Study of a constrained-film bubble absorber under cycle operating conditions Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/xp68kj48f

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  • An experimental and numerical study of absorption of ammonia vapor bubbles into a constrained thin film of ammonia-water solution is presented in the context of a 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 75ºC. A counter-flowing coolant in a minichannel removes the generated heat of absorption. The mass flow rate of the weak solution, vapor flow rate, coolant inlet temperature, and mass flow rate of the coolant solution are varied. Two absorber channel geometries are considered: 1) a smooth 600 µm channel, and 2) a stepped geometry that has 2-mm deep trenches across the width of one of the channel walls. The 1-D, steady state species and energy transport equations, are solved for the smooth-channel absorber 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. Experimental results indicate that overall heat transfer coefficients vary from 700 W/m²-K to 2,300 W/m²-K, while the mass transfer conductances range from 0.024 kg/s-m² to 0.24 kg/s-m². The coolant inlet temperature has a significant effect on the mass transfer rates. At the highest inlet coolant temperature of 58ºC, up to 1.5 g/min of vapor is at best absorbed into 35 g/min of weak solution for the smooth absorber plate, while at the lowest coolant temperature of 30ºC, up tp 3 g/min of vapor is absorbed in 35 g/min of weak solution with the same absorber plate. For the stepped absorber geometry, only 1 g/min of vapor is absorbed into 35 g/min of weak solution for the highest coolant temperature of 58ºC, while for the lowest coolant temperature of 30ºC, 5 g/min of vapor is absorbed in 40 g/min of weak solution. Trends of local variation of temperature and convected vapor from the numerical parametric study complement experimental results and provide further insight into the performance of the absorber. Based on the experimental results, a preliminary size estimate for the absorber to operate in a cooling cycle with a 6 kW evaporator load is provided. Other considerations such as strong solution exit subcooling and porous plate pressure drop are also addressed.
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