A study on the MHD (magnetohydrodynamic) micropump with side-walled electrodes

This paper presents the continuous flow MHD(magnetohydrodynamic) micropump with side walled electrodes using Lorentz force, which is perpendicular to both magnetic and electric fields, for the application of microfluidic systems. A theoretically simplified MHD flow model includes the theory of fluid dynamics and electromagnetics and it is based upon the steady state, incompressible and fully developed laminar flow theory. A numerical analysis with the finite difference method is employed for solving the velocity profile of the working fluid across the microchannel under various operation currents and magnetic flux densities. In addition, the commercial CFD code called CFD-ACE has been utilized for simulating the MHD micropump. When the program was run(CFD-ACE), the applied current and magnetic flux density were set to be the variables that affected the performance of the MHD micropump. The MHD micropump was fabricated by using MEMS technology. The performance of the MHD micropump was obtained by measuring the flow rate as the applied DC current was changed from 0 to 1mA at 4900 and 3300 Gauss for the electrodes with the lengths of 5000, 7500 and 10000 μm, respectively. The experimental results were compared with the analytical and the numerical results. In addition, with the theoretical analysis and the preliminary experiments, we propose a final model for a simple and new MHD micropump, which could be applicable to microfluidic systems. This paper was recommended for publication in revised form by Associate Editor Seungbae Lee Bumkyoo Choi received a B.S. degree in mechanical engineering, M.S. in mechanical design engineering from Seoul National University, Seoul, Korea in 1981 and 1983 respectively, and PhD in engineering mechanics from the University of Wisconsin, Madison in 1992. From 1992 to 1994, he was a technical staff member of CXrL (Center of X-ray Lithography) in the University of Wisconsin where he developed a computer code for thermal modeling of X-ray mask membrane during synchrotron radiation. He is currently a professor in the Dept. of Mechanical Engineering of Sogang Univ., Seoul, Korea. His research interest includes microelec-tromechanical system (MEMS), micromatching and microfabrication technologies, and modeling issues. Sangsoo Lim received a B.S. degree in mechanical engineering from Sogang University, Seoul, Korea in 2005. He currently works at Hyundai Motors.     

Computational study of flow in a curved pipe with circular cross section

Laminar imcompressible flows in a circular sectioned pipe were investigated by seeking numerical solutions to the Navier-Stokes equations. Three semi-circular pipes of radius ratios 0.05, 0.143 and 0.148 were studied. These calculations covered the Dean number ranging from 183 to 3847. In the range of low and medium Dean numbers, a steady-state solution was obtained; when the Dean number was high, a three-dimensional separation and the associated secondary flow were clearly observed far downstream near the outlet. Extensive flow visualizations were made to depict the computed results.     

Effect of surface roughness on static and dynamic characteristics of MHD couple stress lubrication of parabolic slider bearing

The effect of surface roughness on static and dynamic characteristics of parabolic slider bearing lubricated with couple stress fluid in the presence of magnetic field is theoretically analysed in this paper. The modified stochastic MHD couple stress Reynolds-type equation is derived on the basis of Christensen stochastic theory and considered two types of roughness pattern namely longitudinal and transverse. Expressions for steady pressure and load, dynamic stiffness and damping coefficients are obtained for both roughness patterns. Compared to smooth surface, transverse roughness pattern provides larger load-carrying capacity, dynamic stiffness and damping coefficients, whereas longitudinal roughness pattern has adverse effects. The presence of couple stress and applied magnetic field improves the bearing performance.     

非圆变截面管道流场的数值仿真

管流式动海水腐蚀试验装置主要用于模拟材料长时间浸泡在海水中,受海水冲刷的腐蚀情况。若其过渡段的形状、尺寸设计不合理将会严重影响试验数据的准确性和可靠度,且极易导致试验装置管壁发生空化气蚀现象,缩短试验装置的使用寿命。运用计算流体力学软件Fluent对非圆变截面管道内流场进行了数值仿真和研究。结果表明,过渡段的长度对管道内介质流速影响不大,而对管道接口处气蚀现象影响较大。将分析结果应用于某企业管流式动海水腐蚀试验装置的设计生产中,取得了良好的使用效果。     

Experimental investigation of particle distribution in a flow through a stenosed artery

The particle distribution of a dilute solid-liquid suspension through a stenosed arterial geometry was investigated. Particle image velocimetry (PIV) was used to determine the velocity as well as to acquire the flow images. The light intensity scattered by particles was evaluated to determine the particle distribution. Flow separation exists where the flow emerges from the stenosis throat. From the PIV images, the particle density distribution exhibited differing non-uniform characteristics which vary with flow rate, particle size and particle concentration. At low flow rates, a particle-free layer is formed. As the flow rate is increased, particles accumulate in concentric recirculation orbits within the downstream vortex. Particles with larger size and higher concentration tend to accumulate more towards the center of the vortex.     

连续变截面横梁回弹特性及控制

连续变截面板各处厚度不同,冲压成形后的回弹特性与等厚板有很大差异,如何减小回弹并使各处回弹趋于平衡成为连续变截面板应用中的重要问题。利用大型通用有限元软件ANSYS及其参数化设计语言APDL,通过编程建立横梁的变截面有限元模型;利用ANSYS/LS-DAYNA对连续变截面横梁和等厚板冲压卸载后的回弹进行了数值模拟,模拟结果分析表明:变截面板的回弹规律与等厚板的回弹规律有很大不同;最后利用分块压边圈技术,使横梁的回弹量趋于平衡,同时使整体回弹量下降。     

Experimental investigation of air spindle unit thermal characteristics

This paper presents a report on the first stage of a research on the thermal analysis of spindle units with aerostatic bearings and an experimental investigation of temperature distributions. The objective of the present paper is to provide background information for further analysis. A test machine was used running to a maximum rotational speed of 20000 min⁻¹ with a 60 mm diameter spindle supported by aerostatic journal bearings of 20 μm radial clearance, and the temperatures of the housing, bush and the interface between the bush and air film were measured. In addition, the inlet and outlet air temperatures, the air film pressures and the deformations of the housing and spindle were measured. The experimental results show that the heat flow pattern is essentially radial flow, although axial heat flow was observed. The circumferential temperature distribution can be considered to be uniform, and the temperatures are proportional to the square of the spindle speed.     

A study of the flow characteristics of developing turbulent pulsating flows in a curved duct

Flow characteristics of turbulent pulsating flows in a square-sectional curved duct were experimentally investigated. Experimental studies for air flow were conducted to measure axial velocity profiles, secondary flow and pressure distributions in a square-sectional 180° curved duct by using an LDV system with a data acquisition and processing system which includes a Rotating Machinery Resolve (RMR) and PHASE software. Measurements were made at the seven cross-sections from the inlet (ø=0°) to the outlet (ø=180°) of the duct with 30° intervals. Pressure was measured by using a magnetic differential pressure gage. The experiment was conducted in nineteen sections from the inlet to the outlet of the duct at 10° intervals.Velocity profiles for turbulent pulsating flows were large at the outer wall for a bend angle of ø=30° because of the centrifugal force. The velocity profiles were similar to those of turbulent steady flows. The secondary flow of the turbulent pulsating flow had a positive value at a bend angle of 150° without regarding the phase. The dimensionless value of the secondary flow became gradually weak and approached to zero in the region of a bend angle of 180° regardless of the ratio of velocity amplitude. The pressure difference of turbulent pulsating flows was the largest near the region of a bend of angle of 90° in the case of the middle region and became small beyond 90.     

An investigation on the cooling characteristics of impingement-reversed convection film cooling in a curved section

An experiment was designed at the curving section of a combustion chamber to study the effects of cooling effectiveness on impingement-reversed convection film cooling with and without pin fins. Numerical simulations were also carried out. The effectiveness of compound cooling increased with the rise in blowing ratio. With regard to the effect of the pin fins, impingement pin fin-reversed convectionfilm cooling was more effective than the method without pin fins, particularly for smaller blowing ratio conditions. The number and arrangement of fin pin rows had a small effect on cooling efficiency under the same blowing ratio value. Simulation results agreed well with experimental data and could be used to optimize basic design.     

Experimental and numerical investigation of the cavitation-induced choked flow in a herschel venturi-tube

Due to their simple geometry, cavitating Venturi nozzles (CV) are a long time subject of experimental as well as numerical investigations. However, research mostly focused on certain aspects like the comparison of experimental data with numerical cavitation models or the spray development of diesel injection nozzles, but rarely on the choked flow condition itself, especially with regard to liquid flow measurement.If the pressure decreases due to the local acceleration of the flow to the respective vapor pressure, a choked flow condition similar to the well-known critical flow Venturi-nozzles (CFVN) develops. For the purpose of gaining further insight into the choked flow condition with respect to liquid flow measurement, high-speed camera investigations of a transparent Herschel Venturi-tube configuration, also known as classical Venturi-tube, were performed. Together with pressure and flow rate measurements, they demonstrated the overall stable flow behavior under choked conditions. With additional numerical investigations, phenomena during the onset of the choked condition were clarified. Furthermore, a simple correlation for the calculation of the actual flow rate during the choked condition, including a temperature correction was proposed.     

A numerical investigation of flow and performance characteristics of a small propeller fan using viscous flow calculations

The present work is aimed at investigating an unusual variation in flow and performance characteristics of a small propeller fan at low flow rates. A performance test of the fan showed dual performance characteristics, i.e., radial type characteristics at low flow rates and axial type at high flow rates. Dual performance characteristics of the fan are numerically investigated using viscous flow calculations. The Finite Volume Method is used to solve the continuity and Navier-Stokes equations in the flow domain around a fan. The performance parameters and the circumferentially averaged velocity components obtained from the calculations are compared with the experimental results. Numerical values of the performance parameters show good agreement with the measured values. The calculation simulates the steep variations of performance parameters at low flow rates and shows the difference in the flow structure between high and low flow rates. At a low flow coefficient of ϕ=0.2, the flow enters the fan in an axial direction and is discharged radially outward at its tip, which is much like the flow characteristics of a centrifugal fan. The centrifugal effect at low flow rates makes a significant difference in performance characteristics of the fan. As the inlet flow rate increases, flow around the fan changes into the mixed type at ϕ=0.24 and the axial discharge at ϕ=0.4.     

Numerical investigation of fluid flow and heat transfer characteristics in a helically-finned tube

In order to investigate the characteristics of flow and heat transfer rate in a Helically-finned tub (HFT), we used continuity, momentum and energy equations under a steady, three-dimensional and incompressible fluid flow assumptions. For the performance metrics, we considered the Darcy friction factor, Colburn j-factor, volume goodness factor and area goodness factor of the HFT. We could also evaluate the effect of geometry parameters on the results of local pressure coefficient, fluid vorticity and Nusselt number of the HFT. We carried out the CFD calculation for a range of laminar flow (Re = 100) and turbulent flow (Re = 2000 and 10000). In a laminar and turbulent flow regime, the friction factor increases with increasing the each geometric parameter. While the Colburn j-factor decreases as increasing these geometric parameters. Consequently, the thermal performance of HFT is poorer than that of single straight circular tube type because of having a small volume and area goodness factor as increasing the Reynolds numbers.     

半球形屋面结构风荷载特性试验研究

球形屋面网壳结构是常用的大跨结构屋面形式.在同济大学TJ-3大气边界层风洞中,通过1∶200比例的刚性测压模型,研究了A,B两类风场下半球形屋面结构表面的平均以及脉动风压分布特征.试验结果表明风场类型对平均风压分布影响较小,但对脉动风压分布会有相对较大的影响.同时通过研究子午线上平均及脉动风压分布,表明结构顶部的突出圆顶（采光顶）将增大结构局部风荷载,设计时需特别考虑.最后给出了适合工程应用的分块体型系数,以供结构工程师设计参考.     

Experimental investigation of heat transfer enhancement in a circular duct with circumferential fins and circular disks

The characteristics of heat transfer and pressure drop for fully developed turbulent flow in a tube with circumferential fins and circular disks were experimentally studied. The various spacing and sizes of circumferential fins and circular disks were selected as design parameters, while the effects of these parameters on heat transfer enhancement and pressure drop were investigated. In order to quantify the effect of heat transfer enhancement and the increase of pressure drop due to the fins and disks in a tube, the Nusselt numbers and the friction factors for various configurations and operating conditions were compared to those for a corresponding smooth tube. The results showed that the heat transfer rate was significantly enhanced by increasing the height of circumferential fins and decreasing the pitch of circumferential fins. On the other hand, the influence of the disk size and the fin-disk spacing were not significant. Based on the experimental results, a correlation for estimating the Nusselt number was suggested.     

Experimental investigation on aero-acoustic characteristics of a centrifugal compressor for the fuel-cell vehicle

A variety of centrifugal compressors are used in various fields of industry such as aircraft, home appliances, and vehicles. Comfort and quietness are important in these uses. As a result, noise has become an important consideration in compressor design besides the conventional performance parameters such as efficiency and pressure ratio. However, compressor noise has been difficult to understand because of the lack of information. The aim of this paper is to investigate the aero-acoustic characteristics of a centrifugal compressor for the fuelcell vehicle by experiments. The existing compressor system is modified to measure the internal pressure fluctuation at the impeller inlet, the impeller outlet and the diffuser outlet. Four microphone probes are also installed to determine the external noise levels and spectra of the compressor in an airtight room according to the RPM and mass flow rate. The test results show the possibility to tell the relative noise level of a centrifugal compressor with the internal pressure data. The external microphone signals have relation to the internal pressure signals. They have similar patterns and spectra. It is a noteworthy phenomenon because it is easier and inexpensive to predict pressure behaviors than noise characteristics of centrifugal compressors. The dominant noise source is the tonal noise during normal operation. But the broadband noise component due to the turbulent flow in the compressor increases during low flow rate operation. Computational simulations are carried out to describe these phenomena and to identify noise indicators. The turbulence kinetic energy and the pressure distribution obtained from CFD results may be indicative of the relative noise intensity of the compressor. The experimental facility, instrumentation and simulation conditions are described, and the results are presented in this paper.     

Status and possibilities of the time-of-flight measurement technique using long scintillation counters with a small cross section (Review)

Results of the analysis of the state-of-the-art time-of-flight measurement technique with the use of long scintillation counters with a small cross section (the length of the counter far exceeds its width in the plane of the hodoscope and thickness along the beam direction), which are used as basic elements of time-of-flight detectors for large physical installations and intended for identifying secondary particles generated during collisions of high-energy particles. The following issues are considered in this review. Various methods for identifying particles are compared, and it is pointed out that the time-of-flight method has certain advantages for secondary particles with momenta higher than ～3–5 GeV/c. Some elements incorporated in scintillation counters and affecting its time resolution are considered: optical fibers, optical contacts in the scintillator-fiber-photodetector system, and high-frequency cables. Their characteristics are presented. The characteristics of all elements of a counter (scintillator, photodetector (PD), and electronics) and the processes occurring in them are discussed. The presented experimental data show that, under the conditions of high counting rates and strong magnetic fields, the working capacity of counters holds at a time resolution of ～100–200 ps. The results of measuring the operating characteristics of counters are analyzed. The dependences of the time resolution on such variables as the coordinate of the particle transit through a counter along its length, the counter length, the light-absorption length in the scintillator, the quality of treatment of the scintillator’s surface, and the energy deposited by a particle in the scintillator substance are considered. The main characteristics of individual time counters and average parameters of time-of-flight detectors in certain physical facilities are presented. Possible ways to improve the time resolution and reduce it to ～50–100 ps are considered.     

Experimental investigation and mechanism of material removal in nano finishing of MMCs using abrasive flow finishing (AFF) process

Aluminum alloy and its composites appear to have a good future as a candidate material for engineering and structural components. Finishing of these materials is a big challenge as they are heterogeneous in nature having abrasive particles randomly distributed and oriented in the matrix material. Metal matrix composite (MMC-aluminum alloy and its reinforcement with SiC) workpieces were initially ground to a surface roughness value in the range of 0.6 ± 0.1 μm, and later were finished to the R_a value of 0.25 ± 0.05 μm by using Abrasive Flow Finishing (AFF) process. The effects of different process parameters, such as extrusion pressure, number of cycles and viscosity of the medium were studied on a change in average surface roughness (ΔR_a) and material removal. The relationship between extrusion pressure and ΔR_a shows an optimum at about 6 MPa. In the same way, the relationship between weight percentage of processing oil (plasticizer) and ΔR_a also shows an optimum at 10 wt%. Further, an increase in workpiece hardness requires more number of cycles to achieve the same level of improvement in ΔR_a. Material removal also increases with an increase in extrusion pressure and number of cycles while it decreases with an increase in processing oil content in the medium. It is also concluded that the mechanism of finishing and material removal in case of alloys is different from that in case of MMC.     

Numerical investigation on the transient characteristics of sediment-laden two-phase flow in a centrifugal pump

The objective of this work was to determine pressure fluctuation and transient flow characteristics, which aims to provide references to improve noise and vibration performance for the pump design and optimization when delivering sediment-laden flow. The three-dimensional (3D) transient simulations were simulated by SST k-ω turbulence model combined with Homogeneous equilibrium model (HEM). The experimental and numerical data was compared to validate the numerical accuracy. The simulation results predicted that the concentration shows strong effects on the external performance, velocity, pressure, turbulent kinetic energy distribution and peak amplitude of pulsation frequency, which all perform increasing trend with the rise of concentration. Meanwhile, the effect of the diameter size of particles on the flow field was relatively minor, which can also evidently influence the internal flow, but the effect is not simply proportional to the diameter size. The effect of diameter size on silt flow needs to be taken into account associated with the concentration distribution. The dominant frequency of solid-liquid approximately equals 0.8 times that of pure water, and the transient characteristics of sediment-laden flow perform low frequency with high amplitude features.     

Experiments on the turbulent shear flow in a turn-around duct(II)—The structure of turbulence

Detailed measurements including two-component mean velocities (U andV), RMS of turbulent fluctuations (u′ andv′) and turbulent cross-correlation (uv) were made throughout a turn-around duct. Two-component velocity data were obtained using a Laser Doppler Velocimeter. With the aid of a digitized data acquisition system, energy spectra were estimated using a Fast Fourier Transform. Along the outer wall, the flow is affected more than the half across the channel height due to the centrifugal instability. The measured results are consistent with the turbulence production mechanism for stabilizing and destabilizing curved flows. Energy production and dissipation are reduced along the convex wall and amplified along the concave wall. In the quasi-laminar region, turbulent fluctuations and cross-correlations are damped. The mean flow and turbulence structure in this region are influenced mainly by the streamwise pressure gradient rather than curvature. The flow in the downstream part of the turn is dominated by the inertial effect. The turbulent large eddy motions along the concave wall are strongly anisotropic.     

Experimental and CFD investigation of 100 mm size cone flow elements

This paper presents performance characteristics of 100 mm line size cone flow elements having beta ratios of 0.4, 0.5, 0.6, 0.7 and 0.8. A magnetic flow meter is used as a reference standard for flow measurement in vertical test section. A series of experiments have been conducted using water at in-house Flow Calibration Facility (FCF) to cover the Reynolds number ranging from 20,000 to 200,000. The performance characteristics of 100 mm line size cone flow elements with different beta values have been evaluated experimentally. It is found that the discharge coefficient of the cone flow element is nearly independent of the specified range of Reynolds number. Testing of the cone flow element in accordance with new API 5.7 is carried out at flow calibration facility. The testing requirements in the standard explain the conditioning effect of the cone flow element having gate valve disturbance upstream of the cone at various locations. The effect of the upstream velocity profile has been investigated by placing a gate valve upstream of the cone flow element at a distance of 0D and 28D and performing experiments at 25%, 50% and 100% opening of gate valve. The value of the discharge coefficient is not affected when the cone is placed at a distance of 0D and for 100% opening of gate valve. The uncertainty results of the cone testing are discussed. For studying pressure and velocity distributions, cone elements are modeled using computational fluid dynamics (CFD) code PHOENICS. Pressure and velocity profiles for different sizes of cone elements are plotted. From the pressure profile, it can be seen that the pressure recovery downstream of the cone is within a distance of 3D. The velocity profile downstream of the cone signifies the use of flow element as a signal conditioner. For measurement of flow through a 100 mm line, differential pressure across the cone is measured using a Differential Pressure Transmitter (DPT). Experiments were repeated by replacing the cone element for obtaining different β values.