International Journal of Thermophysics最新文献

The paper presents a complete theory for a new method for the determination of the thermal diffusivity of a bulk solid in the form of a cylinder using a pulse of energy of finite duration delivered on one face and the subsequent temperature rise detected on a parallel face. It is an important feature of the method that the departure from equilibrium in the solid sample is small so that the temperature rise is no more than a few degrees Kelvin. The energy pulse may be of any temporal distribution and the detection of the temperature rise can be conducted at any point on the opposing face of the sample. The theory explicitly accounts for heat losses at all the surfaces of the sample and enables absolute measurement of the thermal diffusivity of the sample. A prototype instrument is described to realize this theory in which the heating pulse is generated by an array of light emitting diodes in a circular configuration which is then guided by a light pipe so that a uniform distribution is ensured across the flat face of the solid sample being tested. The instrument is designed for operation over the temperature range from ambient to 1300 K but, in the current proof of principle, measurements are conducted at room temperature on a sample of Pyroceram™ 9606.¹ In this case, the detection is performed with a micro-thermocouple at the center of the sample. Several different rectangular heating pulse durations are employed to show that the theory provides an appropriate description of the experiment. The potential for future applications of the technique is demonstrated.

The mathematical tool “Parametrical Identity Mapping (PIM)” is presented in detail. Its tasks are to effectively assists (1) in choosing the best form of a measurement model, e.g., for the transient hot-wire method and (2) in completely adjusting the selected model by two global correction factors. These factors control the amplitude and time-response of the adjusted model in a way that this form of the model optimally predicts the experimental data points. The goodness of the adjusted model can easily be determined by a simple statistic, the level of similarity, (R^{2} le 1). (R^{2}) is known as the coefficient of determination. Here, it describes the proportion of the variation in the observed values that is predictable from the estimated values. PIM is able to create an adjusted model of a level of similarity that comes very close to unity, (R^{2} approx 1).

This study introduces a novel method for calculating the thermal conductivity of graphene using a Monte Carlo approach to evaluate anisotropic three-phonon interactions. The phonon dispersion relation is derived using a force constant model that incorporates up to fifth-order nearest neighbor interactions, while the phonon density of states (DOS) is computed via a generalized Gilat–Raubenheimer method. A quantitative relationship for the scaling exponent of the specific heat capacity at low temperatures is established, emphasizing the unique two-dimensional characteristics of graphene. To address anisotropic effects, the Monte Carlo approach efficiently identifies three-phonon combinations that adhere to the conservation laws of energy and momentum. The findings highlight the pivotal role of anisotropic phonon interactions in graphene’s thermal conductivity. The thermal conductivity values obtained through the iterative method exhibit strong agreement with previous three-phonon calculations, thereby validating the model. Nevertheless, discrepancies with experimental data suggest that incorporating higher-order phonon processes, such as four-phonon scattering, may further improve predictive accuracy.

A new fundamental equation of state expressed as a function of the Helmholtz energy is presented for 3,3,3-trifluoroprop-1-ene (R-1243zf). The equation is valid from the triple-point temperature (122.35 K) to 430 K at pressures up to 35 MPa. The expected uncertainties ((k = 2)) in calculated properties from the equation of state are 0.1 % for vapor pressures, 0.1 % for liquid densities, 1 % for vapor densities, 0.3 % for saturated liquid densities, 1 % for saturated vapor densities, 0.06 % for vapor-phase sound speeds, and 2 % for liquid-phase isobaric heat capacities. Differences between experimental and calculated vapor pressures are within 2 kPa in most cases. Uncertainties for caloric properties are particularly improved from the former equations of state. Various plots of constant-property lines demonstrate that not only does the equation exhibit correct behavior over all temperatures and pressures within the range of validity, but also that it shows reasonable extrapolation behavior at extremely low and high temperatures, and at high pressures.

Efficient use of heating systems is necessary from an environmental and economic perspective. This work analyses the Joule effect and the thermal transport properties of carbon nanofibers dispersed in ethylene–glycol aligned by applying a constant AC electric field. We tested several weight fraction concentrations from 0.1 % to 1 % wt of carbon nanofibers. The evolution of temperature and electric current as a function of time was analyzed. The amount of heat generated was quantified using Joule's law equation, and we estimated the thermal conductivity as a function of the concentration before and after the voltage application. The dependence of the temperature increase on the concentration of carbon nanofibers and electric voltage was investigated. Our work explores the viability of using carbon nanofiber dispersed in ethylene glycol in developing intelligent fluids useful for heat generation and release, with applications in heat management systems, such as those used for deicing.

Understanding the physical properties of diesel fuel blends is essential for evaluating spray characteristics, engine performance, and exhaust emissions of internal combustion engines. Moreover, higher alcohols (n-butanol, n-pentanol, and n-octanol) have recently garnered attention as promising oxygenated additives for enhancing the fuel characteristics of diesel fuel in various combustion applications. For these reasons, in this study, density (ρ), kinematic viscosity (ν), and refractive index (n_D) values of pseudo-binary blends (diesel fuel + n-butanol, diesel fuel + n-pentanol, and diesel fuel + n-octanol) are measured at different temperatures (288.15 K–323.15 K with 5 K interval) and over the entire range of composition (mole fractions). Experimental results for n-butanol, n-pentanol, and n-octanol obtained in this study are consistent with literature values, showing average absolute percentage deviation less than 0.11 %, 3.94 %, and 0.14 % for density, viscosity, and refractive index, respectively. The studied blends meet density and kinematic viscosity limits imposed by the diesel fuel standard (EN 590). Derived from the experimental data, excess molar volumes, viscosity deviations, and refractive index deviations are calculated. These deviation from ideality are fitted using the Redlich–Kister polynomial equation. Refractive index data of pseudo-binary blends are predicted using different models (Lorentz–Lorenz, Gladstone–Dale, Newton, Eykman, Heller, and Edwards). These models have low average absolute percentage deviation (less than 0.67%) for all studied pseudo-binary blends and temperature ranges (293.15 K–308.15 K), which shows they give excellent fitting results between measured data and calculated values.

The progressive phase-out of high-GWP refrigerants as mandated by the Kigali Amendment to the Montreal Protocol and the EU F-gas Regulation necessitates the exploration of sustainable alternatives within the HVAC&R industry. A recent proposal by the Council and the European Parliament aims to significantly reduce Hydrofluorocarbons (HFCs) consumption by 2050, including specific bans on high-GWP fluorinated gases in heat pumps and small air conditioning units. Heat pumps, pivotal in mitigating climate change, are expected to see a significant rise in residential applications. However, R134a, widely employed in these systems, has a high GWP of 1530, highlighting the need for more eco-friendly substitutes. Hydrofluoroolefins (HFOs) and natural fluids, particularly hydrocarbons (HCs), have emerged as promising fourth-generation refrigerants due to their negligible ozone depletion potential (ODP) and very low global warming potential (GWP). Despite the potential of these new refrigerants, an optimal replacement for R134a in heat pumps has yet to be found. In this regard, this study investigates the potential of the low-GWP HFO/HC mixture R1234yf/R600a (0.85/0.15) as a drop-in replacement for R134a in water-to-water heat pumps. The research conducts a comparative analysis between R134a and the nearly-azeotropic mixture, assessing their performance under identical heating conditions across 20 different combinations of heat sink and heat source temperatures, ranging from 35 °C to 70 °C and from 10 °C to 20 °C respectively. The R1234yf/R600a mixture exhibited a lower pressure ratio and higher mass flow rates compared to R134a. Additionally, the mixture showed favorable performance in terms of power consumption and compressor outlet temperatures, with slightly lower COP compared to the baseline fluid. These findings suggest that with proper optimization, the R1234yf/R600a mixture could be a viable and sustainable alternative to R134a in residential heat pump applications.

Modern multiparameter equations of state (MP EOSs) enable accurate computation of thermodynamic properties of fluids in broad ranges of temperature and pressure. Between the saturated vapor and saturated liquid densities, a pressure vs. density isotherm computed with an MP EOS exhibits several oscillations with large amplitudes. This is not a problem for most engineering computations, because this portion of isotherm is replaced with a horizontal line, representing an equilibrium mixture of the vapor and liquid phases. However, for computing properties in metastable states, modeling phase interfaces with gradient theory, and certain models of fluid mixtures, an isotherm with a single maximum and a single minimum (single van der Waals loop) is needed. As a step toward an accurate, single-loop EOS, we propose a generalized four-parameter cubic (G4C) EOS. The four parameters are temperature functions which are fitted, at subcritical temperatures, to the second virial coefficient, saturation pressure, liquid density and compressibility, and, in the supercritical region, to the second virial coefficient and derivatives of pressure. Fitted data were generated from thermodynamic property formulation for propane (Lemmon et al. J Chem Eng Data 54:3141, 2009). The G4C EOS provides a representation of thermodynamic properties of propane in the gaseous, liquid, and supercritical regions, which is sufficiently accurate for the intended applications. The equation can be extrapolated to high temperatures. Between 85.5 K and 296 K, the density and compressibility of the saturated liquid are represented with an average absolute relative deviation (AARD) of, respectively, 0.04 and 0.25 percent, the density and compressibility of saturated vapor show AARD of 0.31 and 0.39 percent, and the saturation pressure deviates by 0.23 percent. Features to be improved in future are temperature dependencies of the third- and higher-order virial coefficients at low temperatures, the curvature of isotherms in the liquid region, liquid density at very high pressures, and the critical region. Developed G4C EOS was successfully used in a new mixture model (Hrubý Int J Thermophys 44:130, 2023) to model volumetric behavior and vapor–liquid equilibrium in asymmetric mixtures with propane as a low-volatile component.

Thermoelectric materials play a significant role in the electronic industry and energy production. However, temperature-dependent material properties make the theoretical analysis challenging. This paper investigates the thermo-mechanical performance of thermoelectric generators with temperature-dependent material properties by differential transform method (DTM). The nonlinear distribution of temperature-dependent thermal conductivity, Seebeck coefficient, and electric resistivity are considered. DTM is used to construct analytical approximate solutions of the nonlinear differential equation governing the temperature distribution of the thermoelectric element. The thermal performance of the thermoelectric element including temperature distribution, temperature gradient, heat flux, power output per area, and energy conversion efficiency are predicted by DTM. And, the proposed method is utilized to analyze the thermal stress of the thermoelectric element. Compared with numerical solutions, the results indicate that DTM has a fast convergence speed and a high accuracy. The findings reveal that the maximum energy conversion efficiency and thermal stress enhance with the increase of temperature difference.

The laser periodic heating method is widely used to measure the thermal diffusivity of various thin films. In this technique, surface temperature responses are monitored using either an infrared (IR) camera or a thermocouple (TC) detector. Under air pressure, the impact of air heat loss on these two measurement methods warrants further examination. In this study, we measured the in-plane thermal diffusivity of a polyethylene terephthalate (PET) film under air pressure using both a non-research-grade IR camera and a microscale TC. Results indicate that air heat loss significantly influenced the TC measurements, yielding an abnormally high thermal diffusivity. Comparatively, the thermal diffusivity measured by the IR camera decreased slightly as modulation frequency increased from 0.1 Hz to 1 Hz. When the thermal diffusion length was approximately three times the film thickness, the diffusivity values from the IR camera closely matched those obtained under vacuum, indicating that the non-contact IR method can effectively suppress the impact of air heat loss.