HEAT TRANSFER STUDIES IN POOL BOILING OF LIQUIDS

Abstract

An investigation of heat transfer in nucleate pool boiling for atmospheric and subatmospheric pressures has been carried out both analytically and experimentally, Traditionally, the study of heat transfer in nucleate pool boiling are considered empirically, without regard for bubble dynamics. But in the present investigation, following the fact that the heat transfer in nucleate boiling is charac terised by the induced turbulence due to- nucleation sites on the heating surface, bubble size and their emission fre quency, a mathematical analysis has led to equation for predicting the values of absolute heat transfer coefficient, Eq, {^,ZZ), This resultant equation relates heat transfer coefficient to the wall heat flux, system pressure and the pertinent physico-thermal properties of boiling fluids through the heating surface characteristics. But this equation is useful for calculating the absolute values of heat transfer coefficient f only if , heating surface characteristics are known as required for constant M and the values of nf for the determination of exponent a. Since the surface charac teristics and the value of nf are extremely unpredictable for industrial surfaces and they differ from surface to surface, a considerable built-in difficulty is inherited in this equation. Therefore, it appears impossible to provide a 11 panacea for predicting the absolute values of heat transfer coefficient. However, for a given heating surface it is poss ible to determine the value of constant M and exponent a empirically which can be used for the calculation of absolute values of transfer coefficients, The value of exponent a has been evaluated as 0,3683, and the expression for Mare represented by the set of equations, Eq. (6,3), Thus the equation for calculating the absolute values of heat transfer coefficient is as follows* while f is obtained from the respective equation, Eq. (515 ) or Eq, (5,19) for Jakob number less and greater than 100. 2,33 .0,3683 2 5 0-5 \ 1'2 s -Mkf§/ ^ ' J {~* j J h * M Vt3;'5 o- ^ Vk|r g/ \Cg ) I t fi (6,M The constant M represents the combined effects of pressure and surface-liquid combination on boiling heat transfer. A procedure has also been devised for the calcula tion of h*/hj f-cf, Eq(6.6)]. It has been found that the values of h*/h£ depend upon wall heat flux, system pressure and pertinent physico-thermal properties of boiling fluids. They do not depend on surface-liquid combinations. Hence this equation is useful to compare the data of different investiga tors obtained on differing surface-liquid combinations. It was found that this equation correlated the present data and those of Cryder and Finalborgo i3h Raben, Beaubouef and Ill Cctnaerford {%] within a maximum deviation of ± 20 per cent. This equation also provides the facility for the computation of absolute values of heat transfer coefficient at subatmospheric pressures, without resort to experimentation, if the value of heat transfer coefficient at normal boiling point is available, Equation (6,6) is as follows: 1.5 hi -0.3997825 0,958316k5(P /P±) f 2.5. p /T _ \ 1.2 f'55 ,_ . (?) 5a 0,3 0.3 CJk l*w 0,3683 (6.6) Since the present study is for atmospheric and subatmospheric pressures.obviously the resultant equation, Eq,(6,6) might not correlate the boiling data for higher pressures, A computer program was written and calculations were made to compute the heat transfer coefficients from the above resultant equations. Apart from the analytical analysis the purpose of the investigation was also to obtain experimental data in order to verify the resultant equations from the analysis and to generate the new experimental data for subatmospheric pressures which are scanty in the literature. The experimental investigation involved the determination of heat transfer coefficient from iilO ASIS stainless steel heating surface to the boiling fluids! iv distilled water, isopropanol, ethanol and methanol for the pressures ranging from 11,33 kN/m2 to 98,UU kN/m2 and heat flux ranging from 6870 W/hT to M730 W/m , The excellent consistency between the experimental data and the predicted values sufficiently proves that the present mathematical analysis based on the governing equations for nucleation sites, bubble growth, bubble size and bubble emission frequency provides an adequate procedure for heat transfer coefficient in nucleate pool boiling of fluids for the range of parameters investigated,