International Journal of Thermophysics

There are two promising candidates as alternative refrigerants for air-conditioners and heat pumps. The first is R407C, which is composed of HFC-32 (23 mass%), HFC-125 (25 mass%), and HFC-134a (52 mass%). The second is R410A, which is composed of HFC-32 (50 mass%) and HFC-125 (50 mass%). In this study, formation conditions of clathrate compounds between water and HFC alternative refrigerants such as HFC-32, HFC-125, HFC-134a, and their mixtures, R407C and R410A, were investigated. Phase diagrams of clathrates of these HFC alternative refrigerants and their mixtures were determined. From the phase diagrams, the critical decomposition temperature and the critical decomposition pressure were determined. The relationship between the critical decomposition points for the clathrates of HFC-32, HFC-125, HFC-134a, R410A, and R407C were studied. It is found that R407C and R410A form clathrate compounds with water under the evaporating temperature condition in the refrigeration cycle of air-conditioners and heat pumps.

Laser-supported processes can be used to modify the electrical and thermal properties of ceramic substrates locally. These processes are characterized by a strong thermal interaction between the laser beam and the ceramic surface which leads to localized melting. During the dynamic melting process, an additive material is injected into the melt pool in order to modify the physical properties. The heat and mass transfer during this dynamic melting and solidification process has been studied numerically in order to identify the dominating process parameters. Simulation tools based on a finite volume method have been developed to describe the heat transfer, fluid flow, and the phase change during the melting and solidification of the ceramic. The results of the calculations have been validated against experimental results.

Measurements of the radiance temperature of graphite at 655 nm have been performed in the vicinity of its triple point by means of a rapid pulse-heating technique. The method is based on resistively heating the specimen in a pressurized gas environment from room temperature to its melting point in less than 20 ms by passing an electrical current pulse through it and simultaneously measuring the radiance temperature of the specimen surface every 120 μs by means of a high-speed pyrometer. Results of experiments performed on two different grades of POCO graphite (AXM-5Q1 and DFP-1) at gas pressures of 14 and 20 MPa are in good agreement and yield a value of 4330±50 K for the radiance (or brightness) temperature (at 655 nm) of melting graphite near its triple point (triple-point pressure, ∼ 10 MPa). An estimate of the true (blackbody) temperature at the triple point is made on the basis of this result and literature data on the normal spectral emittance of graphite.

An experimental measurement program was performed to determine thermophysical properties of aluminum-based foam metals. The effective thermal conductivity k e and permeability K were investigated in detail. Experimental facilities were fabricated, and the measurement procedures and methodologies were evaluated. One-dimensional heat conduction was considered to determine k e . The results indicate that k e increases as the porosity ε decreases. However, no noticeable changes in k e were detected from variations of the cell size of the foam metal at a fixed porosity ε. The permeability K is substantially affected by both ε and the cell size. An empirical correlation for the friction factor f is proposed based on the concepts of K and inertial effect.

In order to determine the density of tantalum over the entire liquid phase (at the pressure applied) and several hundred K into the super-heated region, the method of ohmic pulse-heating was applied. For this purpose, images of the thermal radial expansion of the resistively heated sample wires were taken with an adapted CCD system. A newly integrated high-power photoflash and improved triggering of the experiment allowed the acquisition of high-contrast shadow images of the expanding wires. To reduce the uncertainty arising from simultaneous pyrometric temperature measurement, the change in normal spectral emissivity as a function of temperature was additionally taken into account. In this work, the density versus temperature relationship of tantalum is reported and compared to existing literature data. From the newly obtained liquid-phase density, critical point data of tantalum, such as critical temperature and critical density, were estimated via an extrapolation procedure. Furthermore, an estimate of the phase diagram in the density versus temperature plane is given. The work is concluded by a rigorous density uncertainty estimation according to the guide to the expression of uncertainty in measurement (GUM).

Experimental data for the thermal conductivity of the complex hydrides NaAlH4, LiBH4, NaBH4, and KBH4, in dense, solid form over wide ranges in temperature and pressure are presented. These materials contain high volume and mass fractions of hydrogen and are considered possible candidates as future hydrogen storage materials for mobile applications. The pressure–temperature phase diagrams of several materials as obtained from thermal-conductivity studies are briefly discussed, and the temperature and pressure dependencies of the thermal conductivity of the bulk materials are also discussed using simple theoretical models.

A new method is presented for calculating the effective thermal conductivity of a composite material containing spherical inclusions. The surface of a large body is assumed kept at a uniform temperature. This body is in contact with a composite material of infinite extent having a lower temperature far from the heated body. Green's theorem is then used to calculate the rate of heat transfer from the heated body to the composite material, yielding $$k_e /k = 1 + \frac{{3(\alpha - 1)}}{{[\alpha + 2 - (\alpha - 1)\phi ]}}\{ \phi + f(\alpha )\phi ^2 + 0(\phi ^3 )\} $$ where k e is the effective thermal conductivity, k is the thermal conductivity of the continuous phase, α is the ratio of the thermal conductivity of the spherical inclusions to k, and φ is the volume fraction occupied by the dispersed phase. The function f(α) is presented in this work. Although a similar result has been found previously by renormalization techniques, the method presented in this paper has merit in that a decaying temperature field is used. As a result, only convergent integrals are encountered, and a renormalization factor is not needed. This method is more straightforward than its predecessors and sheds additional light on the basic properties of two-phase materials.

The densities of solid and liquid Cu $$_{48}$$ Zr $$_{52}$$ and the viscosity of the liquid were measured in a containerless electrostatic levitation system using optical techniques. The measured density of the liquid at the liquidus temperature (1223 K) is (7.02 $$\pm $$ 0.01) g $$\cdot $$ cm $$^{-3}$$ and the density of the solid extrapolated to that temperature is (7.15 $$\pm $$ 0.01) g $$\cdot $$ cm $$^{-3}$$ . The thermal expansion coefficients measured at 1223 K are (6.4 $$\pm $$ 0.1) $$\,\times \,10^{-5}$$ K $$^{-1}$$ in the liquid phase and (3.5 $$\pm $$ 0.3) $$\,\times \,10^{-5}$$ K $$^{-1}$$ in the solid phase. The viscosity of the liquid, measured with the oscillating drop technique, is of the form $$A\exp \left[ \left( {{E}_{0}}+{{E}_{1}}\left( 1/T-1/{{T}_{0}} \right) \right) \times \left( 1/T-1/{{T}_{0}} \right) \right] $$ , where $${{T}_{0}}=1223$$ K, $$A= (0.0254 \pm 0.0004)$$ Pa $$\cdot $$ s, $${{E}_{0}}$$ =  (8.43 $$\pm $$ 0.26) $$\,\times \,10^3$$ K and $${{E}_{1}}$$ =  (1.7 $$\pm $$ 0.2) $$\,\times 10^7$$ K $$^{2}$$ .