Molecular Thermal Conductivity Research Articles

Molecular Orientation and Thermal Conductivity in Liquid Crystalline Epoxy Resins by Anionic Ring-Opening Polymerization

Molecular Orientation and Thermal Conductivity in Liquid Crystalline Epoxy Resins by Anionic Ring-Opening Polymerization

New ways to improve the efficiency of railway transportation of viscous petroleum products

The paper proposes a new approach to solving the problem of increasing the economic efficiency of railway transportation of viscous petroleum fuels (fuel oils) at low air temperatures. The physical properties of fuel oils allow them to be obtained when poured into the tank of a tank wagon in a stratified state, when their density in the upper part of the tank is significantly less than in its lower part. This blocks the natural convection of hot fuel oil on the cold walls of the tank, and it cools only due to the molecular thermal conductivity which is very small. Upon cooling down, a relatively thin highly viscous layer forms on the inner walls of the tank of a tank wagon, which acts as a heat-insulating shell, and the bulk of it (more than 90%) retains high temperature and fluidity throughout the period of transportation. The thermal and hydrodynamic calculations were performed using modern computer technologies (the ANSYS 5.6 software package). The results obtained show that the need to heat up the fuel oil during unloading remains, but already requires significantly less time and heat energy. The proposed energy-saving technologies for the delivery of viscous petroleum products are especially relevant in Russia with its cold continental climate, long-haul transportation, and the current structure of the country’s wagon fleet. The value of the results obtained lies in the fact that the proposal can be implemented on tank wagons in circulation with minimal change to their design. The technology of operation of the drain equipment at unloading points will not change either.

Modulating thermal and electrical conductivities in polymers: An approach toward extracting molecular design rules through atomistic simulations

Polymers are extensively employed in diverse industries, including electrical equipment and electronic devices. Recent technological advancements have intensified the demand for dielectric polymers with both high insulation resistance and high thermal conductivity. We employed molecular dynamics simulations to clarify the intricate relationship between molecular structures, thermal conductivity, and ionic mobility from an atomistic point of view. Examined polymers include polyethylene, polyvinyl alcohol, polyvinyl chloride, polyvinylidene fluoride, polytetrafluoroethylene, polychlorotrifluoroethylene, polyoxymethylene, and polyethylene oxide. Based on the elucidated correlations among force field parameters, we found that the parameters can be clustered into four groups: group 1 (atomic and bond parameters and force constant of the bond angle), group 2 (equilibrium angle and force constant of dihedral angle), and group 3 (side chain atom charges). Thermal conductivity showed relationships with parameters in group 1, with correlation coefficients mostly exceeding 0.7 in the absolute value. Considering the correlations between the parameters, we systematically altered the parameters within each group and computed thermal conductivity and ionic mobility. When altering the force field parameters of groups 1 and group 2, a trade-off relationship between thermal conductivity and ionic mobility becomes evident. Conversely, altering the force field parameters in group 3 increased thermal conductivity while decreasing ionic mobility, breaking the trade-off relationship. The proposed clustered-parameter variation method can predict the changes in the electrical and thermal conductivity of polymers through molecular structure modifications. The method, being a general and first-principles approach, is likely to have significant advantages in the molecular design across a diverse range of polymers.

Simulation of lighting characteristics of pulse discharge in cesium vapor

Mathematical modeling of characteristics of intense light source on the basis of high pressure pulse discharge in cesium vapor is performed. The investigated discharge is realized in a long ceramic tube from Al2O3 with an internal diameter of 5 mm. The amount of cesium in the discharge is determined by the value of the saturation pressure at the cold end of the tube. A current pulse with amplitude of 40 A to 250 A is periodically passed through the discharge plasma, with a frequency of 400- 1700 Hz. In the work, a mathematical model based on the equations of radiation gas dynamics is used to simulate discharge. When calculating heat exchange by radiation in plasma and light characteristics of discharge, direct integration of the radiation transfer equation is performed. The model takes into account two mechanisms for heating the walls of the gas discharge tube. Firstly, it is the molecular thermal conductivity of the near wall plasma and secondly, it is the absorption of discharge plasma radiation by walls. As a result of the simulation, the dependencies of plasma pressure and temperature, luminous flux and luminous efficacy of the discharge on the temperature of the cold tube end were investigated for different values of amplitudes of current pulses. It is shown that at all current amplitudes there is a pronounced maximum of luminous efficacy. In all studied modes of discharge combustion color rendering index exceeds Ra > 95. It is shown in particular that if the power embedded per unit of discharge length increases to 400 W/cm, the luminous flux reaches 36000 lm/cm.

A predictive approach for the compositional and temperature representation of thermal conductivity in multicomponent molten salt systems for advanced energy applications

This study presents a theoretical model, based on kinetic theory, for accurately predicting thermal conductivity in molten salt mixtures, a vital factor in designing advanced energy systems such as concentrating solar power, thermal energy storage, and advanced nuclear reactors. Applicable to non-complex, non-reciprocal systems, the model represents thermal conductivity with respect to composition and temperature. We evaluated the model using molecular dynamics simulations on binary systems from fluoride, chloride, bromide, iodide salt families. The model predictions were also compared against existing experimental datasets for those families as well as binary nitrates and carbonates. The model demonstrated an improvement in predictive accuracy over the ideal linear estimation and showed reasonable agreement with the reliable experimental data. Deviations from ideal conductivity, analyzed using the Redlich-Kister method, pointed to the molecular weight and thermal conductivity of individual compounds as primary influencing factors. When applied to predict the thermal conductivity of higher order eutectic salt mixtures relevant to energy applications, the model showed predictive accuracy comparable to empirical methods. The model was further applied to generate compositional mapping to quantify deviations from ideality over the compositional space. This advance enhances new molten salt mixtures evaluation efficiency and extends the model to complex mixtures.

Laboratory equipment for practice learning in the framework of educational course “Molecular Physics and Thermodynamics”

This article presents laboratory equipment for practice learning of physics specialties students in the framework of educational course “Molecular Physics and Thermodynamics”. The presented laboratory equipment allows measuring the temperature dependence of the thermal conductivity coefficient of molecular liquids under atmospheric pressure and saturated vapors pressure in the temperature range from 150K to 400K. A general scheme of the developed experimental setup for measuring the thermal conductivity of liquids by the steady-state method under isobaric condition has been described. The created laboratory setup uses general design solutions of the coaxial cylinder method, preserving its features and measurement precision in general. The measurements are carried out on coaxial-geometry cell under atmospheric pressure. A modified heat potentiometer method with one thermometer was used, which allowed us to minimize the error in the estimation of the thermal conductivity coefficient due to the uncontrollable heat flows and different calibration of thermometers. The total systematic error in the measurement of the molecular liquids thermal conductivity does not exceed 3%. The procedures for direct measurement and calculation of the thermal conductivity coefficient of molecular liquids using the developed laboratory equipment are described.

Investigation of the heat transfer enhancement and deterioration induced by vortex generators in low Prandtl number sodium-potassium alloy liquid

Liquid metals, being characterized by low molecular viscosity and high molecular thermal conductivity, appear to be a stupendous choice for heat transfer fluids in deep-space nuclear reactors. Nevertheless, because of space nuclear reactor weight and volume constraints, the heat transfer capability of the heat exchanger must be substantially improved for a deeper space exploration mission. In this study, the influence of the vortex generator applied to the liquid metal was investigated to further promote the heat transfer capability of the liquid metal. And the thermo-fluid behaviors of the sodium-potassium alloy (NaK-78) are compared among the channel with the Body-Centered Cubic (BCC) lattice array, the rectangular ring rib array, and the Kagome lattice array under identical porosity. The numerical results indicate that the heat transfer deterioration phenomena occurred in the case of rectangular ring rib channel, which is a relatively rare occurrence in conventional fluids with the local Nu number even being lower than that of the smooth channel at the same location. Therefore, the rectangular ring rib should be carefully considered and evaluated when it is applied to heat transfer enhancement in NaK alloy fluid. The Kagome structure and the BCC structure could effectively promote the heat transfer performance of the NaK alloy fluid by means of the induced composite vortex system. In comparison to the smooth channel, the average Nu of the cases with the Kagome array and the BCC array yield 17.26–37.32% and 51.98–78.85% higher, respectively.

Molecular Weight Segregation and Thermal Conductivity of Polydisperse Wax-Graphene Nanocomposites.

Paraffin waxes are a promising material for heat storage with high energy density. Their low thermal conductivity, which limits the speed of charging and discharging in heat buffers, was previously shown to be improved by adding graphene nanofillers. In the present study, using molecular dynamics simulations, the segregation by molecular weight of polydisperse paraffin near graphene flakes is investigated. In liquid bidisperse paraffin composed of decane and triacontane, an aligned layer containing mainly triacontane was observed next to the graphene. Upon slow cooling, the wax crystallised into distinct layers parallel to the graphene sheet, with much stronger segregation by molecular weight than in the crystallised bidisperse wax without graphene. For polydisperse wax, the segregation effect was much less pronounced. The molten paraffin had a somewhat higher concentration of the longest chains in the first layers next to the graphene, but during crystallisation, the molecular weight segregation was only slightly increased. Measurements of crystallinity using an alternative version of the method developed by Yamamoto showed that the layers of wax were highly aligned parallel to the graphene, both in the solid state with all wax crystallised and in the liquid state with one layer of aligned wax above and below the graphene. Thermal conductivity was increased in planes parallel to the graphene flakes. The strong segregation of chain lengths in the bidisperse wax resulted in clear differences in thermal conductivity in the segregated regions. The less segregated polydisperse wax showed less variation in thermal conductivity.

MAVEN/NGIMS Dayside Exospheric Temperatures Over Solar Cycle and Seasons: Role of Dayside Thermal Balances in Regulating Temperatures

AbstractSeveral important processes of the Mars upper atmosphere are regulated by exospheric temperature (Texo) variations, including atmospheric escape rates. From the MAVEN mission, significant and largely periodic variability of Mars dayside Texo is now revealed by Neutral Gas and Ion Mass Spectrometer (NGIMS) data sets collected throughout solar cycle 24. Two complementary methods are utilized to extract temperatures from NGIMS data sets. These NGIMS dayside mean exospheric temperatures are shown to vary by ∼80 K (∼180–260 K) over solar cycle 24. This corresponds to a ΔTexo/ΔEUV sensitivity of ∼38 K m2 mW−1, where Lyman‐α is the solar index. Previous Mars Global Surveyor derived Texo values for solar cycle 23 yielded a sensitivity of ∼45 in the same units. This close correspondence suggests that the underlying dayside energy balances are similar yet slightly different over these two solar cycles. Corresponding Mars Global Ionosphere‐Thermosphere Model simulations show that molecular thermal conduction largely balances EUV heating for the Mars dayside thermosphere, while CO2 15‐µm cooling is secondary in importance, along with global winds. It is valuable to compare this Mars sensitivity to that computed for the dayside thermosphere of Venus. Pioneer Venus dayside data sets imply a sensitivity of ∼17.5 K m2 mW−1 units for solar cycle 21, a factor of ∼2.2 smaller than computed for Mars. This is because Venus CO2 cooling strongly balances EUV heating near its peak, thereby serving as a more efficient thermostat regulating dayside temperatures. CO2 cooling is much weaker for Mars.

Tidal Structures in the Mesosphere and Lower Thermosphere and Their Solar Cycle Variations

We studied the correlations between the migrating and non-migrating tides and solar cycle in the mesosphere and lower thermosphere (MLT) regions between 60° S and 60° N, which are in LAT-LON Earth coordinates, by analyzing the simulation datasets from the thermosphere and ionosphere extension of the Whole Atmosphere Community Climate Model (WACCM-X). A least squares fitting method was utilized to obtain the daily mean migrating tides and non-migrating tides. The Pearson linear correlation coefficient was used to analyze the correlations between tides and solar activity. Our analysis shows that the negative correlations between tides and solar activity are mostly impacted by the first symmetrical structure of the tidal modes for both migrating and non-migrating components. The coefficient of molecular thermal conductivity for the first symmetrical structure is small at low solar flux, so the tides dissipate more slowly when the F10.7 cm radio flux level is low. Thus, the amplitudes of tidal variations under a solar minimum condition are larger than those under a solar maximum condition. The correlation between tides and solar activity could also be influenced by some other factors, such as geomagnetic activity and the density of carbon dioxide CO2 on Earth. The tidal variations can be influenced by westward background wind, which grows stronger as geomagnetic activity rises. Further, dissipation of the tides decreases because the heat conduction and molecular viscosity are weakened in the cooling thermosphere caused by increasing CO2, which results in larger tidal amplitudes under the solar maximum condition. It is found that the correlations between tides and solar cycle vary at different altitudes and latitudes. The negative correlations are most possibly influenced by the first symmetrical structure of tidal variations and may also be impacted by geomagnetic activity. The positive correlations are impacted by the density of CO2.

Molecular Dynamics Study on Structure, Vibrational Properties, and Transport Coefficients of Liquid Alumina

The structure, vibrational density of states, and transport coefficients of liquid alumina were studied using molecular dynamics simulations. At the temperature of 2500 K, 3000 K, 3500 K, and 4000 K, systems with three different densities were constructed, respectively, including the configurations with densities of 2.81 g/cm3 and 3.17 g/cm3, and the relaxed ones with nearly zero pressure at each temperature. With the changes in temperature or density, the transformations on the structural, vibrational and transport properties were discussed. The Born-Mayer-Huggins type of atomic interactions was used, with newly optimized parameters. The analysis of the interatomic correlations indicated that the short-range order of liquid alumina was mainly constructed by AlO4 tetrahedra, also a certain number of AlO3 and AlO5 was present. Meanwhile, the structural transitions on the elemental units occurred as either the temperature or density increased. Two primary frequency bands were observed in each vibrational density of states spectrum, with the higher frequency bands produced by the O atom vibrations, and the lower frequency ones generated by the Al atom vibrations. Self-diffusion coefficients were estimated using the linear behavior of the mean-squared displacement at long time, while by using the Green-Kubo relation during equilibrium molecular dynamics simulations, thermal conductivities and viscosities were calculated. Significantly, the viscosity at 2500 K with a density of 2.81 g/cm3 was equal to 25.23 mPa s, which was very close to the experimental finding.

Hough Mode Extensions (HMEs) and Solar Tide Behavior in the Dissipative Thermosphere

AbstractThis paper quantifies and interprets how thermosphere dissipation in the form of molecular viscosity, molecular thermal conductivity and collisions with ions (“ion drag”) determines the height versus latitude structures of a variety of migrating and non‐migrating solar thermal tides, pole‐to‐pole up to 400 km altitude. This is done through computation of thermosphere Hough Mode Extensions (HMEs); that is, solutions to the linearized momentum, thermal energy, continuity, state and hydrostatic balance equations wherein the horizontal structure of troposphere forcing for each HME corresponds to the eigenfunction (Hough function) of Laplace's tidal equation for a particular tidal mode, and the background state and thermosphere dissipation are specified for nominal solar minimum, average and maximum activity conditions. The broad features revealed, different for each HME, include changes in vertical(horizontal) structure with latitude(height), degree of vertical penetration to the middle and upper thermosphere, changes in vertical wavelength (λz) with height due to the transition in background thermal gradient around the mesopause, and insights into role of ion drag in determining tidal amplitudes and their solar cycle variability. The HMEs are particularly useful for fitting measurements of tides, and for estimating tidal fields beyond those dependent variables actually fit and including those outside the fitting domain. The HMEs reported here were created as part of the Ionospheric CONnection (ICON) mission to serve as observation‐based lower boundary conditions for Thermosphere Ionosphere Mesosphere General Circulation Model‐ICON. Access to the full set of HMEs in tabular and graphical form is provided.

Spectral Analysis of Gravity Waves in the Martian Thermosphere during Low Solar Activity Based on MAVEN/NGIMS Observations

Gravity waves (GWs) are important for vertical coupling between the lower and upper atmosphere on Mars. Saturated GWs will promote the acceleration of mean flow and turbulence generation, causing diffuse transport of energy and momentum. We analyze the spectral characteristics of GWs in the thermosphere on Mars during the low solar activity of Martian Year (MY) 35, based on CO2 density measured by the Neutral Gas and Ion Mass Spectrometer (NGIMS) on board Mars Atmosphere and Volatile EvolutioN (MAVEN). The results are compared with those of MY33 at the higher solar activity. The vertical wavenumber spectral density obtained in the Martian thermosphere is similar to the semi-empirically predicted saturation spectrum in the Earth’s atmosphere, with a logarithmic spectral slope around −3. The average spectral density of GWs shows saturation between 160 and 200 km and decreases with increasing altitude. Compared to MY33, GW activity is larger and less attenuated in MY35 as it propagates upward, implying that waves experience more favorable propagation conditions during low solar activity. Also, the dependence of GW activity on local time during this period is found to be different from previous studies, with the strongest GWs in the spring of MY35 occurring within 6–12 hr on the dayside, suggesting that the amplitude of the GWs in the Martian thermosphere during low solar activity is less controlled by the background temperature and is subject to dissipative effects of other factors, such as molecular viscosity and thermal conduction.

Dynamic and kinematic characteristics of unsteady motion of a water-in-oil emulsion droplet in collision with a solid heated wall under conditions of convective heat transfer

The paper presents the experimental results of spreading, fingering and corona-like splashing of water-in-oil emulsion droplets based on n-decane, isoparaffin oil and distilled water in the range of Weber numbers We = 100–900, wall temperatures Tw = 20–80 °C and in collision with a smooth solid surface. The spreading features and separate convective rolling structures in emulsion droplets are examined at the late stages of wetting. The effect of Tw on the viscous dissipation energy and the maximum spreading coefficient is established. The validity of the assumptions explaining the contradictory behavior of the maximum spreading diameter of emulsion droplets with increasing temperature is studied. The assumption of unstable behavior of the emulsion droplet rim during spreading is proposed. The maximum emulsion corona diameter and height decrease linearly with the Tw growth due to the longer corona lifetime. The maximum corona diameter increases following the power law with increasing the Brinkman number due to an increase in the liquid viscosity and a decrease in Tw. The results help to predict the corona's behavior by determining the relationship between the viscous dissipation of the spreading liquid and the energy transferred to the liquid from the wall due to molecular (thermal) conduction.

Fluid properties impact on energy separation in Ranque–Hilsch vortex tube

This paper examines the energy separation in vortex tubes which is a passive device that can split a pressurized room temperature gas stream to hot and cold streams. The paper employs numerical simulations to investigate the impact of various working fluids such as helium, air, oxygen, nitrogen, and carbon dioxide on the energy separation in the vortex tube, using the SST k{-}omega turbulence model with viscous heating. A three-dimensional numerical investigation is sued to examine the effect of a single fluid property on vortex tube performance, while keeping the rest of the fluid properties unchanged, which is impossible to achieve via experimental study. The numerical investigation examines the influence of molecular weight, heat capacity, thermal conductivity, and dynamic viscosity on energy separation. The results show that energy separation performance improves with lower molecular weight and heat capacity, and higher dynamic viscosity of the working fluids, while no impact of the thermal conductivity is observed. Out of five gases tested in this study, helium has yielded the maximum temperature separation, while carbon dioxide has yielded the lowest performance. Results show that viscous dissipation contributes to the temperature separation in vortex tube.

Quantum-mechanical approach to simulation of molecular crystals thermal conductivity

This article is devoted to the implementation of scientific achievements into the educational process of physics specialties students in the framework of study course “Solid State Physics”. In this work, based on our previous scientific results, we present a quantum-mechanical approach that can adequately describe the temperature dependences of the dielectric crystals thermal conductivity. The basic provisions of quantum-mechanical approach are studied by students in the framework of university study course “Solid State Physics” and are based on Einstein and Debye classical models. This approach is based on the assumption that in dielectric crystals heat is transferred due to the phonons (Debye model) and thermal diffusion between the thermally activated neighboring quantum mechanical oscillators directly from site to site on a time scale of one-half of the oscillation period (Einstein model). In term of this consideration the thermal conductivity of molecular crystals are simulated in the framework of thermal conductivity model where heat is transferred by low-frequency phonons with taking into account phonon–rotation coupling, and above the phonon mobility edge by “diffusive” modes. For this purpose the theoretical temperature dependences of the isochoric thermal conductivity have been calculated numerically in the interval near or over the Debye temperature and compared with experimental results for solid C6H12, CHCl3 and CH2Cl2. Using simple molecular crystals as an example it is shows the dualism of the nature of heat transfer processes in the temperature region of the order of the Debye temperature and above. The obtained results will be useful for implementation in the educational process in the study course “Solid State Physics” in particular for understanding the features of heat transfer in the high-temperature range of dielectric crystals existence.

Insights into thermal characteristics of spiral carbon-based nanomaterials: From heat transport mechanisms to tunable thermal diode behavior

The discovery of new ways and new materials to control heat transfer has always received significant attention. Carbon-based nanomaterials due to their outstanding thermal characteristics have been a promising candidate for thermal transport and thermal rectification. A man-made spiral carbon-based nanomaterial (SCBN) possesses unique geometry characteristics and mechanical response gaining immense popularity for usage in nanodevices and nanocomposites. Using molecular dynamics simulation, thermal conductivity and thermal rectification of SCBNs have been scrutinized considering different geometry, hydrogenation effect, and mechanical response. Results show that the thermal conductivity of SCBN is utterly depended on geometry properties and heat direction; thus, thermal conductivity has obtained a wide variety from 4.1 W/mK to 45.4 W/mK. In addition, the gradient, full/pristine, and 30%hydrogenation/pristine of graphene helicoid (GH), as a special type of SCBNs, influence thermal rectification. Also, the results affirm a sharper increase in the hydrogenation ratio leads to higher thermal rectification. Interestingly, the strain has a remarkable impact on the thermal rectification which changes from 18.1% to 51.5% for the 30%/pristine hydrogenated GH. Our result revealed that tunning heat transfer is completely possible by applying strain. Thus, SCBNs benefited from inherent novel properties have a great potential for usage in thermal management applications.

Correct definition of internal energy at the phenomenological construction of model of multicomponent continuous medium by methods of nonequilibrium thermodynamics

Taking into account the methods of thermodynamics of irreversible processes using the Onsager principle, a model of a multicomponent continuous medium is constructed, the internal energy of which is "free" of the kinetic energy of diffusion. The model is designed for an imperfect continuous medium with chemical reactions in the field of conservative external forces. Generalized StefanMaxwell relations are obtained, which represent a system of hydrodynamic equations of motion of a mixture with true inertial forces. The proposed thermodynamic technique made it possible to obtain a number of algebraic relations known from the kinetic theory of gases for the transfer coefficients, relating, in particular, the coefficients of multicomponent diffusion with binary diffusivitys, thermal diffusion ratios with thermal diffusion coefficients and multicomponent diffusion coefficients, true (molecular) thermal conductivity coefficient for multicomponent mixture with partial thermal conductivity coefficient, which indicates their versatility. The results obtained are intended for modeling not only liquid imperfect solutions, but also gas-dust mixtures with a finely dispersed dust component.

Thermal conductivity of graphite nanofibers electrospun from graphene oxide-doped polyimide

Aromatic polyimide (PI)-based graphite nanofibers were obtained from the graphitization of graphene oxide (GO)-doped electrospun PI nanofibers. GO improves the PI molecular orientation, crystalline structure and thermal conductivity of the resulting nanofibers. The degree of PI molecular orientation in the nanofibers is increased by the GO during fiber preparation. This improvement in molecular orientation produces an increase in the thermal conductivity of the graphite nanofibers, and the addition of only 0.1% GO has a significant effect. The GO not only affects the thermal conductivity, but improves the PI molecular orientation and its role as nucleation centers during graphitization. This approach and the resulting high thermal conductivity materials show great potential for practical applications.

Adapting the MPAS Dynamical Core for Applications Extending Into the Thermosphere

AbstractTo extend the nonhydrostatic global Model for Prediction Across Scales (MPAS) for deep‐atmosphere (geospace) applications, we have modified the model equations and numerics to include variable atmospheric composition and (potentially large) molecular viscosity and thermal conductivity. The split‐explicit numerical integration techniques in MPAS remain stable in idealized test cases for atmospheric domains extending into the upper thermosphere and continue to provide an efficient numerical framework for nonhydrostatic simulations. Variations in the atmospheric constituents influence the dynamical equations by altering the heat capacity and ideal gas constants. These feedbacks require little alteration of the dynamical equations although our testing reveals that the amplitude of disturbances may be sensitive to even small variations in the thermodynamic coefficients. Although the potential temperature is no longer formally conserved for adiabatic flow, it remains effective as a prognostic thermodynamic variable in the model equations. Molecular viscosity and thermal conductivity are dominant influences in the upper thermosphere and are represented implicitly in the model numerics. Because of the large magnitude of these terms, their treatment, though stable, may significantly under represent the true magnitude of their damping effects. Further consideration of these deep‐atmosphere extensions to MPAS will be explored in more realistic simulations of thermospheric dynamics.