Search

Abstract The rotary-type energy recovery device (RERD) is a promising energy recovery device which can significantly reduce the energy consumption of the Reverse Osmosis system in seawater desalination. A two-dimensional particle image velocimetry (PIV) measurement has been carried out in our previous study to visualize and understand the flow dynamics in the duct of the RERD. Based on the PIV experimental results, a computational fluid dynamics (CFD) method was applied to study flow characteristics and mixing process in the RERD. Three different turbulence modeling approaches (standard k - ε , RNG k - ε , and Realizable k - ε) were compared and validated by PIV experimental data. As a result, the simulation method was able to capture major features of the flow characteristic in the duct, such as the jet flow and the vortex structure. The Realizable k - ε turbulence model, presenting a good agreement with the experimental data, was recommended for analyzing the detailed information of flow characteristics. The simulation results revealed a swirling flow formed in the duct. This three-dimensional flow structure caused high flow turbulence and significant mass transfer in the mixing zone. These results would be beneficial to researchers in better understanding the mixing process in the duct of the RERD. Highlights • Three different k - ε turbulence model results were compared with experimental data. • Numerical results enriched the finding of the "entrance effect" observed by PIV method. • The Realizable k - ε model showed better predictions of vortex structures in the duct center. • A high concentration gradient and a swirling flow were in the duct transverse plane. [ABSTRACT FROM AUTHOR]

Abstract: In a gas–solid fluidized bed, the particle vortexes play an important role in determining the heat transfer capability, while the coherent structures dominate the hydrodynamic characteristics of the turbulent flow field. A two-dimensional Eulerian–Eulerian model integrating the kinetic theory of granular flow (KTGF) is used to simulate the particle vertical fluctuating velocity in a gas–solid fluidized bed, based on which the continuous wavelet transform technique is for the first time introduced to analyze the particle vortex behavior. The results show that the time scale of a particle vortex has a linear relationship with the wavelet scale. Some particle vortexes perform periodic motion and merge with or separate from others until the end of the vortex period. As the superficial gas velocity and bed height increase, particle vortexes are stretched and their boundaries become unclear. Furthermore, to investigate the intermittency of the gas–solid flow field, the probability density function (PDF) of wavelet coefficients is computed. It is found that the wavelet coefficients and the probabilities of intermittent events both improve as the wavelet scale, superficial gas velocity and bed height increase. Since intermittency is caused by coherent structures in the flow field, the effects of coherent structures on the intermittency are studied through a coherent structure extracting method and the extended self-similarity (ESS) scaling law. Before extraction, wavelet coefficients at the same scale increase as the superficial gas velocity and bed height increase, while the scaling law curve at U g =0.75m/s deviates from those at other superficial gas velocities, but shows no obvious change with the bed height. After extraction, the discriminations of scaling law curves under different operating conditions are eliminated and all curves merge in one. The effects of bubble formation, their size and propagation on particle vortex behavior are also analyzed. Bubble motion becomes more violent as gas velocity and bed height increase, which enhance the particle vortex size and activity and coherent structure generation, leading to stronger intermittency of granular flow field. As gas velocity increases, the amplitude of pressure fluctuation first increases and then decrease, while one sharp peak changes to be uniform lower peaks. As bed height increases, the major frequency decreases slightly and the amplitude increases slightly. This work provides deep insights into the turbulent flow field in a gas–solid fluidized bed by CFD simulation and the continuous wavelet transform technique. [Copyright &y& Elsevier]

Abstract: This work presents a novel swirler with variable blade configuration for gas turbine combustors and industrial burners. The flow dynamics downstream the swirler was explored using Large Eddy Simulation (LES). The resolved turbulence kinetic energy in the region where the flow exhibits the main flow phenomena was well above 80% of the total turbulent kinetic energy of the flow. It was evidently shown that the new swirler produces a central recirculation zone and a Rankine vortex structure which are necessary for swirl flame stabilization. Two Reynolds-averaged NavierStokes (RANS) simulation cases utilizing the standard and realizable k-ε turbulence models were also conducted for two objectives. The first is to demonstrate the validity of RANS/eddy-viscosity models in predicting the main characteristics of swirling flows with comparison to the LES results. The second objective is to comparatively investigate the flow features downstream the new swirler in both co-rotating and counter-rotating blade configurations. The results show that the counter-rotating configuration produces higher turbulence kinetic energy and more compact recirculation zone compared to the co-rotating configuration. [Copyright &y& Elsevier]