Vapour condensation in presence of a non-condensing gas and aerosols

Abstract

Paper presented at the 7th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Turkey, 19-21 July, 2010.

In the study of growth/evaporation of particles due to (water) vapour in the atmosphere, it is usual to assume that the vapour is diluted and the temperature is uniform enough in the environment. These assumptions cannot be applied to heat exchanger condensers, where the gas mixture is enriched in water vapour (vs. the concentration of an incondensable). An added difficulty is the large value of water latent heat. Therefore, the equations that describe the phenomenon in the atmosphere should be corrected We present a model for vapour condensation in a double pipe best exchanger. The main physical processes in the gas/aerosol atmosphere flowing between coaxial cylinders, being the internal one cooled by a refrigerant, are modellized. The model takes into account: conduction and convection; film vapour condensation; the aerosol particle size distribution (PSD); heterogeneous nucleation in non-diluted conditions; some particle phenomena: gravitational and turbulent deposition, thermophoresis and difussiophoresis; and mass, momentum and energy balances in the heat exchanger. ln the model, the particles (liquid water coming from condensed vapour with a log-normal PSD) are injected in the system at the heat exchanger inlet. Future model improvements will include particles formation by homogeneous nucleation. Results of the model application are truly plausible. For example, the mean size of the particles grows as the flow runs towards the end of the exchanger, but some of them disappesr eventually. AB it could be expected, the main condensation occurs at the liquid film interface of the interior due to the greater temperature difference at these points. The presence of particles leads to a diminishing of the condensed vapour as compared with the case without particles (only vapour in gas phase).