Assessment of the use of DNS modeling for the geometrical evaluation of variable geometry ejectors
Creators: name: Gonçalves, Karla
Publisher: name: Zenodo
Year: 2022
Subjects: ejector
DNS
turbulence model

description: Air-conditioner (AC) devices account for 20% of overall energy consumption in buildings.<br> Thus, there is a strong potential to effectively improve building energy performance by<br> developing more efficient and sustainable solutions. The mechanical compressor is the<br> main source of energy consumption. Furthermore, this equipment comprises movable<br> parts, which increases the cost of maintenance. Reducing the compressor work is a<br> straightforward way to decrease the energy consumption of the AC and heat-pump<br> systems. The ejector is a simple device that can be applied to assist the mechanical<br> compressor in a transcritical system, or to replace it completely (ejector cooling cycle) and<br> use renewable energy, like solar heat, to drive it. Using a Variable Geometry Ejector (VGE)<br> is the optimum choice for achieving the best performance when employing an ejector in<br> a cooling system operating under variable conditions.<br> To increase ejector performance, one needs to understand the very details of the flow<br> processes inside these devices, which is compressible, turbulent and characterized by a<br> series of shock waves. Experimental observation of the detailed flow structure is very<br> challenging, therefore accurate mathematical models, such as Computational Fluid<br> Dynamics (CFD), have a special interest. This seminar provides a comprehensive study of<br> state-of-art in ejector research with special attention to the different models to simulate<br> the flow inside these devices.<br> Reviewing the open literature revealed that most CFD models for ejector simulations are<br> based on different versions of the semi-empirical Reynolds Averaged Navier-Stokes<br> equations (e.g. Realizable k-ε, RNG k-ε, or SST k-ω) and assuming ideal gas behavior. The<br> results these models show that they lack the accuracy the local flow structure inside the<br> ejector is concerned. Considerable predicting errors are associated to the velocity and<br> pressure profiles, shock wave location and intensity, etc. Even for global ejector<br> performance predictions there are discrepancies between the various RANS methods.<br> Direct Numerical Simulation (DNS) requires no model for the turbulence behavior and<br> therefore can capture the flow physics on a fundamental level. DNS simulations however<br> are constrained by the available computational power. Literature review shows that DNS<br> has not been applied to ejector simulation before. This is because flows with large number<br> of Reynolds require extremely fine space discretion to capture small eddies an alternative<br> option is to use a hybrid approach. In a hybrid method, the flow domain is discretized<br> using a fine mesh in regions where, e.g., DNS is used to and a coarse mesh in parts where<br> RANS is used.<br> In the existing literature, there is a lack of CFD models for ejector simulations that are not<br> based on the RANS approach. Because there is a need for fully understanding ejector<br> operation in refrigeration systems, alternative approaches using DNS and hybrid DNS<br> approaches should be looked at. Therefore, it is proposed in this seminar to carry out a<br> Ph.D. work with the principal objective to develop an alternative CFD method for ejector<br> flow simulation, using a more fundamental approach than RANS, and to use this model<br> to improve the performance of variable geometry ejectors.