Gas Thermal Diffusion Research Articles

Peridynamics modeling of coupled gas convection transport and thermal diffusion in heterogeneous porous media

Peridynamics modeling of coupled gas convection transport and thermal diffusion in heterogeneous porous media

Effect of heat diffusivity for driving chain stitching of dual-type hybrid organosilica-derived membranes

In this study, the usage of dual hybrid organosilica is proposed for discussing the influence of thermal-driven chain stitching on the organosilica membrane structure for the first time. The influence of the gas thermal diffusivity on the hybrid organosilica membrane for separating H2 from CH4, C2H6, and C3H8, is discussed in detail. The characterization and permeation tests revealed that the optimal condition of the organosilica membrane fabrication is the usage of nitrogen as calcination atmosphere owing to the mild reaction. In addition, it is found that when N2 is used as the calcining gas and calcination is performed at a lower temperature, it is beneficial for the separation of small-molecule gases. However, calcination at higher temperatures is beneficial to the separation of gas pairs with a big difference in kinetic diameter. The as-prepared M−N2−300 exhibited the maximum selectivity of 37.32 for CO2/N2, 82.37 for CO2/CH4, 47.51 for H2/N4, and 109.28 for H2/CH4, and ideal H2 permeance of 2.24 × 10−8 mol/(m2sPa). Moreover, the as-prepared M−N2−500 exhibited the maximum selectivity for the following gases: 438.99 and 301.47 toward H2/C2H6 and H2/C3H8, respectively, at the ambient environment, and satisfactory H2 permeance of 5.14 × 10−8 mol/(m2sPa). This shows the potential and industrial relevance for gas-separation concerning applications like H2 energy production and olefins/paraffins recovery.

A combined model of heat and mass transfer for the in situ extraction of volatile water from lunar regolith

Chemical analysis of lunar soil samples often involves thermal processing to extract their volatile constituents, such as loosely adsorbed water. For the characterization of volatiles and their bonding mechanisms it is important to determine their desorption temperature. However, due to the low thermal diffusivity of lunar regolith, it might be difficult to reach a uniform heat distribution in a sample that is larger than only a few particles. Furthermore, the mass transport through such a sample is restricted, which might lead to a significant delay between actual desorption and measurable outgassing of volatiles from the sample. The entire volatiles extraction process depends on the dynamically changing heat and mass transfer within the sample, and is influenced by physical parameters such as porosity, tortuosity, gas density, temperature and pressure. To correctly interpret measurements of the extracted volatiles, it is important to understand the interaction between heat transfer, sorption, and gas transfer through the sample. The present paper discusses the molecular kinetics and mechanisms that are involved in the thermal extraction process and presents a combined parametrical computation model to simulate this process. The influence of water content on the gas diffusivity and thermal diffusivity is discussed and the issue of possible resorption of desorbed molecules within the sample is addressed. Based on the multi-physical computation model, a case study for the ProSPA instrument for in situ analysis of lunar volatiles is presented, which predicts relevant dynamic process parameters, such as gas pressure and process duration.

Correlations of medium physical properties and process performance in solid-state fermentation

The medium of solid-state fermentation (SSF) is a multiphase system, whose complex physical properties cause difficulties of the cognition of SSF process and the process control. This paper aimed to deeply analyze the mechanical, structural and thermal properties of solid-state medium, explored relationships between the medium’s physical properties and fermentation performance, then to guide the process control and culture medium optimization for SSF. A mechanical property index (Imp), which was the product of resilience, cohesiveness, and springiness, was established to fully characterize physical properties of medium. Results showed at the initial stage of SSF, there were positive correlations of Imp with thermal conductivity and water retention, negative correlations with gas permeability and thermal diffusivity, and a parabolic relationship with biomass content with a symmetry-axis at Imp=4.37×10−2. Correlations were further verified in SSF dynamic process. During the first 100h, Imp≤4.37×10−2 was positively correlated with heat conduction and water retention in medium, which was beneficial for cell growth; while when Imp>4.37×10−2 during 100–168h, medium heat accumulation, poor heat diffusivity and poor gas permeability indicated the reduction of fermentation performance. Results revealed medium physical properties significantly affected heat and mass transfer and further influenced fermentation performance in SSF. Mechanical property index Imp could well comprehensively characterize physical properties of medium, which could be useful for guiding initial culture medium preparation and SSF process control.

Why a steady state void size distribution in irradiated UO2? A modeling approach

In UO2 pellets irradiated in standard water reactor, Xe nano-bubbles nucleate, grow, coarsen and finally reach a quasi steady state size distribution: transmission electron microscope (TEM) observations typically report a concentration around 10−4nm−3 and a radius around 0.5nm. This phenomenon is often considered as a consequence of radiation enhanced diffusion, precipitation of gas atoms and ballistic mixing. However, in UO2 thin foils irradiated with energetic ions at room temperature, a nano-void population whose size distribution reaches a similar steady state can be observed, although quasi no foreign atoms are implanted nor significant cation vacancy diffusion expected in conditions. Atomistic simulations performed at low temperature only address the first stage of the process, supporting the assumption of void heterogeneous nucleation: 25keV sub-cascades directly produce defect aggregates (loops and voids) even in the absence of gas atoms and thermal diffusion. In this work a semi-empirical stochastic model is proposed to enlarge the time scale covered by simulation up to damage levels where every point in the material undergoes the superposition of a large number of sub-cascade impacts. To account for the accumulation of these impacts, simple rules inferred from the atomistic simulation results are used. The model satisfactorily reproduces the TEM observations of nano-voids size and concentration, which paves the way for the introduction of a more realistic damage term in rate theory models.

Effect of CO2 on the Methane Combustion over a Perovskyte Catalyst at High Pressure

The effect of the CO2 content (up to 30%) on the methane catalytic combustion over a perovskyte-based monolith has been investigated at high pressure (up to 8 bar). Catalytic tests have been performed by varying the preheating temperature and the operating pressure. The CO2 adsorption over the catalytic surface has been evaluated through temperature-programmed desorption tests coupled to a kinetic model. Results show that the presence of CO2 retards ignition, shifting the ignition pressure at higher values. However, once ignition has been attained, the catalytic surface temperature increases upon an increasing CO2 content. These results have been explained by a double role of CO2: on one hand, it affects the ignition through adsorption over the catalytic surface, and on the other hand, it decreases the gas thermal diffusivity, thus reducing the velocity of heat removal from the surface.

An initial assessment of a mechanistic model, GRASS-SST, in U–Pu–Zr metallic alloy fuel fission-gas behavior simulations

A mechanistic kinetic rate theory model originally developed for the prediction of fission gas behavior in oxide nuclear fuels under steady-state and transient conditions has been assessed to investigate its applicability to model fission gas behavior in U–Pu–Zr metallic alloy fuel. In order to capture and validate the underlying physics for irradiated U–Pu–Zr fuels, the mechanistic model was applied to evaluate fission gas release, fission gas and fission product induced swelling, and detailed gas bubble size distributions in three different fuel zones: the outer α-U, the intermediate, and the inner γ-U zones.Due to its special microstructural features, the α-U zone in U–Pu–Zr fuels is believed to contribute the largest fraction of fission gas release among the different fuel zones. It is shown that with the use of small effective grain sizes, the mechanistic model can predict fission gas release that is in reasonable consistence with (though slightly lower than) experimentally measured data. These simulation results are comparable to the experimentally measured fission gas release since the mechanism of fission gas transport through the densely distributed laminar porosity in the α-U zone is analogous to the mechanism of fission gas transport through the interconnected gas bubble porosity utilized in the mechanistic model.Detailed gas bubble size distributions predicted with the mechanistic model in both the intermediate zone and the high temperature γ-U zone of U–Pu–Zr fuel are also compared to experimental measurements from available SEM micrographs. These comparisons show good agreement between the simulation results and experimental measurements, and therefore provide crucial guidelines for the selection of key physical parameters required for modeling these two zones. Material properties such as fuel grain size and thermal diffusivity of gas and model parameters such as di-atom nucleation probability and gas bubble re-solution constant are predicted by these comparisons. In addition, the results of parametric studies for several parameters are presented for both the intermediate zone and the γ-U zone simulations in order to clarify the sensitivities of simulation results on these parameters.

Review of scattering and extinction cross-sections, damping factors, and resonance frequencies of a spherical gas bubble

Perhaps the most familiar concepts when discussing acoustic scattering by bubbles are the resonance frequency for bubble pulsation, the bubbles' damping, and their scattering and extinction cross-sections, all of which are used routinely in oceanography, sonochemistry, and biomedicine. The apparent simplicity of these concepts is illusory: there exist multiple, sometimes contradictory definitions for their components. This paper reviews expressions and definitions in the literature for acoustical cross-sections, resonance frequencies, and damping factors of a spherically pulsating gas bubble in an infinite liquid medium, deriving two expressions for "resonance frequency" that are compared and reconciled with two others from the reviewed literature. In order to prevent errors, care is needed by researchers when combining results from different publications that might have used internally correct but mutually inconsistent definitions. Expressions are presented for acoustical cross-sections associated with forced pulsations damped by liquid shear and (oft-neglected) bulk or dilatational viscosities, gas thermal diffusivity, and acoustic re-radiation. The concept of a dimensionless "damping coefficient" is unsuitable for radiation damping because different cross-sections would require different functional forms for this parameter. Instead, terms based on the ratio of bubble radius to acoustic wavelength are included explicitly in the cross-sections where needed.

Application of the Three-Omega Method to Measurement of Thermal Conductivity and Thermal Diffusivity of Hydrogen Gas

Preliminary investigations have been conducted to discuss the possibility of measuring the thermal conductivity of hydrogen gas by the three-omega method. A one-dimensional analytical solution for the 3ω component is derived which includes the effect of the wire heat capacity. It is shown that it is very important to take into account the wire heat capacity in the calculation to measure the thermal conductivity of gas by the three-omega method. In contrast, the wire heat capacity is less important for the thermal conductivity of the liquid or solid phase. The importance of the wire heat capacity is found to increase with increasing frequency and decrease if the sample thermal conductivity is high. In order to measure the thermal conductivity of hydrogen gas at atmospheric pressure, a wire of diameter less than 1μm is necessary if the properties of the wire are to be neglected.

Effect of carbon dioxide on flame propagation of pulverized coal clouds in CO2/O2 combustion

CO 2 /O 2 combustion of pulverized coal is one of the promising new technology in order to reduce the emission of CO 2 and NO x from coal combustion furnaces. However, several experiments with pulverized coal burners show that temperature and stability of pulverized coal flame is reduced in this condition. CO 2 has distinctive thermodynamic and optical property compared with that of other gas, and it is important to know the effect of CO 2 on the flame stability of pulverized coal. In this study, effect of CO 2 on flame propagation velocity of pulverized coal clouds were studied experimentally using micro-gravity condition, and also numerically considering detailed radiation heat exchange using Monte Carlo method. Experiments were made by using spherical chamber with inner diameter of 200 mm. Micro-gravity condition was used in order to achieve uniform pulverized coal cloud in a chamber. Flame propagation velocity was measured from the photographic image of the flame front by using high speed camera. Results show that flame propagation velocity of pulverized coal cloud in CO 2 /O 2 mixture gas decreases to about 1/3–1/5 of that in N 2 /O 2 mixture gas at the same oxygen concentration. By using Ar/O 2 mixture gas, it is revealed that thermal diffusivity of gas seems to have a large effect on flame propagation velocity. From the numerical analysis using Monte Carlo method, effect of absorption of radiation by CO 2 gas is proven to be relatively small compared with that of thermodynamic property especially for heat capacity of CO 2 . Consequently, it is clarified that reduction of flame stability in CO 2 /O 2 combustion is mainly due to the larger heat capacity of CO 2 gas.

Monitoring of gas diffusion in air using the TWI technique: Thermal diffusivity measurements made easy

The thermal wave interference technique (TWI) has been explored in recent years for the investigation of the thermal and transport properties of gases and liquids. In this article we address ourselves to the quantitative understanding of the transient thermal wave interference signal of air:hydrocarbon vapor mixtures. This is the situation one faces when placing a given portion of liquid hydrocarbon inside an initially air-filled thermal wave interference. What is observed in this case is the thermal wave interference signal decay as a function of the time as a result of the change of the gas thermal diffusivity with time due to the increase of the hydrocarbon vapor concentration in air. In this article we show that value of the thermal diffusivity of the saturated mixture is readily obtained from the saturation value of the normalized signal amplitude, without the need of performing the conventional L scan.

Energy (mass) transfer processes in low-temperature plasma as applied to forming ordered structures

The methods of irreversible thermodynamics are adopted to analyze the energy (mass) transfer in gases (their mixtures) over a wide temperature range up to 20000 K. An energy transfer process is considered as a path in the state space, and a concept of non-Euclidean state space is postulated. Experimental data on the gas thermal conductivity over a wide temperature range is generalized by the methods of thermodynamic similarity theory. Thermal conductivities of CH4-O2 and CH4−H2O mixtures are calculated for technological applications. The process of forming a space-time structure is analyzed by using the experimental data on the gas thermal conductivity over a wide temperature range. An attempt is made to interpret the mechanism of the thermal diffusion in gases due to the forming of ordered structures. A contribution of the diffusional thermoeffect to the energy transfer processes in a three-component nitrogen plasma is calculated.

Pyroelectric Thermal-Wave Resonant Cavity: A Precision Thermal Diffusivity Sensor for Gases and Vapors

A novel thermal-wave resonant cavity (TWRC) was constructed and used for thermophysical measurements of gases and vapors, with an AC current-heated thin-film resistive element acting as a thermal-wave source. A thin-film pyroelectric element was used both as a cavity wall and as a signal transducer. A theoretical model of the cavity length-scanned thermal-wave field was developed to quantify the standing-wave resonance antinode patterns in the demodulated lock-in signal output in-phase and quadrature channels. These resonance extrema were used to measure precisely the thermal diffusivity of the intracavity gas or vapor. Seven high-purity gases (nitrogen, dry air, oxygen, methane, hydrogen in nitrogen, pure hydrogen, and helium) were measured using the cavity. Fourth-significant-figure precision was obtained for this parameter, with standard deviations less than 0.32% for the five measurements performed with each gas. Furthermore, three grades of gasoline vapors from Imperial Oil were studied with the cavity. The measured thermal diffusivities showed that the TWRC can monitor fundamental evaporation kinetics as an analytical quality-control instrument. These results, together with the simplicity of TWRC sensor fabrication, are indicative of its potential to become a new standard measurement instrument for the determination of gas thermal diffusivity with improved precision, and a new in situ monitor of chemical evaporation kinetics over conventional methodologies, such as gas chromatography and mass spectrometry.

Thermal wave resonator : In situ investigation by photothermal deflection technique

The photothermal deflection technique applied to a gas thermal wave resonator seems to be one of the most powerful techniques to investigate in situ the thermal diffusivity of the gas. After a brief description of what a thermal wave resonator really is, a discussion is presented of the advantages and disadvantages of its use for measuring the gas thermal diffusivity.

Turbulent barodiffusion, turbulent thermal diffusion, and large-scale instability in gases

Two effects, turbulent barodiffusion and turbulent thermal diffusion in gases, are discussed. These phenomena are related to the dynamics of a gaseous admixture in compressible turbulent fluid flow with low Mach numbers. Turbulent barodiffusion causes an additional mass flux of the gaseous admixture directed to the maximum of the mean fluid pressure, while turbulent thermal diffusion results in an accumulation of the gaseous admixture in the vicinity of the minimum of the mean temperature of the surrounding fluid. At large

Thermal-wave resonator cavity

A thermal-wave resonant cavity was constructed using a thin aluminum foil wall as the intensity-modulated-laser-beam induced oscillator source opposite a pyroelectric polyvilidene fluoride wall acting as a signal transducer and cavity standing-wave-equivalent generator. It was shown that scanning the frequency of oscillation produces the fundamental and higher overtone resonant extrema albeit with increasingly attenuated amplitude—a characteristic of thermal-wave behavior. Experimentally, scanning the cavity length produced a sharp lock-in in-phase resonance with simple linewidth dependencies on oscillation (chopping) frequency and intracavity gas thermal diffusivity. The thermal diffusivity of air at 294 K was measured with three significant figure accuracy: 0.211±0.004 cm2/s. The novel resonator can be used as a high-resolution thermophysical property sensor of gaseous ambients.

Performance of gas saturators in the presence of exit stream temperature gradients and implications for chemical vapor deposition saturator design

When a carrier gas is passed through a gas saturator at temperature T 0 containing liquid or solid reagent under conditions leading to its saturation with an equilibrium vapor pressure p∘ of the reagent, and then into a tube where the exit stream temperature is increased to T ∞, the downstream partial pressure of reagent, p ∞, may be less than p∘ due to thermal diffusion of the reagent driven by the temperature gradient. The mass transfer process has been modeled for two cases: (1) a linear temperature gradient over a tube length L 2 and (2) a tube length L 1 of constant temperature T 0 followed by a tube length L 2 with a linear temperature gradient. Solution of case (1) is defined by the dimensionless Peclet number Pe 2 = υ L 2/ D and a dimensionless “thermal diffusion number” Td = α ln ( T ∞/ T 0), where υ is average gas velocity in the tube, D is the reagent gas—carrier gas binary diffusion coefficient, and α is the reagent gas—carrier gas thermal diffusion factor. Solution of case (2) is characterized by Td, Pe 2, and Pe 1 = υ L 1/ D. Chemical vapor deposition processes used in the fabrication of electronic and optoelectronic semiconductor devices require the reproducible control of reagent partial pressures to better than ± 0.4%. Gas saturators for these processes are commonly operated under conditions where p ∞ is depedent on both flow rate and the downstream temperature profile, and where p ∞ can be as much as 10% lower than p∘. The reproducible control of reagent partial pressure is best brought about by designing the saturator system so that Pe 1 > 10, conditions leading to p ∞ ≅ p∘ independent of flow rate or downstream temperature.

Experimental heat exchanger performance in a thermoacoustic prime mover

The effectiveness of heat transfer between the thermoacoustic stack and a typical copper fin heat exchanger is determined by the detailed gas motion and thermal diffusion, and the fin geometry. This effectiveness depends strongly on acoustic amplitude and heat load, as does the ability of the copper to conduct the heat. These and other nonlinear effects contribute to limiting the amplitude of a prime mover. A modular prime mover experiment has been built so that the limiting amplitude can be measured with a variety of heat exchanger geometries, with the goal of finding the best geometry. The liquid nitrogen temperature experiment uses a heavier gas, such as neon, at a low pressure in order to minimize the heat load, and the temperature defects associated with copper conduction, while allowing large Mach numbers and gas particle displacements. In preliminary measurements, peak pressure amplitudes are in access of 20% of the mean pressure and the peak‐to‐peak acoustic displacements are comparable to the stack length. Such large displacements take the study of prime movers into a poorly explored region. [Work supported by the Naval Research Laboratory and the Office of Naval Research.]

A nonisothermal film model for mass-transfer-limited selective epitaxial growth of silicon on patterned surfaces

A two-dimensional film model for mass-transfer-controlled selective epitaxial growth (SEG) of Si on bare and regularly patterned wafers was developed, and the effects of bulk gas composition, pressure, substrate temperature, thermal diffusion, and oxide coverage were analyzed. SEG rates decrease with a lower concentration of the source gas and with the addition of HCl, which eventually leads to etching. The effects of pressure and substrate temperature on mass-transfer-limited SEG rates are determined by the way in which the equilibrium composition at the deposition surface depends on these variables. Thermal diffusion contributes to larger net fluxes away from the deposition surface. The effect of thermal diffusion is to inhibit SEG rates at lower bulk HCl concentrations and to extend the growth region to higher bulk HCl concentrations. Higher oxide coverages produce higher deposition rates with reduced mass transfer limitations. These model predictions are qualitatively consistent with reported observations.

Optimization and Calculation of Gas Thermal Diffusion Column

Abstract Analytical dependencies, permitting the optimization of a gas thermal diffusion column (TDC), are obtained. A method for calculation of the geometry and gas flows in TDC or a cascade of TDC (TD-cascade) has been proposed.