Two-phase Ejector Research Articles

Modelling and thermodynamic analysis of ejector flow for application at design and off design operating conditions in an ejector air conditioner

Ejector flow in an ejector air conditioning system using R245fa is analysed for entrainment ratio and potential refrigeration effect, at varying temperature and heat input conditions in the generator ranging from 60C to 100C and 2kW to 5kW respectively. The effect of varying generator temperature in cooling capacity of the system when the vapour ejectoris operating at design evaporator and condenser temperatures of 10C and 35C respectively is investigated. The mathematical model of the vapour ejector with optimum area ratio is developed and validated. A critical entrainment ratio of 0.385 is obtained corresponding to generator temperature of 100C. When the generator temperature is varied from 60C to 100C, the cooling capacity range from 0.3kW at generator heat input of 2 kW to 1.78 kW at 5 kW heat input. Further, the operation of the system is analysed for off design operating condition corresponding to reduced heat input rate in the generator. In that case the state of primary refrigerant flow in ejector inlet will be two phase and a mathematical model for two-phase ejector flow is developed and validated. Ejector flow analysis revealed the minimum quality of flow at ejector inlet to maintain adequate backpressure for condensation to occur range from 0.72 at 60C to 0.22 at 100C. The corresponding refrigeration refrigeration effect produced is less than the respective designed operation value byits 12.2% to 8%. Further, analysis of the system shows that at least 7 kW heat input at 100C is required to produce 1 ton of cooling effect.
 Ejector flow in an ejector air conditioning system using R245fa is analysed for entrainment ratio and potential refrigeration effect, at varying temperature and heat input conditions in the generator ranging from 60C to 100C and 2kW to 5kW respectively. The effect of varying generator temperature in cooling capacity of the system when the vapour ejectoris operating at design evaporator and condenser temperatures of 10C and 35C respectively is investigated. The mathematical model of the vapour ejector with optimum area ratio is developed and validated. A critical entrainment ratio of 0.385 is obtained corresponding to generator temperature of 100C. When the generator temperature is varied from 60C to 100C, the cooling capacity range from 0.3kW at generator heat input of 2 kW to 1.78 kW at 5 kW heat input. Further, the operation of the system is analysed for off design operating condition corresponding to reduced heat input rate in the generator. In that case the state of primary refrigerant flow in ejector inlet will be two phase and a mathematical model for two-phase ejector flow is developed and validated. Ejector flow analysis revealed the minimum quality of flow at ejector inlet to maintain adequate backpressure for condensation to occur range from 0.72 at 60C to 0.22 at 100C. The corresponding refrigeration refrigeration effect produced is less than the respective designed operation value byits 12.2% to 8%. Further, analysis of the system shows that at least 7 kW heat input at 100C is required to produce 1 ton of cooling effect.

Novel flow modulation method for R744 two-phase ejectors – Proof of concept, optimization and first experimental results

The cooling industry involves various essential applications, such as food preservation, medicine storage and air conditioning. However, its significant direct and indirect contribution to global warming is bound to increase in years to come, leading to the need for highly efficient cooling units using eco-friendly working fluids. Consequently, carbon dioxide (R744) is achieving resounding success as a refrigerant for various medium- and large-capacity applications, as some of the available expansion work is recovered with the aid of two-phase ejectors. However, its adoption is being limited for small-capacity solutions (e.g. condensing units) due to the current lack of a suitable flow modulation technique for two-phase ejectors installed in these units. Therefore, the goal of this work is to bridge this knowledge gap by formulating and experimentally proving an innovative flow control mechanism for two-phase ejectors, being based on the pulse-width modulation (PWM) of the refrigerant flow through the ejector. All the experimental evaluations were carried out at the compressor speed of 50 Hz, water temperature at the gas cooler inlet of 35 °C and R744 evaporating temperature of about −5 °C.The first experimental data revealed that the high pressure can be controlled appropriately as well as varied from about 87 bar to 112 bar, demonstrating the effectiveness of the proposed technique. In addition, the effect of the muffler volume as well as the PWM period on the ejector and system performance were investigated. It was found that the influence of both the muffler volume and the PWM period was not significant. Compared to the solution employing the passive ejector (i.e. without flow modulation technique), the unit with the PWM ejector presented enhancements in coefficient of performance (COP) by more than 5% at the optimum operation conditions. It is worth mentioning that its today’s available competitors, i.e. needle-based ejector and vortex-based ejector, feature COP enhancements by 2%–4% as contrasted with the passive ejector. As benchmarked to the standard unit (i.e. with flash gas by-pass valve and without ejector), the PWM ejector could improve the COP by more than 10% at the optimal running conditions. Also, the results obtained suggest that at present the proposed solution should operate with a PWM period of 2 s and no mufflers. Finally, the PWM ejector is characterized by low cost, simplicity, low vulnerability to clogging and no practical size or application constraints.

Development of ejector performance map for predicting fixed-geometry two-phase ejector performance for wide range of operating conditions

Recent studies have expanded the operational envelope of ejector by utilizing both pressure recovery and liquid recirculation mechanisms. This necessitates finding a consistent representation of fixed-geometry ejector performance data using only operational variables for predicting ejector performance. Taking inspiration from the compressor performance maps, the study explores the appropriate motive, performance, and characterization variable for representing ejector performance while conducting experiments on a transcritical CO2 ejector system. The existing ejector efficiency is revisited by conducting ejector power analysis. Two possible motive variables, along with a new ejector efficiency is introduced. The trends of existing and new ejector efficiencies are studied in controlled motive variable experiments while changing suction flow conditions. The ejector performance can be represented with the help of a single curve, termed as ejector performance map, using the new ejector efficiency as performance variable and the volumetric entrainment ratio as characterization variable. The prediction accuracy for both the single-phase and the two-phase suction inlet conditions are evaluated while developing performance map using only the single-phase suction inlet data points. The methodology can predict 89.3% of the data within 20% accuracy. The applicability of the methodology is assessed for other datasets involving different refrigerants, cycle architectures, and applications. The proposed ejector performance map can be utilized in numerical ejector system analysis for investigating new cycle architectures based on ejector experimental data, thus, improving system model fidelity. It can also help system design engineers in making ejector system selection decisions after thorough system performance analysis.

Dynamic modelling of a CO2 transport refrigeration unit with multiple configurations

Abstract The paper presents the dynamic characterization of a novel CO2 system designed for the refrigerated transport sector, when equipped to a medium-size refrigerated truck, during a daily typical long-distance delivery mission. The system takes advantage of a two-phase ejector to improve its performances and an auxiliary evaporator to extend the ejector operating range towards mild ambient temperatures. The system is designed allowing the possibility to switch between different configurations during operation, in order to maximize the COP or the refrigerating capacity, as a function of internal air and external environmental conditions. A numerical model of the system and its control strategy is developed using a 0-D dynamic commercial software, and coupled with a validated model of a refrigerated body. Simulation results show an average COP equal to 1.60 and an overall Duty Cycle of 10% for the delivery mission. The 96.6% of the total cooling energy is provided exploiting both the main and the auxiliary evaporator; however, dynamic simulations allowed highlighting that using the configuration exploiting both the evaporators during the hottest moments of the day can become counterproductive. Moreover, the behavior of the two-phase ejector under different environmental air temperature conditions is investigated. The maximum values of the average ejector efficiency (10.1%) and of the average pressure lift (1.59 bar) are reached for the highest external ambient temperatures (36.0 °C). Finally, to avoid the temperature and pressure drift inside the low pressure side of the refrigerating system during a long time of inactivity, an innovative safety control system based on the definition of a maximum allowed pressure level inside the low pressure side of the cooling unit is proposed.

Impact and quantification of various individual thermodynamic improvements for transcritical R744 supermarket refrigeration systems based on advanced exergy analysis

Climate change mitigation actions have led transcritical R744 refrigeration systems to take center stage in supermarket applications worldwide. Various measures aimed at making these solutions highly performing in any climate context, in fact, have been developed in the last few years. However, their benefits have been evaluated and quantified almost uniquely with the aid of conventional tools, e.g. energy and environmental assessments. Therefore, in this work the most power thermodynamic tool for such a purpose, i.e. the advanced exergy analysis, was applied to the state-of-the-art transcritical R744 supermarket refrigeration systems for the first time ever. Three advanced solutions, namely R744 unit with parallel compression and a R744 system using parallel compression and overfed evaporators with and without two-phase ejectors, were selected in addition to two baselines, being conventional and improved basic R744 units. The investigation was based on the outdoor temperature of 40 °C and operating conditions derived from field measurements. The results obtained suggested that the conventional basic R744 unit features total irreversibilities of 194.49 kW and total unavoidable irreversibilities of 81.96 kW, whereas total irreversibilities and total unavoidable irreversibilities respectively amounted to 168.80 kW and 81.44 kW for the improved basic R744 system. As parallel compression was considered, potential reductions from 143.30 kW down to 67.20 kW, i.e. approximately 18% lower in comparison with the conventional basic unit and the improved basic system, were assed. The use of overfed evaporators was found to lead to potential decrements in total irreversibilities down to 63.79 kW, which were around 5% lower in comparison with parallel compression and 22% lower than with the conventional basic unit and the improved basic system. In addition, the outcomes associated with advanced exergy analysis implementation revealed the need to adopt two-phase ejectors as well as more efficient parallel compressors. The first measure would permit decreasing the total irreversibilities potentially from 115.04 kW down to 44.35 kW, resulting in decrement by about 46% compared to conventional and improved basic R744 systems, 34% in relation to the unit with parallel compressors and 30.5% over the solution with parallel compressors and overfed evaporators. Also, if 10% more efficient parallel compressors had been available, total irreversibilities, total unavoidable irreversibilities and total avoidable irreversibilities of the solution with two-phase ejectors would have been further reducible by 9.3%, 2.6% and 13.5%, respectively. It could be concluded that ejector-based transcritical R744 supermarket refrigeration systems possibly with high performing parallel compressors are capable of providing great thermodynamic performance at high sink temperatures. In addition, the vapour ejectors were found to be responsible for 22.9% of the total avoidable inefficiencies. These irreversibilities could be decremented by almost uniquely enhancing the performance of the ejectors themselves. However, unlike the results derived from conventional energy and environmental analyses, the outcomes of the advanced exergy methodology did not reveal a significant contribution on the part of the overfed evaporators to the thermodynamic performance enhancement of transcritical R744 supermarket refrigerating plants.

An experimental investigation of performance and instabilities of the R744 vapour compression rack equipped with a two-phase ejector based on short-term, long-term and unsteady operations

In the case of R744 refrigeration technology, the system equipped with a two-phase ejector is one of the most efficient refrigeration systems, allowing utilisation of an R744-based unit in hot and tropical climates. However, the experimental analyses of the ejector-based system have only been performed for system performance evaluation, mapping the performance of ejectors or validation of the ejector numerical simulation. Therefore, the main goal/scope of this paper is to enhance the experimental knowledge of the state-of-the-art R744 vapour compression rack equipped with an efficient two-phase ejector, the relationship between the ejector and other components, and the influence of unsteady system work on the ejector performance and cooling demand. The test campaign was carried out on the R744 ejector-based test rig with a cooling capacity of 50 kW, high-side pressures from 60 bar to 100 bar and temperatures from 15 °C to 40 °C. The experimental analysis was conducted for three different system operating conditions, namely, short-term, long-term, and unsteady-state conditions, for refrigeration and air-conditioning applications. Utilisation of the ejector influenced the system instability, especially at the internal heat exchanger control. In addition, the low-pressure lift below 4 bar allowed the most efficient control strategy of the cooling demand for the operation below the critical point. For the transcritical unsteady operation, the most efficient operation was obtained for the defined pressure lift of 6 bar.

Exergy and Energy Analyses of Dual-Temperature Evaporator Split AC System Incorporated A Capillary Tube and A Two-Phase Ejector

This study was aimed to investigate performance of a split type air conditioning (SAC) system applying exergy destruction method. A numerical model was established based on exergy destruction analysis of a Condenser Outlet Split-Split Air Conditioning (COS-SAC) system integrated with dual-temperature evaporator and incorporated capillary tube and ejector as expansion devices. An experimental test system was also established to experimentally validate the model. Two type of refrigerants R-290 and R-22 were involved in the evaluations. The Coefficient of Performance (COP) of the ejector COS-SAC system, exergy destruction, and exergy efficiency were determined and compared with those in the SAC system utilizing capillary tube. The results showed there was a significant improvement in the overall exergy efficiency of the COS-SAC systems. The COP of the COS-SAC system was also found to be better than the COP of the SAC system.

Modified CO2-based combined cooling and power cycle with multi-mode and adjustable ability

To fulfill the diversified demand of power and cooling, the multi-mode combined cooling and power cycle based on CO2 is studied in this research, which can realize full power mode, simultaneous power and refrigeration mode and full refrigeration mode. A two-phase ejector is introduced to ameliorate the configurations of conventional combined cycles. A refrigerated truck is selected as the research objective and two refrigeration conditions are considered. Through the energy and exergy analyses, great improvement in performance and considerable potential in diversified energy supply can achieve by the modified systems. Therein, the modified system with single-stage compression achieves 4.9% improvement of power output and 21.7% improvement of refrigeration output versus the baseline system under refrigeration condition. It can provide extra 15.1 kW power for the objective refrigerated truck after satisfying the refrigeration demand. Besides, the adjustability performance is also discussed in this study, which reveals that the modified system with single-stage compression also possesses the best adjustability performance for its highest cooling to power ratio and the convenient mode transforming and controlling.

A detailed review on [formula omitted] two-phase ejector flow modeling

Ejector-equipped vapor-compression systems for refrigeration and cooling, relying solely on CO2 (R744) as a natural working fluid, are perceived to be an eco-friendly and highly efficient solution for many applications. However, the complexity of two-phase ejector flows makes it very challenging to find realiable and efficient ejector designs. Improved design methods are necessary in order to achieve higher performance in R744 units compared to the traditional compressor-based systems with refrigerants that put a high strain on the environment. Consequently, the development of advanced models and tools for an accurate design of the R744 ejectors has been a highly prioritized research topic. To the best of the authors’ knowledge, the current status of R744 ejector models and their limitations has not been thoroughly evaluated yet. To summarise the current state of the art and knowledge gaps, this work presents an exhaustive overview of the available numerical models applied to R744 two-phase ejectors, i.e. multiphase flow modeling, turbulence aspects, numerical solution methods, applications of models, to further encourage the adoption of R744 vapor-compression solutions. Finally, a thorough discussion of different focus points for future research as well as the main challenges in the field is presented.

Performance evaluation of a modified R290 dual-evaporator refrigeration cycle using two-phase ejector as expansion device

This paper presents a modified dual-evaporator ejector expansion refrigeration cycle (MDEEC) in which a two-phase ejector is used as the expansion device to recover the expansion work. Based on constant-pressure mixing theory, a thermodynamic model is developed to investigate the effects of various parameters on the system performance. Also a comparative study of the modified cycle with a conventional one (CDVCC) and a previous ejector cycle (COSEC) is conducted in terms of energy and exergy analysis. The results show that COP of the modified cycle is improved by about 10% and the exergy destruction of expansion device is reduced by more than 90% compared with the COSEC. At the operating condition of Tc = 45 °C and Te2 = 5 °C, the COP and exergy efficiency of the MDEEC can reach 6.515 and 0.3184, respectively, which are 42.6% and 36.7% higher than those of the CDVCC. It is also revealed the performance improvements become more significant at higher condensing temperatures or lower evaporating temperatures. Finally, the effects of the ejector components efficiency and the performance under off-design conditions are discussed.

One-dimensional analysis of the convergent-divergent motive nozzle for the two-phase ejector: Effect of the operating and design parameters

Two-phase ejectors are used in the refrigeration cycles to decrease the throttling losses and improve the performance. The subject of this paper is the motive nozzle which is the critical component of the ejector since the pressure distribution throughout the motive nozzle affects the secondary fluid to be entrained into the ejector. A simplified version of the one of the previously established one-dimensional (1-D) converging-diverging motive nozzle models is developed in this paper to calculate the pressure, temperature, velocity, and Mach number distributions. 1-D model of the nozzle is established based on the conservation equations of mass, momentum, and energy and the equation of state under steady, frictional, and adiabatic flow assumptions with the homogeneous equilibrium condition. Maximum differences of the pressure drop throughout the nozzle between the literature data and the calculated results are around 6% and 8% for CO2 and R134a nozzles, respectively. The main objective is investigating the effects of the subcooling temperature difference, inlet pressure (condenser temperature or pressure), mass flow rate, and motive nozzle outlet diameter for R134a, R1234yf, and R1234ze(E). Parametric comparisons and evaluations are used to lead the motive nozzle designs for the further numerical and experimental studies including these new generation refrigerants for R134a replacement.

A review on current status of capacity control techniques for two-phase ejectors

Abstract The adoption of highly efficient vapour-compression heating, ventilation, air conditioning and refrigeration (HVAC&R) systems is compulsory to achieve a low-carbon society. Expansion work recovery using a two-phase ejector is widely recognized as one of the most promising measures to improve the energy efficiency of HVAC&R units. This holds true for all operation conditions provided that an effective capacity control technique is implemented. In this work a thorough critical review on the current status of the presently available capacity control strategies for two-phase ejectors was carried out. In addition, their pros and cons as well as the comparison of their performance were reported. It was concluded that two-phase ejectors can be properly capacity controlled in large- and medium-scale vapour-compression units. However, a suitable capacity control mechanism for small-scale vapour-compression solutions still requires a major breakthrough and is being intensively discussed among experts in the field.

Experimental Performance of a Two-Phase Ejector: Nozzle Geometry and Subcooling Effects

This paper presents the results of an experimental study on a two-phase ejector. The main objective is to assess the effects of the nozzle’s divergent and the throat diameter on performance under various working conditions. Under the same conditions, ejector operation with a convergent nozzle, results in higher critical primary mass flow rate and lower critical pressure than with a convergent-divergent nozzle version. Experiments show as well that the flow expansion is higher in the convergent-divergent nozzle. The throat diameter turns out to have an important impact only on the amount of the critical mass flow rate. The nozzle geometry has no impact on its optimal position in the ejector. Globally, the ejector with the convergent-divergent nozzle provides a higher entrainment ratio, due to a reduced primary mass flow rate and an increased secondary flow induction. Tests also show that the ejector with a lower throat diameter provides a higher entrainment ratio, due to better suction with less primary flow. Unlike the convergent-divergent nozzle, the convergent nozzle permits an entrainment ratio almost insensitive to a wide range of primary inlet sub-cooling levels. Primary and secondary mass flow rates increase proportionally with the subcooling level and result in a quasi-constant entrainment ratio.

Review on the simulation models of the two-phase-ejector used in the transcritical carbon dioxide systems

The two-phase-ejector is a prominent approach of performance improvement for transcritical CO2 refrigeration by recovering pressure losses into compression power; however, no clear understanding of the involved complicated thermodynamic mechanism is the uppermost block to its popularization. Although comprehensive numerical study has been published aiming at the performance analysis of the two-phase ejector as well as the integrated systems, the omnifarious simulation methods were rarely summarized. This review reported and systematically categorized the state-of-the-art simulation methods aiming at two-phase ejectors. The core concepts and unsolved problems of zero/one-dimension and three-dimension models were clearly illustrated, respectively. Besides, a historical perspective of the models’ emphasis in different groups was comprehensively summarized, by which the development history of the simulation models was clearly generalized. This review could provide an overview of the simulation methods for scholars, developers and manufacturers using the most appropriate one into certain applications of ejector technology.

Modelling the performance of a new cooling unit for refrigerated transport using carbon dioxide as the refrigerant

• Development of a dynamic numerical model of a CO 2 cooling unit. • 3 layouts, including a back-pressure valve, ejector and auxiliary evaporator. • The ejector can give a COP increase of +15.9% at T amb = 42 °C, T i = −5 °C. • The auxiliary evaporator can extend the operating range of the ejector to lower T amb . • The auxiliary evaporator gives a max COP increase of 21.0% for T amb = 25 °C, T i = 5 °C. This paper provides a theoretical assessment of the thermal performance of a new CO 2 vapour-compression system for refrigerated transport applications. Three different configurations are investigated: the standard back-pressure with low pressure receiver lay-out and two arrangements integrating a two-phase ejector. In particular, the use of an auxiliary evaporator in the outlet line of the ejector is considered, to extend the ejector operating range. A numerical model of the system is developed and its theoretical performance is discussed for different values of internal space temperature and external ambient temperature. Simulations’ results show that the ejector cycle configuration is convenient when the system is operating in a hot climate with a maximum COP increase (compared to the traditional configuration) equal to 15.9%, at 42 °C ambient temperature and -5 °C internal space temperature. The use of an auxiliary evaporator can extend the operating range of the ejector to lower values of ambient temperature, with a maximum COP improvement (over the traditional configuration) equal to 21.0% at 25 °C ambient temperature and 5 °C internal cargo space.

A review on current status of capacity control techniques for two-phase ejectors

A review on current status of capacity control techniques for two-phase ejectors

A review on current status of capacity control techniques for two-phase ejectors

A review on current status of capacity control techniques for two-phase ejectors

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

Abstract 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.

Experimental Investigation on Heat Transfer Mechanism of Air-Blast-Spray-Cooling System with a Two-Phase Ejector Loop for Aeronautical Application

This paper presents an air-oriented spray cooling system (SCS) integrated with a two-phase ejector for the thermal management system. Considering its aeronautical application, the spray nozzle in the SCS is an air-blast one. Heat transfer performance (HTP) of air-water spray cooling was studied experimentally on the basis of the ground-based test. Factors including pressure difference between water-inlet-pressure (WIP) and spray cavity one (PDWIC) and the spray volumetric flow rate (SVFR) were investigated and discussed. Under a constant operating condition, the cooling capacity can be promoted by the growth factors of the PDWIC and SVFR with the values from 51.90 kPa to 235.35 kPa and 3.91 L ⋅ h − 1 to 14.53 L ⋅ h − 1 , respectively. Under the same heating power, HTP is proportional to the two dimensionless parameters Reynolds number and Weber number due to the growth of droplet-impacting velocity and droplet size as the increasing of PDWIC or SVFR. Additionally, compared with the factor of the droplet size, the HTP is more sensitive to the variation in the droplet-impacting velocity. Based on the experimental data, an empirical experimental correlation for the prediction of the dimensionless parameter Nusselt number in the non-boiling region with the relative error of only ± 10 % was obtained based on the least square method.

Experimental study on the primary flow expansion characteristics in transcritical CO2 two-phase ejectors with different primary nozzle diverging angles

This paper presented an experimental study to explore the primary flow expansion characteristics in the transcritical CO2 two-phase ejectors for various primary nozzle diverging angles (NDAs) and secondary flow pressure. The phase change of the primary flow was visualized, and the effect of the primary flow expansion state on the entrainment ratio was evaluated. The results disclosed that the primary flow changed from the under-expanded state to the over-expanded state with increasing the NDA, and the primary flow expansion state was insensitive to the secondary flow pressure. The visualization images showed that the phase change position moved towards the nozzle exit visibly when the NDA was 0.0°, and the effect of secondary flow pressure on the phase change position was insignificant. Besides, the results revealed that the over-expanded state of primary flow had a negative effect on the entrainment ratio, and the entrainment ratio obtained the maximum when the NDA was 2.0°. The present work is indispensable to realize a comprehensive understanding of the primary flow expansion mechanism inside the transcritical CO2 ejector, and the insights gained from this study may be of assistance to optimize the ejector structure and improve the ejector performance.