Zhi Gang Feng

Inclusion of heat transfer computations for particle laden flows

A newly developed direct numerical simulation method has been used to study the dynamics of nonisothermal cylindrical particles in particulate flows. The momentum and energy transfer equations are solved to compute the effects of heat transfer in the sedimentation of particles. Among the effects examined is the drag force on nonisothermal particles, which we found strongly depends on the Reynolds and Grashof numbers. It was observed that heat advection between hotter particles and fluid causes the drag coefficient of particles to significantly increase at relatively low Reynolds numbers. For Grashof number of 100, the drag enhancement effect diminishes when the Reynolds number exceeds 50. On the contrary, heat advection with colder particles reduces the drag coefficient for low and medium Reynolds number (Re<50) for Grashof number of −100. We used this numerical method to study the problem of a pair of hot particles settling in a container at different Grashof numbers. In isothermal cases, such a pair of ...

A Correlation of the Drag Force Coefficient on a Sphere with Interface Slip at Low and Intermediate Reynolds Numbers

The hydrodynamics of a sphere with interface slip has been numerically investigated for flows of Reynolds number ranging 0 < Re ≤ 75. A simple correlation of the drag force coefficient in the present of interface slip has been derived based on our numerical simulations. The correlation takes the slip coefficient and Reynolds number as two input parameters. By comparing results found in the literature, we believe that the present correlation is more accurate; it provides a source for future experiment study and for numerical simulations of large multi-particle system where the interface slip is important.

On the drag force of a viscous sphere with interfacial slip at small but finite Reynolds numbers

We investigate the hydrodynamic drag force on a viscous sphere in a fluid of different viscosities at small but finite Reynolds numbers when interfacial slip is present at the surface of the sphere. The sphere is small enough for it to retain its spherical shape, as is the case with most small droplets. By using a singular perturbation method, the exterior flow field of the droplet is decomposed into an inner region, where the viscous effects dominate, and an outer region, where the inertia is important. The interior flow of the viscous sphere is also solved analytically. By applying appropriate boundary conditions to the surface of the viscous sphere and matching the conditions between the inner and outer flow fields, stream functions up to the order of Re2 log Re for both the exterior and the interior flow are obtained. Thus, an analytical expression for the drag force coefficient of the viscous droplet is derived. This general expression yields, as special cases, several other expressions that are applicable to spheres that translate rectilinearly under more restrictive conditions. One of the practical conclusions from this study is that the presence of interfacial slip can significantly reduce the drag force on a droplet.

A Three-Dimensional Resolved Discrete Particle Method for Studying Particle-Wall Collision in a Viscous Fluid

Particle collisions with the walls are very important in understanding the fluid-particle behavior near the walls and determining the boundary conditions of the particulate phases in two-fluid models. In this paper, we examine the velocity characteristics of several types of particles near solid walls by applying a resolved discrete particle method (RDPM), which also uses the immersed boundary approach to model the solid particles. We assume that the particles are spherical with an initial velocity that is prescribed. The particles are allowed to traverse part of the viscous fluid until they collide with the solid wall. The collision force on the particle is modeled by a soft-sphere collision scheme with a linear spring-dashpot system. The hydrodynamic force on the particle is solved directly from the RDPM. By following the trajectories of several particles, we investigate the effect of the collision model parameters to the dynamics of particle close to the wall. We report here the rebound velocity of the particle, the coefficient of restitution, and the particle slip velocity at the wall as functions of the collision parameters. DOI: 10.1115/1.4002432 Interparticle and wall collisions are very important in particlefluid flows. The dynamic behavior of such flows is decided by these collisions, especially when the flow is dense and the particles move at high velocity. Particulate flow modeling, as an effective and robust tool to study many issues associated with particular flows segregation, agglomeration, and clustering, requires the ability of accurately resolving interparticle and wall collisions. The correct handling of these collisions is an essential element for the discrete particle method DPM, which will not work without an accurate knowledge of the collisions 1. Generally, a grid used in a DPM simulation is not fine enough to resolve the lubrication force that develops between the particles or between a particle and a solid boundary. Therefore, an artificial mechanism is necessary to be introduced in the numerical simulation to account for the collision force during collision processes. Without such a mechanism in the model, it is likely that the particles will penetrate significantly into each other’s computational boundary, thus, terminating the computation or rendering the results meaningless. There are a few collision schemes that have been developed and applied in the numerical simulation of particulate flows: Ladd 2,3 suggested using lubrication force to repel two particles when

Direct Numerical Simulation of Forced Convective Heat Transfer From a Heated Rotating Sphere in Laminar Flows

The laminar forced convection of a heated rotating sphere in air has been studied using a three-dimensional immersed boundary based direct numerical simulation method. A regular Eulerian grid is used to solve the modified momentum and energy equations for the entire flow region simultaneously. In the region that is occupied by the rotating sphere, a moving Lagrangian grid is used, which tracks the rotational motion of the particle. A force density function or an energy density function is introduced to represent the momentum interaction or thermal interaction between the sphere and fluid. This numerical method is validated by comparing simulation results with analytical solutions of heat diffusion problem and other published experimental data. The flow structures and the mean Nusselt numbers for flow Reynolds number ranging from 0 to 1000 are obtained. We compared our simulation results of the mean Nusselt numbers with the correlations from the literature and found a good agreement for flow Reynolds number greater than 500; however, a significant discrepancy arises at flow Reynolds number below 500. This leads us to develop a new equation that correlates the mean Nusselt number of a heated rotating sphere for flows of 0≤Re≤500.

Condensation Analysis of Exhaust Gas Recirculation System for Heavy-Duty Trucks

The exhaust gas recirculation (EGR) system has been widely used in the automotive and heavy-duty trucks to reduce NOx ,S Ox, and other controlled emissions. A liquid-cooled or air-cooled heat exchanger is the main constituent of the EGR system. The heat exchanger decreases the temperature of the exhaust gases mixture that flows through the EGR channels and the lower temperatures reduce the content of the controlled gas emissions. Condensation of water vapor is an undesirable by-product of the EGR systems because, in combination with the emission gases, it forms the corrosive sulfuric and nitric acids. The U.S. EPA has suggested that engine makers should turn off their EGR systems periodically to avoid the formation of the corrosive sulfuric and nitric acids. In order to accurately predict the corrosion process, a condensation model has been developed to investigate the rates of formation and diffusion of nitric acid and sulfuric acid to the cold tube surface. A three-dimensional computational fluid dynamics (CFD) simulation has been conducted for a typical EGR cooler during normal operating conditions of Tier 4 heavy-duty trucks. A lumped, 1D heat and mass transfer model has also been developed to study the most important physical aspects of the condensation process. The CFD and the analytical results of the rate of condensation and local fluid properties are an important and inexpensive complement to more expensive experimental measurements and testing. Such models may be used to improve the design and to optimize the operating conditions of the EGR systems and may become valuable tools in the design and manufacturing of the next generation of EGR systems for diesel engines. The model developed is general and the techniques and numerical results of this study may be extended to engine reliability, corrosion reduction, and damage prevention of other industrial engines. [DOI: 10.1115/1.4004745]

Modified kinetic theory applied to the shear flows of granular materials

Granular materials are characterized by large collections of discrete particles, where the particle-particle interactions are significantly more important than the particle-fluid interactions. The current kinetic theory captures fairly accurately the granular flow behavior in the dilute case, when only binary interactions are significant, but is not accurate at all in the dense flow regime, where multi-particle interactions and contacts must be modeled. To improve the kinetic theory results for granular flows in the dense flow regime, we propose a Modified Kinetic Theory (MKT) model that utilizes the contact duration or cutoff time to account for the complex particle-particle interactions in the dense regime. The contact duration model, also called TC model, was originally proposed by Luding and McNamara [“How to handle the inelastic collapse of a dissipative hard-sphere gas with the TC model,” Granular Matter 1, 113 (1998)] to solve the inelastic collapse issue existing in the inelastic hard sphere model...

Application of a Three-Dimensional Immersed Boundary Method for Free Convection From Single Spheres and Aggregates

A three-dimensional immersed boundary method (IBM) is applied for the solution of the thermal interactions between spherical particles in a viscous Newtonian fluid. At first, the free convection of an isolated isothermal sphere immersed in a viscous fluid is analyzed as a function of the Grashof number. A new correlation for the heat transfer rate from a single sphere is obtained, which is valid in the ranges 0.5 ≤ Pr ≤ 200 and 0 ≤ Gr ≤ 500. Second, the free convection heat transfer rate from pairs of spheres (bispheres) and from small spherical clusters immersed in air (Pr = 0.72) is investigated using this numerical technique. For bispheres, their orientation and the thermal plume interactions within a range of interparticle distances may cause the enhancement of the heat transfer rate above the values observed for two isolated spheres. For the simple triangular particle clusters, where the particles are in contact, it was observed that the average heat transfer rate per sphere decreases with the increased number of spheres in the cluster.

An Experimental Study on Fluidization of Binary Mixture in Particulate Flows

In this article, we studied experimentally the fluidization of binary mixture in particulate flows. A laboratory fluidized bed is built with water being used as carrying fluid. Three types of solid particles, nylon, glass, and aluminum of the same size and different densities, were used in the experiments. We investigated the wall effect to the fluidization of a single particle, the fluidization of binary mixture of nylon and aluminum with a density ratio of 0.42, and the fluidization of binary mixture of glass and aluminum with a density ratio of 0.91. Our results show that the presence of narrow walls reduces the minimum fluidization velocity for a single particle up to 40%. We also found that in the case of binary mixture of glass and aluminum particles, uniform mixing can be easily achieved with no segregation observed; on the other hand, in the case of binary mixture of nylon and aluminum particles, significant segregation and separation are observed. Based on experimental results, correlation relationships between fluidization velocity and bed height are also reported.

Shape Effects on the Drag Force and Motion of Nano and Micro Particles in Low Reynolds Number Flows

Particulate flows are commonly found in a variety of applications. For example, nanoparticles have been used in targeted drug delivery systems and improving heat transfer in nanofluids. Crucial to the development of technologies that incorporate nanoparticles is to understand the effect of a nanoparticle’s shape on its motion. The effect of shape on nanoparticles used in drug delivery, in particular, is a very active area of experimental investigation. Also, the determination of the coefficients of hydrodynamic forces or drag forces on nanoparticles of different shapes is crucial in designing effective nanoparticle-mediated therapies. In this study we present a resolved discrete particle method (RDPM), which is also called the Direct Numerical Simulation (DNS), to investigate the effect of shape on drag force in a vicious fluid. Three different shapes of particles are studied: a sphere, a probate ellipsoid, and an oblate ellipsoid. These particles have the same volume and are placed in contact with the bottom wall in simple shear flows. Their drag forces are computed numerically; it is found that the particle shape has a significant effect on the drag forces. In the case of a spherical particle, our results agree very well with the analytical results found in the literature. The motion of three particles of the same volume but different shape in a simple shear flows are also simulated. It shows that different particle shapes cause particles to experience different hydrodynamics forces, leading them to different velocities and paths.Copyright