Application of an ejector in autocascade refrigeration cycle for the performance improvement

This paper presented a novel autocascade refrigeration cycle (NARC) with an ejector. In the NARC, the ejector is used to recover some available work to increase the compressor suction pressure. The NARC enables the compressor to operate at lower pressure ratio, which in turn improves the cycle performance. Theoretical computation model based on the constant pressure-mixing model for the ejector is used to perform a thermodynamic cycle analysis for the NARC with the refrigerant mixture of R23/R134a. The effects of some main parameters on cycle performance were investigated. The results show the NARC has an outstanding merit in decreasing the pressure ratio of compressor as well as increasing the COP. For NARC operated at the condenser outlet temperature of 40 °C, the evaporator inlet temperature of −40.3 °C, and the mass fraction of R23 is 0.15, the pressure ratio of the ejector reaches to 1.35, the pressure ratio of compressor is reduced by 25.8% and the COP is improved by 19.1% over the conventional autocascade refrigeration cycle.     

Performance characteristics of an irreversible magnetic Brayton refrigeration cycle

Based on the thermodynamic properties of a paramagnetic salt, an irreversible model of the magnetic Brayton refrigeration cycle is established, in which the working substance is a special paramagnetic material. The expressions of the important performance parameters, such as the coefficient of performance, refrigeration load and work input, are derived. Moreover, the optimal performance parameters are obtained at the maximum coefficient of performance. The results obtained here may include the ones of the magnetic Brayton refrigeration cycle using the magnetic material obeyed the Curie law as the working substance, the magnetic Brayton refrigeration cycle without regeneration and the eversible magnetic Brayton refrigeration cycle. Therefore, the results obtained here have general significance and will be helpful to deeply understand the performance of a magnetic Brayton refrigeration cycle.     

Modélisation des données thermodynamiques du mélange ammoniac/eauModelling of the thermodnyamic properties of the ammonia/water mixture

The present work deals with the modeling of the thermodynamic properties of the ammonia/water mixture using the Gibbs free energy function. For the liquid phase, a three constant Margules model of the excess free enthalpy is formulated. The vapour phase is considered as a perfect mixture of real gases, each pure gas being described by a virial equation state in pressure truncated after the third term. The model developed describes with a good accuracy the mixture in the three states, subcooled liquid, superheated vapour and liquid-vapour saturation for temperatures from 200 to 500 K and pressures up to 100 bar.     

Thermodynamic evaluation of new absorbent mixtures of lithium bromide and organic salts for absorption refrigeration machines

Mixtures of lithium bromide and organic salts of sodium and potassium (formate, acetate and lactate) have been evaluated as alternative absorbents for absorption refrigeration machines. The main objective is to overpass the limitations of lithium bromide and improve the characteristics and the efficiency of the refrigeration cycle. In order to select the mixture that presents better properties for its employment in absorption refrigeration cycles, a thermodynamic analysis have been done. Density, viscosity, enthalpies of dilution, solubility and vapour pressure data of the proposed mixtures have been measured. A simulation program has been developed to evaluate temperatures, heats exchanged in the different sections and the efficiency of the cycle.     

Numerical investigation of a diffusion absorption refrigeration cycle

A thermodynamic model was developed for an ammonia–water diffusion absorption refrigeration (DAR) cycle with hydrogen or helium as the auxiliary inert gas, manufactured by Electrolux Sweden (currently known as Dometic). The performance of the system was examined parametrically by computer simulation. Mass and energy conservation equations were developed for each component of the cycle and solved numerically. The model was validated by comparison with previously published experimental data for DAR systems. Investigation of cycle performance under different conditions indicated that the best performance was obtained for a concentration range of the rich solution of 0.2–0.3 ammonia mass fraction and that the recommended concentration of the weak solution was 0.1. It was also found that as the degree of rectification decreased, the performance of the DAR cycle decreased. Finally, the study showed that helium was superior to hydrogen as the inert gas: the coefficient of performance of a DAR unit working with helium was higher by up to 40% than a cycle working with hydrogen.     

Development and validation of “grey-box” models for refrigeration applications: A review of key concepts

“Grey-box” modelling combines the use of first-principle based “white-box” models and empirical “black-box” models, offering particular benefits when: (a) there is a lack of fundamental theory to describe the system or process modelled; (b) there is a scarcity of suitable experimental data for validation or (c) there is a need to decrease the complexity of the model. The grey-box approach has been used, for example, to create mathematical models to predict the shelf life of chilled products or the thermal behaviour of imperfectly mixed fluids, or to create models that combine artificial neural networks and dynamic differential equations for control-related applications. This paper discusses the main characteristics of white-box, black-box and their integration into grey-box models, the requirements and sourcing of accurate data for model development and important validation concepts and measures.     

A theoretical study on a novel combined power and ejector refrigeration cycle

A new combined power and refrigeration cycle is proposed for the cogeneration, which combines the Rankine cycle and the ejector refrigeration cycle by adding an extraction turbine between heat recovery vapor generator (HRVG) and ejector. This combined cycle could produce both power output and refrigeration output simultaneously, and could be driven by the flue gas from gas turbine or engine, solar energy, geothermal energy and industrial waste heats. Parametric analysis and exergy analysis are conducted to examine the effects of thermodynamic parameters on the performance and exergy destruction in each component for the combined cycle. The results show that the condenser temperature, the evaporator temperature, the turbine inlet pressure, the turbine extraction pressure and extraction ratio have significant effects on the turbine power output, refrigeration output, exergy efficiency and exergy destruction in each component in the combined cycle. It is also shown that the biggest exergy destruction occurs in the heat recovery vapor generator, followed by the ejector and turbine.     

Optimising the refrigeration cycle with a two-stage centrifugal compressor and a flash intercooler

The optimisation of a refrigeration process with a two-stage centrifugal compressor and flash intercooler is presented in this paper. The two-stage centrifugal compressor stages are on the same shaft and the electric motor is cooled with the refrigerant. The performance of the centrifugal compressor is evaluated based on semi-empirical specific-speed curves and the effect of the Reynolds number, surface roughness and tip clearance have also been taken into account. The thermodynamic and transport properties of the working fluids are modelled with a real-gas model. The condensing and evaporation temperatures, the temperature after the flash intercooler, and cooling power have been chosen as fixed values in the process. The aim is to gain a maximum coefficient of performance (COP). The method of optimisation, the operation of the compressor and flash intercooler, and the method for estimating the electric motor cooling are also discussed in the article.     

Modelling of the thermodynamic properties of the water–ammonia mixture by three different approaches

This paper deals with the modelling of the thermodynamic properties of the water–ammonia refrigerant mixture. Three different approaches are formulated and compared. The first is an empirical approach based on a free enthalpy model of the mixture considered as the resultant of the properties of its pure components and of an excess term corresponding to the deviation to the ideal solution concept. Secondly, a semi-empirical approach based on the PATEL and TEJA cubic equation of state is considered. Finally, a theoretical approach formulated as PC-SAFT (perturbed chain statistical associating fluid theory) equation of state is treated. A comparison of these three methods proves the superiority of PC-SAFT in predicting and extrapolating the thermodynamic properties of the water–ammonia system up to very high temperatures and pressures.     

Experimental investigation of mass recovery adsorption refrigeration cycle

The study investigates the performance of silica gel–water adsorption refrigeration cycle with mass recovery process by experimental prototype machine. In an adsorption refrigeration cycle, the pressures in adsorber and desorber are different. The mass recovery cycle utilizes the pressure difference to enhance the refrigerant mass circulation. Moreover, novel cycle was proposed for improvement of cooling output. In our previous study, simulation analysis shows that mass recovery cycle has the advantage over conventional single-stage. Experiments with prototype machine were conducted to investigate the performance improvement of mass recovery cycle in the present paper. Specific cooling power (SCP) and coefficient of performance (COP) were calculated with experimental data to analyze the influences of operating conditions. The proposed cycle was compared with the single-stage cycle in terms of SCP and COP. The results show that SCP of mass recovery cycle is superior to that of conventional cycle and mass recovery cycle is effective with low temperature heat source.     

A new thermoeconomic approach and parametric study of an irreversible regenerative Brayton refrigeration cycle

The detailed parametric study of an irreversible regenerative Brayton refrigerator cycle using the new thermoeconomic approach is presented in this paper. The external irreversibility is due to finite temperature difference between the cycle and the external reservoirs while the internal irreversibilities are due to the nonisentropic compression and expansion processes and the regenerative loss. The thermoeconomic objective function defined as the cooling load per unit cost is optimized with respect to the state point temperatures for a typical set of operating conditions. The power input and cooling load are found to be decreasing functions of the expansion outlet temperature (T₁), while it is the reverse in the case of COP. On the other hand, there are optimal values of the temperature T₁, cooling load, power input and COP at which the cycle attains the maximum objective function for a typical set of operating parameters. Again, the objective function, COP and cooling load further enhance, while the power input goes down, as the various values of the effectiveness or efficiency components are increased.     

Thermodynamic analysis of a liquid-flooded Ericsson cycle cooler

A novel approach to implementing a gas Ericsson cycle heat pump was developed. The concept, termed a liquid-flooded Ericsson cooler (LFEC), uses liquid flooding of the compressor and expander to approach isothermal compression and expansion processes. Analytical models of liquid-flooded compression and expansion processes were developed using ideal gas, constant specific heat, and incompressible liquid assumptions. Special considerations for use of positive displacement compressors with fixed volume ratios are detailed. The unique behavior of a liquid-flooded compressor was explored, including the discovery of an optimum liquid flooding rate that minimizes compression power. A computer model of the LFEC cycle was developed using ideal gas, incompressible liquid, and constant specific heat assumptions. The model was used for a thorough parametric study. The purpose of the study was to explore the feasibility of the concept, identify the optimum operating parameters, and to provide a basis for the design of an experimental system.     

Analysis of room temperature magnetic regenerative refrigeration

Results of a room temperature magnetic refrigeration test bed and an analysis using a computational model are presented. A detailed demonstration of the four sequential processes in the transient magnetocaloric regeneration process of a magnetic material is presented. The temperature profile during the transient approach to steady state operation was measured in detail. A 5 °C evolution of the difference of temperature between the hot end and the cold end of the magnetocaloric bed due to regeneration is reported. A model is developed for the heat transfer and fluid mechanics of the four sequential processes in each cycle of thermal wave propagation in the regenerative bed combined with the magnetocaloric effect. The basic equations that can be used in simulation of magnetic refrigeration systems are derived and the design parameters are discussed.     

Mixing and separation characteristics of isobutane with refrigeration oil

This paper discusses the transient mixing and separation characteristics of isobutane with/from refrigeration oil. The mixing/separation processes are observed and investigated experimentally in a glass cylindrical vessel. Since liquid isobutane is less dense than refrigeration oil, the mixing process proceeds one dimensionally by diffusion from the interface between isobutane gas and refrigeration oil. The progress of mixing, therefore, is very slow compared with a combination of halocarbon refrigerant and refrigeration oil having convection flow during the mixing process. The diffusion process can be analyzed using a one-dimensional diffusion model with an appropriate diffusion coefficient, which increases linearly with temperature. The separation of isobutane from the oil–refrigerant mixture occurs at the interface and the denser oil from which isobutane is separated causes a convective flow. Bubble generation under the depressurized conditions is unstable, but in the most cases, it tends to start when a high super saturation degree is reached. The temperature change during the separation process is estimated using latent heat as the separation heat of refrigerant.     

Refrigeration Carnot-type cycle based on isothermal vapour compression

A refrigeration Carnot-type cycle based on isothermal compression and two reversible expansions is proposed. Although ideal, this cycle is close to a realistic one which could be designed with existing hardware.     

Thermodynamics of magnetic refrigeration

A comprehensive treatment of the thermodynamics of cyclic magnetic refrigeration processes is presented. It starts with a review of the work, heat and internal energy of a magnetized specimen in a magnetic field, and a list of the thermodynamic potentials is given. These are based on the very recent discovery of an alternative Kelvin force. It is shown that this force is compatible with the internal energy proposed by Landau and Lifshitz. New formulas for the specific enthalpies are presented. Cyclic processes are discussed in detail, e.g. the Brayton, Ericsson and Carnot cycles. Magnetic refrigeration and magnetic heat pump cycles are preferably designed by applying the cascade or/and regeneration principle. Cascade systems allow wider temperature ranges to be obtained. The main objective of this article is to yield a theoretical basis for an optimal design of new magnetic refrigeration and heat pump devices.     

Transcritical CO2 refrigeration cycle with ejector-expansion device

An ejector expansion transcritical CO₂ refrigeration cycle is proposed to improve the COP of the basic transcritical CO₂ cycle by reducing the expansion process losses. A constant pressure mixing model for the ejector was established to perform the thermodynamic analysis of the ejector expansion transcritical CO₂ cycle. The effect of the entrainment ratio and the pressure drop in the receiving section of the ejector on the relative performance of the ejector expansion transcritical CO₂ cycle was investigated for typical air conditioning operation conditions. The effect of different operating conditions on the relative performance of the ejector expansion transcritical CO₂ cycle was also investigated using assumed values for the entrainment ratio and pressure drop in the receiving section of the ejector. It was found that the COP of the ejector expansion transcritical CO₂ cycle can be improved by more than 16% over the basic transcritical CO₂ cycle for typical air conditioning operation conditions.     

Thermodynamic analysis of the V-type Stirling-cycle refrigerator

The thermodynamic analysis of a V-type Stirling-cycle refrigerator is performed. The Stirling-cycle refrigerator consists of expansion and compression spaces, cooler, heater and regenerator, and divided into 14 fixed control volumes subjected to a periodic mass flow. The conservation of mass and energy equation are written for each control volume. A computer program is prepared in FORTRAN, and the basic equations are solved iteratively. The mass, temperature and density of working fluid in each control volume are calculated for a given charge pressure, engine speed, and fixed heater and cooler surface temperatures, and the results are obtained from a PC. The heat transfer coefficients are assumed constant. The work, instantaneous pressure and COP of the Stirling-cycle refrigerator are also calculated. The steady cyclic conditions are obtained for temperature after few cycles and the results are given by diagrams.     

Performance of a packed bed absorber for aqua ammonia absorption refrigeration systemPerformance d''un matelas dispersant dans un système frigorifique à absorption à ammoniac/eau

A mathematical model of a packed bed absorber for aqua-ammonia absorption refrigeration system is presented. The model is used to predict the performance of the bed at various design and operating conditions. The governing equations and the boundary conditions are derived to predict the bed performance. A numerical integral method and an iteration scheme are used to solve the governing one dimensional, non-linear simultaneous differential equations which are subjected to three point boundary value problem. A computer program is prepared and carefully debugged to solve the governing equations with the help of some supporting equations to describe the properties of the working fluids and the heat and mass transfer coefficients in the bed. The analysis show that the absorption process is affected by the following parameters: the volumetric heat rejection model, bed height, vapor and solution flow rates to the bed and the inlet conditions; and packing material type. The effect of changing each of those parameters on the performance of the bed is studied after suggesting a model for the volumetric heat rejection from the bed. The results showed that changing the bed pressure and/or the vapor inlet temperature have negligible effect on the performance of the bed. Changing other parameters are found to affect the performance of the bed by different degrees. Also, the results show that within the present range of parameters, a bed height less than 0.7 m guarantees an absorption efficiency better than 91%.

Résumé

On présente un modèle mathématique d'un matelas dispersant dans un système frigorifique à absorption à ammoniac/eau. On utilise ce modèle pour prévoir la performance du matelas utilisant diverses conceptions et sous diverses conditions de fonctionnement. On établit des équations qui décrivent ce processus et les conditions limites afin de prévoir la performance du matelas. On utilise une méthode numérique intégrale et un schéma d'itération afin de résoudre les équations unidimensionnelles, non-linéaires, simultanées et différentielles, qui sont soumises au problème des limites à trois points. Un programme informatique est préparé et débogué afin de résoudre les équations qui gouvernent le processus étudié, avec l'aide de quelques équations supplémentaires qui décrivent les caractéristiques des fluides actifs et les coefficients de transmission thermique et de transfert d'énergie massique du matelas. L'analyse montre que le processus d'absorption est influencé par des paramètres suivants: le modèle de rejet de chaleur volumétrique, la hauteur du matelas, les débits d'écoulement de la vapeur et de la solution vers le matelas, les conditions d'entrée et le matériau dispersant utilisé. On étudie également l'influence de la variation de chacun de ces paramètres sur la performance du matelas apres avoir proposé un modèle de rejet de chaleur volumétrique par le matelas. Les résultats montrent que si on change la pression dans le matelas et/ou la température de la vapeur à l'arrivée, de tels changements ont un effet negligeable sur le matelas. Suite au changement d'autres paramètres, la performance du matelas a été modifée de diverses façons. Les résultats montrent également qu'avec les paramètres adoptés ici, une hauteur du matelas inférieure à 0,7 m assure un taux d'absorption supérieur à 91%.     

Modeling absorption of pure refrigerants and refrigerant mixtures in lubricant oil

This paper addresses the problem of absorption of refrigerant vapor in a stagnant layer of lubricant oil. The bulk motion of the solute is described in terms of apparent diffusion coefficients that encompass both molecular diffusion and possible macroscopic motion induced by liquid density instability and surface tension. In absorption of refrigerant mixtures, diffusion in the vapor and liquid phases are coupled with a thermodynamic model for interfacial equilibrium. Results are compared with experimental data available in the literature for absorption of several refrigerants in polyol ester oil (POE68). The adequacy of the formulation is assessed in the light of its basic assumptions and performance of the model.