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A prototype ejector chiller with nominal cooling power of 40 kW, designed according to the “CRMC” criterion, has been tested by our research group since 2011. The prototype has undergone several refinements and finally has been upgraded to R1233zd(E), which is non-flammable, has low-GWP and favourable thermodynamic properties, i.e. “dry-expansion” and moderate pressure at generator. In terms of saturation pressure curve, the new fluid is similar to R245fa, which was previously used in the prototype. Hence, R1233zd(E) has been basically used as a “drop-in” replacement. The ejector chiller was tested at different evaporation temperatures (2.5 to 10°C), typical of air conditioning applications. Saturation temperatures at generator from 95 to 105°C have been selected as representative of waste-heat recovery. The experimental findings prove that the CRMC ejector has a satisfactory performance with the new working fluid.

This paper numerically investigates the performance of a high capacity, industrial ejector chiller working with the non-flammable, low GWP refrigerant R1233zd(E). The test procedures adopted to acquire the experimental data are explained in part 1 of this work. Different turbulence models were examined in order to accurately predict the ejector off-design transition and critical pressure. A peculiar difference has been noticed between the numerical results obtained when initializing the CFD simulation from the solution of a case with lower or higher condenser pressure. This difference may partly explain the difficulties in the accurate predictions of the critical conditions reported by this and other works in the literature. A detailed discussion is presented on the nature and possible cause of this phenomenon, which is identified as typical of non-linear fluid dynamics.

An impulse facility for analysis of water vapour nozzle flows using both shock tunnel and Ludwieg tube operating modes has been developed and tested at the University of Southern Queensland. Unique high-speed flow visualisation of the water vapour condensation shock has been acquired in the throat region of a nominally two-dimensional convergent-divergent nozzle with a 120 mm2 throat area. This paper presents the facility performance and the time-resolved visualisation results for the nozzle flow, and the results are analysed with the aid of image post-processing tools and quasi-one-dimensional (Q1D) thermodynamic calculations. The experiments have produced qualitative and quantitative data on the flow conditions at four different relative humidity values from 25% to 100%, and over a range of nozzle supply temperatures between 293 K and 343 K. The direct measurement of the pressures and wave speeds within the tunnel resulted in a good agreement with the ideal gas Q1D calculations, particularly for the first incident shocks and expansion waves. Differences progressively increased with the number of reflections, due to the effect of viscous dissipation and the non-ideal end-wall interaction caused by the presence of the nozzle inlet. Moreover, flow visualizations of the location and orientation of both the weak disturbance waves and the condensation shock enabled the investigation of the supersonic water vapour condensation within the nozzle of this impulse facility. Based on these measurements, the nozzle flow downstream of the condensation shock was demonstrated to have a lower Mach number relative to the dry nitrogen flow case. This phenomenon is due to the phase change heat release, which appeared to be more significant when the condensation shock intensity was higher. Although further development of the apparatus is possible, the impulse test-rig configuration that we have developed permits a relatively high flow rate of test gas for a short period of time, leading to significantly reduced operating expenses relative to a continuous flow facility with the same mass flow rate. It is therefore suggested that future studies further improve and test the impulse facility techniques for studying condensation phenomena as a potential alternative to high-power, continuous flow facilities.

The present paper describes a novel CFD approach for the flashing of CO2 through nozzles and ejectors. The novelty of the method is represented by the possibility of defining both the liquid and vapor phases as compressible materials. The properties of each phase are obtained via look-up tables calibrated against standard fluid libraries and are valid in the whole domain of interest, including the supercritical, subcritical and metastable regions. The model has been implemented within a commercial CFD solver and is completely general, i.e., it can be applied to any type of compressible multiphase flow. In the present study, the proposed approach has been validated against an experimental test-case available in literature.

The paper presents the numerical simulation results of the surface roughness influence on gas-dynamic processes inside flow parts of a supersonic ejector. These simulations are performed using two commercial CFD solvers (Star- CCM+ and Fluent). The results are compared to each other and verified by a full-scale experiment in terms of global flow parameters (the entrainment ratio: the ratio between secondary to primary mass flow rate - ER hereafter) and local flow parameters distribution (the static pressure distribution along the mixing chamber and diffuser walls). A detailed comparative study of the employed methods and approaches in both CFD packages is carried out in order to estimate the roughness effect on the logarithmic law velocity distribution inside the boundary layer. Influence of the surface roughness is compared with the influence of the backpressure (static pressure at the ejector outlet). It has been found out that increasing either the ejector backpressure or the surface roughness height, the shock position displaces upstream. Moreover, the numerical simulation results of an ejector with rough walls in the both CFD solvers are well quantitatively agreed with each other in terms of the mean ER and well qualitatively agree in terms of the local flow parameters distribution. It is found out that in the case of exceeding the “critical roughness height” for the given boundary conditions and ejector’s geometry, the ejector switches to the “off-design” mode and its performance decreases considerably.

The article presents some trends of multistage steam-driven ejectors improvement and examples of its implementation. Three relevant directions were picked out: increasing of reliability, profitability and efficiency. Regarding the reliability increasing, existing serial ejector designs of various manufactures were analyzed. A number of new technical solutions were designed. For profitability raising two approaches were considered: decreasing of primary steam parameters and effect of high ejector productivity to the vacuum deviation in the condenser. Efficiency increasing comprises jet devices improvement. For these reasons a design method was refined. In this case geometrical parameters of existing serial ejectors were summarized, CFD research was performed. The improved design method along with new design solutions are implemented in several ejectors successfully. New ejectors are set at thermal power plants and have shown a high technical-economical effect. © The Authors, published by EDP Sciences, 2020.

The present work is about CFD modelling of condensing flow inside a supersonic ejector. The geometry used for the simulations reproduces a small-scale prototype ejector chiller built at Georgia Institute of Technology (Atlanta). The working fluid is R134a, whose expansion inside the primary nozzle and mixing chamber can lead to non-equilibrium condensation phenomena. These alter the pressure and Mach profiles along the ejector, thus generating severe thermodynamic losses. The numerical analysis of non-equilibrium condensation requires modelling of the microscopic behaviour of the fluid with a high level of fidelity. In this study, the condensation of R134a is simulated by means of two in-house developed numerical models. The first considers equilibrium conditions between the phases whereas the latter reproduces the non-equilibrium behaviour of the phase transition. Comparisons are made to understand the limitations and advantages of both approaches.

Research activity on ejectors is ongoing at the University of Florence since the late nineties. The most important achievement is a 40 kW ejector chiller designed according to the “CRMC” criterion. The experimentally validated CFD simulations have given some hints about some possible improvements, i.e. refine the surface finish of the ejector, study the effect of heat transfer and improve the final part of the diffuser, which in its present shape does not produce a measurable compression. The prototype has been recently filled with low-GWP refrigerant R1233zd, as a drop-in replacement of previously used R245fa. Both fluids are “dry-expanding” and hence significantly easier to model in CFD simulations. Synthetic low-GWP refrigerants may be an option for ejector chillers, due to their ability to reach below-zero temperature and high volumetric refrigerant capacity. Some lessons learned with synthetic refrigerants can be transferred to the project of a steam ejector chiller, which remains one of our future targets. Herein we resume the principal findings gathered by means of experimental and numerical activity on our prototype and propose a few ideas for the future research.

In this paper we present a set of experimental data acquired on a CO2 flashing ejector designed for expansion loss recovery applications. Experimental results are compared with CFD simulations obtained using a Homogeneous Equilibrium Model (HEM) and a recently developed mixture model that treats both the liquid and vapour phases as compressible and metastable materials. The models are implemented within a commercial CFD software via user defined subroutines. Pro and cons of each of the two approaches are critically discussed in order to understand the relative performances in terms of numerical stability and accuracy. From this comparison, it can be shown that while the HEM is faster and intrinsically stable, the mixture model has the potential to better reproduce the experimental data, especially in terms of mass-flow rates predictions.

Supersonic ejectors can be used in heat powered chillers to transfer mechanical energy between the motive and the inverse cycle. Within the ejector, momentum is exchanged between a high speed flow produced by a primary nozzle and a slow current coming from the chiller evaporator. Due to the supersonic regime of the primary flow, the mixing of the two streams causes significant loss and impairs the system efficiency. Up to now, second law efficiency of ejector chillers is quite low and optimization is highly needed. The fluid dynamics of the whole ejector involves turbulent mixing, shock trains and complex wall flow, requiring CFD analyses for an adequate description. However, in order to attempt an optimization, mathematically workable models are advantageous. An analytical scheme that captures the basic features of the turbulent mixing zone is discussed here in view of a Constructal design of an ejector chiller.

The Constant Rate of Momentum Change (CRMC) criterion attempts to improve the design of supersonic ejectors, that can be used in heat-powered chillers for industrial or air-conditioning use. Moving from its original formulation, the CRMC design method can be advanced accounting for friction irreversibilities and real gas behaviour, as done in a previous work by our research group. Here we present an upgraded version of this analysis, supported by experimental data from a prototype chiller using R245fa as working fluid. The analysis is extended to other fluids (water, isobutane, 5 HFCs and 3 HFOs) whose performance is calculated on a wide range of heat source/sink temperatures. The existing literature, based generally on ideal gas simulations, suggests that water yields poor results in terms of COP. This paper shows that this result may be argued. Low GWP fluid HFO1233zd also gives good results.

The prediction of the supersonic ejector dynamics implies the accurate description of all the complex flow features discussed in Chap. 2. Unfortunately, the theoretical modeling approach necessitates a number of simplifying assumptions and empirical constants that introduce significant uncertainty and reduce the capability of capturing a number of relevant flow features. In this respect, computational fluid dynamics may represent a tool to overcome these difficulties and analyze the flow details of arbitrary ejector geometries.

In the present paper, a numerical model for the simulation of wet-steam flows has been developed and implemented within a commercial CFD code (ANSYS Fluent) via user defined functions. The scheme is based on a single-fluid approach and solves the transport equations for a homogeneous mixture coupled with conservation equations for the droplets number and liquid volume fraction. The model is validated against a steam nozzle test-case and then compared with experimental data from a steam ejector with a significant amount of generated liquid phase. The simulations show a good agreement both in terms of mass flow rates and pressure profile data. Some of the modeling assumptions are also reviewed and discussed.

The global behavior of the ejector results from a combination of complex flow features including shock trains, turbulent mixing layers bounded by wall regions, shock-induced separations, boundary layers subject to adverse pressure gradients, non-equilibrium phase change, etc. It is because of this complexity that ejector design and performance have thus far been difficult to characterize and optimize.

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.

A thermodynamic analysis is performed on open, regenerated, inverse air-cycles, focussing on low temperature refrigeration, in order to provide general optimization criteria, support for a rational configuration choice and potential performance improvement for this specific application. The effect of high regenerator effectiveness (0.95) is explored, showing that it may double the air cycle COP in the case at hand. Low and high pressure configurations (i.e. featuring only one heat exchanger on the cold or hot cycle side) are compared. High pressure configuration, normally preferred in the field of low temperature refrigeration, has a lower efficiency, but the gap reduces as regenerator effectiveness increases. Avoidance of a cold heat exchanger eliminates frosting problems and power input for the circulation fan, so that the small COP decrease suffered by the high pressure cycle can easily be compensated for and eventually yield higher average plant efficiency. The feasibility of such a highly effective regenerator, already reported in the literature, is demonstrated using classic Kays and London (1964) data.

This paper describes and evaluates the development of generalised steady state and transient mathematical models for thermostatic expansion valves (TEVs) of the types used in commercial refrigeration systems. The model is of a generalised nature, because it is not necessary to input performance or geometrical data for a particular valve to operate the model. However, if required, the models can provide an accurate correlation of valve manufacturer's data. Derivations are provided and validating data is presented. The mathematical models described in the paper form part of computer software that simulates the thermal operation of whole refrigeration systems. The software is titled Vapour Compression Refrigeration System (VCRS) simulator, and by way of example the paper presents results from the VCRS simulator which are used to aid a discussion of operating faults, such as hunting and under damping and their possible causes, where these can be attributed to the expansion valve.

Ejector refrigeration has been studied at DIEF (Dipartimento di Ingegneria Industriale Firenze) since the ‘90s. Use of environmentally safe fluids (steam) was addressed. A two-stage prototype with cooling capacity 5 kW was optimized and built. Later, the CRMC prescription for the design of the supersonic diffuser was focused. By a gradual reduction of the fluid velocity and a continuous profile, the CRMC design promises a reduction of the normal shock that usually develops in the mixing chamber. A second 40 kWf prototype was designed in 2010 for an industrial partner (Frigel Firenze s.p.a.). The design procedure used a thermodynamic code accounting for real gas behavior. This code gives a first design of the mixing chamber and diffuser according to the CRMC criterion. It also gives an estimation of the friction loss along the diffuser. A comparison between different operating fluids was performed and resulted in the selection of R245fa. A first design of the ejector was manufactured in carbon fiber. The primary nozzle is mounted on a movable support, in order to change its axial position with respect to the mixing chamber. In terms of COP, first results were below the values predicted by the simulation code. Meanwhile a numerical simulation was in progress with FLUENT. From the first CFD results it was decided that the diffuser throat had to be moved forward from the primary nozzle exit, in order to allow a complete mixing between the primary and secondary flows, and enlarged, the losses encountered in the mixing process being higher than expected and hence the fluid density lower. This produced a second ejector design, which was manufactured and tested in 2012, showing improved performance. These results suggested a third design, with a further lengthened diffuser, which has undergone a complete testing campaign, allowing validation of the CFD results. The activity performed till now suggests that ejector refrigeration plants have a robust operation and can be easily manufactured at relatively low cost with off-the-shelf components, a part from the ejector itself, which however represents a small fraction of the system cost. However, the COP is lower with respect to absorption refrigeration and hence needs substantial improvement through detailed thermodynamic and CFD design optimization.

A 40 kWc jet-pump chiller has been built in Florence at Frigel s.p.a. The plant is intended as a test bench for future jet-pump chillers for applications in the industrial temperature control market and powered by waste heat from CHP systems. The generator and evaporator of the prototype unit are water-heated and the condenser is water cooled. R245fa has been chosen as the working fluid. The prototype's jet-pump has a movable primary nozzle and 7 static pressure probes on the mixing chamber/diffuser duct. This latter has been manufactured in two versions, both characterized by a profile inspired by the CRMC (Constant Rate of Momentum Change) criterion. This paper describes the prototype and presents a preliminary set of experimental test results.

Ejector chillers may represent a competitor for absorption chillers, as soon as their cost per unit cooling power becomes equal or lower. This target is not far from our present achievements. If input energy is waste or renewable heat, the system operating cost is mainly due to the investment in heat exchangers. Therefore, a cost reduction requires an increase of COP. This latter may be improved by a careful design of the ejector, which requires a deep insight into the thermodynamics and fluid-dynamics of its complex physics. The tools and the knowledge for an advanced design are already available and the improvement potential is significant.

Adiabatic compressed air energy storage (A-CAES) represents a valuable and environmentally friendly option for massive energy storage. Existing examples of CAES refer to underground storage at medium pressure level. But for widespread utilization, independent from the availability of underground storage volumes, artificial reservoirs would be required. This requires rather high air pressure within the storage, which in turn will demand a carefully optimized recovery of the thermal energy released in the compression phase. Starting from a detailed thermodynamic analysis of the relevant design parameters and their influence on the system efficiency, we propose a comprehensive set of criteria for the design of the system, with particular attention to heat transfer devices.

A design procedure, based on a one-dimensional simulation, is presented for an ejection refrigeration system. Heat exchangers are included in the calculation, accounting for temperature differences between the fluids and for pressure losses. The ideal gas assumption, which is quite common in the literature concerning ejector systems, is avoided. Furthermore, the supersonic diffuser is designed with a continuous profile, without cylindrical piece, controlling the variation of momentum along the flow path and accounting for friction. At design conditions, this should reduce the irreversibility due to the normal shock. A comparison between different operating fluids is presented and R245fa is selected. The results of the design procedure and the expected performance, in terms of first and second law efficiency, are presented.

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 , 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 thermodynamic analysis is presented for an energy storage system without combustion and including thermal recuperation (adiabatic compressed air energy strong (CAES)). The storage volume is optimized and a system layout with thermal storage (TES) and variable configuration is designed. The proposed system is simulated in order to have a first estimate of its energy recovery efficiency.

Supersonic steam ejectors are commonly used in many applications, for example suction of non-condensable gases in steam power plants or heat-powered chillers. In the specific case of ejector chillers, CFD studies of the ejector are necessary for optimization of this device because ejector performances have a direct impact on the COP of the chiller. The complex ejector flow and the high Mach numbers make characterization of the phase change inside this component difficult. Metastability effects also have to be accounted for and some CFD commercial software provide built-in wet-steam models for this purpose. A simpler approach for numerical modeling of multiphase ejector is the Homogeneous Equilibrium Model (HEM) in which the phase change occurs in equilibrium conditions, i.e. metastability is neglected. These second kinds of models are still important tools for preliminary analysis of condensing ejectors. In the present paper a comparison between commercial software wet-steam models and an in-house developed model based on HEM is presented.

Analysing how energy is delivered from its primary sources to final users, it may be seen that the evolution of technology, driven by economic considerations, has mainly rewarded those systems that have intense energy fluxes through their main sections. On the other hand, renewable energy sources are prevented from being widespread by their low energy density. If a high energy flux is a recognized target for energy use, one may try to characterise the various devices encountered along the energy path according to the concentration obtained of the energy flow. In this way, apart from measuring the energy loss suffered within a given device, it can be decided if this loss is adequate with respect to the gain in terms of energy density.

An innovative repowering concept for combined cycle power plants is presented. The design concept consists in adding one or more gas turbines to the combined cycle, integrated by steam injection into the existing gas turbine. The steam is produced in a simplified heat recovery steam generator fed by the additional turbine’s exhaust gas. The scheme is quite simple and easy to adapt to various types of combined cycles. The efficiency of the repowered plant compares favorably with that of the original combined cycle and far surpasses that obtained by simply adding the gas turbine with no integration. Furthermore, the additional gas turbine enhances the plant operating flexibility as any power output intermediate between the original and the repowered capacity can be readily attained with no significant efficiency penalty. A thermodynamic and economic evaluation of the system feasibility is presented.

Ejector chillers are being studied at Department of Industrial Engineering of Florence (DIEF) since 2000, both theoretically and experimentally. This chapter discusses the application of solar-powered chillers in air conditioning and details the fundamental parameters. This technology is not far from being a technically feasible alternative to commercially available single-stage absorption chillers, but obviously a huge effort is still needed to improve its performance and gain access to the market.

The capture and sequestration of the CO2 emitted from fossil-fuelled power plants is gaining widespread interest for controlling anthropogenic emissions of greenhouse gases. Among technology options for CO2 capture, membrane-based gas separation systems are noteworthy owing to their low energy requirements, promising technology evolution and effective integration with power plants. This paper presents a mathematical model for membrane-based separation systems that is able to cover the most significant membrane types and configurations. This model has been integrated in a general simulation method for analysing and optimizing advanced energy conversion systems. Performance of these simulation tools is demonstrated by evaluating the influence of different operating conditions on the behaviour of pre-and post-combustion separation units, based on metallic or polymeric membranes. Finally, the feasibility of integrating a metallic membrane system into a chemically recuperated gas turbine (CRGT) power plant is explored, obtaining encouraging results for CO2 capture.

Compressed Air Energy Storage is recognized as a promising technology for applying energy storage to grids which are more and more challenged by the increasing contribution of renewable such as solar or wind energy. The paper proposes a medium-size ground-based CAES system, based on pressurized vessels and on a multiple-stage arrangement of compression and expansion machinery; the system includes recovery of heat from the intercoolers, and its storage as sensible heat in two separate (hot/cold) water reservoirs, and regenerative reheat of the expansions. The CAES plant parameters were adapted to the requirements of existing equipment (compressors, expanders and heat exchangers). A complete exergy analysis of the plant was performed. Most component cost data were procured from the market, asking specific quotations to the industrial providers. It is thus possible to calculate the final cost of the electricity unit (kWh) produced under peak-load mode, and to identify the relative contribution between the two relevant groups of capital and component inefficiencies costs.

The feasibility of integrating a commercially available reheat gas turbine with a methane steam reformer is analyzed. A slight modification to the original reheat design is proposed to improve the methane conversion rate in the reforming process and, consequently, the efficiency in recovering waste exhaust heat from the gas turbine. Two solutions are proposed for the heat recovery scheme: a first reformer has a single pressure level while the second has two in order to match the different pressures of the combustors. While the single pressure scheme gives good performance with respect to the stand alone gas turbine, the dual pressure reformer can give a further benefit, as far as an accurate optimization of the steam management is performed.

An open-cycle engine approximating the Stirling cycle but retaining external combustion is being developed. The Stirling engine is being built on a motorcycle V-engine; however, the heat exchangers have been designed from scratch. Some thermodynamic considerations justifying the use of the open cycle are presented. A numerical simulation of the proposed open cycle is described. An optimization of the heat exchanging system, making the use of an entropic parameter, is attempted, and the results are presented. The design of the experimental motor has been completed and is waiting for experimental testing to verify the cycle effectiveness and the optimization criteria. >