Enhancement of nucleate boiling heat transfer has been studied with the structured surfaces composed of interconnected internal cavities in the form of tunnels and small pores connecting the pool liquid and the tunnels. The boiling curves of R-11, water and nitrogen show 80 to 90 percent reduction of wall superheat required to transfer the same heat flux as that on plain surfaces, when the pore diameter is set around 0.1 mm. The experimental data on bubble formation showed a significant contribution of latent heat transport to the enhancement. A visualization study made with a transparent structured model suggested that the liquid suction into the tunnel is triggered by the bubble growth at active pores and subsequent evaporation inside the tunnel plays a vital role in driving the bubble formation cycle. This observation led to a conception of the dynamic model expounded in Part II.

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Based on the experimental results reported in Part I, an analytical model of the dynamic cycle of bubble formation is proposed. The cycle consists of a waiting period and a bubble growth period; in the former the pressure in the tunnel is increased due to evaporation from internally held meniscuses and in the latter a certain amount of pool liquid is sucked in the tunnel to be subsequently evaporated. The equations are formulated with the adoption of a moderate number of empirical constants. Their solutions give the predictions of latent heat flux due to internal evaporation, population density of active sites, and frequency of bubble formation. The population density is then used to estimate convective heat flux on the outer surface from the empirical correlation established in Part I. The analytical predictions are compared with the experimental data of Part I. The results are encouraging and indicate the course of future study to implement the model. Hd^^ ^^ t™™ 1

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The porous surface structure was manufactured with precision for the experimental study of nucleate boiling heat transfer in R-11. Boiling curves and the data of bubble formation were obtained with a variety of geometrical and operational parameters; the pore diameters were of 50, 100, 150 μm, there was a combination of pores of different sizes; and the system pressures were of 0.04, 0.1, 0.23 MPa. The boiling curves exhibit certain trends effected by the diameter and population density of pores. A combination of high system pressure and pore sizes of 100 or 150 μm dia enables boiling to persist even when the wall superheat is reduced to an extremely low level of 0.1 K. A noteworthy feature of porous surface boiling is that intense bubble formation does not necessarily yield a high heat-transfer performance. Examination of the data indicates that liquid suction and evaporation inside the cavities are a proable mechanism of boiling with small temperature differences.