AUGMENTATION OF CONDENSATION HEAT TRANSFER OF R-113 BY INTERNALLY FINNED TUBES AND TWISTED TAPE INSERTS

Heat transfer and pressure drop data were taken during condensation of R-113 inside seven horizontal tubes. One tube was smooth, four were internally finned with different outside diameter and different fin geometries, and the remaining two had twisted tape inserts with different pitch to diameter ratios. The smooth tube results were used to identify, among existing smooth tube correlations, the best predictors of the experimental measurements of this investigation. For each of the augmentation techniques tested, new appropriate modifiers were identified and applied to the smooth tube correlations to bring about the best agreement between the measurements of the augmented tubes and predictions. The results are summarized in the following: (1) Over the mass flux range tested, an enhancement in the heat transfer coefficients as high as 51% over the smooth tube results, on a nominal area basis, was obtained with the internally finned tubes tested. The heat transfer enhancement was accompanied by a modest increase in the pressure drop. (2) A correlation equation was proposed for predicting the overall average heat transfer coefficient during condensation inside internally finned tubes. (3) The twisted tape inserts enhanced the heat transfer coefficients by as much as 23% over the smooth tube, on a nominal area basis. The heat transfer enhancement was accompanied by a significant increase in the pressure drop. (4) A correlation equation was proposed for predicting the overall average heat transfer coefficient during condensation inside horizontal tubes with twisted tape inserts. (5) A pressure drop correlation was proposed for predicting the pressure drop during condensation inside the finned tubes. It was based on the introduction of a new modifier to the friction coefficient in the Dukler frictional pressure drop correlation. The proposed correlation predicted most of the experimental data of the present study and studies of other investigators to within (+OR-) 40%. (6) The pressure drop data of tubes with twisted tape inserts were satisfactorily correlated by existing correlations.