Effect of downward vapor stream on falling film evaporation of R134a in a tube bundle

The influence of downward vapor stream on the falling film evaporation with nucleate boiling outside a horizontal enhanced tube bundle arrayed in a vertical column is experimentally investigated. The variations in local and bundle averaged heat transfer coefficients with vapor velocity are presented. To simulate the practical condition in a falling film evaporator, the flow passages of the liquid/vapor are constructed using two quartz glass sheets and two columns of dummy half tubes. The test ranges are saturation temperature of 6 °C, film flow rate of 0.035-0.1 kgm-1s-1, heat flux of 20, 30, 40 kWm-2. It is found that (1) the downward vapor effects are either positive or negative depending on the tube position, film flow rate, heat flux and vapor velocity; (2) the bundle effect becomes stronger under the effect of downward vapor stream; (3) the downward vapor has a significant effect on the heat transfer of the lower tubes than the upper tubes; and (4) the vapor stream effect on the bundle averaged heat transfer coefficient varies with the levels of film flow rate and heat flux. The experimental results provide significant guidance in the design of the vapor escape channel in a falling film evaporator.