International Journal of Thermophysics最新文献

Reliable density measurements of molten metals are crucial in understanding the evolution and formation of their phase structures. However, they are very challenging with container methods due to the possible parasitic reactions, wetting and meniscus formation, biased volume estimates, etc. Electrostatic levitation (ESL) is a containerless technique that alleviates these limitations while keeping the molten metals close to spheres for accurate volume measurements. This paper uses image analysis techniques to perform single-view bidirectional diameter extraction of non-ideal spherical specimens in ESL through ellipse fitting and Legendre polynomial fitting. More importantly, the proposed single-camera single-view workflow can reach an accuracy level comparable to dual-camera 3D reconstruction approaches reported in the literature, while requiring only one optical access. After image inversion, edge detection, and contour extraction, the droplet contour is fitted using two models: an ellipse and a Legendre polynomial curve. From these fits, the horizontal and vertical diameters are obtained and used to estimate the droplet volume. To make the workflow practical for large datasets, we implemented the entire pipeline in code and developed a fully automated batch program. It processes all images in a folder without manual intervention, generates annotated output images for each frame, and exports the key measurement results to an Excel file. Tests on heating–cooling images of levitated zirconium show that both fitting methods work reliably, while ellipse fitting is typically more stable when the contour is clean and continuous. Overall, the proposed approach improves the efficiency and consistency of diameter/volume extraction and supports high-throughput analysis for thermophysical property characterization.

The density and viscosity of binary mixtures of triethanolamine (TEA) and glycerol were investigated over the full composition range at temperatures from 293.15 K to 323.15 K, under atmospheric pressure. The experimentally measured density and viscosity data were correlated with temperature-dependent equations. The excess molar volume (V^E) and viscosity deviation (Δη) were determined and fitted using the Redlich–Kister polynomial equation. In addition, thermodynamic parameters, including partial molar volumes, apparent molar volumes, and thermal expansion coefficients, were evaluated to provide further insight into the mixing behavior of the system. Negative values of V^E and Δη were observed over the entire range of temperatures and compositions investigated, indicating the presence of strong specific interactions between TEA and glycerol molecules. These interactions were further elucidated through Density Functional Theory (DFT) calculations. The computational results are consistent with the experimental observations, providing molecular-level support for the non-ideal volumetric and viscosity behavior of the mixtures.

This study investigates the thermophysical characteristics of binary mixtures involving the low-global warming potential refrigerant 3,3,3-trifluoropropene (R-1243zf) and two polyol ester (POE) lubricants (RL 32 and RL 68). Experimental measurements of solubility, liquid density, and dynamic viscosity were conducted over a temperature range of 298–353 K. These outcomes imply that R-1243zf exhibits complete miscibility with both POE oils across the studied conditions, with higher solubility observed in POE RL 32 compared to POE RL 68. The dissolution of R-1243zf significantly reduces the flow characteristics of the lubricants, particularly in the oil-rich phase, while its effect on liquid density is relatively minor. A satisfactory correlation of the phase equilibrium data was achieved with the non-random two-liquid model, and mixture densities and viscosities were accurately represented using an excess-property approach combined with Redlich–Kister expansions. Additionally, Daniel charts were constructed to illustrate the viscosity–pressure–temperature–concentration relationships for both mixtures, providing practical guidance for the selection of lubricating oils in R-1243zf-based refrigeration systems. The findings suggest that POE RL 68 offers better viscosity retention under high-temperature conditions, making it more suitable for severe operating environments.

Microstructures and thermal properties of solid alloys of the Bi–Ge system with a Ge content of 15.1 at.%, 40.8 at.%, 51.8 at.%, 70.3 at.%, and 85.4 at.% were investigated in the present study. Microstructural analysis was performed using scanning electron microscopy (SEM) combined with energy dispersive spectrometry (EDS). It was noticed that phase morphology of the primary germanium varies based on alloy composition from polygonal faceted to plate- and rod-like structures. Thermal diffusivity was measured by the xenon flash method in the temperature interval from 25 °C to 150 °C. The nearly constant thermal conductivity in the range of about 7 W·m⁻¹·K⁻¹ to 9 W·m⁻¹·K⁻¹ was observed over a wide compositional interval of Bi–Ge alloys, followed by its rapid increase with higher Ge content. Thermal conductivity of the studied alloys slightly decrease with the temperature increasing. Density dependence on composition at room temperature was determined using indirect Archimedean method. The obtained results from the alloy’s density measurements indicate the existence of a positive excess volume. Phase transition temperatures and latent heat of eutectic melting were measured using differential scanning calorimetry (DSC) and compared with the results of thermodynamic calculation based on the CALPHAD (calculation of phase diagram) method. Empirical equation for the estimation of the latent heat for Bi–Ge alloys was obtained.

Multiparameter equations of state (MP EOSs) are available for many fluids. Despite their accuracy, they are not suitable for all applications. In contrast to MP EOSs, cubic equations offer a Van der Waals loop with a single inflection, which is suitable for modeling phase interfaces. They are also amiable for modeling mixtures. We present a method of tailoring a cubic equation of state to a MP EOS. Four temperature-dependent parameters of the previously developed Generalized 4-Parameter Cubic Equation of State (G4C EOS) are matched to the second virial coefficient, liquid density, compressibility, and Gibbs energy. Consequently, the G4C EOS is more accurate than conventional cubic equations of state. The liquid properties are matched along a “sew-on line”, which coincides with the saturated liquid density at lower reduced temperatures and bypasses the critical region. The method is tested for argon, propane, and water. The range of temperatures in which the parameters of G4C EOS can be determined (feasibility interval) covers the validity range of the reference MP EOS with exception for water, where the high-temperature limit is 754 K or 909 K depending on the variant of data used for the second virial coefficient. Parameters of the G4C EOS and ideal gas properties are tabulated in the Supplementary Information and an interpolation scheme is provided. All thermodynamic properties can be computed for the tested fluids with provided relations and tables. Since the method has been successfully applied to three fluids with very different molecular interactions, we assume that it is applicable to a wide range of fluids. It can also be used to other equations of state with four temperature-dependent parameters.

Proton exchange membrane fuel cell (PEMFC) faces inherent trade-offs among efficiency, durability, and stability, especially under wide operating conditions. Achieving multi-objective collaborative optimization under wide operating conditions remains a key engineering challenge. To address the above challenges, this study investigates the effects of temperature and back pressure on performance through electrochemical impedance spectroscopy (EIS), electrode kinetics, and entropy generation analysis. A physics-based model is developed to support fault diagnosis and durability assessment. A performance trade-off analysis is conducted from the perspectives of power output, energy consumption, and operational stability. From an electrochemical perspective, temperature primarily affects the intrinsic exchange current density and the transport activity of water/protons, while back pressure mainly regulates the partial pressure of reactants and mass transfer processes. Further systematic investigation of exergy losses inside the PEMFC from the perspective of irreversible thermodynamics reveals that the cathode, anode, and membrane are the primary sources of these irreversible losses. Moreover, entropy production varies exponentially with temperature and exhibits a linear dependence on pressure. All these provide mechanistic foundations for performance optimization strategies in PEMFC engineering.

Hydrophobic wax is commonly used to enhance the dimensional stability of wood; however, its influence on the thermal and combustion behavior of wood under controlled heat flux conditions remains insufficiently understood. This study investigates the ignition, mass-loss characteristics, and carbonization behavior of pine wood pressure impregnated with paraffin and polyethylene (PE) waxes using a mass loss calorimeter at 50 kW·m⁻². Scanning electron microscopy (SEM) was employed to examine wax penetration and structural degradation before and after combustion. PE wax-treated specimens exhibited the longest time to ignition and the lowest mass loss, indicating superior thermal stability compared to untreated and paraffin wax-treated wood. Wax-filled lumens burned off prior to cell wall degradation, allowing the wood matrix to retain its structural morphology during combustion. Surface cracking and carbonization depth were significantly reduced in wax-treated specimens, particularly those treated with PE wax. These findings demonstrate that wax modification can delay thermal decomposition and improve combustion resistance, suggesting the potential of wax-treated timber as a bio-based material with improved combustion resistance for sustainable building applications.

Building Energy Simulation (BES) typically represents heat transfer through building envelopes using one-dimensional (1D) conduction models to maintain computational efficiency. However, this simplification prevents BES from capturing the dynamic thermal effects produced by thermal bridges (TBs), particularly in high-performance envelopes where localized heat-flow paths significantly influence transient behavior. Existing approaches such as equivalent U-value methods overlook dynamic inertia effects, while detailed multi-dimensional simulations provide high fidelity but are too computationally demanding for whole-building analysis. To address this gap, this study develops a Thermal Bridge Transfer Function Model (TBTFM) that expresses transient TB heat flow in a compact linear time-invariant (LTI) form compatible with standard 1D BES frameworks. The model is constructed using system identification techniques applied to detailed 3D transient simulation data, enabling the dynamic influence of thermal bridges to be incorporated without modifying existing BES structures. Validation shows that the proposed TBTFM accurately reproduces TB heat-flux profiles with varying model orders, and that a third-order representation achieves an effective balance between accuracy and computational efficiency. The proposed approach provides a practical means to quantify the dynamic contribution of thermal bridges during transient periods, offering improved fidelity for building-energy assessments while preserving the simplicity of 1D BES workflows.

Ceramic matrix composite phase change materials suffer from inherently low thermal conductivity due to solid particle contact limitations in ceramic skeleton, restricting concentrated solar energy storage applications. This study presents a bioinspired heterostructured composite that emulates the hierarchical sieve tube structures in Bombax ceiba vascular bundles to create anisotropic phonon transport channels. Integrating directionally aligned silicon carbide fibers within a porous SiC matrix via centrifugal flow-assisted alignment achieves three-dimensional anisotropic thermal conductivity. This design bypasses the high thermal resistance of sintered particle junctions using biomimetic "sieve plate-like" fiber networks. Elongated SiC fibers act as unidirectional heat conduits, minimizing phonon scattering at grain boundaries. At the expense of 9.4% total energy storage capacity, this anisotropic CPCM significantly optimized equipment performance in the required heat transfer direction. Compared to pure sodium acetate trihydrate and ordinary CPCM, the thermal conductivity in the expected direction improved by 281.19 % and 29.63 %, respectively. The energy storage rate increased by 60.54 % and 36.6 %. The maximum temperature difference in the heat transfer direction decreased by 50.39 % and 31.75 %, and the junction temperature difference reaching the working limit was reduced by 66.83 % and 52.18 %. Therefore, this CPCM efficiently and stably realized thermal energy storage, providing a research basis for building and distributed energy storage.

Fuel blends incorporating oxygenated additives are increasingly explored to enhance combustion efficiency and reduce greenhouse gas emissions. Understanding the thermodynamic behavior of such mixtures is essential for optimizing their formulation. In this study, the excess molar enthalpy (({H}_{m}^{E})) a key property reflecting molecular interactions and non-ideality was measured for four ternary blends containing 2-(2-methoxyethoxy)ethanol, 2-(2-ethoxyethoxy)ethanol, 2-methoxyethanol, and 2-phenoxyethanol, each mixed with ethanol, at 298.15 and 313.15 K under 0.1 MPa using a quasi-isothermal flow calorimeter. The experimental results were correlated using the Redlich–Kister, NRTL, and UNIQUAC models, while the predictive performance of the Modified UNIFAC (Dortmund) model was also assessed. Positive ({H}_{m}^{E}) values were obtained for all mixtures, indicating endothermic mixing and dominant dispersive–dipolar interactions. Among the applied models, the Redlich–Kister equation provided the best correlation with experimental data. The results contribute valuable thermodynamic benchmarks for modeling the energetics of oxygenated fuel blends and improving predictive approaches for complex liquid mixtures.