Performance of a Constrained Microscale Film Bubble Absorber Under System Operating Conditions

Absorption of ammonia vapor bubbles into a constrained thin film of ammonia-water solution is presented in the context of potential reduction in size of a heat-actuated heat pump component. A large-aspect-ratio channel with a depth of 600 μm restricts the thickness of the weak solution film, while ammonia vapor bubbles are injected from a porous wall. Experiments are performed at a nominal system pressure of 6.2 bar absolute and at an inlet weak solution temperature of 75 °C. A counter flowing coolant at a fixed inlet temperature of 58 °C removes the generated heat of absorption. The mass flow rate of the weak solution, vapor flow rate, and mass flow rate of the coolant solution are varied. Results indicate that a desirable operating condition for the absorber considering both heat and mass transfer attributes is obtained for a flow rate of 1.5 g/min of vapor and 35 g/min of weak solution. Variation of the coolant flow rate does not significantly affect the overall heat transfer coefficient at a low vapor flow rate of 1 g/min. Under these operating conditions, preliminary geometric scaling estimates indicate that a 11.3 kW heat load absorber would require a heat exchange surface area of 0.88 m². The excess pressure drop penalty on the vapor side across the porous plate for this absorber needs to be considered in terms of the overall absorption cycle performance.