Relative Thermal Diffusivity Research Articles

Influence of Circular through Hole in Pt-Rh Bushing on Temperature Propagation at High Temperature.

In the fiberglass industry, Pt-Rh bushings made of platinum and rhodium have very good characteristics, such as high temperature resistance, corrosion resistance, oxidation resistance, and creep resistance. In this paper, a semi-infinite lath structure model is constructed, and the expression of the surface temperature distribution of a Pt-Rh alloy plate with a circular through hole is obtained based on the non-Fourier heat conduction equation, complex function method and conformal mapping method. At the same time, the influence of the position of the circular through hole in the Pt-Rh bushing and the parameters of the incident light source (Non-diffusion incident wave number and relative thermal diffusion length) on the surface temperature distribution of the Pt-Rh bushing is studied by using this formula. It is found that: 1. heat concentration and fracture are occur easily at the through hole; 2. when the through hole is in the asymmetric center, the greater the asymmetry, the smaller the maximum temperature amplitude; 3. when the buried depth of the through hole increases, the maximum temperature amplitude decreases; 4. when the incident wave number and the relative thermal diffusion length of the incident light source are larger, the maximum temperature amplitude is smaller. The numerical results are almost consistent with those of ANSYS thermal simulation. The expression of the surface temperature distribution of the semi-infinite lath structure proposed in this paper can effectively reduce the loss of precious metal materials and the time of thermal simulation in the experimental process, as well as provide important significance for structural design, quality inspection, process optimization, and service life improvement of Pt-Rh bushings.

Modeling of the Optical Properties of Vermiculite-Sungulite Conglomerates Subject to Thermal Activation in Electric Furnaces

This paper reviews the fundamentals of the technology and theory of the process of thermal activation of vermiculite-sungulite conglomerates contained in the waste of the Kovdor phlogopite-vermiculite field. An analytical physical model has been developed to describe a single-layer flow of vermiculite-sungulite conglomerate vibro-transported in a thermal field using a vibration hearth platform of an electric furnace. The absorptivity and reflectivity of a dense single-layer array of the processed conglomerate located on a plane under thermal radiation conditions have been determined. Using sungulite, pyroxene, olivine, and a number of other minerals, the correlation of their absorptivity with relative thermal diffusivity and thermal conductivity was demonstrated.

Modeling of the optical properties of the vermiculite-sungulitovyh conglomerates with their thermal activation in electric furnaces

The fundamentals of technology and theory of the process of thermal activation of a vermiculite-sungulite conglomerate contained in the waste of the Kovdorsky phlogopitevermiculite deposit are considered. An analytical-physical model of a single-layer flow of vermiculite-sugulite conglomerate, vibro-transported in a thermal field using a vibration hearth platform of an electric furnace, has been built. The absorption and reflectivity of a dense singlelayer array of the processed conglomerate located on a plane under thermal radiation conditions are determined. For example, Sungulite, Pyroxene, Olivine, and a number of other minerals, the correlation of their absorption capacities with the relative thermal diffusivity and thermal conductivity of these minerals is shown. Ill. 2. Ref. 14.

INFLUENCE OF THERMAL DIFFUSION ON KINETICS OF OSCILLATING IR DRYING

The relative thermal diffusion coefficient δt was calculated from the obtained moisture sorption isotherm for the seeds of onions. It allowed estimating the contribution of thermal diffusion with respect to the flow of mass conductivity in the considered version of the oscillating IR drying, and it turned out to be 5%. Hence, the thermal diffusion can be ignored for engineering calculations of the kinetics of the oscillating IR drying.

Characterization of the porosity in TRISO coated fuel particles and its effect on the relative thermal diffusivity

A heat transfer model for a Tri-structural Isotropic (TRISO) coated fuel particle was developed using Image Based Modelling (IBM). Computed X-ray tomography at a resolution of 0.7μm was used to quantify the porosity of each layer. In order to study the thermal diffusivity of these coatings, an internal heat pulse was simulated in the kernel and the temperature, as a function of time, was measured from the surface of the Outer Pyrolitic Carbon (OPyC). Consequently, the half rise time of the temperature increase was found. The novel idea behind this technique is that once a heat pulse propagates through the particle layers, the half rise time of the temperature can be obtained from different spots on the OPyC surface. The article presents a correlation between the pore size distribution (measured by X-ray tomography) and the relative thermal diffusivity. The average porosities in OPyC (relative to the volume of the ceramic coating) measured using helium pycnometry, mercury intrusion porosimetry and X-ray tomography were 12.3vol%, 9.0vol% and 11.1vol%, respectively.

Precise measurements of radial temperature gradients in the laser-heated diamond anvil cell

A new spectroradiometry system specialized for measuring two-dimensional temperature gradients for samples at high pressure in the laser heated diamond anvil cell has been designed and constructed at UCLA. Emitted light intensity from sample hotspots is imaged by a videocamera for real time monitoring, an imaging spectroradiometer for temperature measurement, and a high-dynamic-range camera that examines a magnified image of the two-dimensional intensity distribution of the heated spot, yielding precise measurements of temperature gradients. With this new system, most systematic errors in temperature measurement due to chromatic aberration are bypassed. We use this system to compare several different geometries of temperature measurement found in the literature, including scanning a pinhole aperture, and narrow-slit and wide-slit entrance apertures placed before the imaging spectrometer. We find that the most accurate way of measuring a temperature is to use the spectrometer to measure an average hotspot temperature and to use information from the imaging charge coupled device to calculate the temperature distribution to the hotspot. We investigate the effects of possible wavelength- and temperature-dependent emissivity, and evaluate their errors. We apply this technique to measure the anisotropy in temperature distribution of highly oriented graphite at room temperature and also at high pressures. A comparison between model and experiment demonstrates that this system is capable of measuring thermal diffusivity in anisotropic single crystals and is also capable of measuring relative thermal diffusivity at high pressures and temperatures among different materials. This shows the possibility of using this system to provide information about thermal diffusivity of materials at high pressure and temperature.

Numerical analysis of melting with constant heat flux heating in a thermal energy storage system

Melting in a finite slab with a second kind boundary condition is studied numerically in order to simulate the charging process of a thermal energy storage system. A dimensionless model is given, from which it is concluded that the main factors that influence the melting process are the dimensionless heating flux, the modified Stefan number, the relative thermal diffusivity and the relative thermal conductivity. The influence of preheating or solid subcooling is studied. It is found that though preheating does not have very important effects on the melting time, it does influence the interface marching velocity significantly. The melt fraction and the melting time are calculated extensively for various dimensionless numbers. The numerical results show that the ratio of the thermal conductivity of the solid to that of the liquid has little effect on the melting time, and the time for finishing melting can be expressed as a function of the dimensionless heating flux, the modified Stefan number and the relative thermal diffusivity, and the possible function form is suggested.

Thermal fractionation of air in polar firn by seasonal temperature gradients

Air withdrawn from the top 5–15 m of the polar snowpack (firn) shows anomalous enrichment of heavy gases during summer, including inert gases. Following earlier work, we ascribe this to thermal diffusion, the tendency of a gas mixture to separate in a temperature gradient, with heavier molecules migrating toward colder regions. Summer warmth creates a temperature gradient in the top few meters of the firn due to the thermal inertia of the underlying firn and causes gas fractionation by thermal diffusion. Here we explore and quantify this process further in order to (1) correct for bias caused by thermal diffusion in firn air and ice core air isotope records, (2) help calibrate a new technique for measuring temperature change in ice core gas records based on thermal diffusion [Severinghaus et al., 1998], and (3) address whether air in polar snow convects during winter and, if so, whether it creates a rectification of seasonality that could bias the ice core record. We sampled air at 2‐m‐depth intervals from the top 15 m of the firn at two Antarctic sites, Siple Dome and South Pole, including a winter sampling at the pole. We analyzed 15N/14N, 40Ar/36Ar, 40Ar/38Ar, 18O/16O of O2, O2/N2, 84Kr/36Ar, and 132Xe/36Ar. The results show the expected pattern of fractionation and match a gas diffusion model based on first principles to within 30%. Although absolute values of thermal diffusion sensitivities cannot be determined from the data with precision, relative values of different gas pairs may. At Siple Dome, δ40Ar/4 is 66 ± 2% as sensitive to thermal diffusion as δ15N, in agreement with laboratory calibration; δ18O/2 is 83 ± 3%, and δ84Kr/48 is 33 ± 3% as sensitive as δ15N. The corresponding figures for summer South Pole are 64 ± 2%, 81 ± 3%, and 34 ± 3%. Accounting for atmospheric change, the figure for δO2/N2/4 is 90 ± 3% at Siple Dome. Winter South Pole shows a strong depletion of heavy gases as expected. However, the data do not fit the model well in the deeper part of the profile and yield a systematic drift with depth in relative thermal diffusion sensitivities (except for Kr, constant at 34 ± 4%), suggesting the action of some other process that is not currently understood. No evidence for wintertime convection or a rectifier effect is seen.

Thermal diffusion in hydrogen-helium mixtures

Relative thermal diffusion factors of the binary systems He/sup 4/--HT, He/sup 4/-DT, and He/sup 4/--T/sub 2/ were measured in a thermal diffusion column at three temperatures: 325, 355, and 390 un. Concent 85% K. The experiments show that in this small region a temperature dependence exists, which cannot be explained with the Chapman-Enskog theory. Moreover, the experiments suggest that the diffusion coefficient of the He/sup 4/--HT mixture is much smaller than of the He/sup 4/--DT and He/sup 4/--T/sub 2/ systems. (auth)

Development of Safe Cooling Schedules for Structural Clay Products

A method is described whereby safe cooling schedules for structural clay products are developed, based on a criterion for thermal failure. Resistance to thermal failure may be indicated by the following equation: Resistance =MR a/Ea, where MR is the modulus of rupture, E is the modulus of elasticity, a is the thermal diffusivity, and a is the coefficient of linear thermal contraction. The modulus of rupture, modulus of elasticity, linear thermal contraction, and relative thermal diffusivity of three commercial bodies were measured in the temperature range 1600°F. to room temperature. Using the experimental values, relative cooling schedules are developed in terms of ware temperature versus dimensionless time, the total time for cooling being a function of size and shape. The order of importance of the physical properties is discussed with respect to cooling schedules and it is concluded that the coefficient of linear thermal contraction has the greatest effect. The mechanisms acting in clay bodies during cooling are discussed and the idea of quartz separation is presented. The hysteresis of the modulus of elasticity and coefficient of thermal expansion while heating and cooling is correlated with quartz separation and the literature is cited to support the idea. Some tests are described to show that rebonding of the quartz occurs on reheating to sufficiently high temperatures.