制冷剂比定压热容的推算理论研究

为了探索比定压热容具有良好精度的统一的理论预测模型,把比定压热容看成导出热力性质,采用余函数法,分别基于通用性好、精确度高的Peng-Robinson状态方程(P-R)和改进型的M-H方程(M-H81)推导了比定压热容的计算公式并编写了程序。对于M-H81方程,比定压热容数值的获得需基于最佳化的方程常数,在广泛的温度范围内算得了10种制冷剂的M-H81方程各常数。针对具体的制冷剂研究对象,利用该比定压热容的推算理论就可以进行推算和进一步的验证了。     

Thermal Properties of n-Hexane at Temperatures of 298.15–363.5 K and Pressures of 0.098–147 MPa

The thermal-expansion coefficient and isobaric heat capacity c _p of n-hexane at 298.15–363.5 K and 0.098–147 MPa are measured by flux calorimetry. Observed and calculated c _p data are compared, and a general equation for determining c _p is suggested.     

Catalytic Combustion of the Stoichiometric n-Butane/Air Mixture on Isothermally Heated Platinum Wire

The catalytic combustion of the stoichiometric n-butane–air mixture per se or diluted with N₂, on a platinum wire at different initial pressures (10–70 kPa) and temperatures (690–1,080 K) was studied. The chemical heat flow rate, dQ _r/dt, of the surface reaction was measured in isothermal and isobaric conditions and the overall kinetic parameters were evaluated for both steady state and initial transient catalytic combustion. At low total pressure (10 kPa), the temperature dependence of dQ _r/dt indicated a normal (Arrhenius) behavior for 690 < T < 900 K, while at higher temperatures, over 900 K, an anti-Arrhenius behavior was found. The obtained results are consistent with a diffusion-controlled process, accompanied by reactant depletion around the catalytic surface, at higher temperatures.     

Drying paper by impinging jets of superheated steam. Part 1: Constant drying rate in superheated steam

Over a wide range of operating conditions, the drying of paper by impinging jets of superheated steam proceeds by a constant rate period followed by a falling rate period. The constant drying rate, investigated here in the jet temperature range 150 ≤ T_j/ ≤ 465°C and over jet Reynolds numbers of 1000 ≤ Re_j ≤ 12000, is predicted within + 12% by a heat transfer expression using Martin's (1977) correlation for the heat transfer coefficient corrected for mass transfer by the Couette flow approximation factor, and a property ratio to account for the large temperature difference between the jet and the paper.     

Cohesion energy densities and solubility parameters for the n-alkanes expressed as a new homomorph equation

A new correlation formula for the calculation of the cohesive energy density for n-alkanes and the corresponding homomorphs for polar solvents has been developed. Constants of the Antoine equation and literature values of heat of vaporisation at the boiling point and at 25°C were used. For the C₂-C₁₆ n-alkanes the new correlation equation describes the cohesion energy density as a function of molal volume and reduced temperature within the accuracy of experimental data. In the range of reduced temperatures 0.4 < T_r < 0.7 and molal volumes 50 < V <300 cm³ the maximum deviation for this fit is less than ± 0.05 J^1/2 cm^?3/2. In the region of T_r and V for polar homomorphs to the n-alkanes at 25°C the deviations are typically less than half this value.     

A correlation for hydrogen solubility in alicyclic and aromatic solvents

A correlation for hydrogen solubility in aromatic, alicyclic, and heterocyclic hydrocarbon solvents is presented. This correlation, employing corresponding state theory, provides reliable estimates for hydrogen solubility, in simple and multicomponent solvents, over the temperature interval 0.45 ≤ T_r ≤ 0.97, and for pressures from 0.1 to 30.0 MPa. The correlation was found to have an average absolute error of 5.9% for 14 simple solvents, and 6.8% for 11 multicomponent solvents which include coal liquids and bitumen. Solvent specific coefficients are not employed, and the correlation is well suited for predicting hydrogen solubility in multicomponent ill defned solvents, or in simple solvents where experimental data are unavailable.     

Specific Heat Capacity Measurements of Frying Oil Using Modulated Differential Scanning Calorimetry

The specific heat capacity (c_p) of frying oils is of practical importance in engineering work associated with refining operations and the thermal resistance during application. The objective of this study was to use modulated differential scanning calorimetry (MDSC) to measure the specific heat capacity of frying oil. Samples were exposed to a cyclic heating profile that was generated by a linear heating rate while simultaneously superimposing a sinusoidally varying time–temperature wave. The c_p variation of three commercial frying oils during frying was tested over a temperature range between 0 and 250 °C, and the correlation of c_p with triacylglycerol (TAG) polymer contents, total polar compounds (TPC), and polar fractions was studied. Results indicated that the specific heat capacity of frying oils increased linearly as temperature increased. During frying, c_p had a positive relation with TAG polymers and TPC at the beginning, but finally decreased when frying ended. This was possibly associated with the variation of polar fractions as frying continued. The large molecular compounds and small molecular compounds of polar fractions were considered to contribute oppositely to c_p, which led to its final decrease.     

Second-order-transition like characteristic contributes to strain temperature stability in (K,Na)NbO3-based materials

High strain and good temperature stability are contradictory properties in (K, Na)NbO₃ (KNN)-based materials. Herein, good temperature stability with high strain is obtained in a multiphase coexistent [ie, orthorhombic-tetragonal (O-T) and rhombohedral-orthorhombic-tetragonal (R-O-T)] KNN. A second-order transition-like characteristic should contribute to the temperature stability, in which an intrinsic lattice structure forms a bridge between them. The observed second-order transition-like characteristic is due to the reduced discrepancy among different lattice symmetries and a broadened temperature region for the phase transition. These integrated factors can slow the latent heat in a first-order transition and extend it over a wide temperature region, thereby exhibiting second-order transition-like behavior. Correspondingly, the abrupt increase in strain near the phase transition temperature significantly slows. In addition, the appearance of pure tetragonal symmetry (P4mm) is deferred to a much higher temperature than T_O-T, in which the strain will inevitably decrease. As a result, good temperature stability with a high strain response can be realized in multiphase coexistent KNN materials, including d₃₃*=448 pm/V, -27.5%≤fluctuation≤4.2% for O-T, and d₃₃*=446 pm/V, -17.5%≤fluctuation≤7.6% for R-O-T, over the whole temperature range 25 °C-190 °C.     

Prediction of isobaric and isothermal vapour–liquid equilibria from limited experimental data

Malesinski's method for prediction of isobaric vapour–liquid equilibrium data using a single experimental datum on a T–x isobar has been modified and extended to a more general case where the molar entropies of vaporisation of the components are not necessarily equal and the non-ideality in the vapour phase is considered. However, it is assumed that the solution behaves like a strictly regular one over the temperature range in question, that is, the constant A in the expression for excess free energy: g^E=Ax₁x₂ is independent of temperature. The method has been illustrated for several systems and is found to be highly satisfactory for non-polar–non-polar as well as polar–non-polar systems in which the boiling points of the pure components are not much different. Incorporating temperature dependence of the constants in the Redlich–Kister equation for excess free energy, a method has been developed for predicting isothermal vapour–liquid equilibrium data at several temperature levels from equilibrium values at a single pressure. For testing the validity of this method, predicted results have been compared with the available experimental data for zeotropic as well as azeotropic systems comprising non-polar–non-polar, polar–non-polar and polar–polar mixtures, and the method has been found to be satisfactory for all systems.     

Specific heat capacities of some polyepoxides

Specific heat capacity measurements were made in a differential scanning calorimeter on a series of eight crosslinked epoxy/diamine polymers over a range of temperatures chosen, for each polymer, to include the glass transition. The tabular data at 5°C intervals was then fitted to a five-parameter empirical equation that represents the data with a deviation less than the experimental uncertainty of the measurements. The measured change in specific heat at the glass transition was an average of 1.9 cal/mol°C for each bead in the polymer repeat unit compared with 2.6 cal/mol°C bead found by Wunderlich for linear polymers. The measurements were then analyzed in terms of the molecular components of the polymers, assuming that the specific heat contribution of each component is independent of its neighbors, i.e., that specific heat is an additive property. In calculating empirical component values as a function of temperature, the polymer specific heats should be plotted as a function of T–T_g rather than T alone. In this manner, component specific heats as functions of T–T_g were determined over a range from the glassy to the rubbery state.     

On the derivation and application of a new vapor pressure equation

Starting from the Clapeyron equation and making use of the equivalent expression for the reference state for latent heat of vaporization of pure liquids, namely at 0°K., a new vapor pressure equation is derived as follows: _χ(T_r) has been tabulated for T_r values in the range 0.50 < T_r <0.99 after programming on an IBM 1620 computer. The new vapor pressure equation has been tested with a variety of liquids both polar and non-polar, and found to predict vapor pressure data in the orthobaric range with an average error of ± 1.90% and maximum error of ± 3.20%. A significant outcome of this investigation is the identification of Riedel's factor α_o and Pitzer's acentric factor ω These have been shown to be functions of critical properties and reduced boiling point.     

BaTiO3–Bi(Mg2/3Nb1/3)O3 Ceramics for High‐Temperature Capacitor Applications

Solid solutions of (1?x)BaTiO₃–xBi(Mg_2/3Nb_1/3)O₃ (0 ≤ x ≤ 0.6) were prepared via a standard mixed‐oxide solid‐state sintering route and investigated for potential use in high‐temperature capacitor applications. Samples with 0.4 ≤ x ≤ 0.6 showed a temperature independent plateau in permittivity (ε_r). Optimum properties were obtained for x = 0.5 which exhibited a broad and stable relative ε_r ~940 ± 15% from ~25°C to 550°C with a loss tangent <0.025 from 74°C to 455°C. The resistivity of samples increased with increasing Bi(Mg_2/3Nb_1/3)O₃ concentration. The activation energies of the bulk were observed to increase from 1.18 to 2.25 eV with an increase in x from 0 to 0.6. These ceramics exhibited excellent temperature stable dielectric properties and are promising candidates for high‐temperature multilayer ceramic capacitors for automotive applications.     

Reaction kinetics modeling and thermal properties of epoxy–amines as measured by modulated‐temperature DSC. II. Network‐forming DGEBA + MDA

Reaction‐induced vitrification takes place in the network‐forming epoxy–amine system diglycidyl ether of bisphenol A (DGEBA) + methylenedianiline (MDA) when the glass‐transition temperature (T_g) rises above the cure temperature (T_cure). This chemorheological transition results in diffusion‐controlled reaction and can be followed simultaneously with the reaction rate in modulated‐temperature DSC (MTDSC). To predict the effect of T_cure and the NH/epoxy molar mixing ratio (r) on the reaction rate in chemically controlled conditions, a mechanistic approach was used based on the nonreversing heat flow and heat capacity MTDSC signals, in which the reaction steps of primary (E_1OH = 44 kJ mol^?1) and secondary amine (E_2OH = 48 kJ mol^?1) with the epoxy–hydroxyl complex predominating. The diffusion factor DF as defined by the Rabinowitch approach expresses whether the chemical reaction rate or the diffusion rate determines the overall reaction rate. A model based on the free volume theory together with an Arrhenius temperature dependency was used to calculate the diffusion rate constant in DF as a function of conversion (x) and T_cure. The relation between x, r, and T_g, needed in this model, can be predicted with the Couchman equation. An experimental approximation for DF is the mobility factor DF* obtained from the heat capacity signal at a modulation frequency of 1/60 Hz, normalized for the effect of the reaction heat capacity in the liquid state and the change in C_p in the glassy region with x and T_cure. In this way, an optimized set of diffusion parameters was obtained that, together with the optimized kinetic parameters set, can predict the reaction rate for different cure schedules and for stoichiometric and off‐stoichiometric mixtures. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2814–2833, 2004     

Electrical resistivity and magnetoresistivity of protonic acid (h2SO4 and HCl)‐doped polyaniline at low temperature

Electrical‐transport properties of protonic acid (H₂SO₄ and HCl)‐doped polyaniline (PANI) in an aqueous ethanol medium were investigated in the temperature range 1.8 K ≤ T ≤ 300 K and in a magnetic field up to 8 T. The room‐temperature resistivity of HCl‐doped PANI is larger than that of H₂SO₄‐doped PANI. The resistivity ratios ρ_r = ρ(1.8 K)/ρ(300 K) of the samples are high. The samples in the insulating region show a crossover from a Mott to an Efros–Shklovskii variable range hopping conduction at T = 9.8 K for H₂SO₄‐doped PANI and 8.5 K for HCl‐doped PANI. The magnetoresistivity of these samples is also explained by the variable range hopping theory. The different physical parameters were calculated from the experimental data. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1480–1486, 2000     

Temperature‐Stable Relative Permittivity from −70°C to 500°C in (Ba0.8Ca0.2)TiO3–Bi(Mg0.5Ti0.5)O3–NaNbO3 Ceramics

Temperature‐stable relaxor dielectrics have been developed in the solid solution system: 0.45Ba_0.8Ca_0.2TiO₃–(0.55 ? x)Bi(Mg_0.5Ti_0.5)O₃–xNaNbO₃. Ceramics of composition x = 0 have a relative permittivity ?_r = 950 ± 15% over a wide temperature range from +70°C to 600°C. Modification with NaNbO₃ at x = 0.2 decreases the lower limiting temperature to ?70°C, but also decreases relative permittivity such that ?_r ~ 600 ± 15% over the temperature range ?70°C to 500°C. For composition x = 0.3, the low‐temperature dispersion in loss tangent, tan δ, (at 1 kHz) shifts to lower temperature, giving tan δ values ≤0.02 across the temperature range ?60°C to 300°C in combination with ?_r ~ 550 ± 15%. Values of dc resistivity for all samples are of the order of 10¹⁰ Ω m at 250°C and 10⁷ Ω m at 400°C.     

The enthalpy and specific heat of yttrium oxide at 1300–2100°K

Conclusions The temperature dependence of the enthalpy and heat capacity of Y₂O₃ over the range 1300–2100°K was determined by a combination of experimental and analytical methods. The equation obtained can be interpolated in a first approximation for the temperature range 298–1300°K.Translated from Ogneupory, No. 2, pp. 16–18, February, 1974.     

Laminar heat transfer in the thermal entrance region

Series solutions for heat transfer in the thermal entrance region are presented for laminar flow in conduits (plane ducts, tubes, annular passages). The present solution of the equation of energy has been obtained for either a step change in heat flux or wall temperature and is valid for all time-independent non-Newtonian fluids, providing that the flow-field is such that v_θ = 0, v_r = 0, v_g = v_g(r) From its general nature with regard to the flow-field, such a solution is a convenient alternative to the eigenvalue solutions when applied in the thermal entrance region where a large number of eigenvalues and eigencoefficients are required.     

Effect of dissolved gases on subcooled flow boiling heat transfer

The influence of various dissolved gases (He, N₂, Ar, CO₂, C₃H₈) on subcooled boiling heat transfer was investigated for flow of water and of heptane in an annulus with a heated core. Flow velocity, liquid bulk temperature, system pressure, gas partial pressure and heat flux were all varied over a wide range.In comparing the measured heat transfer coefficients with those for subcooled boiling of the corresponding degassed liquids, it was found that the coefficients were always increased owing to the desorption of the dissolved gases. The extent of the increase depended on the solubility of the given gas in the given liquid and could be as much as several hundred per cent. In addition, the solid surface temperature required for the inception of bubble formation was reduced considerably, in some cases far below the saturation temperature of the pure liquid.Attempts were made to extend the prediction of incipient boiling temperatures to cases where gases are dissolved in the liquid.     

Synthesis of augmented biofuel processes using solar energy

A method for synthesizing augmented biofuel processes, which improve biomass carbon conversion to liquid fuel (η_carbon) using supplemental solar energy as heat, H₂, and electricity is presented. For a target η_carbon, our method identifies augmented processes requiring the least solar energy input. A nonconvex mixed integer nonlinear programming model allowing for simultaneous mass, heat, and power integration, is built over a process superstructure and solved using global optimization tools. As a case study, biomass thermochemical conversion via gasification/Fischer–Tropsch synthesis and fast‐hydropyrolysis/hydrodeoxygenation (HDO) is considered. The optimal process configurations can be categorized either as standalone (η_carbon ≤ 54%), augmented using solar heat (54% ≤ η_carbon ≤ 74%), or augmented using solar heat and H₂ (74 ≤ η_carbon ≤ 95%). Importantly, the process H₂ consumption is found to be close to the derived theoretical minimum values. To accommodate for the intermittency of solar heat/H₂, we suggest processes that can operate at low and high η_carbon. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2533–2545, 2014     

Equation of state for gaseous ethane determined from isotropic model potentials

A four-term virial equation of state was combined with isotropic model potentials to predict accurate volumetric and caloric thermodynamic properties of ethane in the gas phase. The parameters of the model potentials were determined from a fit to speed-of-sound data alone; no other data were used. The approximation used for the fourth virial coefficient included all interactions that contain up to two triplet potentials. Predicted ordinary second and third virial coefficients are in agreement with the data of Funke et al. [8]; we believe that predicted fourth virial coefficients are reliable and accurate. In the subcritical temperature region, the equation of state predicted compressibility factors that deviate by less than 0.04 percent at densities of up to 2.7 mol/dm³ (≈ 0.4ρ _c ). At supercritical temperatures, compressibility factors deviate by less than 0.02 percent at densities of up to 2.6 mol/dm³; also, in this region predicted isobaric heat capacity agrees with available data to within uncertainties of 0.4 percent at densities above 3 mol/dm³. We demonstrated that the four-term virial equation is more accurate than the three-term analogue.