Transport property measurements with different moisture contents in sheared granular flows

Wen-Lung Yang and Shu-San Hsiau

Department of Mechanical Engineering

National Central University Taiwan

Keywords: granular flows, shear cell, self-diffusion coefficient.

ABSTRACT

Granular flows are widely found in nature, such as ice flow, soil liquefaction, landslides and river sedimentation. In industry, granular flows can be found in the mixing and transport processes of foodstuffs, coal, pellets, metal mine and so on. If the particles of flows are wet, the flows become more viscous. The mixing and transport properties would be influenced seriously. This paper discusses a series of experiments performed in a shear cell device with seven different moisture contents. The glass spheres with a 3 mm mean diameter were used as granular materials. A high-speed camera recorded the motions of the granular materials. Using the image processing technology and particle tracking method, the average and fluctuation velocities in the streamwise and transverse directions could be measured. Continuously tracking the particles’ displacements, the self-diffusion coefficient could be found from the history of the displacements of particles. The self-diffusion coefficients and fluctuations in the streamwise direction were much higher than those in the transverse direction. Besides, the self-diffusion coefficients were increased with the increase of the velocity fluctuations and the shear rates. From the experiments, the friction angle of wet particles was larger than that of dry particles. Also, for the wetter granular material flows, the average and fluctuation velocities were also greater. This paper will discuss the influences of the moisture contents on these transport properties in detail.

INTRODUCTION

A granular material is a collection of many discrete solid particles. Because the random motion of particles in a granular flow is analogous to the motion of molecules in a gas, the dense-gas kinetic theory and molecular dynamic simulations (Campbell 1989) are borrowed to analyze and model the granular flow behavior. All granular flows are naturally highly dissipative. The energy supplied to a granular flow, through vibration, gravity, or shearing is rapidly dissipated into heat. Thus, work must constantly be done on the system to maintain a granular flow. The Couette granular flow was very suitable for fundamental research (Savage Sayed 1984). In our earlier study (Hsiau & Yang 2002), the effect of wall condition was discussed. In this paper, the focus will be on the dependence of transport properties on the flow with different moisture contents.

EXPERIMENTAL SETUP

Figure 1 schematically shows the experimental apparatus. The velocity of the lower wall u0 of 0.88 m/sec was used in this study. The soda lime beads with an average diameter dp of 3 mm were used as granular materials in the experiments, and the channel height h is fixed as 1.6 cm. The dimensionless liquid bridge volume, V* = Vw/Vw+p, is used as a control parameter, where Vw is the volume of the water and Vw+p is the volume of the water and particles. Different moisture of granular materials are put in the channel in seven series of tests to form the flows with different moisture contents: (1): V*= 0; (2): V*= 0.008; (3): V* = 0.017; (4): V*= 0.025; (4): V*= 0.033; (4): V*= 0.042; (4): V*= 0.050.

FIGURE 1 Schematic Drawing of the Experimental Apparatus.

In this study, the commercially available Jenike Shear Tester (FT-5STEH) was also used to determine the internal and wall friction coefficients. The flow motions were recorded by a high-speed camera. The image processing was employed to investigate the granular flow behaviours in a shear cell.

RESULTS AND DISCUSSIONS

Figure 2 (a) shows the ensemble velocity distributions in the streamwise and transverse directions for the seven tests. The transverse velocities are 0 and the streamwise velocity decreases with the channel height. The case with higher dimensionless liquid volume has the smaller velocities. Note that there exist slip velocities along the top and the lower walls. Figure 2 (b) shows the fluctuation velocity distributions in the channels for the seven cases. The fluctuations are not isotropic with the greater values in the streamwise direction (Hsiau Shieh 1999). The flow with the higher dimensionless liquid volume induces relatively greater fluctuations in both directions, due to the higher shear rates.

FIGURE 2 (a) The Ensemble Velocity Distributions and (b) the Fluctuation Velocity Distributions in the Channels for the Seven Cases.

FIGURE 3 (a) The Wall Friction Coefficients and (b) the Internal Friction Coefficients in the Channels for the Seven Cases.

Figures 3 (a) and (b) show the wall and internal friction coefficients varied with the seven cases. The wall and internal friction coefficients increase with increasing the value of V*. Besides, the internal friction is greater than the wall friction coefficient at the same V*. Besides, figures 3 (a) and (b) indicate that the case with higher value of V* has greater viscous force. Many studies confirm that the viscous force increase by increase the moisture between particles at the status with a little wet (Yang & Hsiau 2001, and Oulahna et al. 2003).

We divided the test section into three regions according to the ratio of streamwise velocity to the lower wall velocity u0. Figures 4 (a) and (b) show the diffusion coefficients Dxx and Dyy at the three regions versus the channel height for the cases using the seven moisture contents. The diffusion coefficients are greatest in the high shear region, and the values in the uniform region are smallest, though the differences are not significant. The diffusion coefficients increase with the increase of the dimensionless liquid volume. In a wet flow the case with greater viscous force dues to the particles more close to each other, the collisional frequency is higher resulting in the higher granular temperature. The anisotropy of the diffusion coefficients with the greater value in the streamwise direction can be found from both figures.

FIGURE 4 (a) The Diffusion Coefficients Dxx and (b) the Diffusion Coefficients Dyy at the Three Regions Versus the Channel Height for the Seven Tests with Different Moisture Contents.

CONCLUSION

This paper studied the flow behavior of granular materials in a two-dimensional shear cell. There existed three flow regimes in the test section: the uniform, the low shear and the high shear regimes. The anisotropy of the transport properties were demonstrated from the experimental results that was against the assumptions in the dense-gas kinetic theory. The wall and internal friction coefficients increase by increasing the value of V*, and the internal friction coefficient is greater than the wall friction coefficient. The fluctuations were found to increase with increasing the dimensionless liquid volume, and the diffusion coefficients were greater in the flow with a higher value of V*.

REFERENCES

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Oulahna, D., Cordier, F., Galet, L. and Dodds, J. A. 2003. Wet Granulation: the Effect of Shear on Granular Properties, Powder Tech. 130 : 238-246.

Savage, S. B. and Sayed, M. 1984. Stresses Developed by Dry Cohesionless Granular Materials Sheared in An Annular Shear Cell, J. Fluid Mech. 142 : 391-430.

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