ENHANCED BOILING OF SATURATED LIQUIDS ON COPPER COATED HEATING TUBE SURFACE

Abstract

This thesis presents an experimental investigation related to nucleate pool boiling of saturated liquids from plain as well as copper coated mild steel heating tubes of various thicknesses at atmospheric and subatmospheric pressures. It also includes the effect of heat flux, pressure and coating thickness on boiling heat transfer coefficient of various liquids on coated tube surfaces. In addition, it prescribes a criterion for enhanced boiling of liquids on copper coated tubes. The experimentation includes saturated pool boiling of water, methanol and isopropanol from an electrically heated horizontal mild steel tube having 18 mm I.D., 32 mm O.D. and 145 mm effective length. It has been coated with copper by means of plasma spraying technique. Calibrated PTFE (polytetrafluoroethylene) coated copper-constantan thermocouples of 30 gauge have been used to measure tube surface and liquid temperatures. Thermocouples have been positioned in wall thickness of heating tube at top-, sides- and bottom- position to measure temperature of heating tube. Thermocouple probes have been located outside superheated boundary layer surrounding the tube corresponding to surface thermocouples' positions to measure liquid bulk temperature. A digital multimeter has been used to measure e.m.f. of thermocouples. Adequate precautions have been taken to ensure radial flow of heat from heating surface to liquid pool and negligible heat loss to surrounding. Experiments have been conducted by following standard operating procedure which includes thermal stabilization of tube surface, removal of dissolved air from liquid pool, attainment of steady state condition and recording of power input, pressure and liquid and surface temperatures. The reproducibility of experimental data has been checked by rotating the heating tube surface and found no change in data. Heat flux has been increased progressively from 15,670.20 to 43,151.57 W/m2 in six steps and pressure from 23.02 to 98.68 kN/m2 in five steps. The thicknesses of copper coating on mild steel tube surface have been 19, 26, 33, 41 and 60 |am. The maximum uncertainty associated with average heat transfer coefficient has Abstract been found to be ± 3.98%. Thus, experimental data of this investigation have been considered to be within acceptable tolerance limits. Data analysis has shown surface temperature to increase from bottom to side to top position of a plain heating tube for a given value of heat flux at atmospheric and subatmospheric pressures. However, liquid temperature remains uniformly constant irrespective of heat flux at a given pressure. Further, local heat transfer coefficient, at a given circumferential position, has been found to vary with heat flux according to power law relationship, hv oc q°7 for all the pressures of this investigation. Adimensional equation of the form: hy = Cv q07p032 has been developed for all the boiling liquids where C is a constant whose value depends upon circumferential position on heating tube, its surface characteristics and boiling liquid. Average value of heat transfer coefficient of a plain heating tube has been found to be related with heatflux by the relationship, h oc q07. A dimensional equation for the boiling of all the liquids: h = Ci q07p032 has been developed by the method of least squares within a maximum error of ± 7% where Ct is a constant representing surfaceliquid combination factor. Above equation has been reduced into a nondimensional form: (hVhi*) = (p/p-,)032 to overcome the difficulty in the estimation of constant, Ci owing to its improbable nature. This equation has been tested against the experimental data of [4,10,23,32,99,100] for saturated boiling of various liquids on heating surfaces of differing characteristics at various pressures and found to match excellently. Besides, it can also be used to generate the value of heat transfer coefficient for boiling of liquids at subatmospheric pressures without resorting to experimentation from the knowledge of experimentally-determined values of heat transfer coefficient at atmospheric pressure only. Above correlation can also be used for examining the consistency of experimental data of boiling heat transfer irrespective of heating surface and boiling liquid involved. The experimental data for boiling of liquids at pressures higher than one atmosphere could not be correlated by above equation due to the fact that power law relationship between heat transfer coefficient and heat flux, h <x q07 does not hold true for pressures greater than one atmosphere. Abstract Experimental data on copper coated mild steel heating tubes for saturated boiling of water, methanol and isopropanol at atmospheric and subatmospheric pressures have been analysed and found to disobey power law relationship, hocq07 which holds true usually for the boiling of liquids on a plain surface. Instead, it is governed by the relationship, h<xqn where the value of exponent, n depends upon coating thickness and boiling liquid. In fact, its value is less than 0.7. Boiling of methanol and isopropanol have resulted similar behaviour. Further, an increase in thickness of coating has lowered the value of exponent of q for all the liquids of this investigation on coated surfaces. But the value of exponent for water has always been found to remain more than that of methanol and isopropanol. The value of exponent, n for methanol and isopropanol has been found to be identical for all the thicknesses of copper coating. A dimensional correlation relating heat transfer coefficient with heat flux and pressure for saturated boiling of a liquid on a mild steel tube coated with copper of a given thickness has been obtained by least squares method in the following form: h = C2 qxpy where the value of constant, C2 and exponents, x and y depend upon liquid, heating surface characteristics and thickness of coating on heating tube surface. Comparison of boiling characteristics on a coated tube with those on an uncoated tube surface has shown significant enhancement in the value of heat transfer coefficient for the boiling of all the liquids except that of a 60 ^m thick coated heating surface. Heat transfer coefficient of a copper coated tube has increased upto a certain value and thereafter decreased with increase in thickness of coating. In fact, heat transfer coefficient of a thick coated surface becomes even lower than that of an uncoated one for some values of heat flux. As regards, the effect of pressure the behaviour is same. However, the region of heat flux for which heat transfer coefficient of a thick coated surface may be lower than that of an uncoated one at atmospheric pressure shrinks with decrease in pressure and ultimately may vanish. These observations have been consistently obtained for all the liquids -water, methanol and isopropanol. The ratio of boiling heat transfer coefficient at top position to that at bottom position of a coated tube surface has been found to be lower than that 111 Abstract of a plain heating tube surface subjected to same values of heat flux and pressure. Hence, magnitude of thermal stress in coated tubes is likely to be lower than that of plain tube. In view of above, employment of copper coating on a mild steel heating tube surface for boiling of liquids is likely to enhance heat transfer coefficient and reduce thermal stress to increase its service life. Therefore, use of coating on a surface is advantageous from thermal as well as material engineering point of view. The thermal performance, %of a coated heating tube surface has been evaluated by defining it as a ratio of heat transfer coefficient on a heating surface coated with a given thickness of copper to that of an uncoated one for the boiling of a liquid subjected to same value of heat flux and pressure. It is related to heat flux and pressure by the following dimensional equation, %= C3 qapp where constant, C3 and exponents, a and (3 depend upon heat flux, pressure, boiling liquid and thickness of coating. Based upon the logic of ^>1 for the suitability of a surface, a criterion q'ap'p < C3 has been established for enhanced boiling of a liquid on a mild steel heating tube surface coated with copper of a given thickness. This criterion can be used to determine the range of heat flux for enhanced boiling of liquids on a copper coated tube surface at a given pressure. Alternatively, it can also be used to obtain the range of pressure for enhanced boiling of liquids at a given value of heat flux.