Measurement Of Effective Thermal Conductivity Research Articles

204 定常法による金属基複合材料の有効熱伝導率測定と熱伝導シミュレーション

204 定常法による金属基複合材料の有効熱伝導率測定と熱伝導シミュレーション

Effect of the addition of date palm fibers on thermal properties of plaster concrete: experimental study and modeling

Simultaneous measurement of effective thermal conductivity, thermal diffusivity, specific heat, and thermal effusivity of date palm fibers reinforced plaster concrete have been studied by CT meter device using a ring sensor technique. Samples of different weight percentages 1, 1.5, and 2% for different length fibers have been considered. It is found that the thermal conductivity, thermal diffusivity, and effusivity decrease as the fraction of fibers increases, even an increase for the specific heat is recorded. A modeling application reveals good correspondence with the experimental results. Moreover, a scanning microscopy study has shown a good interaction between the gypsum matrix and date palm fiber cells. Finally, it should be noted that date palm fibers and plaster are mutually compatible and many new building components can be made by incorporating these materials.

Comparison between ambulatory measurement of effective thermal conductivity and laser Doppler flowmetry method to assess skin microcirculatory activity

The main objective of this paper was to assess the performance of the ambulatory device μHematron to measure indirectly skin blood flow relative to the well-established Laser Doppler flowmetry method. The μHematron device is dedicated to the non-invasive measurement of effective thermal conductivity of living tissues, based on the thermal clearance method. Its major advantage is its ambulatory functionality, as available methods for evaluation of microcirculatory activity are non-ambulatory methods. An experiment was conducted on ten healthy women exposed for one hour in three different thermal environments (22°C, 25°C and 30°C). Skin microcirculatory activity was analyzed after an acclimatization period of 30minutes. The time between each exposure was at least one hour. Performances of the μHematron device were assessed and a comparative study with a laser Doppler perfusion monitor (LDPM) was performed. Good correlation coefficients between the two devices (r=0.71 at T1=22°C, r=0.77 at T2=25°C and r=0.83 at T3=30°C) were obtained while the LDPM signal was filtered by a low pass filter (0.1Hz). These results showed that continuous monitoring of effective thermal conductivity was possible in neutral and warm ambiences. Then, the μHematron device could be considered as a complementary tool to Doppler techniques for the investigation of skin blood flow, when ambulatory conditions are required.

Measuring the thermal conductivity of heat transfer fluids via the modified transient plane source (MTPS)

Heat transfer fluids are often a critical performance component in industrial processes and system design. Fluids are used in heat dissipation to maintain stable operating temperatures in a variety of applications, such as diesel engines, chemical production, asphalt storage, and high-power electric transformers. A wide range of fluids specific to various applications are available, thus a reliable and accurate thermal conductivity characterization is extremely important. Thermal conductivity analysis of heat transfer fluids with traditional methods is time-consuming and error-prone due to the impact of convection. Convection often distorts effective thermal conductivity measurement as an additional source of heat transfer. The modified transient plane source method implemented in the C-Therm Technologies TCi Analyzer provides an easy way to accurately measure the thermal conductivity and distinguish this form of heat transfer in negating the impact of convection by (a) employing the shortest test time in commercially available sensors (0.8 s), (b) offering a minimal sample volume requirement (1.25 mL), and (c) employing a low-energy power flux to the specimen under test (approximately 2,600 W m−2). This work presents thermal conductivity results generated on three types of heat transfer fluids over a wide temperature range and discusses the significance of the data in relevance to the application.

Effective Thermal-Conductivity Measurement on Germanate Glass–Ceramics Employing the $$3\omega $$ 3 ω Method at High Temperature

It can be noted that the germanate glass–ceramic is a functional material with excellent thermal stability which can be used in optical devices. The temperature-dependent effective thermal conductivities of CaO–BaO–CoO–Al $$_{2}$$ O $$_{3}$$ –SiO $$_{2}$$ –GeO $$_{2}$$ glass–ceramics from 295.5 K to 780 K are determined using a $$3\omega $$ method. One of the main advantages for the $$3\omega $$ method is to diminish radiation errors effectively when the temperature is as high as 1000 K. Thermal conductivities of CaO–BaO–CoO–Al $$_{2}$$ O $$_{3}$$ –SiO $$_{2}$$ –GeO $$_{2}$$ increase with a rise in temperature. Effective thermal conductivities of a sample increase from $$1.55~\hbox {W}\cdot \hbox {m}^{-1}\cdot \hbox {K}^{-1}$$ at 295.5 K to $$7.64~\hbox {W}\cdot \,\hbox {m}^{-1}\cdot \hbox {K}^{-1}$$ at 698.1 K. The effective thermal conductivity of CaO–BaO–CoO–Al $$_{2}$$ O $$_{3}$$ –SiO $$_{2}$$ –GeO $$_{2}$$ glass–ceramic increases with a rise of temperature. This investigation can be used as a basis for the measurement of thermal properties of ceramic materials at higher temperature.

Measurement and Estimation of High-Vacuum Effective Thermal Conductivity of Polyimide Foam in the Temperature Range from 160 K to 370 K for Outer Space Applications

Polyimide foam (PF) is a low-thermal conductivity and lightweight material with high resistances against heat, protons, and UV irradiation. A new thermal insulation composed of PFs and multiple aluminized films (PF–MLI) has potential to be used in outer space as an alternative to conventional multilayer insulation (MLI). As fundamental numerical data, the effective thermal conductivity of PF in wide ranges of density and temperature need to be determined. In the present study, thermal-conductivity measurements were performed by both the periodic heating method and the guarded hot-plate method in the temperature range from 160 K to 370 K and the density range from 6.67 \(\mathrm{kg} \cdot \mathrm{m}^{-3}\) to 242.63 \(\mathrm{kg}\cdot \mathrm{m}^{-3}\). The experiments were carried out in a vacuum and under atmospheric pressure. For confirmation of the validity of the present guarded hot-plate apparatus under atmospheric pressure, the effective thermal conductivity of the lowest-density PF was measured with the aid of the heat flow meter apparatus calibrated by the standard reference material (NIST SRM 1450c) in the temperature range from 303 K to 323 K. In order to cross-check the present experimental results, the temperature and density dependences of the effective thermal conductivity of PF were estimated by means of the lattice Boltzmann method based on a dodecahedron inner microscopic complex structure model which reflects a real 3D X-ray CT image of PF.

Temperature Influence on the Effective Thermal Conductivity of Cellular Metals Measured in Stagnant Air

For the optimum use of cellular metals, good knowledge of the properties is a necessary precondition. One of the significant thermophysical properties is the effective thermal conductivity representing the various heat transfer processes. Furthermore, cellular metals have complex structures, which make the experimental investigation of this effective thermal conductivity rather complicated. For the present investigations, hollow sphere structures from different steel alloys and open cell structures made of FeCrAl have been measured with two instruments, the transient plane source technique and the panel test technique up to about 250°C and 800°C in air, respectively. The results of both methods show good agreement. The effective thermal conductivity measurements are influenced by the effects of temperature, porosity and pore diameter.

Rock Thermal Conductivity of Natural Gas Hydrate Deposits in the Muli Basin of Chinese Qinghai-Tibet Plateau

This study reports laboratory measured thermal conductivity values of sandstones from 4 wells (predominantly hydrae exploration boreholes). The objective of the present work is to report the values of the thermal conductivity that were measured in the cores from the borehole that was estimated for the geological formations intercepted by it. The measurements were made on drill cores using the guarded parallel-plate apparatus at a temperature range from 285 to 290 K at Atmospheric pressure. Bulk thermal conductivities of sandstones corrected for in site thermal conditions range between 2.1 and 2.8 W/m/K. In general, the estimated uncertainty of the Effective thermal conductivity measurements is 2%. These data can be used for design of gas recovery form gas hydrate using heat injection technology.

Experimental study of evaporative heat transfer in sintered powder structures at low superheat levels

Evaporation at the evaporator of a heat pipe plays an important role in its overall heat transfer performance, especially at low superheat levels. Used sintered powder structures as wicks, this study investigated the correlations between superheat levels and heat fluxes. The parameters included powder sizes of 45μm, 75μm, 150μm, and powder shapes of spherical, dendritic. A two-part measurement in this study consisted of effective thermal conductivity and evaporative heat transfer. For the experiment of evaporative heat transfer, an apparatus consisting of a thermal guard test chamber, a direct sintering design, a pressure control loop, and a data acquisition system was used to measure heat fluxes and corresponding superheat levels. The effective thermal conductivity measurement showed that smaller powder sizes achieved higher effective thermal conductivities for both powder shapes. Spherical powder structures achieved twice the effective thermal conductivity of dendritic powder ones for each powder size. Furthermore, the evaporative heat transfer measurement showed that the heat fluxes increased proportionally with the superheat between 2 and 6K. At the same superheat level, structures of smaller powder size and dendritic powder shape achieved higher heat fluxes. In conclusion, the effect of thin-film evaporation may be the primary factor affecting evaporative heat transfer among these structural parameters.

Heat transfer between pebbles by taking self-heating of each pebble into consideration

The effective thermal conductivity of pebble bed has been measured by many researchers by means of various measurement techniques. Almost no measurement of effective thermal conductivity has been performed by using self-heating pebbles. In actual blanket, the pebbles are heated by neutron interaction. In that case, the temperature profile in each pebble caused by overall heat flow in the pebble bed, for example linear profile, is overlapped with the individual temperature profile originated from the nuclear heating. The driving force to transfer the heat between contacting pebbles is the temperature gradient near the contact region of both pebbles. In this paper, the effect of temperature profile of each contacting pebble on the heat transfer between pebbles is considered and the contribution of the direct heat conduction to the heat transfer between the pebbles is estimated.

Thermal conductivity of single biological cells and relation with cell viability

This paper presents measurements of effective thermal conductivity of single biological cells (HeLa, NIH-3T3 J2, and hepatocyte). Such cellular-level measurements have not been performed previously, due to the small size (∼10 pl) of cells. The measurement uses the three-omega method with a nanoscale thermal sensor and a micro-well structure that confines the cell. The results provide a fundamental property to investigate the thermodynamics of cells. We also measured thermal conductivity of dead cells; results were significantly different from those of live cells. The difference means that thermal conductivity can be used for probing cell viability.

Influence of CdS Nano Additives on the Thermal Conductivity of Poly(vinyl chloride)/CdS Nanocomposites

PVC/CdS nanocomposites have been prepared by solution casting method taking 2, 4, 6 and 8 wt% of CdS nanoparticles. PVC/CdS nanocomposites were characterized through TEM measurement. The measurement of effective thermal conductivity of PVC/CdS nanocomposites has also been done using transient plane source (TPS) method. The effects of concentration of CdS nanoparticles and temperature on the effective thermal conductivity of polymer PVC/CdS nanocomposites have been studied. The variation of effective thermal conductivity with the concentration of filler particles and temperature has also been discussed in terms of dispersion of filler particles into polymer matrix and phonon scattering mechanism, respectively.

Effective Thermal-Conductivity Measurements on Supporting Structures of the Mercury Probe Bepi Colombo

Measurements were done on a carbon fiber reinforced composite (CFC) sample tested for the space probe Bepi Colombo by using the guarded hot-plate (GHP) method. The values of interest were the thermal transmittance through the samples, (56.3 ± 3.6) W · m−2 · K−1, and the effective thermal conductivity (1.06 ± 0.07) W · m−1 · K−1. The samples consist of a light honeycomb core attached to thicker surface plates. Due to this construction, the effective thermal conductivity parallel to the face plates is higher than the effective thermal conductivity through the sample. This leads to lateral heat gains or losses during the GHP measurement, which in return can lead to erroneous results. Furthermore, due to the high rigidity of the CFC material, there will be high contact resistances between the samples and the GHP apparatus plates. The separation of these thermal contact resistances from the total measured thermal resistance is essential in order to achieve correct results. Good results were achieved using a special measurement setup and a lateral correction method designed to reduce errors due to lateral heat flows.

Vacuum insulation properties of phenolic foam

Characteristic properties of phenolic foam as the interstitial material of a vacuum insulation panel are investigated experimentally. For the measurement of effective thermal conductivity, a vacuum guarded hot plate (VGHP) apparatus is used and the conductivity is measured at various vacuum levels. Radiative properties are found using a Fourier transform infrared spectroscopy (FT-IR) device. Solid conductivity is estimated using the porosity of the foam. Effective thermal conductivity at high level of vacuum is measured to be 5mW/mK which is sum of solid conductivity (2.56mW/mK) and radiative conductivity (2.44mW/mK) with 5% of measurement uncertainty. The pore size of the foam is estimated to be 260μm using rarefied gas conduction theory. This ensures insulation performance of phenolic foam up to about 10−3atm. Other practical characteristics of phenolic foam as the VIP core material are also discussed.

Experimental Analysis of Thermal Conductivity for Building Materials Depending on Moisture Content

The presence of moisture in building envelopes can have many causes and may lead to deterioration of useful thermophysical characteristics of the materials, to weakening of the building structure, and to facilitating growth of mold. The International Standard ISO 13788 establishes a calculation procedure for the determination of hygrometric characteristics of building components and materials, assuming that the influence of moisture content on the thermal field across walls, ceilings, and roofs may be neglected. However, condensed water increases the effective thermal conductivity of building materials, thus modifying the temperature profiles across the building envelope. This effect is analogous to the one due to the material aging. In this paper, the authors show the results of effective thermal-conductivity measurements in some commonly adopted building materials as a function of moisture content, in order to assess the potential significance of interstitial condensation on thermal losses and to verify if the maximum allowed moisture content reported is useful to prevent the decay of the thermal properties of building materials.

Simplified Transient Hot-Wire Method for Effective Thermal Conductivity Measurement in Geo Materials: Microstructure and Saturation Effect

The thermal conductivity measurement by a simplified transient hot-wire technique is applied to geomaterials in order to show the relationships which can exist between effective thermal conductivity, texture, and moisture of the materials. After a validation of the used &#x201c;one hot-wire&#x201d; technique in water, toluene, and glass-bead assemblages, the investigations were performed (1) in glass-bead assemblages of different diameters in dried, water, and acetone-saturated states in order to observe the role of grain sizes and saturation on the effective thermal conductivity, (2) in a compacted earth brick at different moisture states, and (3) in a lime-hemp concrete during 110 days following its manufacture. The lime-hemp concrete allows the measurements during the setting, desiccation and carbonation steps. The recorded <svg style="vertical-align:-2.21957pt;width:57.025002px;" id="M1" height="13.5625" version="1.1" viewBox="0 0 57.025002 13.5625" width="57.025002" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(1.25,0,0,-1.25,0,13.5625)"> <g transform="translate(72,-61.15)"> <text transform="matrix(1,0,0,-1,-71.95,63.41)"> <tspan style="font-size: 12.50px; " x="0" y="0">Δ</tspan> <tspan style="font-size: 12.50px; " x="9.0271664" y="0">𝑇</tspan> <tspan style="font-size: 12.50px; " x="15.903816" y="0">/</tspan> <tspan style="font-size: 12.50px; " x="21.455149" y="0">l</tspan> <tspan style="font-size: 12.50px; " x="24.930983" y="0">n</tspan> <tspan style="font-size: 12.50px; " x="33.270481" y="0">(</tspan> <tspan style="font-size: 12.50px; " x="37.433983" y="0">𝑡</tspan> <tspan style="font-size: 12.50px; " x="41.34742" y="0">)</tspan> </text> </g> </g> </svg> diagrams allow the calculation of one effective thermal conductivity in the continuous and homogeneous fluids and two effective thermal conductivities in the heterogeneous solids. The first one measured in the short time acquisitions (&#x3c;1&#x2009;s) mainly depends on the contact between the wire and grains and thus microtexture and hydrated state of the material. The second one, measured for longer time acquisitions, characterizes the mean effective thermal conductivity of the material.

Experimental measurement of effective thermal conductivity of packed lithium-titanate pebble bed

Lithium titanate is a promising solid breeder material for the fusion reactor blanket. Packed lithium titanate pebble bed is considered for the blanket. The thermal energy; that will be produced in the bed during breeding and the radiated heat from the reactor core absorbed must be removed. So, the experimental thermal property data are important for the blanket design. In past, a significant amount of works were conducted to determine the effective thermal conductivity of packed solid breeder pebble bed, in helium atmosphere, but no flow of gas was considered. With increase in gas flow rate, effective thermal conductivity of pebble bed increases. Particle size and void fraction also affect the thermal properties of the bed significantly. An experimental facility with external heat source was designed and installed. Experiments were carried out with lithium-titanate pebbles of different sizes at variable gas flow rates and at different bed wall temperature. It was observed that effective thermal conductivity of pebble bed is a function of particle Reynolds number and temperature. From the experimental data two correlations have been developed to estimate the effective thermal conductivity of packed lithium-titanate pebble bed for different particle Reynolds number and at different temperatures. The experimental details and results are discussed in this paper.

PARAMETRIC STUDY ON TRANSIENT HOT-WIRE METHOD TO MEASURE NANOFLUID CONDUCTIVITIES

Recently, nanofluid is a hot research topic among thermal engineers to enhance the heat transfer. However, there are deviations between research groups in the effective thermal conductivity measurements by the transient hot-wire method. Since there has been no report for the researchers to select a proper data range of temperature to estimate the thermal conductivities, the deviation could be partly attributed to the poor selection of the temperature data range. In this study, the impacts of the data range selection, the power supplier response delay, the thermal coefficient of resistors, the hot-wire type and test section size on the thermal conductivity measurement using the transient hot-wire method are analyzed. The proper selection of the temperature data range to be used to estimate the thermal conductivities of fluids using transient hot-wire method is suggested by investigating the impact of it on the measurement by both computationally and experimentally. The thermal coefficient of resistor is found to affect the measurement seriously whereas the type of hot-wire does not. The test section size of the measuring device is compared with the thermal penetration depth.

Measurement of in-plane effective thermal conductivity in PEM fuel cell diffusion media

Gas diffusion media used in polymer electrolyte membrane (PEM) fuel cells are highly anisotropic with significantly different transport property values in the through- and in-plane directions. In this study, experimental measurements of the in-plane effective thermal conductivity k for gas diffusion media used in PEM fuel cells have been carried out using a parallel thermal conductance technique. Conductivity values are measured at a mean sample temperature of 70 °C for six different material types and two different orientations in order to quantify the effect of PTFE content on thermal conductivity and to reveal any anisotropic behavior. The results vary from a minimum of k = 3.54 W/(m °C) to a maximum value of 15.1 W/(m °C) for various samples and configurations tested in this study, with an uncertainty between 1% and 2% for all the cases investigated.

Effect of in and Zn additives on some thermal properties of a-Se

In the present work, the effect of In and Zn on some thermo–physical properties (thermal conductivity, diffusivity and specific heat per unit volume) of amorphous Se (a-Se) have been studied. For this, simultaneous measurements of effective thermal conductivity ( λ e) and effective thermal diffusivity ( χ e) are used at room temperature for twin pellets of Se, Se 90In 10 and Se 90Zn 10 alloys using transient plane source (TPS) technique. It has been found that In and Zn additives changes significantly the values of thermo-physical properties (thermal conductivity, diffusivity and specific heat per unit volume) of a-Se studied in the present work. The results have been analyzed in terms of average bond strength and effective molecular weight of the binary alloys.