NUCLEATE POOL BOILING OF SATURATED LIQUIDS ON HORIZONTAL TUBE SURFACES COATED WITH TEFLON

Abstract

The present investigation deals with an experimental investigation related to pool boiling heat transfer from uncoated as well as teflon coated heating tube surface to saturated liquids at atmospheric and subatmospheric pressures. It also includes a semi- theoretical analysis of heat transfer in various stages of ebullition cycle of boiling to obtain an equation for the prediction of heat flux. This investigation includes experiments on boiling heat transfer from a heating tube laid horizontally in a pool of saturated distilled water, methanol, ethanol and isopropanol. The heating tube was a brass cylinder of 18 mm ID, 32 mm OD and 161 mm effective length. It was heated by an home made cartridge heater. Calibrated teflon coated copper-constantan thermocouples of 30 gauge were used to measure surface and liquid temperature. They were-located at top, side, bottom and side positions of the heating tube surface. Du Pont teflon green enamel 850-314 / 851-214 has been used for coating the outer surface of plain heating tube according to guidelines of Du Pont for teflon (ready to coat pack ) enamel. The experimental parameters included heat flux from 15288.393 W/m2 to 43680.062 W/m2, and pressure from 26.56 kN/m2 to 99.31 kN/m2. The thicknesses of teflon coating were 21, 39 and 51 microns. Data analysis indicated heat loss to surrounding to be negligible and the condition of radial flow of heat from the heater to the pool ofliquid to exist. The maximum value of uncertainty associated with average heat transfer coefficient has been found to be ± 3.848%. Thus experimental data of present investigation were considered to be reproducible and reliable. Experimental data for nucleate pool boiling of distilled water, methanol, ethanol and isopropanol from a horizontal uncoated brass tube surface have been found to obey well established seven-tenth power law of boiling heat transfer correlating heat transfer coefficient with heat flux. These data ABSTRACT when compared with those of earlier investigators [24, 36, 78, 137, 172, 201] showed a similar trend but formed a distinct group owing to differing surface liquid combination factor involved in each investigation. Further, a dimensional equation correlating heat transfer coefficient with heat flux and pressure through surface-liquid combination factor has been obtained by using regression analysis. From the analysis of experimental data of teflon coated surfaces, it has been found that boiling power law of the form h oc q" holds good, where the value of exponent, n depends upon the thickness of coating, pressure, heat flux and boiling liquid. In fact, its value has been found to be different than 0.7 usually observed for boiling of liquids from uncoated surfaces. In case of distilled water it has been found to be lower than 0.7 for high values of heat flux and pressure and greater than 0.7 for low values of heat flux and pressure. However, in case of alcohols value of exponent, n has been less than 0.7 irrespective of pressure and heat flux. Dimensional equations of the form h = C qr P5 have been developed for boiling of distilled water and alcohols over a given thickness of coating by using regression analysis. In this equation, C is a constant whose value depends on coating thickness, boiling liquid and heating surface characteristics. Teflon coated heating surface data when compared with those of uncoated surface indicated boiling to be greatly affected by the coating. As a matter of fact, 21 microns thick teflon coated surface has been found to offer the highest heat transfer coefficient out of all the three thicknesses investigated for boiling of distilled water, methanol and ethanol irrespective of heat flux and pressure. But, in case of isopropanol, this behavior holds true only at atmospheric pressure. Further, variation of heat transfer coefficient with thickness of teflon coating has been observed to depend upon the boiling liquid, pressure and the range of heat flux. In fact, pressure and heat flux contribute in such a way that coating of teflon on plain heating tube improves heat transfer coefficient for boiling of liquids at low values of heat flux and pressure. Hence, use of teflon coated heating surface is recommended for ABSTRACT boiling heat transfer equipment especially in low pressure applications such as food processing, juice concentrators, etc. for reducing their size and thereby cost. Using the above results on teflon coated heating tube, teflon coated surface has been evaluated in terms of its effectiveness factor which has been defined as the ratio of heat transfer coefficient of coated surface to that of uncoated one at the same operating conditions of heat flux and pressure. Further analysis has resulted in the establishment of various criteria for the determination of range of heat flux and pressure for the enhancement of boiling heat transfer coefficient by the application of teflon coating of given thickness on plain heating tube. Asemi-theoretical analysis has been carried out to describe the process of heat transfer in waiting as well as growth period of the ebullition cycle. Using experimental data of Siegel &Keshok [177] for the growth of vapor bubble in saturated boiling of distilled'water at atmospheric pressure, following empirical equation has been obtained for instantaneous diameter of the vapor bubble during the growth period. Ja D=!664—V^ (1) Eq.(l) has been found to correlate the experimental data for boiling ofdistilled water due to Siegel & Keshok [177] at 101.3 kN/m2 pressure, Saini et. al. [163] at 101.3 kN/m2 pressure , Plesset & Zwick [153] at 1013 kN/m2 pressure, Cole &Shulman [44] at 12.93 kN/m2 pressure, for boiling of carbon tetrachloride due to Cole & Shulman[44] at 18.21 kN/m2 pressure and Akiyama [cf 163] at 40.11 kN/m2 pressure and for boiling of n-pentane at 101.3 kN/m2 pressure, toluene at 6.33 kN/m2 pressure and acetone at 29.30 kN/m2 pressure due to Cole &Shulman [44] within a maximum error of ±20%. in ABSTRACT Using the above expression thickness of evaporating microlayer existing at the base of vapor bubble and then heat transfer rate per bubble during growth period has been calculated. Considering natural convection from portion unaffected by the bubble activities following expression for the computation of heat flux in isolated bubble region for nucleate pool boiling of liquids at atmospheric and subatmospheric pressures has been obtained ; Total Boiling 2.554 k,ATwJa (133.3 Pr°.' 0.0267 y \\i Pr0J Ja P J v 133.3 A f a g(Pl-pv), a PA g(pi-Pv). 0.5 + N A + hll-A^ —|AT_ N A (2) Predictions from this equation have been found to agree well with the experimental values due to Wang & Dhir[201] for boiling of distilled water, Gaertner[68] for boiling of distilled water, Kurihara & Myers[l 10] for the boiling of distilled water, acetone and carbon tetrachloride and Kirby & Westwater[105] for the boiling of carbon tetrachloride within ±20% . Hence, this semi-theoretical analysis has established that in the regime of isolated bubble, all the three heat transfer mechanisms - transient conduction from heater to surrounding liquid, -microlayer evaporation and -natural convection occur simultaneously during the process of boiling of liquids from heating surface submerged in the pool of licjuids.