David Didion

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.

Evaluation of Suction Line-Liquid Line Heat Exchange in the Refrigeration Cycle

Abstract The paper presents a theoretical evaluation of the performance effects resulting from the installation of a liquid-line/suction-line heat exchanger (LLSL-HX). It examines cycle parameters and refrigerant thermodynamic properties that determine whether the installation results in improvement of COP and volumetric capacity. The study shows that the benefit of application of the LLSL-HX depends on a combination of operating conditions and fluid properties — heat capacity, latent heat, and coefficient of thermal expansion — with heat capacity being the most influential property. Fluids that perform well in the basic cycle are marginally affected by the LLSL-HX, and the impact on the coefficient of performance and volumetric capacity may be either positive or negative. Fluids performing poorly in the basic cycle benefit the from LLSL-HX installation through increase of the coefficient of performance and volumetric capacity.

Role of refrigerant mixtures as alternatives to CFCs

Abstract Refrigerant mixtures may provide some solutions to the problem of the very limited number of fluids which have suitable properties to provide alternatives to CFCs. Mixing of refrigerants allows the adjustment or tuning of the most desirable properties of the mixture by varying the molar fraction of the components. There are three categories of mixtures which can be used as working fluids: azeotropes, near-azeotropes and zeotropes. Azeotropes are usually binary and have long been used in the refrigeration industry; it is unlikely that new combinations will now be found. Near-azeotropes have a much greater potential for development, but under leakage conditions may alter their composition and properties. Near-azeotropes have the most potential as drop-in alternatives to CFCs. Zeotropes have potential for improvements in energy efficiency and capacity modulation, but, as they require hardware design changes can only be considered in new system designs. However, with the long-term concerns about global warming, energy efficiency, and therefore the use of zeotropes, will probably have a great influence on the refrigeration industry.

Trade-offs in refrigerant selections : past, present, and future

Abstract Recent attention to depletion of stratospheric ozone, by chemicals containing bromine and chlorine, resulted in an international accord to halt their production. The most widely used refrigerants are among them. Chemical and equipment manufacturers mounted aggressive research and development programs to introduce alternative and transition refrigerants, associated lubricants and desiccants, and redesigned equipment. The already difficult criteria became even more complex, with subsequent linkage of chemical emissions from human activities to global climate change. The very successful response to protect the ozone layer has led some regulators and users to assume that ideal substitutes will be found. Such chemicals should be free of all environmental and safety concerns, be chemically and thermally stable, and perform efficiently. The analyses presented in this paper demonstrate that the outlook for discovery or synthesis of ideal refrigerants is extremely unlikely. Trade-offs among desired objectives, therefore, are necessary to achieve balanced solutions. The paper also shows that fragmented regulation of the chemicals involved, to address individual issues, jeopardizes the prospect of solving subsequently addressed problems. The paper reviews the history of refrigerants, their roles in ozone depletion and global climate change, and necessary trade-offs in refrigerant selections.

Performance Evaluation of Two Azeotropic Refrigerant Mixtures of HFC-134a With R-290 (Propane) and R-600a (Isobutane)

The reduction in chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) production and the scheduled phase-out of these ozone-depleting refrigerants require the development and determination of environmentally safe refrigerants for use in heat pumps, water chillers, air conditioners, and refrigerators. This paper presents a performance evaluation of a generic heat pump with two azeotropic refrigerant mixtures of HFC-134a (1,1,1,2-tetrafluoroethane) with R-290 (propane) and R-600a (isobutane); R-290/134a (45/55 by mass percentage) and R-134a/600a (80/20 by mass percentage). The performance characteristics of the azeotropes were compared with pure CFC-12, HFC-134a, HCFC-22, and R-290 at the high temperature cooling and heating conditions including those using liquid-line/suction-line heat exchange. The coefficient of performance of R-290/134a is lower than that of HCFC-22 and R-290, and R-134a/600a shows higher coefficient of performance than CFC-12 and HFC-134a. The capacity for R-290/134a is higher than that for HCFC-22 and R-290, and R-134a/600a exhibits higher system capacity than CFC-12 and HFC-134a. Experimental results show that the discharge temperatures of the studied azeotropic mixtures are lower than those of the pure refrigerants, CFC-12 and HCFC-22.

Glide matching with binary and ternary zeotropic refrigerant mixtures Part 1. An experimental study

Abstract An improvement of the coefficient of performance (COP) of the refrigeration cycle can be realized when temperature profiles of the refrigerant mixture and the heat transfer fluid (HTF) are matched. For the same temperature lift, the benefit of glide matching increases as the application glide increases. High-glide binary mixtures composed of components far apart in boiling points tend to have a non-linear relationship between temperature and enthalpy in the two-phase region. The introduction of an intermediate boiler as a third component can linearize this relationship and, theoretically, increase the cycle COP when heat-source and heat-sink fluids are substantially linear (e.g. water, brines, dry air). The research described in this paper was directed at exemplifying this characteristics of ternary mixtures by experimental evaluation of the performance of an R23/142b binary mixture and an R23/22/142b ternary mixture in a generic laboratory breadboard refrigeration system.

Mathematical model of an air-to-air heat pump equipped with a capillary tube

Abstract This paper described in general a computer model for simulation of steady-state performance of a split, residential, air-to-air heat pump. Organization of the model is discussed and approach to modelling of main heat pump components is explained. The modelling effort emphasis was on the local phenomena to be described by fundamental thermodynamic, heat transfer and fluid mechanic relationships. The model has been verified in a wide range of operating conditions from high temperature cooling to low temperature heating.

Glide matching with binary and ternary zeotropic refrigerant mixtures Part 2. A computer simulation

Abstract The glide-matching study presented in Part 1 was a laboratory investigation which demonstrated the evaporator performance in detail. However, since it was not possible to instrument the condenser sufficiently, some computer simulation work was conducted using a semi-theoretical model cycle -11, which has been under continual development at NIST for the past five years. As in the experimental effort, R22, R142b, R22/142b, R23/22/142b and R23/142b working fluids were investigated, but the simulation work did not include heat pump operation with liquid-line/evaporator heat exchange. By utilizing the model to quantify entropy generation at various state points within the cycle, it was possible to locate the likelihood of temperature profile pinch points in both the condenser and evaporator. This information clarified the impact of non-linearities on the system performance.

Thermophysical-property needs for the environmentally acceptable halocarbon refrigerants

The need for and uses of thermodynamic and transport properties in the selection of working fluids for the vapor compression cycle and in equipment design are reviewed. A list of hydrogen-containing halocarbons, as well as their mixtures, is presented as alternatives to the environmentally harmful, fully halogenated chlorofluorocarbons. These fluids range from well-characterized, widely available refrigerants to materials available only by custom synthesis about which very little is known. Data priorities for these fluids are presented; most essential are critical point, vapor pressure, liquid density, ideal-gas heat capacity, and vapor p-V-T data. A critical need exists for these data on a number of candidate working fluids in order not to lose the opportunity to select the best set of future refrigerants.