INVESTIGATION OF HEAT TRANSFER MECHANISM IN NUCLEATE POOL BOILING

Abstract

A number of investigators have proposed heat transfer models to explain highly efficient heat transport in nucleate boiling heat transfer. Comparison of some heat transfer models reveal conflicting opinions concerning the mechanisms. Early models developed were based on bubble agitation that promotes the heat transfer between the surface and liquid. However the heat transfer phenomena differs considerably in different regimes of nucleate boiling. It has been found that a single mechanism can not explain the heat transfer process in• the entire nucleate boiling regime. This work was taken up with the objective of investigating the phenomenon and to develop heat transfer model for nucleate pool boiling on the realistic evaluation of relative contributions of various heat transfer components. The heat transfer is considers to occur by the' combination of natural convection, enhanced convection and latent heat transport. At heat flux lower than that required for boiling incipience almost all the heat is transferred by natural convection. Under low boiling heat flux conditions, the natural convection term still dominates whereas at high heat flux, the natural convection component is negligibly small. The enhanced convection plays a. dominant role in the intermediate heat flux range whereas at high heat flux, since the bubbles begin to interfere and start to impede the flow of liquid to the heated surface, this decreases the enhanced convection contribution. Under these conditions, however the latent heat transport Iii component plays the dominant .role. The relative contributions of these components have been evaluated on the basis of suitable expressions for the bubble dynamical parameters both for water and for organic fluids. It was found that a. judicious choice of suitable expressions for. the evaluation of these parameters is very important to arrive at suitable heat transfer relationship. A heat transfer relationship has been developed on the basis of most suitable expressions for the heat transfer components that represent the boiling mechanism in a. realistic way. . The values of • heat transfer coefficients predicted by the proposed relationship and those by the correlations proposed by some previous investigators have been compared with experimental values. The agreement between the heat transfer coefficients predicted by the proposed relationship and the experimental values has been found to be satisfactory. The standard deviation between the values predicted by the proposed relationship and the experimental values has been found to be lower than the deviations between the values predicted by the correlations of other investigators and the experimental values. It can therefore be concluded that a single mechanism is incapable of explaining the heat transfer process in the entire nucleate boiling because the heat transfer process differs considerably in different regimes of boiling. Therefore, heat transfer relationship developed on the •.basis of relative contributions can be considered to be satisfactory.