Miriam Johansson

Non-evaporating black liquor falling film hydrodynamics

Knowledge of wave behaviour is a key factor for understanding heat transfer in falling film evaporators. Here the industrially important fluid black liquor has been examined, together with water which has been used as a reference. Notable results are that there are large local fluctuations in the film thickness; for instance at a Re number of 4100 in the black liquor falling film, the thickness varies between 0.2 and 8.0 mm. For the water falling film, both correlations (Brotz and Brauer) are mostly within the 95% confidence interval of the measured average film thickness. The black liquor film tends to be thicker than the film thickness calculated with the correlations, especially for higher Re numbers.

Turbulence models for falling film hydrodynamics and heat transfer compared with experimental values for black liquor

Increased knowledge about the hydrodynamics and the heat transfer in falling film evaporation of black liquor is an important support for design of energyand cost-effective evaporation plants. Two comprehensive turbulence models for falling film hydrodynamics are compared with experimental results, gained in this study, for heat transfer in falling film evaporation of black liquor. The turbulence models provide increased understanding about the velocity, temperature and heat transfer profile in the evaporating black liquor falling film. The largest temperature gradients are, according to the turbulence model, in the region close to the wall and the region close to the film surface. The heat transfer coefficients calculated with the turbulence models were in agreement with the experimental water heat transfer data. However, they did not agree well with the experimental heat transfer coefficients for black liquor. Several possible reasons for the deviation are discussed.