A comprehensive study on the effects of porous char particles on drag coefficients under combustion based on Micro-CT and pore-resolving simulation

Abstract

This study examines the influence of real porous coal char particle morphology on drag coefficients under combustion conditions using micro-CT imaging and pore-resolving computational fluid dynamics (CFD) simulations. Traditional drag models, which rely on simplified geometries such as spheres, ellipsoids, and pipes, may not fully capture the aerodynamic behavior of real char particles. To evaluate these limitations, pore-resolving simulations were conducted at Reynolds numbers of 20, 40, 60, 100 and 200, comparing real char particles with various idealized shapes. Results demonstrate that the drag coefficient of real char particles, with irregular surfaces and complex internal pores, cannot be well represented by the ideal shapes. The poorly connected internal pores had negligible effects on drag, unlike well-connected pores in ideal pipe structures, which influenced aerodynamic behavior. Flow directionality along the same axis had minimal impact on drag for real particles, with less than 1 % error in most cases. However, different axes showed significant errors when Re is less than 200, ranging from 4.59 % to 31.98 %, emphasizing the need for realistic morphology and orientation in predictive models. As Reynolds number increased, discrepancies between drag coefficients of real and ideal particles diminished, suggesting potential use of simplified models at higher Reynolds numbers (with an overall error of 6.35 %). Four classical 1D models based on ideal shapes showed considerable errors in predicting drag coefficients, especially for complex orientations and porosities. While the Schiller model aligned better along the major axis with 10.41 % error and the Ouchene model along the minor axes with 10.08 % error, both still deviated significantly in predicting overall aerodynamic behavior. The need for optimizing drag coefficient models to incorporate realistic morphologies is emphasized in this research.