| Wavy falling films are an important aspect of absorption refrigeration systems. Much larger mass and heat transfer rates have been observed in such films than predicted by smooth-film theory. A review of the existing theories is followed by a simple model for the absorption of water vapor in an aqueous solution of lithium bromide flowing down a vertical-tube absorber. The presence of high-frequency capillary waves is accounted for through empirical transport coefficients from the literature.;Inertial, roll waves were identified during experimental validation of the 1-D model. Image-processing studies confirmed their presence and an appropriate hydrodynamic description of roll waves was sought. Results from the solution of the flow equations, obtained for different Reynolds numbers, were then extended using Fourier series expansions and continuity at each point to yield the complete periodic velocity field.;Transient, 2-D governing equations were formulated for the simultaneous heat and mass transfer during film absorption, with the hydrodynamics as input. The equations, coupled non-linearly at the vapor-liquid interface, were solved by an iterative finite-difference scheme. Enhancement due to roll waves by a factor of as much as 5.8 was predicted for the isothermal absorption of carbon-dioxide in water, at a Reynolds number of 880 and a Schmidt number of 465.;Previous to the present work, no complete study existed of simultaneous heat and mass transfer enhancement due to waves. The model was run for the laminar, capillary-wave and roll-wave regimes, i.e. over a Reynolds number range from 20 to 600. Average heat and mass transfer coefficients were extracted from the results for the fully developed region of the tube, and compared to experimental as well as theoretical data from the literature. Excellent agreement with penetration theory was obtained for the smooth film, but in the roll-wave regime, the model predicted much higher transport. Evidence of this enhancement has also been found in recent experimental data.;The results of the modeling effort indicate that the normal convective flux due to the transverse velocity in its inward phase, coupled with the effect of the corresponding streamwise-velocity phase, is responsible for transport enhancement in wavy films. |
