Fluid-solid drag models selection for simulating wheat straw particle movement in anaerobic digester

Computational fluid dynamics(CFD)has been utilized to simulate the movements of wheat straw particles for agitator speed selection in full-scale wet digestion.Previous research has found that the current drag model generally used for depicting the motion of spherical particles cannot match the movement behavior of wheat straw particles with their non-spherical shape.In this study,the sedimentation experiment and horizontal flow experiment of straw particles were determined using a V20-3D camera and a micro Particle Image Velocimetry(PIV)system.With analyses of the experimental data and CFD simulation results,the prediction accuracies of the non-spherical drag models of Hölzer and Sommerfeld(HS),Kishore and Gu(KG),Haider and Levenspiel(HL),Richter and Nikrityuk(RN),and Fabio Dioguardi(FD)were evaluated by the motion of individual straw particles.The results showed that the KG model has a significant advantage over the other drag models,both simulating the particle settling velocities in a one dimensional settling experiment and simulating the predictable trajectory in a two-dimensional horizontal flow experiment.Therefore,the KG drag model was selected to simulate with CFD the wheat straw particle movement to select agitator speeds.Additionally,the realizable k-turbulence model was proven to be superior to the other turbulence models for simulating the continuous phase flow with CFD.

Online single particle analysis of chemical composition and mixing state of crop straw burning particles： from laboratory study to field measurement

Fresh straw burning （SB） particles were generated in the laboratory by the combustion of rice straw and corn straw. The chemical composition and mixing state of the fresh SB particles were investigated by an Aerosol Time-of-Flight Mass Spectrometer （ATOFMS）. Based on the mass spectral patterns, the SB particles were clustered into four major types： Salt, Organic Carbon （OC）, Elemental Carbon （EC）, and internally mixed particles of EC and OC （EC-OC）. In addition, particles containing ash, polycyclic aromatic hydrocarbons, heavy metals or nicotine were also observed. Physical and chemical changes of the SB particles immediately after the emission were analyzed with highly time-resolved data. During the aging processes, the average particle size increased steadily. Freshly emitted organic compounds were gradu- ally oxidized to more oxygenated compounds in the OC- containing particles. Meanwhile, an important displace- ment reaction （2KCI＋ SO24- KzSO4 ＋ 2C1-） was observed. The marker ions for SB particles were optimized and applied to identify the SB particles in the ambient atmosphere. The fluctuation of the number fraction of ambient SB particles sorted by ATOFMS agrees well with that of water soluble K＋ measured by an online ion chromatography, demonstrating that the optimized marker ions could be good tracers for SB particles in field measurements.