ENHANCED BOILING OF WATER-METHANOL MIXTURES ON COATED SURFACES

Abstract

This thesis presents an experimental investigation on nucleate pool boiling of methanol, distilled water and their binary mixtures on plain as well as copper coated stainless steel tubes at atmospheric and subatmospheric pressures. Basically, it deals with the effect of operating parameters; viz. heat flux, pressure and composition of mixture on heat transfer coefficient for the boiling of methanol, distilled water, and their mixtures on a stainless steel heating tube surface. Further, it also includes the effect of coating thickness along with the other parameters for the boiling of these liquids on stainless steel heating tubes surfaces coated with copper. In addition, thermal effectiveness of coated tubes has also been studied to obtain the range of heat flux and pressure for which enhanced boiling of liquids and their binary mixtures may occur. The experimentation includes saturated boiling of methanol, distilled water and their binary mixtures on an electrically heated horizontal plain as well as copper coated stainless steel heating tube surfaces. The heating tube has been made of AISI 304 stainless steel cylinder having 18 mm I.D., 31.94 mm O.D. and 150 mm effective length. It is heated by placing a laboratory made electric heater inside it. Wall and liquid temperatures were measured by polytetraflouroethylene (PTFE) coated 30 gauge copper-constantan calibrated thermocouples. The thermocouples are placed inside four holes drilled at a pitch circle diameter of 25 mm in the wall thickness of heating tube for measurement of surface temperature. Similarly, thermocouple probes are placed in liquid pool corresponding to wall thermocouple positions in heating tube for the measurement of liquid temperature. A digital multi-meter measures e.m.f. of thermocouples. The compositions of binary liquid mixtures and those of boiling liquid and vapor were measured by using HPLC system. A Novel Pack, C18 column of size 3.9 mm x 150 mm was used to measure the concentration of methanol in the binary mixture. Power input to heater is increased gradually from 240 Wto 640 Win six equal steps and pressure from Abstract 44.40 kN/m2 to 97.71 kN/m2 in five steps. Three thicknesses of copper coating; viz. 22, 43 and 67 |im have been employed over plain heating tube by electroplating technique. The maximum uncertainty associated with the measured value of average heat transfer coefficient is of the order ±1.13%. Experimental data for saturated boiling of distilled water on plain and copper coated tubes of various thicknesses at atmospheric and subatmospheric pressures have been processed to obtain local as well as average heat transfer coefficient. Analysis of experimental data has shown surface temperature to increase from bottom to side to top position of heating tube for a given value of heat flux at atmospheric and subatmospheric pressures. However, liquid temperature remains uniformly constant at all values of heat flux for a given pressure. Further, at a given value of heat flux, local heat transfer coefficient increases from top to side to bottom position on a heating tube surface and has been found to vary with heat flux according to power law relationship, hv <x q07. Above observations are consistent for all the liquids of this investigation. Average value of heat transfer coefficient of a plain heating tube has also been found to be related with heat flux by the power law relationship h oc q07 for all the pressures of this investigation. This corroborates the findings of earlier investigators [4, 18, 19, 33, 34, 39, 56, 62, 83, 92, 105, 149, 150]. Adimensional equation, h=C1q07p032 for saturated boiling of liquids has been obtained by regression analysis within a maximum error of ±5%, where, Ci is a constant whose value depends up on the type of boiling liquid and heating surface characteristics. To overcome the difficulty in the estimation of constant, Ci owing to its improbable nature, above expression has been modified to the following non-dimensional form: (jn'/h])=(p/Pi)032. It has been tested against experimental data of various investigators [4, 8, 16, 33, 39, 105, 106, 148, 149] for saturated boiling of several others liquids on heating surfaces with differing characteristics at various pressures, and found to correlate them excellently. The experimental data for the pool boiling of methanol-distilled water binary mixtures at atmospheric and subatmospheric pressure showed Abstract analogous boiling characteristic as that of pure liquids. The functional relationship of heat transfer coefficient with heat flux and pressure is same as observed for liquids and therefore, a dimensional equation, h=C2q07p032 for the boiling of a binary mixture, for atmospheric and subatmospheric pressures, has been developed by regression analysis. Further, above equation has also been reduced into a non-dimensional form: (h"/h',)=(p/pj32, alike pure liquids, and found to match the experimental data of Pandey [102] within an error ranging from -12 to +20%. Furthermore, a reduction in heat transfer coefficient has been observed for the boiling of methanol-distilled water binary mixtures than the interpolated values of heat transfer coefficients of pure liquids present in the mixture. This has been due to the occurrence of mass transfer along with heat transfer in the process. Thus, an equation h/hid =[1 +|y-x|(a/D)05]<08 °2) has been developed for the prediction of heat transfer coefficient of a binary mixture. This equation correlates all the experimental data of this investigation within an error of ±15% as well as those predicted by correlations of [22, 50, 60, 76, 114, 121, 132, 134] within an average error of ±25%. Analysis of experimental data reveals that coating of copper on a stainless steel tube enhances heat transfer coefficient for the boiling of distilled water at atmospheric and subatmospheric pressures. In fact, enhancement is found to depend upon the thickness of coating. It has also been found that for a given value of heat flux, heat transfer coefficient increases with increase in coating thickness up to a certain value and thereafter decreases. However, increase in heat transfer coefficient is not proportional to increase in coating thickness. A functional relationship amongst heat transfer coefficient, heat flux and pressure has been established as h = C3qrps, where the value of constant, C3 and exponents, r and s depend upon heating surface characteristics and thickness of coating on heating tube surface. Further, enhancement on a 43 urn thick coated tube surface is found to be more than any other coated surface of this investigation. Hence, a 43 urn thick coated tube surface has been selected to conduct experiments for the boiling of methanol and various compositions of methanol-distilled water binary IV Abstract mixtures. Boiling of methanol and the binary mixtures on a 43 urn thick coated tube at atmospheric and subatmospheric pressures has also shown similar behavior as observed on a plain tube. However, increase in magnitude of heat transfer coefficient has differed due to difference in physico-thermal properties of methanol, distilled water and their binary mixtures. A dimensional relationship, h = C4q060p039, correlating heat transfer coefficient, heat flux and pressure is of the same form as obtained for liquids, where constant, C4 depends upon the composition of methanol in the mixture and heating surface characteristics. In addition, it has been found that application of copper coating on stainless steel heating tube surface does not alter the methanol turnaround concentration. Therefore, the correlation, h/hid =[l +|y-x|(a/D)05]"(08x+02) developed for boiling of methanol-distilled water binary mixtures on a plain tube is also valid for the boiling of those on a 43 urn thick copper coated tube as well. This correlation has been compared against the experimental data for the boiling of methanol-distilled water mixtures on a 43 urn thick copper coated tube and found to match within an error of ±20%. Performance of a coated heating tube surface has been evaluated in terms of thermal effectiveness, C, which is defined as a ratio of heat transfer coefficient on a heating surface coated with a given thickness of copper to that of a plain one for the boiling of a liquid subjected to same value of heat flux and pressure. It has been related to heat flux and pressure by the following equation, <^ = kqapp, where constant, k and exponents, a and p depend upon heat flux, pressure, boiling liquid and thickness of coating. Using the condition, C, >1, a criterion q"ap"p <k has been established for enhanced boiling of a liquid on a stainless steel heating tube surface coated with copper of a given thickness. This criterion can also be used to determine the range of heat flux for enhanced boiling of liquids on a copper coated stainless steel 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. This criterion is also applicable to enhanced boiling of methanol-distilled water mixtures on a stainless steel heating tube surface coated with copper of a given thickness.