Effect of Channel Spacing on Mixed Convection in Vertical Convergent Channels

2006 ◽  
Author(s):  
Nicola Bianco ◽  
Giovanni Lacasa ◽  
Oronzio Manca
Keyword(s):  
Reynolds Number ◽  
Pressure Drop ◽  
Aspect Ratio ◽  
Mixed Convection ◽  
Wall Temperature ◽  
Forced Flow ◽  
Flat Plates ◽  
Convergent Channel ◽  
Aspect Ratios ◽  
Converging Angle

Mixed convection in air in a convergent channel with the two principal flat plates at uniform heat flux is analyzed numerically by Fluent code. In the considered system two parallel adiabatic extensions are placed downstream of the convergent channel. The forced flow is obtained by imposing a pressure drop between the inlet and the outlet of the channel. The flow in the channel is assumed to be two-dimensional, turbulent and incompressible. A k-ε turbulent model is employed. Results in terms of dimensionless wall temperature distribution as a function of the walls converging angle, the Grashof number, the pressure drop and the channel aspect ratio are presented in the ranges: 0° ≤ θ ≤ 10°; 4.10 102 ≤ Gr ≤ 32.1 105, 0 ≤ ΔP ≤ 8.82·107, 10.15 < Lw/bmin < 58.0. Results show that Reynolds number, and then the mass flow rate flowing in the channel, increases at decreasing aspect ratios, Lw/bmin. The converging angle that optimizes the fluid-dynamic within the channel is equal to 5°. Dimensionless maximum wall temperature values decreases at increasing Reynolds number and the larger the aspect ratio, the larger the decrease. The Reynolds number over which natural convection become negligible, with respect to forced convection, increases at increasing converging angle and at decreasing aspect ratio.

Analysis of Mixed Convection in Vertical Convergent Channels

Volume 1 ◽  
2004 ◽  
Author(s):  
Nicola Bianco ◽  
Oronzio Manca ◽  
Alfonso W. Mauro ◽  
Vincenzo Naso
Keyword(s):  
Reynolds Number ◽  
Pressure Drop ◽  
Free Convection ◽  
Mixed Convection ◽  
Forced Flow ◽  
Uniform Heat Flux ◽  
Turbulent Model ◽  
Flat Plates ◽  
Convergent Channel ◽  
Converging Angle

Air mixed convection in a convergent channel with the two principal flat plates at uniform heat flux is analyzed numerically. In the considered system two parallel adiabatic extensions are placed downstream the convergent channel. The forced flow is obtained by imposing a pressure drop between the inlet and the outlet of the channel. The flow in the channel is assumed to be two-dimensional, turbulent and incompressible. A k-ε turbulent model is employed. Results in terms of dimensionless wall temperature distribution as a function of the walls converging angle, the Grashof number and the pressure drop are presented in the ranges: 0 ≤ ΔP ≤ 2.2·107, 2.8·104 ≤ Gr ≤ 2.1·105. Results show that increasing the angle of converging the Reynolds number increases at the same pressure drop. The larger the pressure drop the smaller the contribution of the free convection to the Reynolds number. Increasing the converging angle only slightly increases the ΔP value for which the effect of free convection is negligible.

scholarly journals Leading-Edge Vortex Characteristics of Low-Aspect-Ratio Sweptback Plates at Low Reynolds Number

2021 ◽  
Vol 11 (6) ◽  
pp. 2450
Author(s):  
Jong-Seob Han ◽  
Christian Breitsamter
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Leading Edge ◽  
Flapping Wing ◽  
Flat Plates ◽  
Leading Edge Vortex ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Oil Flow ◽  
Edge Vortex

A sweptback angle can directly regulate a leading-edge vortex on various aerodynamic devices as well as on the wings of biological flyers, but the effect of a sweptback angle has not yet been sufficiently investigated. Here, we thoroughly investigated the effect of the sweptback angle on aerodynamic characteristics of low-aspect-ratio flat plates at a Reynolds number of 2.85 × 104. Direct force/moment measurements and surface oil-flow visualizations were conducted in the wind-tunnel B at the Technical University of Munich. It was found that while the maximum lift at an aspect ratio of 2.03 remains unchanged, two other aspect ratios of 3.13 and 4.50 show a gradual increment in the maximum lift with an increasing sweptback angle. The largest leading-edge vortex contribution was found at the aspect ratio of 3.13, resulting in a superior lift production at a sufficient sweptback angle. This is similar to that of a revolving/flapping wing, where an aspect ratio around three shows a superior lift production. In the oil-flow patterns, it was observed that while the leading-edge vortices at aspect ratios of 2.03 and 3.13 fully covered the surfaces, the vortex at an aspect ratio of 4.50 only covered up the surface approximately three times the chord, similar to that of a revolving/flapping wing. Based on the pattern at the aspect ratio of 4.50, a critical length of the leading-edge vortex of a sweptback plate was measured as ~3.1 times the chord.

Reynolds-number scaling of vortex pinch-off on low-aspect-ratio propulsors

2016 ◽  
Vol 799 ◽  
Author(s):  
John N. Fernando ◽  
David E. Rival
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Time Scales ◽  
Early Stage ◽  
Reynolds Numbers ◽  
Growth Time ◽  
Flat Plates ◽  
Low Aspect Ratio ◽  
Drag Forces ◽  
Aspect Ratios

Impulsively started, low-aspect-ratio elliptical flat plates have been investigated experimentally to understand the vortex pinch-off dynamics at transitional and fully turbulent Reynolds numbers. The range of Reynolds numbers investigated is representative of those observed in animals that employ rowing and paddling modes of drag-based propulsion and manoeuvring. Elliptical flat plates with five aspect ratios ranging from one to two have been considered, as abstractions of propulsor planforms found in nature. It has been shown that Reynolds-number scaling is primarily determined by plate aspect ratio in terms of both drag forces and vortex pinch-off. Due to vortex-ring growth time scales that are longer than those associated with the development of flow instabilities, the scaling of drag is Reynolds-number-dependent for the aspect-ratio-one flat plate. With increasing aspect ratio, the Reynolds-number dependency decreases as a result of the shorter growth time scales associated with high-aspect-ratio elliptical vortex rings. Large drag peaks are observed during early-stage vortex growth for the higher-aspect-ratio flat plates. The collapse of these peaks with Reynolds number provides insight into the evolutionary convergence process of propulsor planforms used in drag-based swimming modes over diverse scales towards aspect ratios greater than one.

Analysis of mixing performances in microchannel with obstacles of different aspect ratios

2019 ◽  
Vol 233 (5) ◽  
pp. 1045-1051 ◽  
Author(s):  
Bappa Mondal ◽  
Sukumar Pati ◽  
PK Patowari
Keyword(s):  
Reynolds Number ◽  
Pressure Drop ◽  
Aspect Ratio ◽  
Flow Condition ◽  
Schmidt Number ◽  
Mixing Efficiency ◽  
Rectangular Microchannel ◽  
Mixing Performance ◽  
Aspect Ratios ◽  
Flow Through

In this study, the mixing performance and pressure drop characteristics have been numerically analyzed for flow through rectangular microchannel with obstacles in the walls arranged in a staggered manner. Three different aspect ratios (AR) of the obstacles are considered, namely 4:1, 1:1, and 1:4. The effects of aspect ratio of the obstacles on the mixing efficiency and the pressure drop are analyzed and compared with that of the channel without obstacle. The results are presented in terms of Reynolds number (Re) and Schmidt number (Sc) in the following range: 0.2 ≤ Re ≤ 1 and 500 ≤ Sc ≤ 1500. Enhanced mixing efficiency is achieved for the case of microchannel with obstacles and the corresponding pressure drop is also found to be higher. The mixing efficiency as well as the pressure drop is maximum for AR = 1:4 among all the geometries considered in the analysis in same flow condition. Furthermore, for a given configuration of the microchannel the mixing efficiency is governed by the mass Peclet number and, accordingly, the mixing efficiency increases with the decrease in Schmidt number for a given Reynolds number.

Heat Transfer for High Aspect Ratio Rectangular Channels in a Stationary Serpentine Passage With Turbulated and Smooth Surfaces

2013 ◽  
Author(s):  
Matthew A. Smith ◽  
Randall M. Mathison ◽  
Michael G. Dunn
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Aspect Ratio ◽  
Flow Behavior ◽  
Reynolds Numbers ◽  
Hydraulic Diameter ◽  
Internal Cooling ◽  
Number Range ◽  
Aspect Ratios ◽  
Parallel Configuration

Heat transfer distributions are presented for a stationary three passage serpentine internal cooling channel for a range of engine representative Reynolds numbers. The spacing between the sidewalls of the serpentine passage is fixed and the aspect ratio (AR) is adjusted to 1:1, 1:2, and 1:6 by changing the distance between the top and bottom walls. Data are presented for aspect ratios of 1:1 and 1:6 for smooth passage walls and for aspect ratios of 1:1, 1:2, and 1:6 for passages with two surfaces turbulated. For the turbulated cases, turbulators skewed 45° to the flow are installed on the top and bottom walls. The square turbulators are arranged in an offset parallel configuration with a fixed rib pitch-to-height ratio (P/e) of 10 and a rib height-to-hydraulic diameter ratio (e/Dh) range of 0.100 to 0.058 for AR 1:1 to 1:6, respectively. The experiments span a Reynolds number range of 4,000 to 130,000 based on the passage hydraulic diameter. While this experiment utilizes a basic layout similar to previous research, it is the first to run an aspect ratio as large as 1:6, and it also pushes the Reynolds number to higher values than were previously available for the 1:2 aspect ratio. The results demonstrate that while the normalized Nusselt number for the AR 1:2 configuration changes linearly with Reynolds number up to 130,000, there is a significant change in flow behavior between Re = 25,000 and Re = 50,000 for the aspect ratio 1:6 case. This suggests that while it may be possible to interpolate between points for different flow conditions, each geometric configuration must be investigated independently. The results show the highest heat transfer and the greatest heat transfer enhancement are obtained with the AR 1:6 configuration due to greater secondary flow development for both the smooth and turbulated cases. This enhancement was particularly notable for the AR 1:6 case for Reynolds numbers at or above 50,000.

Numerical Prediction of Contaminant Removal from Cavity in Horizontal Channel by Constrained Interpolated Profile Method

2014 ◽  
Vol 695 ◽  
pp. 384-388
Author(s):  
Nor Azwadi Che Sidik ◽  
A.S. Ahmad Sofianuddin ◽  
K.Y. Ahmat Rajab
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Removal Rate ◽  
Contaminant Removal ◽  
Square Cavity ◽  
Profile Method ◽  
Aspect Ratios ◽  
Contaminants Removal ◽  
Parabolic Flow ◽  
High Removal Rate

In this paper, Constrained Interpolated Profile Method (CIP) was used to simulate contaminants removal from square cavity in channel flow. Predictions were conducted for the range of aspect ratios from 0.25 to 4.0. The inlet parabolic flow with various Reynolds number from 50 to 1000 was used for the whole presentation with the same properties of contaminants and fluid. The obtained results indicated that the percentage of removal increased at high aspect ratio of cavity and higher Reynolds number of flow but it shows more significant changes as increasing aspect ratio rather than increasing Reynolds number. High removal rate was found at the beginning of the removal process.

Power Augmentation of a HAWT by Mie-type Tip Vanes, considering Wind Tunnel Flow Visualisation, Blade-Aspect Ratios and Reynolds Number

2003 ◽  
Vol 27 (3) ◽  
pp. 183-194 ◽  
Author(s):  
Yukimaru Shimizu ◽  
Edmond Ismaili ◽  
Yasunari Kamada ◽  
Takao Maeda
Keyword(s):  
Reynolds Number ◽  
Wind Tunnel ◽  
Aspect Ratio ◽  
Reynolds Numbers ◽  
Flow Visualisation ◽  
Velocity Measurements ◽  
Horizontal Axis Wind Turbine ◽  
Aspect Ratios ◽  
Detailed Surface ◽  
Blade Tips

Wind tunnel results are reported concerning the effects of blade aspect ratio and Reynolds number on the performance of a horizontal axis wind turbine (HAWT) with Mie-type1 tip attachments. The flow behaviour around the blade tips and the Mie-type tip vanes is presented. Detailed surface oil film visualization and velocity measurements around the blade tips, with and without Mie vanes, were obtained with the two-dimensional, Laser-Doppler Velocimetry method. Experiments were performed with rotors having blades with different aspect ratio and operating at different Reynolds numbers. The properties of the vortices generated by the Mie vanes and the blade tips were carefully studied. It was found that increased power augmentation by Mie vanes is achieved with blades having smaller aspect ratio and smaller Reynolds number.

The Effect of the Top Wall Temperature on the Laminar Natural Convection in Rectangular Cavities With Different Aspect Ratios

2009 ◽  
Vol 131 (5) ◽  
Author(s):  
Wenjiang Wu ◽  
Chan Y. Ching
Keyword(s):  
Natural Convection ◽  
Aspect Ratio ◽  
Wall Temperature ◽  
Vertical Wall ◽  
Flow Region ◽  
Temperature Profiles ◽  
Aspect Ratios ◽  
Vertical Walls ◽  
C And N ◽  
Laminar Natural Convection

The effect of the top wall temperature on the laminar natural convection in air-filled rectangular cavities driven by a temperature difference across the vertical walls was investigated for three different aspect ratios of 0.5, 1.0, and 2.0. The temperature distributions along the heated vertical wall were measured, and the flow patterns in the cavities were visualized. The experiments were performed for a global Grashof number of approximately 1.8×108 and nondimensional top wall temperatures from 0.52 (insulated) to 1.42. As the top wall was heated, the flow separated from the top wall with an undulating flow region in the corner of the cavity, which resulted in a nonuniformity in the temperature profiles in this region. The location and extent of the undulation in the flow are primarily determined by the top wall temperature and nearly independent of the aspect ratio of the cavity. The local Nusselt number was correlated with the local Rayleigh number for all three cavities in the form of Nu=C⋅Ran, but the values of the constants C and n changed with the aspect ratio.

Three-Dimensional Thermocapillary Convection in Open Cylindrical Annuli

2000 ◽  
Author(s):  
Bok-Cheol Sim ◽  
Abdelfattah Zebib
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Critical Frequency ◽  
Thermocapillary Convection ◽  
Three Dimensional ◽  
Time Dependent ◽  
Infinite Layer ◽  
Aspect Ratios ◽  
Temperature Oscillations ◽  
Good Agreement

Abstract Three-dimensional, time-dependent thermocapillary convection in open cylindrical containers is investigated numerically. Results for aspect ratios (Ar) of 1, 2.5, 8, and 16 and a Prandtl number of 6.84 are obtained to compare the results of numerical simulations with ongoing experiments. Convection is steady and axisymmetric at sufficiently low values of the Reynolds number (Re). Transition to oscillatory states occurs at critical values of Re which depend on Ar. With Ar = 1.0 and 2.5, we observe, respectively, 5 and 9 azimuthal wavetrains travelling clockwise at the free surface near the critical Re. With Ar = 8.0 and 16.0, there are substantially more, but pulsating waves near the critical Re. In the case of Ar = 16.0, which approaches the conditions in an infinite layer, our results are in good agreement with linear theory. While the critical Reynolds number decreases with increasing aspect ratio in the case of azimuthal rotating waves, it increases with increasing aspect ratio in the case of azimuthal pulsating waves. The critical frequency of temperature oscillations is found to decrease linearly with increasing Ar. We have also computed supercritical time-dependent states and find that while the frequency increases with increasing Re near the critical region, the frequency of supercritical convection decreases with Re.

Experimental Analysis of Opposing Flow in Mixed Convection in a Channel With an Open Cavity Below

2005 ◽  
Author(s):  
Oronzio Manca ◽  
Sergio Nardini ◽  
Kambiz Vafai
Keyword(s):  
Reynolds Number ◽  
Mixed Convection ◽  
Flow Visualization ◽  
Vortex Structure ◽  
Open Cavity ◽  
Forced Flow ◽  
Heated Wall ◽  
Heated Plate ◽  
Thermal Plume ◽  
Recirculation Flow

In this paper mixed convection in an open cavity with a heated wall bounded by a horizontal unheated plate is investigated experimentally. The cavity has the heated wall on the opposite side of the forced inflow. The results are reported in terms of wall temperature profiles of the heated wall and flow visualization for Reynolds number (Re) from 100 to 2000 and Richardson number (Ri) in the range 4.3–6400; the ratio between the length and the height of cavity (L/D) is in the range 0.5–2.0 and the ratio between the channel and cavity height (H/D) is equal to 1.0. The present results show that at the lowest investigated Reynolds number the surface temperatures are lower than the corresponding surface temperature for Re = 2000, at same the ohmic heat flux. The flow visualization points out that for Re = 1000 there are two nearly distinct fluid motions: a parallel forced flow in the channel and a recirculation flow inside the cavity. For Re = 100 the effect of a stronger buoyancy determines a penetration of thermal plume from the heated plate wall into the upper channel. Moreover, the flow visualization points out that for lower Reynolds numbers the forced motion penetrates inside the cavity and a vortex structure is adjacent to the unheated vertical plate. At higher Reynolds number the vortex structure has a larger extension at same L/D value.

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