Mark O. McLinden

A study of flow boiling heat transfer with refrigerant mixtures

Abstract Mixture effects are studied on horizontal flow boiling heat transfer with both azeotropic and non-azeotropic refrigerant mixtures. More than 2000 local heat transfer coefficients are obtained with the azeotropie R 12/R 152a mixture and compared against the previously measured data with the non-azeotropic R22/R114 mixture. In a convective evaporation region, small mass transfer resistance is found for mixtures. The variation of physical properties due to mixing is responsible for almost all of the heat transfer degradation. In a partial boiling region, however, severe degradation of heat transfer with mixtures, similar to that in nucleate pool boiling heat transfer with mixtures, is found. A suppression of nucleate boiling at lower qualities due to loss of wall superheat with mixtures is responsible for this degradation. An analysis is developed to predict a transition quality by using Hsus onset of nucleate boiling theory. The prediction agreed well with observed transition qualities for both pure and mixed refrigerants. Correlations, based on the supposition of Chen and using only phase equilibrium data to consider mixture effects, are developed with mean deviations of 7.2 and 9.6% for pure and mixed refrigerants.

Horizontal flow boiling heat transfer experiments with a mixture of R22/R114

Abstract An experimental study on horizontal flow boiling heat transfer for pure R22, R114 and their mixtures under uniform heat flux condition is reported. More than 1200 local heat transfer coefficients are obtained for annular flow at a reduced pressure of 0.08. The ranges of heat flux and mass flow rate are 1045 kW m −2 and 16–46 g s −1 , respectively. The results indicate a full suppression of nucleate boiling for pure and mixed refrigerants beyond transition qualities and the majority of the data belongs to the convective evaporation region. The heat transfer coefficients of mixtures in this region are as much as 36% lower than the ideal values under the same flow condition. Non-ideal variations in physical properties account for 80% of the heat transfer degradation seen with mixtures and the other 20% (less than 10% of the heat transfer coefficient) is believed to be caused by mass transfer resistance in this region. A composition variation of up to 0.07 mole fraction in the annular liquid film was measured between the top and bottom of the tube, which causes a corresponding circumferential variation of wall temperature with mixtures.

A simplified cycle simulation model for the performance rating of refrigerants and refrigerant mixtures

Abstract A simulation program, CYCLE11, which is useful for the preliminary evaluation of the performance of refrigerants and refrigerant mixtures in the vapour compression cycle is described. The program simulates a theoretical vapour-compression cycle and departures from the theoretical cycle that occur in a heat pump and in a refrigerator. The cycles are prescribed in terms of the temperatures of the external heat-transfer fluids with the heat exchangers generalized by an average effective temperature difference. The isethalpic expansion process is assumed. The program includes a rudimentary model of a compressor and a representation of the suction line and liquid line heat exchange. Refrigerant thermodynamic properties are calculated by using the Carnahan-Starling-DeSantis equation of state. Refrigerant transport properties are not included in the simulations. The program can generate merit ratings of refrigerants for which limited measured data are available. An example of simulation results stresses the need for careful application of simplified models and consideration for the assumptions involved.

An International Standard Equation of State for the Thermodynamic Properties of Refrigerant 123 (2,2‐Dichloro‐1,1,1‐Trifluoroethane)

A modified Benedict–Webb–Rubin (MBWR) equation of state has been developed for Refrigerant 123 (2,2‐dichloro‐1,1,1‐trifluoroethane) based on recently measured thermodynamic property data and data available from the literature. Single‐phase pressure‐volume‐temperature (PVT), heat capacity, and sound speed data, as well as second virial, vapor pressure, and saturated liquid and saturated vapor density data, were used with multiproperty linear least squares fitting techniques to fit the 32 adjustable coefficients of the MBWR equation. Coefficients for the equation of state and for ancillary equations representing the vapor pressure saturated liquid and saturated vapor densities, and ideal gas heat capacity are given. While the measurements cover differing ranges of temperature and pressure, the MBWR formulation is applicable along the saturation line and in the liquid, vapor, and supercritical regions at temperatures from 166 to 500 K with pressures to 40 MPa and densities to 11.6 mol/L (1774 kg/m3). This fo...

An improved extended corresponding states method for estimation of viscosity of pure refrigerants and mixtures

Abstract The extended corresponding states method for calculating the viscosity of pure refrigerants and mixtures is investigated. The accuracy of pure fluid viscosity values is significantly improved by introducing a third shape factor evaluated using available pure fluid viscosity data. A modification to the method of Huber and Ely (Fluid Phase Equilibria, 1992, 80 , 45–46) is proposed for estimation of the viscosity of mixtures; this modification eliminates the possibility of discontinuities at the critical point, ensures that the pure component viscosity is provided in the limit of a component mole fraction approaching 1, and improves the overall accuracy of the method. The method has been applied to 12 pure refrigerants including three hydrocarbons and mixtures. The average absolute deviations between the calculated and experimental viscosity values are within 4% for all of the pure fluids and most of the mixtures investigated.

Methods for comparing the performance of pure and mixed refrigerants in the vapour compression cycle

Abstract Methods of comparing pure and mixed refrigerants are considered by computing the coefficient of performance and the heating capacity for an ideal vapour compression cycle for R22/R11 aand R22/R11 mixtures. For comparisons based only on one characteristic condensation temperature and one evaporation temperature, the results depend entirely on how the characteristic temperatures are defined. A method specifying the heat transfer fluid temperatures and a total heat exchanger area per unit capacity is thought to offer a comparison applicable to both pure and mixed refrigerants. Using this method, the effects of compressor superheat, heat exchanger approach temperatures, and the match of refrigerant and heat transfer fluid temperatures are discussed.

A Modified Benedict–Webb–Rubin Equation of State for the Thermodynamic Properties of R152a (1,1‐difluoroethane)

A modified Benedict–Webb–Rubin (MBWR) equation of state has been developed for R152a (1,1‐difluoroethane). The correlation is based on a selection of available experimental thermodynamic property data. Single‐phase pressure–volume–temperature (PVT), heat capacity, and sound speed data, as well as second virial coefficient, vapor pressure, and saturated liquid and saturated vapor density data, were used with multi‐property linear least‐squares fitting to determine the 32 adjustable coefficients of the MBWR equation. Ancillary equations representing the vapor pressure, saturated liquid and saturated vapor densities, and the ideal gas heat capacity were determined. Coefficients for the equation of state and the ancillary equations are given. Experimental data used in this work covered temperatures from 162 K to 453 K and pressures to 35 MPa. The MBWR equation established in this work may be used to predict thermodynamic properties of R152a from the triple‐point temperature of 154.56 K to 500 K and for pressu...

Equations of state for the thermodynamic properties of R32 (difluoromethane) and R125 (pentafluoroethane)

Thermodynamic properties of difluoromethane (R32) and pentafluoroethane (R125) are expressed in terms of 32-term modified Benedict-Webb-Rubin (MBWR) equations of state. For each refrigerant, coefficients are reported for the MBWR equation and for ancillary equations used to fit the ideal-gas heat capacity and the coexisting densities and pressure along the saturation boundary. The MBWR coefficients were determined with a multiproperty fit that used the following types of experimental data: PVT: isochoric, isobaric, and saturated-liquid heal capacities; second virial coefficients; and properties at coexistence. The respective equations of stale accurately represent experimental data from 160 to 393 K and pressures to 35 MPa for R32 and from 174 to 448 K and pressures to 68 MPa for R125 with the exception of the critical regions. Both equations give reasonable results upon extrapolation to 500 K and 60 MPa. Comparisons between predicted and experimental values are presented.

An extended corresponding states model for the thermal conductivity of refrigerants and refrigerant mixtures

Abstract The extended corresponding states (ECS) model of Huber et al. (Huber, M.L., Friend, D.G., Ely, J.F. Prediction of the thermal conductivity of refrigerants and refrigerant mixtures. Fluid Phase Equilibria 1992;80:249–61) for calculating the thermal conductivity of a pure fluid or fluid mixture is modified by the introduction of a thermal conductivity shape factor which is determined from experimental data. An additional empirical correction to the traditional Eucken correlation for the dilute-gas conductivity was necessary, especially for highly polar fluids. For pure fluids, these additional factors result in significantly improved agreement between the ECS predictions and experimental data. A further modification for mixtures eliminates discontinuities at the pure component limits. The method has been applied to 11 halocarbon refrigerants, propane, ammonia, and carbon dioxide as well as mixtures of these fluids. The average absolute deviations between the calculated and experimental values ranged from 1.08 to 5.57% for the 14 pure fluids studied. Deviations for the 12 mixtures studied ranged from 2.98 to 9.40%. Deviations increase near the critical point, especially for mixtures.

Thermodynamic properties for the alternative refrigerants

Abstract Models commonly used to calculate the thermodynamic properties of refrigerants are summarized. For pure refrigerants, the virial, cubic, Martin-Hou, Benedict-Webb-Rubin, and Helmholtz energy equations of state and the extended corresponding states model are discussed. High-accuracy formulations for 16 refrigerants are recommended. These models may be extended to mixtures through the use of mixing rules applied either to the parameters of the equation of state or to some property of the mixture components. Mixtures of a specific composition may also be modeled as a pseudo-pure fluid. Five mixture models, employing four distinct approaches, have been compared by a group working under the auspices of the International Energy Agency. These comparisons show all five models to be very capable in representing mixture properties. No single model was best in all aspects, but based on its combination of excellent accuracy and great generality, we recommend the mixture Helmholtz energy model as the best available. Experimental data are essential to both fit the adjustable parameters in property models and to assess their accuracy. We present a survey of the data available for mixtures of the HFC refrigerants R32, R125, R143a, R134a, and R152a and for mixtures of the natural refrigerants propane, butane, isobutane, and carbon dioxide. More than 60 data references are identified. Further data needs include caloric data for additional mixtures, comprehensive pressure-density-temperature data for additional mixture compositions, and improved accuracy for vapor-liquid equilibria data.