Numerical investigation of liquid dispersion by hydrophobic/hydrophilic mesh packing using particle method
Graphical abstract
Introduction
Liquid dispersion is a significant approach used in chemical processes to increase mass or heat transfer (Science and Britain, 1995). As an efficient gas–liquid mass transfer device, a rotating packed bed (RPB) can provide a high centrifugal force to allow liquid to flow into porous materials (packing zone) for liquid dispersion and full contact between gas and liquid. Packing zone is the main work space of RPBs, which accounts for up to 70% of the effective mass transfer area of a whole RPB (Guo et al., 2014). The liquid introduced into the rotating packing zone will rapidly develop into thin films and tiny droplets near the inner edge and then synchronously moves with the packing, the gas-liquid mass or heat transfer is also going on at the same time (Luo, 2017, Zheng et al., 2016). Improving of liquid dispersion performance of the packing could enlarge the interface area between liquid and gas and enhance the mass and heat transfer of RPBs.
Extensive experimental studies have been carried out to study the flow regularity within RPBs and enhance RPB mass transfer efficiency (Burns and Ramshaw, 1996, Sang et al., 2017a, Sang et al., 2017b, Yan et al., 2012, Liu et al., 2017). Some studies show that the mass transfer efficiency in an RPB can be affected by the packing surface wettability. Zheng et al. (2016) prepared a surface-modified nickel foam packing (SNP), which is hydrophobic, and a nonmodified nickel foam packing (NNP), which is hydrophilic. They experimentally, separately investigated the mass transfer performance in an RPB with these two kinds of packing materials. The results show that the RPB with SNP had a higher mass transfer performance because of enhanced liquid dispersion. Considering that the mass transfer coefficient of mesh packing is the highest among various shapes (Chen et al., 2006), Zhang et al. (2017) carried out visual experiments to observe the liquid dispersion created by a surface-modified stainless mesh (SSM) and nonsurface-modified stainless mesh (NSM). The results show that the SSM is more conducive to forming a liquid dispersion than the NSM, and the mass transfer performance in an RPB with SSM packing is better than that with NSM packing.
The studies above have provided credible and practical results to improve the liquid dispersion and increase the mass transfer performance in an RPB. Further mechanism analysis is needed to investigate liquid dispersion patterns using hydrophobic and hydrophilic packing. Computational fluid dynamics (CFD) can provide visible flow traces and detailed information inside the flow field. As one branch of CFD, particle-based methods such as smoothed-particle hydrodynamics (SPH) (Monaghan, 1992), macro-scale pseudo-particle method (MaPPM) (Wei and Jinghai, 2001), and moving particle semi-implicit (MPS) (Koshizuka and Oka, 1996) discretize continuum mechanics into particles, whose coordinates are updated every time step to match the shape of the fluid. Thus, particle-based methods have the advantage of simulating large deformation (Xiang and Chen, 2015, Xu and Deng, 2016), multiphase flow (Xiong et al., 2010, Xiong et al., 2011), free surface flow (Yanget al., 2016) and even large-scale flow (Xiong et al., 2013, Chen and Wan, 2019).
The MPS method was proposed by (Koshizuka and Oka, 1996) and was specifically applied to solve the incompressible Navier-Stokes equations for an incompressible fluid simulation. To date, the MPS method has been successfully used in many industrial fields (Natsui et al., 2014, Gambaruto, 2015). For example, the mixing process of two viscous liquids in an agitator and the liquid transformation in a rotating atomizer were analyzed using the MPS method (Sun et al., 2009a, Sun et al., 2017). Considering the hold-up phenomenon in a packed bed in blast furnaces, Kon et al. (2015) employed the MPS method to study the packed bed flow. For small-scale problems, Sun et al. (2009b) introduced a surface tension model, CSF (continuum surface force), to the MPS method and simulated binary collisions (Xi and Sun, 2018). More researchers have added different physical models to the MPS method for further application of the MPS method.
In this study, the MPS method with the surface tension model was employed to separately analyze the mechanism of liquid dispersion by mesh packing under hydrophobic and hydrophilic conditions. First, three basic flow patterns, single wire flow, orthogonal wire flow and aperture flow, were employed to investigate the wetting mechanism of the mesh packing. Then, the liquid dispersion process by hydrophobic and hydrophilic mesh packing with three initial jet velocities was simulated. Finally, comparisons of the surface particle number and liquid dispersion range between hydrophobic and hydrophilic conditions are shown, and the influence of wettability on the liquid dispersion performance is discussed.
Section snippets
Governing equations
The governing equations for incompressible flow motion are continuity, and the Navier-Stokes equation is shown as follows:where u is velocity [m/s], t is time [s], ρ is density [kg/m3], p is pressure [N/m2], μ is the kinetic viscosity coefficient [Pa·s], g is the acceleration of gravity [m/s2], fs is surface tension [N/m3] translated into a force per unit fluid volume, ∇ is the gradient, and ∇2 is the Laplacian.
Discretization
In the MPS method (Koshizuka and Oka, 1996), the
Flow mechanism of wire circumferential motion
The mechanism of liquid dispersion by hydrophilic/hydrophobic mesh packing, which has seldom been analyzed by experimental methods, was investigated using the MPS method in this study. The complex process of liquid dispersion by mesh packing was decomposed into three basic flow models: single wire flow, orthogonal wire flow, and single aperture flow. In this section, these basic flow models were simulated and analyzed to obtain the basic rules of liquid dispersion by mesh packing. The
Conclusion
In the liquid dispersion process by mesh packing, the wettability of the solid surface plays a significant role in affecting the dispersion performance. In this study, the moving particle semi-implicit (MPS) method with a surface tension model was employed to simulate the process of liquid dispersion using typical hydrophobic and hydrophilic mesh packing. A three-dimensional mesh packing model was established, and an inlet boundary condition was applied to simulate a continuous jet flow. The
Conflict of interest
There is no conflict of interest.
Acknowledgement
We thank De-lin Chai, Tong-sheng Wang, Kai Zhang, Yun-zhang Song, Ya-li Chen and Yong Zhang for helpful discussions. This work is supported by the Natural Science Foundation of China (NSFC) Project (No. 51576154).
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