The Efficient Computational Tools For The Design Process Of The Transcritical Two-Phase Ejectors For Natural-Based Working Fluids

Naturalbased working fluids for refrigeration are becoming a standard commercial solution due to the dynamic research and development in this area as well as law regulations. The stateoftheart ejector technology for R744 systems reached current status due to a significant interest on modelling approaches and effective regulation concepts. The development path of the fast and efficient design tools based on the numerical simulations could be described as a key feature for the R744 commercial technology. In this study, one of the most effective numerical approaches dedicated for the twophase CO2 ejector design and analysis is discussed. Namely, homogeneous equilibrium and relaxation model for high motive pressures and mixture approach for lower motive pressures were reviewed. According to the requirements of the effective design tools, the comparison also included a prediction of the vapour quality at given operating conditions and the corresponding computational costs. Moreover, several research studies on swirling and bypassing solutions as well as commercial applications of multiejector device and reduced order models for regulation systems where aforementioned models were used was described. Conclusions on a potential of the reviewed approaches were formulated having regard possible utilisation for the design process of the ejector based R744 systems

Heat Transfer Process Within The R744 Two-phase Ejector: Numerical And Experimental Study

The proposed three dimensional CFD model to simulate the influence of the heat transfer on the R744 two-phase ejector performance is presented. The numerical model was developed based on the homogeneous real fluid flow assumption with the enthalpy-based formulation of the energy equation. The R744 two-phase ejector was designed to evaluate the temperature profile within the ejector walls. The prototype R744 ejector for experimental investigation was manufactured by Institute of Thermal Technology and ATM in Poland. The performance measurements were carried out at a R744 test facility at SINTEF/NTNU in Norway. The foregoing ejector was equipped with the thirteen thermocouples located inside the ejector to measure the wall temperature in different ejector section i.e. the motive nozzle, the suction nozzle, the mixing section and the diffuser. The experimental test campaign at different operating conditions typical for refrigeration application was carried out and the uncertainty of the measurement was defined. Moreover, the experimental data are applied to validate the CFD results at defined operating conditions. The numerical results were set to evaluate the influence of the wall temperature on the two-phase flow parameters. In addition, the heat transfer coefficient of the two-phase flow within the ejector was estimated. The analysis of the heat transfer process within the R744 two-phase ejector let to investigate the influence of the ambient conditions and the different temperature levels of the motive and suction streams on the ejector performance

Thermal analysis of 8.5 MVA disk-type power transformer cooled by biodegradable ester oil working in ONAN mode by using advanced EMAG–CFD–CFD coupling

Power transformers are the first devices used to transfer the electrical energy produced in power plants to the grid to supply the industrial and individual receivers with electricity. The heat generation in windings and core, being an effect of the power losses, is usually dissipated in large units by using mineral oils, which are harmful to the environment. Nowadays, the industry and global society seek environmentally-friendly alternatives. One of the most promising substitute for their high biodegradability, safety in operation, and favourable thermo-physical properties are natural ester oils. For this reason, a numerical study of 8.5 MVA disk-type power transformer cooled using conventional mineral oil and a commercially used rapeseed ester oil is presented in this paper. Moreover, due to different thermal behaviour of the considered oils, the comparison was made for the unit working in different seasons of hot and moderate climate zones (Argentina and Poland). In the numerical approach, electromagnetic (EMAG) and computational fluid dynamics (CFD) models were used for a detailed study of the selected device. In particular, a novel and very efficient EMAG–CFD–CFD coupling procedure was developed to assess the cooling of the large power transformer. Such a coupled computational procedure allowed for the detailed investigation of the power loss, oil flow characteristics, and temperatures with a satisfying computational effort. The results showed that the average windings temperatures are higher by 2–9 K when the ester oil is used, dependent on the ambient conditions. The hotspot temperature in the low voltage windings increased by up to 9 K and up to 18 K in the high voltage windings using ester oil. According to the results, the oil duct construction requires modification in the high voltage region for transformers cooled using mineral oil in cold climate conditions.Fil: Stebel, Michal. Silesian University Of Technology; PoloniaFil: Kubiczek, Krzysztof. Silesian University Of Technology; PoloniaFil: Rios Rodriguez, Gustavo Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Centro de Investigaciones en Métodos Computacionales. Universidad Nacional del Litoral. Centro de Investigaciones en Métodos Computacionales; ArgentinaFil: Palacz, Michal. Silesian University Of Technology; PoloniaFil: Garelli, Luciano. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Centro de Investigaciones en Métodos Computacionales. Universidad Nacional del Litoral. Centro de Investigaciones en Métodos Computacionales; ArgentinaFil: Melka, Bartlomiej. Silesian University Of Technology; PoloniaFil: Haida, Michal. Silesian University Of Technology; PoloniaFil: Bodys, Jakub. Silesian University Of Technology; PoloniaFil: Nowak, Andrzej J.. Silesian University Of Technology; PoloniaFil: Lasek, Pawel. Silesian University Of Technology; PoloniaFil: Stepien, Mariusz. Silesian University Of Technology; PoloniaFil: Pessolani, Francisco. Tadeo Czerweny S.a.; ArgentinaFil: Amadei, Mauro. Tadeo Czerweny S.a.; ArgentinaFil: Granata, Daniel. Tadeo Czerweny S.a.; ArgentinaFil: Storti, Mario Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Centro de Investigaciones en Métodos Computacionales. Universidad Nacional del Litoral. Centro de Investigaciones en Métodos Computacionales; ArgentinaFil: Smolka, Jacek. Silesian University Of Technology; Poloni

Development of the natural working fluid‐based refrigeration system for domestic scale freeze‐dryer

In this work, the analysis of the refrigeration system designed for the FrostX 10 freeze‐dryer is presented. The main goal of this study was to experimentally investigate the reference R452a freeze‐dryer and prepare recommendations for a machine based on the R290 refrigeration unit. In order to guarantee the temperature requirements and efficient operation of that unit, the analysis of suitable natural refrigerants was performed. Consequently, propane (R290) was selected. In addition, a number of modifications were introduced for the prototype system. System analysis showed that the replacement of the refrigerant in the existing system improves the system energy efficiency by approximately 18%. During the experimental campaign of the basic refrigeration unit, an unstable operation of the evaporator was found. The concept of a new cooling system for a prototype device was presented. The configuration and type of heat exchanger to maximise the performance of the ice trap of the freeze‐dryer were proposed.Development of the natural working fluid‐based refrigeration system for domestic scale freeze‐dryeracceptedVersio

Non-equilibrium approach for the simulation of CO2 expansion in two-phase ejector driven by subcritical motive pressure

A non-equilibrium approach was proposed for highly accurate modelling of the expansion process during two-phase flow in the convergent-divergent motive nozzle of an R744 ejector. Comprehensive mapping of the coefficients used in the source terms of the additional transport equation of the vapour quality was provided on the basis of four ejector geometries. The calibration range contained motive pressures from 50 bar to 70 bar, where the prediction quality of the homogeneous equilibrium (HEM) and relaxation (HRM) models, was unsatisfactory. The calibrated model was validated on the basis of experimental mass flow rate data collected from 150 operating points. The mapping results were utilised for final model derivation in the form of an approximation function for R744 expansion. The validation process resulted in satisfactory relative error below 10% for the vast majority of the cases. Moreover, 70% of the simulated cases were considered with a mass flow rate discrepancy below 7.5% in the inaccuracy. Finally, the selected cases were compared and discussed with the HEM approach on the basis of field results. © 2020 Elsevier Ltd and IIRacceptedVersio

Experimental analysis of the R744 vapour compression rack equipped with the multi-ejector expansion work recovery module

Present supermarket refrigeration systems, with carbon dioxide as a refrigerant, indicate high-efficiency performance and they are commonly installed in throughout Europe. The refrigeration systems with R744 have still a large potential to limit power consumption. One of solutions is ejector-based expansion work recovery module. Aim of this thesis is to present an experimental investigation of standard R744 supermarket refrigeration system, with the high-pressure electronic valve (HPV), and refrigeration system with multi-ejector expansion pack on the same vapour compression rack. Comparison of the R744 multi-ejector refrigeration system, was carried out based on energy performance characteristics: refrigeration capacity, power consumption, COP, and exergy efficiency. Apart from the system performance comparison, influence of the pressure level in the flash tank on the system performance for both alternatives was analysed. The experimental results indicated COP and exergy efficiency improvement of the multi-ejector refrigeration system up to 7% and 13.7%, respectively. The multi-ejector system was able to operate in smaller range of the tanks pressure lift than the standard system dependent on the refrigeration load and the exit gas cooler section parameters. The highest values of COP and exergy efficiency were obtained by the multi-ejector refrigeration system for the tanks pressure lift value close to the limit value. The values of the overall compressor efficiencies were significantly differentiated, dependent on the operation module (cooling load and heat rejection conditions), which strongly influenced the values of COP and exergy efficiency. Therefore, it was not possible to clearly define the optimum pressure in the flash tank. It was concluded that improvement of compressors efficiencies utilized in the multi-ejector system will indicate high energy performance of the refrigeration system

A gas ejector for CO2 supercritical cycles

he CO2 ejectors are recently often used as the main expansion device in the modern refrigeration cycles. On the other hand, according to the newest literature the implementation the ejectors into supercritical CO2 power cycles increase its performance. The recent studies showed that in case of the power cycles the ejector pressure lift and mass entrainment ratio are relatively high. Therefore, the main scope of this paper is the investigation of the possibilities of designing the ejector for supercritical Brayton CO2 system. The CFD based computational tool was used to design the ejector for the considered cycle. The system analysis was used to define the ejector on design point. The results of that analysis showed that the required pressure lift and must be equal to 103 bar and mass entrainment ratio equal to 0.995, respectively. The CFD-based evaluation of the proposed ejector showed that these values are impossible to achieve. Therefore, the modifications of the crucial ejector dimensions was performed to increase its performance. Nevertheless, the maximum possible pressure lift for the proposed ejector was equal to 60 bar The analysis of the gathered results showed that the design of the ejector fulfilling the system requirements may be impossible to achieve. © 2018 Elsevier Ltd. All rights reserved.A gas ejector for CO2 supercritical cyclesacceptedVersio

Modified homogeneous relaxation model for the r744 trans-critical flow in a two-phase ejector

acceptedVersio

A modified homogeneous relaxation model for CO2 two-phase flow in vapour ejector

In this study, the homogenous relaxation model (HRM) for CO2 flow in a two-phase ejector was modified in order to increase the accuracy of the numerical simulations The two- phase flow model was implemented on the effective computational tool called ejectorPL for fully automated and systematic computations of various ejector shapes and operating conditions. The modification of the HRM was performed by a change of the relaxation time and the constants included in the relaxation time equation based on the experimental result under the operating conditions typical for the supermarket refrigeration system. The modified HRM was compared to the HEM results, which were performed based on the comparison of motive nozzle and suction nozzle mass flow rates. © Published under licence by IOP Publishing Ltd