Transient Hot-wire Method Research Articles

An Experimental Study on the Thermal Conductivity of Carbon Nanotubes/Oil (TECHNICAL NOTE)

[if gte mso 9]> In the present work, the thermal conductivity coefficients of nanoparticle-oil suspensions for two types of carbon nanotubes, single-walled (SWNTs) and multi-walled (MWNTs) carbon nanotubes at 0.1, 0.2 and 0.3 wt.% were measured by a modified transient hot wire method (KD2-pro thermal property meter). Results showed that the thermal conductivity of suspension containing single-walled carbon nanotubes is higher than that of suspension containing multi-walled carbon nanotubes. It was also observed that the thermal conductivity coefficients of both nanofluids increase with increasing temperature.

Thermal conductivity and heat transfer performance enhancement of phase change materials (PCM) containing carbon additives for heat storage application

In thermal storage system, a high thermal conductivity of Phase Change Materials (PCM) is required to complement the crystallization phenomenon of the PCM during the discharging process. In this study, PCM with carbon additives, Multi-walled Carbon nanotube, Graphite and Graphene, are manufactured and the thermal conductivity of the PCM is measured by the transient hot-wire method for thermal storage application. The thermal conductivity of the PCM is enhanced by adding the carbon additives, and the effect of Poly Vinyl Pyrrolidone (PVP) as a dispersion stabilizer on the thermal conductivity is evaluated. It is found that the heat transfer rate enhances up to 3.35 times in the case of Graphite at 5.0 vol%. It is finally concluded that Graphite is the most promising candidate for heat transfer enhancement of stearic acid among three carbon additives even though Graphene gives the highest thermal conductivity enhancement.

Analysis of the Transient Hot-Wire Method to Measure Thermal Conductivity of Silica Aerogel: Influence of Wire Length, and Radiation Properties

When the transient hot-wire method is used to measure the thermal conductivity of very low thermal conductivity silica aerogel (in the range of 10 mW/m·K at 1 atm) end effects due to the finite wire size and radiation corrections must be considered. An approximate method is presented to account for end effects with realistic boundary conditions. The method was applied to small experimental samples of the aerogel using different wire lengths. Initial conductivity results varied with wire length. This variation was eliminated by the use of the end effect correction. The test method was validated with the NIST (National Institute of Standards and Technology) Standard Reference Material 1459, fumed silica board to within 1 mW/m·K. The aerogel is semitransparent. Due to the small wire radius and short transient, radiation heat transfer may not be fully accounted for. In a full size aerogel panel radiation will augment the phonon conduction by a larger amount.

Temperature dependence of the stability and thermal conductivity of an oil-based nanofluid

Abstract Oil-based nanofluids were prepared by dispersing copper nanoparticles into a high-temperature synthetic oil using a two-step method. The nanofluid stability was evaluated by measuring the effective thermal conductivity over time. The thermophysical properties of the nanofluids were investigated by measuring the effective thermal conductivity up to a maximum temperature of 210 °C using the transient hot wire method. The thermal conductivity was increased with increasing Cu nanoparticle volume fraction and decreased with temperature between 30 and 210 °C. A maximum thermal conductivity enhancement of about 18% was obtained at 30 °C with the 0.02 volume fraction of Cu nanoparticles. The experimental data below 150 °C compares well with predictions of the Jang and Choi model. Above 150 °C, the effective thermal conductivity enhancement is much lower than predicted by the model, so new mechanisms are needed to explain this phenomenon.

EXPERIMENTAL INVESTIGATION OF PARAMETERS AFFECTING NANOFLUID EFFECTIVE THERMAL CONDUCTIVITY

The present research reports nanofluid effective thermal conductivity enhancements (ETCE) using an accurate transient short hot wire method system. Preparation of nanofluids was carried out through a two-step method with highly powered pulses similar to that for nanoparticle dispersion in base fluids. Parameters affecting nanofluid heat conductivity such as concentration, sizes, and material of nanoparticleş type of base fluid, temperature, ultrasonic mixing time, and elapsed time after preparation were studied. In the present study, nanoparticles of Al, Al2O3, CuO, SnO2, TiO2, and SiO2 with base fluids of water and ethylene glycol were used. Parameters like concentration, size, temperature, and the type of base fluid showed more noticeable effect on the effective thermal conductivity than the others, and mixing time had the least effect. The results showed that any increase in concentration and temperature, and also any decrease in size of nanoparticles and time elapsed after nanofluid preparation, leads to the ETCE of the nanofluid. However, the effects of nanoparticle material, base fluid, and mixing time on thermal conductivity of the nanofluid showed varying trends. Last, a number of mathematical models for prediction of thermal conductivity of nanofluids were applied.

Thermal conductivity measurements of a phase change material slurry under the influence of phase change

Phase change material slurry is widely used in such applications as thermal energy storage and thermal management. Thermal conductivity of phase change material slurry is one of the most important thermo-physical properties that are necessary for system design and performance evaluation. In the present study, thermal conductivity of a phase change material slurry, tetra-n-butylammonium bromide (TBAB) clathrate hydrate slurry (CHS), is experimentally measured by using transient hot-wire method. The theoretical model of thermal conductivity measurement of phase change material slurry under the influence of phase change is proposed and numerically analyzed to obtain real thermal conductivity. It is found that phase change significantly affects the measurement of thermal conductivity in that the real thermal conductivity is smaller than that obtained directly from the experiments because phase change enhances heat transfer during the measurement. The smaller the solid fraction of TBAB CHS, the larger the influence of phase change on thermal conductivity is. Such effect is apparently ubiquitous, which should also be taken into consideration in thermal conductivity measurement of other phase change material slurries.

Structural, optical and thermal properties of PVA/CdS nanocomposites synthesized by radiolytic method

Monodispersed spherical CdS nanoparticles stabilized in PVA solution were synthesized by the gamma radiolytic method and found the average particle size increased from 12 to 13nm with the increment of dose from 10 to 40kGy. The XRD results show that it has crystalline planes of cubic structure with crystal lattice parameter of 5.832Å. The optical reflectance revealed a band-edge of CdS nanoparticles at about 475nm and the reflectance wavelength red shifted with increasing dose due to increasing particle size. The thermal conductivity of CdS/PVA nanocomposites measured by the transient hot wire method that revealed a decrement of the thermal conductivity with an increase of dose caused by effect of radiation on crystallinity of the polymer structure.

A novel method to evaluate dispersion stability of nanofluids

The thermal conductivity of nanofluids is known to be a function of the nanoparticle concentration and the particle size and is known to vary with time due to sedimentation and aggregation induced by particle–particle interaction. Therefore, it is crucial to measure the nanofluid thermal conductivity together with temporal variation of the volume fraction. However, researchers have reported only the estimation of the initial volume fraction based on the particle mass added to the base fluids and the primary particle size, which could be the crucial cause of the deviation between the thermal conductivity measurements from different research groups.In this study, a new method is introduced to track the temporal changes of the particle volume fraction and size distribution, and the effects of the change of the nanofluid thermal conductivity are scrutinized. Five different nanofluids are generated: DI water/Al2O3, DI water/SiO2, DI water/Ag, EG/Al2O3, and EG/ZnO. The particle volume fraction change is identified by monitoring the suspension density using a hydrometer, and the particle size distribution change is analyzed using the dynamic light scattering (DLS) method. The thermal conductivity of each of the nanofluids is measured using the transient hot-wire method. From the analysis of the data, the nanofluid thermal conductivity is found to be affected by the changes in concentration and the particle size distribution.

비정상열선법에서 열선의 절연코팅이 선형구간의 초기시점에 미치는 영향

본 논문에서는 절연된 열선을 이용한 비정상열선법에서 열선의 절연코팅이 선형구간의 초기시점에 미치는 영향을 살펴보기 위하여 이론적인 연구를 수행해 보았다. 이를 위해, 절연된 열선의 온도상승에 관한 해석적 해에서 선형구간의 초기시점에 영향을 미치는 중요한 인자들을 파악해 보았으며, 이러한 인자들의 영향이 무시할 만큼 작아지는 임계시간(critical time)에 대한 연구를 수행해 보았다. 이론적인 연구 결과, 열선의 반경 및 코팅의 두께가 작을수록 절연코팅의 영향이 빠르게 감소하였고, 또한 절연물질의 열전도도가 크고 열확산계수가 작을수록 절연코팅의 영향이 빠르게 감소함을 알 수 있었다. 본 연구의 결과는 절연된 열선을 이용한 비정상열선법에서, 선형구간의 초기시점을 결정하는데 도움이 될 것으로 사료된다.

016 Integration of transient hot-wire method into scanning cryomacroscopy in the study of thermal conductivity of dimethyl sulfoxide

A pilot study was conducted to investigate the dependency of thermal conductivity on temperature, phase of state, and solution concentration in the range of 2 M (classical cryopreservation) and up to 10 M Me 2 SO (simulative of highly concentrated cocktails used for vitrification). Cryoprotective agents (CPAs), such as dimethyl sulfoxide (Me 2 SO) are used to control ice formation—the cornerstone of cryoinjury. When cooled, the CPA may crystallize in low concentrations and relatively low cooling rates, or vitrify (form glass—be trapped in an amorphous state) in high concentrations and relatively high cooling rates. Analysis of cryopreservation protocols and explanation of related physical events may be assisted by computer simulations of the thermal process. Unfortunately, the physical properties of CPAs necessary for thermal analysis represent a largely uncharted area. Furthermore, the difference in thermal conductivity between an amorphous and crystalline material of the same composition can vary by orders of magnitude. A transient hot-wire technique was used to measure thermal conductivity of Me 2 SO solutions in a controlled-rate cooler. Physical events in the sample along the cryopreservation protocol were recorded with the application of the scanning cryomacroscope to confirm whether the sample was vitrified (transparent) or crystallized (opaque). Selected cooling and rewarming rates were chosen to promote either vitrification or crystallization, based on literature data. Thermal conductivity measurements were made continuously during the rewarming phase of the protocol, in the temperature range of −100 to +20 °C. Thermal conductivity of crystalline Me 2 SO was found to be five-fold higher than that of amorphous Me 2 SO, with relatively little dependency on the concentration. In the amorphous state, thermal conductivity decreases with the increasing concentration: at 15°C ranging from 0.24 to 0.31 W/m K for 10 M and 7.05 M Me 2 SO, respectively, and at −100 °C ranging from 0.22 to 0.26 W/m K for 10 M and 7.05 M Me 2 SO, respectively. The decrease in thermal conductivity with decreasing temperature is consistent with literature data for other amorphous materials such as SiO 2 , Se, and PMMA. The measured thermal conductivity of 2 M Me 2 SO above −4 °C is consistent with the dependency of thermal conductivity on concentration for the other solutions in the liquid phase. The onset of crystallization based on a water–Me 2 SO phase diagram, −4 °C, correlates well with the measured increase in thermal conductivity of the mixture around this temperature. The increased thermal conductivity upon phase transition is consistent with literature data on pure water, although thermal conductivity of pure water increases by a factor of four (from 0.566 to 2.25 W/m K), whereas the thermal conductivity of 2 M Me 2 SO increases by a factor of three, from 0.47 W/m K to higher than 1.4 W/m K. The integration of the transient hot-wire method into scanning cryomacroscopy developed in this study represents the only available method for measuring thermal conductivity in cryogenic temperature while visually verifying the phase of state. Source of funding: This study is supported, in part by, Award Number R21RR026210, National Center for Research Resources (NCRR) and R21GM103407, National Institute of General Medical Sciences (NIGMS). Conflict of interest: None declared. rabin@cmu.edu

Influence of the oxidation treatment and the average particle diameter of graphene for thermal conductivity enhancement

Nanofluid is a new-concept solution wherein nm-sized solid particles of excellent, higher-than-base-fluid thermal conductivity are stably suspended. Recently, high-thermal-conductivity graphene nanoparticles have attracted particularly close attention from researchers. In the present study, the thermal conductivity and viscosity properties of three kinds of graphene nanofluid added to distilled water — two graphenes of differing size and oxidized graphene — were compared and analyzed. The thermal conductivities of the nanofluids, formulated in the usual manner by adding graphene to distilled water and subjecting the mixture to ultrasonic dispersion, were measured by the transient hot-wire method, and the viscosities were determined using a rotational digital viscometer. As a result, we concluded that the nanofluid of small average particle diameter and that which incorporates oxidized graphene have outstanding properties as heat transfer media, due to their excellent thermal conductivity and viscosity, compared with the other graphene nanofluids.

The Prediction of Thermal Conductivity of Agricultural Residues from Straw for Biomass Energy

Agricultural residues from straw are widely used for energy and other applications. The thermal conductivity is one of the most important thermophysical properties considered when using agricultural residues, such as rice straw, in renewable energy engineering. In this paper, the thermal conductivity of rice straw was measured using a thermal probe by the transient hot wire method at selected moisture contents, temperatures and dry densities. The moisture contents of the samples ranged from 0 to 21.47 percent wet basis and the dry densities ranged from 90.7 to 136.4 kg/m3 and the temperature ranged from 0 to 170°C. Under those conditions, the thermal conductivity was measured and analyzed. Experiment results showed that the thermal conductivity increases with the increases of the density, moisture content and temperature, and the relationship among them is approximately described in a linear way. A new model to predict the thermal conductivity of agricultural residues from straw was proposed. The calculated results by the proposed model are in good agreement with the experimental data.

그래핀과 탄소나노튜브의 형상에 따른 나노유체의 열전도도 특성 비교 연구

Recently, high-thermal-conductivity graphene and carbon nanotube nanoparticles have attracted particularly close attention from researchers. In the present study, the thermal conductivity and viscosity properties of two kinds of graphene and carbon nanotube nanofluids added to distilled water - two graphenes and carbon nanotubes of differing size - were compared and analyzed. The thermal conductivities of the nanofluids, formulated in the usual manner by adding graphene and carbon nanotube to distilled water and subjecting the mixture to ultrasonic dispersion, were measured by the transient hot-wire method, and the viscosities were determined using a rotational digital viscometer. As a result, we concluded that the nanofluid of small particle diameter of graphene have outstanding properties as heat transfer media, due to their excellent thermal conductivity and viscosity, compared with the other nanofluid.

시간경과, 교반 및 분산제 첨가에 따른 물-Al2O3나노유체 열전도도 변화에 관한 실험적 연구

A water-<TEX>$Al_2O_3$</TEX> nanofluid was manufactured, and its thermal conductivity was measured in this study. The measurement was performed at volumetric concentrations of 0.5%, 1%, 2%, and 3%, and the nanoparticle sizes were 20 nm and 70 nm. Experimental test equipment, using the transient hot wire method, was installed to measure the thermal conductivity of the nanofluid, and the measured results were confirmed by measuring pure water with a measurement error of 0.92% at <TEX>$20^{\circ}C$</TEX>. The thermal conductivity enhancement ranged from 4.8% to 13.6% for the 20 nm particle size, and from 3.1% to 8.8% for the 70 nm particle size at a concentration range of 0.5% to 3%. The enhancement increased with a decrease in particle size and an increase in concentration. With the elapse of time after manufacturing the nanofluid, the thermal conductivity enhancement decreased significantly from 5 to 9 h, and this trend was measured under all of the measurement conditions. After 24 h, the enhancement ranged from 1.2% to 3.5% for the 20 nm particles, and from 0.6% to 2.3% for the 70 nm particles. The enhancement trends with the elapse of time were almost identical with and without stirring the nanofluid. SDBS (Sodium Dodecyl Benzene Sulfonate) was added as a dispersing agent, and the decrease in the thermal conductivity enhancement was delayed.

Enhancement of thermal conductivity of silver nanofluid synthesized by a one-step method with the effect of polyvinylpyrrolidone on thermal behavior

A one-step chemical method has been developed for the preparation of stable, non-agglomerated silver nanofluids. Silver nanofluid is prepared by using silver nitrate as a source for silver nanoparticles, distilled water as a base fluid, and sodium borohydride and hydrazine as reducing agents by means of conventional heating using polyvinylpyrrolidone (PVP) as surfactant. This is an in situ, one-step method that seems to be valuable with high yield of product with less time consumption. The characterization of the nanofluid is done by particle size analyzer, X-ray diffraction topography, UV-visible absorption spectroscopy, and transmission electron microscopy followed by the study of thermal conductivity of nanofluid by the transient hot wire method. The results show that Ag-water nanofluids with low concentration of nanoparticles, i.e., below 1000 ppm, have noticeably higher thermal conductivities than the water base fluid without Ag. Moreover, the amount of PVP in silver nanofluid can have a significant effect on magnitude and behavior of the thermal conductivity enhancement.

Characterization and thermal conductivity of modified graphite/fatty acid eutectic/PMMA form-stable phase change material

We prepared and characterized a form-stable composite phase change material (PCM) with higher thermal conductivity. Capric acid(CA)-myristic acid(MA) eutectic as core, poly-methyl methacrylate (PMMA) as supportive matrix and modified graphite (MG) powders serving as the thermal conductance improver were blended by bulk-polymerization method. The composite PCMs with different MG mass fraction (2%, 5%, 7%, 10% and 15%) were characterized by FT-IR, SEM, DSC technique and mechanical tests. Thermal conductivities of the composites were measured by transient hot-wire method. The results indicate that MG powders have been successfully inserted into the CA-MA/PMMA matrix without any chemical reaction with each other. The MG/CA-MA/PMMA composites maintain good thermal storage performance while the thermal conductivity has been enhanced significantly. The composite PCM added with 15 wt% MG powders increases approximately as 195.9% in thermal conductivity. Moreover, the thermal conductivity improvement of the composite PCMs is also verified by the melting-freezing experiment, which is profitable for the heat transfer efficiency in latent heat thermal energy storage system.

Thermal conductivity of nanofluids containing high aspect ratio fillers

The effective thermal conductivity of three water based nanofluids (NFs) consisting of large aspect ratio fillers – carbon nanotubes (CNTs), silver nanowires, copper nanowires – were measured by transient hot wire method. The results show that silver nanofluid has the highest thermal conductivity compared with copper and CNTs nanofluids, while the latter two present almost the same thermal conductivity at the same volume fraction. The experiment indicates that particle shape has a substantial effect on the effective thermal conductivity of suspension and shape factor is one of the most important factors that leads to the large discrepancies among the experimental values of the thermal conductivities. Our results reveal that material with higher thermal conductivity is not a decisive factor and not always effective to improve the thermal transport properties of nanofluids.

Effects of various carbon nanofillers on the thermal conductivity and energy storage properties of paraffin-based nanocomposite phase change materials

The effects of adding various carbon nanofillers on the thermal conductivity and energy storage properties of paraffin-based nanocomposite phase change materials (PCMs) for thermal energy storage were investigated experimentally. These included short and long multi-walled carbon nanotubes, carbon nanofibers, and graphene nanoplatelets (GNPs). For each type of the nanofillers, nanocomposite PCM samples with mass concentrations of 1–5wt.% at an increment of 1wt.% were prepared. The thermal conductivity of the samples in solid phase was measured using the transient hot-wire method at elevated temperatures. The energy storage properties, including melting/solidification temperatures and enthalpies, were measured using a differential scanning calorimeter. It was shown that the presence of the nanofillers slightly decreases the phase change enthalpies and has negligible influence on the phase change temperatures. The thermal conductivity of the nanocomposite PCMs was found to increase with raising the loading, while the relative enhancement strongly depends on the size and shape of the nanofillers. Of the four types of carbon nanofillers examined, GNPs were observed to cause greatest relative enhancement up to 164% at the loading of 5wt.%, due to their two-dimensional planar structure that leads to reduced filler/matrix thermal interface resistance, in contrast to the moderately decreased energy storage capacity of GNP-based nanocomposite PCMs.

Measurement of Thermal Conductivity of Liquid Based on Transient Double-Hot Wire Technique

A new experimental apparatus has been developed for measuring the thermal conductivity of liquid by the two transient hot-wire method, which is based on the theory of one-dimensional unsteady heat conduction in infinite medium. The glass tubes, the heating line source and temperature measurement devices are designed in this experiment. As the temperature controller, low-temperature bath provides a constant water bath environment and current calibrator is the power source. Wheatstone bridge is used to test the relation between the temperature changes and resistance. Data acquisition and control unit is data acquisition devices of the Wheatstone bridge. As the temperature coefficient of platinum wire resistance is calibrated. It can be used to measure 32 groups of data from 10°C to 40°C at the every 10°C according to the experimental steps.

Transport and thermal properties of quaternary phosphonium ionic liquids and IoNanofluids

The transport and thermal properties of phosphonium ionic liquids (ILs) and thermally stable suspensions of phosphonium ILs and multiwalled carbon nanotubes (MWCNT) were determined. The thermal conductivity was measured over the temperature range T=(280 to 360)K, and at atmospheric pressure, using the transient hot-wire method with an accuracy estimated as 5% from (0.2 to 2)W·m−1·K−1 and less than ±0.01W·m−1K−1 from (0.02 to 0.2)W·m−1·K−1. The phosphonium ILs studied were trihexyltetradecylphosphonium phosphinate [(C6)3PC14)][Phosph], tretrabutylmethylphosphonium methylsulfate, [(C4)3PC1)][C1SO4], trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide, [(C6)3PC14][NTf2], and trihexyltetradecylphosphonium trispentafluoroethyltrifluorophosphate [(C6)3PC14][FAP]. For all ILs studied, the thermal conductivities were found to lie within the range (0.12 to 0.16)W·m−1·K−1. Our measurements combined with selected values from the open literature allowed us to determine a linear increase of combined properties (thermal conductivity, density and molar mass) with the increasing of molar mass. The addition of only (0.1 to 0.2)wt.% of MWCNT to ILs (IoNanofluids) revealed a slight increasing of the thermal conductivity from 0.5% to 1%. There was also observed a slight decrease of the thermal conductivity with increasing temperature for the ILs alone and respective IoNanofluids. The rheological behaviour of [(C6)3PC14)][Phosph], and of [(C6)3PC14][NTf2] based IoNanofluids was evaluated at T=(298.15 to 333.15)K and for shear rates in the range γ̇=(3 to 30)s−1. All the samples showed shear thinning behaviour. The thermal stability of the IoNanofluids compared to that of ILs alone was studied by High Resolution Thermogravimetric Analysis. It was found that pure [(C6)3PC14)][Phosph] is a thermally stable IL, whose thermal decomposition under the conditions of this study (2K·min−1) and starts above T=513K. It exhibits a one step thermogravimetric profile ([(C6)3PC14][NTf2] that was already investigated in a previous study). A small shift towards lower temperatures was observed in the thermogravimetric curves of IoNanofluids compared to those of the respective ILs alone indicating comparable thermal stabilities. The heat capacities determined by Modulated Differential Scanning Calorimetry within a temperature interval ranging from ca. (310 to 515)K with an estimated uncertainty of less than ±0.012J·K−1·g−1.