Hydraulic Characteristics and Thermal Properties of Metal Foam Sandwich Structure for High Temperature Heat Exchanger Applications

Open pore foams can be used as gas or liquid filters, catalyst supports, and heat exchangers due to their high gas permeability. Nickel-based superalloy foams have excellent high temperature oxidation resistance and strength and can be used for high temperature applications in air up to 1050 °C. In this study, alloy 625 skins were deposited on each side of a sheet of metal foam (10 pores per linear inch, ppi) by HVOF to form a sandwich structure in order to use in high temperature heat exchanger applications. The microstructure of the coatings and the interface between the struts and skins were investigated. Results showed well-adhered and dense skins on each side of the foam sheet. The foam struts were imbedded deeply into the coatings deposited by HVOF. The hydraulic characteristics and heat conductivity of the heat exchanger sandwich structure were experimentally measured. The pressure drops across the heat exchanger were measured and recorded at various coolant flow velocities. The pressure drop was proportional to the square of the coolant velocity. The pressure drops across the foam with 0.22 meter length at maximum coolant flow velocity, 1.8 m/s, was around 0.2 bar/m. The least square fitting approach was used to solve for the permeability ,K, and the form coefficient ,C, of the foam. The permeability of the foam was 28.8x10^-10 m² and the form coefficient of the foam was 1892 m^-1. The variations of the dimensionless Nusselt number at various coolant flow velocities were measured and calculated. The maximum Nusselt number was 30 at coolant flow velocity of 1.8 m/s.