Ejector cycle control
Ejector cycle performance is usually sensitive to working condition changes which are common in automotive systems. Different working conditions require different ejector geometries to achieve maximum performance. Slightly different geometries may result in substantially different COPs under the same conditions. Therefore, it is desirable to introduce an adjustable feature to the ejector so that ejector cycle performance can be optimized under different working conditions, which could make ejector technology more suitable for real world applications (Sumeru et al. [1]).
The ejector motive nozzle throat diameter is one of the key dimensions that affect ejector cycle COP. It has a direct impact on motive mass flow rate. Other important ejector geometric parameters that affect ejector efficiency and ejector cycle COP include motive nozzle position, constant area diameter of the mixing chamber and suction chamber converging angle. Additional information can be found in Sarkar [2]. One way to adjust the motive nozzle throat diameter in order to optimize ejector cycle performance according to the working conditions is by using a needle which moves back and forth so that the nozzle throat diameter can be varied, as illustrated in Figure 1. Elbel and Hrnjak [3] were the first researchers to publish experimental results of introducing a variable two-phase ejector to a transcritical R744 system by installing a needle in the motive nozzle to control the motive nozzle throat diameter. The needle mechanism allowed control of gas cooler high-side pressure, which is an important task for a transcritical cycle to get optimum performance. However, nozzle and ejector efficiencies were impaired because of the additional frictional losses introduced by the needle. It was found that the benefits of high-side pressure control offset the losses in nozzle and ejector efficiencies. A variable geometry ejector with adjustable needle in the motive nozzle can optimize ejector cycle performance under different conditions, but this design is complicated and costly, and more frictional losses are incurred because of the additional surface area introduced which results in lower nozzle and ejector efficiencies. This provides motivation to develop a new technology to control the motive nozzle restrictiveness. Zhu and Elbel [4] were the first to introduce vortex control to ejector for the control of ejector cooling cycles. A vortex ejector which employs the vortex control to adjust motive nozzle restrictiveness differs from a conventional ejector in that an adjustable vortex is generated at the ejector motive inlet, as shown in Figure 2. The motive inlet vortex can be created by injecting part of the motive flow tangentially. After injection the tangential flow is mixed with the axial motive flow. The total mass flow rate passing through the vortex nozzle is equal to the sum of mass flow rates entering through the nozzle’s axial and tangential flow inlets. The ejector cooling cycle using a vortex ejector, as shown in Figure 3, is almost the same as the conventional ejector cooling cycle. The only difference is that the flow at the condenser outlet of the vortex ejector cooling cycle is separated into two streams. One stream enters the vortex ejector through the motive flow tangential inlet and another enters through the motive flow axial inlet. In such a way, a vortex is created at the ejector motive inlet. The ratio of mass flow rates through the two inlets can be adjusted by a valve installed at the motive flow tangential inlet, thereby changing the vortex strength. The pressure drop across the control valve is usually small. It can be assumed that the thermodynamic state at the motive nozzle inlet after the vortex is introduced (downstream of the tangential inlet valve) is the same as the refrigerant state at the condenser outlet. |
Figure 1. Variable ejector with adjustable needle in the motive nozzle.
Figure 2. (a) Conventional ejector and (b) vortex ejector.
Figure 3. Vortex ejector cooling cycle.
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Zhu and Elbel’s experiments [4] on vortex nozzle with initially subcooled R134a show that the strength of the nozzle inlet vortex can change the restrictiveness of the two-phase nozzle without the need of changing the nozzle geometry. The nozzle becomes more restrictive as the strength of the vortex increases. The mass flow rate can be reduced by 36% with vortex control under the same inlet and outlet conditions. The control range of inlet pressures and mass flow rates that can be achieved by vortex control appears to be large enough to be applicable for real world applications.
[1] Sumeru, K., Nasution, H., and Ani, F. N., “A review on two-phase ejector as an expansion device in vapor compression refrigeration cycle,” Renewable and Sustainable Energy Reviews, 16(7): 4927-4937, 2012.
[2] Sarkar, J., “Ejector enhanced vapor compression refrigeration and heat pump systems - A review,” Renewable and Sustainable Energy Reviews, 16(9): 6647-6659, 2012.
[3] Elbel, S., and Hrnjak, P., “Experimental validation of a prototype ejector designed to reduce throttling losses encountered in transcritical R744 system operation,” International Journal of Refrigeration, 31(3): 411-422, 2008.
[4] Zhu, J. and Elbel, S., “A New Control Mechanism for Two-Phase Ejector in Vapor Compression Cycles for Automotive Applications Using Adjustable Motive Nozzle Inlet Swirl,” SAE Int. J. Passeng. Cars - Mech. Syst. 9(1): 2016, doi:10.4271/2016-01-0243.
[1] Sumeru, K., Nasution, H., and Ani, F. N., “A review on two-phase ejector as an expansion device in vapor compression refrigeration cycle,” Renewable and Sustainable Energy Reviews, 16(7): 4927-4937, 2012.
[2] Sarkar, J., “Ejector enhanced vapor compression refrigeration and heat pump systems - A review,” Renewable and Sustainable Energy Reviews, 16(9): 6647-6659, 2012.
[3] Elbel, S., and Hrnjak, P., “Experimental validation of a prototype ejector designed to reduce throttling losses encountered in transcritical R744 system operation,” International Journal of Refrigeration, 31(3): 411-422, 2008.
[4] Zhu, J. and Elbel, S., “A New Control Mechanism for Two-Phase Ejector in Vapor Compression Cycles for Automotive Applications Using Adjustable Motive Nozzle Inlet Swirl,” SAE Int. J. Passeng. Cars - Mech. Syst. 9(1): 2016, doi:10.4271/2016-01-0243.