Experimental study of falling film evaporation in tube bundles of doubly-enhanced, horizontal tubes

Falling film evaporation (FFE) involves complicated physical phenomena and mechanisms such as wavy liquid film, bubbly flow, capillary-driven evaporation and nucleate boiling. FFE heat transfer characteristics in four doubly-enhanced tube bundles were investigated experimentally with R134a. For single tube, heat transfer coefficient (HTC) first increases then decreases with increase in heat flux, the turning points occurs around 20 kW/m². Tubes with different positions in tube bundle own similar HTCs when tested individually. In tube bundle, with decreasing film Reynolds number (Re_Γ), HTC firstly keeps a quasi-plateau stage (increasing or keeping constant for upper tubes, decreasing for lower tubes), then after a certain threshold film Reynolds number, HTC decreases sharply with Re_Γ. At lower heat fluxes (10 kW/m² and 20 kW/m²), tubes with different positions exhibit similar HTCs and threshold Re_Γ. At higher heat fluxes (30 kW/m² and 40 kW/m²), bottom tubes own much smaller HTCs and larger threshold Re_Γ than upper ones due to partial dryout occurrence. The tube bundle with top plate exhibits higher HTCs and lower threshold Re_Γ than those of the open-ended tube bundle indicating that counter-current vapor flow can deteriorate the heat transfer of FFE. Effect of heat flux on the bundle-averaged HTC increases with tube pitch.