Heat Transfer Enhancement of Falling Film Evaporation on a Horizontal Tube by Thermal Spray Coating

Abstract

Falling film evaporators are gaining popularity as substitutes to typical flooded evaporators because of their low refrigerant charge. It is important to form and keep a thin liquid film on the heat transfer surface to ensure their high heat transfer performance. In this study, as a heat transfer enhancement surface, a fine porous surface processed using thermal spray coating was applied to a smooth copper tube with an outer diameter of 19.05 mm. Heat transfer coefficients of falling film evaporation on a single horizontal tube were experimentally evaluated using the HFC-134a refrigerant. The experiments were performed at a saturation temperature of 20 °C with the heat flux ranging from 10 to 85 kW·m−2 and for film Reynolds numbers up to 673. The study aimed to clarify the effect of the coating on the heat transfer characteristics of falling film evaporation. The results revealed that the coating could suppress partial dry out and enhance nucleate boiling in the falling film. The maximum heat transfer enhancement factor was 5.2 in the experimental range. It was further noted that the effect of the coating was especially strong under a low heat flux condition.

Falling film boiling of refrigerants over nanostructured and roughened tubes: Heat transfer, dryout and critical heat flux

“…No literature has been found for the use of surfaces that can be categorised as nanostructured in falling film-boiling conditions. Reduced dryout has been successfully achieved on 3D micro-enhanced and micro-porous tubes tested under falling film-boiling conditions with refrigerants, with plateau regions more insensitive to flow rate changes [16,17,24] and critical dryout at lower film flow rates [16,17,25,26] than for plain tubes. The improved wetting performance has been suggested to be through the ability of microstructured surfaces to better distribute the thin liquid films across the heat transfer surfaces [25] with interconnected internal pores distributing liquid [16] and nucleating bubbles further assisting by sucking liquid through the capillary pores to the nucleation sites [26].…”