Measurements of the isochoric heatc v at the critical point of SF6 under microgravity: Results of the German Spacelab Mission D2

During the Second German Spacelab Mission D2 (April 26 to May 6, 1993) the isochoric specific heatc _v of SF₆ was measured along the critical isochore under microgravity conditions with a newly developed scanning radiation calorimeter. This calorimeter provided the possibility to perform comparable heating and cooling runs with variable ramp rates since the spherical sample cell was heated and cooled only by radiation. During the experimental time of 220 h, 11 heating and cooling runs with different ramp rates were performed in a temperature range ofT–T _c=±6 K. ApproachingT _c by cooling from the homogeneous one-phase region avoided significant temperature and density gradients in the fluid, which would have distorted the integral measurement ofc _v. The inhomogenities introduced by a finite ramp rate were greatly reduced by the fast dynamic temperature propagation (critical speeding up). Thec _v data achieved with slow cooling runs are in remarkably good agreement with the theoretical prediction more than one order of magnitude closer to the critical point than anyc _v measurements done so far. The preliminary value for the critical exponent is 0.107±0.02, and for the amplitude ratio we obtainedA ^–/A ⁺=1.94±0.07. In contrast to the cooling runs, the heating runs showed a strong hysteresis ofc _v. A comparison to 1g measurements is provided.Invited paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, USA.     

Isochoric Heat-Capacity Measurements for Pure Methanol in the Near-Critical and Supercritical Regions

Isochoric heat-capacity measurements for pure methanol are presented as a function of temperature at fixed densities between 136 and 750 kg·m⁻³. The measurements cover a range of temperatures from 300 to 556 K. The coverage includes the one- and two-phase regions, the coexistence curve, the near-critical, and the supercritical regions. A high-temperature, high-pressure, adiabatic, and nearly constant-volume calorimeter was used for the measurements. Uncertainties of the heat-capacity measurements are estimated to be 2–3% depending on the experimental density and temperature. Temperatures at saturation, T _S(ρ), for each measured density (isochore) were measured using a quasi-static thermogram technique. The uncertainty of the phase-transition temperature measurements is 0.02 K. The critical temperature and the critical density for pure methanol were extracted from the saturated data (T _S,ρ_S) near the critical point. For one near-critical isochore (398.92 kg·m⁻³), the measurements were performed in both cooling and heating regimes to estimate the effect of thermal decomposition (chemical reaction) on the heat capacity and phase-transition properties of methanol. The measured values of C _V and saturated densities (T _S,ρ_S) for methanol were compared with values calculated from various multiparametric equations of state (EOS) (IUPAC, Bender-type, polynomial-type, and nonanalytical-type), scaling-type (crossover) EOS, and various correlations. The measured C _V data have been analyzed and interpreted in terms of extended scaling equations for the selected thermodynamic paths (critical isochore and coexistence curve) to accurately calculate the values of the asymptotical critical amplitudes ( and B ₀).     

Experimental and theoretical studies of the crossover behavior of the specific heat Cv,x of ethane,propane, and their mixture at critical isochores

The specific heat at constant volume, C_v, of propane, and mixture with a concentration ofx=0.0081 cool fraction of propane was measured along the critical isochores at temperatures between 290 and 400 K. The measurements were performed with a high-temperature constant-volume adiabatic calorimeter. The uncertainty of most of the measurements is estimated to be less than 1.5%. Measurements have been carried out in both the one- and two-phase regions. The results for the pure components are compared with earlier measurements. Crossover equations forC, obtained on the basis of the renormalization-group method and -expansion are applied to represent our experimental specific heat data for ethane, propane. and its mixture along the critical isochores.     

The process of heat and mass transport at the critical point of pure fluids

The effects of fast isentropic temperature propagation, called the piston effect, or critical speeding up, and slow mass diffusion, called critical slowing down, are investigated. A temperature propagation experiment in a spherical cell filled with pure SF₆ at critical density was performed during the Second German Spacelab Mission D2 in 1993. The results evidently confirm the presence of the piston effect both in the one-phase region and in the two-phase region. The numerical simulations are in remarkable good quantitative agreement with the experimental results.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Specific heat of a liquid mixture near the consolute point in the bulk phase and in a porous medium

The critical behavior of the specific heat of the mixture 2.6-lutidine+water near the consolute point was investigated in the bulk phase and in a porous medium. The measurements of the bulk specific heat yield a critical exponent =0.111±0.018 and a universal amplitude ratio A ^–/A⁺=1.77, in good agreement with theoretical predictions. Using previous experimental data for nitroethane+isooctane, we also determined the two-scale-factor ratio X ⁺=0.271 between the critical amplitude of the specific heat and of the correlation length in agreement with the results reported for other fluid systems. The specific heat in the porous medium with a pore size of about 100 nm was measured. The behavior of the specific heat differs from that of the bulk specific heat. This may be the result of finite size effects, of a wetting layer in the pores, and of a distortion of the coexistence curve.     

Scaled equation for the isochoric heat capacity of methane-ethane mixtures in the critical region

Results of an experimental study of the isochotic heat capacity of methaneethane mixtures in a wide region around the liquid-gas critical line are presented. The data are represented by a universal scaled equation for critical mixtures.     

Universal crossover behavior of fluids and fluid mixtures in the critical region

A new universal scaled equation of state for one-component fluids, binary mixtures, and ionic solutions, which represents the thermodynamic behavior of fluids in a wide range of temperatures and densities including the critical region, is proposed. Near the critical point this equation reduces to a theoretically based scaled equation including the leading nonasymptotic (Wegner) correction and a correction accounting for the asymmetry of fluids with respect to the critical isochore. Far away from the critical point the new equation goes over into the classical Landau expansion (van der Waals equation). The new equation is applied to represent experimental P-V-T data for H₂O and CO₂, as well as to represent C _v,x data for dilute aqueous solutions of NaCl. A crossover from fluctuation to mean-field behavior is observed at increased concentrations of NaCl. A universal crossover function for the heat capacity C _v,x of one-component fluids and binary mixtures is presented.     

Estimation and experimental study of the density and specific heat for alumina nanofluid

This study analyses the density and specific heat of alumina (Al₂O₃)/water nanofluid to determine the feasibility of relative calculations. The Al₂O₃/water nanofluid was produced by the direct-synthesis method with cationic chitosan dispersant served as the experimental sample, and was dispersed into three concentrations of 0.5, 1.0 and 1.5?wt.%. This experiment measures the density and specific heat of nanofluid with weight fractions and sample temperatures with a liquid density meter and a differential scanning calorimeter (DSC). To assess the availability of these equations, it then compares the experimental data with the calculated results according to the concepts of mixing theory and statistical mechanism. Comparing the calculated results of density and specific heat with the experimental data, the deviation of density fell within the range of ?1.50% to 0.06% and 0.25% to 2.53%, whereas the deviation of specific heat fell within the range of ?0.07% to 5.88% and ?0.35% to 4.94%, respectively. Calculated results of density and specific heat show a trend of greater deviation with an increased concentration of nanofluid. However, two kinds of density and specific heat of the calculated results fall within an acceptable deviation range in this study.     

Thermodynamic properties of 1,1,1,2-tetrafluoroethane (R134a) in the critical region

A theoretically based simplified crossover model, which is capable of representing the thermodynamic properties of fluids in a large range of temperatures and densities around the critical point, is presented. The model is used to predict the thermodynamic properties of R134a in the critical region from a limited amount of available experimental information. Values for various thermodynamic properties of R134a at densities from 2 to 8 mol·L^–1 and at temperatures from 365 to 450 K are presented.     

An improved parametric crossover model for the thermodynamic properties of fluids in the critical region

An improved parametric equation for the thermodynamic properties of fluids is presented that incorporates the crossover from singular thermodynamic behavior in the immediate vicinity of the critical point to regular thermodynamic behavior far away from the critical point. Based on a comparison with experimental data for ethane and methane, it is demonstrated that the crossover model is capable of representing the thermodynamic properties of fluids in a large range of temperatures and densities around the critical point.     

Thermodynamic behavior of mixtures of methane and ethane in the critical region

An isomorphic equation of state for near-critical binary mixtures is used to represent equation of state and specific heat data for mixtures of methane and ethane in a substantial range of densities and temperatures around the critical line.     

Measurement of the enthalpy and specific heat of a Be2C-graphite-UC2 reactor fuel material to 1980 K

The enthalpy and specific heat of a Be₂C-Graphite-UC₂ composite nuclear fuel material have been measured over the temperature range 298–1980 K using both differential scanning calorimetry and liquid argon vaporization calorimetry. The fuel material measured was developed at Sandia National Laboratories for use in pulsed test reactors. The material is a hot-pressed composite consisting of 40 vol% Be₂C, 49.5 vol% graphite, 3.5 vol% UC₂, and 7.0 vol% void. The specific heat was measured with the differential scanning calorimeter over the temperature range 298–950 K, while the enthalpy was measured over the range 1185–1980 K with the liquid argon vaporization calorimeter. The normal spectral emittance at a wavelength of 6.5×10^–5 cm was also measured over the experimental temperature range. The combined experimental enthalpy data were fit using a spline routine and differentiated to give the specific heat. Comparison of the measured specific heat of the composite to the specific heat calculated by summing the contributions of the individual components indicates that the specific heat of the Be₂C component differs significantly from literature values and is approximately 0.56 cal · g^–1 · K ^–1 (2.3×10³J · kg^–1 · K ^–1) for temperatures above 1000 K.     

A generalized scaled equation of state for n-alkanes (methane to n-nonane) in the critical region

A generalized scaled equation of state has been developed to calculate thermodynamic properties of n-alkanes from methane (CH₄) to n-nonane (C₉H₂₀) in the critical region. The equation is valid in the reduced density range 0.7 _c1.3 at T=T _c and up to 1.2T _c at = _c.     

Thermodynamic and transport properties of fluids and fluid mixtures in the extended critical region

A practical representation of the thermodynamic properties and the transport coefficients related to diffusion, heat conduction, and their cross-processes in pure fluids and binary mixtures near the liquid-vapor critical line is developed. Crossover equations for the critical enhancement of those coefficients incorporate the scaling laws near the critical point and are transformed to the regular background far away from the critical point. The crossover behavior of the thermal conductivity and the thermal diffusion ratio in binary mixtures is also discussed. A comparison is made with thermal-conductivity data for pure carbon dioxide, pure ethane, and carbon dioxide add ethane mixtures.