scholarly journals Analysis of Aspect Ratio in a Miniature Rectangle Channel for Low Frictional Resistance

Micromachines ◽  
2021 ◽  
Vol 12 (12) ◽  
pp. 1580
Author(s):  
Takashi Fukuda ◽  
Makoto Ryo Harada
Keyword(s):  
Surface Roughness ◽  
Reynolds Number ◽  
Aspect Ratio ◽  
Rectangular Channel ◽  
Frictional Resistance ◽  
Experimental System ◽  
Cross Sectional ◽  
Friction Resistance ◽  
Calculation Results ◽  
Volume Range

We conducted a theoretical investigation of the cross-sectional aspect ratio of a rectangular channel to have sufficiently low frictional resistance under less than 150 of the Reynolds number. From the theoretical consideration, it was clarified that 3.40 or more is recommended as a criterion for determining the aspect ratio. This addresses the problem of determining the interval of rectangle channels, installed in a plate reactor. There is a concern that the real system does not follow the analytical solution, assuming laminar flow, since the higher aspect ratio leads to disturbances of the flow such as the emergence of vortices. However, in the channel’s volume range of (W × H × L) = (7.0 mm × 0.38 mm × 0.26 m), such a turbulence was not observed in the detailed numerical calculation by CFD, where both calculation results were in agreement to within 3% accuracy. Moreover, even in an experimental system with a surface roughness of ca. 7%, friction resistance took agreement within an accuracy of ±30%.

Heat Transfer in a Rotating Two-Pass Rectangular Channel Featuring Reduced Cross-Sectional Area After Tip Turn (Aspect Ratio = 4:1 to 2:1) With Profiled 60 deg Angled Ribs

2019 ◽  
Vol 141 (7) ◽  
Author(s):  
Andrew F Chen ◽  
Chao-Cheng Shiau ◽  
Je-Chin Han ◽  
Robert Krewinkel
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Aspect Ratio ◽  
Flow Direction ◽  
Rectangular Channel ◽  
Secondary Flows ◽  
Transfer Coefficients ◽  
First Passage ◽  
Cross Sectional ◽  
Internal Surfaces

The present study features a two-pass rectangular channel with an aspect ratio (AR) = 4:1 in the first pass and an AR = 2:1 in the second pass after a 180-deg tip turn. In addition to the smooth-wall case, ribs with a profiled cross section are placed at 60 deg to the flow direction on both the leading and trailing surfaces in both passages (P/e = 10, e/Dh ∼ 0.11, parallel and in-line). Regionally averaged heat transfer measurement method was used to obtain the heat transfer coefficients on all internal surfaces. The Reynolds number (Re) ranges from 10,000 to 70,000 in the first passage, and the rotational speed ranges from 0 to 400 rpm. Under pressurized condition (570 kPa), the highest rotation number achieved was Ro = 0.39 in the first passage and 0.16 in the second passage. The results showed that the turn-induced secondary flows are reduced in an accelerating flow. The effects of rotation on heat transfer are generally weakened in the ribbed case than the smooth case. Significant heat transfer reduction (∼30%) on the tip wall was seen in both the smooth and ribbed cases under rotating condition. Overall pressure penalty was reduced for the ribbed case under rotation. Reynolds number effect was found noticeable in the current study. The heat transfer and pressure drop characteristics are sensitive to the geometrical design of the channel and should be taken into account in the design process.

A Modified Chezy Formula for One-Dimensional Unsteady Frictional Resistance in Open Channel Flow

2021 ◽  
Vol 143 (5) ◽  
Author(s):  
Junwei Zhou ◽  
Weimin Bao ◽  
Geoffrey R. Tick ◽  
Hamed Moftakhari ◽  
Yu Li ◽  
...  
Keyword(s):  
Unsteady Flow ◽  
Frictional Resistance ◽  
Mitigation Strategies ◽  
Flood Routing ◽  
Flood Events ◽  
Complex Flow ◽  
Cross Sectional ◽  
Friction Resistance ◽  
Flow Unsteadiness ◽  
Better Than

Abstract It has been observed in literature that for unsteady flow conditions the one-to-one relationships between flow depth, cross-sectional averaged velocity, and frictional resistance as determined from steady uniform flow cases may not be appropriate for these more complex flow systems. Thus, a general friction resistance formula needs to be modified through the addition of new descriptive terms to account for flow unsteadiness, in order to eliminate errors due to uniform and steady-flow assumptions. An extended Chezy formula incorporating both time and space partial derivatives of hydraulic parameters was developed using dimensional analysis to investigate the relationship between flow unsteadiness and friction resistance. Results show that the proposed formula performs better than the traditional Chezy formula for simulating real hydrograph cases whereby both formula coefficients are individually identified for each flood event and coefficients are predetermined using other flood events as calibration cases. Although the extended Chezy formula as well as the original Chezy formula perform worse with the increasing degree of flow unsteadiness, its results are less dramatically affected by unsteadiness intensity, thereby improving estimations of flood routing. As a result, it tends to perform much better than traditional Chezy formula for severe flood events. Under more complex conditions whereby peak flooding events may occur predominantly under unsteady flow, the extended Chezy model may provide as a valuable tool for researchers, practitioners, and water managers for assessing and predicting impacts for flooding and for the development of more appropriate mitigation strategies and more accurate risk assessments.

Effect of Cross Aspect Ratio on Flow in Diverging and Converging Microchannels

2017 ◽  
Vol 139 (6) ◽  
Author(s):  
V. S. Duryodhan ◽  
Shiv Govind Singh ◽  
Amit Agrawal
Keyword(s):  
Reynolds Number ◽  
Pressure Drop ◽  
Aspect Ratio ◽  
Numerical Study ◽  
Three Dimensional ◽  
Hydraulic Diameter ◽  
Working Fluid ◽  
Cross Sectional ◽  
Flow Acceleration ◽  
Three Dimensional Models

Aspect ratio is an important parameter in the study of flow through noncircular microchannel. In this work, three-dimensional numerical study is carried out to understand the effect of cross aspect ratio (height to width) on flow in diverging and converging microchannels. Three-dimensional models of the diverging and converging microchannels with angle: 2–14 deg, aspect ratio: 0.05–0.58, and Reynolds number: 130–280 are employed in the simulations with water as the working fluid. The effects of aspect ratio on pressure drop in equivalent diverging and converging microchannels are studied in detail and correlated to the underlying flow regime. It is observed that for a given Reynolds number and angle, the pressure drop decreases asymptotically with aspect ratio for both the diverging and converging microchannels. At small aspect ratio and small Reynolds number, the pressure drop remains invariant of angle in both the diverging and converging microchannels; the concept of equivalent hydraulic diameter can be applied to these situations. Onset of flow separation in diverging passage and flow acceleration in converging passage is found to be a strong function of aspect ratio, which has not been shown earlier. The existence of a critical angle with relevance to the concept of equivalent hydraulic diameter is identified and its variation with Reynolds number is discussed. Finally, the effect of aspect ratio on fluidic diodicity is discussed which will be helpful in the design of valveless micropump. These results help in extending the conventional formulae made for uniform cross-sectional channel to that for the diverging and converging microchannels.

Performance Simulations of Tesla Microfluidic Valves

2007 ◽  
Author(s):  
S. Zhang ◽  
S. H. Winoto ◽  
H. T. Low
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Flow Rate ◽  
Three Dimensional ◽  
Model Performance ◽  
Hydraulic Diameter ◽  
Cross Sectional ◽  
Pressure Flow ◽  
Aspect Ratios ◽  
Tesla Valve

A three-dimensional (3-D) parametric model of Tesla-type valves is proposed. A geometrical relationship is derived for optimization study, and based on the model, performance investigations in terms of diodicity and pressure-flow rate characteristics of the valve are numerically carried out with same hydraulic diameter and different aspect ratios (of the model cross-sectional dimensions) ranging from 0.5 to 4. The 3-D computational simulations show that, for the same hydraulic diameter, the unity aspect ratio gives higher diodicity at Reynolds number less than 500 and higher will be achieved with bigger aspect ratio when the Reynolds number is above 500. Investigations of pressure-flow rate characteristics of the Tesla valve show that Tesla valve with high aspect ratio gives more flow control ability.

Flow Friction Behavior in Porous Channels

2005 ◽  
Author(s):  
T. M. Jeng ◽  
T. Y. Wu ◽  
P. L. Chen ◽  
S. F. Chang ◽  
Y. H. Hung
Keyword(s):  
Reynolds Number ◽  
Friction Factor ◽  
Rectangular Channel ◽  
Experimental Studies ◽  
Darcy Number ◽  
Metallic Foam ◽  
Friction Behavior ◽  
Cross Sectional ◽  
Porous Flow ◽  
Flow Friction

A series of experimental studies on the flow friction behavior in a rectangular channel filled with various porous metallic foam materials have been performed. The rectangular channel has a cross-sectional area 60mm × 25.4mm with a length of 60mm. The parameters and conditions of interest in the study are the Reynolds number (Re) and medium porosity/pore density (ε/PPI). The ranges of the above-mentioned parameters are: Re=2058-6736 and ε=0.7-0.93/5-40PPI. Their effects on flow friction characteristics in such porous metallic foam channels have been systematically explored. In the study, the porous flow parameters including the Darcy number (Da), inertia coefficient (CF) and Darcy friction factor (f) are investigated. The combined effects of foam porosity and Reynolds number are examined in detail. From the results, the relevant new empirical correlations of Da and CF are proposed, respectively; and a new correlation of the friction factor in terms of ε, Da and Re is presented. Besides, the results reveal that all the ratios of f/fε=1 are much greater than unity and reach the orders of around hundreds to thousands. This manifests that it needs more pumping power to maintain the same flow rate as in a hollow channel. Finally, the experimental data of f/fε=1 is correlated in the study.

Parametric and Numerical Study of Fully-Developed Flow and Heat Transfer in Rotating Rectangular Ducts

1990 ◽  
Author(s):  
H. Iacovides ◽  
B. E. Launder
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Aspect Ratio ◽  
Initial Conditions ◽  
Numerical Study ◽  
Suction Side ◽  
Rossby Number ◽  
Constant Density ◽  
Pressure Side ◽  
Cross Sectional

This work is concerned with fully-developed constant-density turbulent flow through rectangular straight ducts rotating in an orthogonal mode. Ducts of both square and 2:1 aspect ratio cross-sections have been examined. For the square duct, predictions have been performed for Reynolds numbers of 33,500 and 97,000 and for the 2:1 aspect ratio duct the computations were carried out for a Reynolds number of 33,500. Values of the inverse Rossby number (Ro = ΩD/Wb) ranged from 0.005 to 0.2. Except in the immediate vicinity of the wall, the standard high-Reynolds-number version of the k-ε model is used to account for the effects of turbulence. Across the near-wall sublayer the damping of turbulence is modelled through a low-Reynolds-number one-equation model. Low rotational speeds cause the formation of a pair of symmetric streamwise vortices. At higher rotational speeds, flow instabilities on the pressure side lead to transition to a more complex four-vortex structure. The transition point depends on both the cross-sectional geometry and the flow Reynolds number. Moreover, over a range of Rossby number, either two- or four-vortex solutions are possible depending upon initial conditions. The rotation leads to significant differences between the values of friction factor and Nusselt number on the suction and pressure surfaces of the duct. The degree of heat transfer augmentation on the pressure side is found to depend on the Reynolds number as well as on Rossby number. In contrast, heat-transfer attenuation on the suction side is only Rossby-number dependent.

Heat Transfer at High Rotation Numbers in a Two-Pass 4:1 Aspect Ratio Rectangular Channel With 45-Degree Skewed Ribs

2006 ◽  
Author(s):  
Fuguo Zhou ◽  
Sumanta Acharya
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Aspect Ratio ◽  
Rotation Number ◽  
Rectangular Channel ◽  
Published Data ◽  
Channel Measurements ◽  
First Passage ◽  
Square Channel ◽  
Smooth Channel

Heat transfer measurements are reported for a rotating 4:1 aspect ratio (AR) coolant passage with ribs skewed 45 degree to the flow. The study covers Reynolds number (Re) in the range of 10,000–70,000, rotation number (Ro) in the range of 0–0.6, and density ratios (DR) between 0.1–0.2. These measurements are done in a rotating heat transfer rig utilizing segmented copper pieces that are individually heated, and thermocouples with slip rings providing the interface between the stationary and rotating frames. The results are compared with the published data obtained in a square channel with similar dimensionless rib-geometry parameters, and with the results obtained for a 4:1 AR smooth channel. As in a 1:1 AR channel, rotation enhances the heat transfer on the destabilized walls (inlet-trailing wall and outlet-leading wall), and decreases the heat transfer ratio on the stabilized walls (inlet-leading wall and outlet-trailing wall). However, the rotation-induced enhancement/degradation for the 4:1 rectangular channel is much weaker than that in the square ribbed channel, especially in the inlet (the first passage). The results on the inlet-leading wall are in contrast to that in the smooth channel with the same AR, where rotation causes heat transfer to increase along the inlet-leading wall at lower Reynolds number (Re = 10,000 and 20,000). Higher DR is observed to enhance the heat transfer on both ribbed walls in the inlet (the first passage) and the outlet (the second passage), but the DR effects are considerably weaker than those in a ribbed square channel. Measurements have also been parameterized with respect to the buoyancy parameter and results show the same general trends as those with respect to the rotation number. In addition, pressure drop measurements have been made and the thermal performance factor results are presented.

Parametric and Numerical Study of Fully Developed Flow and Heat Transfer in Rotating Rectangular Ducts

1991 ◽  
Vol 113 (3) ◽  
pp. 331-338 ◽  
Author(s):  
H. Iacovides ◽  
B. E. Launder
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Aspect Ratio ◽  
Initial Conditions ◽  
Numerical Study ◽  
Suction Side ◽  
Rossby Number ◽  
Constant Density ◽  
Pressure Side ◽  
Cross Sectional

This work is concerned with fully developed constant-density turbulent flow through rectangular straight ducts rotating in an orthogonal mode. Ducts of both square and 2:1 aspect ratio cross sections have been examined. For the square duct, predictions have been performed for Reynolds numbers of 33,500 and 97,000 and for the 2:1 aspect ratio duct the computations were carried out for a Reynolds number of 33,500. Values of the inverse Rossby number (Ro = ΩD/Wb) ranged from 0.005 to 0.2. Except in the immediate vicinity of the wall, the standard high-Reynolds-number version of the k–ε model is used to account for the effect of turbulence. Across the near-wall sublayer the damping of turbulence is modeled through a low-Reynolds-number one-equation model. Low rotational speeds cause the formation of a pair of symmetric streamwise vortices. At higher rotational speeds, flow instabilities on the pressure side lead to transition to a more complex four-vortex structure. The transition point depends on both the cross-sectional geometry and the flow Reynolds number. Moreover, over a range of Rossby number, either two– or four–vortex solutions are possible depending upon initial conditions. The rotation leads to significant differences between the values of friction factor and Nusselt number on the suction and pressure surfaces of the duct. The degree of heat transfer augmentation on the pressure side is found to depend on the Reynolds number as well as on Rossby number. In contrast, heat transfer attenuation on the suction side is only Rossby-number dependent.

Expansion Ratio Effects on Three-Dimensional Separated Flow and Heat Transfer Around Backward-Facing Steps

2006 ◽  
Vol 129 (9) ◽  
pp. 1141-1155 ◽  
Author(s):  
Aya Kitoh ◽  
Kazuaki Sugawara ◽  
Hiroyuki Yoshikawa ◽  
Terukazu Ota
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Aspect Ratio ◽  
Rectangular Channel ◽  
Three Dimensional ◽  
Separated Flow ◽  
Expansion Ratio ◽  
Temperature Fields ◽  
Number Range ◽  
Flow And Heat Transfer

Direct numerical simulation methodology clarified the three-dimensional separated flow and heat transfer around three backward-facing steps in a rectangular channel, especially effects of channel expansion ratio ER upon them. ER treated in the present study was 1.5, 2.0, and 3.0 under a step aspect ratio of 36.0. The Reynolds number Re based on the mean velocity at inlet and the step height was varied from 300 to 1000. The present numerical results for ER=2.0 were found to be in very good agreement with the previous experimental and numerical ones in the present Reynolds number range for both the steady and unsteady flow states. The time averaged reattachment length on the center line increases with a decrease of ER. The flow became unsteady at RE=700, 600, and 500 for ER=1.5, 2.0, and 3.0, respectively, accompanying the remarkable increase of the three-dimensionality of the flow and temperature fields in spite of a very large step aspect ratio of 36.0. The Nusselt number increases in the reattachment flow region, in the neighborhood of the sidewalls, and also in the far downstream depending on both Re and ER.

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