Convection-condensation heat transfer characteristics of air/water vapor mixtures with ash particles along horizontal tube bundles

Lignite drying is an effective way of clean and efficient use of lignite. In the waste heat recovery processes of dryer exhausts, ash particles widely exist. In order to provide a reference for the designation of waste heat recovery heat exchangers of dryer exhausts, air/water vapor mixture with ash particles was used to simulate actual dryer exhausts in this paper, and its convection-condensation heat transfer characteristics along horizontal tube bundles were experimentally investigated under various conditions. The effects of concentration of ash particles, water vapor mass fraction, mainstream velocity and temperature were discussed. In the presented experimental conditions, as the convection-condensation proceeded, the convection-condensation heat transfer coefficients of air/water vapor mixture with ash particles reduced and then gradually became stable due to the ash deposition. It was found that the convection-condensation heat transfer coefficients increased with the increase in the water vapor mass fraction and the mainstream velocity, and decreased with the increase in the concentration of ash particles and the mainstream temperature. As the concentration of ash particles increased from 0 to 6.5 g/m3, the convection-condensation heat transfer coefficient decreased from 339.6 to 315.5 W/(m2·K) when the water vapor mass fraction was 10%, decreased 7.10%. And with increasing the water vapor mass fraction from 5% to 25%, the convection-condensation heat transfer coefficient increased from 201.8 to 960.0 W/(m2·K) when the concentration of ash particles was 5.4 g/m3, increased 4.76 times. A normalized correlation of convection-condensation heat transfer coefficient of air/water vapor mixture with ash particles were derived, with the deviation of −12.5% to +12.8%.