Gas Tangential Velocities Research Articles

CFD microscale modelling of flow behavior in different parts of a rotating packed bed

Process intensification (PI) is playing a key role in alleviating the challenge of reducing carbon footprint of many chemical processes and bringing down their development costs. Over the years, many PI technologies have been investigated with rotating packed bed (RPB) technology receiving much of the attention for its potential of significant intensification in terms of capital expenditure, operating costs, and hardware size. In this study, microscale CFD simulations of a rotating packed bed were conducted, and the results were validated with experimental data. The results show the strong relation between the reverse flow at the packing outer periphery and the gas maldistribution factor. The latter is mainly caused by the accelerating flow in the outer cavity. Inside the wire mesh packing, the gas flow is found to be almost fully uniform for nearly half of the total packing depth. Also, turbulent kinetic energy (TKE) levels at the packing outer edge are strongly linked to the slip tangential velocity component, while at its inner edge, they depend mainly on the radial packing velocity. The so-called gas end effect zone is detected by observing the TKE profiles near the packing outer edge. The latter accounts for less than 10% of the total packing depth. The validity of the widely used porous media model in RPBs’ packing for both radial and tangential directions is confirmed by the obtained results, but this excludes the packing inner and outer edges. In the inner cavity region, gas exhibits two distinctive behaviors and transits from free vortex flow to swirling flow as the flow becomes close to the vortex core. As a result of this transition, the increase in shear stress accelerates the decrease in the gas tangential velocity in the vortex core and help speed up the favorable pressure gradient and flow establishment beyond the vortex core.

Effects of the inlet particle spatial distribution on the performance of a gas-solid cyclone separator

The present study was aimed at the inlet particle spatial distribution of a gas-solid separator with high solid loading and investigated its effects on cyclone performance and the inner flow pattern. The flow fields of a cyclone with different rectangular particle flow areas on the inlet surface were numerically simulated using a four-way coupling method. The simulated results indicate that reducing the inlet particle flow area and lowing the inlet particle position can effectively reduce the scattered particles inside the cyclone separator and enhance the separation performance. Vertically gathering the particles to the centerline can also weaken the particle back-mixing. The particles near the roof account for the swirling particle ceiling phenomenon. The inlet particle spatial distribution affects the pressure drop mainly by affecting the gas tangential velocity in the cylinder body. Moreover, compared to the horizontal particle distribution on the inlet surface, the vertical particle distribution has greater effects on cyclone performance.

Effect of gas-induced inlet design on flow pattern and energy loss for small-scale countercurrent vortex reactor: Numerical simulation and experimental investigation

Small-scale vortex reactors have gained attention in the fields of gas-particle multiphase reactions because of the great potential of process intensification. Their performances highly depend upon the vortex flow-induced structure. However, its role has not yet been completely understood. To address the effect of the vortex flow-induced design of a small-scale countercurrent vortex reactor, the gas-induced inlet design with varying aspect ratio (the ratio of inlet height to inlet width of 0.56, 2.25, and 5.06) was designed to numerically and experimentally investigate the flow distribution, energy loss, and process intensification. It was found that for a given flow rate (inlet velocity), and with increasing aspect ratio the maximum tangential velocity is increased whereas the decay of the vortex flow is decreased. The semi-empirical equations were further developed to characterize the spatial distribution of the gas tangential velocity. There is no significant differentiation in axial velocity, but the static pressure near the wall increases significantly. Moreover, the pressure drop of the reactor increases as the inlet aspect ratio increases. Finally, an aspect ratio of 2.25 was found to be optimum for potential multiphase mixing and mass transfer. The results may provide a positive reference for the vortex-flow-based process intensification.

Effects of the inlet duct length on the flow field and performance of a cyclone separator with a contracted inlet duct

The inlet duct length may affect the gas flow field and performance of a cyclone separator with a contracted inlet duct. In this study, a numerical simulation using the Eulerian-Lagrangian approach with a one-way coupling method was conducted to evaluate the gas flow field and separation efficiency of the cyclone separators with contracted inlet ducts of different lengths. The simulation results indicated that increasing the inlet duct length led to higher gas tangential velocity in the cyclone body. The effect of the inlet duct length on gas axial velocity was inferior to that on gas tangential velocity. A longer inlet duct reduced both the pressure drop and the separation efficiency. The reductions of the expansion loss at the inlet of the cylinder and friction loss inside the cyclone body contributed the most reduction of the pressure drop as the inlet duct length increased. Generally, when the inlet duct was 1.5 times longer than the cylinder diameter, the effect of inlet duct length on the gas flow field and performance of a cyclone separator became slight.

Numerical analysis of hydrodynamic characteristics and interphase coupling in a gas‐solid cyclone reactor

AbstractA down‐flow gas‐solid cyclone reactor (GSCR) for fluidized catalytic cracking process was proposed to solve the side reactions. Comprehensive three‐dimensional numerical models were established to interpret its gas‐solid flow features, where the continuous phase and dispersed phase were modelled in the Eulerian grid and Lagrangian framework, respectively. Simulation results show that the four‐way approach has a better prediction of gas‐solid flow hydrodynamics in GSCR than the two‐way approach, and the numerical pressure drop agreed well with the experimental data. Most catalysts accumulated on the wall when descending and the phase interface structure composed of internal pure gas and external mixtures was subsequently formed. The radial distribution of tangential velocity was almost hump shaped. Tangential and axial kinetic energies accounted for 92.49%–95.45% of total kinetic energy and were thus dominant. In the dilute condition, the oscillatory behaviour of particle concentration was more severe than that of gas velocities, and particle concentration and gas tangential velocity had the same dominant frequency (~2.6 Hz). As the particulate loading increased tenfold, the hydrodynamic characteristics became rather complex.

Structural optimization of vortex finder for a centrifugal air classifier

To further improve the centrifugal air classifier performance, effects of the fully-inserted vortex finders with slots are investigated. Experimental results show that the classifier with the non-uniform slotted vortex finder has the higher particle selectivity, smaller cut size and narrower product particle size distribution than the classifier with the uniform slotted vortex finder and the base partly-inserted vortex finder. The classifier obtains the particle classifying sharpness of 1.72 when the primary and secondary gas velocities are 12 m/s and 2.4 m/s. The experimental curves present a same distorted S-shape with the fish-hook dip at about 4 μm. Simulation results indicate that the non-uniform slotted vortex finder could accelerate the swirling flow with the gas tangential velocity averagely increased by 3 m/s. The static pressure varies from −350 Pa to 1700 Pa. Meanwhile, the slotted vortex finder can reduce the influence of the short circuit flow on coarse particles. The DPM model can well predict the cut size with a maximum error of 4.9%, but not the fish-hook effect. The obtained results can provide a guideline for the vortex finder design of the centrifugal air classifier.

A Deep X-Ray Look at Abell 2142—Viscosity Constraints From Kelvin–Helmholtz Eddies, a Displaced Cool Peak That Makes a Warm Core, and A Possible Plasma Depletion Layer

Abstract We analyzed 200 ks of Chandra ACIS observations of the merging galaxy cluster A2142 to examine its prominent cold fronts in detail. We find that the southern cold front exhibits well-developed Kelvin–Helmholtz (KH) eddies seen in the sky plane. Comparing their wavelength and amplitude with those in hydrodynamic simulations of cold fronts in viscous gas, and estimating the gas tangential velocity from centripetal acceleration, we constrain the effective viscosity to be at most 1/5 of Spitzer isotropic viscosity, but consistent with full Braginskii anisotropic viscosity for magnetized plasma. While the northwestern front does not show obvious eddies, its shape and the structure of its brightness profile suggest KH eddies seen in projection. The southern cold front continues in a spiral to the center of the cluster, ending with another cold front only 12 kpc from the gas density peak. The cool peak itself is displaced ∼30 kpc from the brightest cluster galaxy (BCG) (the biggest such offset among centrally peaked clusters), while the X-ray emission on a larger scale is still centered on the BCG, indicating that the BCG is at the center of the gravitational potential and the cool gas is sloshing in it. The specific entropy index of the gas in the peak (K ≈ 49 keV cm2) makes A2142 a rare “warm core”; apparently the large displacement of the cool peak by sloshing is the reason. Finally, we find a subtle narrow, straight channel with a 10% drop in X-ray brightness, aligned with the southern cold front—possibly a plasma depletion layer in projection.

Influence of a laminarizer at the inlet on the classification performance of a cyclone separator

Abstract A laminarizer consisting of a bank of 15 tubes was assembled at the entrance of a cyclone. Experiments and numerical simulations were performed to examine the influence of the laminarizer on the pressure drop across the cyclone, partial separation efficiency, gas tangential velocity distribution, particle distribution at the entrance, and 50% cut size. Using the laminarizer slightly increased the pressure drop across the cyclone and slightly reduced the gas tangential velocity. The laminarizer altered the air mass flow rate and particle flow distribution at the entrance. It increased the air mass flow rate through the outer part of the entrance. At an inlet gas velocity of 11 m/s, installing the laminarizer reduced the number of particles flowing through the inner part of the entrance by 20.2%. Higher particle concentrations were found at the outer part of the entrance, where the particles were more easily captured after entering the main body. At an inlet gas velocity of 11 m/s, using the laminarizer increased the pressure drop across the cyclone by 8.1% and reduced the 50% cut size of the cyclone from 2.04 μm to 1.89 μm.

Pressure Drop Model on Rotating Zigzag Bed as a New High-Gravity Technology

The pressure drop across “Higee”, a synonym for high-gravity technology, is one of the most important characteristics, and the liquid flow gives rise to intriguing and unusual effects on the pressure drop. The pressure drop of the wet Higee can be substantially lower than that of the dry one. This trend is quite different from the conventional packed and trayed tower. And the unusual effects also occur in the rotating zigzag bed (RZB), a novel type of Higee. The pressure drop of RZB was studied by two methods of the pressure drop model, based on gas tangential velocity and empirical correlation. By use of an air–water system, experiments were carried out in a RZB with a rotor that had an inner diameter of 214 mm, outer diameter of 486 mm, and axial height of 104 mm. The radial distribution of gas tangential velocity in the RZB rotor was measured by a five-hole pitot probe. Lliquid flow reduced the gas tangential velocity due to the drag exerted by liquid droplets on the gas. The gas tangential velocities were correlated by an empirical equation on which the centrifugal pressure drop was predicted accurately. The correlation of the frictional pressure drop was subsequently obtained. As a result, the pressure drop model based on gas tangential velocity was established, from which the phenomenon that the wet rotor pressure drop is lower than the dry rotor pressure drop was reasonably explained. The rotor and overall pressure drops of RZB were correlated by empirical equations with good agreement. Deviation of the rotor pressure drops based on experiments and on gas tangential velocity was less than that based on experiments and on empirical equations. Variation of the overall pressure drop of RZB was similar to that of the rotor pressure drop. The pressure drop model based on gas tangential velocity can also be applied to the conventional rotating packed bed, which is beneficial to the theory of the pressure drop of Higee.