CFD-DEM simulation of nanoparticle agglomerates fluidization with a micro- jet

Nanoparticles can be fluidized as agglomerates, but for some materials this is cumbersome due to the cohesive nature. Micro-jets are shown to be effective for improving the fluidization in such cases (1). In this study, the mechanisms of micro-jet assistance are investigated by using an adhesive CFD-DEM (Computational Fluid Dynamics – Discrete Element Modelling) model. In previous studies, the complex agglomerates found in a fluidized bed are treated as the discrete elements (2). Here we use the simple agglomerates as the discrete elements, which are the building blocks of the larger complex agglomerates. The collision of the simple agglomerates are modeled by including collision mechanisms of elastic-plastic, cohesive and viscoelastic forces. Particles with =40 and =250 are used to represent the simple agglomerates. The cohesive force is expressed by the non-dimensional parameter , definded by the ratio of der Waals force over the particle gravity. A fluidized bed with dimension of 3 mm × 0.4 mm × 12 mm containing ~120,000 particles is simulated. At different cases, a micro-jet with horizontal cross-section size of 20 x 20 pointing downwards is turned ON or OFF (36 m/s) while the gas velocity to the bed is set as 2.8 cm/s or 4 cm/s, respectively. The schematic of the microjet in the bed is shown in Figure 1. In this way, like in our previous study, we keep the total amount of gas provided to the bed equal (2). Please click Additional Files below to see the full abstract

Solitary waves on falling liquid films in the inertia-dominated regime

We offer new insights and results on the hydrodynamics of solitary waves on inertia-dominated falling liquid films using a combination of experimental measurements, direct numerical simulations (DNS) and low-dimensional (LD) modelling. The DNS are shown to be in very good agreement with experimental measurements in terms of the main wave characteristics and velocity profiles over the entire range of investigated Reynolds numbers. And, surprisingly, the LD model is found to predict accurately the film height even for inertia-dominated films with high Reynolds numbers. Based on a detailed analysis of the flow field within the liquid film, the hydrodynamic mechanism responsible for a constant, or even reducing, maximum film height when the Reynolds number increases above a critical value is identified, and reasons why no flow reversal is observed underneath the wave trough above a critical Reynolds number are proposed. The saturation of the maximum film height is shown to be linked to a reduced effective inertia acting on the solitary waves as a result of flow recirculation in the main wave hump and in the moving frame of reference. Nevertheless, the velocity profile at the crest of the solitary waves remains parabolic and self-similar even after the onset of flow recirculation. The upper limit of the Reynolds number with respect to flow reversal is primarily the result of steeper solitary waves at high Reynolds numbers, which leads to larger streamwise pressure gradients that counter flow reversal. Our results should be of interest in the optimisation of the heat and mass transport characteristics of falling liquid films and can also serve as a benchmark for future model development

Artificial viscosity model to mitigate numerical artefacts at fluid interfaces with surface tension

AbstractThe numerical onset of parasitic and spurious artefacts in the vicinity of fluid interfaces with surface tension is an important and well-recognised problem with respect to the accuracy and numerical stability of interfacial flow simulations. Issues of particular interest are spurious capillary waves, which are spatially underresolved by the computational mesh yet impose very restrictive time-step requirements, as well as parasitic currents, typically the result of a numerically unbalanced curvature evaluation. We present an artificial viscosity model to mitigate numerical artefacts at surface-tension-dominated interfaces without adversely affecting the accuracy of the physical solution. The proposed methodology computes an additional interfacial shear stress term, including an interface viscosity, based on the local flow data and fluid properties that reduces the impact of numerical artefacts and dissipates underresolved small scale interface movements. Furthermore, the presented methodology can be readily applied to model surface shear viscosity, for instance to simulate the dissipative effect of surface-active substances adsorbed at the interface. The presented analysis of numerical test cases demonstrates the efficacy of the proposed methodology in diminishing the adverse impact of parasitic and spurious interfacial artefacts on the convergence and stability of the numerical solution algorithm as well as on the overall accuracy of the simulation results

Thermographic laser Doppler velocimetry

We propose a point measurement technique for simultaneous gas temperature and velocity measurement based on thermographic phosphor particles dispersed in the fluid. The flow velocity is determined from the frequency of light scattered by BaMgAl10O17:Eu2+ phosphor particles traversing the fringes like in conventional laser Doppler velocimetry. Flow temperatures are derived using a two-color ratio method applied to the phosphorescence from the same particles. This combined diagnostic technique is demonstrated with a temperature precision of 4%–10% in a heated air jet during steady operation for flow temperatures up to 624 K. The technique provides correlated vector-scalar data at high spatial and temporal resolution

Development of the thermographic laser doppler velocimetry technique

Simultaneous measurements of flow temperature and velocity are crucial in characterising turbulent heat transport processes. The advancement of particle-based velocimetry methods has provided both qualitative and quantitative description of turbulent flows. In recent studies, the use of thermographic phosphors particles as flow tracers further supports these advancements due to the additional temperature information they provide. These particles have been employed to obtain planar measurements of flow temperature and velocity in an approach termed thermographic particle image velocimetry. Similarly, a point-based measurement approach has been demonstrated to achieve simultaneous measurements of these flow vector-scalar properties. This paper further describes and characterises the point-based joint measurement technique called thermographic laser Doppler velocimetry (thermographic LDV) technique for flow temperature and velocity measurements. The flow metrology uses both Mie-scattered light and the optical properties of the phosphorescence emission that results from successive interactions between continuous wave laser light and individual 2 µm BaMgAl10O17:Eu2+ thermographic phosphor particles, which are seeded into the flow as a tracer. Photomultipier tubes (PMTs) are used to detect the signals collected from the measurement volume. The flow velocity is determined from frequency of the Doppler bursts obtained when particles traverses the fringes of two crossed visible laser beams as in conventional LDV. Luminescence in the form of Gaussian bursts that occurs after excitation of the same particles by an overlapped UV laser beam is simultaneously detected. Flow temperatures are evaluated from these acquired luminescence signals using the two-colour ratio, where two PMTs, each fitted with interference filters, transmits different parts of the temperature dependent emission spectral profile. The ratio of the two detected intensities has a monotonic dependence on temperature and is used to infer the particle temperature using previously acquired calibration data. Potential cross dependencies that affect temperature measurements such as seeding density and laser fluence are investigated. The technique is then applied to acquire combined vector-scalar profile measurement at the exit of turbulent heated jet to evaluate the accuracy of the temperature measurements. A deviation better than 2% is achieved between mean temperature profile measurements obtained using a thermocouple and the point-based technique. Thermographic LDV is shown to serve as a valuable tool to turbulent heat transfer research