Experimental and Computational Study of Turbulent Flows in a Channel With Two Pairs of Turbulence Promoters in Tandem

1990 ◽  
Vol 112 (3) ◽  
pp. 302-310 ◽  
Author(s):  
T.-M. Liou ◽  
Y. Chang ◽  
D.-W. Hwang
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Turbulent Flows ◽  
Turbulence Intensity ◽  
Computational Models ◽  
Computational Study ◽  
Reattachment Length ◽  
Transfer Enhancement ◽  
Mean Velocity ◽  
Pitch Ratio

Measurements and computations are presented of mean velocity and turbulence intensity for an arrangement of two pairs of turbulence promoters mounted in tandem in developing channel flow. The Reynolds number (ReD) and the pitch ratio (PR) were varied in the range of 1.2 × 104 to 1.2 × 105 and 1 to 100, respectively. The three pitch ratios 5, 10, 15 were found to provide three characteristic flows which are a useful test of the computational models. The effects of PR on the reattachment lengths and the pressure loss as well as the influence of ReD on the reattachment length were documented in detail. It was found that PR=10 was preferable to PR = 5 and PR = 15 from the standpoint of heat transfer enhancement.

Structure of Turbulent Flows Over Forward Facing Steps With Adverse Pressure Gradient

2016 ◽  
Vol 138 (11) ◽  
Author(s):  
Hassan Iftekhar ◽  
Martin Agelin-Chaab
Keyword(s):  
Reynolds Number ◽  
Pressure Gradient ◽  
Turbulent Flows ◽  
Large Scale ◽  
Adverse Pressure Gradient ◽  
Reynolds Numbers ◽  
Step Height ◽  
Reattachment Length ◽  
Mean Velocity ◽  
The Mean

This paper reports an experimental study on the effects of adverse pressure gradient (APG) and Reynolds number on turbulent flows over a forward facing step (FFS) by employing three APGs and three Reynolds numbers. A particle image velocimetry (PIV) technique was used to conduct velocity measurements at several locations downstream, and the flow statistics up to 68 step heights are reported. The step height was maintained at 6 mm, and the Reynolds numbers based on the step height and freestream mean velocity were 1600, 3200, and 4800. The mean reattachment length increases with the increase in Reynolds number without the APG whereas the mean reattachment length remains constant for increasing APG. The proper orthogonal decomposition (POD) results confirmed that higher Reynolds numbers caused the large-scale structures to be more defined and organized close to the step surface.

Heat Transfer and Pressure Loss Characteristics of Zig-Zag Channel With Rib-Turbulators

2013 ◽  
Author(s):  
Sin Chien Siw ◽  
Minking K. Chyu ◽  
Mary Anne Alvin
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Enhancement ◽  
Pressure Loss ◽  
Hydraulic Diameter ◽  
Test Cases ◽  
Transfer Enhancement ◽  
Mean Velocity ◽  
Smooth Channel ◽  
Rib Turbulators

This paper describes a detailed experimental study of rib-turbulators in a novel four-pass channel with 110 degree turns that exhibits a “zig-zag” pattern, and hence the name. The rectangular cross-sectional channel has the cross-section of 63.5mm by 25.4mm, corresponding to the aspect ratio of 2.5:1. This specific design with several turns will generate additional secondary vortices, while providing longer flow path that allows coolant to remove a greater heat load before being discharged downstream. Heat transfer is further enhanced by the presence of rib-turbulators. Four test cases with different rib-configurations are explored and compared to the baseline case, the smooth zig-zag channel. For the first three cases, the rib pitch-to-height (p/e) ratio is 10 and height-to-hydraulic diameter (e/Dh) of 0.044. Larger ribs are used in the fourth case, giving the p/e ratio of 5, and e/Dh of 0.088. The local heat transfer coefficient of the entire zig-zag channel is determined using the transient liquid crystal technique. The Reynolds number is based on the hydraulic diameter of the channel and bulk mean velocity ranges from 15,000 up to 30,000. The heat transfer in the zig-zag channel is enhanced due to the additional vortices and bulk flow mixing induced by the presence of turns and ribs in the entire channel. The highest heat transfer is observed along each rib-turbulator, followed by the region immediately behind the rib-turbulators. However, the heat transfer at the corner of each turn remains low and unaffected by the presence of rib-turbulators. The zigzag channel with larger rib-turbulators exhibits the highest heat transfer enhancement at approximately 4.0–4.8 times higher than that of the smooth channel, followed by the other test cases with the heat transfer enhancement ranging from 2.5–3.5. In each zig-zag channel test case, although rib-turbulators have profound impact toward the heat transfer enhancement, with rather minimal increment in pressure loss within the tested Reynolds number.

Numerical Investigation and Thermal Transfer on a Wall Corrugated without and with Artificial Roughness

2019 ◽  
Vol 392 ◽  
pp. 189-199
Author(s):  
Rabia Ferhat ◽  
Ahmed Zine Dinne Dellil ◽  
M. Kamal Hamidou
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Exchange ◽  
Heat Transfer Enhancement ◽  
Turbulent Flows ◽  
Exchange Process ◽  
Three Dimensional ◽  
Artificial Roughness ◽  
Transfer Enhancement ◽  
Corrugated Channel

The objective of this study is to give the designer an appreciation of the heat transfer enhancement in turbulent flows through corrugated channels in a heat plate exchanger. Precisely, the influence of a new technic named the artificial roughness is probed on corrugated walls, with their variable wall amplitudes for assessing the effectiveness of the heat exchange. For that purpose, a numerical simulation approach is adopted. The rectangular, triangular, trapezoidal and sinusoidal corrugated wall and artificial roughness wall shapes are investigated, in order to determine the optimal wall profile resulting in significance increase in the heat exchange process with a minimum friction loss. The numerical results are presented in the form of isotherms, streamlines, contour, Nusselt number (Nu) and friction coefficient (Cf) using commercial software ANSYS- Fluent where the Reynolds number is in the range from 3 000 to 12 000. Our simulations reveal that the sinusoidal-corrugated channel has the highest heat transfer enhancement followed by rectangular, triangular and trapezoidal-corrugated channel. In addition, introduction of artificial roughness in the wavy channel induces stronger secondary flow which makes the flow three-dimensional and improve the heat transfer by a maximum 40% at a Reynolds number equal to 12 000. This may indicate benefits for designing heat plate compact exchangers capable of higher performances in the turbulent flow regimes.

Symmetric and Asymmetric Turbulent Flows in a Rectangular Duct With a Pair of Ribs

1988 ◽  
Vol 110 (4) ◽  
pp. 373-379 ◽  
Author(s):  
T.-M. Liou ◽  
C.-F. Kao
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbulent Flows ◽  
Laser Doppler Velocimetry ◽  
Boundary Layer Thickness ◽  
Critical Reynolds Number ◽  
Doppler Velocimetry ◽  
Rectangular Duct ◽  
Reattachment Length ◽  
Mean Velocity

Laser-Doppler velocimetry (LDV) measurements are presented of mean velocity and turbulence intensity for turbulent flows past a pair of ribs in a rectangular duct of aspect ratio 2. The Reynolds number based on the duct hydraulic diameter was varied in the range of 2.0 × 103 to 7.6 × 104. The experiments cover ribs with rib height to duct height ratios from 0.13 to 0.33 and with rib width to height ratios from 1 to 10. The critical rib height above which and the critical Reynolds number below which the flow patterns become asymmetric were determined from the results. In addition, the effects of the rib width and boundary layer thickness on the formation and the size of the separation bubbles on the top surface of the ribs as well as on the reattachment length behind the ribs were documented. Furthermore, the degree of turbulence enhancement was compared between the asymmetric and the symmetric flows.

scholarly journals Effect of Integrating Metal Wire Mesh with Spray Injection for Liquid Piston Gas Compression

Energies ◽  
2021 ◽  
Vol 14 (13) ◽  
pp. 3723
Author(s):  
Barah Ahn ◽  
Vikram C. Patil ◽  
Paul I. Ro
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Enhancement ◽  
Metal Wire ◽  
Wire Mesh ◽  
Transfer Enhancement ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Gas Compression ◽  
Liquid Piston

Heat transfer enhancement techniques used in liquid piston gas compression can contribute to improving the efficiency of compressed air energy storage systems by achieving a near-isothermal compression process. This work examines the effectiveness of a simultaneous use of two proven heat transfer enhancement techniques, metal wire mesh inserts and spray injection methods, in liquid piston gas compression. By varying the dimension of the inserts and the pressure of the spray, a comparative study was performed to explore the plausibility of additional improvement. The addition of an insert can help abating the temperature rise when the insert does not take much space or when the spray flowrate is low. At higher pressure, however, the addition of spacious inserts can lead to less efficient temperature abatement. This is because inserts can distract the free-fall of droplets and hinder their speed. In order to analytically account for the compromised cooling effects of droplets, Reynolds number, Nusselt number, and heat transfer coefficients of droplets are estimated under the test conditions. Reynolds number of a free-falling droplet can be more than 1000 times that of a stationary droplet, which results in 3.95 to 4.22 times differences in heat transfer coefficients.

Impingement Cooling Flow and Heat Transfer Under Acoustic Excitations

1997 ◽  
Vol 119 (4) ◽  
pp. 810-817 ◽  
Author(s):  
C. Gau ◽  
W. Y. Sheu ◽  
C. H. Shen
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Turbulence Intensity ◽  
Pressure Level ◽  
Unstable Wave ◽  
Impingement Cooling ◽  
Cooling Flow ◽  
Acoustic Excitations ◽  
Excitation Pressure

Experiments are performed to study (a) slot air jet impingement cooling flow and (b) the heat transfer under acoustic excitations. Both flow visualization and spectral energy evolution measurements along the shear layer are made. The acoustic excitation at either inherent or noninherent frequencies can make the upstream shift for both the most unstable waves and the resulting vortex formation and its subsequent pairing processes. At inherent frequencies the most unstable wave can be amplified, which increases the turbulence intensity in both the shear layer and the core and enhances the heat transfer. Both the turbulence intensity and the heat transfer increase with increasing excitation pressure levels Spl until partial breakdown of the vortex occurs. At noninherent frequencies, however, the most unstable wave can be suppressed, which reduces the turbulence intensity and decreases the heat transfer. Both the turbulence intensity and the heat transfer decreases with increasing Spl, but increases with increasing Spl when the excitation frequency becomes dominant. For excitation at high Reynolds number with either inherent or noninherent frequency, a greater excitation pressure level is needed to cause the enhancement or the reduction in heat transfer. During the experiments, the inherent frequencies selected for excitation are Fo/2 and Fo/4, the noninherent frequencies are 0.71 Fo, 0.75 Fo, and 0.8 Fo, the acoustic pressure level varies from 70 dB to 100 dB, and the Reynolds number varies from 5500 to 22,000.

Heat Transfer Enhancement in a Rectangular (AR = 3:1) Channel With V-Shaped Dimples

2012 ◽  
Vol 135 (1) ◽  
Author(s):  
C. Neil Jordan ◽  
Lesley M. Wright
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Liquid Crystal ◽  
Pressure Drop ◽  
Heat Transfer Enhancement ◽  
Thermal Performance ◽  
Secondary Flows ◽  
Transfer Enhancement ◽  
Reduced Pressure

An alternative to ribs for internal heat transfer enhancement of gas turbine airfoils is dimpled depressions. Relative to ribs, dimples incur a reduced pressure drop, which can increase the overall thermal performance of the channel. This experimental investigation measures detailed Nusselt number ratio distributions obtained from an array of V-shaped dimples (δ/D = 0.30). Although the V-shaped dimple array is derived from a traditional hemispherical dimple array, the V-shaped dimples are arranged in an in-line pattern. The resulting spacing of the V-shaped dimples is 3.2D in both the streamwise and spanwise directions. A single wide wall of a rectangular channel (AR = 3:1) is lined with V-shaped dimples. The channel Reynolds number ranges from 10,000–40,000. Detailed Nusselt number ratios are obtained using both a transient liquid crystal technique and a newly developed transient temperature sensitive paint (TSP) technique. Therefore, the TSP technique is not only validated against a baseline geometry (smooth channel), but it is also validated against a more established technique. Measurements indicate that the proposed V-shaped dimple design is a promising alternative to traditional ribs or hemispherical dimples. At lower Reynolds numbers, the V-shaped dimples display heat transfer and friction behavior similar to traditional dimples. However, as the Reynolds number increases to 30,000 and 40,000, secondary flows developed in the V-shaped concavities further enhance the heat transfer from the dimpled surface (similar to angled and V-shaped rib induced secondary flows). This additional enhancement is obtained with only a marginal increase in the pressure drop. Therefore, as the Reynolds number within the channel increases, the thermal performance also increases. While this trend has been confirmed with both the transient TSP and liquid crystal techniques, TSP is shown to have limited capabilities when acquiring highly resolved detailed heat transfer coefficient distributions.

Thermal Performance Assessment of Turbulent Flow Through Dimpled Tubes

2010 ◽  
Author(s):  
Pornchai Nivesrangsan ◽  
Somsak Pethkool ◽  
Kwanchai Nanan ◽  
Monsak Pimsarn ◽  
Smith Eiamsa-ard
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Friction Factor ◽  
Heat Transfer Rate ◽  
Thermal Performance ◽  
Transfer Rate ◽  
Staggered Arrangement ◽  
Plain Tube ◽  
Pitch Ratio ◽  
Surface Patterns

This paper presents the heat transfer augmentation and friction factor characteristics by means of dimpled tubes. The experiments were conducted using the dimpled tubes with two different dimpled-surface patterns including aligned arrangement (A-A) and staggered arrangement (S-A), each with two pitch ratios (PR = p/Di = 0.6 and 1.0), for Reynolds number ranging from 9800 to 67,000. The experimental results achieved from the dimpled tubes are compared with those obtained from the plain tube. Evidently, the dimpled tubes with both arrangements offer higher heat transfer rates compared to the plain tube and the dimpled tube with staggered arrangement shows an advantage on the basis of heat transfer enhancement over the dimpled tube with aligned arrangement. The increase in heat transfer rate with reducing pitch ratio is due to the higher turbulent intensity imparted to the flow between the dimple surfaces. The mean heat transfer rate offered by the dimpled tube with staggered arrangement (S-A) at the lowest pitch ratio (PR = 0.6), is higher than those provided by the plain tube and the dimpled tube with aligned arrangement (A-A) at the same PR by around 127% and 8%, respectively. The empirical correlations developed in terms of pitch ratio (PR), Prandtl number (Pr) and Reynolds number, are fitted the experimental data within ±8% and ±2% for Nusselt number (Nu) and friction factor (f), respectively. In addition, the thermal performance factors under an equal pumping power constraint of the dimple tubes for both dimpled-surface arrangements are also determined.

Direct numerical simulation of turbulent channel flow up to

2015 ◽  
Vol 774 ◽  
pp. 395-415 ◽  
Author(s):  
Myoungkyu Lee ◽  
Robert D. Moser
Keyword(s):  
Numerical Simulation ◽  
Reynolds Number ◽  
Direct Numerical Simulation ◽  
Turbulent Flows ◽  
Channel Flow ◽  
Turbulent Channel Flow ◽  
Streamwise Velocity ◽  
Mean Velocity ◽  
Layer Structures ◽  
High Reynolds

A direct numerical simulation of incompressible channel flow at a friction Reynolds number ($\mathit{Re}_{{\it\tau}}$) of 5186 has been performed, and the flow exhibits a number of the characteristics of high-Reynolds-number wall-bounded turbulent flows. For example, a region where the mean velocity has a logarithmic variation is observed, with von Kármán constant ${\it\kappa}=0.384\pm 0.004$. There is also a logarithmic dependence of the variance of the spanwise velocity component, though not the streamwise component. A distinct separation of scales exists between the large outer-layer structures and small inner-layer structures. At intermediate distances from the wall, the one-dimensional spectrum of the streamwise velocity fluctuation in both the streamwise and spanwise directions exhibits $k^{-1}$ dependence over a short range in wavenumber $(k)$. Further, consistent with previous experimental observations, when these spectra are multiplied by $k$ (premultiplied spectra), they have a bimodal structure with local peaks located at wavenumbers on either side of the $k^{-1}$ range.

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