Experimental study on ash deposition characteristics of dryer exhaust along horizontal tube bundles using digital imaging techniques

In this study, wet air with ash particles was used to simulate real dryer exhaust from lignite-drying process, and its ash deposition characteristics along horizontal tube bundles were experimentally studied to provide theoretical guidance for the design and operation of waste heat recovery devices. The ash deposition on the longitudinal section of the tube bundles was considered, and the ash deposition thickness under different experimental conditions was quantitatively measured using digital imaging techniques. The spatial and temporal distributions of the ash deposition thickness and its variation with mainstream inlet temperature, mainstream velocity, water vapor mass fraction, and ash particle concentration were presented. The ash accumulated on the windward side of the tube bundles and mainly concentrated on the first five rows in the inlet section, especially on the first two rows. The rapid growth of ash deposition mainly occurred in the first 3000 s and then gradually stabilized. The ash deposition thickness was insensitive to the mainstream inlet temperature and velocity, whereas the water vapor mass fraction and ash particle concentration had significant influences on the ash deposition thickness. When the water vapor mass fraction increased from 5% to 15%, the average ash deposition thickness decreased from 7.67 mm to 1.87 mm. As the ash particle concentration increased from 1.0 g·m⁻³ to 14.8 g·m⁻³, the average ash deposition thickness increased from 1.91 mm to 26.0 mm. The change in ash deposition thickness was significantly larger than that in heat transfer coefficient under different conditions.