Effect of Co-flowing Vapor during Vertical Falling-film Evaporation

Interfacial shear stress, heat transfer and bubble appearance in falling film evaporation

“…An extensive study of the viscosity as a function of the solid content is presented in Mura et al [3]. The wetting rate of the product has been adapted to ensure a total wettability.…”

“…Values of DC that exceed 30% of the product exhibit a non-Newtonian viscosity (typical of shear thinning fluids) that can be characterized by a yield stress factor (s y ), according to the Herschel-Bulkely equation: [3] proposed the following correlation to evaluate the yield stress factor as a function of the dry solid content at 55°C:…”

“…The fluid product falls downward along the inside of tubes (or outside, depending on the application: milk, black liquor) [1][2][3] driven by the force of gravity, ensuring the wettability of the entire heated surface. The heat for the evaporative process is provided by film condensation of saturated steam on the opposite side of the tube.…”

“…The vapor that is produced will flow in parallel with the fluid, and the flow rate of the vapor will progressively increase, i.e., the flow is continuous, accelerating in the same direction as the film (co-flowing vapor). The momentum shear between the coflowing vapor and evaporating surface [3] can affect the mechanical equilibrium of the bubble. In industrial applications, such as in the dairy industry, the evaporator tube can be up to 18 m long.…”