Chemical Engineering Science 60 (2005) 4265 – 4274 www.elsevier.com/locate/ces Wet granular materials in sheared flows Wen-LungYang, Shu-San Hsiau ∗ Department of Mechanical Engineering, National Central University, Chung-Li, 32054 Taiwan, ROC Received 13 December 2004; received in revised form 3 February 2005; accepted 1 March 2005 Available online 26 April 2005 Abstract The transport properties of wet granular materials in a shear cell apparatus have been studied. If the particles are wet, the flow becomes more viscous forming liquid bridges between particles. The dynamic liquid bridge forces are considered as the cohesive forces between particles to restrict their movements. The cohesive forces make the particles stick tighter with each other and hamper the movement of particles. The mixing and transport properties are influenced seriously by the amount of moisture added in the flow. This paper discusses a series of experiments performed in a shear cell device with five different moisture contents using 3-mm glass spheres as the granular materials. The motion of granular materials was recorded by a high-speed camera. Using the image processing technology and particle tracking method, the average and fluctuation velocities in the streamwise and transverse directions could be measured. The self-diffusion coefficient could be found from the history of the particle displacements. The self-diffusion coefficients and fluctuations in the streamwise direction were much larger than those in the transverse direction. Three bi-directional stress gages were installed to the upper wall to measure the normal and shear stresses of the granular materials along the upper wall. For wetter granular material flows, the fluctuation velocities and the self-diffusion coefficients were smaller.  2005 Elsevier Ltd. All rights reserved. Keywords: Moisture content; Cohesive; Granular flows; Shear cell; Self-diffusion coefficient; Stress gage 1. Introduction Granular materials are collections of discrete solid par- ticles dispersed in vacuum or in an interstitial fluid. The voids between particles are filled with a fluid-like air or wa- ter. Therefore, the flow behavior of granular material can be treated as a multiphase flow. Granular flows are widely found in nature, such as avalanche, soil liquefaction, land- slides and river sedimentation. In industries, granular flows include mixing and transport processes of foodstuffs, coal, pellets, and metal mine. In chemical industry, more than 30% of the products are formed as particles (Shamlou, 1988). All granular flows are highly dissipative. The energy supplied to a granular flow, through vibration, gravity, or shearing is rapidly dissipated into heat. Thus, energy must be constantly supplied to the system to maintain a granular flow. ∗ Corresponding author. E-mail addresses: s9343010@cc.ncu.edu.tw (W.-L. Yang), sshsiau@cc.ncu.edu.tw (S.-S. Hsiau). 0009-2509/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2005.03.001 The dominant mechanism effecting the flow behavior is the random motion of particles resulted from the interactive collision between particles (Campbell, 1990). Since the ran- dom motion of particles in a granular flow is analogous to the motion of molecules in a gas, the dense-gas kinetic the- ory (Savage and Jeffrey, 1981; Jenkins and Savage, 1983; Lun et al., 1984; Jenkins and Richman, 1985) and molecu- lar dynamic simulations (Campbell, 1989; Lan and Rosato, 1995) are utilized to analyze and model the granular flow behavior. However, the granular flow is not completely anal- ogous to molecular dynamic theory. In this study, we use the dense-gas kinetic theory to analyze the granular flow. The presence of a small amount of interstitial fluid in the system introduces another degree of complexity due to the cohesive forces between particles in addition to the core repulsive force and the force of friction in a dry granular matter. An increase in repose angle is the most well known effect of the presence of interstitial fluid in a granular sys- tem and has become a topic of current interest (Tegzes et al., 1999; Halsey and Levine, 1998). The interstitial fluid also