CFD modelling of biomass ash deposition under multiple operation conditions using a 2D mass-conserving dynamic mesh approach

A dynamic CFD model with conjugate heat transfer that considers particle sticking and erosion is developed and used for predicting the effect of multiple operation conditions on ash deposition for a model biomass fly ash species (K2Si4O9) in a lab-scale entrained flow reactor. In order to achieve stable dynamic mesh morphing, a globally mass-conserving smooth method is proposed using a multiple-point weighted moving average algorithm and a growth scaling factor. A new method to estimate particle count for Lagrangian particle tracking is also proposed, which is based on particle impaction efficiency and face count of the mesh of deposition tube. Particle sticking is predicted based on the two-body collision method and particle erosion is evaluated using the empirical model that is dependent on temperature, particle diameter and velocity, and impaction angle. The proposed smoothing method incorporated with the particle count estimated can accomplish stable dynamic mesh morphing for all the 37 deposition cases without the need to loop the smoothing process and sub-group/averaging the growth rate. The prediction results using the proposed ash deposition model agree reasonably with the experimental observation of the effects of flue gas temperature, tube surface temperature, flue gas velocity, fly ash flux and deposition time on deposit formation rate. R-2 of the predicted and measured deposit formation rates is approximately 0.68. The results also show that the predicted erosion rate essentially correlates negatively with the experimental deposit formation rate. Interestingly, coarse particles are numerically seen to reduce the deposition rate caused by smaller particles via erosion.

Automated summary

A dynamic CFD model with conjugate heat transfer is developed to predict the effect of multiple operation conditions on ash deposition. The model considers particle sticking and erosion, and achieves stable dynamic mesh morphing using a smoothing method. The results show good agreement with experimental observations, and suggest that coarse particles reduce the deposition rate caused by smaller particles via erosion.