In this study, a numerical model will be used to investigate the evaporation and flow characteristics of heated liquid sheets. The liquid will be modeled as water and as a non-Newtonian fluid with the rheological properties of black liquor, a byproduct produced by paper mills that is concentrated in falling film evaporators. However the effectiveness of these evaporator is reduced by fouling on heat transfer surfaces.
Two flow arrangements are to be investigated. A thin sheet of liquid sliding down a vertical heated plane with a free surface exposed to a gas, known as a falling film. This arrangements is currently used in paper mills. The black liquor and steam are separated by the heat transfer surface. Scale builds up on the black liquor side of the surface and causes an increase in the resistance to heat transfer. The other arrangement is a liquid curtain, a thin sheet of liquid falling under the influence of gravity heated from all free surfaces by an envelope of hot gas. In this arrangement the liquid and gas come into direct contact. There is no fouling since there is no heat transfer surface. This type of arrangement is not currently used in paper mills and is being investigated in this work to determine its feasibility.
The fluid system is modeled by solving the governing equations for mass, momentum, and energy using the finite volume method with a combined single-fluid field to capture the multiphase transport and liquid-gas interface. The computational model is parallelized and able to run over multiple cores allowing for refined spatial and temporal discretization.
The model will be used to study the structure of capillary waves in the presence of heat transfer, and to study the effect of evaporation on the instability of liquid sheets for both water and black liquor as a falling film and a liquid curtain. This study will investigate how the breakup mechanisms of a liquid curtain are affected by the temperature dependent fluid properties, such as surface tension and viscosity, and how the breakup into droplets and strands influences the evaporation of the liquid curtain. A power-law viscosity model will be used to simulate the non-Newtonian rheology of black liquor.
Results from this study will be used to compare the energy requirements and vaporization rate for falling film and liquid sheet arrangements. From this comparison, the feasibility of falling liquid curtain evaporators will be determined.