Review of convective heat transfer enhancement with nanofluids

Prediction of turbulent forced convection of a nanofluid in a tube with uniform heat flux using a two phase approach

Hydrodynamics of binary fluidization in a riser : CFD simulation using two granular temperatures

Computational fluid dynamics (CFD) for fluidization is reaching maturity (Roco, 1998). It has become common practice to compare time-averaged solid velocities and concentrations to measurements of fluxes and densities. The dynamic behavior of the riser, however, has not been previously compared to experiments. This article shows that the dynamics of solids flow in the riser is in the form of clusters, but the time-averaged particle concentrations and fluxes give us the core-annular flow regime in agreement with measurements. The computed clusters, which are essentially compressible gravity waves, produce major frequencies of density oscillations in agreement with measurements. The model and the CFD code compute granular temperature distributions, agreeing qualitatively with data. For volume fraction around 3–4%, which is the average particle concentration in the riser, the computed viscosity agrees with our experimental measurements.

Riser hydrodynamics: Simulation using kinetic theory

To consider the critical effect of mesoscale structure on the drag coefficient, this paper presents a drag model based on the energy-minimization multiscale (EMMS) approach. The proposed structure parameters are obtained from the EMMS model, and then the average drag coefficient can be calculated from the structure parameters and further incorporated into the two-fluid model to simulate the gas-solid flow in a circulating fluidized-bed riser. Simulation results indicate that the simulated flow structures are different for the EMMS-based drag model and the hybrid model using the Wen and Yu correlation and the Ergun equation. The former shows its improvement in predicting the solids entrainment rate, the mesoscale heterogeneous structure involving clusters or strands, and the radial and axial voidage distributions. The simulation results support the idea that the average drag coefficient is an important factor for the two-fluid model and suggest that the EMMS approach could be used as a kind of multiscale closure law for drag coefficient.

Simulation of Heterogeneous Structure in a Circulating Fluidized-Bed Riser by Combining the Two-Fluid Model with the EMMS Approach

The hydrodynamics of gas-solid flow, usually referred to as circulating fluidizedbed flow, was studied in a 7.5-cm clear acrylic riser with 75-μm FCC catalyst particles. Data were obtained for three central sections as a function of gas and solids flow rates. Fluxes were measured by means of an extraction probe. Particle concentrations were measured with an X-ray densitometer. In agreement with previous investigators, these data showed the flow to be in the core-annular regime, with a dilute rising core and a dense descending annular region. However, unlike the previous studies conducted worldwide, the data obtained in this investigation allowed us to determine the viscosity of the suspension. The viscosity was a linear function of the volume fraction of solids. It extrapolates to the high bubbling-bed viscosities.

Aubrey Miller

Papers

Dense, vertical gas‐solid flow in a pipe