STUDIES OF NUCLEATE POOL BOILING HEAT TRANSFER FROM HORIZONTAL FINNED TUBES TO SATURATED LIQUIDS

Abstract

This represents an experimental investigation dealing with the nucleate poo boiling heat transfer from a horizontal heating tube to the pool of saturate( liquids under atmospheric and subatmospheric pressures. The heating tubes employed are both plain and finned having wolverim type low integral fins of 8, 12, 16, 19 and 24 FPI. All the tubes are of bras: having inside diameter of 18 mm, outside diameter of 34 mm with the effective length of 161 mm and are manufactured from the same lot of material in order to ensure similar surface characteristics of each tube. The liquid investigated are saturated distilled water and isopropanol uncle' pressures ranging from 26.66 kN/m2 to 97.99 kN/m2. The heat rate iE varied in steps from 240 W to 640 W, which correspond to heat flux of 14,815 W/nf to 39,506 W/m2. The present investigation involves the measurement of wall- and liquid-temperatures. The wall temperature of the heating tube is measured along the circumference of the tube at the top-, the side- and the bottom -positions by the copper constantan thermocouples placed in the holes drilled in the wall thickness of the tube. The liquid bulk temperature is also measured by the copper constantan thermocouples. For this, they are positioned in the bulk of liquid pool. Before conducting the experiments the heating tube surface is thermally stabilized. For this, about 72 hours of submergence and 48 hours of boiling are required. In the case of boiling of liquids on the plain tube under atmospheric and subatmospheric pressures, it has been established that the average boiling heat transfer coefficient changes with heat flux raised to the power of 0.7 and pressure of 0.32. As regards the surface-liquid combination factor, it is independent of pressure, unlike the case when the boiling takes place in the pool of saturated liquids under high pressures. A generalized equation has been obtained to predict the values of boiling heat transfer coefficient from a horizontal plain tube to a boiling liquid under subatmospheric pressures from the knowledge of its value on the same Further, a procedure has been developed to calculate heat transfer coefficient for the boiling of saturated isopropanol from the knowledge of boiling heat transfer coefficient of distilled water from a given heating tube. As a matter of fact, the ratio of boiling heat transfer coefficient of distilled water to that of isopropanol for a given condition of heat flux, pressure and heating tube is 1.10. As regards the boiling of saturated liquids on horizontal finned tubes of differing fin spacings of 8, 12, 16, 19 and 24 FPI, the heat transfer coefficient varies with heat flux raised to the power of 0.81 and pressure of 0.27. In this respect the observation is in contrast to that for the boiling of liquids on plain tube. The surface-liquid combination factor, is found to be independent of pressure as for the boiling of liquids on plain tube. Further, the heat transfer coefficient increases with the FPI and it is found to be maximum corresponding to the FPI of 19, indicating that the fins, still act independent of each other. But the further increase in FPI i.e. for 24 FPI, the interference between the fins seems to set in. Due to this; the heat transfer coefficient corresponding to FPI of 24 decreases. It is also established that the heat transfer coefficient for tubes of 8, 12, 16. and 24 FPI bears a constant ratio with that for the tube of 19 FPI. The values of these respective ratios are 0.692, 0.720, 0.781 and 0.958. As regards the thermal performance of finned tubes vis-a-vis that of plain tube, the boiling heat transfer coefficients from the finned tubes investigated are found to be always greater than those from plain tube. The present investigation has succeeded to recommend a correlation in terms of heat flux and physico-thermal properties of boiling liquids, capable to predict the wall superheats of the wolverine type low integral finned tube when boiling of saturated liquids, viz. distilled water and isopropanol takes place on them.