The impact of filling level resolved: Capillary-assisted evaporation of water for adsorption heat pumps

Abstract

Evaporation of water at low pressures is usually limited by low heat transfer coefficients. Poor heat transfer can severely restrict the performance of cooling devices such as adsorption heat pumps and chillers. The heat transfer can be enhanced by using structures such as fins and porous coatings. These structures provide capillary action to wet the heat exchanger tubes surface with a thin water film leading to high heat transfer coefficients. Hence, the evaporation performance strongly depends on the thickness of the thin water film and consequently on the filling level of the heat exchanger. In this work, the evaporation performance is investigated systematically for horizontal copper tubes with macroscopic fin structures, microporous coatings and the combination of both structures. In particular, an experimental setup is introduced to study continuously varying filling levels. The influence of evaporation temperature and heat flow on the heat transfer coefficient is studied. The heat transfer is found to depend strongly on the filling level and the temperature, whereas the heat flow has no significant influence at the studied measurement conditions. It is shown that the heat transfer is directly proportional to the tube surface wetted by capillary action. The evaporation performance of thermally-coated tubes can reach heat transfer coefficients similar to falling film evaporators. Thus, the presented tube structures allow for improved evaporator designs for future adsorption heat pumps using water as refrigerant.

Closed‐Loop Adsorption‐Based Upgrading of Heat from 90 to 110 °C: Experimental Demonstration and Insights for Future Development

“…14 identical, capillary active copper tubes covered with circular fins and connected serially in a single layer [ 33 ]…”

Optimization of carbon dioxide refrigerant charge in capillary radiation air conditioning system based on genetic algorithm

“…18 When the refrigerant supercooled liquid from the condenser passes through the capillary tube, it becomes a low temperature and low pressure vapor-liquid two-phase state, 19 due to the friction resistance in the tube, and then enters the evaporator. Generally, the location and heat transfer of capillary in refrigeration system are different, 20 which can be divided into two categories: one is adiabatic capillary, the other is nonadiabatic capillary. When the capillary is exposed in the surrounding environment, it can be regarded as adiabatic capillary; when the capillary and the suction pipe are combined as regenerative heat exchanger, it can be regarded as nonadiabatic capillary.…”

Performance of finned tubes used in low-pressure capillary-assisted evaporator of adsorption cooling system

“…They claimed that the evaporation heat transfer coefficient was about 5000 W/(m 2 K). Lanzerath et al [25,26] studied a combination of finned tubes and thermal coating for capillaryassisted evaporation at low pressures. Their investigation showed a strong dependency of the total heat transfer coefficient on the water filling level.…”