Transverse particle motion analysis as a basis for heat transfer investigations in rotary kilns with flights. Part I: Optical evaluation method

Heat transfer analysis in rotary kilns requires the determination of heat transfer surfaces and the hydraulic cross-sectional area. In flighted kilns, particle curtains introduce additional gas–solid contact, raising the question whether the inner or outer curtain surface is decisive for heat transfer. To address this, two limiting flow scenarios are defined: gas flow through the particle curtain and gas flow around the curtain. Based on these assumptions, corresponding heat transfer surfaces and hydraulic cross-sectional areas for axial gas flow are derived. An optical image-based methodology is used to quantify the relevant parameters. Representative results show that the number of particles in the airborne phase and the resulting inner curtain surface increase with rotational speed, while particle diameter affects the relative relevance of inner and outer curtain surfaces. Furthermore, the results indicate that flight length ratio and number of flights significantly influence curtain formation and heat transfer-relevant surfaces. The presented approach provides a consistent and reproducible basis for heat transfer calculations and supports the analysis and design of flighted rotary kilns.