Evaporation of Pure Liquids at High Prandtl Numbers in a Scale-Up Capable Falling Film Evaporator

In industrial application heat transfer to temperature sensitive products in falling film evaporation is often linked to the evaporation at elevated viscosities. In the present study a scale-up capable falling film evaporator has been used to investigate the heat transfer to liquids with Prandtl numbers up to 150. The focus was on heated falling films during surface evaporation. Film-Reynolds numbers were varied from 48 to 10,000. As pure liquids water and cyclohexanol were used. In the results a distinct transition zone between laminar and turbulent flow can be observed for elevated Prandtl numbers. The comparison to literature models shows that more parameters have to be taken into account to properly predict the heat transfer in falling film evaporators for different equipment and fluids. The optical monitoring of the film on the inside of the evaporation tube through an endoscope showed that the fully turbulent regime could not been reached for high viscosities.