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Patrick E Hopkins - One of the best experts on this subject based on the ideXlab platform.
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the influence of titanium adhesion layer oxygen stoichiometry on Thermal Boundary conductance at gold contacts
Applied Physics Letters, 2018Co-Authors: David H. Olson, Keren M Freedy, Stephen Mcdonnell, Patrick E HopkinsAbstract:We experimentally demonstrate the role of oxygen stoichiometry on the Thermal Boundary conductance across Au/TiOx/substrate interfaces. By evaporating two different sets of Au/TiOx/substrate samples under both high vacuum and ultrahigh vacuum conditions, we vary the oxygen composition in the TiOx layer from 0 ≤ x ≤ 2.85. We measure the Thermal Boundary conductance across the Au/TiOx/substrate interfaces with time-domain thermoreflectance and characterize the interfacial chemistry with x-ray photoemission spectroscopy. Under high vacuum conditions, we speculate that the environment provides a sufficient flux of oxidizing species to the sample surface such that one essentially co-deposits Ti and these oxidizing species. We show that slower deposition rates correspond to a higher oxygen content in the TiOx layer, which results in a lower Thermal Boundary conductance across the Au/TiOx/substrate interfacial region. Under the ultrahigh vacuum evaporation conditions, pure metallic Ti is deposited on the substrate surface. In the case of quartz substrates, the metallic Ti reacts with the substrate and getters oxygen, leading to a TiOx layer. Our results suggest that Ti layers with relatively low oxygen compositions are best suited to maximize the Thermal Boundary conductance.
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energy confinement and Thermal Boundary conductance effects on short pulsed Thermal ablation thresholds in thin films
Physical Review B, 2017Co-Authors: John A Tomko, Ashutosh Giri, Brian F Donovan, D M Bubb, S M Omalley, Patrick E HopkinsAbstract:For this paper, single-pulse ablation mechanisms of ultrafast laser pulses ($25\phantom{\rule{0.222222em}{0ex}}\mathrm{ps}$) were studied for thin gold films ($65\phantom{\rule{0.222222em}{0ex}}\mathrm{nm}$) on an array of substrates with varying physical properties. Using time-domain thermoreflectance, the interfacial properties of the thin-film systems are measured: in particular, the Thermal Boundary conductance. We find that an often used, and widely accepted relation describing threshold fluences of homogeneous bulk targets breaks down at the nanoscale. Rather than relying solely on the properties of the ablated Au film, the ablation threshold of these Au/substrate systems is found to be dependent on the measured Thermal Boundary conductance; we additionally find no discernible trend between the damage threshold and properties of the underlying substrate. These results are discussed in terms of diffusive Thermal transport and the interfacial bond strength.
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analytical model for Thermal Boundary conductance and equilibrium Thermal accommodation coefficient at solid gas interfaces
Journal of Chemical Physics, 2016Co-Authors: Ashutosh Giri, Patrick E HopkinsAbstract:We develop an analytical model for the Thermal Boundary conductance between a solid and a gas. By considering the Thermal fluxes in the solid and the gas, we describe the transmission of energy across the solid/gas interface with diffuse mismatch theory. From the predicted Thermal Boundary conductances across solid/gas interfaces, the equilibrium Thermal accommodation coefficient is determined and compared to predictions from molecular dynamics simulations on the model solid-gas systems. We show that our model is applicable for modeling the Thermal accommodation of gases on solid surfaces at non-cryogenic temperatures and relatively strong solid-gas interactions (e(sf) ≳ k(B)T).
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molecular dynamics studies of material property effects on Thermal Boundary conductance
Physical Chemistry Chemical Physics, 2013Co-Authors: Xiaowang Zhou, John C Duda, Reese E Jones, Patrick E HopkinsAbstract:Thermal Boundary resistance (inverse of conductance) between different material layers can dominate the overall Thermal resistance in nanostructures and therefore impact the performance of the Thermal property limiting nano devices. Because relationships between material properties and Thermal Boundary conductance have not been fully understood, optimum devices cannot be developed through a rational selection of materials. Here we develop generic interatomic potentials to enable material properties to be continuously varied in extremely large molecular dynamics simulations to explore the dependence of Thermal Boundary conductance on the characteristic properties of materials such as atomic mass, stiffness, and interfacial crystallography. To ensure that our study is not biased to a particular model, we employ different types of interatomic potentials. In particular, both a Stillinger–Weber potential and a hybrid embedded-atom-method + Stillinger–Weber potential are used to study metal-on-semiconductor compound interfaces, and the results are analyzed considering previous work based upon a Lennard-Jones (LJ) potential. These studies, therefore, reliably provide new understanding of interfacial transport phenomena particularly in terms of effects of material properties on Thermal Boundary conductance. Our most important finding is that Thermal Boundary conductance increases with the overlap of the vibrational spectra between metal modes and the acoustic modes of the semiconductor compound, and increasing the metal stiffness causes a continuous shift of the metal modes. As a result, the maximum Thermal Boundary conductance occurs at an intermediate metal stiffness (best matched to the semiconductor stiffness) that maximizes the overlap of the vibrational modes.
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Thermal transport across solid interfaces with nanoscale imperfections effects of roughness disorder dislocations and bonding on Thermal Boundary conductance
International Scholarly Research Notices, 2013Co-Authors: Patrick E HopkinsAbstract:The efficiency in modern technologies and green energy solutions has boiled down to a Thermal engineering problem on the nanoscale. Due to the magnitudes of the Thermal mean free paths approaching or overpassing typical length scales in nanomaterials (i.e., materials with length scales less than one micrometer), the Thermal transport across interfaces can dictate the overall Thermal resistance in nanosystems. However, the fundamental mechanisms driving these electron and phonon interactions at nanoscale interfaces are difficult to predict and control since the Thermal Boundary conductance across interfaces is intimately related to the characteristics of the interface (structure, bonding, geometry, etc.) in addition to the fundamental atomistic properties of the materials comprising the interface itself. In this paper, I review the recent experimental progress in understanding the interplay between interfacial properties on the atomic scale and Thermal transport across solid interfaces. I focus this discussion specifically on the role of interfacial nanoscale “imperfections,” such as surface roughness, compositional disorder, atomic dislocations, or interfacial bonding. Each type of interfacial imperfection leads to different scattering mechanisms that can be used to control the Thermal Boundary conductance. This offers a unique avenue for controlling scattering and Thermal transport in nanotechnology.
Philippe Marti - One of the best experts on this subject based on the ideXlab platform.
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the asymptotic equivalence of fixed heat flux and fixed temperature Thermal Boundary conditions for rapidly rotating convection
Journal of Fluid Mechanics, 2015Co-Authors: Michael A Calkins, Kevin Hale, Keith Julien, David Nieves, Derek Driggs, Philippe MartiAbstract:The influence of fixed temperature and fixed heat flux Thermal Boundary conditions on rapidly rotating convection in the plane layer geometry is investigated for the case of stress-free mechanical Boundary conditions. It is shown that whereas the leading-order system satisfies fixed temperature Boundary conditions implicitly, a double Boundary layer structure is necessary to satisfy the fixed heat flux Thermal Boundary conditions. The Boundary layers consist of a classical Ekman layer adjacent to the solid boundaries that adjust viscous stresses to zero, and a layer in Thermal wind balance just outside the Ekman layers that adjusts the normal derivative of the temperature fluctuation to zero. The influence of these Boundary layers on the interior geostrophically balanced convection is shown to be asymptotically weak, however. Upon defining a simple rescaling of the Thermal variables, the leading-order reduced system of governing equations is therefore equivalent for both Boundary conditions. These results imply that any horizontal Thermal variation along the boundaries that varies on the scale of the convection has no leading-order influence on the interior convection, thus providing insight into geophysical and astrophysical flows where stress-free mechanical Boundary conditions are often assumed.
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the asymptotic equivalence of fixed heat flux and fixed temperature Thermal Boundary conditions for rapidly rotating convection
arXiv: Fluid Dynamics, 2015Co-Authors: Michael A Calkins, Kevin Hale, Keith Julien, David Nieves, Derek Driggs, Philippe MartiAbstract:The influence of fixed temperature and fixed heat flux Thermal Boundary conditions on rapidly rotating convection in the plane layer geometry is investigated for the case of stress-free mechanical Boundary conditions. It is shown that whereas the leading order system satisfies fixed temperature Boundary conditions implicitly, a double Boundary layer structure is necessary to satisfy the fixed heat flux Thermal Boundary conditions. The Boundary layers consist of a classical Ekman layer adjacent to the solid boundaries that adjust viscous stresses to zero, and a layer in Thermal wind balance just outside the Ekman layers adjusts the temperature such that the fixed heat flux Thermal Boundary conditions are satisfied. The influence of these Boundary layers on the interior geostrophically balanced convection is shown to be asymptotically weak, however. Upon defining a simple rescaling of the Thermal variables, the leading order reduced system of governing equations are therefore equivalent for both Boundary conditions. These results imply that any horizontal Thermal variation along the boundaries that varies on the scale of the convection has no leading order influence on the interior convection.
John C Duda - One of the best experts on this subject based on the ideXlab platform.
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Thermal Boundary conductance accumulation and interfacial phonon transmission measurements and theory
Physical Review B, 2015Co-Authors: Ramez Cheaito, John C Duda, Ashutosh Giri, John T Gaskins, Matthew E Caplan, Brian F Donovan, Brian M Foley, Chester J Szwejkowski, Costel Constantin, Harlan James BrownshakleeAbstract:The advances in phonon spectroscopy in homogeneous solids have unveiled extremely useful physics regarding the contribution of phonon energies and mean-free paths to the Thermal transport in solids. However, as material systems decrease to length scales less than the phonon mean-free paths, Thermal transport can become much more impacted by scattering and transmission across interfaces between two materials than the intrinsic relaxation in the homogeneous solid. To elucidate the fundamental interactions driving this Thermally limiting interfacial phonon scattering process, we analytically derive and experimentally measure a Thermal Boundary conductance accumulation function. We develop a semiclassical theory to calculate the Thermal Boundary conductance accumulation function across interfaces using the diffuse mismatch model, and validate this derivation by measuring the interface conductance between eight different metals on native oxide/silicon substrates and four different metals on sapphire substrates. Measurements were performed at room temperature using time-domain thermoreflectance and represent the first-reported values for interface conductance across several metal/native oxide/silicon and metal/sapphire interfaces. The various metal films provide a variable bandwidth of phonons incident on the metal/substrate interface. This method of varying phonons' cutoff frequency in the film while keeping the same substrate allows us to mimic the accumulation of Thermal Boundary conductance and thus provides a direct method to experimentally validate our theory. We show that the accumulation function can be written as the product of a weighted average of the interfacial phonon transmission function and the accumulation of the temperature derivative of the phonon flux incident on the interface; this provides the framework to extract an average, spectrally dependent phonon transmissivity from a series of Thermal Boundary conductance measurements. Our approach provides a platform for analyzing the spectral phononic contribution to interfacial Thermal transport in our experimentally measured data of metal/substrate Thermal Boundary conductance. Based on the assumptions made in this work and the measurement results on different metals on native oxide/silicon and sapphire substrates, we demonstrate that high-frequency phonons dictate the transport across metal/Si interfaces, especially in low Debye temperature metals with low-cutoff frequencies.
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ion irradiation of the native oxide silicon surface increases the Thermal Boundary conductance across aluminum silicon interfaces
Physical Review B, 2014Co-Authors: Caroline S Gorham, John C Duda, Thomas E Beechem, Ramez Cheaito, John T Gaskins, Khalid Hattar, Jon F Ihlefeld, Laura Biedermann, Edward S Piekos, Douglas L MedlinAbstract:The Thermal Boundary conductance across solid-solid interfaces can be affected by the physical properties of the solid Boundary. Atomic composition, disorder, and bonding between materials can result in large deviations in the phonon scattering mechanisms contributing to Thermal Boundary conductance. Theoretical and computational studies have suggested that the mixing of atoms around an interface can lead to an increase in Thermal Boundary conductance by creating a region with an average vibrational spectra of the two materials forming the interface. In this paper, we experimentally demonstrate that ion irradiation and subsequent modification of atoms at solid surfaces can increase the Thermal Boundary conductance across solid interfaces due to a change in the acoustic impedance of the surface. We measure the Thermal Boundary conductance between thin aluminum films and silicon substrates with native silicon dioxide layers that have been subjected to proton irradiation and post-irradiation surface cleaning procedures. The Thermal Boundary conductance across the Al/native oxide/Si interfacial region increases with an increase in proton dose. Supported with statistical simulations, we hypothesize that ion beam mixing of the native oxide and silicon substrate within $\ensuremath{\sim}2.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ of the silicon surface results in the observed increase in Thermal Boundary conductance. This ion mixing leads to the spatial gradation of the silicon native oxide into the silicon substrate, which alters the acoustic impedance and vibrational characteristics at the interface of the aluminum film and native oxide/silicon substrate. We confirm this assertion with picosecond acoustic analyses. Our results demonstrate that under specific conditions, a ``more disordered and defected'' interfacial region can have a lower resistance than a more ``perfect'' interface.
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molecular dynamics studies of material property effects on Thermal Boundary conductance
Physical Chemistry Chemical Physics, 2013Co-Authors: Xiaowang Zhou, John C Duda, Reese E Jones, Patrick E HopkinsAbstract:Thermal Boundary resistance (inverse of conductance) between different material layers can dominate the overall Thermal resistance in nanostructures and therefore impact the performance of the Thermal property limiting nano devices. Because relationships between material properties and Thermal Boundary conductance have not been fully understood, optimum devices cannot be developed through a rational selection of materials. Here we develop generic interatomic potentials to enable material properties to be continuously varied in extremely large molecular dynamics simulations to explore the dependence of Thermal Boundary conductance on the characteristic properties of materials such as atomic mass, stiffness, and interfacial crystallography. To ensure that our study is not biased to a particular model, we employ different types of interatomic potentials. In particular, both a Stillinger–Weber potential and a hybrid embedded-atom-method + Stillinger–Weber potential are used to study metal-on-semiconductor compound interfaces, and the results are analyzed considering previous work based upon a Lennard-Jones (LJ) potential. These studies, therefore, reliably provide new understanding of interfacial transport phenomena particularly in terms of effects of material properties on Thermal Boundary conductance. Our most important finding is that Thermal Boundary conductance increases with the overlap of the vibrational spectra between metal modes and the acoustic modes of the semiconductor compound, and increasing the metal stiffness causes a continuous shift of the metal modes. As a result, the maximum Thermal Boundary conductance occurs at an intermediate metal stiffness (best matched to the semiconductor stiffness) that maximizes the overlap of the vibrational modes.
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effect of dislocation density on Thermal Boundary conductance across gasb gaas interfaces
Applied Physics Letters, 2011Co-Authors: Patrick E Hopkins, John C Duda, S P R Clark, C P Hains, Thomas J Rotter, Leslie M Phinney, Ganesh BalakrishnanAbstract:We report on the Thermal Boundary conductance across structurally-variant GaSb/GaAs interfaces characterized by different dislocations densities, as well as variably-rough Al/GaSb interfaces. The GaSb/GaAs structures are epitaxially grown using both interfacial misfit (IMF) and non-IMF techniques. We measure the Thermal Boundary conductance from 100 to 450 K with time-domain thermoreflectance. The Thermal Boundary conductance across the GaSb/GaAs interfaces decreases with increasing strain dislocation density. We develop a model for interfacial transport at structurally-variant interfaces in which phonon propagation and scattering parallels photon attenuation. We find that this model describes the measured Thermal Boundary conductances well.
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role of dispersion on phononic Thermal Boundary conductance
Journal of Applied Physics, 2010Co-Authors: John C Duda, Thomas E Beechem, Justin L Smoyer, Pamela M Norris, Patrick E HopkinsAbstract:The diffuse mismatch model (DMM) is one of the most widely implemented models for predicting Thermal Boundary conductance at interfaces where phonons dominate interfacial Thermal transport. In the original presentation of the DMM, the materials comprising the interface were described as Debye solids. Such a treatment, while accurate in the low temperature regime for which the model was originally intended, is less accurate at higher temperatures. Here, the DMM is reformulated such that, in place of Debye dispersion, the materials on either side of the interface are described by an isotropic dispersion obtained from exact phonon dispersion diagrams in the [100] crystallographic direction. This reformulated model is applied to three interfaces of interest: Cr–Si, Cu–Ge, and Ge–Si. It is found that Debye dispersion leads to substantially higher predictions of Thermal Boundary conductance. Additionally, it is shown that optical phonons play a significant role in interfacial Thermal transport, a notion not previously explored. Lastly, the role of the assumed dispersion is more broadly explored for Cu–Ge interfaces. The prediction of Thermal Boundary conductance via the DMM with the assumed isotropic [100] dispersion relationships is compared to predictions with isotropic [111] and exact three-dimensional phonon dispersion relationships. It is found that regardless of the chosen crystallographic direction, the predictions of Thermal Boundary conductance using isotropic phonon dispersion relationships are within a factor of two of those predictions using an exact three-dimensional phonon dispersion.
Thomas E Beechem - One of the best experts on this subject based on the ideXlab platform.
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ion irradiation of the native oxide silicon surface increases the Thermal Boundary conductance across aluminum silicon interfaces
Physical Review B, 2014Co-Authors: Caroline S Gorham, John C Duda, Thomas E Beechem, Ramez Cheaito, John T Gaskins, Khalid Hattar, Jon F Ihlefeld, Laura Biedermann, Edward S Piekos, Douglas L MedlinAbstract:The Thermal Boundary conductance across solid-solid interfaces can be affected by the physical properties of the solid Boundary. Atomic composition, disorder, and bonding between materials can result in large deviations in the phonon scattering mechanisms contributing to Thermal Boundary conductance. Theoretical and computational studies have suggested that the mixing of atoms around an interface can lead to an increase in Thermal Boundary conductance by creating a region with an average vibrational spectra of the two materials forming the interface. In this paper, we experimentally demonstrate that ion irradiation and subsequent modification of atoms at solid surfaces can increase the Thermal Boundary conductance across solid interfaces due to a change in the acoustic impedance of the surface. We measure the Thermal Boundary conductance between thin aluminum films and silicon substrates with native silicon dioxide layers that have been subjected to proton irradiation and post-irradiation surface cleaning procedures. The Thermal Boundary conductance across the Al/native oxide/Si interfacial region increases with an increase in proton dose. Supported with statistical simulations, we hypothesize that ion beam mixing of the native oxide and silicon substrate within $\ensuremath{\sim}2.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ of the silicon surface results in the observed increase in Thermal Boundary conductance. This ion mixing leads to the spatial gradation of the silicon native oxide into the silicon substrate, which alters the acoustic impedance and vibrational characteristics at the interface of the aluminum film and native oxide/silicon substrate. We confirm this assertion with picosecond acoustic analyses. Our results demonstrate that under specific conditions, a ``more disordered and defected'' interfacial region can have a lower resistance than a more ``perfect'' interface.
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role of dispersion on phononic Thermal Boundary conductance
Journal of Applied Physics, 2010Co-Authors: John C Duda, Thomas E Beechem, Justin L Smoyer, Pamela M Norris, Patrick E HopkinsAbstract:The diffuse mismatch model (DMM) is one of the most widely implemented models for predicting Thermal Boundary conductance at interfaces where phonons dominate interfacial Thermal transport. In the original presentation of the DMM, the materials comprising the interface were described as Debye solids. Such a treatment, while accurate in the low temperature regime for which the model was originally intended, is less accurate at higher temperatures. Here, the DMM is reformulated such that, in place of Debye dispersion, the materials on either side of the interface are described by an isotropic dispersion obtained from exact phonon dispersion diagrams in the [100] crystallographic direction. This reformulated model is applied to three interfaces of interest: Cr–Si, Cu–Ge, and Ge–Si. It is found that Debye dispersion leads to substantially higher predictions of Thermal Boundary conductance. Additionally, it is shown that optical phonons play a significant role in interfacial Thermal transport, a notion not previously explored. Lastly, the role of the assumed dispersion is more broadly explored for Cu–Ge interfaces. The prediction of Thermal Boundary conductance via the DMM with the assumed isotropic [100] dispersion relationships is compared to predictions with isotropic [111] and exact three-dimensional phonon dispersion relationships. It is found that regardless of the chosen crystallographic direction, the predictions of Thermal Boundary conductance using isotropic phonon dispersion relationships are within a factor of two of those predictions using an exact three-dimensional phonon dispersion.
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effects of surface roughness and oxide layer on the Thermal Boundary conductance at aluminum silicon interfaces
Physical Review B, 2010Co-Authors: Patrick E Hopkins, Leslie M Phinney, Justin R Serrano, Thomas E BeechemAbstract:In nanosystems, the primary scattering mechanisms occur at the interfaces between the material layers. As such, the structure and composition around these interfaces can affect scattering rates and, therefore, Thermal resistances. In this work, we measure the room-temperature Thermal Boundary conductance of aluminum films grown on silicon substrates subjected to various pre-Al-deposition surface treatments with a pump-probe thermoreflectance technique. The Si surfaces are characterized with atomic force microscopy to determine mean surface roughness. The measured Thermal Boundary conductances decrease as Si surface roughness increases. In addition, stripping of the native oxide layer from the surface of the Si substrate immediately prior to Al film deposition causes the Thermal Boundary conductance to increase. The measured data are compared to an extension of the diffuse mismatch model that accounts for interfacial mixing and structure around the interface in order to better elucidate the Thermal scattering processes affecting Thermal Boundary conductance at rough interfaces.
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effects of surface roughness and oxide layer on the Thermal Boundary conductance at aluminum silicon interfaces
2010 14th International Heat Transfer Conference Volume 6, 2010Co-Authors: Patrick E Hopkins, Leslie M Phinney, Justin R Serrano, Thomas E BeechemAbstract:Thermal Boundary resistance dominates the Thermal resistance in nanosystems since material length scales are comparable to material mean free paths. The primary scattering mechanism in nanosystems is interface scattering, and the structure and composition around these interfaces can affect scattering rates and, therefore, device Thermal resistances. In this work, the Thermal Boundary conductance (the inverse of the Thermal Boundary resistance) is measured using a pump-probe thermoreflectance technique on aluminum films grown on silicon substrates that are subjected to various pre-Al-deposition surface treatments. The Si surfaces are characterized with Atomic Force Microscopy (AFM) to determine mean surface roughness. The measured Thermal Boundary conductance decreases as Si surface roughness increases. In addition, stripping the native oxide layer on the surface of the Si substrate immediately prior to Al film deposition causes the Thermal Boundary conductance to increase. The measured data are then compared to an extension of the diffuse mismatch model that accounts for interfacial mixing and structure around the interface.Copyright © 2010 by ASME
Emily S C Ching - One of the best experts on this subject based on the ideXlab platform.
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Thermal Boundary layer equation for turbulent rayleigh benard convection
Physical Review Letters, 2015Co-Authors: Olga Shishkina, Susanne Horn, S R Wagner, Emily S C ChingAbstract:We report a new Thermal Boundary layer equation for turbulent Rayleigh-Benard convection for Prandtl number Pr>1 that takes into account the effect of turbulent fluctuations. These fluctuations are neglected in existing equations, which are based on steady-state and laminar assumptions. Using this new equation, we derive analytically the mean temperature profiles in two limits: (a) Pr≳1 and (b) Pr≫1. These two theoretical predictions are in excellent agreement with the results of our direct numerical simulations for Pr=4.38 (water) and Pr=2547.9 (glycerol), respectively.
