Experimental investigations of water droplet transient phase changes in flue gas flow in the range of temperatures characteristic of condensing economizer technologies

Abstract

The heating and phase changes of water droplets in 140 °C air flow humidified up to water vapor volumetric fraction of 0.3 were investigated experimentally in the context of heat recovery from flue gases with condensing economizers. The experiments were performed in experimental set-up, where atmospheric air of certain temperature and humidity passed over a water droplet suspended on thermocouple ball. It was demonstrated that initial temperature of droplet and humidity of gas flow are defining factors in transitional phase changes. However, initial droplet temperature had no effect on the equilibrium evaporation regime. The experiments performed confirmed that humidification of atmospheric air flow causes changes in magnitude of heating and phase change process of water droplets suspended in the air flow. These changes translate into increased temperature of equilibrium evaporation of the droplet, formation of favorable conditions for the condensation regime, and a change in dynamics of the droplet's geometrical parameters in initial stage of phase changes. In additionally humidified air flow, the droplet volume increased by three percent, and evaporation temperature of the droplet increased by 17 °C. Practical recommendations for water injection were produced to ensure optimal heat recovery from wet exhaust gases in a condensing economizer.