Nucleate boiling of various fluids in a thermosiphon reboiler Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/73666698j

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  • A study of nucleate boiling of water, four alcohols and three ethanol-water mixtures was conducted in a vertical thermosiphon reboiler. The test section consisted of a 1/2- inch diameter electrically heated stainless-steel rod jacketed by a 15-inch long pyrex glass tube. Two different inside diameters of the glass tube corresponding respectively to annuli of equivalent diameters 0.225 and 0.750 inch were used. The purpose of the study was to investigate whether or not there was an effect of the annulus gap on heat transfer to the boiling liquids. Heat fluxes ranged from 17,000 to 91,000 Btu/hr.ft². Velocities were measured in an attempt to simulate the natural-circulation data by means of a micro-pump and a rotameter. The velocities so determined varied from 0.093 to 1.171 ft/sec. Calculations have shown wall superheats (wall-minus-saturation temperature) from 17.4 to 58.6°F, vapor qualities from 0.9 to 7.6% and heat transfer coefficients from 518 to 2473 Btu/hr.ft². °F. An equation expressing the relationship between the heat flux and the wall superheat raised to a power n was developed for each fluid-diameter combination following a statistical analysis of the data. It was observed that for each fluid, the value of the exponent n was less for the smaller diameter of the annulus and approached a value of three for the larger diameter. This was attributed to a much lower contribution of the convective heat transfer component compared to the nucleate boiling heat transfer component for the larger tube. It was also observed that for the pure organic liquids, no significant effect of the diameter was observed whereas for water an increase in the annular width noticeably decreased the heat transfer coefficient at a given degree of saturation. For ethanol-water mixtures, it was observed particularly at high heat flow rates, that the diameter effect tended to disappear as the weight fraction of ethanol in the solution was increased. However, although consistent within itself, this result probably needs further investigation. To check the validity of the experimental data, comparisons were made with two commonly used equations in boiling heat transfer: Chen's correlation and Rohsenow's correlation. It was found that Chen's correlation usually predicted higher heat transfer rates than the experimental values. On the other hand, Rohsenow's correlation mostly predicted lower heat fluxes at De = 0.225 inch and mostly higher heat fluxes at De = 0.750 inch than the experimental data. Mean deviations for Rohsenow's equation were always lower than those from Chen's equation for both diameter sizes.
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