International Journal of Thermophysics最新文献

The research aimed to explore the thermal performance of a miniature hexagonal tube heat sink (MHTHS) by utilizing three different binary nanofluids. These nanofluids incorporated nanoparticles such as MgO, Al₂O₃, and CuO, dispersed in base fluids of de-ionized water (DIW) (80 %) and ethylene glycol (EG) (20 %) at different concentrations (0.5 vol %, 1.0 vol %, 1.5 vol %, and 2.0 vol %). Variations in volume flow rate (VFR) and temperature spanned from 10L/h to 50L/h and 10 °C to 50 °C, respectively. Throughout the study, nanofluids circulated through the hexagonal tube side (HTS) at VFR ranging from 10L/h to 50L/h, while hot DIW flowed through the mini passage (MPS) at a constant VFR of 30L/h. Notably, CuO–DIW/EG nanofluid exhibited an 8.7 % increase in density, and MgO–DIW/EG nanofluids demonstrated a 14 % increase in thermal conductivity at a particle concentration of 2.0 vol %. However, at a higher particle concentration of 2.0 vol %, MgO–DIW/EG nanofluids exhibited a 5.6 % decrease in specific heat. Furthermore, MgO–DIW/EG nanofluids displayed a 79.6 % increase in heat transfer coefficient and a 66.7 % increase in Nusselt number. Although the pumping power and friction factor showed 5.1 % to 20.4 % and 7.5 % increases in particle concentration and Reynolds number, this negative impact did not affect the overall thermal performance of the heat sink. Finally, the study determined that MgO–DIW/EG nanofluid stands out as the most suitable heat transfer fluid for the heat sink.

In this study, a technique to estimate the thermal properties of a suspended copper wire using the 3ω method was proposed and its operation was demonstrated. This approach used a digital multimeter with a large measurement buffer to implement a procedure in which an appropriate window function and a discrete Fourier transform (DFT) were applied. This significantly reduces the noise level to a few nV, especially in the lower-frequency regions (less than 1 Hz), even if a longer measurement time is required. The third-harmonic voltage signal containing the thermal properties information for the 3ω method was clearly observed with a high signal-to-noise (S/N) ratio, and the thermal conductivity and diffusivity were estimated from 60 K to 300 K from the current frequency dependence of the third-harmonic voltage. The thermal conductivities of the copper wire were determined to be 423.0 and 385.9 W/mK at 100 and 300 K, respectively. Specific heat was calculated from thermal conductivity and diffusivity, and the Debye temperature was estimated to be 346 K from the temperature dependence of the specific heat. These values were in good agreement with previous research. Measurement of thermal properties using a digital lock-in amplifier with identical configuration resulted in overestimation of thermal conductivity in entire temperature regions owing to a low S/N ratio throughout the analysis of the DFT. The technique based on DFT with a window function is therefore more reliable for detecting the third-harmonic voltage in the low-frequency region of less than 1 Hz because it obtains a high S/N ratio.

We present hybrid predictive-correlative engineering correlations for the calculation of the viscosity and thermal conductivity of tetrahydrofuran (THF) in the fluid phase. They incorporate critically evaluated experimental data where available, and predictive methods in regions where there are no data and can be applied over the gas, liquid, and supercritical phases. The viscosity correlation is validated from 195 K to 353 K, and up to 30 MPa pressure, while the thermal conductivity is validated in the temperature range 174 K to 332 K, and up to 110 MPa pressure. Both correlations are designed to be used with a recently published equation of state that extends from the triple point to 550 K, at pressures up to 600 MPa. The estimated uncertainty (at a 95 % confidence level) for the viscosity is 10 % for the low-density gas (up to atmospheric pressure), and 6 % for the liquid at temperatures up to 353 K and pressures up to 30 MPa. For thermal conductivity, the expanded uncertainty is estimated to be 15 % for the low-density gas, and 2 % for the liquid phase from the triple-point temperature to 330 K at pressures up to 15 MPa, rising to 4 % at 110 MPa. Due to the extremely limited data available, these correlations should be considered preliminary until further experimental data become available.

In this study, the solubilities of hydrogen (H₂), nitrogen (N₂), and carbon dioxide (CO₂) in the eutectic mixture of diphenylmethane (1) and biphenyl (2) with mass fraction w₁ = 0.64947 were determined by the isochoric saturation method at pressures ranging from 0.918 MPa to 6.208 MPa, and temperatures ranging from 293 K to 363 K. The results indicate that the solubilities of the gases in the eutectic mixture follow the order CO₂ > H₂ ≈ N₂. The gas solubility data were correlated using the Krichevsky-Kasarnovsky (K-K) equation. The absolute average relative deviations (AADs) of the experimental values from the calculated data for the H₂ + eutectic mixture, N₂ + eutectic mixture, and CO₂ + eutectic mixture systems were 1.98 %, 1.74 %, and 1.74 %, respectively. Henry’s constants for the dissolution of the three gases in the eutectic mixture at different temperatures were calculated. Finally, thermodynamic parameters (the solution enthalpy, solution Gibbs free energy, solution entropy, and solution specific heat capacity) were calculated and discussed.

The theory and application of the vibrating-wire technique for the measurement of viscosity, as well as both viscosity and density, are reviewed. Theory is presented in the form of practical working equations and well-established limitations on their ranges of validity. The cases of both transient and steady-state excitation of the vibrating wire are considered in detail. For the steady-state mode, we describe a variant of the method in which the density is also measured. Practical details including wire materials, magnet systems and instrumentation are discussed, and several design examples from the literature are reviewed. Relative uncertainties in vibrating-wire viscometry vary from, at best, 0.2 % to about 2 % at 95 % confidence. In an appropriately designed instrument, density can be measured simultaneously with a relative uncertainty of about 0.2 %.

Single-isotherm n(p, T₉₀) results are reported for the gases Ar, N₂, H₂O, and D₂O at vacuum wavelength (lambda = 1542.383(1)) nm. The argon and nitrogen isotherms were measured near 303 K; the water isotherms were measured near 373 K. Combined with the two previous articles of this series, the present results beget several insights via dispersion analyses. The argon result is highly consistent with static measurement plus ab initio calculation of dispersion polarizability. The nitrogen result is nominally consistent with one recent experiment and the dipole oscillator strength distributions, but the present work offers a refined estimate of the molar refractivity at optical wavelengths. For ordinary and heavy water, the dispersion trend is nominally consistent with existing liquid measurements. However, water’s absorption features in the near-infrared preclude a reliable comparison of the present result with literature.

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.

Graphical Abstract

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.