不同构形构造分布器的数值研究

Seven distributors with different configurations are designed and optimized by constructal approach. Their flow distribution performance and energy dissipation are investigated and compared by computational fluid dynamics (CFD) simulation. The reliability of CFD simulation is verified by experiments on the distributor that has all distributing rectangle channels on a plate. The results show that the symmetry of the distributing channels has decisive influence on the performance of flow distribution. Increasing the generations of channel branching will improve the flow distribution uniformity, but on the other hand increase the energy dissipation. Among all the seven constructal distributors, the distributor that has dichotomy configuration, Y-type junctions and straight intercon-necting channels, is recommended for its better flow distribution performance and less energy dissipation.     

Numerical optimization for blades of Intermig impeller in solid–liquid stirred tank

The multiphase flow in the solid-liquid tank stirred with a new structure of Intermig impeller was analyzed by computational fluid dynamics(CFD).The Eulerian multiphase model and standard k-ε turbulence model were adopted to simulate the fluid flow,turbulent kinetic energy distribution,mixing performance and power consumption in a stirred tank.The simulation results were also verified by the water model experiments,and good agreement was achieved.The solid-liquid mixing performances of Intermig impeller with different blade structures were compared in detail.The results show that the improved Intermig impeller not only enhances the solid mixing and suspension,but also saves more than 20% power compared with the standard one.The inner blades have relatively little influence on power and the best angle of inner blades is 45°,while the outer blades affect greatly the power consumption and the optimized value is 45°.     

A CFD model for predicting the heat transfer in the industrial scale packed bed

Compared to the traditional lumped-parameter model,computational fluid dynamics (CFD) attracted more attentions due to facilitating more accurate reactor design and optimization methods when analyzing the heat transfer in the industrial packed bed.Here,a model was developed based on the CFD theory,in which the heterogeneous fluid flow was resolved by considering the oscillatory behavior of voidage and the effective fluid viscosity.The energy transports in packed bed were calculated by the convection and diffusion incorporated with gaseous dispersion in fluid and the contacting thermal conductivity of packed particles in solids.The heat transfer coefficient between fluid and wall was evaluated by considering the turbulence due to the packed particles adjacent to the wall.Thus,the heat transfer in packed bed can be predicted without using any adjustable semi-empirical effective thermal conductivity coefficient.The experimental results from the literature were employed to validate this model.     

Influence of nanoparticle concentrations on flow boiling heat transfer coefficients of Al2O3/R141b in micro heat exchanger by direct metal laser sintering

Al2O3/R141b+Span-80 nanorefrigerant for 0.05 wt.% to 0.4 wt.% is prepared by ultrasonic vibration to investi-gate the influence of nanoparticle concentrations on flow boiling heat transfer of Al2O3/R141b+Span-80 in micro heat exchanger by direct metal laser sintering.Experimental results show that nanoparticle concentrations have significantly impact on heat transfer coefficients by homogeneity test of variances according to mathemat-ical statistics.The heat transfer performance of Al2O3/R141b+Span-80 nanorefrigerant is enhanced after adding nanoparticles in the pure refrigerant R141b.The heat transfer coefficients of 0.05 wt.%,0.1 wt.%,0.2 wt.%,0.3 wt.% and 0.4 wt.% Al2O3/R141b+Span-80 nanorefrigerant respectively increase by 55.0%,72.0%,53.0%,42.3% and 39.9% compared with the pure refrigerant R141b.The particle fluxes from viscosity gradient,non-uniform shear rate and Brownian motion cause particles to migrate in fluid especially in the process of flow boiling.This mi-gration motion enhances heat transfer between nanoparticles and fluid.Therefore,the heat transfer performance of nanofluid is enhanced. It is important to note that the heat transfer coefficients nonlinearly increase with nanoparticle concentrations increasing.The heat transfer coefficients reach its maximum value at the mass concentration of 0.1% and then it decreases slightly.There exists an optimal mass concentration corresponding to the best heat transfer enhancement. The reason for the above phenomenon is attributed to nanoparticles deposition on the minichannel wall by Scanning Electron Microscopy observation.The channel surface wettability increases during the flow boiling experiment in the mass concentration range from 0.2 wt.% to 0.4 wt.%.The channel surface with wettability increasing needs more energy to produce a bubble.Therefore,the heat transfer coefficients decrease with nanoparticle concentrations in the range from 0.2 wt.% to 0.4 wt.%.In addition,a new correlation has been proposed by fitting the experimental data considering the influence of mass concentrations on the heat trans-fer performance.The new correlation can effectively predict the heat transfer coefficient.     

Numerical study on non-uniform heat transfer deterioration of supercritical RP-3 aviation kerosene in a horizontal tube

The convective heat transfer of supercritical-pressure RP-3(Rocket Propellant 3) aviation kerosene in a horizontal circular tube has been numerically studied, focusing mainly on the non-uniform heat transfer deterioration along the circumferential direction. The governing equations of mass, momentum and energy have been solved using the pressure-based segregated solver based on the finite volume method. The re-normalization group(RNG) k-ε turbulence model with an enhanced wall treatment was selected. Considering the heat conduction in the solid wall, the mechanism of heat transfer deterioration and the buoyancy effect on deteriorated heat transfer were discussed. The evolution of secondary flow was analyzed. Effects of the outer-wall heat flux,mass flux, pressure and tube thermal conductivity on heat transfer were investigated. Moreover, the buoyancy criterion and the heat transfer correlation were obtained. Results indicate that the poor flow performance of near-wall fluid causes the pseudo-film boiling, further leads to the heat transfer deterioration. The strong buoyancy has an effect of enhancing the heat transfer at the bottom of tube, and weakening the heat transfer at the top of tube, which results in the non-uniform inner-wall temperature and heat flux distributions. Decreasing the ratio of outer-wall heat flux and mass flux, increasing the pressure could weaken the heat transfer difference along the circumferential direction, while the effect of thermal conductivity of tube on the circumferential parameters distributions is more complicated. When the buoyancy criterion of(Gr_q/Gr_(th))_(max)≤ 0.8 is satisfied, the effect of buoyancy could be ignored. The new correlations work well for non-uniform heat transfer predictions.     

流体流动型态对膜通量影响的实验和计算流体力学研究

This paper reports a study on the role of fluid flow pattern and dynamic pressure on the permeate flux through a micro filtration membrane in laboratory scale. For this purpose, a dead-end membrane cell equipped with a marine type impeller was used. The impeller was set to rotate in the clockwise and counter clockwise directions with the same angular velocities in order to illustrate the effect of rotation direction on permeate flux. Consequently, permeate fluxes were measured at various impeller rotational speeds. The computational fluid dynamics (CFD) pre-dicted dynamic pressure was related to the fluxes obtained in the experiments. Using the CFD modeling, it is proven that the change in dynamic pressure upon the membrane surface has direct effect on the permeate flux.     

规整填料内单相流的LDV实验研究

To date, many models have been developed to calculate the flow field in the structured packing by the computational fluid dynamics （CFD） technique, but little experimental work has been carried out to serve the vali-dation of flow simulation. In this work, the velocity profiles of single-phase flow in structured packing are measured at the Reynolds numbers of 20.0, 55.7 and 520.1, using the laser Doppler velocimetry （LDV）. The time-averaged and instantaneous velocities of three components are obtained simultaneously. The CFD simulation is also carried out to numerically predict the velocity distribution within the structured packing. Comparison shows that the flow pattern, velocity distribution and turbulent kinetic energy （for turbulent flow） on the horizontal plane predicted by CFD simulation are in good agreement with the LDV measured data. The values of the x-and z-velocity components are quantitatively well predicted over the plane in the center of the packing, but the predicted y-component is sig-nificantly smaller than the experimental data. It can be concluded that experimental measurement is important for further improvement of CFD model.     

Influence of bio-inspired flow channel designs on the performance of a PEM fuel cell

Performance of the proton exchange membrane fuel cell(PEMFC) is appreciably affected by the channel geometry. The branching structure of a plant leaf and human lung is an efficient network to distribute the nutrients in the respective systems. The same nutrient transport system can be mimicked in the flow channel design of a PEMFC, to aid even reactant distribution and better water management. In this work, the effect of bio-inspired flow field designs such as lung and leaf channel design bipolar plates, on the performance of a PEMFC was examined experimentally at various operating conditions. A PEMFC of 49 cm~2 area, with a Nafion 212 membrane with a 40% catalyst loading of 0.4 mg·cm-2 on the anode side and also 0.6 mg·cm~(-2) on the cathode side is assembled by incorporating the bio-inspired channel bipolar plate, and was tested on a programmable fuel-cell test station.The impact of the working parameters like reactants' relative humidity(RH), back pressure and fuel cell temperature on the performance of the fuel cell was examined; the operating pressure remains constant at 0.1 MPa. It was observed that the best performance was attained at a back pressure of 0.3 MPa, 75 °C operating temperature and 100% RH. The three flow channels were also compared at different operating pressures ranging from 0.1 MPa to 0.3 MPa, and the other parameters such as operating temperature, RH and back pressure were set as 75 °C,100% and 0.3 MPa. The experimental outcomes of the PEMFC with bio-inspired channels were compared with the experimental results of a conventional triple serpentine flow field. It was observed that among the different flow channel designs considered, the leaf channel design gives the best output in terms of power density. Further,the experimental results of the leaf channel design were compared with those of the interdigitated leaf channel design. The PEMFC with the interdigitated leaf channel design was found to generate 6.72% more power density than the non-interdigitated leaf channel design. The fuel cell with interdigitated leaf channel design generated5.58% more net power density than the fuel cell with non-interdigitated leaf channel design after considering the parasitic losses.     

低比转速高速复合离心叶轮的流动模拟和试验研究

Based on the Navier-Stokes equations and the Spalart-Allmaras turbulence model, three-dimensional turbulent flow in four low-specific-speed centrifugal impellers are simulated numerically and analyzed. The relative velocity distribution, pressure distribution and static pressure rise at the design point are obtained for the regular impeller with only long blades and three complex impellers with long, mid or short blades. It is found that the back flow region between long-blade pressure side and mid-blade suction side is diminished and is pushed to pressure side of short blades near the outlet of impeller at suction side by the introduction of mid, short blades, and the size of back flow becomes smaller in a multi-blade complex impeller. And the pressure rises uniformly from inlet to outlet in all the impellers. The simulated results show that the complex impeller with long, mid and short blades can improve the velocity distribution and reduce the back flow in the impeller channel. The experimental results show that the back flow in the impeller has an important influence on the performance of pump and a more-blade complex impeller with long, mid and short blades can effectively solve low flow rate instability of the low-specific-speed centrifugal pump.     

交叉间断肋片与凹槽强化传热的实验与数值研究(英文)

Experimental and numerical investigations have been conducted to study turbulent flow of water and heat transfer characteristics in a rectangular channel with discontinuous crossed ribs and grooves.The tests investigated the overall heat transfer performance and friction factor in ribbed and ribbed-grooved channels with rib angle of 30°.The experimental results show that the overall thermo-hydraulic performance for ribbed-grooved channel is increased by 10%-13.6% when compared to ribbed channel.The investigation on the effects of different rib angles and rib pitches on heat transfer characteristics and friction factor in ribbed-grooved channel was carried out using Fluent with SST(shear-stress transport) k-ω turbulence model.The numerical results indicate that the case for rib angle of 45° shows the best overall thermo-hydraulic performance,about 18%-36% higher than the case for rib angle of 0°.In addition,the flow patterns and local heat transfer characteristics for ribbed and ribbed-grooved channels based on the numerical simulation were also analyzed to reveal the mechanism of heat transfer enhancement.     

Application of CFD for simulation of a baffled tubular membrane

Computational fluid dynamics (CFD) investigation of a tubular membrane channel containing a set of baffles was conducted for predicting turbulent flow. Simulation was performed using an array of baffles oriented either in the flow or in the reverse direction. A range of local parameters such as stream function, velocity, static pressure, wall shear stress, turbulent kinetic energy, and turbulent dissipation energy on the membrane surface was computed using CFD code FLUENT. The simulation results indicate that the presence of baffle can improve the local shear stress on the membrane surface and produces eddy activities which enhance the filtration performance. The observed flux enhancement can be attributed to the intense fluctuations of wall velocity and shear stress which can disrupt the growth of boundary layer on the membrane surface. The experimental evaluation was performed through cross flow microfiltration of titanium dioxide suspension which showed an acceptable agreement with the CFD predictions.     

Comparison of Different Pitch Lengths on Static Promoters for Flux Enhancement in Tubular Ceramic Membrane

Abstract

The use of turbulence promoters in membrane based processes have been investigated and are increasingly been used in industrial applications to minimize fouling and enhance the membrane flux. The efficiency of crossflow microfiltration is limited by membrane fouling and concentration polarization leading to flux decline during operation. A detailed study was carried out in the microfiltration of yeast suspensions using an in‐house rig and three different static turbulence promoters of varying pitch lengths. The design of the promoters incorporates a helical thread around the length of the insert, which induces turbulent flow through the membrane. This promotes good mixing of the feed fluid and minimizes concentration polarization effects. The testing of tubular membranes with the static inserts has been carried out and the results are included in the report. The pitch lengths used were 7 mm, 10 mm, and 14 mm and the parameters investigated included temperature, CFV, concentration of feed suspension and pressure. The flux decline data was recorded over a 50 minute filtration cycle and the cleaning protocol was employed after every cycle to restore the permeability of the membrane. A comparison of the membrane performance and efficiency of the three swirls inserts of varying pitch lengths together with a comparison of the degree of total, reversible, and irreversible fouling data amongst others are reported and discussed. The results obtained during the investigations of flux enhancement via static turbulence promoters into the tubular membranes are presented and are selected to differentiate the efficiency of the inserts and the degree of fouling associated with them.     

Flux Improvement during Cross-flow Microfiltration of Wheat Starch Suspension using Turbulence Promoter

The objective of this study was to investigate the effects of the process variables (transmembrane pressure, flow rate, and concentration) on the permeate flux during the microfiltration of model starch suspensions, and to determine the conditions under which the use of Kenics statics mixer as a turbulence promoter is justified. A response surface methodology was used to examine the influence of the selected operating conditions on starch suspension microfiltration using a single channel ceramic membrane with 200 nm pore size. The experimental results clearly show that the improved performance of starch suspension cross-flow microfiltration can be obtained by using a Kenics static mixer, especially at lower flow rates. Compared to the operation without the turbulence promoter, the average permeate flux improvement during the filtration period ranged from 30% to 230%. As a result of the statistical analysis, the optimal conditions for starch suspension microfiltration were determined and applied to microfiltration of starch industry wastewater.     

Influence of Geometry Structures on the Transfer and Flow Characteristics of Membrane Modules

The effects of different inlet/outlet features and helical structures on the flow pattern and transfer performance of membrane modules were investigated with both three‐dimensional computational fluid dynamics and experimental methods. In a circular flat membrane module, two types of inlet/outlet with single‐port or three‐port configuration were used to evaluate the influence of small differences in the opening area structure of the inlet/outlet on the flow patterns. Although the transfer regions differ, large flow swirls appear in both systems, leading to stagnation regions and causing significant back‐mixing. To alleviate the swirls and improve the flow pattern, a helix clapboard was used to separate the membrane feed channel as spiral channel. Both the numerical and experimental results indicate that the helical structure can greatly enhance the transfer performance and increase the permeation rate while the increment in energy consumption is very small.     

Performance Characteristics of a Suspended‐Catalyst Oscillatory Membrane Photocatalytic Reactor

The performance characteristics of an oscillatory membrane photocatalytic reactor were investigated using dye degradation over a suspended ZnO catalyst as a model reaction. Both flat‐surface membranes and ones with transverse turbulence promoters (TP) were used. Application of oscillatory motion can be effective in enhancing the performance of suspended‐catalyst membrane photocatalytic reactors. The eddy formation and vortex shedding when using membranes with TP gave rise to several synergistic effects by providing effective removal of catalyst deposits from the membrane surface, which enhanced the flux and increased the suspended‐catalyst fraction in solution, which consequently enhanced the reaction rate. The effective mixing in the reaction channel minimized particles sedimentation and agglomeration which further enhanced the catalyst suspension and increased its effective reaction area. The specific energy consumption favorably compared to a membrane cross‐flow filtration system.     

Experimental studies on heat transfer and friction factor characteristics of turbulent flow through a circular tube fitted with helical screw-tape inserts

Experimental investigation of heat transfer and friction factor characteristics of circular tube fitted with full-length helical screw element of different twist ratio, and increasing and decreasing order of twist ratio set have been studied with uniform heat flux under turbulent flow conditions. The Reynolds number was varied from 2700 to 13 500. The experimental data obtained are compared with those obtained from plain tube published data. The maximum Nusselt number for the twist of 1.95 was obtained. The performance of the helical twist insert was compared with the twisted tape performance reported in the literature and found that it is better than twisted tape performance. The heat transfer augmentation for helical twist of increasing and decreasing order twist were also presented. The empirical correlations developed relating twist ratio and Reynolds number, are fitting the experimental data within ±13% and ±15% for Nusselt number and friction factor, respectively. The performance evaluation study has been presented to check the potential of using the helical twist insert.     

柱式膜组件结构的CFD优化设计

采用Euler模型与多孔介质模型对不同结构的柱式膜组件内的流体流动进行了计算模拟。研究了曝气孔数目（开孔率为1.92%保持不变）与膜组件高度对膜组件膜丝填充区域内的气液两相分布、壁面剪应力、湍流黏度以及液相速度场的影响。计算模拟数据与实验结果吻合良好。计算模拟表明：通过减小曝气孔直径,增加曝气孔数目的方式能够促进气液两相流场与液相速度场的均匀分布,以及壁面剪应力与湍流黏度的增强;增加膜组件的高度,有利于增加单支膜组件膜面积的同时充分利用曝气擦洗过程中气液两相流对膜丝壁面进行高效的气擦洗。综合考虑膜组件的安装运输、膜丝通量分布以及能耗等因素,对于直径250 mm的膜组件采用曝气孔的直径为6.32 mm,数目为30个,长度在2~2.5 m之间为最优。     

射流强化螺旋通道内流体流动与换热的数值模拟

采用CFD软件模拟了射流作用下圆形截面螺旋通道内流体的流动及强化传热特性,模拟结果与实验结果吻合较好。研究了无量纲曲率?=0.061、无量纲螺距?=0.121的螺旋通道内复合涡旋结构及其演变过程,考察了射流入射角度?=π/6~π/3、射流速比?j=3~6时射流对螺旋通道换热的强化效果。结果表明,射流的初始阶段,射流的冲击作用抑制了单一螺旋通道内的离心二次涡旋,生成一对与其旋转方向相反的射流诱导涡旋,随流动发展,射流诱导涡旋先由两涡演变为单涡结构而后逐渐耗散消失。射流作用显著强化了螺旋通道内侧壁面附近流体的换热,随着?减小或?j增大,强化传热效果增强。?j≥4时,不考虑射流流量增加时综合强化传热因子JF1=1.26~1.67,考虑射流流量增加时JF2=1.008~1.19。     

集成反应器微滤过程中湍流促进器的研究

采用湍流促进器对陶瓷膜分离TiO2超细粒子悬浆液微滤过程进行了强化研究。考察了湍流促进器结构参数对强化过程的影响,确定了合适的湍流促进器为螺旋式湍流促进器,其结构参数为螺杆直径2 mm,螺距20 mm;探讨了湍流促进器的强化机理,同时对湍流促进器的有效性进行了实验考察。研究表明,采用这种方法提高了渗透通量。     

The use of dean vortices for crossflow microfiltration: basic principles and further investigation

The effect of Dean vortices on the filtration flux and efficiency of crossflow microfiltration processes in sinusoidal curved and helically coiled tubular membranes was investigated by several research groups. Experimental results are presented showing an enhanced filtrate flux and efficiency of the filtration process. CFD simulations were performed, revealing two important effects of secondary flow: an increased wall shear stress causing a reduced particle deposition and an enhanced mass transfer between the boundary layer and the bulk phase.