Paper online: CFD-DEM simulation of drying of food grains with particle shrinkage

CFD-DEM simulation of drying of food grains with particle shrinkage

Jannatul Azmir¹, Qinfu Hou¹* and Aibing Yu^1,2

¹ARC Research Hub for Computational Particle Technology, Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia

² Centre for Simulation and Modelling of Particulate Systems, Southeast University - Monash University Joint Research Institute, Suzhou, Jiangsu 215123, PR China

Abstract

Food grains naturally undergo physical and structural changes during a drying process. The volumetric change of particles or particle shrinkage is one of the important and complicated physical changes in drying. In this work, a shrinkage model for particle diameter reduction is incorporated into the computational fluid dynamics- discrete element method (CFD-DEM) drying model for food grains. First, mixing, general drying and shrinkage characteristics including particle and air moisture content, and particle diameter variation are reproduced. Then, the model is tested by comparing the predicted moisture reduction and volume shrinkage curve with the experimental data of wheat from the literature. The results demonstrate the capability of the current model in predicting drying and particle shrinkage characteristics. Finally, the effects of inlet air temperature and velocity on drying and particle shrinkage are studied. It is revealed that the shrinkage rate increases significantly with increasing air temperature but increases slightly with increasing inlet air velocity. The uniformity of grain size, quantified here by the standard deviation of the particle diameter distribution, increases with decreasing air temperature or increasing air velocity. This grain scale drying model with particle shrinkage should be useful for the design and control of many drying processes.

Keywords: CFD-DEM; food grain drying; particle shrinkage; shrinkage rate; particle diameter distribution.

Janna's second paper, well done!

[Paper accepted Powder Technology] Discrete particle simulation of food grain drying in a fluidised bed

Discrete particle simulation of food grain dryingin a fluidised bed

Jannatul Azmir¹, Qinfu Hou¹* and Aibing Yu^1,2

¹ARC Industrial Transformation Research Hub for Computational

Particle Technology, Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia

² Centre for Simulation and Modelling of Particulate Systems, Southeast University - Monash University Joint Research Institute, Suzhou, Jiangsu 215123, PR China

*Corresponding author. Tel: +61-3-99050845; Email address: qinfu.hou@monash.edu

ABSTRACT

Drying is a common practice for post-harvest processing of food grains. Fluidised beds are often adopted for this purpose. It is of importance to understand the fluidised bed drying process for improving its energy efficiency. This work establishes a numerical drying model based on the combined approach of computational fluid dynamics and discrete element method for describing heat and mass transfer in the gas-solid flow system. Water evaporation is modelled in resemblance to a chemical reaction, thereby requiring fewer model parameters. The model is first described in detail. Then it is tested by comparing model predictions with those experimental data of corn kernel from the literature. General drying characteristics including grain and air moisture contents are reproduced qualitatively. The predicted drying rate curves are quantitatively comparable with those of experimental data. Finally, the effects of inlet air velocity and temperature are examined. The model predictions confirm that the drying rate increases with both the inlet air velocity and temperature. However, the drying product quality, here represented by the standard deviation of grain moisture distribution, increases with increasing air velocity or decreasing air temperature. This grain scale model would be useful to the design and control of the drying process.

Keywords: CFD-DEM; fluidised bed; drying rate; moisture distribution.

Link: http://www.sciencedirect.com/science/article/pii/S0032591017308124

Note Janna was awarded the “Fell Consulting Prize” for the best paper presented by a student in CHEMECA 2017 held at the Melbourne Convention and Exhibition Centre in July.

http://qfhou.blogspot.com.au/2017/07/the-fell-consulting-prize-awarded-to.html

[Paper online] DEM-based virtual experimental blast furnace: A quasi-steady state model

DEM-based virtual experimental blast furnace: A quasi-steady state model

Q. F. Hou,¹* D. Y. E,¹ S. B. Kuang,¹ Z. Y. Li,¹ and A. B. Yu^1,2*

Corresponding authors, Email: qinfu.hou@monash.edu (QFH) and aibing.yu@monash.edu (ABY).

¹Laboratory for Simulation and Modelling of Particulate Systems, Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia

²Centre for Simulation and Modelling of Particulate Systems, Southeast University - Monash University Joint Research Institute, Suzhou 215123, PR China

http://www.sciencedirect.com/science/article/pii/S0032591016308853

Abstract

Intensive heat and mass transfer between continuum fluids and discrete particulate materials is quite common in many chemical processes. To understand and improve the operation of these processes, discrete particle models are very helpful when they are combined with the flow, heat transfer and chemical reaction models. Here, a quasi-steady state model for investigating thermo-chemical behaviors is established and tested for an experimental blast furnace (BF). First, the new treatments and assumptions are discussed in detail. Then, the model is tested against available experimental data under comparable conditions in terms of in-furnace flow state, temperature distribution, and the characteristics of the cohesive zone. Finally, a discussion of further development is presented. Such a model can be used to study the effects of burden distribution, inlet gas composition and material properties on the operation and energy efficiency of a BF. Such particle scale modeling can be extended to other chemical processes such as fluidized beds and rotary kilns not only for better fundamental understanding but also for better process design and control.

Keywords: Blast furnace, Heat and mass transfer, Chemical reaction, Discrete element method, Computational fluid dynamics

Review: Particle Scale Study of Heat Transfer in Packed and Fluidized Beds

A review is available online in Advances in Chemical Engineering. It is considered very useful for those researchers who want to study heat transfer from the particle scale.

Link: http://www.sciencedirect.com/science/article/pii/S0065237715000071

Particle Scale Study of Heat Transfer in Packed and Fluidized Beds

Qinfu Hou*, †, Jieqing Gan*, Zongyan Zhou*, Aibing Yu*

doi:10.1016/bs.ache.2015.10.006

Abstract

Understanding and modeling coupled flow and heat transfer in a particulate system at a particle scale is a rapidly developing research area, in connection with the development of discrete particle simulation techniques and computer technology. The approach based on the discrete element method plays an important role in this area. This approach can provide detailed dynamic information of particulate systems, such as realistic packing structure, transient/static deformation, and relative velocities between fluid and individual particles. The information is directly related to the prediction of thermal behavior. In this chapter, the development of this approach in our laboratory is briefly reviewed, together with the discussion of different case studies. It is concluded that this particle scale approach is effective for studying the coupled fluid flow and heat transfer in particulate systems, although further developments are necessary to be generally applied to industrial processes.

Keywords

Discrete particle simulation; Computational fluid dynamics; Heat transfer; Fluidized bed; Packed bed

[Paper online] Micromechanical modeling and analysis of different flow regimes in gas fluidization

Micromechanical modeling and analysis of different flow regimes in gas fluidization
Q.F. Hou, Z.Y. Zhou, A.B. Yu
Abstract
The micromechanics of different particle-fluid flow regimes, such as fixed, expanded and fluidized beds, in gas fluidization is investigated for group A and B powders. To establish the connection between macroscopic and microscopic descriptions of complex particle-fluid flows, focus is given to the following two aspects: the formation of a stable expanded bed in relation to the interparticle cohesive, sliding and rolling frictional forces, and the correlation between coordination number (CN) and porosity (ε). The method employed is the combined approach of three-dimensional (3D) discrete element method (DEM) and two-dimensional (2D) computational fluid dynamics. The results show that compared to 2D DEM, 3D DEM is more reliable in investigating the micromechanics of granular media, although both can capture key features of different flow regimes. The roles of various forces between particles and between particles and fluid are examined, and the origin of different flow regimes is discussed. It is shown that the cohesive force is critical to the formation of a static expanded bed, while the sliding and rolling frictional forces also play a role here. The criterion for bed expansion is analyzed at bulk and particle scales, and the deficiency at a bulk scale is identified. CN, as a key measure of local structure, is analyzed. It is found that the CN-ε relationship for group A powders has a transitional point between the expanded and fluidized bed flow regimes at a bulk scale, unlike group B powders. A new phase diagram is established in terms of CN-ε relationship that has two branches representing expanded and fluidized (bed) states, which corresponds to the one in terms of interparticle forces.

Highlights
► Flow regimes such as fixed, expanded and fluidized beds are numerically reproduced.
► The effects of various forces is analysed corresponding to different flow regimes.
► The contacts among particles and associated flow structures are studied in details.
► A new phase diagram is established to describe the flow regimes.

Keywords
Fluidization; Computational fluid dynamics; Discrete element method; Granular materials; Microstructure; Simulation

http://www.sciencedirect.com/science/article/pii/S0009250912005568?v=s5

Paper online: Computational study of the effects of material properties on heat transfer in gas fluidization

Ind. Eng. Chem. Res., 2012, 51 (35), pp 11572–11586

ABSTRACT

Heat transfer characteristics of different powders in gas fluidization are investigated by means of a combined approach of discrete element method and computational fluid dynamics. First, the heat transfer characteristics in three flow regimes of group A powders are examined. Then, the effects of the Hamaker constant and particle size, both related to the van der Waals force, are investigated in detail. The results confirm that the convective heat transfer is dominant, and radiative heat transfer becomes important when the bed temperature is high. However, conductive heat transfer also plays a role depending on the flow regimes and material properties. Significant effects of the Hamaker constant and particle size are observed under certain conditions. Finally, an effort is made to quantify the effects of the Hamaker constant, particle size and inlet gas velocity. The findings should be useful for better understanding and predicting the heat transfer in gas fluidization.

Keywords: fluidization; heat transfer; mathematical modeling; multiphase flow; cohesive force; discrete element method

Link:  http://pubs.acs.org/doi/abs/10.1021/ie3015999

Snapshots showing heating process of particles by hot air uniformly injected at the bed bottom in different flow regimes: (a) fixed bed (u_f/u_mf = 0.5); (b) expanded bed (u_f/u_mf = 1.4); and (c) fluidized bed (u_f/u_mf = 6.0). Particles are colored by their dimensionless temperatures.

A heat transfer paper in AIChE Journal is online

Computational study of heat transfer in a bubbling fluidized bed with a horizontal tube

1. Q.F. Hou, 
2. Z.Y. Zhou, 
3. A.B. Yu^,†

DOI: 10.1002/aic.12700

Keywords:

• Discrete particle simulation;
• Heat transfer;
• Fluidization;
• Horizontal tube

Abstract

A combined approach of discrete particle simulation and computational fluid dynamics is used to study the heat transfer in a fluidized bed with a horizontal tube. The approach is first validated through the good agreement between the predicted distribution and magnitude of local heat transfer coefficient with those measured. Then, the effects of inlet fluid superficial velocity, tube temperature and main particle properties such as particle thermal conductivity and Young's modulus are investigated and explained mechanistically. The relative importance of various heat transfer mechanisms is analysed. The convection is found to be an important heat transfer mode for all the studied conditions. A large convective heat flux corresponds to a large local porosity around the tube, and a large conductive heat flux corresponds to a large number of particle contacts with the tube. The heat transfer is enhanced by the increase of particle thermal conductivity while it is little affected by Young's modulus. Radiative heat transfer becomes increasingly important as the tube temperature is increased. The results are useful for temperature control and structural design of fluidized beds. © 2011 American Institute of Chemical Engineers AIChE J, 2011.