Improving Heat Transfer Performance Of Heat Exchangers

The wetting of solids by liquids is an important aspect in heat exchange and is particularly relevant for condensation heat transfer, as it can have a dramatic impact on heat transfer efficiency. Condensation can be filmwise or dropwise, with filmwise condensation involving the coalescence of condensate droplets into a film, which increases thermal resistance and reduces heat transfer performance. The preference is therefore for dropwise condensation where droplets condense onto the cooled surface and rapidly depart freeing up the surface area for new droplets to form. A key means of promoting dropwise condensation is through the use of hydrophobic/oleophobic (hygrophobic) materials, surfaces or coatings that encourage high contact angles between the surface and the droplet and discourage the formation of liquid films on the surface. Hygrophobic coatings are therefore being considered in the GeoHex program to enhance the heat transfer performance of geothermal heat exchangers, and the program will consider the modification of surface energies as well as micro or nano structuring to achieve hygrophobic surfaces. GeoHex will consider coatings on four substrate materials, aluminium, copper, stainless steel and carbon steel, and will consider microscale bottom-up roughness and nanoscale top-down roughness.