting performance.