Evaluation of heat and mass transfer phenomena in nucleate boiling

Abstract

Based on experimental and theoretical investigations the present paper evaluates heat and mass transfer phenomena in nucleate boiling of pure substances and binary mixtures. It is recognized that microscale phenomena can be very important for the understanding and prediction of macroscopic heat transfer. Modeling strategies are presented that include microscale heat and mass transfer. The nucleate boiling heat transfer coefficient can be well predicted using these models and the process can be better explained taking the microscale phenomena into account. Microscale evaporation experiments of the authors confirm the modeling results on this scale. On the microscale, governing phenomena are: intermolecular forces of adsorption, capillary forces, molecular interfacial phase change resistance, and change of phase equilibrium. In a binary mixture, the phase equilibrium temperature is influenced by the strong gradient of concentration and thus the microscale heat transfer is significantly reduced. The contributions of diffusive mass transfer and variable thermophysical properties are negligible in microscale heat and mass transfer. On the macroscale, the influence of free and forced convection, transient heat conduction, and latent heat depends on the geometry of the evaporator and the boiling conditions. In a binary mixture, parameters are identified that are responsible for the characteristic reduction of heat transfer coefficients at intermediate mole fractions.