1,126 research outputs found
Simulation of copper-water nanofluid in a microchannel in slip flow regime using the lattice Boltzmann method with heat flux boundary condition
Laminar forced convection heat transfer of water–Cu nanofluids in a microchannel is studied using the double population Thermal Lattice Boltzmann method (TLBM). The entering flow is at a lower temperature compared to the microchannel walls. The middle section of the microchannel is heated with a constant and uniform heat flux, simulated by means of the counter slip thermal energy boundary condition. Simulations are performed for nanoparticle volume fractions equal to 0.00%, 0.02% and 0.04% and slip coefficient equal to 0.001, 0.01 and 0.1. Reynolds number is equal to 1, 10 and 50.The model predictions are found to be in good agreement with earlier studies. Streamlines, isotherms, longitudinal variations of Nusselt number and slip velocity as well as velocity and temperature profiles for different cross sections are presented. The results indicate that LBM can be used to simulate forced convection for the nanofluid micro flows. They show that the microchannel performs better heat transfers at higher values of the Reynolds number. For all values of the Reynolds considered in this study, the average Nusselt number increases slightly as the solid volume fraction increases and the slip coefficient increases. The rate of this increase is more significant at higher values of the Reynolds number
Droplet breakup driven by shear thinning solutions in a microfluidic T-Junction
Droplet-based microfluidics turned out to be an efficient and adjustable
platform for digital analysis, encapsulation of cells, drug formulation, and
polymerase chain reaction. Typically, for most biomedical applications, the
handling of complex, non-Newtonian fluids is involved, e.g. synovial and
salivary fluids, collagen, and gel scaffolds. In this study we investigate the
problem of droplet formation occurring in a microfluidic T-shaped junction,
when the continuous phase is made of shear thinning liquids. At first, we
review in detail the breakup process providing extensive, side-by-side
comparisons between Newtonian and non-Newtonian liquids over unexplored ranges
of flow conditions and viscous responses. The non-Newtonian liquid carrying the
droplets is made of Xanthan solutions, a stiff rod-like polysaccharide
displaying a marked shear thinning rheology. By defining an effective Capillary
number, a simple yet effective methodology is used to account for the
shear-dependent viscous response occurring at the breakup. The droplet size can
be predicted over a wide range of flow conditions simply by knowing the
rheology of the bulk continuous phase. Experimental results are complemented
with numerical simulations of purely shear thinning fluids using Lattice
Boltzmann models. The good agreement between the experimental and numerical
data confirm the validity of the proposed rescaling with the effective
Capillary number.Comment: Manuscript: 11 pages 5 figures, 65 References. Textual Supplemental
Material: 6 pages 3 figure. Video Supplemental Materials: 2 movie
Experimental investigation of heat transfer rate in micro-channels
Metal-based MHEs are of current interest due to the combination of high heat transfer performance and improved mechanical integrity. Efficient methods for fabrication and assembly of functional metal-based MHEs are essential to ensure the economic viability of such devices. The present study focuses on the results of heat transfer testing of assembled Cu- and Al- based microchannel heat exchanger (MHE) prototypes. Efficient fabrication of Cu- and Al- based high-aspect-ratio microscale structures (HARMS) have been achieved through molding replication using surface engineered, metallic mold inserts. Replicated metallic HARMS were assembled through eutectic bonding to form entirely Cu- and Al- based MHE prototypes, on which heat transfer tests were conducted to determine the average rate of heat transfer from electrically heated Cu blocks placed outside the MHEs to water flowing within the molding replicated microchannel arrays. Experimentally observed heat transfer rates are higher as compared to those from previous studies on microchannel devices with similar geometries. Further, infrared thermography was conducted to determine the overall cooling rate and time constants. The time constant for the MHE device was found out to be lower for Cu channels with response times around 1-2 seconds. Al MHE device response time was only slightly lower due to the lower thermal conductivity. Experimental results show a great influence of the type of metal, flow rate and the surrounding conditions on the overall cooling performance of the MHEs. The potential influence of microchannel surface profile on heat transfer rates is discussed. The present results illustrate the potential of metal-based MHEs in wide ranging applications. A two-dimensional thermal lattice Boltzmann model was developed to simulate the heat transfer phenomenon in Cu- and Al- based microchannels. The LBM results were compared with 3D and 2D fluent models. Additionally, attempts were made to visualize the flow field inside an assembled Cu micro-channel at very low flow rates using oil-in-water solution
The Optimum Surface Pattern to Enhance Flow Oscillation in Micro-channel
Mixing of analytes and reagents in microfluidic devices is often crucial to the effective functioning of lab-on-a-chip. It is possible to affect the mixing in microfluidics by intelligently controlling the thermodynamic and chemical properties of the substrate surface. Numerous studies have shown that the phase behavior of mixtures is significantly affected by surface properties of microfluidics. For example, the phase separation between the fluids can be affected by heterogeneous patterns on the substrate. The patterned substrate can offer an effective means to control fluid behavior and in turn to enhance mixing. In this study, we numerically studied the effect of optimum surface pattern on mixing in a micro channel and found that the flow oscillation was enhanced apparently when the ratio of hydrophobic and hydrophilic boundary follows certain ratios
Effect of aspect ratio on transverse diffusive broadening: A lattice Boltzmann study
We study scaling laws characterizing the inter-diffusive zone between two
miscible fluids flowing side by side in a Y-shape laminar micromixer using the
lattice Boltzmann method. The lattice Boltzmann method solves the coupled 3D
hydrodynamics and mass transfer equations and incorporates intrinsic features
of 3D flows related to this problem. We observe the different power law regimes
occurring at the center of the channel and close to the top/bottom wall. The
extent of the inter-diffusive zone scales as square root of the axial distance
at the center of the channel. At the top/bottom wall, we find an exponent 1/3
at early stages of mixing as observed in the experiments of Ismagilov and
coworkers [Appl. Phys. Lett. 76, 2376 (2000)]. At a larger distance from the
entrance, the scaling exponent close to the walls changes to 1/2 [J.-B. Salmon
et al J. Appl. Phys. 101, 074902 (2007)]. Here, we focus on the effect of
finite aspect ratio on diffusive broadening. Interestingly, we find the same
scaling laws regardless of the channel's aspect ratio. However,the point at
which the exponent 1/3 characterizing the broadening at the top/bottom wall
reverts to the normal diffusive behavior downstream strongly depends on the
aspect ratio. We propose an interpretation of this observation in terms of
shear rate at the side walls. A criterion for the range of aspect ratios with
non-negligible effect on diffusive broadening is also provided.Comment: 19 pages, 7 figure
Computational inertial microfluidics:a review
Since the discovery of inertial focusing in 1961, numerous theories have been put forward to explain the migration of particles in inertial flows, but a complete understanding is still lacking. Recently, computational approaches have been utilized to obtain better insights into the underlying physics. In particular, fundamental aspects of particle focusing inside straight and curved microchannels have been explored in detail to determine the dependence of focusing behavior on particle size, channel shape, and flow Reynolds number. In this review, we differentiate between the models developed for inertial particle motion on the basis of whether they are semi-analytical, Navier-Stokes-based, or built on the lattice Boltzmann method. This review provides a blueprint for the consideration of numerical solutions for modeling of inertial particle motion, whether deformable or rigid, spherical or non-spherical, and whether suspended in Newtonian or non-Newtonian fluids. In each section, we provide the general equations used to solve particle motion, followed by a tutorial appendix and specified sections to engage the reader with details of the numerical studies. Finally, we address the challenges ahead in the modeling of inertial particle microfluidics for future investigators
Simulations of slip flow on nanobubble-laden surfaces
On microstructured hydrophobic surfaces, geometrical patterns may lead to the
appearance of a superhydrophobic state, where gas bubbles at the surface can
have a strong impact on the fluid flow along such surfaces. In particular, they
can strongly influence a detected slip at the surface. We present two-phase
lattice Boltzmann simulations of a flow over structured surfaces with attached
gas bubbles and demonstrate how the detected slip depends on the pattern
geometry, the bulk pressure, or the shear rate. Since a large slip leads to
reduced friction, our results allow to assist in the optimization of
microchannel flows for large throughput.Comment: 22 pages, 12 figure
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