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.
Highlights
The charge amount of refrigerant is required to be reduced because the most widely used refrigerants have a high global warming potential (GWP)
A thicker liquid film leads to higher heat resistance and wall superheat, which further results in nucleate boiling in the liquid film
The heat transfer coefficient was improved by increasing the Ref because a higher liquid flow rate led to an increase in the wetted area
Summary
The charge amount of refrigerant is required to be reduced because the most widely used refrigerants have a high global warming potential (GWP). One important issue in improving the heat transfer performance of falling film evaporation is to keep a liquid film on the heat transfer tubes because dry out significantly deteriorates heat transfer [1]. A thicker liquid film leads to higher heat resistance and wall superheat, which further results in nucleate boiling in the liquid film. When nucleate boiling occurs in the liquid film, vapor bubbles can cause liquid entrainment from the surface, leading to a decrease in the liquid film flow rate. If these entrained droplets flow into a compressor, they can cause erosion and deteriorate the adiabatic efficiency.
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