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

Abstract

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.