The effect of air velocity on slugs in a confined channel

The present work focuses on the study of slugs occurring in a two-phase flow of a confined rectangular channel: conditions of appearance and effect on the flow behavior. Three-dimensional numerical simulations have been carried out to examine the effect of superficial air velocity on flow behavior. The Volume Of Fluid model (VOF) is used to track the air-water interface. Validation of the numerical model is obtained by comparing the results of the simulated axial velocity with experimental data determined using the Laser Doppler Anemometry (LDA) technique. The numerical results revealed that for a fixed water level and superficial water velocity, higher superficial air velocities generate a slug flow that causes channel blockage. The position of these slugs and the timing of their occurrence were correlated in terms of air and water superficial Reynolds numbers.     

Correlation between friction and flow of lubricating greases in a new tribometer device

A tribometer requires speed and normal force control, as well as a torque measurement, to acquire tribological data. An air bearing‐supported rotational rheometer allows the measurement of the same variables, but in a broader range and with better accuracy. This and the intension to measure Stribeck curves, as well as the static friction with one single instrument, led to the idea to design an accessory turning a rheometer into a high‐resolution tribometer based on a ball on pyramid principle. Speed ramp and static friction measurements of oil‐lubricated systems illustrate the performance of the rheo‐tribometer. The flow behaviour of greases is more complex, and extensive rheological and tribological testing was performed on three greases of the American National Lubrication Grease Institute classes 0, 1 and 2 at the temperatures of 25 and −40°C. The influence of temperature could be monitored, and a correlation between rheology and tribology was found. To link the model system with real applications for greases, a ball bearing fixture for the tribological accessory has been designed for which first results are presented. Copyright © 2009 John Wiley & Sons, Ltd.     

Turbulent flow over thin rectangular riblets

The effect of longitudinal thin rectangular riblets aligned with the flow direction on turbulent channel flow has been investigated using direct numerical simulation. The thin riblets have been modeled using the immersed boundary method (IBM) where the velocities at only one set of vertical nodes at the riblets positions are enforced to be zeros. Different spacings, ranging between 11 and 43 wall units, have been simulated aiming at getting the optimum spacing corresponding to the maximum drag reduction while keeping the height/spacing ratio at 0.5. Reynolds number based on the friction velocityuτ and the channel half depth δ is set to 150. The flow is driven by adjusted pressure gradient so that the mass flow rate is kept constant in all the simulations. This study shows similar trend of the drag ratio to that of the experiments at the different spacings. Also, this research provides an optimum spacing of around 17 wall units leading to maximum drag reduction as experimental data. Explanation of drag increasing/ decreasing mechanism is highlighted.     

Formulation,rheology and thermal ageing of polymer greases—Part I: Influence of the thickener content

The aim of this work is to show the correlation between polymer greases’ rheology and its formulation. The tested polypropylene (PP) thickened greases were evaluated regarding their thickener content and its effect on the rheological properties. An artificial ageing procedure was performed by heating fresh grease samples in an oven to study the thermal degradation. The ageing evaluation was performed through rheological measurements, FT-IR spectra, oil loss, bleed-oil viscosity changes and bleed rate. The rheology measurements were performed on a rotational rheometer, emphasizing on the storage and loss moduli values at the Linear Visco-Elastic (LVE) region. The flow curve of each grease was also measured. A modified Herschel–Buckley model was applied and the data was correlated to the thickener content.     

The Influence of Speed,Grease Type,and Temperature on Radial Contaminant Particle Migration in a Double Restriction Seal

Microparticle image velocimetry (μPIV) is used to measure the grease velocity profile in small seal-like geometries and the radial migration of contaminant particles is predicted. In the first part, the influence of shaft speed, grease type, and temperatures on the flow of lubricating greases in a narrow double restriction sealing pocket is evaluated. Such geometries can be found in, for example, labyrinth-type seals. In a wide pocket the velocity profile is one-dimensional and the Herschel-Bulkley model is used. In a narrow pocket, it is shown by the experimental results that the side walls have a significant influence on the grease flow, implying that the grease velocity profile is two-dimensional. In this area, a single empirical grease parameter for the rheology is sufficient to describe the velocity profile.

In the second part, the radial migration of contaminant particles through the grease is evaluated. Centrifugal forces acting on a solid spherical particle are calculated from the grease velocity profile. Consequently, particles migrate to a larger radius and finally settle when the grease viscosity becomes large due to the low shear rate. This behavior is important for the sealing function of the grease in the pocket and relubrication.     

Discharge prediction in flow measurement flumes with different downstream transition slopes

Experimental testing of 9 different rectangular compound cross-section flow measurement flumes with different downstream slopes was conducted to yield the coefficient of discharge and the approach velocity coefficient. The aim of the experimental research was the determination of stage–discharge relationship in compound cross-section flow measurement flumes with different downstream slopes. One empirical predictive model for each of the coefficient of discharge and the approach velocity coefficient for the 9 cross-sections have been derived using one dimensionless parameter for the coefficient of discharge and another one dimensionless parameter for the approach velocity coefficient as the single independent variable. This approach is preferred as it allows the estimation of discharge by only measuring the water depth at the head measurement section. All obtained predictive models statistics have indicated the high reliability of the derived models in estimating discharge in an open channel flume of a rectangular compound cross-section using the predicted coefficients.     

Impact of Water on the Rheology of Lubricating Greases

The operational life of bearings is often determined by the performance of the lubricating grease. The consistency of the grease prevents it from leaking out of the bearing and provides good sealing properties. The possible ingress of water into the bearing will have a considerable impact not only on this consistency but also on the lubricating ability of the grease. There are numerous applications where water ingress may occur, such as in the steel, food, pulp, and paper industries. Some greases are less sensitive to water than others. No specific guidelines are available to select the proper grease for bearings subjected to water ingress. The goal of the article is to contribute to the development of such guidelines for greases subjected to water ingress by studying the impact of water on grease rheology. Fully formulated, commercially available greases with the most common thickeners and base oils are used as model greases. It will be shown that water strongly influences rheological properties such as zero-shear viscosity, yield stress, and storage modulus. Calcium sulfonate greases were found to become stiffer after absorbing a considerable amount of water, leading to an increase in zero-shear viscosity and yield stress. However, lithium, lithium complex, and polyurea greases were found to soften, with appreciable changes in measured rheological properties.     

Numerical simulation of mold filling and deformation behavior in rheology forming process

Rheology forming is a novel semi-solid processing method, which is different from traditional mold forging and conventional casting processes. The rheological behavior of metallic alloys containing both solid and liquid phases investigated the low and high solid fraction ranges. Rheology forming has several obvious advantages including its excellent forming ability resulting in the easier production of complex work pieces, more flexibility in shaping pieces, and more compact inner quality because of its high pressure. This research paper presents the theory of the rheology forming process and the results of a finite element simulation of rheology forming for aluminum alloys. The algorithms for both a single- and two-phase flow model, developed for this study, gives the die filling patterns, velocities, temperatures and solid fractions of the rheology material during the rheology process. To calculate the velocity and temperature fields, the respective governing equations corresponding to the liquid and solid regions were used. Therefore, the respective numerical models that take the co-existing solid and liquid phases within the rheology material into consideration have been developed to predict the defects of parts manufactured by the rheology process. This study compares of the velocity, temperature, and solid fraction between the single- and two-phase flow models. And, to predict the liquid segregation in the parts, the deviation of velocity between the liquid and solid regions in the two-phase flow model was analyzed.     

Gas Leakage Prediction of Contact Interface in Fabric Rubber Seal Based on a Rectangle Channel Model

In this article, the gas leakage prediction through the contact interfaces between a fabric rubber seal and a metal platen is studied based on numerical simulation. A rectangular channel model based on the contact interface is established. The theoretical prediction about the gas leakage for the initial rectangular channel verifies the result from the computational fluid dynamics (CFD) method. Both the finite element method (FEM) and CFD are used to simulate deformation of the fabric rubber seal and the gas leakage, respectively. The influence of rubber thickness and the modulus of the fabric rubber seal under different loading displacements and different contact stresses is studied. In addition, the rubber thickness does not influence the mass flow rate any more under the same contact stress for the normal rubber seal (h₂ ≥ 2,000 μm).     

μPIV measurement of grease velocity profiles in channels with two different types of flow restrictions

Grease is commonly used to lubricate various machine components such as rolling bearings and seals. In this paper the flow of lubricating grease passing restrictions is described. Such flow occurs in rolling bearings during relubrication events where the grease is flowing in the transverse (axial) direction through the bearing and is hindered by guide rings, flanges etc, as well as in seals where transverse flow occurs, for example during so-called breathing caused by temperature fluctuations in the bearing. This study uses a 2D flow model geometry consisting of a wide channel with rectangular cross-section and two different types of restrictions to measure the grease velocity vector field, using the method of Micro Particle Image Velocimetry. In the case of a single restriction, the horizontal distance required for the velocity profile to fully develop is approximately the same as the height of the channel. In the corner before and after the restriction, the velocities are very low and part of the grease is stationary. For the channel with two flow restrictions, this effect is even more pronounced in the “pocket” between the restrictions. Clearly, a large part of the grease is not moving. This condition particularly applies to the cases with a low-pressure drop and where high consistency grease is used. In practice this means that grease is not replaced in such “corners” and that some aged/contaminated grease will remain in seal pockets.     

湍流边界层内钝体扰流的流动与传热特性

应用大涡模拟方法对小尺度开缝圆柱涡流发生器强化传热和流动减阻的机理进行研究。水平开缝圆柱置于充分发展湍流边界层内,分析不同间隙比对开缝圆柱尾流、湍流边界层拟序结构以及槽道底面流动与换热特性的影响。为验证所采用数值方法的准确性与可靠性,将矩形空槽道的计算结果与前人直接数值模拟结果及与采用相关准则关系式所得结果进行对比。计算结果表明：湍流边界层内钝体扰流的尾迹流与壁面边界层的相互作用能够显著提高槽道的换热性能。与未开缝的基准圆柱相比,间隙比小于2.0时,开缝圆柱通道的整体热性能较好;间隙比为2.0时,其综合性能系数最大;间隙比大于2.0时,整体热性能较差。与矩形空槽道相比,最大努塞尔数可提高17.45%,最小摩擦因数可减小4.94%。     

Design method for flow tube structure of electromagnetic water meter with shrunk measurement tube based on pressure loss-flow restriction

Due to the low flow rate measurement demand for battery-powered electromagnetic water meter, shrunk measurement tubes (such as circular section transition to square section) are often used to enhance flow velocity and measurement performance at small flow rate. However, this will also result in an increase in sensor pressure loss, even exceeding the pressure loss limit. Therefore, it is necessary to study a flow tube structure design method based on pressure loss-flow restriction, and design a flow tube structure which can not only maximize the induced voltage, but also meet the actual pressure loss requirement. Because there are many unknown variables in the formula of pressure loss mechanism, it cannot be directly used in structural design. Therefore, taking DN100 sensor as an example, based on finite element software, the variation of pressure loss with the length, width and height of rectangular section is obtained by orthogonal test method, and the numerical model of pressure loss is established. According to the requirements of industry and induced voltage enhancement, the optimal rectangular section size is found with the established pressure loss numerical model, and the structure of flow tube transition section is further optimized to reduce pressure loss. Finally, the prototype is made according to the optimized structure. Pressure loss experiment shows that the error between simulated value and measured value is within ±2.68% (±0.4 KPa). It means the pressure loss-flow restriction based design method for flow tube structure of electromagnetic water meter with shrunk measurement tube is effective and reliable.     

Prediction of flow crossover in the GDL of PEFC using serpentine flow channel

A serpentine flow channel is one of the most common and practical channel layouts for Polymer electrolyte fuel cells (PEFCs) since it ensures the removal of water produced in the cell with an acceptable parasitic load. The operating parameters such as temperature, pressure and flow distribution in the flow channel and gas diffusion layer (GDL) has a great influence on the performance of PEFCs. It is desired to have an optimum pressure drop because a certain pressure drop helps to remove excess liquid water from the fuel cell, too much of pressure drop would increase parasitic power needed for the pumping air through the fuel cell. In order to accurately estimate the pressure drop precise calculation of mass conservation is necessary. Flow crossover in the serpentine channel and GDL of PEFC has been investigated by using a transient, non-isothermal and three-dimensional numerical model. Considerable amount of cross flow through GDL is found and its influence on the pressure variation in the channel is identified. The results obtained by numerical simulation are also compared with the experimental as well as theoretical solution.     

Flow behaviour analysis and experimental investigation for emitter micro-channels

The existing research of the flow behavior in emitter micro-channels mainly focuses on the single-phase flow behavior.And the recent micro-particle image velocimetry(PIV) experimental research on the flow characteristics in various micro-channels mainly focuses on the single-phase fluid flow.However,using an original-size emitter prototype to perform the experiments on the two-phase flow characteristics of the labyrinth channels is seldom reported.In this paper,the practical flow of water,mixed with sand escaped from filtering,in the labyrinth channel,is investigated.And some research work on the clogging mechanism of the labyrinth channel’s structure is conducted.Computational fluid dynamics(CFD) analysis has been performed on liquid-solid two-phase flow in labyrinth-channel emitters.Based on flow visualization technology-micro-PIV,the flow in labyrinth channel has been photographed and recorded.The path line graph and velocity vector graph are obtained through the post-treatment of experimental results.The graphs agree well with CFD analysis results,so CFD analysis can be used in optimal design of labyrinth-channel emitters.And the optimized anti-clogging structures of the rectangular channel and zigzag channel have been designed here.The CFD numerical simulation and the micro-PIV experiments analysis on labyrinth-channel emitter,make the "black box" of the flow behavior in the emitter channel broken.Furthermore,the proposed research promotes an advanced method to evaluate the emitter’s performance and can be used to conducting the optimal design of the labyrinth-channel emitters.     

Parallel computation of two-phase flow in a microchannel using the lattice Boltzmann method

We present a numerical simulation of two-phase flow in a three-dimensional cross-junction microchannel by using the lattice Boltzmann method (LBM). At first, we validated our LBM code with the velocity profile in a 3-dimensional rectangular channel. Then, we developed a lattice Boltzmann code based on the free energy model to simulate the immiscible binary fluid flow. The parallelization of the developed code is implemented on a PC cluster using the MPI program. The numerical results of two-phase flow in the microchannel reveal droplet formation process, which compares well with corresponding experimental results. The size of droplet decreases with increase of the flow-rate ratio and the capillary number. The movement of a droplet through the microchannel induces three-dimensional circulating flow inside the droplet. This complex flow is thought to enhance the mixing and reaction of reagents.     

Evaluation of Thermal and Rheological Properties of Lubricating Greases Modified with Recycled LDPE

The influence that recycled low-density polyethylene (LDPE) and lithium thickener concentrations exerts on the thermal and rheological properties of lithium lubricating greases was investigated using different rheological techniques in a temperature range of 25–175°C. In this way, different lubricating grease formulations were manufactured by modifying the concentration of lithium 12-hydroxystearate and content of recycled LDPE. These lubricating greases were rheologically characterized through small-amplitude oscillatory shear (SAOS) and viscous flow measurements. In addition, bomb oxidation tests (BOTs) and thermogravimetric (TGA) analysis were carried out. From the experimental results obtained, it can be deduced that modified lithium lubricating greases can be considered thermo-rheologically complex materials. Different behaviors of the viscoelastic modulus with temperature as a function of thickener and recycled LDPE concentration were found. Two types of viscous flow behavior were observed depending on the grease composition: A plateau region appeared in a wide range of shear rates and, in some cases, a minimum in the flow curve was more pronounced at high temperatures. The modified lubricating greases studied showed lower thermal and oxidation stability than unmodified lithium lubricating greases.     

Interconversion of grease flow curves between rheometers

Many fluids of interest to industry are non-Newtonian and some (eg greases) are plastic. This non-Newtonian character causes variations in the shear gradient in most viscometers and therefore makes it difficult to predict behaviour in one geometry from measurements in another. For certain model fluids exact solutions to the flow equations may be obtained but in general these models do not account satisfactorily for the properties of a complex material such as a lubricating grease. Approximate numerical methods to convert flow curves of non-Newtonian liquids between geometries were developed by Krieger and his co-workers. It is shown here that these methods are generally valid even for plastic materials. Simple methods are proposed for their use in interconverting flow curves.     

Discharging capacity of rectangular side weirs in straight open channels

A side weir is a hydraulic control structure used in irrigation and drainage systems and combined sewer systems. A comprehensive laboratory study, including 843 tests for the discharge coefficient of a sharp-crested rectangular side weir in a straight channel, was conducted in a large physical model under subcritical flow conditions. The discharge coefficient is a function of the upstream Froude number, the ratios of weir length to channel width, weir length to flow depth, and weir height to flow depth. An equation was developed considering all dimensional parameters for discharge coefficient of the sharp-crested rectangular side weir. The average error of the proposed equation is 4.54%. The present study data were compared with ten different discharge coefficient equations developed by several researchers. The study also presents water surface profile and surface velocity streamlines.     

卧式半圆柱型涡流发生器的传热与阻力特性及场协同理论分析

为研究卧式半圆柱型涡流发生器的特性,通过数值模拟方法对安装有卧式半圆柱型涡流发生器的矩形通道进行了传热和流阻特性的研究。结果表明:在相同雷诺数Re下,安装有卧式半圆柱型涡流发生器的矩形通道的换热效果明显优于矩形光滑通道,但阻力系数也均大于矩形光滑通道。对比综合性能指标表明,6/8H楞长处的综合特性PEC值最大。通过对场协同理论分析得出,矩形光滑通道和四种不同楞长的卧式半圆柱型涡流发生器的协同数Fc均随着雷诺数Re的增大而减小,而且其变化趋势也越来越平缓,最终逐渐趋近于定值。相同雷诺数Re下,楞长为6/8H时的协同数Fc最大。     

风冷冷板传热及流动特性研究

建立了实验装置对矩形通道内的传热与流动状态进行实验,其结果与层流解析解和湍流实验经验式接近,从而证实了实验装置的可靠性。通过对典型模型进行的吹风实验获得了典型模型的传热与流动阻力分布;在数值模拟上对于典型模型采用取局部通道的方式,简化了计算模型。计算采用FLU-ENT求解器的层流模型和SIMPLE算法,实验结果与计算结果一致性较好。然后通过正交实验设计计算了不同肋高、肋间距、肋长、肋厚的类似计算模型,总结出了一定Re数范围内的叉排肋化表面内冷板内通道的j、f计算关联式。