Experimental Thermal Diffusivity Research Articles

Determination of thermal and mutual diffusivity of n-heptane with dissolved carbon dioxide by dynamic light scattering

In this work, thermal and mutual diffusivity of n-heptane (n-C7H16) with dissolved carbon dioxide (CO2) were simultaneously measured using dynamic light scattering (DLS) method from 298 to 473 K and saturation pressure up to 5.5 MPa. For this purpose, the DLS setup as well as temperature and pressure system were improved. Two hydrodynamic modes in connection with the statistical fluctuations of entropy and species were verified. The average expanded uncertainty of thermal and mutual diffusivity were 9.0% and 4.9% with a coverage factor of k = 2. The thermal and mutual diffusivity versus pressure under isothermal lines were briefly discussed and a comparison with literature was performed. The experimental thermal and mutual diffusivity measured in this work are in reasonable agreement with the literature data. This work is expected to provide fundamental data for engineering process.

Experimental Study and Thermal Diffusivity Analysis on Whiteness Reduction of Natural Fiber Fabric Treated with Superheated Steam

ABSTRACT The family use garment steamers with superheated steam have several advantages but lead natural fiber fabrics whiteness reduction. This study explored the four kinds of natural fiber fabric whiteness reduction both experimentally and theoretically. We first built a temperature controllable 1 atm superheated steam generator, and recorded the whiteness and decrease percentages of cotton, flax, silk and wool fabrics at 150°C, 175°C, and 200°C separately. Then we calculated the monotonically increases thermal diffusivity(α) as a function of fiber volume fraction(), and the α is divided into three regions based on the : average region( <0.3), transition region(0.3 < <0.9) and fiber region( >0.9). The of studied samples are all in the average region, and the whiteness reduction percentage value sequence is opposite with the α, which are a reflection of the samples average properties, which means the design of superheated fabric steamers should depend on the structures or garment types of fabrics rather than fiber types.

Parameters of the Thermal Diffusivity vs. Water Content Function for Mineral Soils of Different Textural Classes

This work provides parameters of the soil thermal diffusivity vs. water content function for eight textural classes. The experimental thermal diffusivity vs. water content curves previously obtained for mineral soils of the European Russia were used. In the course of the study, the dataset for 77 undisturbed soil samples was analyzed. This dataset included loose sand, dense sand, sandy loam, light loam, medium loam, heavy loam, light clay, and medium clay. Thermal diffusivity of soil samples measured using the unsteady-state method varied within the dataset from 0.77 × 10–7 to 10.09 × 10–7 m2/s. To parameterize the soil thermal diffusivity vs. water content dependences the grouping method was applied. Soils were grouped according to textural classes using the classification either after Dolgov or after Kachinskii. For each of the chosen textural classes, the parameters of an average thermal diffusivity vs. water content curve were estimated from all the experimental data points for this class by approximating these points with the previously suggested four-parameter function. The approximation accuracy was estimated using Willmott’s index of agreement between the model-predicted curve and the observed values. The greatest value for the index of agreement (0.845) was obtained for the medium clays defined according to Dolgov and the smallest one (0.532) for sandy loams. The index of agreement for the whole dataset was 0.699 when the textural classes were defined according to Dolgov, and 0.688 when they were defined according to Kachinskii. In addition, the parameters of an average general curve for the whole experimental dataset were obtained. The index of agreement between this general curve and observed values was only 0.554, which confirms the efficiency of soil grouping by textural classes.

TiB2–SiC-based ceramics as alternative efficient micro heat exchangers

Ceramics as functional materials durable at corrosive media or high temperatures are suitable candidates for manufacturing heat exchangers. In the present work, a series of numerical simulations was carried out to investigate the heat transfer in micro heat exchanger made by TiB 2 –SiC and TiB 2 –SiC doped with 2 wt% carbon fiber. Experimental thermal diffusivity data of materials were obtained by laser flash method and used for numerical simulations. This article demonstrates the feasibility of better heat transfer and performance for ceramic heat exchangers. Conduction heat transfer equation in solid walls of heat exchanger along with fluid flow governing equation was used for solid and fluid domains. The governing equations were discretized by the finite element method and solved by COMSOL Multiphysics software. The obtained results showed that the thermal conductivity of both TiB 2 -based composites are approximately the same and small amount of carbon fiber had no considerable effect on the thermal properties of the composites. For the ceramic heat exchanger, a higher heat transfer with 15.5% enhancement occurs using TiB 2 composites compared to utilization of Al 2 O 3 .

Experimental determination of the thermal diffusivity of industrial grade synthetic cryolite between 200 and 850 °C and comparison with theoretical predictions

Molten cryolite mixed with some additives is used as solvent of alumina by aluminium producers. Cryolite represents the dominating compounds in the wall protecting side-ledge and at the bottom part of the frozen crust on the top of the cells. As the heat, generated in the cell, is lost mainly through the top and the lateral sides of the pot, the thermo-physical properties of the cryolite determine thermal equilibrium and its knowledge is crucial for any numerical thermal model of the cell. Unfortunately, there is a serious lack in experimental thermal diffusivity and conductivity data for pure cryolite. The goal of this paper was to determine experimentally the apparent thermal diffusivity of the purest available cryolite in a wide temperature range and compare it with an earlier published theoretical model, based on first principle calculations. Measurement was carried out using monotone heating technique between 200 and 850 °C on low porosity synthetic cryolite, containing 0.9 mass% of alumina. Experiment setup was improved at the level of protection of the thermocouples compared to earlier published article. This permitted to repeat the test more times with the same sample and reach higher temperatures. Results suggest that the thermal diffusivity of α-cryolite increases very slowly from 3 × 10−7 to 3.5 × 10−7 m2 s−1 between 200 and 560 °C where α–β transformation starts to take place. The transformation is completed around 600 °C. The thermal diffusivity of the β-cryolite is somewhat higher and rise significantly faster with temperature compared to the α-crystal. Namely, it increases from 4.1 × 10−7 to 7.3 × 10−7 m2 s−1 between 600 and 850 °C. DSC tests were carried out, to help to evaluate deviations on the monotone heating curves. Experimental results are in good agreement with the earlier published theoretical model.

On the thermal conductivity of CGI and SGI cast irons

The thermal conductivity of Compacted Graphite Iron (CGI) and spheroidal graphite iron (SGI) was established in the temperature range from room temperature up to 500 °C using the experimental thermal diffusivity, density and specific heat values. The influence of nodularity, graphite amount, silicon content and temperature on the thermal conductivity of fully ferritic high-silicon cast irons was investigated. It was found that the CGI materials showed higher thermal conductivity than the SGI materials. The thermal conductivity tended to increase with increasing temperature until it reached a maximum followed by a subsequent decrease as temperature was increased up to 500 °C. Conventional models were applied to estimate thermal conductivity and the predictive accuracy of each model was evaluated. The thermal conductivity could be estimated by the Helsing model. The Maxwell model, Bruggeman model and Hashin–Shtrikman model were also in fair agreement using the thermal conductivity value of graphite parallel to the basal planes in graphite.

Thermal conductivity of the sideledge in aluminium electrolysis cells: Experiments and numerical modelling

During aluminium electrolysis, a ledge of frozen electrolytes is generally formed, attached to the sides of the cells. This ledge acts as a protective layer, preventing erosion and chemical attacks of both the electrolyte melt and the liquid aluminium on the side wall materials. The control of the sideledge thickness is thus essential in ensuring a reasonable lifetime for the cells. The key property for modelling and predicting the sideledge thickness as a function of temperature and electrolyte composition is the thermal conductivity. Unfortunately, almost no data is available on the thermal conductivity of the sideledge. The aim of this work is to alleviate this lack of data. For seven different samples of sideledge microstructures, recovered from post-mortem industrial electrolysis cells, the thermal diffusivity, the density, and the phase compositions were measured in the temperature range of 423 K to 873 K. The thermal diffusivity was measured with a laser flash technique and the average phase compositions by X-ray diffraction analysis. The thermal conductivity of the sideledge is deduced from the present experimental thermal diffusivity and density, and the thermodynamically assessed heat capacity. In addition to the present experimental work, a theoretical model for the prediction of the effective thermal transport properties of the sideledge microstructure is also proposed. The proposed model considers an equivalent microstructure and depends on phase fractions, porosity, and temperature. The strength of the model lies in the fact that only a few key physical properties are required for its parametrization and they can be predicted with a good accuracy via first principles calculations. It is shown that the theoretical predictions are in a good agreement with the present experimental measurements.

Experimental investigation of heat transfer enhancement factors in the oscillating flow heat exchanger using Kurzweg’s and Nishio’s correlations

In the present work, an experimental investigation of heat transfer enhancement parameters of the oscillating flow heat exchanger using Kurzweg’s and Nishio’s correlations is carried out. An oscillatory axial movement of fluid is established within the flow tube using piston cylinder mechanism. The experiments are carried out for seven different frequencies, five tidal displacements and four heat fluxes. It is observed that at a constant tidal displacement (S), experimental effective thermal conductivity (keff) increases progressively with frequency (f) up to a maximum and then decreases. The frequency corresponding to peak keff is an optimum frequency. In addition to this, it is also observed that with increase in S, the point of peak keff is shifted towards lower frequency. A similar trend is observed for axial heat flux (qa) and convective heat transfer coefficient (h). Based on the dimensional analysis and experimental data, an empirical correlation is obtained for experimental effective thermal diffusivity (αeff) as a function of the Womersley number (Wo) and the transition number (β). Finally the result shows that, in the oscillating flow heat exchanger, f, S, Prandtl number (Pr) of fluid, fluid thermal properties to wall thermal properties, and the ratio of length of cooling tube in heat sink (Lc) to S are primary influencing parameters.

English

Soil thermal parameters are mainly inputs for models of soil heat flux. Mathematical models are important tool for predicting the soil heat and water transfer, depending on some fundamentals of soil physical properties. Soil moisture is one of the soil physical properties that have a great effect on thermal parameters. The aim of the work is to describe the behavior of soil thermal parameters under different values of soil moisture, and is to investigate the effect of some fundamentals of soil physical properties on thermal parameters. We could describe the relationship between thermal diffusivity and soil moisture by ∩ shaped curve using a quadratic equation and evaluate the efficiency of this equation, statistically. Experimental thermal diffusivity (Kexp) by direct method and mathematical models were measured. Mathematical models were Chung and Horton model (1987) (Kcal-2), the model of Arkhangel’skaya (2004) (Kcal-3) and the suggested quadratic equation (Kcal-1). Efficiency of the quadratic equation and mathematical models were estimated using the correlation coefficient (R2), Root Mean Square Error (RMSE), and the Nash-Sutcliffe Efficiency (NSE). The values of R2, RMSR and NSE for the quadratic equation were 0.978, 0.24 and 0.95, respectively, for sod-podzolic soil under the study. The quadratic equation is a simple and faster equation for forecasting soil thermal diffusivity. Key words: Soil thermal parameters, thermal diffusivity, thermal conductivity, soil heat capacity, soil moisture, soil bulk density, organic matter, quadratic equation and mathematical models.

Poly(vinyl alcohol) tissue phantoms as a robust in vitro model for heat transfer

ABSTRACTGlutaraldehyde-crosslinked poly(vinyl alcohol) (PVA) was used to create pourable, volume-stable hydrogel tissue phantoms that demonstrate several advantages over traditional poly(acrylamide) phantoms. The crosslinker concentration, curing time, and curing temperature were tuned to produce a tissue phantom whose volume varied by <0.2% over a 25-day span. The thermal conductivity of the PVA phantoms was tuned across the physiological range from 0.475 to 0.795 W m−1°C−1 by incorporating inert particle fillers. Experimental thermal diffusivity trials demonstrated the method’s utility for creating reliable, versatile tissue phantoms for the in vitro study of heat transfer in biologically relevant scenarios.

Low thermal conductivity of Lu4Si2O7N2: Theoretical and experimental studies

Thermal conductivity of bulk and dense oxynitride Lu4Si2O7N2 was measured at various temperatures. Results show that Lu4Si2O7N2 possesses low thermal conductivity and has potential applications as thermal insulation material. Detailed analysis shows that the experimental thermal diffusivity is significantly influenced by defects at low temperature and infrared radiation at high temperature. Therefore, we combined Debye model and Slack's equation to predict the temperature dependent intrinsic lattice thermal conductivity of Lu4Si2O7N2. All parameters indispensable in the calculation (equilibrium crystal structure, second order elastic constants, elastic modulus, Debye temperature, and Grüneisen constant) were predicted with the help of first-principles calculation methods. Finally, factors that influence the thermal conductivity of Lu4Si2O7N2 were discussed and possible explanations for the difference between experimental and theoretical thermal conductivity were illustrated.

Sensitivity of the interpretation of the experimental ion thermal diffusivity to the determination of the ion conductive heat flux

A moments equation formalism for the interpretation of the experimental ion thermal diffusivity from experimental data is used to determine the radial ion thermal conduction flux that must be used to interpret the measured data. It is shown that the total ion energy flux must be corrected for thermal and rotational energy convection, for the work done by the flowing plasma against the pressure and viscosity, and for ion orbit loss of particles and energy, and expressions are presented for these corrections. Each of these factors is shown to have a significant effect on the interpreted ion thermal diffusivity in a representative DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] discharge.

Heat transport in helical RFX-mod plasmas by electron temperature dynamics from soft-x-ray diagnostics

High current plasmas in the RFX-mod experiment are characterized by a 3D helical magnetic topology where internal electron transport barriers, with a temperature up to 1.2 keV, develop in the core. Power balance and heat transport analyses have been performed for the first time using high time resolution electron temperature Te measurements obtained by a double filter technique with a multichords soft-x-ray (SXR) diagnostic. Te profiles are implemented in the heat transport equation in order to determine the electron thermal diffusivity across the helical magnetic surfaces assuming both stationary and dynamic conditions. In order to explain the discrepancy between the experimental thermal diffusivity at the barrier and the lower neoclassical predictions, the role of residual macro and microscale magnetic stochasticity on the temperature gradient is investigated. The fast termination of helical structures, analyzed by a statistical approach, reveals the different time scales involved in the dynamics of magnetic and thermal plasma properties.

Dynamic Thermal Diffusivity Measurements – A Tool for Studying Gas-Solid Reactions

In the present work, the thermal diffusivity measurements of uniaxially cold pressed NiWO4 has been carried out. The measurements were performed isothermally at temperatures between 973 and 1273 K under H2 gas using the laser flash technique. The experimental thermal diffusivity values were found to increase with the reduction progress as well as with increasing temperature. The calculated activation energy was found to be higher than that for chemically controlled reaction. The difference has been attributed to factors like agglomeration of the product as well as sintering of the precursor along with the chemical reaction. In order to sort out the sintering effect on the thermal diffusivity values, complementary experiments have been done on pressed NiWO4 and Ni-W, produced by the reduction of NiWO4 at 1123K, under Argon gas. The porosity change and its effect on thermal diffusivity values have been studied.

Microstructural characterization of porous thermal barrier coatings by laser flash technique

An inversion procedure has been developed to obtain microstructural parameters describing the porosity morphology of porous thermal barrier coatings (TBCs) from the thermal diffusivity measured in different environments by a laser flash technique. The model base for the inversion procedure combines the Knudsen effect (i.e. the thermal conductivity variation of gases within a pore as a function of pore size and shape) with the asymmetric Bruggeman model that describes the effect of porosity on the thermal conductivity of a porous medium in terms of pore shape, orientation and content. A simplified approach in the inversion procedure has been proposed, and the reliability has been checked by simulating different microstructures within the TBC. The inversion procedure has been also applied to experimental thermal diffusivity values of a TBC which were obtained by filling pores with He, N 2, Ar and in vacuo.

Thermal diffusivity of nonfractal and fractal nickel nanowires

The potential of using nanometallic wires inside a matrix as new generation of thermal interface material led us to study the thermal diffusivity of nickel nanowires embedded inside porous alumina template. Thermal diffusivity measurements using a laser flash method showed size dependence for nickel nanowires inside nanochannel alumina (NCA) templates having nominal pore diameters of 200, 100, and 20nm. Nickel nanowires embedded inside these templates showed decreasing diffusivity values of 10.7×10−6, 8.5×10−6, and 6.5×10−6m2s−1 at 300K with decreasing wire diameter when deposited at 40°C. Nanowires fabricated at 60°C showed similar decreasing diffusivity with wire diameter, and a further 42%–48% reduction was observed when compared to 40°C samples. The modified effective medium theory (MEMT) was employed to evaluate the experimental thermal diffusivity. Calculations based on MEMT resulted in mean thermal conductivities of 70.7 and 36.2Wm−1K−1 for nickel nanowires fabricated at 40 and 60°C respectively. These values are ∼20% and 60% lower than the thermal conductivity value of bulk nickel. A strong grain size dependence of thermal diffusivity in the nanowires was observed. It is believed that the decrease in diffusivity in lower temperature wires is associated with defects/dislocations in large single crystals and reduction in wire diameters according to pore diameters of NCA. Whereas in higher temperature wires, the drastic reduction in diffusivity is believed to arise from self-similar fractal morphology composed of nanogranules, close to the dimension of electron mean free path.

Thermal and transport properties of manganites [formula omitted] ( [formula omitted], 0.45)

The temperature dependencies of the specific heat, thermal diffusivity and thermal conductivity in polycrystalline Nd 0.55 Sr 0.45 MnO 3 and Nd 0.5 Sr 0.5 MnO 3 manganites are measured in wide temperature range of 77–300 K. These characteristics reveal peculiarities at magnetic phase transition and transition into charge-ordered (CO) state. It is shown that a change of the phonon spectrum due to lattice contraction is the main reason of a sharp decrease of the thermal conductivity in Nd 0.5 Sr 0.5 MnO 3 at the transition to the antiferromagnetic CO state. Using the experimental thermal diffusivity data and sound velocity data a temperature dependency of the phonon mean free path is calculated.

The effects of phase transformation on the heat conduction of hafnium hydrides

Thermal diffusivity measurements for hafnium hydrides (HfH x ) with different hydrogen concentrations (α + δ’-HfH 0.20, α + δ’-HfH 0.92, α + δ’-HfH 1.15, δ’-HfH 1.48, δ’-HfH 1.57, δ-HfH 1.70, δ + ɛ-HfH 1.76, ɛ-HfH 1.88 and ɛ-HfH 2.02) were carried out within the temperature range of 290–570 K by means of a laser-flash method. The temperature dependence of the thermal diffusivity for α + δ’-HfH 0.20 was the result of a mixture of a hexagonal α-phase and a slightly deformed face-centered cubic (fcc) δ’-phase structure that was similar to that observed in α-Hf. The thermal diffusivities for HfH 0.92–2.02 at temperatures below 373 K drastically changed due to the phase transformation, while those at temperatures above 373 K remained almost the same, 0.06–0.08 cm 2/s, which is approximately half that of α-Hf. In particular, the thermal diffusivities for δ’-HfH 1.48 and δ’-HfH 1.57 changed between 360 and 390 K with the conversion to an ideal fcc structure in the δ-phase. The temperature dependence of the thermal conductivities for α + δ’-HfH 0.20, α + δ’-HfH 0.92, δ’-HfH 1.48, δ-HfH 1.70 and ɛ-HfH 2.02 were evaluated by way of calculations from the experimental thermal diffusivity data and reference data for the specific heat and density.

Determination of the thermal diffusivity of aqueous solutions of methanol in an extended range of temperature by a laser-induced thermal grating technique

Experimental thermal diffusivity data are presented for aqueous solutions of methanol in the temperature range 295–335 K. The thermal diffusivity measurements were performed using a laser-induced thermal grating technique at atmospheric pressure. The aqueous solutions studied have weight fractions of 20, 40, 60, and 80% methanol. Because of the small temperature rise (Δ T < 0.2 K) in the thin sample (sample thickness d = 0.5 mm) during the measurements and the very short measuring time of a few milliseconds, the influence of free convection is negligible. The results converted to thermal conductivity show very good agreement with the experimental data in the literature.

Simulation of transient thermal stress in gypsum-bonded investment.

The aim of this study was to clarify the effect of heating rate on the development of both transient and residual stresses in investment molds. Solid, cylindrical, gypsum-based molds were modeled and theoretical calculations made of temperature distributions with a constant heating rate. These calculations used experimental thermal diffusivity data obtained with a laser flash method. The simulations calculated transient thermal stresses during heating and at the end of heating for two surface conditions: an unrestricted surface, and a surface restricted by a casting ring. The simulation model developed nonuniform strains and stresses at casting temperature; tangential stress was compressive at the surface and tensile at the mid-point for the unrestricted surface model. The surface restricted model developed compressive tangential stresses throughout the mold at the casting temperature. This resulted in significant thermal strain differences compared to the magnitudes of expansion of the mold due to heating.