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CFD modeling of a transcritical CO2 ejector integrated in a heat pump

ABG-87349 Sujet de Thèse
10/09/2019 < 25 K€ brut annuel
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LMFTEUS
Sherbrooke - Canada
CFD modeling of a transcritical CO2 ejector integrated in a heat pump
  • Sciences de l’ingénieur
  • Energie
thermodynamique, Stockage d'énergie, système hybride, air comprimé, dispositif expérimental

Description du sujet

Context: Carbon dioxide (CO2) is an appropriate replacement for conventional refrigerants due to its minimal impacts on climate change. However, the transcritical CO2 compression cycle has a low thermodynamic performance due to large expansion losses. Ejector is a favorable device, which enables the use of CO2 and other environmentally friendly refrigerants. It helps to reduce losses by recovering part of the expansion work in a throttling process and improve the cycle’s efficiency. Since CO2 can operate in a transcritical system due to high working pressure, CO2 heat pumps have the advantage of high vapor density and volumetric heating capacity, which causes a small volume of CO2 to achieve the same heating capacity as other refrigerants and results in more compact systems. Transcritical CO2 heat pumps have the potential to be commercialized [1-2]. The ejector applications in heat pump systems have been investigated for many years but most of them use a single-phase ejector. Only few works considered the integration of a two-phase ejector in such systems by means of simple thermodynamic models [3-5]. Ju et al. [5] reported a Coefficient of Performance (COP) 16% higher than a conventional system. Sarkar [4] developed a thermodynamic model to optimize a standard ejector expansion transcritical CO2 heat pump cycle. More research is then required to improve the performances of such systems.

Objectives: The main objective is to develop an experimental set-up of a transcritical CO2 heat pump integrating a two-phase ejector for heating and cooling applications of industrial buildings. The specific objectives are first an improved knowledge of the flow patterns and exergy exchanges within a transcritical CO2 ejector. A particular emphasis should be put on the phase change (flashing, condensation) and on the 3D mixing of the two streams before suggesting any possible improvement. Secondly, the unsteady nature of the heat and mass transfer within an ejector has never been considered numerically or theoretically whereas it is inherent to all ejector-based refrigeration systems due to the variable operating conditions that drive the cycle, and the turbulence unsteadiness at steady conditions. It will be considered for the first time by CFD (Computational Fluid Dynamics). Thirdly, much larger two-phase ejectors will be considered to obtain performances compatible with higher thermal output requirements to focus on industrial buildings or processes requiring heating and cooling, which are two novelties compared to the most complete experiment to date, where Boccardi et al. [6] focused on the heating of residential buildings only. The CO2 side of the heat pump cycle will be designed to operate at a distance far away from the pseudo-critical region, to get higher COP.

Methodology: The two-phase ejector is the key component in such thermodynamic cycle and an optimal design for a particular application is required to obtain good performances. The project will be then divided into three successive steps. A CFD analysis of the transcritical CO2 two-phase ejector. Based on the tabulated method developed by Fang et al. [7] for the real properties of CO2 in any possible states, Large Eddy Simulations (3D unsteady) will be performed using the AVBP solver and will provide reliable information on the exergy transfer within the ejector. The efficiencies in the mixing and shock regions are key parameters for thermodynamic modeling. It will be extended to 3D unsteady calculations to account for varying inlet pressures and/or temperatures occurring during normal operation. The method of exergy tubes developed by Lamberts et al. [8] will be implemented to the LES frame to better understand the exergy transfers and losses within the ejector, and compared to the original RANS results (closures assumptions). The thermodynamic model developed by Taslimi et al. [9] under the Matlab environment will be improved/calibrated using the CFD results and then used to design the two-phase ejector and run system simulations. The optimal design will be implemented in the 1.5 MW CO2 heat pump system  available in the Hydro-Québec lab (LTE). The optimization will be based on COP, exergetic efficiency and cost under given physical constraints and achieved through genetic algorithms. The experiments on transcritical CO2 heat pumps integrating a two-phase ejector are scarce [6, 10-11] and more research is required.

Required skills: Master in mechanical engineering or in a related field. A good knowledge in fluid mechanics and heat transfer is required. Knowledge in numerical methods, Large Eddy Simulation and a first significant experience in Computational Fluid Dynamics using one of the following softwares, ANSYS Fluent/CFX or ideally OpenFOAM or/and AVBP, would be appreciated. French native speakers or permanent residents of Canada will be given priority. Applications must meet diversity and equity objectives.

 

Institution: This is a joint PhD position. Thus the two workplaces are (i) the faculty of mechanical engineering at Université de Sherbrooke (Québec, Canada) with a strong collaboration with Laboratoire des Technologies de l’Énergie (Hydro-Québec, Shawinigan) and Emerson Canada and (ii) Thermodynamics and FLuid division at Université catholique de Louvain (Belgium). Good salary (net funding of 23000$ per year) and working conditions are offered. The suitable starting date for this position is winter 2020.

Prise de fonction :

01/01/2020

Nature du financement

Financement public/privé

Précisions sur le financement

Il s'agit d'une thèse en cotutelle entre l'Université catholique de Louvain et l'Université de Sherbrooke. Le financement est issu de la chaire CRSNG en efficacité énergétique industrielle supportée par Hydro-Québec, Ressources Naturelles Canada et Emerson Canada.

Présentation établissement et labo d'accueil

LMFTEUS

L'étudiant fera partie du groupe de recherche LMFTEUS en efficacité énergétique lors de son séjour à Sherbrooke.

A Louvain la Neuve, il travaillera dans l'équipe Thermodynamics and fluid mechanics de l'Université catholique de Louvain.

Intitulé du doctorat

Génie mécanique

Pays d'obtention du doctorat

Canada

Etablissement délivrant le doctorat

Université de Sherbrooke

Ecole doctorale

Thèse en cotutelle

Oui

Pays d'obtention du doctorat en cotutelle

Belgique

Etablissement délivrant le doctorat en cotutelle

Université de Sherbrooke

Profil du candidat

Master in mechanical engineering or in a related field. A good knowledge in fluid mechanics and heat transfer is required. Knowledge in numerical methods, Large Eddy Simulation and a first significant experience in Computational Fluid Dynamics using one of the following softwares, ANSYS Fluent/CFX or ideally OpenFOAM or/and AVBP, would be appreciated. French native speakers or permanent residents of Canada will be given priority. Applications must meet diversity and equity objectives.

Date limite de candidature

30/09/2019
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