The influences of thermal stratification and flow interaction on the enhanced natural convective heat transfer at low temperatures

Abstract

This paper applies the modified heat transfer correlation: Nu/Nu iso = f(S) to horizontal cylinders immersed in the thermally stratified vapour over boiling liquid nitrogen. Correlation of the heat transfer data confirms that the thermal stratification parameter, S is the appropriate parameter to account for the observed heat transfer enhancement. When the data are compared with the theory for natural convection in an extensive environment with a linear thermal stratification, it is found that heat transfer in the cryogenic vapour columns is significantly enhanced. This enhancement appears to arise from a secondary flow circulation inducecd by the interaction of the flow field around the heated cylinder with the primary flow of the cold vapour column. The influence of this flow interaction upon the heat transfer has been studied by varying the cylinder and vapour column diameters. The data have been analysed and correlated with the empirical relation: Nu Nu iso =0.80 0.5( D W ) -0.4 All the results on enhanced convective heat transfer obtained at Southampton are reported and the conditions under which heat transfer enhancement takes place are discussed.