Pool boiling heat transfer of R134a on single horizontal tube surfaces sintered with open-celled copper foam

The pool boiling heat transfer performance of refrigerant R134a on single horizontal tube surfaces sintered with open-celled copper foam was studied at Ts = 6 °C and Ps = 3.62 bar. Three pore density values: 40, 80, and 130 PPI; two porosity values: 90% and 97%; and two thickness values: 1.6 and 2.5 mm, characterize the 12 studied tubes. The nucleate boiling data and heat transfer coefficients were obtained. The influences of pore density, porosity, and foam thickness on heat transfer performance were investigated as well. The foam provides activated nucleation sites; however, it also causes transport resistance for the bubbles within the foam to move from the active nucleation sites to the surface. Performance is dependent on whichever of these two prevails in the heat transfer process. The foam-coated tubes show significant performance enhancement compared with plain tubes at heat fluxes below 30 kW/m2. However, a sharp reduction in heat transfer coefficient is encountered for 130 PPI at larger heat fluxes. Tubes coated with thin foam and high porosity, e.g., pore density: 80 PPI, porosity: 97%, and thickness: 1.6 mm, exhibit a comparatively superior heat transfer performance. The current experimental data provide useful information for the design of highly effective boiling surfaces.