Analytical and Numerical Studies of a Steam Ejector on the Effect of Nozzle Exit Position and Suction Chamber Angle to Fluid Flow and System Performance

Nozzle exit position [NXP] plays a vital role in the performance of the ejector, but its values are specified in a range for the required operating condition. In this study instead of the range of values, a specific value, named as entrainment diameter is developed and its effect on the performance of the ejector is studied for several combinations of suction chamber angle using numerical method. The effect of the condenser and boiler pressures on the performance of the ejector are also studied to ensure the off-design operating conditions. The entrainment diameter of an ejector is derived analytically by solving one dimensional compressible fluid flow equations using MATLAB. To study the effect of entrainment diameter on the performance of the ejector, CFD technique is employed. Analytical and numerical results are validated with experimental data available in the previous studies. For 7 kW refrigeration capacity, it is inferred that the suction chamber angle of 18° and the corresponding entrainment diameter 90.8 mm with the NXP of 23.62 mm yield the maximum entrainment ratio. The study predicts that the performance of the ejector is highly influenced by the pressure increment at the exit of the nozzle, while the suction chamber angle is between 12° to 21°

Optimization Of Hydrocarbon Ejector Using Computational Fluid Dynamics

Ejector is a powerful emerging thermo-compressor, which is more effective when used with hydrocarbon refrigerants because of its unique thermophysical properties. Therefore, in the present work, a steam ejector model is designed and validated with experimental results to evaluate its accuracy, followed by a detailed comparative study of hydrocarbons and synthetic refrigerants namely pentane, propane, butane, iso-butane, R1234-ze and R1234-yf by computational fluid dynamics and literature Review. The effectiveness of both classes of refrigerants is measured through entrainment ratio, critical backpressure, and thermophysical properties (Literature Review). Pentane was selected as a working fluid since it has comparatively high combination of entrainment ratio and critical back pressure with refrigeration compatible properties. Lastly, the optimized geometry was simulated by varying diameter of constant area zone, nozzle exit position and nozzle expansion angle through Computational Fluid dynamics. The simulation results provide insight into shockwaves, boundary layer separation, vortex formation of ejector flow

Numerical Optimization of an Ejector for Waste Heat Recovery Used to Cool Down the Intake Air in an Internal Combustion Engine

[EN] In the present paper, a numerical investigation of a jet-ejector is carried out using a real gas model of R1234yf. The prototype under investigation works with specific operating conditions of a jet-ejector refrigeration system intended for waste heat recovery in an internal combustion engine (ICE). In the first instance, the geometry optimization involving nozzle exit diameter, mixing chamber diameter, and nozzle exit position (NXP) is performed. Once the optimum geometry has been obtained, the jet-ejector prototype is tested with different operating pressure ratios to determine its off-design performance. The flow structure in relevant cases has been examined with an emphasis on critical and subcritical modes. The flow phenomena occurring during expansion, entrainment, and mixing processes are discussed so performance degradation can be directly related to physical processes. The analysis has been completed fitting simulated points to critical and subcritical planar surfaces. The results in terms of goodness of fit are satisfactory so the jet-ejector performance in off-design operating conditions can be reflected through simple mathematic models. When the overall cycle is assessed by using previous computational fluid dynamics (CFD) maps, it is observed that the achievable cooling drops significantly when an ambient temperature of 31 degrees C is exceeded.The authors want to acknowledge the institution "Conselleria d'Educacio, Investigacio, Cultura i Esport de la Generalitat Valenciana" and its grant program "Subvenciones para la contratacion de personal investigador de caracter predoctoral" for doctoral studies (ACIF/2018/124).Galindo, J.; Gil, A.; Dolz, V.; Ponce-Mora, A. (2020). Numerical Optimization of an Ejector for Waste Heat Recovery Used to Cool Down the Intake Air in an Internal Combustion Engine. Journal of Thermal Science and Engineering Applications. 12(5):1-13. https://doi.org/10.1115/1.4046906S113125Varga, S., Oliveira, A. C., & Diaconu, B. 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An experimental and theoretical study of a jet-pump refrigeration system designed using a new two-dimensional model for the entrainment region of the ejector. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 227(4), 486-497. doi:10.1177/0957650913477092Zhu, Y., & Jiang, P. (2014). Experimental and analytical studies on the shock wave length in convergent and convergent–divergent nozzle ejectors. Energy Conversion and Management, 88, 907-914. doi:10.1016/j.enconman.2014.09.023Zhu, Y., & Jiang, P. (2014). Experimental and numerical investigation of the effect of shock wave characteristics on the ejector performance. International Journal of Refrigeration, 40, 31-42. doi:10.1016/j.ijrefrig.2013.11.008Sargolzaei, J., Pirzadi Jahromi, M. R., & Saljoughi, E. (2010). Triple-Choking Model for Ejector. Journal of Thermal Science and Engineering Applications, 2(2). doi:10.1115/1.4002752Armstead, J. R., & Miers, S. A. (2013). Review of Waste Heat Recovery Mechanisms for Internal Combustion Engines. Journal of Thermal Science and Engineering Applications, 6(1). doi:10.1115/1.4024882Luján, J. M., Climent, H., Dolz, V., Moratal, A., Borges-Alejo, J., & Soukeur, Z. (2016). Potential of exhaust heat recovery for intake charge heating in a diesel engine transient operation at cold conditions. Applied Thermal Engineering, 105, 501-508. doi:10.1016/j.applthermaleng.2016.03.028Aghaali, H., & Ångström, H.-E. (2015). A review of turbocompounding as a waste heat recovery system for internal combustion engines. Renewable and Sustainable Energy Reviews, 49, 813-824. doi:10.1016/j.rser.2015.04.144Hsiao, Y. Y., Chang, W. C., & Chen, S. L. (2010). A mathematic model of thermoelectric module with applications on waste heat recovery from automobile engine. Energy, 35(3), 1447-1454. doi:10.1016/j.energy.2009.11.030In, B. D., & Lee, K. H. (2015). A study of a thermoelectric generator applied to a diesel engine. 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Preliminary analysis of organic Rankine cycles to improve vehicle efficiency. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 228(10), 1142-1153. doi:10.1177/0954407014528904Zegenhagen, M. T., & Ziegler, F. (2015). Feasibility analysis of an exhaust gas waste heat driven jet-ejector cooling system for charge air cooling of turbocharged gasoline engines. Applied Energy, 160, 221-230. doi:10.1016/j.apenergy.2015.09.057Novella, R., Dolz, V., Martín, J., & Royo-Pascual, L. (2017). Thermodynamic analysis of an absorption refrigeration system used to cool down the intake air in an Internal Combustion Engine. Applied Thermal Engineering, 111, 257-270. doi:10.1016/j.applthermaleng.2016.09.084Galindo, J., Dolz, V., Tiseira, A., & Ponce-Mora, A. (2019). Thermodynamic analysis and optimization of a jet ejector refrigeration cycle used to cool down the intake air in an IC engine. 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Flow structure of a steam ejector influenced by operating pressures and geometries. International Journal of Thermal Sciences, 46(8), 823-833. doi:10.1016/j.ijthermalsci.2006.10.012Bartosiewicz, Y., Aidoun, Z., Desevaux, P., & Mercadier, Y. (2005). Numerical and experimental investigations on supersonic ejectors. International Journal of Heat and Fluid Flow, 26(1), 56-70. doi:10.1016/j.ijheatfluidflow.2004.07.003Mazzelli, F., Little, A. B., Garimella, S., & Bartosiewicz, Y. (2015). Computational and experimental analysis of supersonic air ejector: Turbulence modeling and assessment of 3D effects. International Journal of Heat and Fluid Flow, 56, 305-316. doi:10.1016/j.ijheatfluidflow.2015.08.003Mazzelli, F., & Milazzo, A. (2015). Performance analysis of a supersonic ejector cycle working with R245fa. International Journal of Refrigeration, 49, 79-92. doi:10.1016/j.ijrefrig.2014.09.020Croquer, S., Poncet, S., & Aidoun, Z. (2016). 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T., & Ziegler, F. (2015). A one-dimensional model of a jet-ejector in critical double choking operation with R134a as a refrigerant including real gas effects. International Journal of Refrigeration, 55, 72-84. doi:10.1016/j.ijrefrig.2015.03.013Besagni, G., Mereu, R., Chiesa, P., & Inzoli, F. (2015). An Integrated Lumped Parameter-CFD approach for off-design ejector performance evaluation. Energy Conversion and Management, 105, 697-715. doi:10.1016/j.enconman.2015.08.029Gagan, J., Smierciew, K., Butrymowicz, D., & Karwacki, J. (2014). Comparative study of turbulence models in application to gas ejectors. International Journal of Thermal Sciences, 78, 9-15. doi:10.1016/j.ijthermalsci.2013.11.009Hakkaki-Fard, A., Aidoun, Z., & Ouzzane, M. (2015). A computational methodology for ejector design and performance maximisation. Energy Conversion and Management, 105, 1291-1302. doi:10.1016/j.enconman.2015.08.070Besagni, G., & Inzoli, F. (2017). 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Review on Ejector Efficiencies in Various Ejector Systems

The ejector efficiencies affect the ejector expansion systems significantly. This paper provides a literature review on ejector efficiency in various ejector systems, such as refrigeration and heat pump systems (transcritical systems and subcritical systems), solar systems, and steam systems. The definitions of ejector efficiency and ejector component efficiencies in literature are summarized. The assumed constant ejector component efficiencies used in ejector modeling, and the empirical correlations of ejector efficiencies derived based on measured data are summarized and compared; the methods of determining energy efficiencies are summarized. The effects of ejector geometries, operation conditions and working fluid characteristics (single phase or two-phase, zeotropic or azeotropic) on ejector efficiencies are discussed. The prospects of ejector application in and beyond the field of refrigeration are also discussed. The potential of ejector used in the low-pressure working fluids ejector systems are discussed based on variation of ejector efficiencies. This review will be useful for the further research on ejector efficiency, optimum operation and control of ejector expansion systems

Constant Rate of Momentum Change Ejector: simulation, experiments and flow visualisation

An ejector is a momentum-transfer device that requires no external mechanical input or moving parts. However, ejectors have low performance due to irreversibilities such as viscous losses and shocks in the primary stream and diffuser. It has previously been argued that by maintaining a constant rate of momentum change along the ejector duct, shock losses could be eliminated or at least minimised, and so the Constant Rate of Momentum Change (CRMC) ejector was introduced. The CRMC configuration appears to have significant potential, but the CRMC design prescription relies on: (1) an arbitrary choice for the constant rate of momentum change along the length of the duct; and (2) complete mixing between primary and secondary streams at the entrance to the duct. This thesis investigates the themes of shock losses and mixing within a CRMC ejector using physical experiments and computational simulation. The CRMC ejector duct and the primary nozzle were manufactured using 3D printing technology and then an experimental test bench using air as the working fluid was assembled and successfully tested. The primary nozzle had a throat diameter of 3.2mm and an exit diameter of 13.6 mm; the CRMC duct had a throat diameter of 25.48 mm. Extensive experimental tests were carried out for primary pressure between 200 kPa and 270 kPa, and secondary pressure between 0.6 kPa and 5 kPa. The results demonstrate the primary nozzle exit position within the entrainment region has a limited effect on the ejector performance in terms of the entrainment ratio and critical back pressures. A gas dynamic model was used to compare the performance of the present CRMC ejector with different ejector profiles (both conventional and CRMC) working with different fluids. The CRMC ejector showed a slightly better performance in terms of entrainment ratio and compression ratio. When CFD simulations of the present CRMC ejector were compared with a conventional ejector at a similar operating condition, the total pressure of the CRMC ejector remained 15% larger than the conventional ejector but this higher performance was due to different primary flow shock structures, not due to improvements in the compression process within the diffuser. Differences in the primary flow structure are thought to be caused by the different contraction angle of the secondary flow area. Higher entrainment ratio and compression ratio were simulated for the CRMC ejector relative to the conventional ejector but were not as high as expected from the CRMC design. To investigate the mixing of the flow within the CRMC ejector, a laser-based visualization technique was developed. A transparent CRMC ejector test section was designed, fabricated, and operated in the ejector system using air as the working fluid. The laser-based flow visualisation used a laser light beam of diameter of 1mm to illuminate the seeded secondary flow and thus, the unmixed primary flow was defined. The wall static pressure of the seeded flow agrees well with that of the unseeded flow which indicates that the seeding has a very small effect on the flow. Analysis of the images by digital image processing tools enabled identification of the jet core flow length which was found to lie between 65mm and 95mm from the nozzle exit at the selected operating conditions. The primary and secondary flows entering the CRMC duct are certainly not fully mixed as assumed in the CRMC design prescription. Furthermore, enhancement of the distribution of the wall static pressure and centreline total pressure is not directly attributable to the CRMC prescription. The modest performance improvements associated with the present CRMC design relative to the performance of a conventional duct should be balanced against the added complexity associated with manufacturing a CRMC duct when considering the CRMC design for future applications

Supersonic ejector simulation and optimisation

The aims of this project were the implementation of Computational Fluid Dynamics (CFD) to the study of supersonic ejectors, and the investigation of the flow processes that occur. The conventional ejector has been in existence for more than a century yet the design has remained largely unchanged and is difficult to optimise. This has been attributed to a lack of understanding of the complex flow processes and phenomena that occur. CFD provides the ability to study these processes, and to rapidly assess geometrical influence upon operational performance. The CFD model was assessed through systematic appraisal of the numerical parameters that influence solution stability and simulation accuracy. Two proprietary CFD codes were utilised; a structured segregated code and an adaptive mesh coupled code. Assessed parameters included; mesh dependency, discretisation schemes, turbulence models, and boundary layer models that are shown highly influential. Simulation was validated through comparison of predicted and experimental entrainment values. Simulations of an ejector that is part of a steam-jet refrigeration cycle were used to assess the influence of geometry and operating conditions. The structured code was found suitable for geometrical studies however the coupled code was required for detailed flow analysis. Geometrical studies showed current ejector design guidelines to be well set. Operational studies highlighted the dominant influence of motive fluid flow rate upon entrainment levels. Shock systems and flow processes could be clearly identified. Simulations of ejectors utilised in vacuum and thrust augmenting applications were also conducted in assessment of the general applicability of CFD. CFD has the potential to be an effective and powerful tool III simulating and understanding ejectors. Qualitative and quantitative results can be obtained dependent upon the optimisation and validation of the mathematical model. This however can only be performed properly if the user fully understands the t10w physics and applied numerics