STUDIES OF BOILING HEAT TRANSFER FROM PLAIN AND INTEGRAL-FIN TUBES TO BINARY AND TERNARY LIQUID MIXTURES

Abstract

The present investigation deals with experimental and theoretical studies related to nucleate pool boiling heat transfer of single component liquids and their binary and ternary mixtures on a single horizontal plain as well as integral-fin tubes at atmospheric pressure with a view to understand their boiling characteristics. The single component liquids investigated are acetone, isopropanol and water. As regards the binary liquid mixtures, they include aqueous mixtures namely; acetone-water and isopropanol-water and hydrocarbon mixtures of acetoneisopropanol, whereas ternary liquid mixtures consisting of acetone, isopropanol and water. In fact, the experimental data have been obtained for boiling of 22 binary and 12 ternary compositions as well as their pure components on three horizontal heating tubes one by one. One of the tubes is of plain surface and the remaining ones are of integral-fin surfaces of 1mm height trapezoidal shape, but of different fin densities, of 748 and 1024 fpm. Each heating tube has an axial hole of 18 mm diameter for its cartridge electric heater to supply heat for boiling of liquid pool. The heated length of each tube is 108.0 mm, and the outer/ envelope diameter of plain/integral-fin tube is 31.2 mm. The experimental set-up has been carefully designed, fabricated and commissioned to ensure reliable and accurate experimental data. For this, surface area of the condenser, liquid pool height over the heating tube, placement of wall and liquid thermocouples, deaeration oftest liquid, thermal stabilization ofheating tube surface, homogeneity of heating surface, and various precautions during experimentation have been the main considerations. The experimental data were obtained for heat fluxes varying from 57,624 W/m2 to 10,391 W/m2, in decreasing order at atmospheric pressure. While carrying out experiments with liquid mixtures, the composition of mixtures were determined before and after a given experimental run. It was found that two values were almost the same within ±0.01 mole fraction. Analysis of experimental data shows that heat transfer coefficients of binary and ternary liquid mixtures depend upon their compositions in addition to heat flux. The heat transfer coefficients for boiling of single component liquids and also of their binary and ternary liquid mixtures on plain tube get enhanced when the surface of tube is finned and the enhancement is more pronounced when fin density is 1024 fpm as compared to 748 fpm. Further, the data obtained and reported as part ofthis investigation provide valuable and unique information about nucleate pool boiling heat transfer coefficients. Itis revealed that addition of acomponent leads to degradation in boiling heat transfer coefficient of mixtures on plain as well as integral-fin tubes, having approximately the similar trend. This degradation, expressed as degradation factor, (h/h.d), happens to be a function of IY-XI for agiven binary/ternary liquid mixture, irrespective of heating surface. The heat transfer coefficient prediction accuracies of the existing correlations have been evaluated by employing the earlier and present experimental data of liquid mixtures boiling on plain tube. It is noted that correlation proposed by Schlunder (1982) provides the best predictions of heat transfer coefficient, if the Stephan-Abdelsalam correlation is employed to calculate ideal heat transfer coefficient, h.d. However, predictions due to Palen- Small (1964) and Thome-Shakir (1987) are also reasonably well. A new correlation for the prediction of nucleate pool boiling heat transfer coefficients of binary mixtures on horizontal plain tube, based on the postulated mechanism of heat and mass diffusion to growing vapour bubbles on heating surface, has been derived. It is important to emphasize that this correlation is simple and convenient since it is free from such empirical constants, whose determination, as is noted in available correlations, generally involves a tedious exercise. Thus, this new correlation is unique. It is also important to underline that this correlation accounts for the variation in physico-thermal properties ofthe mixtures through die Stephan-Abdelsalam correlation which is employed to estimate ideal heat transfer coefficient, hjd. The influence of the phase equilibrium on mixture boiling due to preferential evaporation of the more volatile component is taken care of through the quantity known as 'boiling range'. An excellent agreement is noted between experimental and predicted heat transfer coefficients from the new correlation. For this, the experimental data of present and earlier investigations available in open literature have been employed. Further, it is also established that the derived correlation predicts better than the best of the existing correlations. Another new correlation has also been derived for the boiling of mixtures on integral-fin tubes by incorporating surface correction factor. This correlation also correlates reasonably well the experimental data of the present investigation and those from open literature. It is important to note that the derived correlations in the present investigation can be successfully extended to predict boiling heat transfer coefficient of ternary liquid mixtures with a good agreement between experimental and predicted values. u