Performance Analysis of the R290 Variable Geometry Gas Ejector Application for Other Refrigerants

Ejector refrigeration systems are promising alternative to standard vapour compression refrigeration systems. They can be driven with low-grade heat or solar systems, which make them even more renewable solution aligned with global energy transition. By implementing the controllable ejector, it can adapt to variable operating conditions, ensuring the high efficiency of both the device and the overall performance of the system. However, as it is fluid-driven device, its geometry has to be designed for particular fluid and typically requires redesigning when being applied for new applications. The R290 variable geometry gas ejector has been thoroughly tested for various spindle positions which ensured its highly efficient operation at different conditions. In this study, the same geometry was tested for other natural refrigerants of similar thermodynamic properties, i.e.R600a and R1270. The CFD analysis was based on a set of operating points for ejector-based air conditioning system working during the summer period with characteristic temperatures at evaporator and condenser. The controllable ejector was simulated for all the points with similar motive and suction nozzle parameters and the critical temperature at the outlet was determined. The analysis showed that the ejector can be used with other refrigerants maintaining high efficiency without any changes in geometry but for lower number of spindle positions. The obtained critical temperature indicates that with all the tested refrigerants the ejector-based cycle is able to work for cooling purposes during typical summer conditions for a wide range of temperatures

A comprehensive validation of CFD-based wet steam approach for the modelling of steam ejectors

Ejector-based system performance is closely related to the complex fluid-dynamic behavior inside ejectors, which is usually optimized through CFD simulations. The rapid expansion in ejector’s primary nozzle leads to the condensation of the working fluid, and since it strongly affects system performance, it has to be properly accounted. In this study, the built-in Wet Steam model of the CFD software Ansys Fluent is used to simulate the non-equilibrium condensation inside steam ejectors. Usually, the condensation inside steam ejectors is not considered as it is a complex phenomenon and difficult to model, hence the ideal gas model is generally used, without considering the presence of liquid droplets affecting the thermodynamic profiles inside the domain and therefore the system performances. To assess the performance of wet steam model from a local and global point of view, the entrainment ratio and pressure distribution of both wet steam and ideal gas models have been respectively compared. A set of experimental benchmarks from literature has been used to validate the model with different ejector geometries and operating conditions. The wet steam model can outperform the ideal gas model and predict entrainment ratios closer to experimental result in on-design conditions with errors lower than 10%. However, the wet steam model was found to be very dependent on the geometry of the suction chamber and the nozzle exit position, achieving the best results when the nozzle was placed near the constant area mixing section or with suction chamber with not abrupt changes in its cross-sectional area

An Object-Oriented R744 Two-Phase Ejector Reduced-Order Model for Dynamic Simulations

The object-oriented two-phase ejector hybrid reduced-order model (ROM) was developed for dynamic simulation of the R744 refrigeration system. OpenModelica software was used to evaluate the system’s performance. Moreover, the hybrid ROM results were compared to the results given by the non-dimensional and one-dimensional mathematical approaches of the R744 two-phase ejector. Accuracy of all three ejector models was defined through a validation procedure for the experimental results. Finally, the dynamic simulation of the hybrid ROM ejector model integrated with the R744 refrigeration system was presented based on the summer campaign at three different climate zones: Mediterranean, South American and South Asian. The hybrid ROM obtained the best prediction of ejector mass flow rates as compared with other ejector models under subcritical and transcritical operating conditions. The dynamic simulations of the R744 ejector-based system indicated the ejector efficiency variations and the best efficiency at the investigated climate zones. The coefficient of performance (COP) varied from 2.5 to 4.0 according to different ambient conditions. The pressure ratio of 1.15 allowed a more stabilised system during the test campaign with an ejector efficiency from 20% to over 30%

Numerical Investigation of the Expansion Devices Applied in Modern Vapour Compression Refrigeration Unit Considering Specific Entropy and Entropy Generation Analyses

Ejector-based refrigeration cycles working with natural refrigerants have already gained industry attention. Replacing throttling valve with an ejector in vapour-compression cycles brings high improvement of the cycle efficiency due to the ejectors potential of recovering part of throttling losses. With the rapidly growing market for heat pumps, they are also being implemented in those cycles, but this requires redesigning the ejector geometry for new natural working fluids for different operating conditions and applications. Typical approach to perform the ejector shape optimization is to use the ejector mass entrainment ratio or overall efficiency as an objective function. However, an entropy generation analysis seems to be more efficient. For this reason, the aim of this work was to perform the numerical analysis of the entropy generation of the two-phase ejector for R744 and assess its potential as a tool for efficiency improvement in the shape optimization algorithms. The ejector PL tool utilizing the homogeneous equilibrium model approach was complemented with the entropy generation model implemented using an additional transport equation to the computational fluid dynamics software. The numerical results of the mass flow rates were used for validation purposes. The entropy generation module allowed for the entropy generation analysis in terms of maximum values and their location showing critical areas of irreversibility characterizing different working fluids usage and ejector applications

Experimental study of a R290 variable geometry ejector

Ejectors are classified as fluid-dynamics controlled devices where the “component-scale” performances are imposed by the local-scale fluid dynamic phenomena. For this reason, ejector performances (measured by the pressure-entrainment ratio coordinate of the critical point) are determined by the connection of operation conditions, working fluid and geometrical parameters. Given such a connection, variable geometry ejector represents a promising solution to increase the flexibility of ejector-based systems. The present study aims to extend knowledge on variable geometry systems, evaluating the local and global performances of the R290 ejector equipped with a spindle. The prototype ejector was installed at the R290 vapour compression test rig adapted and modified for the required experimental campaign. The test campaign considered global parameter measurements, such as the pressure and the temperature at inlets and outlet ports together with the mass flow rates at both inlet nozzles, and the local pressure drop measurements inside the ejector. In addition, the experimental data were gathered for different spindle positions starting from fully open position the spindle position limited by the mass flow rate inside the test rig with the step of 1.0 mm