NUCLEATE POOL BOILING OF SATURATED LIQUIDS ON HORIZONTAL CYLINDERS

Abstract

The present thesis pertains to nucleate pool boil ing heat transfer from electrically heated horizontal cylinder(s) to the pool of saturated liquids : distilled water, benzene and toluene under low heat flux values for atmospheric and subatmospheric pressures both experimen tally and theoretically. The heat flux ranged from 16.168 kW/m to 48.504 kW/m and the pressure from 29.86 kN/m2 to 98.00 kN/m2. Both the heating cylinders were of identical surface characteristics and they were kept apart horizontally one over the other at a distance of 75 mm. The experimental data were conducted for the above parameters for the heat transfer from one of the heating cylinders to saturated liquids. These data provide an equation for the calculation of average wall superheat from the knowledge of heat flux and pressure for a given surface liquid combination. Further, they also reveal that the value of wall superheat is not constant over the cir cumference of the heating cylinder. It increases from top- to side- to bottom- position of the electrically heated cylinders. A semi-theoretical analysis has led to a generalized correlation which provides a procedure to predict the values of average wall superheat for the boiling of a liquid under atmospheric and subatmospheric pressures on a given heating surface from the knowledge of the average wall superheat 11 for the boiling of same liquid on the same heating surface but at a known value of pressure and heat flux. The corre lation has successfully correlated the experimental data for the boiling of distilled water, benzene, toluene, methanol, isopropanol and carbon tetra-chloride conducted on differing heating surfaces at atmospheric and subatmospheric pressures within + 10 per cent error. Thus, it can also be profitably used for checking the consistency of experimental data of nucleate pool boiling heat transfer obtained at different values of atmospheric and subatmospheric pressures. Based on the experimental data,it has been found that the ratio of average wall superheat for the boiling of benzene to that of distilled water for a given heating surface, heat flux and pressure bears a constant value equal to 1.98. However this ratio is equal to 1.85 for the boiling of toluene and distilled water. Thus, the heat transfer data for the boiling of benzene and toluene over a heating surface for the desired values of heat flux and pressure can be predicted from the values of boiling heat transfer data for distilled water on the same heating surface and for the same values of heat flux and pressure. The experimental data, when both the heating cylinders were energized simultaneously, show that the heat transfer from the lower heating cylinder is not affected whereas that from the upper heating cylinder is influenced considerab ly by the vapour bubbles emerging out from the lower heating cylinder. As a result of it the wall superheat of the upper iii heating cylinder decreases from top- to side- to bottomposition of the cylinder indicating that the induced turbu lence in the vicinity of bottom- position becomes more than in the vicinity of the top- position. It is equally import ant to mention that for a given heat flux, pressure and boiling liquid the average wall superheat for the upper heating cylinder is less than that of the lower heating cylinder. Further, for the value of heat flux lying between 16.168 to 24.252 kW/m2, the heat transfer coefficient, h for the boiling of liquids on upper heating cylinder has been found to be a function of heat flux, q raised to the power of 0.55 whereas that of on the lower heating cylinder varies with heat flux according to the relationship, haq0,7. Furthermore, the ratio of heat transfer coefficient for the boiling of distilled water from upper heating cylinder to that of from the lower heating cylinder is 1.75 whereas it is 1.50 for the boiling of benzene and 1.30 for the boiling of toluene. In the range of heat flux from 24.252 to 28.294 kW/m the variation of heat transfer coefficient with heat flux for the boiling of liquids on upper heating cylinder is represented by a 'dcme' shaped curve, however for the lower heating cylinder it remains unaltered i.e. haq0,7. For the region of heat flux from 28.294 to 48.504 kW/m2 the heat transfer coefficient for the boiling of liquids from upper cylinder varied with heat flux raised to the power of 0.45. But, for the lower heating cylinder the variation of heat transfer coefficient with heat flux remains the same as observed for the low values of heat flux.