旋风分离器高温流场的实验测量

引言 对旋风分离器内流场的认识是研究其分离过程的基础.长期以来,人们对旋风分离器内流场的结构、速度分布等进行了大量的测量和分析,认识到旋风分离器内是一个三维强旋旋湍流场.     

A Model for the Prediction of the Collection Efficiency Characteristics of a Small,Cylindrical Aerosol Sampling Cyclone

Velocity data from a previous study were nondimensionalized and used in conjunction with a computer program which solves the equations for particle trajectory to predict the collection efficiency for the cyclone. Results for the prediction of cutpoint at the same Reynolds number as that for which the velocities were measured, both for a large cyclone of 88.9 mm diameter and another geometrically similar at one half the scale, are excellent. The model predicts cutpoints of 10 μm and 5.1 μm for the large and small cyclone, respectively, while the actual cutpoints determined from aerosol tests were 9.9 μm and 5.2 μ m. The efficiency curve generated by the model was steeper (geometric standard deviation of 1.1) than the efficiency curve determined through the aerosol testing (geometric standard deviation of 1.4). A simplification of the Dirgo and Leith equation fitting Barth's design curve is suggested which provides a significantly better fit of the aerosol data (geometric standard deviation of 1.3). At 1.5N _{R Q}, where N _{R Q} = (4pg)/(πμD _c), the error in prediction of the cutpoint in the large cyclone is less than 8% while at 04N _{R Q} the error is less than 2%. Although results are good over a limited range of Reynolds numbers, the model is strictly applicable only for flows which are dynamically similar to those studied here.     

Optimizing swirl in compact uniflow cyclones

This paper presents an experimental examination of the velocity field distribution in the separation chamber of a uniflow cyclone with closed particle outlet to evaluate the swirl characteristics in the vortex finder region based on stereoscopic particle image velocimetry. A cold flow model with a closed particle outlet was used to assess different angles of attack and core size ratios at typical Reynolds numbers for the separation of low loaded gas-solid flows. The focus of the study was on the relationship between swirl strength as well as performance data. At higher angles, the parabolic swirl strength distribution changed to a region with constant high acceleration of the particles in the separation zone. Integral and differential swirl numbers were correlated with the ratio of tangential to radial velocity and to the calculated cut size diameter. At low angles of attack, implying a strong redirection of flow perpendicular toward the main flow direction and small core size ratios, defined by the radial distance between hub and tip, the local differential swirl number can be more than twice as large as in the base configuration. Yet, the integral swirl number hardly changed. The velocity fields showed mean tangential to radial velocity ratios ranging from 0.73 to 6.85 at swirl vane angles of 15 ^° –60 ^° ; core size ratios between 0.125 and 0.625 at vortex finder diameter were measured and calculated cut size diameters between 10 and 90 μm were derived. This data provides the foundation for further validation studies and the development of new design criteria. © 2018 American Institute of Chemical Engineers AIChE J, 65: 766–776, 2019     

Velocity profiles and frictional pressure drop for shear thinning materials in lid-driven cavities with fully developed axial flow

A finite element numerical study has been carried out on the isothermal flow of power law fluids in lid-driven cavities with axial throughflow. The effects of the tangential flow Reynolds number (Re_U), axial flow Reynolds number (Re_W), cavity aspect ratio and shear thinning property of the fluids on tangential and axial velocity distributions and the frictional pressure drop are studied. Where comparison is possible, very good agreement is found between current numerical results and published asymptotic and numerical results. For shear thinning materials in long thin cavities in the tangential flow dominated flow regime, the numerical results show that the frictional pressure drop lies between two extreme conditions, namely the results for duct flow and analytical results from lubrication theory. For shear thinning materials in a lid-driven cavity, the interaction between the tangential flow and axial flow is very complex because the flow is dependent on the flow Reynolds numbers and the ratio of the average axial velocity and the lid velocity. For both Newtonian and shear thinning fluids, the axial velocity peak is shifted and the frictional pressure drop is increased with increasing tangential flow Reynolds number. The results are highly relevant to industrial devices such as screw extruders and scraped surface heat exchangers.     

Performance of a Degassing Cyclone with Main and Subsidiary Chambers

Based on the premise that large bubbles are removed in larger cyclones and small bubbles in smaller cyclones, a combined degassing cyclone with main and subsidiary chambers was designed to enhance liquid degassing. The pressure loss, liquid flow rate at the gas outlet, split ratio, gas flow rate at the liquid outlet, and degassing efficiency of the degassing cyclone were measured and calculated. Pressure loss correlations were established which relates the Euler number to the gas and liquid Reynolds numbers in the main chamber. Most cases exhibit a degassing efficiency greater than 0.998 when the liquid flow rate is more than 0.7 m³h^?1. The contours of pressure loss, split ratio, and degassing efficiency provide an effective guidance for designing a degassing cyclone.     

Modelling of the Pressure Drop in Tangential Inlet Cyclone Separators

This article introduces a new mathematical model that predicts the pressure drop in a tangential inlet cyclone. The model calculates the pressure drop from the frictional losses in the cyclone body, using a wall friction coefficient based on the surface roughness and Reynolds number. The entrance and exit losses are also included in the model by defining new geometrical parameters. The pressure drop coefficient is obtained as a function of cyclone dimensions and operating conditions. The model is validated by studying 12 different cyclones presented in the literature. Comparison of the model results with predictions and measurements published in the literature show that the new model predicts the experimental results quite well for a wide range of operating conditions covering a flow rate of 0.3–220 l/s and a temperature range of 293–1200°K, in different cyclones. The pressure drop coefficient is also examined in view of the outlet pipe diameter, friction coefficient, surface roughness, and Reynolds number.     

Experimental and numerical characterization of viscous flow and mixing in an impinging jet contactor

The laminar flow in an impinging jet contactor is examined as a first step toward the development of new technology for fast mixing of viscous fluids. The flow, velocity, and stretching fields in an impinging jet contactor are quantified for low Reynolds number flow using three-dimensional numerical simulations and particle image velocimetry measurements. Computational and experimental velocity fields are in close agreement, as quantified by the velocity probability density functions. Two steady-state flow regimes are found to exist: for jet Reynolds numbers (Re_j) < 10, the jets do not impinge and the velocity field scales linearly with Reynolds number; for Re_j > 10, the jets begin to impinge and recirculation regions form above and below the impingement point. The magnitude of the rate-of-strain tensor is calculated as a function of Re_j. While areas of essentially zero stretching occupy most of the flow domain, very high rates of stretching occur at specific locations in the flow. The maximum and average rates of stretching in the contactor increase roughly linearly as a function of Reynolds number. Mixing simulations show that no mixing occurs for the steady flow in a symmetric-jet contactor. However, mixing is improved substantially by a slight modification of the impinging jet geometry that disrupts geometric symmetry.     

Fine particle turbulence modulation

Streamwise turbulence intensities of fine particulate suspensions were studied in a 26 mm N.B. horizontal pipe loop. Colloidal silica spheres were prepared in 10^?4M and 1M KNO₃ solutions to control the degree of aggregate formation in the suspension. Using an ultrasonic Doppler velocity profiling sensor, the turbulence intensities of the fine particle suspensions were compared with those of a particle‐free flow over a range of Reynolds numbers. At low electrolyte concentration, the silica particles remain dispersed, with the turbulence intensity of the suspension flow comparable with that of the particle‐free flow. At high electrolyte concentration, increased particle‐particle interaction leads to the formation of particle aggregates which support turbulence augmentation over a critical Reynolds number range. The range of Reynolds numbers over which this turbulence enhancement is observed is limited by both fluid dynamic effects at low Reynolds numbers (Re ≈ 5500) and aggregate breakup at high Reynolds numbers (Re ≈ 8000). © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Modelling of Self-Induced Oscillations in the Mixing Head of a RIM Machine

In this paper, the self-induced oscillations in the mixing head of a RIM machine were modelled. An analytical and simple correlation was derived between the Strouhal number and the averaged pressure drop along the amplitude of the oscillation in the mixing chamber. This correlation theoretically identified that the frequency of the oscillations could be well correlated by the jet Reynolds number, Re_d, and the dimensionless distance between the two jets as proposed by Denshchikov et al., Fluid Dyn. 3 , 460-462 (1983). The flow field dynamics in the mixing head was simulated successfully by Fluent and the computed pressure fluctuations were used to calculate the frequencies of the oscillations in the mixing head. The calculated Strouhal numbers are in good agreement with the dominant frequency from the power spectra of the measured velocity component u_x (Santos, 2003). Finally, the effect of Re_d and Froude number, Fr, on the Strouhal number was investigated in the impingement region. The average Strouhal number showed a decrease with the Reynolds numbers, due to the increasing randomness of flow field in the impingement region. It was also found that the operations at lower values of Fr presented an increasing stability up to the point where the system is unable to present dynamic evolution. The model in this paper provides a theoretical starting point towards understanding of the quantities of the oscillatory flow in the mixing head, as well as a numerical approach to evaluate the dominant frequency in the mixing chamber.     

Micromixing performance in a rotating bar reactor

A rotating bar reactor (RBR) was designed for adjusting micromixing efficiency. It consists of two concentric cylinders of inner rotating and outer fixed cylinders. Two feeding modes, named as radial and tangential feeding modes, were applied to modify the feeding at the bottom section of the RBR. The influence of feeding modes on the micromixing performance was measured with different rotational speeds, flow rates, viscosities, volumetric flow ratios, and hydrogen ion concentrations. Experimental results reveal that the RBR with tangential feeding mode has better micromixing performance than that with radial mode. The higher rotational speed has a positive impact on the segregation index (X_s). The values of X_s increased with the decrease of the flow rate, and the increase of the viscosity, volumetric flow ratio, and hydrogen ion concentration. The micromixing time, estimated by using an incorporation model, was in the range of 5.00 × 10⁻⁶-1.13 × 10⁻⁵ seconds in the RBR with a tangential feeding mode, which indicates a promising prospect of the RBR for process intensification of fast and complex reactions.     

旋风分离器内旋转流的不稳定性

<正>引言旋风分离器内的流场是一个复杂的旋转流流场,以往的流场研究主要集中在稳态流场的时均特性上,重点关注流场的时均速度分布和湍流特性方面,从实验测量和模拟两方面进行,而对瞬态速度的分析不够,对流场的不稳定性尚缺乏分     

Design and performance evaluation of vacuum cleaners using cyclone technology

A cyclone technology for a vacuum cleaner—axial inlet flow cyclone and the tangential inlet flow cyclone — to collect dusts efficiently and reduce pressure drop has been studied experimentally. The optimal design factors such as dust collection efficiency, pressure drop, and cut-size being the particle size corresponding to the fractional collection efficiency of 50% were investigated. The particle cut-size decreases with reduced inlet area, body diameter, and vortex finder diameter of the cyclone. The tangential inlet twin-flow cyclone has good performance taking into account the low pressure drop of 350 mmAq and the cut-size of 1.5 μm in mass median diameter at the flow rate of 1 m³/min. A vacuum cleaner using tangential inlet twin-flow cyclone shows the potential to be an effective method for collecting dusts generated in the household.     

Modeling the gas flow in a cyclone separator at different temperature and pressure

The gas flow field in a cyclone separator, operated within a temperature range of 293 K–1373 K and a pressure range of 0.1–6.5 MPa, has been simulated using a modified Reynolds-stress model (RSM) on commercial software platform FLUENT 6.1. The computational results show that the temperature and pressure significantly influence the gas velocity vectors, especially on their tangential component, in the cyclone. The tangential velocity decreases with an increase in temperature and increases with an increase in pressure. This tendency of the decrease or increase, however, reduces gradually when the temperature is above 1000 K or the pressure goes beyond 1.0 MPa. The temperature and pressure have a relatively weak influence on the axial velocity profiles. The outer downward flow rate increases with a temperature increase, whereas it decreases with a pressure increase. The centripetal radial velocity is strong in the region of 0–0.25D below the vortex finder entrance, which is named as a short-cut flow zone in this study. Based on the simulation results, a set of correlations was developed to calculate the combined effects of temperature and pressure on the tangential velocity, the downward flow rate in the cyclone and the centripetal radial velocity in the short-cut flow region underneath the vortex finder.     

Numerical prediction of wall mass transfer rates in turbulent flow through a 90° two-dimensional bend

The present paper reports a numerical study of mass transfer in a curved channel flow using a low Reynolds number k–ε model modified to include the influence of curvature on the turbulent transport. Predictions were obtained of flow through a 90° 2-D bend for Reynolds numbers of Re = 2 × 10⁴, 10⁵ and 10⁶, curvature ratio's of R_c/d = 1, 1.5 and 3, and Schmidt numbers of Sc = 500, 1 500 and 5 000. Based on the numerial simulations, the value of the mass transfer coefficient, k, was observed to generally decrease and increase along the inner and outer walls, respectively, compared to the value in a straight duct. The behaviour appears to be sensitive to both the curvature ratio and Reynolds number, being most pronounced for the case of tight bends and low Reynolds numbers.     

The confined impeller stirred tank (CIST): A bench scale testing device for specification of local mixing conditions required in large scale vessels

A new stirred tank geometry, the confined impeller stirred tank (CIST), was designed to provide repeatable testing of the effect of mixing on the performance of chemical additives at the bench scale. The CIST (T = 0.076 m, H = 3T) is filled with five or six impellers. Three impeller geometries were tested: A310, Rushton and Intermig. This paper presents the following hydrodynamic characteristics of the CIST: power number, flow number, momentum number, velocity profiles at different locations in the tank and the transition point from fully turbulent to transitional flow. Based on the scaled velocity profiles, the CIST was able to keep the flow turbulent at Re < 2000 for Rushton turbines and 3200 for Intermigs. The ratio ?_max/?_average was lower for the CIST than for a conventional stirred tank, indicating that the energy dissipation is more uniformly distributed in the CIST. The CIST consistently maintains turbulent flow down to a Reynolds number 10× smaller than that needed in a conventional stirred tank.     

Hindered settling in non-newtonian power law liquids

New experimental results on the hindered settling of model glass bead suspensions in non-Newtonian suspending media are reported. The data presented encompass the following ranges of variables: 7.38 × 10^?4 ≤ Re_1∞ ≤ 2; 0.0083 ≤ d/D ≤ 0.0703; 0.13 ≤ C ≤ 0.43 and 1 ≥ n ≥ 0.8. In these ranges of conditions, the dependence of the hindered settling velocity on concentration is adequately represented by the corresponding Newtonian expressions available in the literature. The influence of the power law flow behaviour index is completely embodied in the modified definition of the Reynolds number used for power law liquids.     

Effects of separation space diameter on the performance of a novel reverse flow cyclone

ABSTRACT

A numerical study was carried out to investigate the effect of separation space diameter on the performance of a novel reverse flow tangential inlet cyclone design by using the Eulerian-Lagrangian approach. The design of this cyclone is based on the idea of increasing vortex length and decreasing pressure drop compared with traditional cyclones. This novel cyclone differs from the traditional cyclones with separation space and vortex limiter instead of the conical part. A qualitative numerical study was performed to analyze the effect of separation space diameter on the cyclone performance at different flow rates by evaluating velocity profile, pressure drop, fractional and overall efficiencies. The results show that the collection efficiency of smaller particles increases while pressure drop decreases significantly with the increase in separation space diameter for D₁/D < 0.5.     

循环旋风分离器内气液两相流动数值模拟

采用雷诺应力模型RSM对循环旋风分离器内气液两相流动的情况进行了数值模拟研究,讨论了循环旋风分离器内切向速度、轴向速度、径向速度、压力场、雷诺应力的分布特点以及相同入口速度下分离器内液滴运动轨迹与分离器的分离效率。数值模拟结果表明,循环旋风分离器切向速度呈现明显的驼峰状,轴向速度上行流和下行流明显,径向速度相对较小,压力由轴心向外逐渐升高,雷诺应力分布复杂且无明显规律,分离器对小直径液滴分离效率较低,入口速度对分离效率的影响比较明显。     

Impinging-Jet Ozone Bubble Column Modeling: Hydrodynamics,Gas Hold-up,Bubble Characteristics,and Ozone Mass Transfer

A transient back flow cell model was used to model the hydrodynamic behaviour of an impinging-jet ozone bubble column. A steady-state back flow cell model was developed to analyze the dissolved ozone concentration profiles measured in the bubble column. The column-average overall mass transfer coefficient, k_La (s^?1), was found to be dependent on the superficial gas and liquid velocities, u_G (m.s^?1) and u_L (m.s^?1), respectively, as follows: k_La?=?55.58 · u_G ^1.26· u_L ^0.08 . The specific interfacial area, a (m^?1), was determined as a = 3.61 × 10³ · u_G ^0.902 · u_L ^?0.038 by measuring the gas hold-up (ε _G?=?4.67 · u_G ^1.11 · u_L ^?0.05 ) and Sauter mean diameter, d_S (mm), of the bubbles (d_S?=?7.78 · u_G ^0.207 · u_L ^{? 0.008} ). The local mass transfer coefficient, k_L (m.s^?1), was then determined to be: k_L?=?15.40 · u_G ^0.354 · u_L ^0.118 .     

Air drag on fine filaments at oblique and normal angles to the air stream

Measurements are reported for air drag on fine filaments whose axes are oriented at oblique and normal angles to the air velocity. In terms of the drag coefficient C_DN the data are fit well by the following relation: C_DN = 6.96(Re_DN)^?0.440(d/d₀)^0.404, where Re_DN is the Reynolds number based on flow normal to the fiber axis, and d/d₀ is a dimensionless fiber diameter. A wide range of conditions were tested: filament diameters ranged from 13 to 390 microns, gas velocities ranged from 22 to 83 m/s, and fiber Reynolds numbers ranged from 29 to 2120.