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A unified approach is proposed for the prediction of heat transfer coefficients in turbulent falling films undergoing heating, evaporation or condensation for both of the cases with or without interfacial shear. A modified van Driest eddy viscosity model, which incorporated a damping factor f and takes into account the effect of variable shear stress, is used to predict the hydrodynamics of turbulent falling films. The calculated film thicknesses are in good agreement with the Nusselt-Brauer correlations for the non-sheared film and the Dukler prediction for highly sheared film. Also, by including a van Driest type turbulent Prandtl number model, the asymptotic heat transfer coefficients are accurately predicted and show better agreement with the extensive literature data and correlations than do most of the existing turbulence models proposed to date.

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The low-temperature properties of block copolyetheresters with hard segments of poly(alky1ene p,p'-bibenzoate) and soft segments of poly(tetramethy1ene ether) were investigated by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). In the temperature range of -100 to 60"C, two transition temperatures, a glass transition temperature (T,) and a melting temperature (T,,J, were found by DSC and are attributed to the polyether segments. The Tg monitored by DSC of the polyether segments of the block copolyetheresters is around -68°C and independent of the composition and the type of polyester segment. Thus, the amorphous parts of the polyether segments should be immiscible with the amorphous parts of the polyester segments. The polyether segments of the block copolyetheresters exhibit a lower T,,, and a lower crystallinity than those of the poly(tetramethy1ene ether)glycol due to the presence of the polyester segments. The crystallizability of the polyether segments is dependent on the composition to some extent. The DMA data show that the dynamic modulus drops more abruptly around -10 to 15"C, indicating that the mechanical properties may change significantly due to the melting of the polyether segments. 0 1996 John Wiley & Sons, Inc.

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