Thermal Conductivity Of Gases Research Articles

Correlation of pore size distribution with thermal conductivity of precipitated silica and experimental determination of the coupling effect

Vacuum insulation panels are high-performance insulating materials with considerably better thermal properties than conventional thermal insulation. Unfortunately, their use is limited to applications where their high price does not matter. To extend the application range of vacuum insulation, the authors try to replace the high-priced core material fumed silica with the cheaper precipitated silica. For this purpose, five commercially available precipitated silica samples were tested for their suitability as core material for vacuum insulations. They were pre-pressed with 5 bar and 30 bar each. To compare their thermal performances, a guarded hot plate apparatus for measuring thermal conductivities under vacuum was developed. The pore size distributions of the samples were measured by mercury intrusion porosimetry and used to calculate the gas thermal conductivity as a function of the residual pressure. For this purpose, a correction factor for the measured pore size distribution is introduced. Additionally, the coupling effect between gaseous and solid thermal conductivity could be determined through comparison with measured data. A model is presented to predict the thermal conductivity curve, even of unknown silica samples, solely using mercury intrusion porosimetry data.

Enhanced Photothermal Effect in Ultralow-Density Carbon Aerogels with Microporous Structures for Facile Optical Ignition Applications.

The exact mechanism responsible for the phenomenon known as photoignition with an enhanced photothermal effect in high-surface-area carbon with the addition of a metal catalyst is an open issue. Here, we report the first successful flash ignition of a pure carbon material in ambient air microporous carbon aerogels (CAs) with ultralow density and high surface area. Under flash exposure, the CAs show a strong local heat confinement effect near microporous structures (0.6-2 nm), and the graphite crystallite structures existing in single carbon nanoparticles (∼15 nm) are damaged. The local heat confinement effects are mainly derived from the low gaseous thermal conductivity in micropores and low solid thermal conductivity in low-density CAs. In addition, the limiting effects of the microporous structure on the vibration amplitude of free-state electrons in low-density CAs result in a dramatic increase in optical absorption. Numerical simulations of unsteady temperature fields of CAs with different densities and thicknesses are also performed, and the calculated maximum temperature of a 17 μm-thick 20 mg/cm3 CA bed is 1782 °C. CAs with higher density can also give rise to enhanced photothermal response and ignition with the addition of metal Fe nanoparticles. The metal catalyst increases both the light absorption capacity in the visible-light range and the heat accumulation capacity. These results are important for understanding the mechanism of flash ignition, especially the local high temperature and effects of metal catalyst in carbon materials during the photothermal process.

Direct measurements of steady heat transfer maintained by oscillating pipe flow in thermoacoustic system

This study reports direct measurements of steady heat transfer in a heat exchanger pipe of a thermoacoustic heat pump driven by an oscillating helium gas flow. From simultaneous measurements of gas temperature, pressure, and velocity fluctuations, the steady heat transfer rate is determined from the radial profile of the temporal mean temperature and from the axial distribution of the enthalpy flux density. The heat transfer coefficient h is determined to be given by h=2k/r0 consistently in the range of 1.6≤r0/δα≤6.6, where k is the gas thermal conductivity, r0 is the tube radius, and δα=2α/ω is the characteristic transverse length given by using the thermal diffusivity and angular frequency.

A logarithmic-type constitutive model for carbon fiber papers considering Hertz contact effect

Porous carbon fiber papers, which are formed by randomly distributed carbon fibers, are commonly used as gas diffusion layers of proton exchange membrane fuel cells. Several key properties of the gas diffusion layers, for example, air permeability, thermal conductivity, and electrical resistance, are inevitably controlled by the strain it experienced, which is always a nonlinear function of the applied compressive stress. In the present study, a logarithmic-type constitutive model is proposed to characterize the nonlinear stress–strain relation of carbon fiber papers under compressive loading. A hexahedral configuration of carbon fibers is adopted as a typical microstructure of carbon fiber papers. Contact and bending deformation of carbon fibers are accounted for with use of Hertz’s contact model and beam bending model, respectively. The logarithmic-type constitutive model is eventually obtained by solving a differential equation considering the variation of structural porosity. The stress–strain curves of the present model and experimental data are compared for a number of commercial gas diffusion layer samples. The comparisons indicate that the present model is capable of predicting the mechanical performance of carbon fiber papers, and may be useful for analytical and numerical analysis of gas diffusion layers and fuel cells.

Numerical matching of anisotropic transport processes in porous electrodes of proton exchange membrane fuel cells

Owing to the spatial orientations of carbon fibers, the porous electrodes of proton exchange membrane (PEM) fuel cells exhibit strong structural anisotropy, which affects the transport of species, ions, electrons, liquid water, and heat along the in-plane and through-plane directions. To capture the anisotropies of species transport, charge migration, and heat transport for PEM fuel cells operated at various loads, a two-phase flow, non-isothermal, computational fluid dynamics (CFD) model was developed and experimentally validated. Various anisotropic parameters were separately studied, and their contributions to the overall cell performance at different loads were compared. The results indicated the significance of anisotropic transport processes inside the electrodes, as the isotropic electrode properties overpredicted the cell performance. Among all the studied parameters, the anisotropies of the ion conductivity and gas diffusivity deserve careful consideration due to their significant impact on the cell performance, especially at high current densities. The anisotropies of the electrode permeability for gas transport and thermal conductivity can be neglected because of their limited effects on the cell performance. The anisotropy of the electrode permeability for liquid-water transport under a capillary mechanism had a considerable influence on the cell performance owing to its impact on the water saturation within the electrode.

Platinum Thin Film TCD Designed for 3 Omega Sensing of Gas Mixtures

Over the last few decades, the world of sensors has been revolutionized by exploring the disciplines of nanotechnology and sophisticated Nano-fabrication process. As a result, gas sensors have become smaller and more robust in the terms of sensitivity and accuracy. Furthermore, these sensors are able to cover a broader range of gases and volatile compounds increasing detection capabilities, allowing detection down to single ppm. However, challenges still exist in detecting certain gases, specifically ammonia. The chemical-based gas sensors for ammonia are not reliable due to several factors such as slow recovery time, sensor base-line drifting, vulnerability to degradation and false alarm [1]. On the other hand, the physical-based ammonia sensing approach has the advantage of fast response time and long-term durability [2], while the thermal conductivity detector suffers a lack of high sensitivity and selectivity. In an effort to address these challenges, this study is designed to improve the limit of detection of a micro wire thermal conductivity gas sensor that enables the system to detect 100 parts per million for ammonia. The differential 3-Omega measurement technique uses two sensors, one as sensing sensor and the other as a reference [3,4]. It is a harmonic excitation technique that needs a sinusoidal current can produce high thermal stresses that shorten the sensor life-time. To increase the reproducibility of sensor fabrication process, the micro wire TCD sensor material is replaced with platinum. In summary, the new design of micro wire TCD includes optimized shape to bear the high thermal stresses caused by continuous current needed by 3-omega technique and higher stability of the temperature coefficient of resistance with platinum film. [1] Azad, A. M., Akbar, S. A., Mhaisalkar, S. G., Birkefeld, L. D., & Goto, K. S. (1992). Solid‐state gas sensors: A review. Journal of the Electrochemical Society, 139(12), 3690-3704. [2] Mahdavifar, A., Navaei, M., Hesketh, P. J., Dimandja, J. D., Stetter, J. R., & McMurray, G. (2015). Implementation of a polysilicon micro electro-thermal detector in gas chromatography system with applications in portable environmental monitoring. ECS Journal of Solid State Science and Technology, 4(10), S3062-S3066. [3] Lotfi, A., Mahdavifar, A., Struk, D., Stetter, J. R., Navaei, M., & Hesketh, P. (2017). Ultimate Sensitivity of Physical Sensor for Ammonia Gas Detection Exploiting Full Differential 3-Omega Technique. ECS Transactions, 80(10), 1571-1578 [4] Kommandur, S., Mahdavifar, A., Hesketh, P. J., & Yee, S. (2015). A microbridge heater for low power gas sensing based on the 3-Omega technique. Sensors and Actuators A: Physical, 233, 231-238. Figure 1

Experimental study on differences of heat and mass flux between 10- and 50-nm pore-sized nano-porous ceramic membranes

In this study, the experiment compares the thermal and water-recovery performance of the 10- and 50-nm pore-sized ceramic membrane tubes. The gas mixture consisting of water vapor and nitrogen gets thorough the ceramic membrane tube, and the cooling water flows along the tube exterior surface. The feed gas velocity (Ub,in) at the entrance was changed from 4 to 8 L/min and the inlet temperature (Tb,in) was increased from 323 to 353 K. What’s more, the cooling water (Uc,in) velocity at the entrance was 1, 1.5, and 2 L/min respectively. In conclusion, the performance of heat and mass transfer in the 10-nm pore-sized membrane tube is more remarkable than that in 50-nm pore-sized membrane tube. Additionally, the efficiency of water recovery in the two kinds of ceramic membrane tube increases with the increasing of Tb,in and Uc,in, while it decreases with the increasing of Ub,in. The capabilities of gas adsorption and thermal conduction are influenced by surface area of membranes, which can be attributed to the difference of heat and mass-transfer process between the 10- and 50-nm pore-sized ceramic membrane tubes.

Research on a Fast-Response Thermal Conductivity Sensor Based on Carbon Nanotube Modification.

Aiming at solving the slow-response problem of traditional bead-type thermal conductivity gas sensors, a fast-response thermal conductivity gas sensor can be made by using multiwalled carbon nanotubes (MWNTs), combined with the technology of carrier modification, to modify the performance of the sensor carrier. The carrier material, granular nanoscale γ-Al2O3/ZrO2, was synthesized by chemical precipitation, and its particle size was found to be 50–70 nm through SEM. After the carrier material was wet-incorporated into carbon nanotubes, the composite carrier γ-Al2O3/ZrO2/MWNTs was obtained. The results show that the designed thermal conductivity sensor has a fast response to methane gas, with a 90% response time of 7 s and a recovery time of 16 s. There is a good linear relationship between the sensor output and CH4 gas concentration, with an average sensitivity of 1.15 mV/1% CH4. Thus, the response speed of a thermal conductivity sensor can be enhanced by doping carbon nanotubes into γ-Al2O3/ZrO2.

Prediction of thermal conductivity for characterized oils and their fractions using an expanded fluid based model

A methodology is proposed to predict the thermal conductivity of crude oils (mainly heavy oils) and their fractions based on a distillation assay, asphaltene content, molecular weight, and specific gravity of the fluid. The oils are characterized into a set of pseudo-components and their thermal conductivity is calculated using the Expanded Fluid (EF) thermal conductivity model. The inputs of this model are: the density of the fluid, the pressure, the dilute gas thermal conductivity, and, four parameters that are required for each pseudo-component, ρso, λso, c2λ and c3λ. The dilute gas thermal conductivity and the parameter ρso are calculated from existing correlations. New correlations are proposed for the remaining model parameters and for the binary interaction parameters used in the model mixing rules. The proposed approach was developed and tested on thermal conductivity and density data from the literature for pure hydrocarbons, pure hydrocarbon binaries, bitumen/solvent pseudo-binaries, crude oils, and distillation cuts. In addition, thermal conductivity and density data for pseudo-binaries of C5-asphaltene and toluene were collected in this study at temperatures from 20 to 40 °C and pressures up to 10 MPa. The EF thermal conductivity model with correlated fluid-specific parameters predicted the thermal conductivity of 7 crude oils from disparate geographical locations within 3% of the experimental data. Deviations were reduced to within 1% of experimental data by either tuning ρso to a viscosity data point or tuning λso to a thermal conductivity data point.

Fabrication and Characterizations of Microelectromechanical Systems (MEMS) Thermal Conductivity Gas Sensor

Fabrication and Characterizations of Microelectromechanical Systems (MEMS) Thermal Conductivity Gas Sensor

Curing of large prepreg shell in solar synchronous Low Earth Orbit: Precession flight regimes

We investigate the curing of large shell construction made of epoxy resin/carbon fibers prepreg under free space conditions in solar synchronous Low Earth Orbit. The curing kinetics is described by first order kinetic equation with auto-acceleration and deceleration parameters based on the experimental data. Heating of the shell is provided by solar radiation. The heat distribution in the shell is modelled based on partial absorbance of the solar radiation, the prepreg thermal conductivity and thermal capacity, radiation heat transfer between inner surfaces of the shell and the gas thermal conductivity. The iterated algorithm of curing was developed. Three flight regimes based on the circular motion of a construction have been considered: 1 – the axis of a shell lies in the tangent plane to the orbit and makes a constant angle with the tangent; 2 – under conditions (1), a shell rotates around its axis with a constant angular velocity; 3 – under conditions (2), the axis of the shell precesses around the tangent to the orbit. It was found, that the parameters of the motion (i.e. angular velocity of the rotation around the axis, precession angular velocity and precession angle) could be optimised in such way, that the whole shell can be completely cured under the solar radiation.

Ab initio intermolecular potential energy surface for the CO2-N2 system and related thermophysical properties.

A four-dimensional potential energy surface (PES) for the interaction between a rigid carbon dioxide molecule and a rigid nitrogen molecule was constructed based on quantum-chemical ab initio calculations up to the coupled-cluster level with single, double, and perturbative triple excitations. Interaction energies for a total of 1893 points on the PES were calculated using the counterpoise-corrected supermolecular approach and basis sets of up to quintuple-zeta quality with bond functions. The interaction energies were extrapolated to the complete basis set limit, and an analytical site-site potential function with seven sites for carbon dioxide and five sites for nitrogen was fitted to the interaction energies. The CO2-N2 cross second virial coefficient as well as the dilute gas shear viscosity, thermal conductivity, and binary diffusion coefficient of CO2-N2 mixtures were calculated for temperatures up to 2000 K to validate the PES and to provide reliable reference values for these important properties. The calculated values are in very good agreement with the best experimental data.

Study of cavity effect in micro-Pirani gauge chamber with improved sensitivity for high vacuum regime

Ultra-low pressure application of Pirani gauge needs significant improvement of sensitivity and expansion of measureable low pressure limit. However, the performance of Pirani gauge in high vacuum regime remains critical concerns since gaseous thermal conduction with high percentage is essential requirement. In this work, the heat transfer mechanism of micro-Pirani gauge packaged in a non-hermetic chamber was investigated and analyzed compared with the one before wafer-level packaging. The cavity effect, extremely important for the efficient detection of low pressure, was numerically and experimentally analyzed considering the influence of the pressure, the temperature and the effective heat transfer area in micro-Pirani gauge chamber. The thermal conduction model is validated by experiment data of MEMS Pirani gauges with and without capping. It is found that nature gaseous convection in chamber, determined by the Rayleigh number, should be taken into consideration. The experiment and model calculated results show that thermal resistance increases in the molecule regime, and further increases after capping due to the suppression of gaseous convection. The gaseous thermal conduction accounts for an increasing percentage of thermal conduction at low pressure while little changes at high pressure after capping because of the existence of cavity effect improving the sensitivity of cavity-effect-influenced Pirani gauge for high vacuum regime.

Thermal properties of an electron gas with non-standard dispersion relation: Left-handed and right-handed systems

Thermal conductivity of an electron gas with non-standard dispersion relations is studied by using the Fermi-Dirac statistics. To that end, a cubic transmission line to model a left-handed system and its corresponding right-handed system is used. The heat capacity and the group velocity for both systems are obtained and studied, allowing to discern the behavior of thermal conductivity in the high and low temperature limits. Analytical expressions for these limits are obtained and a numerical analysis that supports the results is performed.

Expanded fluid-based thermal conductivity model for hydrocarbons and crude oils

Thermal conductivity data for crude (mainly heavy) oils and mixtures of crude oils and pure hydrocarbons were collected at temperatures from 20 to 125 °C and pressures up to 10 MPa using a hot wire apparatus. A criterion was established to screen out data that were affected by convection. The screened data and data from the literature were used to develop an Expanded Fluid (EF) based thermal conductivity model for pure hydrocarbons, crude oils and their mixtures. The proposed model is applicable across the entire phase diagram including the critical region; however, it does not predict the critical enhancement of thermal conductivity observed in the vicinity of the critical point. The model inputs are the density of the fluid, the pressure, the dilute gas thermal conductivity, the compressed state density, and three other fluid specific parameters. The gas thermal conductivity is calculated from a well-established correlation. The compressed state density is obtained from the literature or from fitting the EF viscosity model to viscosity data. The three fluid specific parameters are determined by fitting the model to thermal conductivity data. The model fits the data of 63 pure hydrocarbons at temperatures from −150 to 330 °C and pressures up to 200 MPa with average deviation of 4%, except in the vicinity of the critical point (0.97 < reduced temperature < 1.1). The model also fits the data of 7 different crude oils at temperatures and pressures up to 150 °C and 10 MPa to within 0.3% of the experimental data. Mass based mixing rules were proposed for the model parameters of mixtures. The data for 19 pure hydrocarbon binaries at atmospheric pressure were predicted with average deviation of 0.5% and that of 8 bitumen/solvent pseudobinaries, at pressures up to 10 MPa, was predicted with average deviation of 1.6%. The introduction of binary interaction parameters into the mixing rules halved the magnitude of the deviations.

Hollow silica nanospheres as thermal insulation materials for construction: Impact of their morphologies as a function of synthesis pathways and starting materials

Hollow silica nanospheres (HSNS) show a promising potential to become good thermal insulators with low thermal conductivity values for construction purposes. The thermal conductivity of HSNSs is dependent on their structural features such as sizes (inner diameter and shell thickness) and shell structures (porous or dense), which are affected by the synthetic methods and procedures including reaction medium, polystyrene template, and silica precursor. Formation of thermally insulating HSNS was favoured by alkaline reaction, whereby highly porous silica shells were formed, promoting less silica per volume of material, thus a lower solid state thermal conductivity. The Knudsen effect is in general reducing the gas thermal conductivity including the gas and pore wall interaction for materials with pore diameters in the nanometer range, which is also valid for our HSNS reported here. Further decreasing the pore sizes would invoke a higher impact from the Knudsen effect. The additional insulating effect of the inter-silica voids (median diameter D50 ≈ 15 nm) within the shell coating contributed also to the insulating properties of HSNS. The synthesis route with tetraethyl orthosilicate (TEOS) was more robust and produced more porous silica shells than the one with water glass (Na2SiO3, WG), although the latter might represent a greener synthetic method.

Reference Values for the Second Virial Coefficient and Three Dilute Gas Transport Properties of Ethane from a State-of-the-Art Intermolecular Potential Energy Surface

The second virial coefficient and the dilute gas shear viscosity, thermal conductivity, and self-diffusion coefficient of ethane (C2H6, R-170) were determined with high accuracy at temperatures from (90 to 1200) K using advanced computational approaches. The second virial coefficient was calculated semiclassically by means of the Mayer-sampling Monte Carlo technique, while the transport properties were obtained using the classical kinetic theory of polyatomic gases. The required intermolecular potential energy surface was developed as part of this work. It is based on high-level quantum-chemical ab initio calculations and was fine-tuned to reliable experimental data for the second virial coefficient. The computed thermophysical property values are in excellent agreement with the best available experimental data and are recommended as reference values. Correlations based entirely on the calculated values are proposed for practical applications in the low-density gas phase.

Characterization of Grain‐Size Distribution, Thermal Conductivity, and Gas Diffusivity in Variably Saturated Binary Sand Mixtures

Core Ideas Parameterized models are relevant to applications where different porous media mixtures are used. Models include a grain‐size distribution function to describe bimodal behavior for binary mixtures. Improved model describes observed thermal conductivity–saturation relations for binary mixtures. Combined Buckingham–Penman model used to describe observed gas diffusivity–air content relations. This work is relevant for proper simulation of mixed porous media. Characterization of differently textured porous materials, as well as different volumetric porous media mixtures, in relation to mass and heat transport is vital for many engineering and research applications. Functional relations describing physical (e.g., grain‐size distribution, total porosity), thermal, and gas diffusion properties of porous media and mixtures are necessary to optimize the design of porous systems that involve heat and gas transport processes. However, only a limited number of studies provide characterization of soil physical, thermal, and gas diffusion properties and the functional relationships of these properties under varying soil water contents, especially for soil mixtures, complicating optimization efforts. To better understand how mixing controls the physical, thermal, and gas diffusion properties of porous media, a set of laboratory experiments was performed using five volumetric mixtures of coarse‐ and fine‐grained sand particles. For each mixture, the grain‐size distribution (GSD), thermal conductivity, and gas diffusivity were obtained and parameterized using existing and suggested parametric models. Results show that the extended, two‐region Rosin–Rammler particle‐size distribution model proposed in this study could successfully describe the bimodal behavior of the GSD of binary mixtures. Further, the modified Côté and Konrad thermal conductivity model adequately described the thermal conductivity–water saturation relations observed in different mixtures. The proposed simple soil‐gas diffusivity descriptive model parameterized the upper limit, average, and lower limit behavior in gas diffusivity–air content relations in apparently texture‐invariant gas diffusivity data. Results further show a close analogy between gas diffusivity and thermal conductivity and their variation with saturation across different binary mixtures. Overall, the results of the study provide useful numerical insight into the physical, thermal, and gas transport characteristics of binary mixtures, with wide implications for future engineering and research applications that involve multicomponent porous systems.

Cross Second Virial Coefficients and Dilute Gas Transport Properties of the (CH4 + C3H8) and (CO2 + C3H8) Systems from Accurate Intermolecular Potential Energy Surfaces

Accurate values for the cross second virial coefficients and dilute gas shear viscosities, thermal conductivities, and binary diffusion coefficients of the (CH4 + C3H8) and (CO2 + C3H8) systems were determined at temperatures from (150 to 1200) K using state-of-the-art computational approaches. The cross second virial coefficients were calculated semiclassically using the Mayer-sampling Monte Carlo method, while the transport properties were computed by means of the classical trajectory approach in conjunction with the kinetic theory of molecular gases. The required intermolecular potential energy surfaces (PESs) for the CH4–C3H8 and CO2–C3H8 interactions are reported in this work, whereas those for the CH4–CH4, CO2–CO2, and C3H8–C3H8 interactions were taken from our previous work on the pure gases. All PESs are based on high-level quantum-chemical ab initio calculations and were fine-tuned to the best experimental data for the second virial and cross second virial coefficients. Overall, the agreement of ...

Porous carbon-bonded carbon fiber composites impregnated with SiO2-Al2O3 aerogel with enhanced thermal insulation and mechanical properties

To improve the thermal insulation and mechanical properties of lightweight carbon- bonded carbon fiber composites (CBCF), a porous CBCF decorated with nano SiO2-Al2O3 aerogel was prepared by sol-gel impregnation. Results showed that the nano aerogel were uniformly assembled within the pores in the CBCF framework. The aerogel played a significant role on reducing the thermal conductivity of CBCF, which could be mainly ascribed to reduced radiation and gas thermal conductivity of CBCF. Moreover, the aerogel in the CBCF pores hindered the bending fracture or elastic bending/rotation of the fibers and enhanced the mechanical properties of the composites.