Experimental study of the local and average falling film evaporation coefficients in a horizontal enhanced tube bundle using R134a

The falling film evaporation with nucleate boiling in a horizontal enhanced tube bundle arrayed in a vertical column is experimentally investigated. And the influences of film flow rate, heat flux, saturation temperature and bundle depth on the local and average heat transfer coefficients are discussed. The tests are performed at saturation temperatures of 6, 10 and 16 °C, film Reynolds number from 254 to 3700 and heat flux from 20 to 80 kW m⁻². To simulate the actual condition of the falling film evaporator, the passages of liquid/vapor flows are constructed using quartz glass panels and dummy tubes. The results indicate that: the film flow rate, heat flux, saturation temperature and bundle depth have significant effects on the falling film evaporation in the tube bundle; as the film flow rate decreases, the local heat transfer coefficients firstly keep at a plateau and then decrease rapidly and the average heat transfer coefficients firstly keep at a quasi-plateau and then drops sharply; the falling film heat transfer performance becomes worse with increase in heat flux, but becomes better with increase in saturation temperature.