共查询到20条相似文献,搜索用时 46 毫秒
1.
A design concept of a junction contraction is proposed for a T-shaped microchannel to enhance mixing. The proposed design entails contracting the flow passage at the junction of two fluids in a T-shaped microchannel. It is validated numerically by application to a well known T-shaped microchannel. The proposed junction contraction results in a noticeable improvement in mixing. The improvement of mixing is found to be caused by a vortex flow around the junction contraction. A downward flow is shown to form just before the contraction and is followed by an upward flow. The dependence on the position and the size of contraction is also studied. The best degree of mixing is obtained when the contraction is placed in the range of 50–125 μm downstream the junction of the two fluids being mixed. The mixing enhancement is found to increase quadratically with the contraction ratio. However, more contracted junction requires a larger pressure drop. 相似文献
2.
Hydrodynamic and thermal characteristics of flow boiling in a non-uniformly heated microchannel were studied. Experiments were performed with a single microchannel and a series of microheaters to study the microscale boiling of water under axially non-uniform heat input conditions. A simultaneous real time visualization of the flow pattern was performed with the measurement of experimental parameters. Tests were performed over a mass flux of 309.8 kg/m2 s, and heat flux of 200–600 kW/m2. Test results showed different fluctuations of heated wall temperature, pressure drop, and mass flux with variations of the heat input along the flow direction. The unique periodic flow boiling in a single microchannel was observed at all heat flux conditions except for the increasing heat input distribution case which is the nearly uniform effective heat input distribution condition. The instability is correlated with flow pattern transition. For the nearly uniform effective heating condition, no fluctuation of the wall temperature, pressure drop, or mass flux was observed. We can relieve the instability by increasing total heat input along the flow direction and predict the instability using the transition criteria and flow pattern map. 相似文献
3.
Mohammad Hadigol Reza Nosrati Ahmad Nourbakhsh Mehrdad Raisee 《ournal of non Newtonian Fluid Mechanics》2011,166(17-18):965-971
Biofluids which exhibit non-Newtonian behavior are widely used in microfluidic devices which involve fluid mixing in microscales. In order to study the effects of shear depending viscosity of non-Newtonian fluids on characteristics of electroosmotic micromixing, a numerical investigation of flow of power-law fluid in a two-dimensional microchannel with nonuniform zeta potential distributions along the channel walls was carried out via finite volume scheme. The simulation results confirmed that the shear depending viscosity has a significant effect on the degree of mixing efficiency. It was shown as the fluid behavior index of power-law fluid, n, decreases, more homogeneous solution can be achieved at the microchannel outlet. Hence, electroosmotic micromixing was found more practical and efficient in microscale mixing of pseudoplastic fluids rather than those Newtonian and dilatant ones. Furthermore, it was found that increase in Reynolds number results in lower mixing efficiency while electroosmotic forces are kept constant. 相似文献
4.
A numerical investigation is performed into the flow characteristics and mixing performance of electrokinetically driven non-Newtonian fluid in a contraction–expansion microchannel. In the study, the rheological behavior of the fluid is characterized using a power-law model. The results show that the volumetric flow rate reduces as the flow behavior index increases, and thus an improved mixing performance is obtained. Furthermore, it is shown that for all considered values of the flow behavior index, the mixing performance can be enhanced by increasing the ratio of the main channel width to the contraction channel width, extending the length of the contraction channel, assigning a smaller value to the nondimensional Debye–Hückel parameter, and applying an appropriate electric field strength. Finally, it is shown that although the mixing efficiency reduces with a reducing flow behavior index, an acceptable mixing performance can still be obtained given an appropriate specification of the flow conditions and geometry parameters. 相似文献
5.
A simultaneous measurement technique for the velocity and pH distribution was developed by using a confocal microscope and
a 3CCD color camera for investigations of a chemical reacting flow field in a microchannel. Micron-resolution particle image
velocimetry and laser induced fluorescence were utilized for the velocity and pH measurement, respectively. The present study
employed fluorescent particles with 1 μm diameter and Fluorescein sodium salt whose fluorescent intensity increases with an
increase in pH value over the range of pH 5.0–9.0. The advantages of the present system are to separate the fluorescence of
particles from that of dye by using the 3CCD color camera and to provide the depth resolution of 5.0 μm by the confocal microscope.
The measurement uncertainties of the velocity and pH measurements were estimated to be 5.5 μm/s and pH 0.23, respectively.
Two aqueous solutions at different pH values were introduced into a T-shaped microchannel. The mixing process in the junction
area was investigated by the present technique, and the effect of the chemical reaction on the pH gradient was discussed by
a comparison between the proton concentration profiles obtained from the experimental pH distribution and those calculated
from the measured velocity data. For the chemical reacting flow with the buffering action, the profiles from the numerical
simulation showed smaller gradients compared with those from the experiments, because the production or extinction of protons
was yielded by the chemical reaction. Furthermore, the convection of protons was evaluated from the velocity and pH distribution
and compared with the diffusion. It is found that the ratio between the diffusion and convection is an important factor to
investigate the mixing process in the microfluidic device with chemical reactions. 相似文献
6.
A flow and heat transfer numerical simulation is performed for a 2D laminar incompressible gas flow through a constricted
microchannel in the slip regime with constant wall temperature. The effects of rarefaction, creeping flow, first order slip
boundary conditions and hydrodynamically/thermally developing flow are assumed. The effects of Knudsen number and geometry
on thermal and hydrodynamic characteristics of flow in a constricted microchannel are explored. SIMPLE algorithm in curvilinear
coordinate is used to solve the governing equations including continuity, energy and momentum with the temperature jump and
velocity slip conditions at the solid walls in discretized form. The resulting velocity and temperature profiles are then
utilized to obtain the microchannel C
f
Re and Nusselt number as a function of Knudsen number and geometry. The results show that Knudsen number has declining effect
on the C
f
Re and Nusselt number in the constricted microchannel. In addition, the temperature jump on wall and slip velocity increase
with increasing Knudsen number. Moreover, by decreasing the throttle area, the fluid flow characteristics experience more
intense variations in the constricted region. To verify the code a comparison is carried out with available results and good
agreement is achieved. 相似文献
7.
8.
We conducted experimental and numerical studies aimed at developing a microfluidic device capable of simultaneous mixing while
pumping. The proposed multifunctional device makes use of alternating current electroosmotic flow and adopts an array of planar
asymmetric microelectrodes with a diagonal or herringbone shape. The pumping performance was assessed in terms of the fluid
velocity at the center of the microchannel, obtained by micro PIV. To assess the mixing, flow visualizations were carried
out over the electrodes to verify the lateral flows. The mixing degree was quantified in terms of a mixing efficiency obtained
by three-dimensional numerical simulations. The results showed that simultaneous mixing and pumping was achieved in the channels
with diagonal or herringbone electrode configurations. A herringbone electrode configuration showed better pumping compared
with a reference, as well as enhanced mixing. 相似文献
9.
The present study investigates the microchannel heat sinks (MCHSs) with smooth and wavy wall for pure electroosmotic flow
(EOF), pressure-driven flow (PDF) and combined electroosmotic and pressure-driven flow (PDF + EOF). A three-dimensional numerical
analysis was performed for EOF, PDF and combined flow (PDF + EOF) through finite volume analysis. The EOF was combined with
the PDF to enhance the flow rate and to reduce the thermal resistance of the MCHS. The effect of wall waviness on electroosmosis
and thermal performance of the MCHS was critically investigated for flow rate, friction factor, Nusselt number, thermal resistance
and pumping power. The design variables related to the wavelength and amplitude and width of microchannel were investigated
for their effect on the overall thermal performance and pumping power. The electroosmosis not only increases the flow rate
but also suppresses the secondary flow developed due to the topology of the microchannel walls. The non-uniformity of the
velocity and temperature is reduced due to the application of the EOF in a PDF and combined flow (PDF + EOF). 相似文献
10.
Under the pulsatile flow with backward flow (PFBF) conditions, flow mixing and mass transfer characteristics are experimentally
investigated in an axisymmetric wavy-walled tube at a net flow Reynolds number from 50 to 1,000. An electrochemical technique
is employed to measure the mass transfer rate. An optimal Strouhal number corresponding to the peak value of the mass transfer
enhancement factor is observed, which is independent of the oscillatory fraction of the flow rate, but decreases with the
increasing net flow Reynolds number. It was found that the mass transfer enhancement under PFBF has the similar characters
of resonant enhancement in two-dimensional (2-D) channels, but there also exists an essential difference since no self-sustained
oscillation occurs in the wavy-walled tube. 相似文献
11.
In this study, a numerical simulation of copper microchannel heatsink (MCHS) using nanofluids as coolants is presented. The
nanofluid is a mixture of pure water and nanoscale metallic or nonmetallic particles with various volume fractions. Also,
the effects of various volume fractions, volumetric flow rate and various materials of nanoparticles on the performance of
MCHS have been developed. A three-dimensional computational fluid dynamics model was developed using the commercial software
package FLUENT, to investigate the conjugate fluid flow and heat transfer phenomena in micro channel heatsinks. The results
show that the cooling performance of a microchannel heat sink with water based nanofluid containing Al2O3 (vol 8%) is enhanced by about 4.5% compared with micro channel heatsink with pure water. Nanofluids reduce both the thermal
resistance and the temperature difference between the top (heated) surface of the MCHS and inlet nanofluid compared with that
pure water. The cooling performance of a micro channel heat sink with metal nanofluids improves compared with that of a micro
channel heat sink with oxide metal nanofluids because the thermal conductivity of metal nanofluid is higher than oxide metal
nanofluids. Micro channel heat sinks with nanofluids are expected to be good candidates as the next generation cooling devices
for removing ultra high heat flux. 相似文献
12.
助欧拉和拉格朗日方法数值模拟了突扩微尺度管道流中微米颗粒的分离情况. 在采用有限体积法求解电荷密度场、电场和流场的基础上,通过基于改进的Langevin方程研究了微管道中不同位置处的微米颗粒在水动力和介电电泳力综合作用下的运动轨迹. 研究发现:电渗流的驱动能力随着扩张比(ER)的增加而提高,然而其提高的趋势逐渐变小;当微米颗粒仅在水动力作用下时,随着ER的增加,颗粒之间的有效分离距离(ESL)随之线性增加,此时ESL与ER的比值约为5.9; 若是考虑介电电泳力对于微米颗粒运动的影响, ESL与ER的比值下降为4.79, 由此可以看出介电电泳力对突扩微管道流中的微米颗粒的分离效果有着一定的负面影响. 上述结论对于基于介电电泳技术设计的生物粒子分离芯片的优化设计有很大价值. 相似文献
13.
不同形状微尺度管道(圆形、六边形、半圆形、不同宽高比的矩形)中的气体流动特性是微机电系统设计最为关心的问题之一.文中利用信息保存(IP)方法和直接模拟Monte Carlo(DSMC)方法进行研究,给出两种方法的计算结果相互符合,并与其它研究者的BGK模型方程计算结果进行了比较.对于微尺度管道中关心的低Mach数流动, IP方法的统计收敛效率明显优于DSMC方法.通过拟合IP和DSMC结果,给出了圆形、六边形、半圆形、不同宽高比的矩形截面情况下无量纲质量流率与等效Knudsen数的关系. 相似文献
14.
In a recent paper we have investigated mixing and heat transfer enhancement in a mixer composed of two circular rods maintained vertically in a cylindrical tank. The rods and tank can rotate around their revolution axes while their surfaces were maintained at a constant temperature. In the present study we investigate the differences in the thermal mixing process arising from the utilization of a constant heat flux as a boundary condition. The study concerns a highly viscous fluid with a high Prandtl number for which this chaotic mixer is suitable. By solving numerically the flow and energy equations, and using different statistical tools we characterize the evolution of the fluid temperature and its homogenization. Fundamental differences are reported between these two modes of heating or cooling: while the mixing with an imposed temperature results in a homogeneous temperature field, with a fixed heat flux we observe a constant difference between the maximal and minimal temperatures that establish in the fluid; the extent of this difference is governed by the efficiency of the mixing protocol. 相似文献
15.
Stereoscopic micro particle image velocimetry 总被引:1,自引:0,他引:1
A stereoscopic micro-PIV (stereo-μPIV) system for the simultaneous measurement of all three components of the velocity vector in a measurement plane (2D–3C) in a closed microchannel has been developed and first test measurements were performed on the 3D laminar flow in a T-shaped micromixer. Stereomicroscopy is used to capture PIV images of the flow in a microchannel from two different angles. Stereoscopic viewing is achieved by the use of a large diameter stereo objective lens with two off-axis beam paths. Additional floating lenses in the beam paths in the microscope body allow a magnification up to 23×. The stereo-PIV images are captured simultaneously by two CCD cameras. Due to the very small confinement, a standard calibration procedure for the stereoscopic imaging by means of a calibration target is not feasible, and therefore stereo-μPIV measurements in closed microchannels require a calibration based on the self-calibration of the tracer particle images. In order to include the effects of different refractive indices (of the fluid in the microchannel, the entrance window and the surrounding air) a three-media-model is included in the triangulation procedure of the self-calibration. Test measurement in both an aligned and a tilted channel serve as an accuracy assessment of the proposed method. This shows that the stereo-μPIV results have an RMS error of less than 10% of the expected value of the in-plane velocity component. First measurements in the mixing region of a T-shaped micromixer at Re = 120 show that 3D flow in a microchannel with dimensions of 800 × 200 μm2 can be measured with a spatial resolution of 44 × 44 × 15 μm3. The stationary flow in the 200 μm deep channel was scanned in multiple planes at 22 μm separation, providing a full 3D measurement of the averaged velocity distribution in the mixing region of the T-mixer. A limitation is that this approach requires a stereo-objective that typically has a low NA (0.14–0.28) and large depth-of-focus as opposed to high NA lenses (up to 0.95 without immersion) for standard μPIV. 相似文献
16.
Mixing is an essential operation in many microfluidic devices. Droplet-based micromixers utilize droplets for mixing enhancement. In the present study, a novel three-dimensional simulation is conducted which has the ability to capture not only the mixing process, but also the chemical reactions inside liquid droplets. This two-phase model is used for simulating the reacting flow inside a serpentine microchannel and explores the effects of droplet size and reaction rate on the production and consumption of species in droplets. It is observed that the chemical reaction in each droplet, begins from its front area. Furthermore, it is shown that the production of species does not depend on water fraction (ratio of water flow rate to total flow rate) and for all droplet sizes, only depends on the reaction rate. Moreover, different transient generated vortices and secondary flows are studied in the presence and absence of droplets and explained in details based on the position of droplets. It is found that during the passage of droplets through a microchannel bend, the number of cross-sectional vortices changes and the velocity magnitude in these secondary flows increases dramatically. 相似文献
17.
18.
A numerical study is performed on a two-dimensional confined opposed-jet configuration to gain basic understanding of the flow and mixing characteristics of pulsed turbulent opposed-jet streams. The sinusoidal pulsating flows with different temperature are imposed at opposed-jet inlets, which are mixed with each other in a confined flow channel. The current mathematical model taking the effect of temperature-dependent thermo-physical properties of fluid into account can present a good prediction for opposed-jet streams compared with experimental data. The numerical results indicate that introduction of temperature difference between opposed jet flows can lead to an asymmetric flow field immediately after jet impact, and the sinusoidal flow pulsations can effectively enhance mixing rate of opposed jets. Parameter studies are conducted for optimization of pulsed opposed jets. The effect of Reynolds number and flow pulsation as well as the configuration geometry on the mixing performance are discussed in detail. Examination of the flow and thermal field shows that the mixing rate is highly dependent on the vortex-induced mixing and residence time of jet fluid in the exit channel. 相似文献
19.
20.
Thermal degradation of piping induced by high cycle thermal fatigue (HCTF) is of significant importance as operating Nuclear Power Plants (NPP) become older and lifetime extension activities are initiated. In particular, HCTF incidents related to turbulent thermal mixing of fluids in a T-junction piping system are not well understood and could not be adequately monitored using common thermocouple instrumentation. To investigate this phenomenon, an experimental T-junction test facility was commissioned at the University of Stuttgart, known as the Fluid Structure Interaction (FSI) test facility. The paper presents the experimental investigation and the corresponding numerical validation using the large eddy simulation (LES) method to study T-junction flow mixing. Three experimental test cases are investigated with temperature differences (∆T) of 51.5 K (Case 1), 76 K (Case 2) and 97 K (Case 3) between the mixing fluids. A constant mass flow rate ratio (main/branch) of 4:1 is maintained in all the investigated cases. Flow mixing is observed to be incomplete in all the cases, resulting in a thermally stratified flow with an oscillating stratification layer downstream of the T-junction. Mean temperature and root mean square (RMS) temperature fluctuations predicted by LES in the mixing region are found to be in good agreement with measurement data, with the exception of few positions. Amplitudes of temperature fluctuations are observed to be higher near the stratification layer, ranging from 6.3–9.9% of ∆T. Power spectral density (PSD) analyses of temperature fluctuations indicate no dominant frequency (spectral peak) under prevailing flow conditions, an important factor in thermal fatigue analysis, and the energy of these fluctuations are mainly contained in the frequency range of 0.1–2 Hz for all the investigated cases. LES is performed using the CFD software ANSYS CFX 14.0. 相似文献