R134a and R1234ze(E) liquid and flow boiling heat transfer in a high porosity copper foam

Heat dissipation is one of the most important issues for the reliability of electronics equipment. Boiling can be a very efficient heat transfer mechanism, thus it can be used to maintain the junction temperature of electronics equipment at values compatible with the technology using efficient and compact heat sinks. On the other hand, the environmental issues associated with the use of synthetic refrigerants call for a continuous improvement in technical solutions. Recently, new low-GWP refrigerants, in particular R1234ze(E), have been proposed as possible alternatives of the more traditional R134a. Besides, cellular structured materials both stochastic and periodic, particularly open cell metal foams, have been suggested as possible enhanced surfaces to lower electronics temperatures at high heat fluxes. Until now, most of the research on metal foams regards only single phase flow, whereas two phase flow is still almost unexplored. This paper presents an experimental study on the heat transfer of R134a and R1234ze(E) during single phase liquid and flow boiling inside a 5 PPI copper foam. The experimental measurements were carried out by imposing two different heat fluxes: 50 and 100 kW m−2, at constant saturation temperature of 30 °C; the refrigerant mass velocity was varied between 50 and 200 kg m−2 s−1 while the vapor quality varied from 0.2 to 0.95. As a result, the effects of the refrigerant mass flow rate, heat flux, and vapor quality on the heat transfer coefficient, dryout phenomenon, and pressure drop are investigated. Furthermore, the flow boiling heat transfer was observed using a high speed video camera allowing for a detailed analysis of the experimental results.