Investigation of boiling heat transfer on small diameter tubes and tube bundles
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Boiling heat transfer on the outside of small diameter tubes in the range of 1.8-3.Omm has been investigated. Pool boiling was investigated at nominal atmospheric pressure for each of the tubes in isolation. The experiment was varied by investigating the effect of bubbles from a second tube mounted below by varying the heat flux on the upper tube. The upper tube diameter was changed from 3.00 to 2.32 and 1.83mm and in each case the lower tube was 3.00mm. Experimental results showed that the upper tube heat transfer coefficient was enhanced due to the combined mechanism of translating bubbles and turbulent convection at low to moderate heat fluxes. A compact tube bundle made up of 30 stainless steel tubes of outer diameter 3mm, pitch diameter ratio 1.5 and heating length of 50mm. was designed to permit the measurement of flow boiling heat transfer coefficient from tubes within the bundle. The heat flux tested was in the range of 4-21 kW/m2 and mass flux of 5.6-32.8 k g/M2 s using distilled water, R-113 and Flutec PPI at nominal atmospheric pressure as the working fluids. Results obtained showed that the heat transfer coefficient was predominantly dependent on the heat flux as opposed to mass flux. Macro scale models were compared with the experimental results and none of these models predicted the experimental results well. The Confinement number (Co developed for flow boiling inside micro channels was applied to compact tube bundles and it was shown that confinement is expected to be significant for Co>0.63. Photographic studies also showed that the diameter of the bubbles that were generated within the bundle were greater than the tube diameter. As such, the sliding bubbles mechanism played less significant role in contributing to the heat transfer coefficient. The recent three-state correlation developed by Thome et al for flow boiling heat transfer in micro channels was modified to predict the experimental results obtained using a compact tube bundle and it has been shown that the thin film evaporation was the dominant mechanism compared to the nucleate boiling. The results from the twin tube and compact bundle arrangement showed two regions coexist at any point in time; that part of the tube covered with liquid subject to nucleate boiling and the other part completely enveloped with vapour. This latter part is designated by the introduction of a factor p and this has been demonstrated experimentally and theoretically corroborated by a model based on a liquid part (1-p) and vapour part p.