A numerical model for ammonia–water absorption into a constrained microscale film

A one-dimensional, steady state model for absorption of ammonia vapor into a constrained microscale film is presented. A weak solution of ammonia–water flows in a microchannel into which ammonia vapor bubbles are injected in cross flow from a porous wall. A counter flowing coolant solution removes the heat generated due to absorption from the opposite wall. The 1-D, steady state species and energy transport equations are solved to yield, along the length of the channel, concentration and temperature profiles of the solution stream and the temperature profile of the coolant fluid stream. The model is validated from experimental measurements of global parameters. A parametric study of fluid and geometrical parameters based on the validated model is presented. Results show that a balance between the residence time within the absorber and the absorption time scales, by way of adjusting the mass flow rates of the vapor and weak solution, is needed to ensure complete absorption. A lower coolant inlet temperature significantly enhances absorption rate by increasing the local concentration difference between the saturation and bulk values. The absorption rate is more sensitive to the liquid–vapor interfacial area than to the heat transfer area between the solution and the coolant.