Investigation of an experimental ejector refrigeration machine operating with refrigerant R245fa at design and off-design working conditions. Part 2. Theoretical and experimental results

The main results of a theoretical and experimental investigation of the performance characteristics of an ejector and an ejector refrigeration machine (ERM) operating with refrigerant R245fa at design and off-design working conditions are presented. The ejector and ERM were explored theoretically using improved 1D model and the calculated results were validated experimentally on ejector test rig that has been assembled and operated at National Taiwan University. For typical cases, the performance characteristics variation with condensing, generating and evaporating temperatures along with performance maps are presented. The theoretical results are compared with the results of a set of experiments and good qualitative and quantitative agreement is observed.     

An experimental investigation of an ejector for validating numerical simulations

Supersonic ejectors have been used in cooling/refrigeration applications since the early 1900s. Interest in supersonic ejectors has been rekindled by recent efforts to reduce energy consumption; ejector refrigeration systems can be powered by solar energy or by waste heat generated by another process. This paper describes an experimental test bench using R245fa that was assembled and operated at CanmetENERGY in Varennes. The results from this test bench provide a source of reference data that may be used to validate numerical models of ejectors that could be used in refrigeration applications. Limited results from two numerical models are presented for comparison; global results from a 1D model and results from a detailed CFD model that show the flow field inside the ejector.     

Performance analysis of a supersonic ejector cycle working with R245fa

A supersonic ejector chiller for industrial use is currently being developed and tested as part of a project cooperation between Frigel s.p.a and DIEF (Department of Industrial Engineering, University of Florence). The refrigerator was built following a “ready to market” setup criterion and is intended for applications on the industrial refrigeration market or in air conditioning. The plant has a nominal cooling power of 40 kW and is powered by low temperature heat (from 90 up to 100 °C). The ejector is equipped with a movable primary nozzle and 9 static pressure probes along the mixing chamber/diffuser duct. The working fluid is R245fa. An extensive numerical campaign was performed to analyze the internal dynamics of the ejector. All the simulations were carried out by accounting for the real gas properties of the refrigerant. Comparison with experimental data resulted in close agreement both in terms of global and local parameters. Analyses showed that in order to achieve an accurate matching with the experimental data, it is necessary to correctly account for the surface roughness of the ejector. This is especially true for off-design operating conditions.     

Design-theoretical study of cascade CO2 sub-critical mechanical compression/butane ejector cooling cycle

In this paper an innovative micro-trigeneration system composed of a cogeneration system and a cascade refrigeration cycle is proposed. The cogeneration system is a combined heat and power system for electricity generation and heat production. The cascade refrigeration cycle is the combination of a CO₂ mechanical compression refrigerating machine (MCRM), powered by generated electricity, and an ejector cooling machine (ECM), driven by waste heat and using refrigerant R600. Effect of the cycle operating conditions on ejector and ejector cycle performances is studied. Optimal geometry of the ejector and performance characteristics of ECM are determined at wide range of the operating conditions. The paper also describes a theoretical analysis of the CO₂ sub-critical cycle and shows the effect of the MCRM evaporating temperature on the cascade system performance. The obtained data provide necessary information to design a small-scale cascade system with cooling capacity of 10 kW for application in micro-trigeneration systems.     

Ejector expansion refrigeration system: Ejector design and performance evaluation

Use of a two-phase flow ejector as an expansion device in vapor compression refrigeration systems is one of the efficient ways to enhance its performance. The present work aims to design a constant-area two phase flow ejector and to evaluate performance characteristics of the ejector expansion refrigeration system working with R134a. In order to achieve these objectives, a simulation program is developed and effects of operating conditions and ejector internal efficiencies on the system performance are investigated using EES software. Comparison between present results and published experimental data revealed that the developed model can predict the system COP with a maximum error of 2.3%. The system COP increased by 87.5% as evaporation temperature changed from −10 °C to 10 °C. Finally, correlations to size ejector main diameters as a function of operating conditions, system cooling capacity and ejector internal efficiencies are reported.     

Historical and present developments of ejector refrigeration systems with emphasis on transcritical carbon dioxide air-conditioning applications

This paper gives an overview of historical and present developments on how ejectors can be utilized to improve the performance of air-conditioning and refrigeration systems. Research on ejector refrigeration cycles that utilize low-grade energy sources to produce cooling is summarized. Another major class of ejector refrigeration cycles that is described tries to recover expansion work by means of a two-phase ejector. This particular approach appears to be very promising for transcritical carbon dioxide (CO₂, R744) systems with inherently large throttling losses. The paper further presents the latest analytical and experimental results of a comprehensive study carried out to investigate possible performance improvements of transcritical R744 two-phase ejector systems. Relevant operational parameters were varied and effects on performance resulting from different ejector geometries were studied as well. Two-phase mixing shock waves inside the ejector were detected by recoding static wall pressure distributions.     

Numerical investigation of a two-phase CO2 ejector

Using an ejector as an additional component in a vapor compression refrigeration system is a promising way to increase the system efficiency. The efficiency increase of the refrigeration system depends strongly on the ejector. Using CFD simulations, it is possible to obtain a better understanding of two-phase CO₂ ejectors in order to design more efficient ejectors. In this work, a numerical model based on a homogeneous equilibrium approach, which is implemented in OpenFOAM, is used to simulate the CO₂ ejector. The numerical investigation of an ejector operated with and without a suction mass flow is presented and the numerical results are compared to experimental data published in previous works to validate the simulations. If the ejector is operated without a suction flow, no mixing losses occur and the friction losses are one of the main losses affecting the flow. Thus, this operating condition is suitable to validate if the friction losses are determined correctly by the numerical model. Afterwards, an ejector which is operated with a suction flow is simulated in order to validate the accurate prediction of the mixing losses by the numerical model. In the presented data range, the numerical model predicts the driving mass flux within an error margin of 10%. The pressure recovery of the ejector operated without a suction flow is determined with an error of 10%. This error increases to 20% when the ejector is operated with a suction flow.     

A study of working fluids for heat driven ejector refrigeration using lumped parameter models

This paper studies the influence of working fluids over the performance of heat driven ejector refrigeration systems performance by using a lumped parameter model. The model used has been selected after a comparison of different models with a set of experimental data available in the literature. The effect of generator, evaporator and condenser temperature over the entrainment ratio and the COP has been investigated for different working fluids in the typical operating conditions of low grade energy sources. The results show a growth in performance (the entrainment ratio and the COP) with a rise in the generator and evaporator temperature and a decrease in the condenser temperature. The working fluids have a great impact on the ejector performance and each refrigerant has its own range of operating conditions. R134a is found to be suitable for low generator temperature (70–100 °C), whereas the hydrocarbons R600 is suitable for medium generator temperatures (100–130 °C) and R601 for high generator temperatures (130–180 °C).     

Theoretical study and design of a low-grade heat-driven pilot ejector refrigeration machine operating with butane and isobutane and intended for cooling of gas transported in a gas-main pipeline

This paper describes the construction and performance of a novel combined system intended for natural gas transportation and power production, and for cooling of gas transported in a gas-main pipeline. The proposed system includes a gas turbine compressor, a combined electrogenerating plant and an ejector refrigeration unit operating with a hydrocarbon refrigerant. The combined electrogenerating plant consists of a high-temperature steam–power cycle and a low-temperature hydrocarbon vapor power cycle, which together comprise a binary vapor system. The combined system is designed for the highest possible effectiveness of power generation and could find wide application in gas-transmission systems of gas-main pipelines. Application of the proposed system would enable year-round power generation and provide cooling of natural gas during periods of high ambient temperature operation. This paper presents the main results of a theoretical study and design performance specifications of a low-grade heat-driven pilot ejector refrigeration machine operating with butane and isobutane.     

Experimental analysis on the effect of internal heat exchanger in transcritical CO2 refrigeration cycle with two-phase ejector

The performance of CO₂ ejector refrigeration system needs further improvement to make CO₂ more viable than traditional harmful refrigerants. In this research, the effect of internal heat exchanger (IHX) in the performance of ejector refrigeration system was analyzed experimentally and compared with conventional expansion refrigeration system. Experiments were performed at different operating pressure and temperature for the cases of without IHX, 30 cm IHX and 60 cm IHX. The results showed that IHX significantly increased the coefficient of performance (COP) of ejector system. At the conditions used in this research, the ejector system with 60 cm IHX provided the maximum COP improvement of up to 27% compared to similar conventional system. The motive nozzle’s inlet condition had significant effect on the performance of ejector system. The results also confirmed the presence of considerable amount of liquid refrigerant at separator’s gas outlet of ejector system which was deemed possible in our previous research.     

Ejector design and theoretical study of R134a ejector refrigeration cycle

In the present paper, a mathematical model is developed to design R134a ejector and to predict the performance characteristics of a vapor jet refrigeration system over a wide range of the investigated parameters. These parameters include boiling temperature (65–85 °C), condensing temperature (25–40 °C), evaporating temperature (0–10 °C), degrees of superheat (0–15 °C), nozzle efficiency (0.75–0.95) and diffuser efficiency (0.75–0.95). Simulated results showed that the present model data are in good agreement with experimental data in the literature with an average error of 6%. It is found that the ejector area ratio at boiling temperature of 85 °C is about double that at boiling temperature of 65 °C for various evaporating and condensing temperatures. The present results confirm that waste heat sources of temperature ranging from 65 to 85 °C are adequate to operate vapor jet refrigeration system for air-conditioning applications.     

Experimental evaluation of an ejector as liquid re-circulator in an overfeed NH3 system with a plate evaporator

This paper deals with the experimental performance evaluation of an ejector, linked to a manual expansion valve, working as a liquid re-circulator component in an overfeed NH₃ plate evaporator. The evaporator was tested in a single stage system belonging to a cascade refrigeration system prototype. The evaporator is an ALFANOVA HP76 plate heat exchanger with 50 plates. A Phillips ejector with a 1/2″ diameter throat and 1.4 mm diameter nozzle was used. The recirculation rate was experimentally determined for different operating conditions. Experimental data are reported for volumetric flow rate at the manual expansion valve inlet from 0.8 to 1.6 l min⁻¹, evaporating pressure from 0.14 to 0.22 MPa and condensing pressure from 0.85 to 1 MPa. The experimental result showed recirculation rates between 2 and 4. The evaporating capacity varied from 9.48 kW to 18.37 kW. In addition, another two nozzles were tested and the results are also presented and discussed.     

Hybrid vapor compression refrigeration system with an integrated ejector cooling cycle

A refrigeration system was developed which combines a basic vapor compression refrigeration cycle with an ejector cooling cycle. The ejector cooling cycle is driven by the waste heat from the condenser in the vapor compression refrigeration cycle. The additional cooling capacity from the ejector cycle is directly input into the evaporator of the vapor compression refrigeration cycle. The governing equations are derived based on energy and mass conservation in each component including the compressor, ejector, generator, booster and heat exchangers. The system performance is first analyzed for the on-design conditions. The results show that the COP is improved by 9.1% for R22 system. The system is then compared with a basic refrigeration system for variations of five important variables. The system analysis shows that this refrigeration system can effectively improve the COP by the ejector cycle with the refrigerant which has high compressor discharge temperature.     

Experimental results with a variable geometry ejector using R600a as working fluid

Experimental results with the first laboratory scale variable geometry ejector (VGE) using isobutane (R600a) are presented. Two geometrical factors, the area ratio and the nozzle exit position, can be actively controlled. The control of the area ratio is achieved by a movable spindle installed in the primary nozzle. The influence of the spindle position (SP) and condenser pressure on ejector performance are studied. The results indicate very good ejector performance for a generator and evaporator temperature of 83 °C and 9 °C, respectively. COP varied between 0.4 and 0.8, depending on operating conditions. The existence of an optimal SP, depending on the back pressure, is identified. A comparison of the benefit of applying the variable geometry design over a fixed geometry configuration is assessed. For example, for a condenser pressure of 3 bar, an 80% increase in the COP was obtained when compared to the performance of a fixed geometry ejector.     

Prediction of condensation in steam ejector for a refrigeration system

An experimental refrigeration system based on a two-stage steam ejector was set-up in the Thermodynamics and Heat Transfer Laboratory of our Department. The system optimization and realization have been described elsewhere ( [Grazzini and Mariani, 1998] and [Grazzini and Rocchetti, 2008] ). In both stages, primary flows are highly supersonic and reach low pressure and temperature levels. As usual in the literature, an ideal gas model was used during the design process. This paper is intended to check the validity of this assumption. In order to understand the actual working condition of our system, several models have been compared. The presence of high flow speed suggests the existence of metastable conditions. To set the border for the metastable region, the spinodal curve has been drawn. Isentropic expansion of vapour through the nozzle, modelled as ideal gas, seems well within the metastable zone. However, the Classic Nucleation Theory shows that the Wilson line is crossed at the nozzle throat. Condensation produces a marked difference in the conditions at the nozzle exit. Results coming from the present analysis will be used in further optimization of the experimental ejector design.     

A 1D model to predict ejector performance at critical and sub-critical operational regimes

This paper proposes a new 1D model to predict ejector performance at critical and sub-critical operational modes, while most previous 1D models have only predicted ejector performance at critical mode operation. Constant pressure mixing is assumed to occur inside the constant area section of the ejector at critical and sub-critical mode operation, and the effectiveness of the model is verified against four sets of experimental data that include different working fluids and geometries. The results show that the proposed model accurately predicts ejector performance over all ranges of operation, and is a useful tool for predicting ejector performance within larger refrigeration cycle models.     

Research on air-source heat pump coupled with economized vapor injection scroll compressor and ejector

The performance of the heat pump can be improved further when running under low temperature conditions when an ejector is used in a heat pump system coupled with economized vapor injection (EVI) scroll compressor. In this paper, the design method of the heat pump system with ejector (EVIe) is presented, and the process for designing the heat pump with ejector has been summarized. The optimal location of the vapor injection inlets is at the place where the vapor can inject into the working chambers when they just be closed. The reasonable value for the entrainment ratio u of the ejector is between 0.1 and 0.2. One prototype heat pump was designed under the condition of the evaporation temperature of −20 °C, and an experimental setup was established to test the prototype. The measured results demonstrated that the heating EER of the heat pump system with ejector could reach about 4% higher than that of the system without ejector when the heating capacity remained nearly constant.     

Theoretical analysis and optimization of a hybrid CO2 transcritical mechanical compression – ejector cooling cycle

The paper provides the results of a design-theoretical study of a hybrid carbon dioxide (CO₂) transcritical mechanical compression – ejector cooling cycle. The hybrid cooling cycle is a combination of a CO₂ transcritical mechanical compression refrigeration machine (MCRM) powered by electricity, and an ejector cooling machine (ECM) driven by heat rejected from the CO₂ cooling cycle. Refrigerants R245ca, R601b (neopentane) and R717 (ammonia) are investigated as the working fluids of ECM in the present study. A method to determine the optimal design parameters and performance of the hybrid cooling cycle is presented. It is shown, that efficiency growth of the transcritical CO₂ cooling cycle due to ejector cooling cycle use is higher as evaporating temperatures are lower.     

Performance of ejector cooling systems using low ecological impact refrigerants

The theoretical behaviour of an ejector cooling system, using as working fluids propane, butane, isobutane, R152a and R134a, is obtained. The ejector works as a thermo-compressor that is simulated with a validated one-dimensional mathematical model, whose errors are lower than 6%. For a system unitary cooling capacity, a parametric study is carried out varying the generation, condensation and evaporation temperatures. From the obtained data, a complete analysis of the system performance can be achieved when the ejector and system operation parameters are considered. The best performance corresponds to the system using propane, because has the highest system coefficient of performance and its ejector has the maximum entrainment ratio value, the least area ratio value and the highest efficiency value. The considered generation temperature ranging from 70 °C to 95 °C is appropriate for low-grade energy sources assisting thermal cooling systems. After this system performance, come those in which R152a and R134a are employed, with isobutane and butane at the end. The obtained results represent potential design points of an efficient ejector cooling system operation, because to each combination of the above mentioned temperatures corresponds one and only one ejector geometry.     

Evaluation of a novel combined ejector-absorption refrigeration cycle — I: computer simulation

This paper describes a novel refrigeration cycle based on the combination of an absorption cycle with an ejector refrigeration cycle. The combination brings together the advantages of absorption and ejector refrigeration systems and provides high COP for refrigeration and air-conditioning. The combined cycle is particularly suitable for utilising waste thermal energy. A computer simulation program was developed for the combined cycle and used to determine the performance of the system using LiBrH₂O for various generator, condenser and evaporator temperatures. Optimum operating conditions and ejector design data are also provided.