Experimental investigation of gas solid hydrodynamics and heat transfer in a vertical riser for suspension preheater based waste heat recovery

Efficient waste heat recovery in cement suspension preheater systems requires a detailed understanding of gas–solid hydrodynamics and heat transfer, which remain insufficiently characterized in industrial risers. This study presents an experimental investigation of gas solid hydrodynamics and heat transfer in a vertical riser simulating industrial suspension preheaters. Air bauxite mixtures were used to evaluate the influence of gas velocity (6.8 to 14.3 m/s), particle size (106–425 μm), solids loading, and material density on acceleration length (LA), pressure behavior, and thermal performance. The results show that LA increased from 0.67 to 1.2 m with increasing gas velocity and from 1.05 to 1.38 m with increasing particle size, while higher solids loading reduced LA from 0.83 to 0.68 m due to intensified particle interactions. Material density also exhibited a strong effect, with LA increasing from 0.91 m (Run 1) and 1.12 m (Run 2) for quarry dust to 1.09 m (Run 1) and 1.51 m (Run 2) for iron ore. Thermal measurements indicated gas temperature reductions of 165–195°C and solids heating of 120 to 175°C. The Nusselt number increased from 10 to 25 at low solids loading to 50 to 80 at higher loading, indicating enhanced convective heat transfer. The regression analysis yielded coefficients of determination (R2) of 0.95 for the acceleration–length correlation and 0.96 for the Nusselt-number correlation, indicating good fits to the experimental data used to develop the respective correlations. These values represent goodness of fit and should not be interpreted as measures of predictive accuracy for independent data. For the comparison of the acceleration-length trends and of the heat-transfer trends, independent published studies were used, and for the assessment of the applicability limits of the proposed correlations, their broader use was considered. These results offer experimentally supported empirical correlations and understanding for the design and optimization of suspension preheater systems and related gas solid processing applications.