Numerical analysis of absorbers used in LiBr/H2O absorption refrigeration

Vapour absorption of LiBr-H2O solution flowing over water cooled vertical and horizontal tubes in the form of falling film is investigated numerically. Although only LiBr-H2O solution is investigated, the present model can be applied to any other absorbent-absorbate pairs as long as the absorbent is nonvolatile.As analytical investigation of the process without many assumptions is impossible due to the complex and coupled nature of the problem, numerical models using finite volume formulations are created for both geometries. Nusselt?s solution is used for film hydrodynamics for both geometries, however it is modified slightly to allow for a change in film thickness with absorption of vapour and change in physical properties. It is observed that this modification improves accuracy of the simulation by reducing error greatly in conservation of species. Rectangular Eulerian grid is used for vertical absorber model, and body-fitted physical Eulerian grid is used for horizontal absorber model by performing a coordinate transformation. Coupled energy and species transport equations are solved numerically.Because the boundary conditions at the tube wall and film surface are not known, a multi iterative process is employed. Also one dimensional energy balance was employed for the cooling water side and bulk temprature distribution of the cooling water is obtained by an iterative procedure. Local physical properties are updated at each iteration. Results for both models are compared with experimental data available in the literature and it is shown that agreement is very good under most common conditions.