Investigation of complex mass and heat transfer transitional processes of dispersed water droplets in wet gas flow in the framework of heat utilization technologies for biofuel combustion and flue gas removal Energy

In this paper, complex processes of water droplet heat and mass transfer are analyzed in a cycle of condensing, transitional evaporation and equilibrium evaporation regimes during phase change which occurs on a droplet’s surface. The dynamics of a heated droplet’s surface temperature is directly related to the change in the regimes. The definition of the dynamics is based on a numerical iterative scheme which depends on the balance of a droplet surface’s heat flux. In this scheme, the energy of phase change and external heat transfer are combined as well as the internal heat transfer occurring in droplets. The numerical investigation results of the water droplets’ phase change were used as a basis while defining the inputs provided by the droplet slipping and the radiation absorbed in the flue gas within the interactions between the processes of complex transitional transfers. For this investigation, the conditions have been set to be typical for heat utilization technologies and biofuel furnaces used in flue gas removal

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

Data availability: Data will be made available on request.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.The authors gratefully acknowledge the partial support from project “Establishment of Nordic-Baltic PhD and researcher mobility network in the field of the bioenergy” supported by Nordic Energy Research under the grant number 10539