Measurements of Hydrogen Thermal Conductivity at High Pressure and High Temperature

The thermal conductivity for normal hydrogen gas was measured in the range of temperatures from 323 K to 773 K at pressures up to 99 MPa using the transient short hot-wire method. The single-wire platinum probes had wire lengths of 10 mm to 15 mm with a nominal diameter of 10 μm. The volume-averaged transient temperature rise of the wire was calculated using a two-dimensional numerical solution to the unsteady heat conduction equation. A non-linear least-squares fitting procedure was employed to obtain the values of the thermal conductivity required for agreement between the measured temperature rise and the calculation. The experimental uncertainty in the thermal-conductivity measurements was estimated to be 2.2 % (k = 2). An existing thermal-conductivity equation of state was modified to include the expanded range of conditions covered in the present study. The new correlation is applicable from 78 K to 773 K with pressures to 100 MPa and is in agreement with the majority of the present thermal-conductivity measurements within ±2 %.     

Gaseous thermal conductivity of difluoromethane (HFC-32), pentafluoroethane (HFC-125), and their mixtures

The gaseous thermal conductivity of dilluoromethane (HFC-32). pentalluoroethane (HFC-125). and their binary mixtures was measured with a transient hot-wire apparatus in the temperature ranges 283–333 K at pressures up to saturation. The uncertainty of the data is estimated to be within I %. The thermal conductivity as a function of composition of the mixtures at constant pressure and temperature is found to have a small maximum near 0.3–0.4 mole fraction of HFC-32. The gaseous thermal-conductivity data obtained for pure HFC-32 and HFC-125 were correlated with temperature and density together with the liquid thermal-conductivity data from the literature, based on the excess thermal-conductivity concept. The composition dependence of the thermal conductivity at a constant temperature is represented with the aid of the Wassiljewa equation.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado. U.S.A.     

Design and Validation of a High-Temperature Comparative Thermal-Conductivity Measurement System

A measurement system has been designed and built for the specific application of measuring the effective thermal conductivity of a composite, nuclear-fuel compact (small cylinder) over a temperature range of 100 °C to 800 °C. Because of the composite nature of the sample as well as the need to measure samples pre- and post-irradiation, measurement must be performed on the whole compact non-destructively. No existing measurement system is capable of obtaining its thermal conductivity in a non-destructive manner. The designed apparatus is an adaptation of the guarded-comparative-longitudinal heat flow technique. The system uniquely demonstrates the use of a radiative heat sink to provide cooling which greatly simplifies the design and setup of such high-temperature systems. The design was aimed to measure thermal-conductivity values covering the expected range of effective thermal conductivity of the composite nuclear fuel from 10 W . m⁻¹ . K⁻¹ to 70 W . m⁻¹ . K⁻¹. Several materials having thermal conductivities covering this expected range have been measured for system validation, and results are presented. A comparison of the results has been made to data from existing literature. Additionally, an uncertainty analysis is presented finding an overall uncertainty in sample thermal conductivity to be 6 %, matching well with the results of the validation samples.     

New Measurements of the Thermal Conductivity of PMMA,BK7, and Pyrex 7740 up to 450K

New measurements of the thermal conductivity of polymethyl methacrylate (PMMA), BK7, and Pyrex 7740 are presented. The technique employed is a refined transient hot-wire technique, based on a full theoretical model with equations solved by finite elements for the exact geometry. At the 95 % confidence level, the standard deviations of the thermal conductivity measurements of PMMA, BK7, and Pyrex 7740 are 0.47 %, 1.0 %, and 0.8 %, respectively. The technique is absolute and is characterized by an uncertainty of <1 %.     

Determining Thermal Conductivity and Thermal Diffusivity of Low-Density Gases Using the Transient Short-Hot-Wire Method

The transient short-hot-wire method for measuring thermal conductivity and thermal diffusivity makes use of only one thermal-conductivity cell, and end effects are taken into account by numerical simulation. A search algorithm based on the Gauss–Newton nonlinear least-squares method is proposed to make the method applicable to high-diffusivity (i.e., low-density) gases. The procedure is tested using computer-generated data for hydrogen at atmospheric pressure and published experimental data for low-density argon gas. Convergence is excellent even for cases where the temperature rise versus the logarithm of time is far from linear. The determined values for thermal conductivity from experimental data are in good agreement with published values for argon, while the thermal diffusivity is about 10 % lower than the reference data. For the computer-generated data, the search algorithm can return both thermal conductivity and thermal diffusivity to within 0.02 % of the exact values. A one-dimensional version of the method may be used for analysis of low-density gas data produced by conventional transient hot-wire instruments.     

An Improved Application of the Transient Hot-Wire Technique for the Absolute Accurate Measurement of the Thermal Conductivity of Pyroceram 9606 up to 420 K

This article describes the final refinements of a novel application of the transient hot-wire technique developed for the absolute, accurate measurements of the thermal conductivity of solids. Although the technique was originally developed five years ago, these new refinements allow a full understanding of the method and hence the performance of measurements with an absolute uncertainty of less than 1 %. New measurements of Pyroceram 9606 up to 420 K are reported. The maximum deviation of the present measurements is 0.54 %, while their standard deviation at the 95 % confidence level is 0.25 %. Since May 2007, Pyroceram 9606 is a European Commission certified thermal-conductivity reference material, designated as BCR-724, with an uncertainty of ±6.5 % at the 95 % confidence level.     

Theoretical and Experimental Analysis of Moisture-Dependent Thermal Conductivity of Lightweight Ceramic Bricks

The moisture-dependent thermal conductivity of two types of lightweight ceramic brick body is analyzed using both theoretical and experimental approaches. The basic physical properties are determined at first. Then, an impulse method is applied for the thermal-conductivity measurement. Initially, the material samples are dried, after that, they are exposed to liquid water for specific time intervals, and finally the moisture content is allowed to homogenize within the whole volume. The thermal-conductivity measurement is performed for different moisture contents achieved in this way. In the theoretical part, the homogenization principles are used for the calculation of the moisture-dependent thermal conductivity, utilizing the distribution functions based on the pore-size distribution measurement. Finally, a comparison of the measured and calculated data is done, and the validity of the applied effective media treatment is assessed.     

Correlation and Prediction of Dense Fluid Transport Coefficients. IX. Ionic Liquids

The aim of this paper is to examine the application of a hard-sphere scheme to the correlation and prediction of the viscosity and thermal conductivity of ionic liquids. Ionic liquids present an excellent case, because of their high viscosity. It was found that, regardless of the fact that the scheme had to be extended by orders of magnitude, it was still an excellent scheme for the correlation and prediction of the properties of these liquids; this fact is attributed to its theoretical basis. A database of 461 viscosity and 170 thermal-conductivity measurements for 19 ionic liquids was considered. The average absolute deviation was 2.31 % for the viscosity and 3.15 % for the thermal conductivity, while the expanded uncertainty at the 95 % confidence level was 4.6 % and 6.3 %, respectively. Moreover, if the thermal-conductivity roughness factor is allowed to be temperature dependent, then the average absolute deviation was reduced to 0.91 % for the thermal conductivity, and the expanded uncertainty at the 95 % confidence level to 1.82 %. As the scheme requires knowledge of the density, 1070 measurements of density were employed to derive a Tait-type equation for every ionic liquid considered.     

Thermal conductivity of halogenated ethanes,HFC-134a,HCFC-123, and HCFC-141b

The gaseous thermal conductivity of three CFC alternatives, HFC-134a (1,1,1,2-tetrafluoroethane), HCFC-123 (1,1-dichloro-2,2,2-trifluoroethane), and HCFC-141b (1,1-dichloro-1-fluoroethane), has been measured in the temperature ranges 273–363 K (HFC-134a) and 313–373 K (HCFC-123, HCFC-141b) at pressures up to saturation. The measurements were performed with a new improved transient hot-wire apparatus. The uncertainty of the experimental data is estimated to be within 1%. The gaseous thermal conductivity obtained in this work together with the liquid thermal-conductivity data from the literature were correlated with temperature and density by an empirical equation based on the excess thermal-conductivity concept. The equation is found to represent the experimental results with average deviations of 2.5 % for HFC-134a, 0.75% for HCFC-123, and 0.55% for HCFC-141b, respectively.     

Constant-power resistive-probe method and measurement of liquid-semiconductor thermal conductivity

An experimental device for measurement of thermal conductivity of liquid semiconductors is described. Results are presented from thermal-conductivity measurements of a number of semiconductor alloys.     

A Transient Hot-Wire Instrument for the Measurement of the Thermal Conductivity of Solids up to 590 K

A novel application of the transient hot-wire technique for measurements of the thermal conductivity of solids up to 590 K is described. The method makes use of a soft silicone material between the hot wires and the solid of interest. Measurement of the transient temperature rise of the wires in response to electrical step heating over a period of 20 s to 20 s allows an absolute determination of the thermal conductivity of the solid, as well as of the silicone paste. The method is based on a full theoretical model with equations solved by a finite-element method applied to the exact geometry. Two sets of thermal-conductivity measurements up to 590 K, employing different silicone pastes and samples of Pyroceram 9606, are reported. At the 95% confidence level, the standard deviation of the thermal conductivity measurements is less than 1.5%.     

Thermal Conductivity and Phase Diagrams of Some Potential Hydrogen Storage Materials Under Pressure

Experimental data for the thermal conductivity of the complex hydrides NaAlH₄, LiBH₄, NaBH₄, and KBH₄, in dense, solid form over wide ranges in temperature and pressure are presented. These materials contain high volume and mass fractions of hydrogen and are considered possible candidates as future hydrogen storage materials for mobile applications. The pressure–temperature phase diagrams of several materials as obtained from thermal-conductivity studies are briefly discussed, and the temperature and pressure dependencies of the thermal conductivity of the bulk materials are also discussed using simple theoretical models.     

Effects of Experimental Method on Aggregation State and Thermal Conductivity of Carbon Nanotube-Based Fluids

The thermal conductivity of liquids with carbon nanotubes (CNTs) is higher than that of liquids with spherical nanoparticles which results from low-resistance heat flow paths formed by CNT–CNT contact. Since CNTs easily precipitate or cluster in base liquids, path formation depends on the dispersion state of CNTs. A model of the thermal-conductivity enhancement of liquids with CNTs is presented by incorporating the aggregate state where such paths are formed. This model concludes that the anomalously wide range of enhancement values that have been observed recently is attributed to aggregate concentration. CNT clustering and sedimentation in base liquids are the causes of the decreased thermal-conductivity enhancement of such liquids due to the increase of the volume fraction of CNTs in aggregates. Predictions based on our model also show that experimental methods of obtaining liquids with uniformly dispersed CNTs can change the CNT geometry and aggregate concentration related to the thermal-conductivity enhancement. Surfactant addition, CNT surface treatment with acid, and sonication have characteristic effects on the CNT state and thermal conduction. The model from this study can prove helpful in explaining the magnitude of such effects quantitatively.     

An Improved Empirical Correlation for the Thermal Conductivity of Propane

New experimental data on the thermal conductivity of propane have been reported since the wide-range correlations proposed by Holland et al. and by Younglove and Ely. These new experimental data, covering a temperature range of 110 to 700 K and a pressure range of 0.1 to 70 MPa, are used together with the previously available data to develop an improved empirical equation for the thermal conductivity of gaseous and liquid propane. The quality of the new data is such that the thermal-conductivity correlation for propane is estimated to have an uncertainty of about ±5% at a 95% confidence level, with the exception of state points near the critical point, where the uncertainty of the correlation increases to ±10%.     

Effects of Laser Irradiation on the Thermal Conductivity and Viscosity of Aqueous Multiwalled Carbon Nanotube Suspensions

The effects of KrF excimer laser irradiation (248 nm) on aqueous suspensions of multiwalled carbon nanotubes (MWCNTs) were experimentally examined. MWCNTs and sodium dodecyl sulfate were added to deionized water at a mass fraction of 0.5 %, and the suspension was ultrasonicated for 30 min. Transmission electron microscopy (TEM) images of the nanotube samples after laser irradiation indicated fractures and network disentanglement. The laser fluence affected the thermal conductivity and viscosity of the suspensions beyond a threshold of 50 mJ · cm⁻². As the irradiation time progressed at a laser fluence of 144 mJ · cm⁻², the thermal conductivity and viscosity decreased until they reached saturation. The thermal-conductivity enhancement decreased from 16 % to 5 %, and the low shear viscosity decreased dramatically to 1/200 the shear viscosity of the non-irradiated sample. Raman spectra and TEM images showed that the defects in the nanotubes increased upon laser irradiation. In conclusion, excimer laser irradiation of a suspension of MWCNTs provided an effective way to tune the heat transfer and rheological characteristics of suspensions.     

Uncertainties Quantification of Effective Thermal Conductivity for Ceramic Fiber Blanket

In the present paper, a probabilistic propagation model for assessing the uncertainty of the effective thermal conductivity was developed based on a combined conduction and radiation heat transfer model of a ceramic fiber blanket composite. The Monte Carlo technique was used to cope with the uncertainties in the material density, radiative properties, and boundary temperatures observed in experimental tests. The calculated effective thermal-conductivity distribution for the sample was compared with the experimental measurements performed on multiple samples, and the predicted mean values were in good agreement with the measured data. The result validates the thermal predictive model and demonstrates the suitability of the stochastic model containing statistical distributions in the input variables. Statistical information also indicates that the uncertainty effect can be enlarged at high temperatures. Response sensitivity analysis between the random inputs and the effective thermal conductivity demonstrates that the randomness in the hot-side temperature, the cold-side temperature, and extinction coefficient of the sample has a significant influence on the variability of thermal-conductivity properties. The extinction coefficient becomes more and more important with an increase of temperature due to the dominant radiative heat transfer contribution at high temperature. The analysis provides good insight into the scattering control in the experimental measurement and theoretical prediction of the effective thermal conductivity of a ceramic fiber composite.     

Experimental study of thermal conductivity of heavy-water vapor at temperatures of 375–600°C and pressures to 250 mPa

Results are presented of an experimental study of the thermal conductivity of heavy water using the plane horizontal-layer method over the temperature range 375–600°C at pressures of 0.1–250 mPa. These are the first published heavy-water thermal-conductivity values at pressures above 100 mPa.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 34, No. 1, pp. 114–117, January, 1978.     

Determining a critical strain for APS thermal barrier coatings under service relevant loading conditions

Despite the huge progress made in recent years in analysing the degradation behavior and the reliability of thermal barrier coating systems, there is still some deficit in the capability to predict damage evolution in terms of crack initiation and crack growth, which ultimately leads to macroscopic delamination and spallation of the coating system. In order to obtain this prediction capability, a fundamental understanding of the damage evolution processes under isothermal, thermo-cyclic and under thermo-mechanical loading conditions has to be developed.The aim of the presented work is to determine the critical strain, i.e. the strain at which cracking initiates, and to analyse the evolution of a network of cracks for widely used atmospheric plasma sprayed (APS) thermal barrier coating (TBC) systems. The TBC system has been exposed in our study to service relevant loading conditions, namely to thermal gradient mechanical fatigue (TGMF). TGMF tests for in-phase as well as out-of-phase loading cycles were performed on hollow cylindrical specimens made of the single crystal super alloy CMSX-4, loaded mechanically in 〈0 0 1〉 orientation, and being coated with a duplex system comprised of a CoNiCrAlY bond coat and a 8 wt.% Yttria partially stabilized Zirconia (YSZ) TBC. The CoNiCrAlY bond coat was deposited by Low Pressure Plasma Spraying (LPPS), while the ceramic top coat was deposited using the APS process. The loading cycles were chosen to represent an industrial gas turbine engine. Critical strains measured for delamination (within the ceramic coating or at the CoNiCrAlY – TBC interface) and through cracking, i.e. segmentation of the ceramic top coat was determined using a special compression test equipped with in situ acoustic emission technique. The mechanical testing was performed at room temperature after TGMF exposure. In order to study the impact of thermally grown oxide (TGO), specimens have been TGMF tested in the “as received” conditions as well as after isothermal aging (up to 3000 h at 1000 °C). To correlate the signal obtained by acoustic emission (AE) with the evolution of (micro-) cracks, the specimens have been carefully sectioned and investigated by standard metallographic means.The measured critical strains are used as a data basis for a strain-based lifetime model developed for isothermal and cyclic oxidation as well as thermo-mechanical loading. The lifetime model considers two failure modes, namely delamination and (vertical) through cracking.Metallographically obtained crack patterns within the TBC system have been incorporated into finite element models to quantify stress–relaxation as a consequence of damage evolution in the TBC system.The observations show that thermal gradient fatigue loading under in-phase loading leads to a shorter lifetime compared to out-of-phase loading.For the delamination mode, the critical strain values of the model are in good agreement with the experimental data of the TGMF experiments. The modeled critical strain for through cracking, on the other hand, is consistently lower than the experimentally determined failure strains, implying that the model describes the failure situation in a conservative manner.     

Modeling of Effective Thermal Conductivity of Dunite Rocks as a Function of Temperature

The thermal conductivity, thermal diffusivity, and heat capacity per unit volume of dunite rocks taken from Chillas near Gilgit, Pakistan, have been measured simultaneously using the transient plane source technique. The temperature dependence of the thermal transport properties was studied in the temperature range from 303 K to 483 K. Different relations for the estimation of the thermal conductivity are applied. A proposed model for the prediction of the thermal conductivity as a function of temperature is also given. It is observed that the values of the effective thermal conductivity predicted by the proposed model are in agreement with the experimental thermal conductivity data within 9%.     

Effects of temperature and clay content on water absorption characteristics of modified MMT clay/cyclic olefin copolymer nanocomposite films: Permeability,dynamic mechanical properties and the encapsulated organic device performance

In the present study, amino-silane modified layered organosilicates were used to reinforce cyclic olefin copolymer to enhance the thermal, mechanical and moisture impermeable barrier properties. The optimum clay loading (4%) in the nanocomposite increases the thermal stability of the film while further loading decreases film stability. Water absorption behavior at 62 °C was carried out and compared with the behavior at room temperature and 48 °C. The stiffness of the matrix increases with clay content and the recorded strain to failure for the composite films was lower than the neat film. Dynamic mechanical analysis show higher storage modulus and low loss modulus for 2.5–4 wt% clay loading. Calcium degradation test and device encapsulation also show the evidence of optimum clay loading of 4 wt% for improved low water vapor transmission rates compared to other nanocomposite films.