Falling film evaporation on a single tube and on a tube bundle

This study is a contribution for the improvement of horizontal falling film heat exchangers. This type of evaporator has the potential to be widely used in the petrochemical industry, for the sea water desalination, or in the large refrigeration systems. An experimental test facility has been constructed in order to study the evaporation of the liquid refrigerant R-134a flowing on a vertical array of horizontal copper tubes with a length of 0.55 meter and a diameter of 19.1mm. Special care has been taken to achieve uniform liquid distribution along the top tube. Four types of tubes were tested including a plain tube, two tubes with mechanically enhanced boiling surfaces (Turbo-BII HP and Gewa-B) and a porous coated tube (High-Flux). Tube arrays with three different tube pitches were tested. Measurements were performed at three different nominal heat flux levels over a wide range of liquid overfeed. For comparison, heat transfer coefficients in pool boiling conditions were measured. The experimental parameters are the following: the liquid film Reynolds numbers from 0 to 3000, heat fluxes between 20 and 60kW/m2 and tube pitches from 22.3 and 25.5mm. The heat transfer coefficients have been measured locally at the mid point of each of the ten tubes. The study shows that there are two types of fundamental behavior characterized by a particular film Reynolds number: Over a threshold value of this Reynolds number, the nucleate boiling in the film governs the heat transfer and this heat transfer coefficient is independent of the Reynolds number. Below this threshold value, the heat transfer coefficients drop drastically due to the formation of local dry patches. The heat transfer coefficient during falling film evaporation was found to be approximately 1.3 times larger than the heat transfer coefficient during pool boiling and this multiplier was found to be dependant on the heat flux and tube spacing but not the flow mode. Finally, the understanding of the physical phenomena governing the falling film evaporation of liquid refrigerants has been improved. Furthermore, a method for predicting the film Reynolds number at the onset of dry patch formation has been developed and a heat transfer correlation for boiling iii falling films proposed. These represent significant improvements for the design of falling film evaporators.