Heat transfer analysis of subcooled flow boiling in copper foam helical coiled heat exchanger – A pore-scale numerical study

High performance thermal energy systems are crucial in fulfilling the growing demand of thermal energy. Consequently, numerous heat transfer enhancements have been proposed. Some of the promising methods are boiling flow heat transfer, helical coiled tube and metal foam insert. Although the performance of each strategy has been extensively studied, based on our literature review, the effectiveness of combining these strategies has not been investigated. This missing information may impede further development of efficient heat transfer systems. This study is therefore performed to investigate subcooled flow boiling heat transfer in helical tube with copper foam insert. A high-resolution three-dimensional pore scale model that considers the shape, porosity and pore density of the metal foam is developed and validated against the published experimental results. Using the model, the effect of inlet temperature, mass flux, wall heat flux, porosity, pore density and half-filled metal foam are studied. The results suggest that operating parameters have mild effects on the heat transfer performance. In contrast, porosity and pore density show significant influence in heat transfer coefficient and pressure drop. It has also been found that total heat transfer surface area and base area are better in characterizing heat transfer in pore-scale level than porosity and pore density at pore-scale level. Moreover, implementing metal foam at outer half of the tube is observed to offer slightly better heat transfer than implementing it at the inner side by around 4.8%. Overall, this study indicates that helical metal foam tubes offer the best performance at mass flux of 200 kg/m2∙s with performance index of 0.72, while straight tubes with 0.90 porosity has the highest performance index of 0.66. Empirical correlations to relate dimensionless parameters to the Nusselt number and friction factor is generated. Finally, the model developed in this study can serve to provide guidelines for the upcoming studies in boiling in metal foam structure and assist in designing metal foam heat exchangers.