Effect of mixed convection and U-bends on the design of double-pipe heat exchangers

For horizontal flow in the laminar flow regime, the interaction between nat ural and forced convection and the effect of the secondary flow induced by an unheated U-bend were considered in developing an empirical correlation f or the nearly uniform heat flux condition. The correlation predicts the loc al peripheral average heat transfer coefficient with an absolute average de viation of 9.9%. In this article the mechanisms involved in laminar flow wi th the presence of a U-bend are explained. The realistic behavior of the co rrelation is illustrated. Also, a brief discussion of some of the existing correlations for laminar flow is presented. Four cases showing the practica lity of using the correlation in the design of horizontal double-pipe heat exchangers are demonstrated.