Particle Motion and Gas–Solid Heat Exchange Enhancement in Rotary Drums with Aligned/Separated Flight

In a waste heat recovery rotary drum with flights (RDF), particle lifting enhances gas–solid contact but also increases the complexity of particle motion in both radial and axial directions. In this study, a long rotary drum model applicable to both aligned and separated flights was developed. The discrete element method was employed to investigate the effects of the inclination angle, feed rate, and rotation speed on particle dynamics and heat exchange performance. Additionally, a gas–solid heat exchange model was formulated to quantitatively assess the system’s heat recovery efficiency, power recovery, and power consumption. The results indicated that particle motion exhibited greater randomness along the axial direction, and the proposed long-drum model effectively captured the key parameters influencing particle dynamics. The heat exchange capacity of the RDF was closely related to the filling degree, which was found to be most sensitive to the inclination angle. Although the separated flight formed a spiral-shaped particle curtain and significantly enhanced the uniformity of the particle distribution, its heat exchange capacity was lower than that of the aligned flight, and it increased the construction cost by more than 30%. Under all operating conditions, the total system power consumption remained below 20% of the recovered power output.