Performance evaluation of a double-pipe heat exchanger with uniform and graded metal foams

Metal foam heat exchangers have attracted a great deal of interest in numerous engineering fields due to their superior thermal capabilities. In the present study, the heat transfer characteristics of a double-pipe heat exchanger with metal foam insert are numerically investigated. The Forchheimer-extended Darcy equation and the local thermal non-equilibrium (LTNE) model are used to predict the fluid and energy transports, respectively. Thermal resistance of the interface solid wall is considered, while the porous-solid boundary follows the continuity principles. The commercial software FLUENT with specific user defined functions (UDFs) is adopted to implement the simulation. Configurations with uniform foam structure are firstly used to analyze the effects of flow arrangement, foam structural parameters (porosity and pore density) and thermal conductivity on the heat exchanger effectiveness and total pressure drop. Then, graded foam structure along the radius is proposed to further make use of the heat transfer potential of metal foam. The overall thermal performance with increasing and decreasing arrangements of porosity and pore density is assessed. The results indicate that the counter flow shows good performance, with 37.5% higher than the parallel flow in effectiveness. The effectiveness and total pressure drop present monotonic variation with the foam structural parameters for the uniform designs, while maximum performance factor occurs at 15 PPI. The effectiveness has a reduction after gradual increase to a peak 0.89 with the increasing of thermal conductivity of foam matrix. For the designs of graded foam structure, using lower porosity and small pore density at both side of the inner pipe wall shows better overall performance with the performance factors of 4.41 and 4.54.