International Journal of Thermophysics最新文献

Viscosity is a thermophysical property of paramount importance, being essential for many scientific and industrial applications. The most common instruments for its measurement are glass capillary viscometers. Therefore, the use of capillary viscometers is widespread both in industry and in research. The range of viscosities of interest range from lower than that of water to several orders of magnitude higher values, the measurement of which requires different capillary viscometers. Most of the practical applications concern routine instruments, mainly for quality control. One main issue for the utilization of capillary viscometers relates to the need for their calibration, assuring its traceability to the water primary viscosity standard, to certify its worldwide validity. The present paper focuses on capillary instruments dedicated to perform viscosity measurements on Newtonian organic liquids at atmospheric pressure, as it is assumed that is the most widespread type of application for these viscometers. Capillary viscometry has a completely well-defined working equation, namely, the Hagen–Poiseuille equation. However, the practical performance of the measuring instruments deviates from that working equation. Most of those deviations are currently considered by many users. However, some of those deviations have not reached that status yet, like those concerning the effects due to the surface tension of the sample on the measurements. All these aspects are summarized and analyzed in the present article, together with a brief general description of the most common types of capillary viscometers, namely, the Ostwald and the constant-level or Ubbelohde instruments.

Understanding of thermophysical and transport properties of H₂-NG blends are needed for the gradual introduction of hydrogen into the national gas grid. A capillary tube viscometer was used to measure the viscosity of hydrogen + methane blends (with hydrogen mole fraction = 0, 0.1000, 0.1997, 0.5019, and 1) at temperatures from 213 to 324 K and pressures up to 31 MPa. A total 147 experimental viscosity measurements were made for the three H₂ + CH₄ blends and compared against the predictions of five different viscosity models: a one-reference corresponding states (Pedersen) model, a two-reference corresponding states (CS2) model, an extended corresponding states (ECS) model, a corresponding states model derived from molecular dynamic simulations of Lennard Jones (LJ) fluids, and a residual entropy scaling (SRES) method. All the model predictions showed a relatively low deviation compared to the measured viscosities. The density required for viscosity model predictions were computed using Multi-Fluid Helmholtz Energy Approximation (MFHEA) equations of state (EoS). To check the experimental procedure and applicability of the viscometer equipment, viscosity validation measurements were carried out for propane, hydrogen, and methane. The measured viscosities of the pure components were in good agreement with the respective viscosity models with AARD of 0.24%, 0.25%, and 0.58% for propane, hydrogen, and methane, respectively.

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This study employs a combined experimental and theoretical approach to investigate the thermophysical properties of eucalyptol (EC) blended with a series of 1-alkanols (ranging from 1-hexanol to 1-nonanol) across a temperature spectrum of 293.15–323.15 K. Density Functional Theory (DFT) calculations at the M05-2x/6-31g(d,p) level of theory are used to optimize the geometry of EC + 1-alkanols and provide insights into the hydrogen bonding interactions between the molecules. The DFT results reveal the significance of alkyl chain length in 1-alkanols on the hydrogen bonding with EC, which is supported by the analysis of geometrical, topological properties, vibrational frequency, NMR, and molecular orbital analysis. The theoretical findings are complemented by experimental measurements of density and viscosity, which show negative deviations from ideality in excess molar volume and viscosity. This study highlights the power of DFT methods in elucidating the molecular-level interactions governing the thermophysical properties of complex binary systems. Furthermore, the DFT results provide a molecular-level understanding of the observed thermophysical behavior, allowing for the development of more accurate predictive models. The integration of experimental and theoretical approaches in this study demonstrates a powerful framework for investigating the properties of complex mixtures.

The current article presents an exploration of the solid–liquid phase diagram for a binary system comprising n-alkanes with an odd number of carbon atoms, specifically n-nonane (n-C₉) and n-undecane (n-C₁₁). This binary system exhibits promising characteristics for application as a phase change material (PCM) in low-temperature thermal energy storage (TES), due to the fusion temperatures of the individual components, thereby motivating an in-depth investigation of the solid–liquid phase diagram of their mixtures. The n-nonane (n-C₉) + n-undecane (n-C₁₁) solid–liquid phase equilibrium study herein reported includes the construction of the phase diagram using Differential Scanning Calorimetry (DSC) data, complemented with Hot–Stage Microscopy (HSM) and low-temperature Raman Spectroscopy results. From the DSC analysis, both the temperature and the enthalpy of solid–solid and solid–liquid transitions were obtained. The binary system n-C₉ + n-C₁₁ has evidenced a congruent melting solid solution at low temperatures. In particular, the blend with a molar composition x_undecane = 0.10 shows to be a congruent melting solid solution with a melting point at 215.84 K and an enthalpy of fusion of 13.6 kJ·mol^–1. For this reason, this system has confirmed the initial signs to be a candidate with good potential to be applied as a PCM in low-temperature TES applications. This work aims not only to contribute to gather information on the solid–liquid phase equilibrium on the system n-C₉ + n-C₁₁, which presently are not available in the literature, but especially to obtain essential and practical information on the possibility to use this system as PCM at low temperatures. The solid–liquid phase diagram of the system n-C₉ + n-C₁₁ is being published for the first time, as far as the authors are aware.

Enhancement of the cooling and heating capabilities of an air conditioning unit (ACU) coupled with a thermal energy storage system of dual phase change materials (PCM) is investigated. The dual PCM, namely SP24E and SP11_gel, are coupled with the ACU outdoor device (condenser/evaporator) during the summer/winter seasons, respectively. Moreover, ACU performance assisted with dual-PCM heat exchanger is compared with a single heat exchanger of SP24E in summer and single heat exchanger of SP11_gel in winter at different PCM capsulation structures (aligned and staggered cylinders). The system dynamic mathematical model is computationally solved using ANSYS software and experimentally validated. Results affirm that charging/discharging periods are minimal for the dual-PCM system and slower for PCM inline cylinder layouts than staggered ones. Inline design yields greater ACU average power savings. In summer, higher inlet air temperature to the PCM system reduces PCM discharging time and ACU power savings, with the opposite effect during winter. ACU COP with PCMs is improved by around 80 % in summer and 40 % in winter, respectively, compared to ACU without PCMs. The maximum average power saving over 4 h of ACU working in summer by single and dual-PCM systems is 21.4 % and 11.8 %, respectively, whereas the results in winter are 18.5 % and 12.8 %, respectively.

The thermophysical properties of molten salts promising for the nuclear industry are crucial, but the available data are limited and contradictory. The thermal diffusivity of the molten mixtures (NaF-KF)_eut–UF₄ containing 30, 40, and 50 mol % UF₄ was measured by the laser flash method. The thermal diffusivity was found to be almost independent of temperature in the range of about 100° (1070 K–1170 K), which makes it possible to extrapolate values to low or high temperatures. The thermal conductivity of the molten mixtures (NaF-KF)_eut–UF₄ was calculated using the thermal diffusivity, density, and heat capacity data. Density was estimated using the molar volume of the molten binary systems NaF-UF₄ and KF-UF₄. The heat capacity of the ternary system NaF-KF-UF₄ was evaluated based on the heat capacity of individual salts, taking into account the additivity law. The thermal conductivity of the molten mixtures 0.7(NaF-KF)_eut–0.3UF₄, 0.6(NaF-KF)_eut–0.4UF₄, and 0.5(NaF-KF)_eut–0.5UF₄ changes slightly with the UF₄ content and temperature; for example, at 1123 K it is 0.48, 0.45, and 0.45 W·m⁻¹·K⁻¹, respectively. The thermal diffusivity of the pure molten UF₄ was estimated using three independent approaches: the concentration, the temperature, and the volume dependences of the thermal diffusivity. The estimated value of the thermal diffusivity for the molten UF₄ was about 0.12 × 10^–6 ± 0.05 × 10^–6 m²·s⁻¹.

There is a paradox involved in the operation of photovoltaic (PV) systems; although sunlight is critical for PV systems to produce electricity, it also elevates the operating temperature of the panels. This excess heat reduces both the lifespan and efficiency of the system. The temperature rise of the PV system can be curbed by the implementation of various cooling strategies. These strategies fall under three categories: passive, active, and hybrid cooling, with similar objectives of regulating excess heat generation. Employing heat pipes can be an example of the passive method, while the use of forced circulation of water flow can represent an active method. A combination of energy storage and forced convection represents an example of hybrid cooling. Most of the research has two objectives, one to obtain higher PV efficiency and another to enhance the life span of the system. This review explores various cooling strategies employed by the researchers i.e., heat pipes, heat sink, air or water channels, water spray, use of phase change material, microchannel for coolant passage, thermoelectric (Peltier) modules. In general, for passive cooling techniques, efficiency enhancement of up to 44.12 % was obtained due to the temperature reduction of around 11 °C. In the case of active cooling techniques reported better performance with PV temperature reduction as high as 55 °C. Hybrid cooling also leads to some promising performance improvements. Characteristics and performance of various cooling methods are explained in this review to provide future researchers with valuable insight and direction. This could lead to much better improvements in these cooling techniques in the near future.

Mass diffusion coefficient measurement techniques with high temporal and spatial resolution have become essential for the research and development of leading-edge technology in a wide range of cross-disciplinary fields, but cannot be achieved using conventional methods. We provide a comprehensive review of the state-of-the-art theoretical and experimental investigations on Soret forced Rayleigh scattering (SFRS), a grating excitation technique (GET) for measuring the mass diffusion coefficient of binary liquid mixtures. SFRS utilizes the Soret effect to create micrometer-order periodic spatial concentration modulation in a sample due to the absorption of an optical interference grating generated by two intersecting heating laser beams. The decay of the concentration modulation by the mass diffusion process within several milliseconds is detected by the diffraction of a probing beam. The theoretical considerations regarding deviations from the ideal mass diffusion conditions are the effects of: (1) the Gaussian beam intensity distribution, (2) the light absorbing material and (3) the cell wall. The proper settings for the optical system are also analyzed, e.g., the effect of coherency and polarization of the heating laser and the effect of the z-direction length of the interference region. We also consider the frame of reference, center of gravity invariance and effect of convection, which are particularly important for mass diffusion experiments. Using the correct implementation of the theory, the optimal SFRS apparatus design and its appropriate use are described in detail. Finally, two successful applications of SFRS are demonstrated using visible light laser heating and mid-wavelength infrared gas laser heating.

The eutectic mixtures consist of dl-menthol and acetic acid were prepared at five molar ratios (3:1, 2:1, 1:1, 1:2, 1:3). The vibrating-tube densimeter was used to measure the density of the DL-menthol/acetic acid mixtures, and the measurements were carried out from 293.15 K to 363.15 K and pressures from 0.1 MPa to 70 MPa. The measured densities of each dl-menthol/acetic acid mixture at different temperature and pressure were correlated by the Tait equation. In addition, the derived properties of dl-menthol/acetic acid mixtures including isothermal compressibility, isobaric thermal expansivity, and internal pressure were calculated. The effects of temperature, pressure, and molar ratios on the derived properties were compared and analyzed.

Measurements of the thermal conductivity were performed as a function of gas pressure from 10^–1 hPa up to 10⁵ hPa on several bimodal silica xerogels. The xerogels exhibit a mesopore and a macropore phase. The measurements were done using a hot-wire apparatus, which can do automated, gas pressure-dependent measurements of the thermal conductivity from 10^–3 up to 10⁵ hPa. Results were fitted with a bimodal gas pressure-dependent thermal conductivity model to gain information on the thermal conductivity of the materials, its various contributions and on structural parameters such as the two main pore sizes, the macro- and mesoporosities. The pore sizes and porosities were compared to values gained from mercury porosimetry and nitrogen adsorption measurements. The porosities from the thermal conductivity measurements are in very good agreement to the other measuring methods. The macropore sizes from the thermal conductivity measurements are mostly in agreement within the given uncertainty range and the mesopore sizes show a good estimate of the order of magnitude of the pores.