scholarly journals Reynolds-number power-law scaling of differential molecular diffusion in turbulent nonpremixed combustion

2018 ◽  
Vol 3 (10) ◽  
Author(s):  
Chao Han ◽  
Haifeng Wang
Keyword(s):  
Reynolds Number ◽  
Power Law ◽  
Molecular Diffusion ◽  
Nonpremixed Combustion ◽  
Turbulent Nonpremixed

Consistent modeling of differential molecular diffusion to yield desired Reynolds-number power-law scaling

2018 ◽  
Vol 30 (8) ◽  
pp. 085108 ◽  
Author(s):  
Chao Han ◽  
Tejas Pant ◽  
Utsav Jain ◽  
Haifeng Wang
Keyword(s):  
Reynolds Number ◽  
Power Law ◽  
Molecular Diffusion

Improving Pin-Fin Heat Transfer Predictions Using Artificial Neural Networks

2013 ◽  
Author(s):  
Jason K. Ostanek
Keyword(s):  
Neural Network ◽  
Heat Transfer ◽  
Neural Networks ◽  
Reynolds Number ◽  
Artificial Neural Networks ◽  
Power Law ◽  
Transfer Data ◽  
Pin Fin ◽  
The Neural Network ◽  
Artificial Neural

In much of the public literature on pin-fin heat transfer, Nusselt number is presented as a function of Reynolds number using a power-law correlation. Power-law correlations typically have an accuracy of 20% while the experimental uncertainty of such measurements is typically between 5% and 10%. Additionally, the use of power-law correlations may require many sets of empirical constants to fully characterize heat transfer for different geometrical arrangements. In the present work, artificial neural networks were used to predict heat transfer as a function of streamwise spacing, spanwise spacing, pin-fin height, Reynolds number, and row position. When predicting experimental heat transfer data, the neural network was able to predict 73% of array-averaged heat transfer data to within 10% accuracy while published power-law correlations predicted 48% of the data to within 10% accuracy. Similarly, the neural network predicted 81% of row-averaged data to within 10% accuracy while 52% of the data was predicted to within 10% accuracy using power-law correlations. The present work shows that first-order heat transfer predictions may be simplified by using a single neural network model rather than combining or interpolating between power-law correlations. Furthermore, the neural network may be expanded to include additional pin-fin features of interest such as fillets, duct rotation, pin shape, pin inclination angle, and more making neural networks expandable and adaptable models for predicting pin-fin heat transfer.

A test of a flamelet model for turbulent nonpremixed combustion☆

1994 ◽  
Vol 96 (1-2) ◽  
pp. 104-120 ◽  
Author(s):  
Y BURIKO ◽  
V KUZNETSOV ◽  
D VOLKOV ◽  
S ZAITSEV ◽  
A URYVSKY
Keyword(s):  
Flamelet Model ◽  
Nonpremixed Combustion ◽  
Turbulent Nonpremixed

scholarly journals Numerical Analysis of the Heterogeneity Effect on Electroosmotic Micromixers Based on the Standard Deviation of Concentration and Mixing Entropy Index

Micromachines ◽  
2021 ◽  
Vol 12 (9) ◽  
pp. 1055
Author(s):  
Alireza Farahinia ◽  
Jafar Jamaati ◽  
Hamid Niazmand ◽  
Wenjun Zhang
Keyword(s):  
Reynolds Number ◽  
Zeta Potential ◽  
Electroosmotic Flow ◽  
Molecular Diffusion ◽  
Slip Surface ◽  
Reynolds Numbers ◽  
Mixing Efficiency ◽  
Slip Coefficient ◽  
Mixing Rate ◽  
Heterogeneous Distribution

One approach to achieve a homogeneous mixture in microfluidic systems in the quickest time and shortest possible length is to employ electroosmotic flow characteristics with heterogeneous surface properties. Mixing using electroosmotic flow inside microchannels with homogeneous walls is done primarily under the influence of molecular diffusion, which is not strong enough to mix the fluids thoroughly. However, surface chemistry technology can help create desired patterns on microchannel walls to generate significant rotational currents and improve mixing efficiency remarkably. This study analyzes the function of a heterogeneous zeta-potential patch located on a microchannel wall in creating mixing inside a microchannel affected by electroosmotic flow and determines the optimal length to achieve the desired mixing rate. The approximate Helmholtz–Smoluchowski model is suggested to reduce computational costs and simplify the solving process. The results show that the heterogeneity length and location of the zeta-potential patch affect the final mixing proficiency. It was also observed that the slip coefficient on the wall has a more significant effect than the Reynolds number change on improving the mixing efficiency of electroosmotic micromixers, benefiting the heterogeneous distribution of zeta-potential. In addition, using a channel with a heterogeneous zeta-potential patch covered by a slip surface did not lead to an adequate mixing in low Reynolds numbers. Therefore, a homogeneous channel without any heterogeneity would be a priority in such a range of Reynolds numbers. However, increasing the Reynolds number and the presence of a slip coefficient on the heterogeneous channel wall enhances the mixing efficiency relative to the homogeneous one. It should be noted, though, that increasing the slip coefficient will make the mixing efficiency decrease sharply in any situation, especially in high Reynolds numbers.

Power Law Velocity Profile in the Turbulent Boundary Layer on Transitional Rough Surfaces

2007 ◽  
Vol 129 (8) ◽  
pp. 1083-1100 ◽  
Author(s):  
Noor Afzal
Keyword(s):  
Boundary Layer ◽  
Functional Equation ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Velocity Profile ◽  
Skin Friction ◽  
Power Law ◽  
Wall Roughness ◽  
Closure Model ◽  
Log Law

A new approach to scaling of transitional wall roughness in turbulent flow is introduced by a new nondimensional roughness scale ϕ. This scale gives rise to an inner viscous length scale ϕν∕uτ, inner wall transitional variable, roughness friction Reynolds number, and roughness Reynolds number. The velocity distribution, just above the roughness level, turns out to be a universal relationship for all kinds of roughness (transitional, fully smooth, and fully rough surfaces), but depends implicitly on roughness scale. The open turbulent boundary layer equations, without any closure model, have been analyzed in the inner wall and outer wake layers, and matching by the Izakson-Millikan-Kolmogorov hypothesis leads to an open functional equation. An alternate open functional equation is obtained from the ratio of two successive derivatives of the basic functional equation of Izakson and Millikan, which admits two functional solutions: the power law velocity profile and the log law velocity profile. The envelope of the skin friction power law gives the log law, as well as the power law index and prefactor as the functions of roughness friction Reynolds number or skin friction coefficient as appropriate. All the results for power law and log law velocity and skin friction distributions, as well as power law constants are explicitly independent of the transitional wall roughness. The universality of these relations is supported very well by extensive experimental data from transitional rough walls for various different types of roughnesses. On the other hand, there are no universal scalings in traditional variables, and different expressions are needed for various types of roughness, such as inflectional roughness, monotonic roughness, and others. To the lowest order, the outer layer flow is governed by the nonlinear turbulent wake equations that match with the power law theory as well as log law theory, in the overlap region. These outer equations are in equilibrium for constant value of m, the pressure gradient parameter, and under constant eddy viscosity closure model, the analytical and numerical solutions are presented.

The Influence of Confinement on the Hydrodynamic Characteristics of a Cylindrical Pillar Within a Microchannel

2015 ◽  
Author(s):  
John O’Connor ◽  
Jeff Punch ◽  
Nicholas Jeffers ◽  
Jason Stafford
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Flow Regime ◽  
Power Law ◽  
Open Channel ◽  
Head Loss ◽  
Heat Fluxes ◽  
Hydrodynamic Characteristics ◽  
Data Set ◽  
Number Range

Microfluidic cooling technologies for future electronic and photonic microsystems require more efficient flow configurations to improve heat transfer without a hydrodynamic penalty. Although conventional microchannel heat sinks are effective at dissipating large heat fluxes, their large pressure drops are a limiting design factor. There is some evidence in the literature that obstacles such as pillars placed in a microchannel can enhance downstream convective heat transfer with some increase in pressure drop. In this paper, measured head-loss coefficients are presented for a set of single microchannels of nominal hydraulic diameter 391μm and length 30mm, each containing a single, centrally-located cylindrical pillar covering a range of confinement ratios, β = 0.1–0.7, over a Reynolds number range of 40–1900. The increase in head-loss due to the addition of the pillar ranged from 143% to 479%, compared to an open channel. To isolate the influence of the pillar, the head-loss contribution of the open channel was extracted from the data for each pillar configuration. The data was curve-fitted to a decaying power-law relationship. High coefficients of determination were recorded with low root mean squared errors, indicating good fits to the data. The data set was surface-fitted with a power law relationship using the Reynolds number based on the cylinder diameter. This was found to collapse the data well below a Reynolds number of 425 to an accuracy of ± 20%. Beyond this Reynolds number an inflection point was observed, indicating a change in flow regime similar to that of a cylinder in free flow. This paper gives an insight into the hydrodynamic behavior of a microchannel containing cylindrical pillars in a laminar flow regime, and provides a practical tool for determining the head-loss of a configuration that has been demonstrated to improve downstream heat transfer in microchannels.

An experiment on the stability of small disturbances in a stratified free shear layer

1972 ◽  
Vol 52 (3) ◽  
pp. 499-528 ◽  
Author(s):  
R. S. Scotti ◽  
G. M. Corcos
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Test Section ◽  
Richardson Number ◽  
Molecular Diffusion ◽  
Free Shear Layer ◽  
Horizontal Beam ◽  
Theoretical Predictions ◽  
Critical Richardson Number ◽  
The Stability

A statically stable stratified free shear layer was formed within the test section of a wind tunnel by merging two uniform streams of air after uniformly heating the top stream. The two streams were accelerated side by side in a contraction section. The resulting sheared thermocline thickened gradually as a result of molecular diffusion and was characterized by nearly self-similar temperature (odd), velocity (odd) and Richardson number (even) profiles. The minimum Richardson numberJ0could be adjusted over the range 0·07 ≥J0≥ 0·76; the Reynolds number Re varied between 30 and 70. Small periodic disturbances were introduced upstream of the test section by a fine wire oscillating in the thermocline. The wire generated a narrow horizontal beam of internal waves, which propagated downstream and remained confined within the thermocline. The growth or decay of these waves was observed in the test section. The results confirm the existence of a critical Richardson number the value of which is in plausible agreement with theoretical predictions (J0≅ 0·22 for the Reynolds number of the experiment). The growth rate is a function of the wavenumber and is somewhat different from that computed for the same Reynolds and Richardson numbers, but the calculation assumed velocity and density profiles which were also somewhat different.

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