Orientation Effects on Pool Boling of Microporous Coating in Water

Copper HTCMC (High-temperature, Thermally Conductive Microporous Coating) with a coating thickness of ~300 µm was created by sintering 67 µm copper particles onto a flat copper surface. This was shown to be the optimum particle size and thickness combination, in terms of boiling heat transfer enhancement with water, during a prior pool boiling study conducted by Jun et al. [1]. The effects of orientation of pool boiling heat transfer in saturated distilled water at 1 atm were tested experimentally and compared with a plain copper surface. An SEM image (top left) shows the porous structure of HTCMC demonstrating reentrant cavities which promote nucleate boiling and lead to significant critical heat flux (CHF) enhancement compared to the plain copper surface (top right). The nucleate boiling incipience heat flux of HTCMC was demonstrated to be 5 kW/m², which was an 8x reduction when compared to a plain copper surface which was found to have an incipience heat flux of 40 kW/m². At this same 40 kW/m2 heat flux, the activated nucleation site density of HTCMC was extremely high, and each bubble appeared much smaller compared to a plain surface. This can be seen in the first row of images, captured with a high speed camera at 2,000 fps. The bubble growth times and departing bubble sizes of 0° and 90° are comparable for both HTCMC and plain surfaces with the order of 10 milliseconds and 100 micrometers. However, when oriented at 180°, the bubble growth time was the order of 100 milliseconds for both HTCMC and plain surface, and the departing bubble size was the order of 10 millimeters. This is due to the growth of a large bubble which coalesced with adjacent bubbles to become a relatively huge bubble which was stretched by buoyance forces before the bubble departed.