Pool boiling on porous surfaces in cryogenic and refrigerant surfaces

Experimental results for nucleate boiling, critical heat flux (CHF) and film boiling on flat plasma sprayed porous surfaces in cryogenic and CFC refrigerant liquids at atmospheric pressure are reported. Such coatings are shown to have excellent heat transfer performances, increasing not only the heat transfer coefficient in the nucleate and film boiling regime but also increasing the CHF. The effect of coating thickness, coating material and surface orientation are investigated. The results for nucleate boiling regime show that in low heat flux region (q < 1 W.cm-2) and for coating thicker than 0.25 mm, the effect of coating thickness on boiling performance is not significant. In contrast, in high heat flux region, there is clearly an optimum coating thickness, which gives the highest heat transfer coefficients for a given value of heat flux. The heat transfer coefficient of the porous surfaces are higher in the cryogenic liquids compared to those in the CFC liquids. As expected the application of coating with higher thermal conductivity gives higher heat transfer coefficient. In the nucleate boiling regime, the heat transfer coefficient is not influenced by surface orientation. The experimental results also show that the coating thickness, coating material, and surface orientations strongly affect the CHF but they do not influence the film boiling performance. Based on the experimental results, boiling mechanism, models of vapour formation inside the coating and correlation equations are proposed. Comparison of the plasma sprayed porous surfaces with arc sprayed surfaces and with other commercially available enhanced surfaces are also discussed.