An investigation of the effects of interfacial shear between the gas and turbulent falling liquid film on the heat transfer coefficient

An improved method is presented for the prediction of heat transfer coefficients in turbulent falling liquid films with or without interfacial shear for both heating or condensation. A modified Mudawwar and El-Masri's semiempirical turbulence model, particularly to extend its use for the turbulent falling film with high interfacial shear, is used to replace the eddy viscosity model incorporated in the unified approach proposed by Yih and Liu. The liquid film thickness and asymptotic heat transfer coefficients against the film Reynolds number for wide range of interfacial shear predicted by both present and existing methods are compared with experimental data. The results show that, in general, predictions of the modified model agree more closely with experimental data than that of existing models. Comparisons of the predictions of the present model with that of existing models and the experimental data show that agreement is fairly good and consistent. A brief summary of the results are as follows: (1) As opposed to the previous model, present model utilizes a continuous linear variation of eddy viscosity near the interfacial surface. For freely falling liquid film, the present model reduce to the Mudawwar and El-Masri's model. (2) The curves of the present model for the heat transfer coefficients with liquid film Reynolds number have positive slopes in turbulent regime while the curves obtained by other models have negative slopes particularly for condensation. (3) The two criteria for transition from laminar to turbulent film flow, one for low interfacial shear and the other for high interfacial shear, respectively are shown to give the best agreement with the data for the present method