The Effect of Cross-sectional Geometry on ZT Enhancement in Rough Silicon Nanostructures

2010 ◽  
Vol 1267 ◽  
Author(s):  
Jyothi Swaroop Sadhu ◽  
Marc G Ghossoub ◽  
Sanjiv Sinha
Keyword(s):  
Thermal Conductivity ◽  
Boundary Condition ◽  
Cross Section ◽  
Silicon Nanowires ◽  
Silicon Nanostructures ◽  
Dramatic Reduction ◽  
Cross Sectional ◽  
The Wire ◽  
Phonon Localization ◽  
The Impact

AbstractThe dramatic reduction in the thermal conductivity of rough silicon nanowires is due to phonon localization in the wire resulting from multiple scattering of phonons from the rough walls. We report the dependence of thermal conductivity of the nanowires as a function of the surface roughness and the diameter of the wire by modeling the nanowire as a waveguide. In addition, we estimate the impact of boundary condition, dimensionality and cross section of rough wire on the thermal conductivity. This theoretical model gives insights for tailoring thermal conductivity and enhancing the ZT of silicon to 1 for its use in thermoelectrics

Large Eddy Simulation-Based Study of the Influence of Thermal Boundary Condition and Combustor Confinement on Premix Flame Transfer Functions

2013 ◽  
Vol 135 (2) ◽  
Author(s):  
Luis Tay-Wo-Chong ◽  
Wolfgang Polifke
Keyword(s):  
Boundary Condition ◽  
Flow Field ◽  
Cross Section ◽  
Thermal Boundary Condition ◽  
Series Data ◽  
Reference Case ◽  
Large Eddy ◽  
Flame Response ◽  
Flame Shape ◽  
The Impact

The influence of the thermal boundary condition at the combustor wall and combustor confinement on the dynamic flame response of a perfectly premixed axial swirl burner is investigated. Large eddy simulations are carried out using the dynamically thickened flame combustion model. Then system identification methods are used to determine the flame transfer function (FTF) from the computed time series data. Two configurations are compared against a reference case with a 90 mm × 90 mm combustor cross section and nonadiabatic walls: (1) a combustor cross section similar to the reference case with adiabatic combustor walls, and (2) a different confinement (160 mm × 160 mm) with nonadiabatic walls. It is found that combustor confinement and thermal boundary conditions have a noticeable influence on the flame response due to differences in the flame shape and flow field. In particular, the FTF computed with an adiabatic wall boundary condition which produces a flame with a significant heat release in both shear layers, differs significantly from the FTF with nonadiabatic walls, where the flame stabilizes only in the inner shear layer. The observed differences in the flow field and flame shape are discussed in relation to the unit impulse response of the flame. The impact of the differences in the FTF on stability limits is analyzed with a low-order thermoacoustic model.

LES-Based Study of the Influence of Thermal Boundary Condition and Combustor Confinement on Premix Flame Transfer Functions

2012 ◽  
Author(s):  
Luis Tay-Wo-Chong ◽  
Wolfgang Polifke
Keyword(s):  
Boundary Condition ◽  
Flow Field ◽  
Cross Section ◽  
Transfer Functions ◽  
Thermal Boundary Condition ◽  
Series Data ◽  
Reference Case ◽  
Flame Response ◽  
Flame Shape ◽  
The Impact

The influence of thermal boundary condition at the combustor wall and combustor confinement on the dynamic flame response of a perfectly premixed axial swirl burner is investigated. Large Eddy Simulations are carried out using the Dynamically Thickened Flame combustion model. Then, system identification methods are used to determine the flame transfer function (FTF) from the computed time series data. Two configurations are compared against a reference case with 90 mm × 90 mm combustor cross section and nonadiabatic walls: 1) combustor cross section similar to the reference case with adiabatic combustor walls, and 2) different confinement (160 mm × 160 mm) with nonadiabatic walls. It is found that combustor confinement and thermal boundary conditions have a noticeable influence on the flame response due to differences in flame shape and flow field. In particular the FTF computed with adiabatic wall boundary condition, which produces a flame with significant heat release in both shear layers, differs significantly from the FTF with nonadiabatic walls, where the flame stabilizes only in the inner shear layer. The observed differences in flow field and flame shape are discussed in relation to the unit impulse response of the flame. The impact of the differences in FTF on stability limits is analyzed with a low-order thermoacoustic model.

scholarly journals Molecular Dynamics Simulations on Influence of Defect on Thermal Conductivity of Silicon Nanowires

2021 ◽  
Vol 9 ◽  
Author(s):  
Hao Li ◽  
Qiancheng Rui ◽  
Xiwen Wang ◽  
Wei Yu
Keyword(s):  
Thermal Conductivity ◽  
Molecular Dynamics ◽  
Silicon Nanowires ◽  
Surface Defects ◽  
Low Frequency ◽  
Simulation Method ◽  
Dynamics Simulation ◽  
Non Equilibrium Molecular Dynamics ◽  
Dynamics Simulations ◽  
The Impact

A non-equilibrium molecular dynamics simulation method is conducted to study the thermal conductivity (TC) of silicon nanowires (SiNWs) with different types of defects. The impacts of defect position, porosity, temperature, and length on the TC of SiNWs are analyzed. The numerical results indicate that SiNWs with surface defects have higher TC than SiNWs with inner defects, the TC of SiNWs gradually decreases with the increase of porosity and temperature, and the impact of temperature on the TC of SiNWs with defects is weaker than the impact on the TC of SiNWs with no defects. The TC of SiNWs increases as their length increases. SiNWs with no defects have the highest corresponding frequency of low-frequency peaks of phonon density of states; however, when SiNWs have inner defects, the lowest frequency is observed. Under the same porosity, the average phonon participation of SiNWs with surface defects is higher than that of SiNWs with inner defects.

scholarly journals Evaluation of Flow Resistance Models Based on Field Experiments in a Partly Vegetated Reclamation Channel

Geosciences ◽  
2020 ◽  
Vol 10 (2) ◽  
pp. 47 ◽  
Author(s):  
Giuseppe Francesco Cesare Lama ◽  
Alessandro Errico ◽  
Simona Francalanci ◽  
Luca Solari ◽  
Federico Preti ◽  
...  
Keyword(s):  
Cross Section ◽  
Cross Sections ◽  
Flow Resistance ◽  
Field Experiments ◽  
Common Reed ◽  
Cross Sectional ◽  
Riparian Plants ◽  
Water Level Measurements ◽  
The Impact ◽  
Resistance Coefficients

This study presents a methodology for improving the efficiency of Baptist and Stone and Shen models in predicting the global water flow resistance of a reclamation channel partly vegetated by rigid and emergent riparian plants. The results of the two resistance models are compared with the measurements collected during an experimental campaign conducted in a reclamation channel colonized by Common reed (Phragmites australis (Cav.) Trin. ex Steud.). Experimental vegetative Chézy’s flow resistance coefficients have been retrieved from the analysis of instantaneous flow velocity measurements, acquired by means of a downlooking 3-component acoustic Doppler velocimeter (ADV) located at the channel upstream cross section, and by water level measurements obtained through four piezometers distributed along the reclamation channel. The main morphometrical vegetation features (i.e., stem diameters and heights, and bed surface density) have been measured at six cross sections of the vegetated reclamation channel. Following the theoretical assumptions of the divided channel method (DCM), three sub-sections have been delineated in the reference cross section to represent the impact of the partial vegetation cover on the cross sectional variability of the flow field, as observed with the ADV measurements. The global vegetative Chézy’s flow resistance coefficients have been then computed by combining each resistance model with four different composite cross section methods, respectively suggested by Colebatch, Horton, Pavlovskii, and Yen. The comparative analysis between the modeled and the experimental vegetative Chézy’s coefficients has been performed by computing the relative prediction error (εr, expressed in %) under two flow rate regimes. Stone and Shen model combined with the Horton composite cross section method provides vegetative Chézy’s coefficients with the lowest εr.

Slip-Flow Pressure Drop in Microchannels of General Cross Section

2009 ◽  
Vol 131 (3) ◽  
Author(s):  
M. Bahrami ◽  
A. Tamayol ◽  
P. Taheri
Keyword(s):  
Boundary Condition ◽  
Pressure Drop ◽  
Cross Section ◽  
Slip Flow ◽  
Homogeneous Boundary Condition ◽  
Slip Boundary Condition ◽  
Arbitrary Cross Section ◽  
Geometrical Parameters ◽  
Cross Sectional ◽  
Slip Boundary

In the present study, a compact analytical model is developed to determine the pressure drop of fully-developed, incompressible, and constant properties slip-flow through arbitrary cross section microchannels. An averaged first-order Maxwell slip boundary condition is considered. Introducing a relative velocity, the difference between the bulk flow and the boundary velocities, the axial momentum reduces to Poisson’s equation with homogeneous boundary condition. Square root of area is selected as the characteristic length scale. The model of Bahrami et al. (2006, “Pressure Drop of Laminar, Fully Developed Flow in Microchannels of Arbitrary Cross Section,” ASME J. Fluids Eng., 128, pp. 1036–1044), which was developed for no-slip boundary condition, is extended to cover the slip-flow regime in this study. The proposed model for pressure drop is a function of geometrical parameters of the channel: cross sectional area, perimeter, polar moment of inertia, and the Knudsen number. The model is successfully validated against existing numerical and experimental data collected from different sources in literature for several shapes, including circular, rectangular, trapezoidal, and double-trapezoidal cross sections and a variety of gases such as nitrogen, argon, and helium.

The cross-section of Johannesburg Securities Exchange listed equity returns (1994-2011)

2015 ◽  
Vol 32 (4) ◽  
pp. 422-444 ◽  
Author(s):  
Jakobus Daniel Van Heerden ◽  
Paul Van Rensburg
Keyword(s):  
Cross Section ◽  
Equity Returns ◽  
Research Approach ◽  
Momentum Effect ◽  
Cross Sectional ◽  
Data Set ◽  
Content Type ◽  
Specific Factors ◽  
The Cross ◽  
The Impact

Purpose – The aim of this study is to examine the impact of technical and fundamental (referred to as firm-specific) factors on the cross-sectional variation in equity returns on the Johannesburg Securities Exchange (JSE). Design/methodology/approach – To reach the objective, the study follows an empirical research approach. Cross-sectional regression analyses, factor-portfolio analyses and multifactor analyses are performed using 50 firm-specific factors for listed shares over three sample periods during 1994 to 2011. Findings – The results suggest that a strong value and momentum effect is present and robust on the JSE, while a size effect is present but varies over time. Multifactor analyses show that value and momentum factors are collectively significant in explaining the cross-section of returns. The results imply that the JSE is either not an efficient market or that current market risk models are incorrectly specified. Practical implications – The findings of the study offers practical application possibilities to investment analysts and portfolio managers. Originality/value – To the authors’ knowledge, this is the first study to use such a comprehensive data set for the specific analyses on the JSE over such a long period. All previously identified statistical biases are addressed in this study. Different approaches are applied to compare results and test for robustness for the first time.

Thermal Conductivity of Silicon Nanowire by Nonequilibrium Molecular Dynamics Simulations

2008 ◽  
Author(s):  
Shuai-Chuang Wang ◽  
Xin-Gang Liang ◽  
Xiang-Hua Xu
Keyword(s):  
Thermal Conductivity ◽  
Molecular Dynamics ◽  
Silicon Nanowire ◽  
Mean Free Path ◽  
Cross Sectional Area ◽  
Nonequilibrium Molecular Dynamics ◽  
Sectional Area ◽  
Cross Sectional ◽  
The Wire ◽  
The Cross

The thermal conductivity of the silicon nanowire was calculated using nonequilibrium molecular dynamics method. The dependence of thermal conductivity on the wire length, cross-sectional area, and temperature was investigated. The Stillinger-Weber potential model and the Nose-Hoover thermostat were used. The surfaces at the wire ends were set free boundary conditions and potential boundaries in other directions. The cross-sectional area range of the nanowires under research is from about 5 nm2 to 19 nm2 and the length range is from about 6 nm to 54 nm. The results agree well with experimental results. The reciprocal of thermal conductivity was found to be linear with that of nanowire length. And our results quantitatively showed that decreasing the cross-sectional area can reduce the phonon mean free path of the nanowire.

Thermoelectric Properties of p and n-type Nanocrystalline Silicon Nanowires with High Doping Levels

2012 ◽  
Vol 1408 ◽  
Author(s):  
F. Suriano ◽  
M. Ferri ◽  
S. Solmi ◽  
L. Belsito ◽  
A. Roncaglia ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Cross Section ◽  
Thermoelectric Properties ◽  
Silicon Nanowires ◽  
Low Cost ◽  
Nanocrystalline Silicon ◽  
Nanowire Diameter ◽  
Vertical Arrays ◽  
Heavily Doped ◽  
Average Size

ABSTRACTAn experimental investigation about the thermoelectric properties of heavily doped p ad n-type nanocrystalline silicon nanowires (NWs) is described. The NWs are produced with low cost CMOS compatible processes, highly customizable in terms of cross-section and placement, which enables the fabrication of both stacked NWs in nearly vertical arrays within nanostructured templates built with SiO2/Si3N4 thin films and individual, freestanding NWs suited for thermal conductivity measurements. The cross-section dimensions of the investigated NWs range between 30 and 120 nm in size and up to about 2 cm in length. The structure of the NWs, as shown by SEM/TEM observations, is nanocrystalline with average size of the nanocrystals in one dimension that is comparable with the nanowire diameter. On the NWs, Seebeck coefficient, electrical resistivity and thermal conductivity have been measured, yielding thermoelectric figure of merit (ZT) values of 0.2 at 300 K for the best case.

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