The modeling of transient phase changes of water droplets in flue gas flow in the range of temperatures characteristic of condensing economizer technologies

Abstract

Heat transfer and phase change processes of water droplets in humid air flow were investigated by performing experiments and numerical simulations of heat recovery from biofuel exhaust gas at 40–250 °C, which is characteristic for condensing heat exchangers. Compared to the experiments, the numerical investigation was performed within a wider range of boundary conditions considering droplet dispersity and flue gas parameters. The reliability of the simulation was justified through the coincidence between the calculated temperature of the equilibrium evaporation of droplets (convection heat transfer) in humid air and the wet-bulb thermometer temperature. In case of droplet combined heating (radiation and convection), the methodology was justified by coincidence between the calculated equilibrium evaporation velocity and experimental results obtained by other authors. When the temperature of radiation source is lower than 150 °C, the vapor flows calculated at the surface of the droplets in equilibrium evaporation in the cases of combined heating and convective heat transfer differs about 0.1%, therefore, the radiation influence can be neglected. Based on the results obtained in the investigation of the droplet's phase changes, the work includes practical recommendations for technological water injection to ensure optimal heat recovery from wet exhaust gas in condensing economizers.