A STUDY OF NUCLEATE BOILING OF SATURATED LIQUIDS ON COPPER COATED HEATING TUBE SURFACE

Abstract

This thesis presents an experimental investigation on nucleate pool boiling of saturated liquids over a copper heating tube surface coated with various thicknesses of copper at atmospheric and sub-atmospheric pressures. Basically, it includes the effect of operating parameters - heat flux, pressure and thickness of coating on boiling heat transfer coefficient of various liquids on coated tube surfaces. In addition, it also points out enhancement in boiling heat transfer coefficient due to copper coating over an uncoated heating tube, and prescribes criteria for enhanced boiling of liquids on coated tubes. The experimentation includes saturated boiling of liquids of widely varying physico-thermal properties on an electrically heated horizontal copper tube coated with copper of various thicknesses. The tube dimensions are18 mm I.D., 31.85 mm. O. D. and 160 mm effective length. It has been coated with copper by means of wire flame spraying technique. Calibrated PTFE coated copper-constantan thermocouples of 30 gauges have been used to measure temperature of tube surface and liquid pool temperature. Thermocouples have been mounted circumferentially in holes drilled in tube wall equispaced at 90° at top, bottom and two sides position of heating tube, to measure temperature therein. Liquid bulk temperature has been measured by placing thermocouple probes outside superheated boundary layer surrounding the heating tube corresponding to surface thermocouples' positions. 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 to minimize heat loss to surrounding. Standard operating procedure has been followed to conduct experiments. It includes thermal stabilization of tube surface, deaeration of liquid under investigation, attainment of steady state condition and recording of power input, pressure and liquid and surface temperatures. Homogeneity of heating tube surface has been checked by rotating the tube through 90° and noting variation in e.m.f. values of a wall thermocouple. No change in a wall thermocouple's e.m.f. value was observed. Abstract This clearly indicates surface to be homogeneous. The operating parameters include heat flux, pressure, thickness of coating and liquid. The heat flux ranged from 16,228.70 W/m2 to 41,324.90 W/m2 and pressure from 43.60 kN/m2 to 98.15 kN/m2. Four thicknesses of copper coating namely; 29, 63, 85 and 118 \im have been employed over uncoated heating tube. Distilled water, methanol and isopropanol are saturated liquids used in this investigation. Heat transfer coefficient for boiling of liquids on coated tube surface is based on substrate temperature. In other words, temperature drop between substrate surface and coated tube surface has not been included in the computing of heat transfer coefficient. Reproducibility of data was ensured by repeating experiments under same operating conditions and noting variation in thermocouples' e.m.f. values, if any. No change in e.m.f. values was noticed. The maximum uncertainty associated with average heat transfer coefficient has been found to be ±1.05%. Analysis of experimental data on an uncoated heating tube has clearly shown that surface temperature increases from bottom to side to top position for a given value of heat flux at atmospheric and sub-atmospheric 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 heating tube surface. It has also been found to vary with heat flux according to power law relationship, h^ a q07 for all the pressures of this investigation. Adimensional equation of the form h^ =Csf(a +bcosip)q°-7p0-32 has been developed for saturated boiling of all the liquids. The constant, Csf has been found to depend upon surface characteristics and boiling liquid. Average value of heat transfer coefficient (hereafter referred as heat transfer coefficient) of uncoated heating tube has also been found to be related with heat flux by the power law relationship h^, <* q0-7. This corroborates findings of earlier investigators [19, 20, 31, 32, 43, 64, 72, 83, 95, 109, 152, 153]. A dimensional equation, h = C1 q°-7p0-32 f0r saturated boiling of liquids on un uncoated in . Abstract heating tube has been developed by the method of least squares within a maximum error of ± 7%. Where, C, is a constant whose value depends up on the type of boiling liquid and heating surface characteristics. Above equation has been reduced into a dimensionless form: NuB =Csf(PeB)07(Kp)07(Pr)~067 and tested against various correlat ions of earlier investigators [3, 44, 58, 78, 93, 96, 97, 106, 109, 146]. As a result, a substantial disagreement amongst all has been noted. Possible reason for this discrepancy lies in differing values of surface-liquid combinations used by various investigators. Thus, it has been concluded that boiling heat transfer correlation developed by an investigator can not be used to correlate experimental data of other investigators. Data analysis for saturated boiling of water, methanol, and isopropanol on copper tube coated with copper of various thicknesses at atmospheric and subatmospheric pressures has been found to be governed by the relationship, h oc qm, where the value of exponent, m depends upon coating thickness and boiling liquid. In fact, its value is less than 0.7 which holds true usually for boiling of liquids on a uncoated surface. Further, an increase in thickness of coating has been found to lower the value of exponent of q for all the liquids of this investigation. An important feature is that both the alcohols - methanol and isopropanol have behaved alike as they are found to be governed by same power law relationship between h and q. However, the value of exponent of q for an alcohol is lower than that for distilled water. Following functional relationship amongst heat transfer coefficient, heat flux and pressure for saturated boiling of liquids on a copper tube coated with copper of a given thickness has been obtained by least squares method: h = C2qmpn, where the value of constant, C2 and exponents, m and n depend upon liquid, heating surface characteristics and thickness of coating on heating tube surface. The constant, C2 and exponents, m and n of above equation have also been expressed by equations involving coating thickness 8 for each of the liquids used in this investigation. In addition, a dimensionless correlation NuB =C*(PeB)x(K )07has also been developed by regression analysis using IV Abstract experimental data of this investigation. The constant, C* and exponent, x have been expressed in terms of a dimensionless group 8d (= 25 / d). Heat transfer coefficient of a coated tube has been found to increase with increase in thickness of coating. However, this phenomena holds true upto a particular coating thickness. Any further addition in coating thickness has been found to decrease heat transfer coefficient. Further, the rate of increase in heat transfer coefficient with heat flux decreases with increase in thickness of coating. These observations have consistently been obtained for all the liquids - water, methanol and isopropanol at atmospheric as well as sub-atmospheric pressures. 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 boiling of all liquids. However, enhancement has been found to depend upon heat flux, pressure and thickness of coating. Performance of a coated heating tube surface has been evaluated in terms of thermal effectiveness, t, which is defined 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 has been related to heat flux and pressure by the following equation: £ = kqapp, where constant, k and exponents, a and [3 depend upon heat flux, pressure, boiling liquid and thickness of coating. An attempt has also been made to develop expressions relating constant k and exponent a and p of a boiling liquid with thickness of coating 5 by the use of method of least squares. The resultant equation is as follows: ^5 =k6 qU6pp&, where the values of constant k§ and exponents a8 and p6 depends upon the thickness of coating. Based upon the criterion, t >1, a criterion q""Ap"(^ <kfi has been established for enhanced boiling of a liquid on a copper 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 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.