| Heat-driven absorption thermodynamic cycles offer a possibility of generating both power and cooling with environment friendly refrigerants, such as ammonia. The absorber in such systems is one of the critical components in terms of size, efficiency and cost. In this work, a new concept of enhancing heat and mass transfer processes in a falling film absorber is proposed that could considerably reduce the absorber size without the penalty of high vapor and coolant side pressure drops. The concept utilizes the vertical spacing between the horizontal tubes to form a falling film using a flow guidance medium such as a screen mesh/fabric. In addition to an increase in the liquid-vapor interface area, the design also enhances falling film stability by preventing coalescence of droplets on the horizontal tubes. Furthermore, the design induces thorough mixing of liquid film while it flows progressively over the mesh/cloth and coolant tubes.;This work details a numerical and experimental analysis of the new design with a microchannel falling film absorber design. A finite difference scheme is presented which models the heat and mass transfer processes in a counter-current flow falling film absorber. The numerical investigation accounts for the liquid and vapor phase mass transfer resistances in the falling film absorption. It also considers the coupled nature of heat and mass transfer processes. Details of the experiments on the proposed concept and the microchannel absorber designs are then presented. The experimental study shows that the absorber heat duty for the proposed design is about 17-26% higher than the conventional microchannel design. The UA value is found to increase by about 50% with an introduction of the screen mesh. This is attributable to the fact that the screen mesh enhances both mixing and wetting action in the liquid film. A comparison of numerical and experimental results is also done, which shows a good agreement with some deviation at low temperatures of the coolant and high flow rates of the weak solution. |
