Effects of gap sizes (3.9-44.3 mm)of vertical annuli on nucleate pool boiling heat transfer of water at atmospheric pressure have been obtained experimentally. Through the tests, the conditions of tube bottom confinement (open or closed) have also been investigated and the whole results are compared with those of a single unconfined tube. According to the results, the annular condition gives much increase in heat transfer at moderate heat fluxes. The Increase is much enhanced as the gap size decreases. At the same tube wall superheat (about 3.2 K) the heat flux of The least gap size is more than four times greater than that of the unconfined tube. Its effect is observed much greater For the bottom-closed tube condition. However, a deterioration of heat transfer occurs at high heat flux for confined Boiling.
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