CFD Study of Effects of Module Geometry on Forced Convection in a Channel with Non‐Conducting Fins and Flow Pulsation

CFD simulations were carried out to investigate the effects of the module geometry on forced convection in a rectangular channel containing series of regularly spaced non‐conducting baffles with flow oscillation. The simulations were performed at constant wall temperature. Steady‐flow Reynolds numbers Re in the range of 200 and 600 were studied. The results of the CFD simulations show that, for the effect fin spacing to be significant on heat transfer enhancement in finned system with oscillating flow, the oscillating flow velocity must be higher than the mean flow velocity. Superposition of oscillation yields increasing heat transfer performance with increasing fin height. Fin geometry with pyramidal shape yields highest performance in terms of the heat transfer effectiveness.     

Measurement and CFD simulation of single-phase flow in solvent extraction pulsed column

A CFD (computational fluid dynamics) model of a solvent extraction pulsed column has been developed and run with a single water phase. The results are compared with experimental measurements taken on a pilot scale column using PIV (particle image velocimetry).The pulsed column investigated had disk-doughnut internals and was operated under pulsing intensities ranging from 10 to 32.5 mm/s. PIV measurements of velocity were used to validate the CFD model and to characterise the pulsing flow of a single phase through the column. The CFD modelling was performed for the same geometry and operating conditions using a 2D computational grid and a low Reynolds Number k-ε turbulence model. An improved velocity prediction was achieved by adding a gap between the doughnut internal and the pulsed column wall. The combined measurements and predictions give insight into the effect of the geometry internals on the flow hydrodynamics in the pulsed column.     

Performance of Kenics static mixer over a wide range of Reynolds number

The present study deals with the numerical simulation of flow patterns and mixing behaviour in Kenics static mixer over a wide range of Reynolds number. Three different sets of Kenics mixer (aspect ratio = 1.5) comprised of 3, 9 and 25 elements each have been characterized. The Reynolds number was varied in the range of 1 to 25,000 (i.e., from laminar to turbulent flow regime). The numerical approach takes into account the aspects of the fluid flow at higher Reynolds number values including circumferential velocity profiles at different cross-sections within the Kenics mixer, which were neglected in previous studies. It was observed that cross-sectional mixing in the turbulent flow regime takes place up to 30% of each element length at element-to-element transition; beyond that velocity profiles were uniform. The experiments were also carried out to measure the circumferential and axial velocity profiles and pressure drop in three different Kenics Mixers using air as fluid. The pressure drop per unit element (ΔP/η) was found to be independent of the number of Kenics mixing elements used in the system. The total pressure drop across Kenics mixer obtained by CFD simulations were compared with the experimental pressure drop values and correlations available in the literature. The numerical results were found in good agreement with the experimental as well as the results reported in the literature. A new pressure drop correlation in the Kenics static mixer has been developed.     

Particle motion and separation in a laminar tube flow with downstream enlargement

Particle classification becomes difficult when the difference in density between particle and fluid is low or negligible and the fluid is viscous. For such applications, a process capable of separating the particles according to their size is needed. Such applications are, e.g. found in biological systems for cell separation or in the removal of gel particles from polymer melts. Particle transport in laminar tube flows at low but non zero Reynolds numbers leads to accumulation of large particles near the tube center and forms a particle free zone near the wall. Small particles find their position on their equilibrium radius. Downstream widening of the flow enhances segregation between large and small particles. Large particles can be collected in a centered collector tube downstream, whereas small particles follow their streamlines around the collector tube and can be removed with the remaining flow. The said particle migration is observed when the ratio of particle to tube diameter is 0.2<d/D<0.51 and the tube Reynolds number is in between 0.2<Re<40. CFD simulations reveal the shape of the streamlines in the downstream enlargement with different tube Reynolds number. The efficiency of the classification process is characterized. Particles need a sufficient transportation length in the tube for proper demixing. This effect is analyzed by a laser sheet illuminated system within an acrylic glass tube.     

Residence Time Distribution of Steady and Pulsed Flow in a Parallel‐Plate Channel with Staggered Fins

The residence time distribution (RTD) in a parallel‐plate channel with staggered fins for both steady and pulsed flow conditions was experimentally determined. Dispersion and tank‐in‐series models were also adopted to characterize the system. The process fluid was water and the experiments were performed at room temperature. A steady Reynolds number Re ranging from 100 to 1000 was studied. The pulsating flow was generated using a frequency f of 6–20 Hz and an amplitude A of 0–2.3 mm. A pulse injection of sodium chloride solution was used as a tracer and the response in the form of electrical conductivity was measured at the outlet stream. The flow in the staggered finned channel approaches nearly plug‐flow behavior with either higher steady‐flow velocity or superposition of oscillation at low Re.     

Thin film flow over structured packings at moderate Reynolds numbers

A model describing the dynamic evolution of waves on laminar falling wavy films at low to moderate Reynolds numbers (Re<30) over corrugated surfaces is presented. This model is based on the integral balance method, which is used to simplify the analysis by assuming a parabolic velocity profile within the bulk of the film. The predictions of this model are compared to those obtained from a computational fluid dynamics (CFD) code for which no assumptions regarding the film velocity profile are made. The film evolution profiles, obtained via numerical solution of the model equations, are used to calculate the ratio of film interfacial area to that of the substrate. The model predicts suppression of wave growth on a corrugated surface. The model is also used to predict the effect of packing geometry on the flow characteristics, which may improve packing design. Solutions obtained for Re∼200 using the CFD code demonstrate the degree of fluid accumulation within the ‘valleys’ of the structured substrate wherein re-circulation occurs.     

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.     

Turbulent mixing in a confined rectangular wake

Liquid-phase turbulent transport in a confined rectangular wake was investigated for a Reynolds number of 37,500 based on bulk velocity and the hydraulic diameter of the test section and a Schmidt number of 1250 using particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF). The velocity and concentration field data were analyzed for flow statistics such as the mean velocity, Reynolds stresses, turbulent kinetic energy, turbulent dissipation rate, mixture-fraction mean, mixture-fraction variance and one-point composition probability density functions (PDF). Computational fluid dynamics (CFD) models, including a two-layer k-ε turbulence model, a scalar gradient-diffusion model and a scalar dissipation rate model were validated against PIV and PLIF data collected at six downstream locations. Low-Reynolds-number effects on turbulent transport were taken into consideration through the mechanical-to-scalar time-scale ratio. The experimental and computational results were found to be in satisfactory agreement.     

The Effects of Power Law Fluid Rheology on Coil Heat Transfer for Agitated Vessel in the Transitional Flow Regime

A CFD model of heat transfer from power‐law fluids to helical cooling coils in the transitional flow regime of a baffled tank mixed with a pitched blade turbine was developed with Fluent^TM. The model captured local temperature and velocity gradients. Simulations were run, varying Re, Pr, K and n. The results indicate that a Sieder‐Tate type correlation, with the exponent on and the coefficient in front of the Reynolds number being a function of n, is recommended for estimating ho. Also, a new two coil bank design was found to be more efficient when 450 < Re < 650.     

CFD simulation and experimental validation of fluid flow and particle transport in a model of alveolated airways

Accurate modeling of air flow and aerosol transport in the alveolated airways is essential for quantitative predictions of pulmonary aerosol deposition. However, experimental validation of such modeling studies has been scarce. The objective of this study is to validate computational fluid dynamics (CFD) predictions of flow field and particle trajectory with experiments within a scaled-up model of alveolated airways. Steady flow (Re=0.13) of silicone oil was captured by particle image velocimetry (PIV), and the trajectories of 0.5 and 1.2 mm spherical iron beads (representing 0.7–14.6 μm aerosol in vivo) were obtained by particle tracking velocimetry (PTV). At 12 selected cross sections, the velocity profiles obtained by CFD matched well with those by PIV (within 1.7% on average). The CFD predicted trajectories also matched well with PTV experiments. These results showed that air flow and aerosol transport in models of human alveolated airways can be simulated by CFD techniques with reasonable accuracy.     

Mass transfer from a contaminated fluid sphere

The unsteady mass transfer from a contaminated fluid sphere moving in an unbounded fluid is examined numerically for unsteady‐state transfer. The effect of the interface contamination and the flow regime on the concentration profiles, inside and outside a fluid sphere, is investigated for different ranges of Reynolds number (0 < Re < 200) and Peclet number (0 < Pe < 10⁵), viscosity ratio between the dispersed phase and the continuous phase (0 < κ < 10), and the stagnant‐cap angle (0° < θ_cap < 180°). It was found that the stagnant‐cap angle significantly influences the mass transfer from the sphere to a surrounding medium. For all Peclet and Reynolds numbers and κ, the contamination reduces the mass transfer flux. The average Sherwood number increases with an increase of stagnant‐cap angle and reaches a maximum equal to the average one for a clean fluid sphere at low viscosity ratio and large Peclet numbers. A predictive equation for the Sherwood number is derived from these numerical results. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Non-Newtonian thin films: Theory and experiment

The flow of thin films of Newtonian and non-Newtonian, Power law fluids down a vertical plate was studied in the laminar and wavy flow regimes. A theoretical development based on the existence of a steady, periodic solution to the equations describing film flow was used to predict the flow characteristics of the film. Double logarithmic plots of film thickness versus Reynolds number (Re_{P. L.}) were linear, as predicted, up to a Re_{P. L.} = 100. The ratio of surface to average velocity was found to be approximately independent of the Reynolds number at values predicted by a laminar velocity profile. The wavelength of stable waves was found to be independent of the Reynolds number for a given fluid.     

Self‐sustained oscillations in opposed impinging jets in an enclosure

The flow of jets in confining enclosures has significant application in many engineering processes. In particular, the impingement of axisymmetric jets in a confined space has been examined using flow visualization, laser Doppler anemometry, and numerical simulations. Several flow regions were found; stable steady, regular oscillatory, and irregular oscillatory. Initially, a steady flow field existed for all arrangements for Re_d < ?90 (based on the nozzle diameter d, the fluid kinematic viscosity v and the volumetric flow rate Q through the nozzle (Q = πd²/4U_avg)) but subsequent increments in the fluid velocity caused a regularly oscillating flow field to emerge. The onset of the oscillations and the upper limit of finite oscillations were found to be a function of the Re_d, and the nozzle diameter to chamber dimension ratio. Steady numerical simulations predicted the steady flow field well and good agreement was obtained in unsteady simulations of the oscillating flow field. The oscillating flow field is considered to be a class of self‐sustaining oscillations where instabilities in the jet shear layer are amplified because of feed back from pressure disturbances in the impingement region.     

纵向内翅片管对流换热特性的实验研究及数值模拟

传热强化技术由于能使换热设备的效率提高,一直受到科技界和工业界的重视．一般认为翅片在强化传热中的作用主要是增加了热阻较大一侧流体的面积,从而减小了以总面积计算的热阻．在管外侧加翅片是强化管外换热的最普遍方式之一．相对而言,利用内翅片管强化换热的研究工作开展得比较少．本文对一种新型内翅片管的传热特性进行了实验研究,并通过数值仿真与实验数据进行了比较和分析．     

Dimensional Analysis of Crossflow Microfiltration Data

Abstract

A new approach to correlating crossflow microfiltration (CFMF) data based on dimensional analysis is presented. The steady state flux was assumed to be a function of the trans‐membrane pressure (ΔP), the crossflow velocity (u), the particle concentration (c), filtrate viscosity (μ), and membrane resistance (R _m). Correlations of the form J/u=K(ΔP/cu ²) ^a (ΔP/μuR _m) ^b were tested on three sets of published data: one for CFMF of dried yeast suspensions in a laminar flow hollow fiber module, one for dried yeast suspensions in a turbulent flow tubular module and one for suspensions of latex particles in a laminar flow flat sheet module. The R ² values for the fits of the correlations to the data were 0.98, 0.94, and 0.91 respectively.     

Validation of CFD simulations of a stirred tank using particle image velocimetry data

Methods for validating CFD simulations based on the Reynolds Average Navier-Stokes equation (RANS) against Particle Image Velocimetry (PIV) measurements are investigated and applied to one of the most common problems in the chemical process industry — the prediction of flow field in a stirred vessel. A total of 1024 sequential instantaneous 2D velocity fields along the central axial plane of a stirred vessel with a P-4 axial impeller are obtained through PIV measurement. From the PIV data, the mean velocity, turbulent kinetic energy, Reynolds stresses and dissipation rate fields are extracted. By introducing several tools to quantify the similarities and differences between two-dimensional fields, CFD predictions of the flow field are validated against PIV data. Furthermore, using PIV and LDV data, the effect of boundary conditions on CFD simulation results is examined. The effect of different Reynolds stress closures on the flow prediction is also studied.     

Hydrodynamic and heat transfer characteristics of a centrally heated cylindrical enclosure: CFD simulations and experimental measurements

Natural convection in enclosures is of importance in many engineering applications. The stratification arising out of natural convection may be desirable/undesirable depending on applications. In order to control the degree of stratification, understanding of flow pattern and temperature profiles is required. In the present work, transient natural convection in a cylindrical enclosure has been investigated for water with CFD simulations and flow visualization [using particle image velocimetry (PIV) and hot film anemometry (HFA)] over a wide range of parameters namely Rayleigh number (1.08 × 10¹¹ ≤ Ra ≤ 3.76 × 10¹³) and aspect ratio (1 ≤ H/R ≤ 2). The effect of various parameters like pressure, tube diameter and aspect ratio on the extent of stratification has been studied. PIV measurements have been performed to understand the transient flow behavior. Multiple thermocouples were used to measure the temperature profiles. CFD simulations have been performed using SST k–ω model and the results have been compared with the PIV measurements. The CFD simulations have been carried out for 2D axi-symmetric cases and the effect of boundary conditions (free-slip and no-slip) has been investigated. An excellent agreement was found between the CFD predictions and the experimental measurements of flow and temperature patterns. The extent of stratification has been quantified using dimensionless parameters like stratification number and stratification time. The kinetic energy profiles and kinetic energy dissipation profiles show that almost 75% of the enclosure is stratified (after different times depending on Ra number and the aspect ratio). The turbulence parameters were found to weaken with time in the stratified region and these predictions are corroborated with HFA measurements.     

柔性Rushton搅拌桨的功耗与流场特性研究

基于传统的Rushton桨,开发了一种柔性叶片Rushton搅拌桨。采用数值模拟方法研究了柔性桨的功耗及层流和湍流流场特性,并分别采用扭矩测量法和粒子图像测速法进行了实验验证。结果表明,对于实验规模的搅拌容器,当介质黏度与甘油接近时,可用橡胶作为柔性桨叶制作材料。Reynolds数≤100时,柔性桨的功耗大于刚性桨;Reynolds数大于该值后,柔性桨的功耗小于刚性桨。柔性桨叶对被搅拌流体具有自适应特性,流固耦合作用下产生的变形增加了流体的径向流动能力。搅拌低黏度流体时,柔性桨能提升近桨区流体的速度,增加桨叶远端流体的循环流动能力;搅拌高黏度流体时,近桨区和桨叶远端流体的速度均大于刚性桨。就尾涡而言,柔性桨产生的涡量较小,耗能少。     

Hydrodynamics characterization of the Maxblend impeller

The hydrodynamic characteristics of the Maxblend^TM impeller have been investigated in the case of viscous Newtonian fluids. Both laboratory experiments and 3D finite element based computational fluid dynamics (CFD) simulations have been carried out. The power consumption, the mixing evolution yielding the mixing time, and the effect of baffles in the laminar and transition flow regimes have been determined. It was found that the limit Reynolds number between the laminar and transition regimes is approximately 25 and 38 for the unbaffled and baffled configurations, respectively. Based on the range of Reynolds numbers studied in this work, the best window performance of the Maxblend^TM mixer where fast and homogenous mixing is achieved is the end of the laminar regime and the early transition regime with baffles.     

The influence of reynolds number on the entry length and pressure drop for laminar pipe flow

A numerical analysis of the laminar flow of a Newtonian fluid in the inlet section of a circular pipe has been carried out for a Reynolds number range of 0 to 500, employing a finite element based CFD code. A new flow phenomenon has been identified at low Reynolds numbers (<50), viz, the existence of a peak in the axial pressure profile at a small distance from the entrance. Existing correlations for the entry length are shown to be insufficient and a new correlation for the entry length is proposed.