Analytical study of falling film absorption on a partially wetted horizontal tube.

Horizontal tube falling-film configuration of the absorbers is usually chosen for the high transfer rates with reduced dimension and low pressure loss. A detailed, widely applicable and time-saving method to predict heat and mass transfer characteristics of a horizontal tube inside a falling film absorber is herein presented. A two-dimensional model is able to capture the physics of the phenomenon and a first comparison with the numerical solution is presented. Based on previous numerical, theoretical and experimental studies about the absorption process of water vapour in aqueous Lithium Bromide solution, simplifying assumptions regarding film hydrodynamics and the linear saturation model are introduced in order to analytically solve the fundamental equations. Moreover, given the extended contact area and small thermal resistance at low specific mass flow rates, falling film absorbers operability at low Reynolds number is attractive to increase absorption system performance and reduce their overall size, but reduction of the contact area due to partial wetting must be considered as a related issue. By means of the inclusion of a film stability criterion and a linear wetting model, partial wetting phenomena are included in the analysis extending the target range of the resulting heat and mass transfer coefficients expressions and increasing their accuracy. Fourier solution method is used and the eigenvalues obtained from the characteristic equation depend on Lewis number and the dimensionless heat of absorption, in turn established with reference to the inlet conditions of the solution, boundary conditions and the pressure of the absorber. Considering a constant temperature at the tube wall, two-dimensional temperature and concentration fields of the laminar film can be expressed analytically as functions of Prandtl, Schmidt, Reynolds numbers, the tube dimensionless diameter and the wetting ratio of the exchange surface. As a result, the expressions of local and average heat and mass transfer coefficients have been obtained and analysed. The paper performs a first screening of the model potential for actual applications’ design and control.