International Journal of Thermophysics最新文献

Inadequate adhesion between the multi-walled carbon nanotube (MWCNT) and the substrate's surface, which will raise the intermediate obstruction, is reported to be a key issue for MWCNT coatings over metallic substrates in the published literature. By utilizing an intermediary layer, the adherence between the metallic substance and the CNT may be strengthened. Emphasis on boiling pools of micro-nano-porous (nanopores on micropores) coverings, particularly MWCNTs on micropores, is currently limited. Two nanocomposites (Cu–Cu) intermediate layers were deposited between the CNTs and a foundation polished metal surface in the current study to increase the bonding between the CNTs and the Cu foundation. Moreover, a three-stage sintering process is used to improve the adhesion between the Cu–Cu–MWCNTs layer and the metallic substrate. The pool boiling of DI water was experimentally investigated with respect to heat transport, bubble behavior, and critical heat flux. The Cu–Cu–MWCNTs-coated substrate achieved the highest heat transfer augmentation and critical heat flux of 374 % and 116 %, respectively, in comparison to a smooth bare surface. With the surface coated with Cu–Cu–MWCNTs, the early signs of nucleate boiling were seen. Highest critical heat flux for the Cu–Cu–MWCNTs-coated substrate was achieved by delayed dryout owing to better rewetting nature of the drier area beneath the created vapor bubble.

A methodology for evaluating experimental uncertainty is presented. Based on the Guide to the Expression of Uncertainty in Measurement (GUM) in conjunction with a sensitivity analysis, this method readily applies to systems of various degrees of complexity. It consists of three steps: (1) to estimate each uncertainty contribution of the system based on GUM; (2) to determine the sensitivity of the calculated results to variations in each of the input measurands in turn, replacing the partial derivatives of the GUM with a purely numerical approach; and (3) to calculate the overall uncertainty using the error propagation principle. Furthermore, the calculated sensitivity coefficients enable a critical evaluation of the investigated system, allowing the detection of possible targeted improvements. For this reason, the presented method is called “the sensitivity analysis method.” This is applied to three case studies with increasing complexity: a mass calibration procedure, a volume calibration procedure, and a gravimetric densimeter characterized by a multi-parameter nonlinear measuring model. When possible, the results are compared to the GUM uncertainty framework or values available in the literature.

Vacuum insulation panel (VIP) is becoming the main resource of thermal insulation material, which has been widely applied in the recent decades. Varies of core material and sealing method have been developed for VIP applications, typically achieving thermal conductivity below 5 mW/mK. To further decrease the thermal conductivity of VIP, the in-depth understanding of the heat transfer mechanism via different components (e.g. core material conduction, gaseous conduction and thermal radiation) is highly necessary. There are reported experimental and modeling works focusing on individual components involved in VIP, but a comprehensive and summative study combining different factors is still missing. Furthermore, most of researches on VIP study the long-term performance evolvement, while initial conductivity, especially in ultralow conductivity region, is less studied but more scientifically interesting. In this work, the existing works on the analytic model of VIP are reviewed and the quantitative comparison between the different contributions to the overall thermal conductivity is presented. This work aims at the low initial conductivity condition and discusses the possible technical routes to further decrease the initial conductivity. The tools provided here will contribute to the future VIP design using novel core materials and manufacturing techniques, to achieve ultralow thermal conductivity.

Surface tension of metals is a data of interest for the simulation of welding or additive manufacturing. In this regard, surface tension of three steels has been measured with an experimental device of aerodynamic levitation with the well-known oscillating drop method (observation of the resonance frequency of the drop). Steel can be very sensitive to evaporation that occurs above melting point which can lead to the modification of the chemical composition and thus of the thermophysical properties. One option to limit evaporation is to reduce the duration of the experiment. In this perspective, a new acoustic method has been tested, which consists in exciting the sample with a frequency close to the resonance frequency for a fraction of second and then observing the frequency naturally adopted by the drop during a short relaxation time. This reduces the time of the experiment to less than 1 s, against about 10 s with the frequency sweep method previously used. Both methods are used and discussed in this article. The solicitation-relaxation method is found to significantly reduce the evaporation and thus provides more consistent results at high temperatures. For the steel on which this new method has been tested, the characteristic increasing–decreasing surface tension with temperature has been observed, which can have an impact on the melt pool dynamic in welding or additive manufacturing and should be considered in numerical simulation for better results.

The speed of sound of liquid squalane is measured with the double-path pulse-echo technique, utilizing a piezoelectric quartz (8 MHz) positioned between two reflectors with distinct path lengths. Calibration of the apparatus is carried out with water, for which highly accurate reference data exist. The present experiments with squalane cover the temperature range from 298.15 K to 493.15 K and a pressure from 0.1 MPa to 20 MPa. The total relative expanded ((k = 2)) uncertainty (U_r(w)) for the speed of sound is estimated to be ±0.1 %. The present data are consistent with literature values, which are, however, only available at ambient pressure. To validate the present speed of sound measurements, the density and isobaric heat capacity are integrated numerically from the sampled data over most of the measurement range, employing rigorous thermodynamic identities. With deviations of about 0.1 %, the resulting density data are favorably compared with literature values. The resulting isobaric heat capacity, for which just a single reference with data at pressures above the ambient exists, deviates by up to 6 %.

The use of hybrid nanofluids is seen as a rarely studied approach in terms of thermal efficiency and still worth investigating. In this article, the effects of ZnO + Pure Water nanofluid and hybrid nanofluid ZnO + CuO + Pure Water nanofluid, used as coolant fluid in a commercial automobile radiator, on radiator cooling performance were experimentally investigated. In addition to this investigation, the effects of using several types of vehicle front grilles on cooling performance were also experimentally examined. In the study, pure water tests used for validation were first conducted, and the prepared nanofluids were tested respectively. The fluid inlet temperature to the radiator was 70 °C, the air inlet speed was 6 m·s⁻¹ to 8 m·s⁻¹ to 10 m·s⁻¹, and the fluid flow rate was 17 L·min⁻¹ to 19 L·min⁻¹ to 21 L·min⁻¹. The fluid concentrations used in the tests were as follows: 100  % pure water, pure water-based nanofluid containing ZnO particles at 0.3 % concentration, and hybrid nanofluid containing 0.15 % ZnO and 0.15 % CuO nanoparticles. At the end of the tests, the cooling performance was calculated by measuring the flow rate, pressure, speed, and temperatures of different coolant fluids and air, with the highest cooling performance achieved in the hybrid nanofluid with a 52 % increase. In addition to using this nanofluid, the effects of using front grilles with decreasing, increasing, and constant cross-sections toward the center on cooling performance were also examined, and the cooling performance was increased by up to 66.5 % by finding the optimum front grille geometry.

Experimental densities and sound speeds at temperatures (293.15, 303.15, 313.15, and 323.15) K and under atmospheric pressure are reported for glycerol + aniline, glycerol + pyridine, and glycerol + piperidine mixtures covering the entire composition ranges. Excess molar volumes and excess isentropic compressibilities were derived from experimental data and correlated using Redlich–Kister polynomial to test the quality of experimental data. Excess properties could be interpreted by considering the differences in molecular structure and hydrogen bonding capacity of the amines. This study utilized the Jouyban–Acree (J–A) model to capture both the compositional and temperature dependencies of mixture properties (density, sound speed, and their related properties, isobaric thermal expansivity, and isentropic compressibility). The average absolute percentage deviation of the correlated values from the experimental ones was better than 0.05 %, 0.011 %, 0.025 %, and 0.023 % for density, sound speed, isobaric thermal expansivity, and isentropic compressibility, respectively, attesting to the robustness of the J–A model to predict mixing behavior under varying conditions. The Perturbed Chain Statistical Associating Fluid Theory Equation of State (PC-SAFT EoS) was used to model the densities of the mixtures using a predictive approach. Schaaff's Collision Factor Theory (SCFT) and Nomoto's Relation (NR) were compared for their ability to model the speed of sound for the binary mixtures. The modeled densities showed good agreement with experimental data. NR outperformed SCFT in modeling speed of sound of current mixtures. The previous models (SCFT and NR) were coupled with PC-SAFT, because they require the input of liquid density data.

This paper considers the necessary conditions for using a derivative of a transient hot-wire (termed a needle probe) approach to measure the thermal conductivity of electrically conducting fluids at high temperatures, especially molten halide salts. The focus is on the development of a new theory based on a multi-layer system necessary to ensure electrical isolation of electrical wires from the surrounding fluid. This includes the use of a thin annulus of fluid to minimize convective heat transfer modes within the fluid of interest, which was inspired by the concentric cylinder method. Good measurements require the following considerations: concentricity of the probe and surrounding crucible to ensure a consistent fluid gap, accounting for corrections for deviation of the model at early times, and modeling radiation heat transfer through transparent fluids. Uncertainties are larger than transient hot-wire methods because of the deviations from experimental conditions that can easily match an analytical approximation. An appropriate estimation of the measurement uncertainty can be obtained through careful design of the instrumentation, thorough uncertainty analysis, and limiting the measurements to only the applicable thermal property ranges of the approach. The 1D model used to interpret measured temperature data has been shown to be reliable for thermal conductivity measurements ranging from at least 0.39 W (mK⁻¹) to 0.92 W (mK⁻¹) and for temperatures from 293 K to 1023 K. The approach is used to present thermal conductivity data of the molten salts NaCl–KCl (51–49 mol%) and LiCl–NaCl (72–28 mol%).

One concept for the safe storage and transport of molecular hydrogen (H₂) is the use of hydrogen carrier systems which can bind and release hydrogen in repeating cycles. In this context, the liquid system based on isopropanol and its dehydrogenated counterpart acetone is particularly interesting for applications in direct isopropanol fuel cells that are operated with an excess of water. For a comprehensive characterization of diluted aqueous solutions of isopropanol or acetone with technically relevant solute amount fractions between 0.02 and 0.08, their liquid density, liquid viscosity, and interfacial tension were investigated using various light scattering and conventional techniques as well as equilibrium molecular dynamics (EMD) simulations between (283 and 403) K. Polarization-difference Raman spectroscopy (PDRS) was used to monitor the liquid-phase composition during surface light scattering (SLS) experiments on viscosity and interfacial tension. For comparison purposes and to expand the database, capillary viscometry and dynamic light scattering (DLS) from bulk fluids with dispersed particles were also applied to determine the viscosity while the pendant-drop (PD) method allowed access to the interfacial tension. By adding isopropanol or acetone to water, density and, in particular, interfacial tension decrease significantly, while viscosity shows a pronounced increase. The behavior of viscosity and interfacial tension is closely related to the strong hydrogen bonding between the unlike mixture components and the pronounced enrichment of both solutes at the vapor–liquid interface, as revealed by EMD simulations. For an aqueous solution with an isopropanol amount fraction of 0.04, minor variations in interfacial tension and viscosity were found in the presence of pressurized H₂ up to 7.5 MPa. Overall, the results from this study contribute to an extended database for diluted aqueous solutions of isopropanol or acetone, especially at temperatures above 323 K.

Nickel-titanium (NiTi)-based shape-memory alloys (SMAs) have various applications in biomedicine, actuators, aerospace technologies, and elastocaloric devices, due to their shape-memory and super-elastic effects. These effects are induced in SMAs by a reversible martensitic phase transformation. This transformation can be achieved by applying stress or temperature differences. It is extremely difficult to measure the temperature of NiTi elements with contact thermometers because they disturb the phase transformation phenomenon. An alternative approach is contactless infrared thermography for which accurate emissivity data are mandatory. To determine the temperature distribution in NiTi components with an infrared camera, a newly constructed apparatus is presented to measure the emissivity of metals. For this purpose, a state-of-the-art infrared camera was employed to analyze the emissivity behavior with temperature, especially during the phase transformation. The emissivity of the NiTi samples was systematically studied by comparing them with a reference-quality black body cavity (BBC) having an emissivity better than 0.995. Calibration measurements revealed that the maximum deviation of the BBC temperature measured with the infrared camera was only 0.15% (0.45 K) from its temperature measured with the built-in contact thermometers. The apparatus was validated by measuring the emissivity of a polished aluminum sample and found to be in good agreement with literature data, particularly for temperatures above 340 K. Finally, the emissivity of two rough and one polished NiTi samples was measured covering the temperature range from 313 K to 423 K with an expanded uncertainty of about 0.02 ((k=2)). The studied NiTi samples have an atomic percent composition of (hbox {Ni}_{42.5}) (hbox {Ti}_{49.9}) (hbox {Cu}_{7.5}) (hbox {Cr}_{0.1}). We observed that the emissivity of NiTi varies between 0.17 and 0.31 depending on temperature. As the temperature rises, the emissivity increases rapidly during the phase transformation, and it decreases gradually beyond the austenite finish temperature. In addition, the emissivity of NiTi depends on the microstructure of the martensite and austenite phases and the surface roughness of the samples.