Evaporation and disintegration of heated thin liquid sheets
Howell, Aaron W.
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In this study, a numerical model is used to investigate the evaporation and flow characteristics of heated liquid sheets and films. The liquid is modeled as water and as black liquor, a byproduct produced by paper mills. In the pulping process, black liquor is concentrated in an evaporator as a falling film. The effectiveness of the evaporator is reduced due to fouling on heat transfer surfaces. Two flow arrangements are studied: falling films, where the liquor and steam are separated by a heat transfer surface; and liquid curtains, which is a thin sheet of liquid falling due to gravity surrounded by steam. For the liquid curtain, the liquid and gas come into direct contact, therefore there is no place for fouling to occur allowing for a more consistent operation of the evaporator. This type of arrangement is not currently used in paper mills but is being investigated in this work to determine its feasibility. The fluid system is simulated using the finite volume method with a single-fluid field to capture the liquid-gas interface. This study investigates how the breakup of a liquid curtain is affected by flow parameters and how the breakup into droplets influences the evaporation characteristics of the liquid curtain. It is found that the falling film evaporator has a much higher liquid evaporation rate than evaporating as a liquid curtain. However the falling film evaporator has an entrance length with no evaporation, and liquid curtains allow for evaporation to start occurring very near the inlet. If reducing length of the evaporator is a priority, liquid curtain evaporators can obtain a higher evaporation rate than falling films within the same distance. Falling film evaporation has a higher steam efficiency than a liquid curtain evaporator. However, for short evaporator lengths the rate at which water is removed from a liquid curtain evaporator is much greater, but at the cost of a higher steam consumption rate.