NUCLEATE POOL BOILING OF LIQUIDS AND THEIR MIXTURES ON COATED HORIZONTAL TUBES

Abstract

The present investigation pertains to an experimental study on nucleate pool boiling of liquid and their binary and ternary mixtures on plain as well copper coated brass heating tubes at atmospheric and subatmospheric pressures. Basically, it deals with the effect of operating variable, namely heat flux, pressure and composition of mixture on heat transfer coefficient for the boiling of iso-propanol, methanol and distilled water and their binary and ternary mixture on a brass heating tube surface. Further, it also includes the effect of coating thickness along with other parameter for boiling of these liquids on brass heating tube surface coated with copper. Finally, a semi-empirical correlation for calculation of heat transfer coefficient of ternary mixture which is free from surface liquid combination factor has been developed. Experiment has been carried out for saturated boiling of iso-propanol, methanol and distilled water and their binary and ternary mixtures on an electrically heated horizontal plain as well as copper coated brass heating tube surfaces. The heating tube is a brass cylinder having 18mm I.D., 30.02mm O.D. and 150 mm effective length it is heated by placing a laboratory made electric heater inside its wall and liquid temperature are measured by poly-tetrafluoro-ethylene (PTFE) coated 30 gauge copper-constantan calibrated thermocouples. The thermocouples are placed inside four holes drilled at a pitch circle diameter of 25mm 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 multimeter measures e.m.f of thermocouples. The composition of binary and ternary liquids mixtures and those of boiling liquid and vapour are measured by using HPLC system. A Novel Pack, C18 column of size 3.9 ×150 mm is used to measure the concentration of methanol and iso-propanol in the binary and ternary mixture. Power input to heater is increased gradually from 220 W- 500W in six steps and pressure from 45.40 kN/m2 to 97.71 kN/m2 Experimental data for saturated boiling of distilled water on a plain as well as copper coated brass heating tube of various thicknesses at atmospheric and subatmospheric pressures , three thickness of copper coating namely 15, 25 and 35 μm have been employed over a plain brass heating tube. The maximum uncertainity associate with the measured value of average heat transfer coefficient is of the order of ±1.12% have been processed to obtain local as well as average heat transfer coefficient. Analysis of these experiment data show that surface temperature increases from bottom to side, side to top 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 given pressure. Further, the local heat transfer coefficient increases from top to side, side to bottom position irrespective of heat flux and the variation in heat transfer coefficient is presented by h q0.7 φα . These observations are consistent for all the liquids of this investigation. Furthermore, average value of heat transfer coefficient of an uncoated heating tube vary according to the power law hα q0.7 at subatmospheric pressure of this investigation this in corroborated the finds of other inve stigators [A4, B6, B11, B12, C22, H6, K11, Y8] . A dimensional equation; 0.7 0.32 1 h = C q p for the boiling of distilled water, methanol, iso-propanol on uncoated brass tube has been develop by regression analysis with a maximum error of ±8%, where C1 * * 0.32 1 1 (h / h ) = (P / P) is a constant whose values depends on the types of boiling liquid and heating surface characteristics. This dimensional equation has been modified to the following non dimensional form: , it has been tested against data reported by various investigator [A2, B12, C12, C22, P8, V2, V12] for saturated boiling of other liquids on heating surfaces with different characteristics at various pressure and found to correlate them excellently with in error of -11 to +9% Experimental data for poo l boiling of methanol-distilled water, iso-propanol binary mixture at atmospheric and subatmospheric pressures on uncoated tube resulted in analogous behavior as that of pure liquids. The functional relationship of heat transfer coefficient with heat flux and pressures is same as observed for liquids and therefore a dimensional equation 0.7 0.32 2 h = C q p for the boiling of binary mixture at atmospheric and subatmospheric pressures, has been developed. Further, this equation has also been reduced into non dimensional form * * 0.32 1 1 (h / h ) = (P / P) alike, pure liquids and found to match the experimental data Pandey [P3], Alam [A2] with in error of -11 to +21% Experimental data for poo l boiling of distilled water-methanol-iso-propanol a ternary mixtures at atmospheric and subatmospheric pressure revealed similar boiling characteristics as that of pure liquids and their binary mixture. A functional relationship of heat transfer coefficient with heat flux and pressure has been developed by regression analysis which is a dimensional equation, 0.67 0.33 3 h = C q p . Further, above equation has been reduced into a non dimensional form * * 0.33 (h / h1 ) = (P / P1) similar to, pure and binary mixture and found to match the experimental data of Nahar and Naess [N1] with in an error ranging from -12% to +9.5%. A reduction in heat transfer coe fficient has been obs erved for bo iling of binary and ternary mixtures as compared to the weighted mean values of heat transfer coefficient of pure liquids present in the mixture. This behavior has been due to the occurrence of simultaneous heat and mass transfer in the process. Hence, and (0.73 1 0.36) 1 1 1 1 2 2 2 2 ( / ) ( / ) [1 | | / | | / ] x id id h h = ΔT ΔT = + y − x α D + y − x α D − + has been developed for the prediction of heat transfer coefficient of a ternary mixture. This equation correlates all the experimental data of this investigation within an error ±18% as well those obtained by other investigators [C22, F1, H8, S3, T5, T7] with an error of ±25%. Analysis of experimental data reveals that coating of copper on a brass tube enhances heat transfer for boiling distilled water at atmospheric and subatmospheric pressures. In fact enhancement is function of 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 and thereafter decreases. However, increase in heat transfer coefficient is not propo rtional to increase in coating thickness. A functional relationship amongst heat transfer coefficient, heat flux and pressure has been developed as 4 h = C qr ps , where the values of constant C4 4 h = C qv pw and exponents (r) and (s) depend upon heating surface characteristics and thickness of coating on brass heating tube surface. Further, enhancement on a 25 μm copper coated brass tube surface has been found to be maximum to the tune of 55% more than that of uncoated brass tube. Hence, a 25 μm thick copper coated tube has been selected to conduct experiment for the boiling of methanol, iso-propanol and their binary and ternary mixtures with distilled water. Boiling of iso-propanol, methanol on 25 μm thick coated heating tube resulted in same features as that of pure liquids. A correlation has been de velope d by regression analys is in dimensional form , where constant C4 0.57 0.36 5 h = C q p and (v) and (w) depend upon the boiling liquids. Further, boiling of methanol-distilled water and iso-propanol-distilled water, binary mixture on 25 μm thick copper coated tube at atmospheric and subatmospheric pressures, has also shown analogous behavior as observed on an uncoated tube. However, increase in magnitude of heat transfer coe fficient has changed due to difference in physico-thermal properties of methanol, iso-propanol, distilled water and their binary mixture. A dimensional relationship correlating heat transfer coefficient, heat flux and pressure, is of the same form as obtained for liquid, where constant C5 0.59 0.36 h = C6q p depend upo n the compos ition in the mixture and heating surface characteristics. Furthermore, boiling o f distilled water-methanoliso- propanol ternary mixture on a 25 μm thick coated tube at atmospheric and subatmospheric pressure has shown similar trend as observed on a plain tube. A dimensional relationship correlating heat transfer coefficient, heat flux and pressure is of the same form as obtain for liquids and their binary mixture, where constant C6 depend upon the concentration of the highest component in the ternary mixture and the surface characteristics. In addition, it has been found that application of copper coating on brass heating tube surface does not change the highe st compos ition (methanol) turnaround concentration. Hence, the correlation has been (0.73 1 0.36) 1 1 1 1 2 2 2 2 ( / ) ( / ) [1 | | / | | / ] x id id h h = ΔT ΔT = + y − x α D + y − x α D − + developed for boiling of ternary mixture on a plain tube is also valid for boiling of those on a 25 μm thick copper coated heating tube as well. Further, this correlation has been compared against the experimental data for the boiling of ternary mixtures of this investigation on a 25μm thick copper coated tube and found to match with in an error of ± 18% and 20% at atmospheric and subatmospheric pressures, respectively.