Modeling of heat transfer deterioration regimes when concentrating solutions in industrial film evaporators

Introduction. Heat transfer deterioration in film evaporators may occur without breaking the integrity of film is termed as boiling crises of the 2-type (dry out). The mechanism and mathematical model of heat transfer at ultrahigh concentrations are developed. Materials and methods. Physical modeling of the thermal-hydrodynamic characteristics of the annular down flowing vapor-liquid streams of highly concentrated sugar solutions is carried out in an experimental set up with a pipe of 9 m long, 30 mm diameter, divided into 20 sections for discrete measurements of the heat flow, fluid concentration and film temperature of the pipe wall. Results and Discussions. The mathematical modeling of heat transfer in flowing films of dense sugar solutions is based on the equations of convection and thermal conductivity with a parabolic velocity profile in the film. The model, shows that the correspondence with the experimental data on heat flux decrease along the evaporation channel occurs due to the periodic film mixing by the large waves (inflows). The mode of heat transfer deterioration is ascribed to the interaction of cyclical processes of increasing concentration on the interphase surface with the correspondent increase in temperature depression. Analytical expressions for calculating the increased interphase concentrations and decrease of the heat flux are provided within the wave cycles. The reduction of the wall heat flux along the entire channel is accounted for by means of zonal calculations with a step corresponding to the length of the large waves. Experiments do not show an abrupt decrease of heat flux along the pipe, but the reduction of HTC at concentrations of 75% and higher. The Prandtl numbers varied within the values of 25-350 along the evaporative pipe, accompanying the decrease in heat flux. The recommended correlations are valid within the heat flux variation of 3-15 kW/m(2), wetting flow rate 0,05-0,3 kg/m s. Maximum deviation of the HTC calculated values from those observed in practice not more 15%. Conclusions. The developed model, depicting the process of heat transfer in film with a super-high concentration of liquids, adequately reflects the processes of heat transfer deterioration to dense films in long channels.