NUCLEATE POOL BOILING OF SATURATED LIQUIDS ON PTFE COATED HORIZONTAL HEATING TUBE

Abstract

This thesis presents an experimental as well as a semi-theoretical investigation on nucleate pool boiling of saturated liquids from heating tubes coated with polytetrafluoroethylene (PTFE) of various thicknesses at atmospheric and subatmospheric pressures. It includes the effect of heat flux, pressure and coating thickness on boiling heat transfer coefficient of various liquids and provides range of above parameters for enhanced boiling condition on PTFE coated heating tubes. A semi-theoretical analysis has led to the development of an equation for the prediction of heat flux during saturated boiling of liquids on a porous surface. Experiments have been carried out on nucleate pool boiling of distilled water, benzene and toluene from an electrically heated horizontal heating tube surface. The heating tube has been made of AISI 304 stainless steel. Its dimensions include 18 mm I.D., 32 mm O.D. and 150 mm effective length. The heating tube surface has been coated with PTFE of 850-314 and 851-214 grade green enamel following the standard procedure of Du Pont Inc. by the use of spraying technique. Calibrated PTFE coated copper-constantan thermocouples of 30 gauge have measured surface and liquid temperature at the bottom, the two sides and the top position of heating tube. A digital multimeter has been used to measure e.m.f. of thermocouples. Experiments have been conducted by the standard operating procedure which included stabilization of tube, removal of dissolved air from liquid pool and attainment of steady state condition. Heat flux have ranged from 15,824 to 42,531 W/m2 and pressure from 20.01 to 98.96 kN/m2. Five thicknesses of PTFE viz., 14, 27, 30, 45 and 50 urn have been coated over the tube surface. Adequate precautions have been taken to get reproducible and reliable data by ensuring radial flow of heat from heating tube surface to liquid pool, complete expulsion of dissolved air from liquid pool and negligible heat loss to surrounding. Maximum uncertainty associated with average heat transfer coefficient has been found to be ± 3.08%. Abstract Analysis of data of saturated boiling of liquids on a plain heating tube surface at atmospheric and subatmospheric pressures has indicated heating tube surface temperature, for a given value of heat flux, to increase from bottom to side to top position. Consequently, local value of heat transfer coefficient has also been found to vary around the circumference of tube. Local heat transfer coefficient, at a given circumferential position on heating tube surface, has been related with heat flux and pressure by equation: h^ =CM,q° 7p°32 where Cv is a constant whose value depends upon circumferential position on the heating tube, its surface characteristics and boiling liquid. Average heat transfer coefficient has been found to vary with heat flux according to the boiling power law, h <x q°7. This corroborates the findings of earlier investigators [12, 19, 20, 28, 42, 48, 56, 72, 103, 104] too. Following functional relationship among average heat transfer coefficient, heat flux and pressure has been developed by regression analysis within a maximum error of ± 7%: h =Cq° 7p°32, where constant C represents surface-liquid combination factor involved in experimentation. To overcome the difficulty in the estimation of constant C owing to its improbable nature, above expression has been modified to the following dimensionless form: (hVh-i*) = (p/p^0,32. It has been tested against experimental data of [3, 4, 28, 101, 103] for saturated boiling of various liquids on heating surfaces of differing characteristics at various pressures and found to match excellently. Above correlation can be used for the generalisation of experimental data of boiling heat transfer irrespective of heating surface and boiling liquid involved. Experimental data for saturated boiling of distilled water, benzene and toluene on heating tubes coated with PTFE of various thicknesses at atmospheric and subatmospheric pressures have been generated. Heat transfer coefficient for the boiling of a liquid, at a given pressure, has been found to vary with heat flux by the relationship, hocqn where the value of exponent, n depends upon coating thickness and boiling liquid. The value of exponent, n has always been found to be less than 0.7 which holds true usually for the boiling of liquids on an uncoated surface. Further, it has also been found to decrease with increase in coating thickness. Following functional relationship Abstract among heat transfer coefficient, heat flux and pressure has been developed for the boiling of saturated liquids on a tube coated with PTFE of a given thickness: h = C'qupv where C is a constant whose value depends upon heating tube surface characteristics and boiling liquids. The values of exponents, u and v have been found to be same for boiling of benzene and toluene on PTFE coated tube surface. Expressions for constant C and exponents u and v have been developed in terms of coating thickness and accordingly a generalised correlation of heat transfer coefficient for saturated boiling of a liquid on PTFE coated tube surfaces has been developed. Comparison of boiling characteristics on a coated tube with those on an uncoated tube surface has shown significant enhancement in the values of heat transfer coefficient for the boiling of distilled water. However, enhancement has been found to depend upon heat flux, pressure and coating thickness. In fact, the range of heat flux for enhanced boiling has been found to shrink with the increase of coating thickness at a given pressure. Lowering pressure has adversely affected the boiling process and thereby shortened the range of heat flux for enhanced boiling of distilled water. In case of boiling of benzene and toluene on PTFE coated tubes at atmospheric and subatmospheric pressures, no enhancement has been observed. Thus, coating of PTFE on a plain tube surface seems to favour enhancement for boiling of distilled water but not for benzene and toluene. Performance of a PTFE coated tube surface has been evaluated in terms of effectiveness factor, C, which is defined as the ratio of heat transfer coefficient on a coated tube with that on an uncoated one for the same values of heat flux and pressure. Expression for C, for boiling of a liquid on a heating tube coated with a given thickness of PTFE has been developed in terms of heat flux and pressure. It is in the following form: £,= kqaqp where the values of constant k and exponents a and p depend upon boiling liquid. Criteria for enhanced boiling of liquids on PTFE coated tube surfaces have been established by considering the condition of C, > 1 in above equation. This is as follows:q"ap~p <k. This criterion can be used to determine the range of heat flux for enhanced boiling of liquids on a PTFE coated tube surface at a given in Abstract pressure. Alternatively, it can also be used to obtain the range of pressure for enhanced boiling of liquids to occur at a given value of heat flux. Considering boiling of saturated liquids on a porous surface to be analogous to that on a plain surface, heat flux has been taken equal to the summation of heat flux due to transient heat conduction from heating surface to liquid, qT; latent heat transport for the growth of vapour bubbles, qx and exterior heat transport from the region unaffected by vapour bubble activity, qex.. The contribution of qT on a porous surface has been found to be negligibly smaller than that due to other heat transfer mechanisms. Contribution due to latent heat transport mechanism has been evaluated by considering countercurrent movement of liquid and vapour in interconnecting channels formed on porous surface and necessary conditions for boiling to occur. The analysis has led to the fact that total heat flux can be governed by latent heat transport mechanism only if boiling occurs on a highly porous surface where the region unaffected by bubble activity is small. Nucleation site density of a porous surface has been computed by using experimental data of Tehvir et al. [97] on the basis of principle of similarity. Following is the resultant expression for the prediction of total heat flux during saturated boiling of liquids on a highly porous surface: „„ i2 „0.18j 0.38 .0.14 q =1.82X10"97 Pf4 ,AT£ d° ;a (Pi-PvWvv+v.) 8011 Above equation has been tested against the experimental data of Tehvir et al. [97] for boiling of R-113 on different porous surfaces (aluminium and copper rod coated by plasma spray), of Tehvir [96] for boiling of R-113 on porous aluminium surface, of Afgan et al. [2] for boiling of R-113, distilled water and ethanol on stainless steel tube coated with chromium-nickel and titanium particles, of Kajikawa et al. [58] for boiling of R-11 on copper porous surfaces, of Nishikawa et al. [79] for boiling of R-11 on copper porous surfaces and of present investigation for boiling of distilled water, benzene and toluene on PTFE coated surfaces and found to match satisfactorily within a maximum error of ± 30%. Since, this equation is free from parameters whose measurement is improbable, it can be employed to determine heat flux for the boiling of liquids irrespective of heating surface involved in the study.