18 research outputs found
Effect Of Pressure Convection On The Energy Separation In Air Vortex Tube: Dimensional Analysis And Numerical Investigation
Coupled Thermodynamic And CFD Approaches Applied To A Supersonic Air Ejector
This paper presents a systematic comparison of ejector performance predictions by a thermodynamic and a CFD model for different operating conditions. The thermodynamic model developed by Galanis and Sorin (2015) assumes the primary flow is always choked, and irreversibilities due to viscous dissipation are taken into account through polytropic efficiencies. The CFD model developed by Croquer et al. (2015) using the software ANSYS Fluent v15.0 has already been validated for supersonic ejectors working with R134a. A standard high Reynolds number k- Ăâ° SST turbulence model coupled with the perfect gas law is used to model the turbulent air flows. The dimensions of the ejector were first determined by the thermodynamic model and then used in the CFD model. The thermodynamic model predicts higher entrainment ratios for double choking operation and somewhat different values of the critical and limiting pressure ratios. The CFD model validates the similarity solutions characteristic of ejectors using perfect gases. The present results show in particular that identical inlet pressure and temperature ratios induce the same entrainment ratio as well as the same critical and limiting pressure ratios. Both models confirm also that similar diameter ratios between the primary nozzle throat and the constant area section lead to the same values of the entrainment ratio. Thus, for double-choking operations, the entrainment ratio depends on the inlet pressure and temperature ratios rather than on the individual values of these four properties as is the case for ejectors with real fluids. It also shows that the position of the shock varies linearly with the compression ratio in qualitative agreement with the assumption used in the thermodynamic model. Finally, the main assumptions made to build the thermodynamic model have been checked and discussed a posteriori using the CFD results
Analyse dynamique (CFD) et thermodynamique combinée dans un éjecteur supersonique en présence de gouttelettes
Abstract : This research project has as main objective to study in detail the internal flow features of single-phase supersonic ejectors for refrigeration applications, and the potential effects of injecting droplets on the performance of the device. To this end, a numerical approach is proposed which has been separated into two parts:
First, a RANS modelling strategy for supersonic ejectors has been outlined combining the NIST real gas equations database [NIST, 2010] and the k â Ï SST turbulence model in its low-Reynolds number formulation. The proposed approach agrees within 5% (resp. 2%) to the experimental entrainment ratio (resp. compression ratio) data of GarcĂa del Valle et al. [2014], properly captures the main internal flow features and has a reasonable computational cost. This RANS model has been applied in the analysis of a supersonic R134a ejector for refrigeration purposes, showing in particular that the secondary flow is entrained by momentum transfer through the mixing shear layer, that the distance between the primary nozzle exit and the shock-waves in the constant area section varies between 9 and 16 times the primary nozzle exit diameter and that the important axial character of the flow limits mixing of both inlet flows until after the shock train. Furthermore, an exergy analysis through the device shows that the mixing and the oblique shock waves are responsible for between 50% and 70% of the generated losses, the latter might be attenuated through droplet injection in the constant area section. Moreover, it has been shown that drop-in replacement of the working fluid with HFOs R1234yf and R1234ze(E) leads to mild changes in the ejector performance but reduces the HDRC system COP (resp. cooling capacity) in average by 7.1% (resp. 23.3%). Lastly, a comparison of the model predictions with the thermodynamic model of Galanis and Sorin [2016] for an air ejector, shows that as the working fluid approaches the ideal gas behaviour, the flow can be adimensionalized in terms of the secondary inlet temperature and pressure, the motive nozzle throat and the entrainment and compression ratios.
In the second part, the influence of droplets has been studied from a local perspective by extending the RANS model to include a discrete phase, which affects the main flow through exchanges of momentum and thermal energy, and from a global perspective by building a thermodynamic model, which predicts the entrainment and limiting compression ratio given a fixed geometry and operating conditions. Both approaches present very good agreement in terms of p, T and M a internal profiles. Results for a supersonic ejector with R134a as baseline working fluid and droplets injected at the constant area section show that the flow structure has perceptible changes only at the highest injection fraction considered 10%, which induces boundary layer detachment, reduces the shock intensity by 8% and diminishes the superheat at the ejector outlet by 15 âŠC. Nonetheless, ejector performance metrics are severely affected as the limiting compression ratio, Elbel efficiency and exergy performance reduce respectively by 5%, 11% and 15%, due mainly to the additional entropy generated through droplet injection and mixing with the main flow.Ce projet de recherche a pour objectif principal dâĂ©tudier en dĂ©tail les caractĂ©ristiques de lâĂ©coulement interne dans des Ă©jecteurs supersoniques monophasiques pour des applications en rĂ©frigĂ©ration, et les effets potentiels de lâinjection de gouttelettes sur les performances de lâappareil. A cette fin, une approche numĂ©rique est proposĂ©e et a Ă©tĂ© sĂ©parĂ©e en deux parties.
Tout dâabord, une stratĂ©gie de modĂ©lisation RANS pour les Ă©jecteurs supersoniques a Ă©tĂ© dĂ©crite en combinant la base de donnĂ©es pour les gaz rĂ©els NIST [NIST, 2010] et le modĂšle de turbulence k â Ï SST dans sa formulation Ă bas nombre de Reynolds. Lâapproche proposĂ©e prĂ©dit avec un accord dâenviron 5% (resp. 2%) le rapport dâentraĂźnement (resp. rapport de compression) avec les donnĂ©es expĂ©rimentales de GarcĂa del Valle et al. [2014]. Il capte Ă©galement correctement les principales caractĂ©ristiques de lâĂ©coulement interne et a un coĂ»t de calcul raisonnable. Ce modĂšle RANS a Ă©tĂ© appliquĂ© Ă lâanalyse dâun Ă©jecteur supersonique au R134a utilisĂ© Ă des fins de rĂ©frigĂ©ration, montrant en particulier que le flux secondaire est entraĂźnĂ© par un transfert dâimpulsion Ă travers la couche de cisaillement, que la position de dĂ©part des ondes de choc dans la section constante se situe dans une plage de 9 Ă 16 fois le diamĂštre de sortie de la buse primaire et que lâimportant caractĂšre axial du flux limite le mĂ©lange des deux Ă©coulements dâentrĂ©e au-delĂ du train dâondes de choc. De plus, une analyse exergĂ©tique Ă travers le dispositif montre que le mĂ©lange et les ondes de choc obliques sont responsables de 50% et 70% des pertes gĂ©nĂ©rĂ©es, ces derniĂšres pouvant ĂȘtre attĂ©nuĂ©es par injection de gouttelettes dans la section Ă zone constante. De plus, il a Ă©tĂ© dĂ©montrĂ© que le remplacement direct du fluide de travail par les HFO R1234yf et R1234ze(E) entraĂźne de lĂ©gers changements dans la performance de lâĂ©jecteur mais rĂ©duit en moyenne le COP du systĂšme HDRC (resp. la capacitĂ© de refroidissement) de 7.1% (resp. 23.3%). Enfin, une comparaison des prĂ©dictions du modĂšle avec le modĂšle thermodynamique de Galanis and Sorin [2016] pour un Ă©jecteur Ă air montre que lorsque le fluide de travail se rapproche du comportement de gaz idĂ©al, lâĂ©coulement peut ĂȘtre normalisĂ© en fonction de la tempĂ©rature et de la pression Ă lâentrĂ©e secondaire, la gorge de la tuyĂšre principale et les rapports dâentraĂźnement et de compression.
Dans la seconde partie, lâinfluence des gouttelettes a Ă©tĂ© Ă©tudiĂ©e dâun point de vue local en Ă©tendant le modĂšle RANS Ă une phase discrĂšte qui affecte le flux principal par des Ă©changes de quantitĂ© de mouvement et dâĂ©nergie thermique, et dâun point de vue global en construisant un modĂšle thermodynamique qui prĂ©dit lâentraĂźnement et le rapport de compression limitant Ă©tant donnĂ© une gĂ©omĂ©trie fixe et les conditions de fonctionnement. Les deux approches prĂ©sentent un trĂšs bon accord en termes de profils internes de p, T et Ma. Les rĂ©sultats pour un Ă©jecteur supersonique au R134a comme fluide de base, avec des gouttelettes injectĂ©es Ă mi-chemin dans la section de la zone constante, montrent que la structure dâĂ©coulement dans cette rĂ©gion prĂ©sente des changements perceptibles seulement Ă la fraction dâinjection la plus Ă©levĂ©e, 10%, en diminuant lâintensitĂ© du choc de 8% et la surchauffe Ă la sortie de lâĂ©jecteur de 15 âŠC. NĂ©anmoins, la performance de lâĂ©jecteur est sĂ©vĂšrement affectĂ©e vu que le rapport de compression, lâefficacitĂ© dâElbel et le performance exergĂ©tique sont rĂ©duites respectivement de 5%, 11% et 15%, principalement en raison de lâentropie supplĂ©mentaire gĂ©nĂ©rĂ©e par lâinjection de gouttelettes et le mĂ©lange avec le flux principal
Exergetic Optimisation of Vortex Tubes using a Thermodynamic Model
This article identifies sources of exergy losses in a vortex tube working with air by using a recently developed thermodynamic model and a reference experiment from the literature. Exergetic efficiency considering transiting exergy is used as the efficiency metrics in this work. When both the cold and hot outlets are useful, the exergetic efficiency reaches its maximum value for a cold mass fraction equal to 0.7. Interestingly, up to 45% of the inlet exergy is lost downstream of the vortex tube under this condition because of pressure losses in the cold tube and through measuring instruments. These losses do not contribute to the energy separation mechanism. Inside the vortex tube, the exergy irreversibly is mainly caused by the dissipation of kinetic exergy. The thermodynamic model is also used to identify the working conditions, which maximize the vortex tube efficiency. The efficiency is always at its maximum value when the inlet Mach number is equal to one. The optimum value of the cold outlet diameter, the mass fraction and the cold outlet axial Mach number changes depending on whether thermal exergy from both outlets can be used or not. Increasing the cold outlet pressure increases the exergetic efficiency as well as changing the optimal condition for all variables except the inlet Mach number. At the end, the optimal vortex tube is twice as efficient as the reference vortex tube. Finally, the model is employed to identify the best vortex tubesâ arrangement to maximize the exergetic efficiency for an open cycle with a fixed inlet pressure of six bar. This analysis demonstrates that the best arrangement is a cascade of vortex tubes, where a vortex tube unit with the maximum efficiency is placed first. Two other vortex tubes are two other vortex tubes are placed to recover waste pressure on the cold and hot streams from the first unit
1D modelling of the liquid-gas jet pump performance
Abstract: The performance of a liquid-gas jet pump, which uses a high-velocity liquid flow to compress and entrain a gas flow, can be divided into two modes, on-design and off-design. The present paper investigates the mentioned modes of performance using a 1D model based on the conservation equations of mass, momentum and energy. Comparisons in terms of compression ratio and efficiency between the present model and experimental data show that the 1D model is capable of predicting the behavior of the liquid-gas jet pump for both modes. The effects of the primary flow velocity head, and areas of the mixing throat and diffuser on the performance of the jet pump are also investigated through a sensitivity study.Communication prĂ©sentĂ©e lors du congrĂšs international tenu conjointement par Canadian Society for Mechanical Engineering (CSME) et Computational Fluid Dynamics Society of Canada (CFD Canada), Ă lâUniversitĂ© de Sherbrooke (QuĂ©bec), du 28 au 31 mai 2023
Numerical Simulation of the Heat Transfer in a Refrigerated Trailer Equipped with Eutectic Plates for Frozen Food Delivery
The present work reports the Computational Fluid Dynamics simulation and analysis of the heat transfer inside a refrigerated truck trailer equipped with three eutectic plates and fans. The numerical model solves the conjugated heat transfer inside the trailer in 2D using the ïżœïżœ âïżœïżœ Shear Stress Transport (SST) turbulence model. It has been already favorably validated against the numerical and experimental data of Lafaye de Micheaux el al. (2015) by Croquer et al. (2019). These simulations are used to improve the configuration of the refrigeration system with the eutectic plates as well as to investigate the feasibility of the eutectic plates for the transport of frozen food products under different operating loads and transport temperature requirements. Three eutectic plates having an optimal inter-plate distance of 6 cm to maximize the air flow between the plates (Croquer et al., 2019) are either placed in series on the roof of the trailer or vertically at its back. For both configurations, fans are blowing the air from the eutectic plates to the inside of the trailer and modeled by adding a source term into the momentum equations. During the door opening period, the configuration with the plates placed on the roof of the trailer without the cargo has noticeably lower area-averaged temperature inside the trailer than the configuration with the plates placed on the back of the trailer due to the presence of the circulation zones and the cold plates located near the doorway. However, introduction of the cargo into the simulations eliminates the formation of the circulation zones that prevents the infiltration of the atmospheric air. Also, the configuration with the plates placed on the roof of the trailer allows the atmospheric air to infiltrate earlier, therefore resulting in an overall higher temperature observed in the cargo
Conséquences du réchauffement climatique sur les structures de protection cÎtiÚre
Parmi les principales consĂ©quences de l'Ă©lĂ©vation du niveau de la mer associĂ©e Ă la variabilitĂ© du climat, se trouvent les impacts sur les structures cĂŽtiĂšres. Jusqu'Ă prĂ©sent, l'attention s'est principalement concentrĂ©e sur les zones cĂŽtiĂšres de faible altitude, car ce sont celles qui subiront de graves inondations. Pourtant, peu d'Ă©tudes ont mis lâemphase sur la performance des structures qui protĂšge ces zones cĂŽtiĂšres. Dans ce travail, nous avons Ă©tudiĂ© l'impact de l'Ă©lĂ©vation du niveau de la mer sur la rĂ©ponse des structures cĂŽtiĂšres et, en particulier sur les principaux modes de dĂ©faillance des brise-lames. Pour cela, nous avons rĂ©alisĂ© une sĂ©rie de tests expĂ©rimentaux en laboratoire et des tests numĂ©riques simulant diffĂ©rents scĂ©narios d'Ă©lĂ©vation du niveau de la mer. Nous pouvons conclure que le dĂ©bit de dĂ©bordement des vagues et les forces appliquĂ©es sur le mur augmentent avec l'Ă©lĂ©vation du niveau de la mer. Cependant, les tests expĂ©rimentaux indiquent que la prĂ©diction d'une relation entre ces deux valeurs n'est pas simple et des programmes expĂ©rimentaux minutieux doivent ĂȘtre rĂ©alisĂ©s Ă l'avenir
Energy and exergy analysis of possible alternatives to R134a in a vapour compression refrigeration cycle of a water cooler unit
Abstract: In this paper, the energy and exergy performance of a vapour-compression refrigeration cycle of a water dispenser unit has been analyzed theoretically using different refrigerants as possible alternative substitutes to R134a. The selected low ?Global Warming Potential (GWP) ? refrigerants are: HydroFluoroOlefins (HFO) R1234yf and R1234ze(E), HydroCarbons (HC) R290 and R600a, and HydroFluoroCarbon (HFC) R152a. The process was evaluated based on the evaporator and condenser temperatures, which range between -10 and 5?C and between 30 and 45?C, respectively. The theoretical model based on the first and second laws of thermodynamics has been developed using the Matlab environment. The performances of the vapor compression cycle are discussed in terms of coefficient of performance, exergy destruction and exergy efficiency. The results show that the maximum COP achieved is 5.12 and 5.10 for R152a and R600a respectively, the highest total exergy destruction is about 82.82 W for R290, the highest exergy efficiency is about 55.12% for R600a.Communication prĂ©sentĂ©e lors du congrĂšs international tenu conjointement par Canadian Society for Mechanical Engineering (CSME) et Computational Fluid Dynamics Society of Canada (CFD Canada), Ă lâUniversitĂ© de Sherbrooke (QuĂ©bec), du 28 au 31 mai 2023
Analyse dynamique (CFD) et thermodynamique combinée dans un éjecteur supersonique en présence de gouttelettes
Abstract : This research project has as main objective to study in detail the internal flow features of single-phase supersonic ejectors for refrigeration applications, and the potential effects of injecting droplets on the performance of the device. To this end, a numerical approach is proposed which has been separated into two parts:
First, a RANS modelling strategy for supersonic ejectors has been outlined combining the NIST real gas equations database [NIST, 2010] and the k â Ï SST turbulence model in its low-Reynolds number formulation. The proposed approach agrees within 5% (resp. 2%) to the experimental entrainment ratio (resp. compression ratio) data of GarcĂa del Valle et al. [2014], properly captures the main internal flow features and has a reasonable computational cost. This RANS model has been applied in the analysis of a supersonic R134a ejector for refrigeration purposes, showing in particular that the secondary flow is entrained by momentum transfer through the mixing shear layer, that the distance between the primary nozzle exit and the shock-waves in the constant area section varies between 9 and 16 times the primary nozzle exit diameter and that the important axial character of the flow limits mixing of both inlet flows until after the shock train. Furthermore, an exergy analysis through the device shows that the mixing and the oblique shock waves are responsible for between 50% and 70% of the generated losses, the latter might be attenuated through droplet injection in the constant area section. Moreover, it has been shown that drop-in replacement of the working fluid with HFOs R1234yf and R1234ze(E) leads to mild changes in the ejector performance but reduces the HDRC system COP (resp. cooling capacity) in average by 7.1% (resp. 23.3%). Lastly, a comparison of the model predictions with the thermodynamic model of Galanis and Sorin [2016] for an air ejector, shows that as the working fluid approaches the ideal gas behaviour, the flow can be adimensionalized in terms of the secondary inlet temperature and pressure, the motive nozzle throat and the entrainment and compression ratios.
In the second part, the influence of droplets has been studied from a local perspective by extending the RANS model to include a discrete phase, which affects the main flow through exchanges of momentum and thermal energy, and from a global perspective by building a thermodynamic model, which predicts the entrainment and limiting compression ratio given a fixed geometry and operating conditions. Both approaches present very good agreement in terms of p, T and M a internal profiles. Results for a supersonic ejector with R134a as baseline working fluid and droplets injected at the constant area section show that the flow structure has perceptible changes only at the highest injection fraction considered 10%, which induces boundary layer detachment, reduces the shock intensity by 8% and diminishes the superheat at the ejector outlet by 15 âŠC. Nonetheless, ejector performance metrics are severely affected as the limiting compression ratio, Elbel efficiency and exergy performance reduce respectively by 5%, 11% and 15%, due mainly to the additional entropy generated through droplet injection and mixing with the main flow.Ce projet de recherche a pour objectif principal dâĂ©tudier en dĂ©tail les caractĂ©ristiques de lâĂ©coulement interne dans des Ă©jecteurs supersoniques monophasiques pour des applications en rĂ©frigĂ©ration, et les effets potentiels de lâinjection de gouttelettes sur les performances de lâappareil. A cette fin, une approche numĂ©rique est proposĂ©e et a Ă©tĂ© sĂ©parĂ©e en deux parties.
Tout dâabord, une stratĂ©gie de modĂ©lisation RANS pour les Ă©jecteurs supersoniques a Ă©tĂ© dĂ©crite en combinant la base de donnĂ©es pour les gaz rĂ©els NIST [NIST, 2010] et le modĂšle de turbulence k â Ï SST dans sa formulation Ă bas nombre de Reynolds. Lâapproche proposĂ©e prĂ©dit avec un accord dâenviron 5% (resp. 2%) le rapport dâentraĂźnement (resp. rapport de compression) avec les donnĂ©es expĂ©rimentales de GarcĂa del Valle et al. [2014]. Il capte Ă©galement correctement les principales caractĂ©ristiques de lâĂ©coulement interne et a un coĂ»t de calcul raisonnable. Ce modĂšle RANS a Ă©tĂ© appliquĂ© Ă lâanalyse dâun Ă©jecteur supersonique au R134a utilisĂ© Ă des fins de rĂ©frigĂ©ration, montrant en particulier que le flux secondaire est entraĂźnĂ© par un transfert dâimpulsion Ă travers la couche de cisaillement, que la position de dĂ©part des ondes de choc dans la section constante se situe dans une plage de 9 Ă 16 fois le diamĂštre de sortie de la buse primaire et que lâimportant caractĂšre axial du flux limite le mĂ©lange des deux Ă©coulements dâentrĂ©e au-delĂ du train dâondes de choc. De plus, une analyse exergĂ©tique Ă travers le dispositif montre que le mĂ©lange et les ondes de choc obliques sont responsables de 50% et 70% des pertes gĂ©nĂ©rĂ©es, ces derniĂšres pouvant ĂȘtre attĂ©nuĂ©es par injection de gouttelettes dans la section Ă zone constante. De plus, il a Ă©tĂ© dĂ©montrĂ© que le remplacement direct du fluide de travail par les HFO R1234yf et R1234ze(E) entraĂźne de lĂ©gers changements dans la performance de lâĂ©jecteur mais rĂ©duit en moyenne le COP du systĂšme HDRC (resp. la capacitĂ© de refroidissement) de 7.1% (resp. 23.3%). Enfin, une comparaison des prĂ©dictions du modĂšle avec le modĂšle thermodynamique de Galanis and Sorin [2016] pour un Ă©jecteur Ă air montre que lorsque le fluide de travail se rapproche du comportement de gaz idĂ©al, lâĂ©coulement peut ĂȘtre normalisĂ© en fonction de la tempĂ©rature et de la pression Ă lâentrĂ©e secondaire, la gorge de la tuyĂšre principale et les rapports dâentraĂźnement et de compression.
Dans la seconde partie, lâinfluence des gouttelettes a Ă©tĂ© Ă©tudiĂ©e dâun point de vue local en Ă©tendant le modĂšle RANS Ă une phase discrĂšte qui affecte le flux principal par des Ă©changes de quantitĂ© de mouvement et dâĂ©nergie thermique, et dâun point de vue global en construisant un modĂšle thermodynamique qui prĂ©dit lâentraĂźnement et le rapport de compression limitant Ă©tant donnĂ© une gĂ©omĂ©trie fixe et les conditions de fonctionnement. Les deux approches prĂ©sentent un trĂšs bon accord en termes de profils internes de p, T et Ma. Les rĂ©sultats pour un Ă©jecteur supersonique au R134a comme fluide de base, avec des gouttelettes injectĂ©es Ă mi-chemin dans la section de la zone constante, montrent que la structure dâĂ©coulement dans cette rĂ©gion prĂ©sente des changements perceptibles seulement Ă la fraction dâinjection la plus Ă©levĂ©e, 10%, en diminuant lâintensitĂ© du choc de 8% et la surchauffe Ă la sortie de lâĂ©jecteur de 15 âŠC. NĂ©anmoins, la performance de lâĂ©jecteur est sĂ©vĂšrement affectĂ©e vu que le rapport de compression, lâefficacitĂ© dâElbel et le performance exergĂ©tique sont rĂ©duites respectivement de 5%, 11% et 15%, principalement en raison de lâentropie supplĂ©mentaire gĂ©nĂ©rĂ©e par lâinjection de gouttelettes et le mĂ©lange avec le flux principal
Thermodynamic Modelling of Supersonic Gas Ejector with Droplets
This study presents a thermodynamic model for determining the entrainment ratio and double choke limiting pressure of supersonic ejectors within the context of heat driven refrigeration cycles, with and without droplet injection, at the constant area section of the device. Input data include the inlet operating conditions and key geometry parameters (primary throat, mixing section and diffuser outlet diameter), whereas output information includes the ejector entrainment ratio, maximum double choke compression ratio, ejector efficiency, exergy efficiency and exergy destruction index. In single-phase operation, the ejector entrainment ratio and double choke limiting pressure are determined with a mean accuracy of 18 % and 2.5 % , respectively. In two-phase operation, the choked mass flow rate across convergent-divergent nozzles is estimated with a deviation of 10 % . An analysis on the effect of droplet injection confirms the hypothesis that droplet injection reduces by 8 % the pressure and Mach number jumps associated with shock waves occuring at the end of the constant area section. Nonetheless, other factors such as the mixing of the droplets with the main flow are introduced, resulting in an overall reduction by 11 % of the ejector efficiency and by 15 % of the exergy efficiency