Peculiarities of thermal and energy state variation in phase change regimes of water droplets in radiating biofuel flue gas flow

Abstract

A mathematical model is provided for the heat and mass transfer processes of a semi-transparent droplet. It evaluates the variation of liquid physical properties, when the droplet warms in a high temperature gas flow. A numerical investigation methodology, based on an iterative algorithm for instantaneous droplet surface temperature, is developed to determine the dynamics of the thermal and energy states of water droplets in phase change regimes. The condensation, transitional and equilibrium evaporation regimes have been modelled for droplets at a temperature of 40ºC, which is common in biofuel combustion technologies. Flue gases were considered as a hot and humid air flow in the 150ºC-1000ºC temperature range and a humidity of 0.4 according to the water vapour volumetric fraction, where droplets are slipping with an initial velocity of up to 60 m/s and their diameter is in the range of 50-1000 μm. In phase change regimes a detailed assessment is provided for heat and mass transfer parameters for droplets slipping in a radiating high temperature humid gas flow. It was confirmed that the thermal and energy state variations of water droplets in a flue gas flow are defined by the change of heat transfer regime, which is caused by the weakening of droplet slipping in the gas flow and radiation absorption weakening during their evaporation.