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CFD-Experiments Integration in the Evaluation of …

CFD-Experiments Integration in the Evaluation of Six turbulence Models for Supersonic Ejectors Modeling Y. Bartosiewicz, Zine Aidoun CETC-Varennes, Natural Ressources Canada, Box 4800, 1615, Boulevard Lionel-Boulet, Varennes (Qc.) J3X 1S6, Canada, Tel: +1 (450) 652 0352 P. Desevaux CREST-UMR 6000, IGE-Parc Technologique, 90000, Belfort, France, Yves Mercadier THERMAUS, Universit de Sherbrooke, 2500 Boul. Universit , Sherbrooke (Qc.), J1K2R1 Canada, Abstract. Supersonic ejectors are widely used in a range of applications such as aerospace, propulsion, and This work evaluates the performance of six well-known turbulence models for the study of supersonic ejector performance and operation.

CFD-Experiments Integration in the Evaluation of Six Turbulence Models for Supersonic Ejectors Modeling Y. Bartosiewicz, Zine Aidoun CETC-Varennes, Natural Ressources Canada, P.O. Box 4800,

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1 CFD-Experiments Integration in the Evaluation of Six turbulence Models for Supersonic Ejectors Modeling Y. Bartosiewicz, Zine Aidoun CETC-Varennes, Natural Ressources Canada, Box 4800, 1615, Boulevard Lionel-Boulet, Varennes (Qc.) J3X 1S6, Canada, Tel: +1 (450) 652 0352 P. Desevaux CREST-UMR 6000, IGE-Parc Technologique, 90000, Belfort, France, Yves Mercadier THERMAUS, Universit de Sherbrooke, 2500 Boul. Universit , Sherbrooke (Qc.), J1K2R1 Canada, Abstract. Supersonic ejectors are widely used in a range of applications such as aerospace, propulsion, and This work evaluates the performance of six well-known turbulence models for the study of supersonic ejector performance and operation.

2 The primary interest of this study is to set up a reliable hydrodynamics model of a supersonic ejector , which may be extended to refrigeration applications. The validation deals particularly with ejectors of zero induced flow. It concentrates on the prediction of shock location, shock strength and the average pressure recovery. In this respect, axial pressure measurements with a capillary probe and performed previously [18,19], have been used in this work for validation and comparison purposes with numerical simulation.

3 A crucial point is that the probe has been included in the numerical model . In these conditions, the RNG and k-omega-sst models have been found to perform satisfactorily for these parameters. For ejectors with an induced flow, preliminary tests have also been performed. Laser tomography pictures were used to evaluate the non-mixing length. This parameter has been numerically evaluated by including an additional transport equation for a passive scalar, which acted as an ideal colorant in the flow. The results have shown significant departures from measurements for secondary pressure with the decrease of the primary pressure.

4 In this condition, the k-omega-sst model has been found to account best for the mixing. Keywords: Shock waves, turbulence modeling, pressure measurements, supersonic ejector , refrigeration. 1. Introduction Supersonic ejectors are simple mechanical components (Fig. 1), which generally allow to perform the mixing and/or the recompression of two fluid streams. The fluid with the highest total energy is the motive or primary stream (stream 1 on fig. 1), while the other, with the lowest total energy (stream 2) is the secondary or the induced stream.

5 Operation of such systems is also quite simple: the motive stream (high pressure and temperature) flows through a convergent divergent nozzle to reach supersonic velocity. By an entrainment-induced effect, the secondary stream is drawn into the flow and accelerated. Mixing, and recompression of the resulting stream then occurs in a mixing chamber, where complex interactions take place between the mixing layer and shocks. In other words, there is a mechanical energy transfer from the highest to the lowest energy level, with a mixing pressure lying between the motive or driving pressure and the induction pressure.

6 Ejectors for compressible fluids are not new and have been known for a long time. Indeed, supersonic ejectors are technological components that have found many applications in engineering. In the aerospace area, they can be used for altitude testing of a propulsion system by reducing the pressure of a test chamber [1]. The pumping effect is also used to mix the exhaust gases with fresh air in order to reduce the thermal signature [2].

7 A most researched area is the application of ejector for thrust augmentation on aircraft propulsion systems [3,4]. Nevertheless, our primary interest in this paper is the use of supersonic ejectors for performing thermal compression in refrigeration cycles. In view of the numerous publications available on this subject, it is perhaps one of the most important application areas for ejectors. A good overview of the different applications in this field may be found in the review article of Sun and Eames [5].

8 In this case, ejectors may either totally replace the mechanical compressors or simply be introduced as a means of cycle optimization [6]. In this particular area, they have become the focus of renewed interest for many scientists, in an attempt to develop energy efficient and environment-friendly techniques, in response to the current practices responsible for environmental damages such as ozone depletion or global warming. Many theoretical and experimental studies have been carried out in order to understand not only the fundamental mechanisms in terms of fluid dynamics and heat transfer, but also ejector operational behaviour.

9 Nevertheless, most of these studies still rely on semi-empirical or one-dimensional models. For theoretical works, Keenan et al. [7,8] set a first step in the one-dimensional analysis by their model of a constant area mixing flow for air and the still used concept of constant pressure mixing. They were followed by Munday and Bagster [9], who introduced the fictive throat, which helped to explain the characteristic ejector maximum capacity limitations. Later, this kind of models was applied to refrigerants and the possibility of the fictive throat being placed in the constant area zone, as assumed by Huang et al.

10 [10]. Very recently, Ouzzane and Aidoun [11] set a step forward by developing a one-dimensional model allowing to track flow properties along the ejector . In their study, fluid properties were evaluated by using NIST [12] subroutines for equations of state of refrigerants. Despite their usefulness and the remarkable progress they provided for the general understanding of ejectors, this kind of studies are still unable to correctly reproduce the flow physics locally along the ejector .


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