Comprehensive approach to predicting the circulation composition of zeotropic refrigerants

The problem of composition shift in zeotropic fluid circulation, which is encountered in modern air conditioning and refrigeration systems, is studied. It reveals that the composition shift phenomena is contributed by fractionation related to four mechanisms. This paper concentrates on the modeling of component fractionation in heat exchangers. Element approach is employed, and the amount of each component holdup is calculated element by element with a proper void fraction model. The circulation concentration is determined from the refrigerant differential holdup in heat exchangers. Simulations are carried out to prove the validity. The results can improve the reliability and efficiency in zeotropic refrigerant applications.

Numerical Investigations on Supercritical Heat Transfer Characteristics of Environmental Friendly Refrigerants

The heat transfer of supercritical fluids is a vastly growing field, specifically to find suitable alternatives to replace conventional R134a, which can be beneficial for climate change. Most of the experimental and numerical investigations have been conducted to explore supercritical water, carbon dioxide and R134a as heat transfer working fluids. Hydrofluoroolefin (HFO) and refrigerants blends have been considered the most environment-friendly refrigerants to replace Chlorofluorocarbons (CFCs), Hydrochlorofluoro-carbons (HCFCs) and Hydrofluorocarbons (HFCs). Their main advantage of zero Ozone Depletion Potential (ODP) and comparatively lower Global Warming Potential (GWP) have attracted growing amount of attention to mitigate environmental issues. This work adopts the computational method and takes the environmentally friendly refrigerants to investigate the heat transfer characteristics under widely used shear-stress transport (SST) model. A comprehensive comparison was performed at reduced pressure of 1.10 for supercritical fluids R515A, R1234ze(E) and R134a. The peaks of heat transfer coefficient occurred in the vicinity of pseudo critical temperature for all of these considered fluids;however, R134a resulted in higher heat transfer coefficient, Reynolds number and Prandtl number in comparison with R515A and R1234ze(E). The higher heat transfer coefficient of supercritical fluid R134a is owing to its thermophysical properties and the specific heat plays crucial role in the heat transfer of supercritical fluids. Owing to environmental issues, R515A can be a considerable replacement of R134a. R1234ze(E) is also promising alternative to R134a;however, safety issues should thoroughly concern its mild flammable characteristics.

Theoretical and Experimental Study of Explosion Limits and the Inhibition of Flammable Refrigerants

In this study, we analyze factors affecting the explosion limits of flammable refrigerants. We conclude that any method used for measuring flammable refrigerant explosion limits has its conditional restrictions. Flammable refrigerants in the atmosphere can also explode under certain conditions, when the concentration is approaching the explosion limits. An experimental study on the explosion limits of six kinds of flammable refrigerants is carried out with a mixture of refrigerant and combustible refrigerant, which has a similar effect to a flame retardant. An experimental apparatus was designed to test the explosion limits of mixtures made from three different nonflammable refrigerants and six different flammable refrigerants. Two practical models were developed to estimate the critical concentration for inhibiting explosion of refrigerant mixtures: one was made up of two flammable components with one nonflammable component, and the second was made up of one flammable component with two nonflammable components.

ENTROPY CHARACTERISTICS OF MAGNETIC REFRIGERANTS

Mathematical models for predicting magnetic entropy,lattice entropy and electron entropy of magnetic refrigerants are carried out from the molecular field appoximation,the Debye model and the electron energy level theory respectively of quantum mechanics.Using theabove models the temperature and magnetic field dependences of the calculated magnetic entropy,lattice entropy,electron entropy and the total entropy of gadolinium,which is a typical magnetic refrigerant near room temperature,are obtained.The Influences of kinds of entropy on the performance of magnetic refrigeration are discussed,and some general rules for selecting magnetic refrlyerants are drawn to hely the develoyment of highly economic magnetic refrigerators.

Needs of thermodynamic properties measurements and modeling in the frame of new regulations on refrigerants

In 1987, the Montreal Protocol prohibited the worldwide use and production of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) and hydro fluorocarbons (HFCs) were proposed as alternative refrigerants. Unfortunately, HFCs have non negligible global warning potential and therefore new refrigerants must be proposed or old refrigerants must be used associated with HFC. Accurate experimental thermodynamic data and predictive techniques are required for better under-standing of the performance of the newly proposed refrigerants. In this communication, experimental techniques based on either analytic or synthetic methods are first described. Data are reported. Then two newly developed predictive models based on thermodynamic approach with the isofugacity criterion and artificial neural network method are presented. The results can provide better evaluation of refrigerants, especially with the aim of studying global warning effects.

Thermodynamic Analysis of Hydrocarbon Refrigerants-Based Ethylene BOG Re-liquefaction System

The present study aims to make a thermodynamic analysis of an ethylene cascade re-liquefaction system that consists of the following two subsystems: a liquefaction cycle using ethylene as the working fluid and a refrigeration cycle operating with a hydrocarbon refrigerant. The hydrocarbon refrigerants considered are propane(R290), butane(R600), isobutane(R600a), and propylene(R1270). A computer program written in FORTRAN is developed to compute parameters for characteristic points of the cycles and the system's performance, which is determined and analyzed using numerical solutions for the refrigerant condensation temperature, temperature in tank, and temperature difference in the cascade condenser. Results show that R600 a gives the best performance, followed by(in order) R600, R290, and R1270. Furthermore, it is found that an increase in tank temperature improves system performance but that an increase in refrigerant condensation temperature causes deterioration. In addition, it is found that running the system at a low temperature difference in the cascade condenser is advantageous.

OPTIMIZATION OF THE COMPOSITIONS FOR CFC ALTERNATIVE MIXTURE REFRIGERANTS

In this work,eight commonly used and recently developed cubic EOSs have beenextensively tested for the calculation of thermodynamic properties,including vapor pressure,vaporand liquid densities and heat of vaporization for 26 pure CFCs and their alternatives.The modifiedDu-Guo EOS is recommended for the vapor-liquid equilibria calculations of mixtures for its goodaccuracy.A method for the development of the optimized mixture compositions of CFC alternativeshas been proposed by using minimization of deviations between the vapor pressures of CFC and thealternative mixtures of interest.As examples,two binary mixtures R22-R142b and R22-R152a,and aternary R22-R142b-R152a have been tested.The results show that the vapor pressure of R12 can bewell duplicated by a mixture refrigerant with the optimized composition.On the other hand,thedeviations between the heat of vaporization are somewhat apparent within 10 percent.Moreover,allthe mixtures discussed here are of the characteristics of being near azeotropic。

AIR CONDITIONING REFRIGERANTS AND OZONE PROTECTION

Until recently, CFC-11 and CFC-12 were the most widely used refrigerant gases in air conditioning systems. Environmental agencies around the world, however, have specifical concern for these gases as a direct cause of the depletion of the Earth’s ozone layer. Consequently, the world’s industrial nations are taking concerted action to decrease the use of CFCs, calling for attention of the engineers and designers involved in the manufacture and application of refrigeration systems for air conditioning. This paper analyses the available alternatives of CFCs in the industry and examines the problem found in process application.

Neural Network Model for Boiling Heat Transfer of R22 and Its Alternative Refrigerants inside Horizontal Smooth Tubes

Accurate prediction of refrigerant boiling heat transfer coefficients is important for the design of evaporators. The generalized correlations have different forms, and could not provide satisfactory results for R22 and its alternative refrigerants R134a, R407C and R410A. This study proposes to use artificial neural network (ANNs) as a generalized correlation model, selects the input parameters of ANNs on the basis of the dimensionless parameter groups of existing correlations, and correlates the in-tube boiling heat transfer coefficients of the above four refrigerants. The results show that the ANNs model with the input and output based on the Liu-Winterton correlation has the best result. The root-mean-square deviations in training and test are 15.5% and 20.2% respectively, and approximately 85% of the deviations are within ±20%, which is much better than that of the existing generalized correlations.

Development of Environmentally Friendly and Energy Efficient Refrigerants for Refrigeration Systems

This paper presents the improvement of eco-friendly and power consumption saving refrigerants for refrigeration systems.The novel azeotropic refrigerant mixtures of HFCs and HCs can replace refrigeration systems,and using the R134,R32,R125,and R1270 refrigerants in several compositions found using the decision tree function of the RapidMiner software(which camefirst in the KDnuggets annual software poll).All refrigerant results are mixed of POE,which is A1 classification refrigerant,non-flammable,and innocuous refrigerant,and using REFPROP software and CYCLE_D-HX software are under the CAN/ANSI/AHRI540 standards.The boiling point of the new refrigerant mix R-No.595 is 4.58%,lower than that of R404A,with a higher refrigerant effect and 50.34%lower GWP value than R404A.The proposed mix R-No.595 can be operated in hot environmental country and has high critical temperature and heat-rejection effects,due to the presence of R32 and R1270.The COPc of R463A is 13.49%,higher than R404A in freeze condition.The novel refrigerant mixes provide alternate refrig-erant options mixed of 1%R1270,and which are related with the development of current refrigerants,containing a compose of HFOs and eco refrigerants for producing low-GWP,zero ODP,high-refrigerant effect,low-operating pressure,and innocuous refrigerants.

A New Excess Free Energy Mixing Rule for Representing Vapor-Liquid Equilibria of Mixed Refrigerants

A suitable mixing rule is important for vapor liquid equilibrium(VLE)investigations for mixed refrigerants.In this work,a new excess free energy mixing rule(MRv)was proposed at zero pressure based on the linear relationship between dimensionless parameter 1/(u-1)and a.MRv mixing rule was explicit adopted variable liquid molar volume.The applicable temperature range of MRv could be extended by means of an empirical method to estimate the liquid molar volume for components at high temperatures.Three mixing rules modified Huron-Vidal mixing rule(MHV1),Wong-Sandler mixing rule(WS),and MRv at two reference pressures were used to compare the VLE data in the calculation of 37 mixed refrigerants.Results demonstrated that MRv had a relatively similar accuracy to MHV1 and WS for component and pressure calculation.Moreover,the average excess Gibbs free energy using the MRv mixing rule for the 37 selected mixed refrigerants(0.0013)was much lower than those using the MHV1(0.0078)and WS(0.0809)mixing rules,which was very valuable for the design and optimization of thermodynamic systems using mixed refrigerants.

Evaluation of zeotropic refrigerants based on nonlinear relationship between temperature and enthalpy

In order to evaluate cycling characters of zeotropic refrigerants in air-con- ditioning operation, and to reveal distribution rules of temperature difference between refrigerants and heat transfer fluids in condenser and evaporator, theoretical researches were carried out based on nonlinear relationship between temperature and enthalpy in period of refrigerants’ phase change. Firstly, a phase changing model of refrigerants was built, and refrigerants state parameters were decided in the air-conditioning operation. Secondly, the state equation of refrigerants was applied for computing relationship be- tween temperature and enthalpy, else based on some suppositions, temperature differ- ences between 15 sorts of refrigerants and heat-transfer fluids were gotten too. Through concluding those temperature differences changing in condenser and evaporator, some rules were found. Lastly, after calculating and comparing the additive exergy loss among 15 sorts of refrigerants, which resulted from the changing of temperature difference, their cycling characters evaluation were presented.

Prospect Evaluation of Low-GWP Refrigerants R1233zd(E)and R1336mzz(Z)Used in Solar-Driven Ejector-Vapor Compression Hybrid Refrigeration System

In this paper,the entrainment ratio,pump work,heat loads of heat exchangers and COPthermal were theoretically evaluated for a solar-driven ejector-vapor compression hybrid refrigeration system with R1233zd(E)and R1336mzz(Z)as the working fluids.The evaluation of the utilization potentials of R1233zd(E)and R1336mzz(Z)was presented by comparing the system performance with that of R245fa,a commonly used refrigerant in the ejector system.The results indicated that the systems with R1233zd(E)and R1336mzz(Z)had a higher entrainment ratio and lower pump work.The pump works when using R1233zd(E)and R1336mzz(Z)can be up to 14.59%and 38.05%lower than those of R245fa,respectively.Meanwhile,the system showed the highest COPthermal utilizing R1233zd(E)followed by that of R245fa,with the R1336mzz(Z)system having the lowest value.The differences between R1233zd(E)and R1336mzz(Z)systems,R1233zd(E)and R245fa systems were 4.33%and 2.0%,respectively.This paper was expected to provide a good reference for the utilizing prospect of R1233zd(E)and R1336mzz(Z)in ejector refrigeration systems.

Simulation study on performance of a dual-source hybrid heat pump unit with alternative refrigerants

To solve the problems of single heat source heat pump systems in severe cold regions,a dual-source hybrid heat pump unit(DSHHPU)is proposed.The mathematical models of the DSHHPU when charging R134a or its alternative refrigerants R32,R290 and R600a were established respectively,and the performance was simulated and analysed.The results showed that the four refrigerants have different performance characteristics in different aspects.In heat pipe mode,the heating capacity and evaporating pressure of R32 are 36.94%and 59.94%higher than those of R134a.The heating capacity and evaporating pressure of R290 are 5.73%and 22.99%lower than those of R134a.The heating capacity and evaporating pressure of R600a are 43.29%and 68.08%lower than those of R134a.In vapour compression heating mode,the discharge temperature of R32,R290 and R600a are 184.88,72.98 and 66.44%of that of R134a.The coefficient of performance(COP)of R32,R290 and R600a are 72.65,111.59 and 117.94%of that of R134a.Finally,the effects of radiation intensity and ambient temperature on key performance parameters of the different refrigerants were analysed.The research results provide a reference for research on refrigerant replacements for multi-heat source composite heat pump systems.

Evaluation on Excess Entropy Scaling Method Predicting Thermal Transport Properties of Liquid HFC/HFO Refrigerants

The application of the excess entropy scaling(EES)method to predict the viscosity,thermal conductivity and thermal diffusivity of HFC/HFO refrigerants is evaluated in this paper.The universal coefficients of the EES model were firstly obtained from the properties of HFC refrigerants,and the accuracy of the model was further investigated with HFO properties.It was suggested that the EES model correlated the viscosity very well with the average absolute deviations(AADs)of most HFC refrigerants lower than 6.55%except R32.The correlations also provided very good prediction on the viscosity for R1234yf and R1234ze(E),but not for R1336mzz(Z).The prediction of thermal conductivity for both HFC and HFO refrigerants was generally well with the maximum AAD of 11.44%.However,the paper also indicated that there were no universal thermal diffusivity coefficients for even HFC refrigerants,and the linear function could not fit the thermal diffusivity curve very well.Therefore,the exclusively two-order polynomial correlations based on the EES model were presented for each HFC/HFO refrigerant.

Performance of a single-stage Linde-Hampson refrigerator operating with binary refrigerants at the temperature level of-60°C

The optimization of the performance of a single-stage Linde-Hampson refrigerator （LHR） operating with six different binary refrigerants （R23/R134a, R23/R227ea, R23/R236ea, R170/R290, R170/R600a and R170/R600） with ozone depletion potentials （ODPs） of zero was conducted using a new approach at the temperature level of-60℃. Among these binary refrig- erants, the 0.55 and the 0.6 mole fractions of R23 for R23/R236ea are the most prospective nonflammable ones for the medium and low suction pressure compressors, respectively. For these two kinds of compressors, the 0.6 and the 0.65 mole fractions of R170 for R 170/R600, respectively, are the most prospective binary refrigerants with low global warming potentials （GWPs）. The results of optimization of pressure levels indicate that the optimum low pressure value for coefficients of performance （COP） is achieved when the minimum temperature differences occur at both the hot and the cold ends of the recuperator at a specified composition and pressure ratio. Two useful new parameters, the entropy production per unit heat recuperated and the ratio of heat recuperating capacity to the power consumption of the compression, were introduced to analyze the exergy loss ratio in the recuperator. The new approach employed in this paper also suggests a promising application even to the optimization of the performance with multi-component refrigerants.

Table-like magnetocaloric effect and large refrigerant capacity of composite magnetic refrigerants based on LaFe_(11.6)Si_(1.4)H_y alloys

Composite magnetic refrigerants were prepared by physical mixing LaFeSiHalloys with different Curie temperatures(Tc). The phase structures of these LaFeSiHalloys were analyzed by X-ray diffraction(XRD) and the magnetocaloric effect(MCE) and refrigerant capacity(RC) of these composite magnetic refrigerants were investigated by experiment and calculation in this paper. The magnetocaloric effect(MCE) and refrigerant capacity(RC) of these composite magnetic refrigerants were investigated by experiment and calculation in this paper. The results indicate the experimental magnetic entropy change(-△S)-Tcurve corresponds reasonably with the(-△S)-Tcurve calculated by the linear combination of(-△S)-T curves of the single material. An optimal mixing ratio can make the composite magnetic refrigerant possess a table-like(-△S)-Tcurve which is beneficial to magnetic Ericsson cycle. When three LaFeSiHalloys with different Tare mixed, the full width at half maximum(△T) of(-△S)-T curves is about 48.7 K and the RC is about 177.76 J/kg under a magnetic field change of 2 T. The composite magnetic refrigerants based on LaFeSiHalloys can be promising candidates for near room temperature magnetic refrigeration and the work will be helpful to develop novel composite magnetic refrigerants with table-like MCE and large RC.

Evaluation of non-azeotropic mixtures containing HFOs as potential refrigerants in refrigeration and high-temperature heat pump systems

With the increasing environmental concern on global warming, hydrofluoro-olefin (HFOs), possessing low GWP, has attracted great attention of many researchers recently. In this study, non-azeotropic mixtures composed of HFOs (HFO-1234yf, HFO-1234ze(z), HFO-1234ze(e) and HFO-1234zf) are developed to substitute for HFC-134a and CFC-114 in air-conditioning and high-temperature heat pump systems, respectively. The cycle performances were evaluated by an improved theoretical cy-cle evaluation methodology. The results showed that all the mixtures proposed herein were favorable refrigerants with excel-lent thermodynamic cycle performances. M1A presented lower discharge temperature and pressure ratio and higher COPc than that of HFC-134a. The volumetric cooling capacity was similar to HFC-134a. It can be served as a good environmentally friendly alternative to replace HFC-134a. M3H delivered similar discharge temperature as CFC-114 did. And the COPh was 3% higher. It exhibits excellent cycle performance in high-temperature heat pump and is a promising refrigerant to substitute for CFC-114. And the gliding temperature differences enable them to exhibit better coefficient of performance by matching the sink/source temperature in practice. Because the toxicity, flammability and other properties are not investigated in detail, ex-tensive toxicity and flammability testing needs to be conducted before they are used in a particular application.

An Improved Prediction Equation of Refrigerants Surface Tension Based on the Principle of Corresponding States

The surface tensions of 21 pure refrigerants have been predicted by a new improved equation based on the principle of corresponding states with double referenced fluids.The average absolute deviation between the calculated surface tensions from the equation and the experimental result is-0.015 mN/m.The surface tensions of 9 binary mixtures were calculated in consideration of certain mixing rules.And the average absolute deviation between the calculated surface tensions from the equation and the experimental result is-0.251 mN/m.The new improved prediction equation can be used for calculating the surface tension of environmental friendly refrigerants.

Chemically inducing room temperature spin-crossover in double layered magnetic refrigerants Pr1.4+xSr1.6-xMn2O7(0.0≤x≤0.5)

The height of total entropy(S)for a magnetic refrigerant material is essentially concerned with the magnetic and structural transitions.However,the participation of such transitions in layered materials is not well understood.Therefore,the purpose of this work is to investigate the interplay between double layer lattice with their single perovskite counterpart,to achieve optimal magnetocaloric performance.A series of self-doped Pr_(1.4+x)Sr_(1.6-x)Mn_(2)O_(7)(0.0≤x≤0.5)Ruddlesden-Popper(R-P)perovskite have been prepared through the solid-state sintering method.With increasing the Pr-stoichiometry,the lattice faults have increased and the double layer lattice dramatically disintegrates into single perovskite structure.Due to the reduction of bilayer R-P phase into single perovskite the spin crossover occurs from weak bilayer(T=304 K)interactions towards the strong three-dimensional(T=308 K)interactions respectively.This series consistently develops thermomagnetic irreversibility in zero-field cooled(ZFC)-field cooled(FC)magnetization,which is indicative of a spin-glass state.The glassy nature has been ascribed collectively to the lattice strain produced because of dislocations and to an antiferromagnetic phase segregated at the surface.The maximum value of temperature average entropy change(TEC)and adiabatic temperature(ΔT)has enhanced nearly by 4 folds from 0.53 J kg^(-1)K^(-1),0.59 K(for x=0.0)up to 1.85 J kg^(-1)K^(-1),10 K(for x=0.5)at 2.5 T,respectively.Additionally,the room temperature relative cooling power has improved from 26.94 J/kg up to 77.84 J/kg with an applied field of 2.5 T.Our findings in this work suggest that the controlled reduction of double layer lattice into single perovskite and/or existence of both phases simultaneously in bilayer R-P manganites may be very effective in obtaining the desirable characteristics of magnetocaloric effects.