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

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.