Buoyant miscible displacement flows in rectangular channels

2017 ◽  
Vol 826 ◽  
pp. 676-713 ◽  
Author(s):  
S. M. Taghavi ◽  
R. Mollaabbasi ◽  
Y. St-Hilaire
Keyword(s):  
Flow Regimes ◽  
Channel Cross Section ◽  
Inertial Effects ◽  
Cross Sectional ◽  
Square Duct ◽  
Rectangular Channels ◽  
Flow Motion ◽  
Flow Features ◽  
Displacement Experiments ◽  
Stationary Interface

Buoyant displacement flows of two miscible fluids in rectangular channels are studied, theoretically and experimentally. The scenario considered involves the displacement of a fluid by a slightly heavier one at nearly horizontal channel inclinations, where inertial effects are weak and laminar stratified flows may be expected. In the theoretical part, a lubrication approximation model is developed to simplify the displacement flow governing equations and furnish a semi-analytical solution for the heavy and light fluid flux functions. Three key dimensionless parameters govern the fluid flow motion, i.e. a buoyancy number, the viscosity ratio and the channel cross-section aspect ratio. When these parameters are specified, the reduced model can deliver the interface propagation in time, leading and trailing front heights, shapes and speeds, cross-sectional velocity fields, etc. In addition, the model can be exploited to provide various classifications such as single or multiple fronts as well as main displacement flow regimes at long times such as no sustained backflows, stationary interface flows and sustained backflows. Focusing on the variation of the buoyancy number, a large number of iso-viscous displacement experiments are performed in a square duct and the results are compared with those of the lubrication model. Qualitative displacement flow features observed in the theory and experiments are in good agreement, in particular, in terms of the main displacement flow regimes. The quantitative comparisons are also reasonable for small and moderate imposed displacement flow velocities. However, at large flow rates, a deviation of the experimental results from the model results is observed, which may be due to the presence of non-negligible inertial effects.

Fluid Flow in a 180 deg Sharp Turning Duct With Different Divider Thicknesses

1999 ◽  
Vol 121 (3) ◽  
pp. 569-576 ◽  
Author(s):  
Tong-Miin Liou ◽  
Yaw-Yng Tzeng ◽  
Chung-Chu Chen
Keyword(s):  
Heat Transfer ◽  
Laser Doppler Velocimetry ◽  
Fluid Flows ◽  
Internal Cooling ◽  
Mean Velocity ◽  
Cross Sectional ◽  
Square Duct ◽  
Measured Velocity ◽  
The Past ◽  
Flow Features

The effect of divider thickness on fluid flows in a two-pass smooth square duct with a 180 deg straight-corner turn is an important issue to the turbine blade internal cooling but has not been explored in the past. Laser-Doppler velocimetry measurements are thus presented for such a study at a Reynolds number of 1.2 × 104 and dimensionless divider thicknesses (Wd*) of 0.10, 0.25, 0.50. Results are presented in terms of various mean velocity components in two orthogonal streamwise planes and three cross-sectional planes, the local and regional averaged turbulent kinetic energy and resultant mean velocity distributions, and complemented by the liquid crystal measured heat transfer coefficient contours. The measured velocity data are able reasonably to explain published and present measured heat transfer results. Wd* is found to have profound effects on the flow features inside and immediately after the turn. The turbulence level and uniformity in the region immediately after the turn respectively decrease and increase with increasing Wd*.

scholarly journals Fluid Flow in a 180 Deg Sharp Turning Duct With Different Divider Thicknesses

1998 ◽  
Author(s):  
Tong-Miin Liou ◽  
Yaw-Yng Tzeng ◽  
Chung-Chu Chen
Keyword(s):  
Heat Transfer ◽  
Laser Doppler Velocimetry ◽  
Fluid Flows ◽  
Internal Cooling ◽  
Mean Velocity ◽  
Cross Sectional ◽  
Square Duct ◽  
Measured Velocity ◽  
The Past ◽  
Flow Features

The effect of divider thickness on fluid flows in a two-pass smooth square duct with a 180 deg straight-corner turn is an important issue to the turbine blade internal cooling but has not been explored in the past. Laser-Doppler velocimetry measurements are thus presented for such a study at a Reynolds number of 1.2 × 104 and dimensionless divider thicknesses (Wd*) of 0.10, 0.25, 0.50. Results are presented in terms of various mean velocity components in two orthogonal streamwise planes and three cross-sectional planes, the local and regional averaged turbulent kinetic energy and resultant mean velocity distributions, and complemented by the liquid crystal measured heat transfer coefficient contours. The measured velocity data are able to reasonably explain published and present measured heat transfer results. Wd* is found to have profound effects on the flow features inside and immediately after the turn. The turbulence level and uniformity in the region immediately after the turn respectively decrease and increase with increasing Wd*.

scholarly journals Numerical and Theoretical Study of Performance and Mechanical Behavior of PEM-FC Using Innovative Channel Geometrical Configurations

2021 ◽  
Vol 11 (12) ◽  
pp. 5597
Author(s):  
Hussein A. Z. AL-bonsrulah ◽  
Mohammed J. Alshukri ◽  
Ammar I. Alsabery ◽  
Ishak Hashim
Keyword(s):  
Fuel Cell ◽  
Cross Section ◽  
Rectangular Channel ◽  
Compressive Load ◽  
Proton Exchange ◽  
Channel Cross Section ◽  
Channel Height ◽  
Cross Sectional ◽  
Triangular Channel ◽  
Higher Power

Proton exchange membrane fuel cell (PEM-FC) aggregation pressure causes extensive strains in cell segments. The compression of each segment takes place through the cell modeling method. In addition, a very heterogeneous compressive load is produced because of the recurrent channel rib design of the dipole plates, so that while high strains are provided below the rib, the domain continues in its initial uncompressed case under the ducts approximate to it. This leads to significant spatial variations in thermal and electrical connections and contact resistances (both in rib–GDL and membrane–GDL interfaces). Variations in heat, charge, and mass transfer rates within the GDL can affect the performance of the fuel cell (FC) and its lifetime. In this paper, two scenarios are considered to verify the performance and lifetime of the PEM-FC using different innovative channel geometries. The first scenario is conducted by adopting a constant channel height (H = 1 mm) for all the differently shaped channels studied. In contrast, the second scenario is conducted by taking a constant channel cross-sectional area (A = 1 mm2) for all the studied channels. Therefore, a computational fluid dynamics model (CFD) for a PEM fuel cell is formed through the assembly of FC to simulate the pressure variations inside it. The simulation results showed that a triangular cross-section channel provided the uniformity of the pressure distribution, with lower deformations and lower mechanical stresses. The analysis helped gain insights into the physical mechanisms that lead to the FC’s durability and identify important parameters under different conditions. The model shows that it can assume the intracellular pressure configuration toward durability and appearance containing limited experimental data. The results also proved that the better cell voltage occurs in the case of the rectangular channel cross-section, and therefore, higher power from the FC, although its durability is much lower compared to the durability of the triangular channel. The results also showed that the rectangular channel cross-section gave higher cell voltages, and therefore, higher power (0.63 W) from the fuel cell, although its durability is much lower compared to the durability of the triangular channel. Therefore, the triangular channel gives better performance compared to other innovative channels.

scholarly journals Fractal-induced 2D flexible net undulation

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Michael Joon Seng Goh ◽  
Yeong Shiong Chiew ◽  
Ji Jinn Foo
Keyword(s):  
3D Reconstruction ◽  
Cross Section ◽  
Channel Cross Section ◽  
Time Varying ◽  
Blockage Ratio ◽  
Transient Time ◽  
Cross Sectional ◽  
Velocity Fluctuations ◽  
Square Grid ◽  
Multilength Scale

AbstractA net immersed in fractal-induced turbulence exhibit a transient time-varying deformation. The anisotropic, inhomogeneous square fractal grid (SFG) generated flow interacts with the flexible net to manifest as visible cross-sectional undulations. We hypothesize that the net’s response may provide a surrogate in expressing local turbulent strength. This is analysed as root-mean-squared velocity fluctuations in the net, displaying intensity patterns dependent on the grid conformation and grid-net separation. The net’s fluctuation strength is found to increase closer to the turbulator with higher thickness ratio while presenting stronger fluctuations compared to regular-square-grid (RSG) of equivalent blockage-ratio, σ. Our findings demonstrate a novel application where 3D-reconstruction of submerged nets is used to experimentally contrast the turbulence generated by RSG and multilength scale SFGs across the channel cross-section. The net’s response shows the unique turbulence developed from SFGs can induce 9 × higher average excitation to a net when compared against RSG of similar σ.

Long waves in a straight channel with non-uniform cross-section

2015 ◽  
Vol 770 ◽  
pp. 156-188 ◽  
Author(s):  
Patricio Winckler ◽  
Philip L.-F. Liu
Keyword(s):  
Boundary Layer ◽  
Wave Propagation ◽  
Cross Section ◽  
Three Dimensional ◽  
Channel Cross Section ◽  
Cross Sectional ◽  
New Model ◽  
One Dimensional ◽  
Uniform Channel ◽  
The Cross

A cross-sectionally averaged one-dimensional long-wave model is developed. Three-dimensional equations of motion for inviscid and incompressible fluid are first integrated over a channel cross-section. To express the resulting one-dimensional equations in terms of the cross-sectional-averaged longitudinal velocity and spanwise-averaged free-surface elevation, the characteristic depth and width of the channel cross-section are assumed to be smaller than the typical wavelength, resulting in Boussinesq-type equations. Viscous effects are also considered. The new model is, therefore, adequate for describing weakly nonlinear and weakly dispersive wave propagation along a non-uniform channel with arbitrary cross-section. More specifically, the new model has the following new properties: (i) the arbitrary channel cross-section can be asymmetric with respect to the direction of wave propagation, (ii) the channel cross-section can change appreciably within a wavelength, (iii) the effects of viscosity inside the bottom boundary layer can be considered, and (iv) the three-dimensional flow features can be recovered from the perturbation solutions. Analytical and numerical examples for uniform channels, channels where the cross-sectional geometry changes slowly and channels where the depth and width variation is appreciable within the wavelength scale are discussed to illustrate the validity and capability of the present model. With the consideration of viscous boundary layer effects, the present theory agrees reasonably well with experimental results presented by Chang et al. (J. Fluid Mech., vol. 95, 1979, pp. 401–414) for converging/diverging channels and those of Liu et al. (Coast. Engng, vol. 53, 2006, pp. 181–190) for a uniform channel with a sloping beach. The numerical results for a solitary wave propagating in a channel where the width variation is appreciable within a wavelength are discussed.

Numerical Study of the Effect of Inlet Constriction on Bubble Growth During Flow Boiling in Microchannels

2005 ◽  
Author(s):  
Abhijit Mukherjee ◽  
Satish G. Kandlikar
Keyword(s):  
Stokes Equations ◽  
Numerical Study ◽  
Flow Boiling ◽  
Navier Stokes ◽  
Channel Cross Section ◽  
Vapor Bubbles ◽  
Reversed Flow ◽  
Cross Sectional ◽  
Parallel Microchannels ◽  
Energy Equations

Flow boiling through microchannels is characterized by nucleation of vapor bubbles on the channel walls and their rapid growth as they fill the entire channel cross-section. In parallel microchannels connected through a common header, formation of vapor bubbles often results in flow maldistribution that leads to reversed flow in certain channels. The reversed flow is detrimental to the heat transfer and leads to early CHF condition. One way of eliminating the reversed flow is to incorporate flow restrictions at the channel inlet. In the present numerical study, a nucleating vapor bubble placed near the restricted end of a microchannel is numerically simulated. The complete Navier-Stokes equations along with continuity and energy equations are solved using the SIMPLER method. The liquid-vapor interface is captured using the level set technique. The results show that with no restriction the bubble moves towards the nearest channel outlet, whereas in the presence of a restriction, the bubble moves towards the distant but unrestricted end. It is proposed that channels with increasing cross-sectional area may be used to promote unidirectional growth of the vapor plugs and prevent reversed flow.

scholarly journals Geometry of the Vocal Tract and Properties of Phonation near Threshold: Calculations and Measurements

2019 ◽  
Vol 9 (13) ◽  
pp. 2755 ◽  
Author(s):  
Lewis Fulcher ◽  
Alexander Lodermeyer ◽  
George Kähler ◽  
Stefan Becker ◽  
Stefan Kniesburges
Keyword(s):  
Vocal Tract ◽  
Wave Model ◽  
Vocal Folds ◽  
Cross Sectional Area ◽  
Inertial Effects ◽  
Sectional Area ◽  
Cross Sectional ◽  
Physical Mechanisms ◽  
Lumped Element ◽  
Geometrical Boundary

In voice research, analytically-based models are efficient tools to investigate the basic physical mechanisms of phonation. Calculations based on lumped element models describe the effects of the air in the vocal tract upon threshold pressure (Pth) by its inertance. The latter depends on the geometrical boundary conditions prescribed by the vocal tract length (directly) and its cross-sectional area (inversely). Using Titze’s surface wave model (SWM) to account for the properties of the vocal folds, the influence of the vocal tract inertia is examined by two sets of calculations in combination with experiments that apply silicone-based vocal folds. In the first set, a vocal tract is constructed whose cross-sectional area is adjustable from 2.7 cm2 to 11.7 cm2. In the second set, the length of the vocal tract is varied from 4.0 cm to 59.0 cm. For both sets, the pressure and frequency data are collected and compared with calculations based on the SWM. In most cases, the measurements support the calculations; hence, the model is suited to describe and predict basic mechanisms of phonation and the inertial effects caused by a vocal tract.

Oscillatory flow regimes around four cylinders in a diamond arrangement

2019 ◽  
Vol 877 ◽  
pp. 955-1006 ◽  
Author(s):  
Chengjiao Ren ◽  
Liang Cheng ◽  
Feifei Tong ◽  
Chengwang Xiong ◽  
Tingguo Chen
Keyword(s):  
Oscillatory Flow ◽  
Bluff Body ◽  
Substantial Reduction ◽  
Period Doubling ◽  
Flow Regimes ◽  
Mode Competition ◽  
Single Type ◽  
Phase Dynamics ◽  
Flow Features ◽  
Four Cylinders

Oscillatory flow around a cluster of four circular cylinders in a diamond arrangement is investigated using two-dimensional direct numerical simulation over Keulegan–Carpenter numbers (KC) ranging from 4 to 12 and Reynolds numbers (Re) from 40 to 230 at four gap-to-diameter ratios (G) of 0.5, 1, 2 and 4. Three types of flows, namely synchronous, quasi-periodic and desynchronized flows (along with 14 flow regimes) are mapped out in the (G, KC, Re)-parameter space. The observed flow characteristics around four cylinders in a diamond arrangement show a few unique features that are absent in the flow around four cylinders in a square arrangement reported by Tong et al. (J. Fluid Mech., vol. 769, 2015, pp. 298–336). These include (i) the dominance of flow around the cluster-scale structure at $G=0.5$ and 1, (ii) a substantial reduction of regime D flows in the regime maps, (iii) new quasi-periodic (phase trapping) $\text{D}^{\prime }$ (at $G=0.5$ and 1) and period-doubling $\text{A}^{\prime }$ flows (at $G=1$) and most noteworthily (iv) abnormal behaviours at ($G\leqslant 2$) (referred to as holes hereafter) such as the appearance of spatio-temporal synchronized flows in an area surrounded by a single type of synchronized flow in the regime map ($G=0.5$). The mode competition between the cluster-scale and cylinder-scale flows is identified as the key flow mechanism responsible for those unique flow features, with the support of evidence derived from quantitative analysis. Phase dynamics is introduced for the first time in bluff-body flows, to the best knowledge of the authors, to quantitatively interpret the flow response (e.g. quasi-periodic flow features) around the cluster. It is instrumental in revealing the nature of regime $\text{D}^{\prime }$ flows where the cluster-scale flow features are largely synchronized with the forcing of incoming oscillatory flow (phase trapping) but are modulated by localized flow features.

A Hybrid Model for Predicting Gas Entrainment in a Small Branch From a Co-Current Flowing Gas-Liquid Regime

2010 ◽  
Author(s):  
Robert Bowden ◽  
Wael Saleh ◽  
Ibrahim Hassan
Keyword(s):  
Digital Imaging ◽  
Three Dimensional ◽  
Flow Distribution ◽  
Critical Conditions ◽  
Inertial Effects ◽  
Cross Sectional ◽  
Gas Entrainment ◽  
Small Branch ◽  
Dip Angle ◽  
Good Agreement

An analytical model was developed to predict the critical conditions at the onset of gas entrainment in a single downward oriented branch. The branch was installed on a horizontal square cross-sectional channel having a smooth stratified co-currently flowing gas-liquid regime in the upstream inlet region. The branch flow was simulated as a three-dimensional point-sink while the downstream run flow was treated with a uniform velocity at the critical dip location. A boundary condition was imposed in the model whereby the flow distribution between the branch and run was obtained experimentally and digital imaging was used to quantify the critical dip location through the dip angle. Three constant dip angles were evaluated in the model and results showed the dip height to have good agreement with experiments between angles of 50 and 60 degrees. The predicted upstream height, however, did not match well with the experimentally determined height due to the omission of shear and inertial effects between the upstream location and critical dip.

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