CFD Analysis of a Primary Nozzle Vortex Generator on Steam Ejector Performance

Cooper, Brodie (2022) CFD Analysis of a Primary Nozzle Vortex Generator on Steam Ejector Performance. [USQ Project]

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Ejectors are considered by many researchers a promising replacement for compressors in air-conditioning and refrigeration applications. They are more economical due to having no moving parts, giving them the advantage of requiring less maintenance and having fewer potential points of failure. With concern surrounding the effects of global warming rapidly increasing, the jet ejector’s ability to operate using environmentally friendly refrigerants and low-grade thermal energy is highly appealing. The main disadvantage observed throughout research is that the ejector has a poor coefficient of performance attributed to the effects of supersonic turbulent mixing.

The University of Southern Queensland commissioned a steam ejector refrigeration apparatus for the purpose of providing flow visualisation for ongoing research aimed at understanding ejector mixing behaviour and factors influencing the coefficient of performance (Al-Doori 2013). This study focuses on improving the performance of the ejector by incorporating a vortex generator into the design of the primary flow nozzle, with the findings of this dissertation intended to contribute to a technical paper currently being prepared by the University of Southern Queensland. Computational fluid dynamics software is capable of providing reliable results at an acceptable level of accuracy with significantly lower associated costs and risks compared to experimental techniques. For this reason, the operation of the steam ejector has been characterised in this study through an entirely computational approach, using ANSYS Fluent.

To achieve this, the dimensions of the UniSQ steam ejector were obtained through previous work completed by Al-Manea (2019) and Al-Doori (2013); these papers also provided the boundary conditions corresponding the UniSQ steam ejector apparatus necessary for the CFD analysis. The key parameters of the CFD simulations were the use of the realisable kϵ turbulence model coupled with advanced wall functions and specified primary, secondary and condenser pressures. Use of the species transport model enabled water-vapour and nitrogen to be assigned to the primary and secondary inlets to remain consistent with the real system; the refrigerants were treated as ideal gases for the purpose of simplicity.

Initially, a 2D ejector model was generated and the geometry was validated by comparing the results obtained through ANSYS Fluent simulations against the existing experimental and CFD results of Al-Manea (2019). A 3D model of the ejector was developed through external CAD software to be used as a standard for comparison for the models equipped with vortex generators. The primary and secondary pressures were set constant at 150kPa and 2.4kPa, respectively, and the condenser pressure was adjusted between 1.8 - 2.8kPa to simulate the operating range of the ejector. The 3D model recorded less than 1% error in choked conditions and up to 10% in unchoked conditions compared to the 2D model.

Following the verification of the standard model, a parametric study was conducted investigating different variations of vortex generators in the primary nozzle. A series of ejector models were created with modified nozzles varying in the vortex generator pitch, profile and quantity. Due to the limited volume of the nozzle and supersonic steam velocity, the models were limited to a maximum of three generators with helical pitches ranging from 17.5 - 53mm, and profiles of 1mm. The influence of the generators geometric characteristics was identified by simulations of the operating range over the same boundary conditions as those used for baseline ejector model.

The results obtained from the simulations revealed that the induced swirling effect decayed rapidly in the mixing chamber and that the presence of the vortex generators reduced primary flow velocity by between 3 and 43.3 m/s. The minimum pressure at the nozzle exit increased over the standard nozzle by between 161.7 and 639.8 Pa for the modified nozzle variations. The combination of these effects resulted in a decreased entrainment ratio of between 0.67 − 2% and an increased calculated mixing efficiency of between 0.22 − 3.7%. Overall, the generator variations resulted in increased recirculation and turbulence that contributed to producing a lower quality primary-secondary mixture. It was determined that, in general, the performance of the ejector was decreased as the quantity of vortex generators increased and the helical pitch was reduced.

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Item Type: USQ Project
Item Status: Live Archive
Faculty/School / Institute/Centre: Current – Faculty of Health, Engineering and Sciences - School of Engineering (1 Jan 2022 -)
Supervisors: Saleh, Khalid
Qualification: Bachelor of Engineering (Honours) (Mechanical)
Date Deposited: 19 Jun 2023 03:01
Last Modified: 20 Jun 2023 01:08
Uncontrolled Keywords: ejectors; compressors; air-conditioning; performance

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