Ejector applications in refrigeration and heating: An overview of modelling, operation and recent developments

Part of: Thermally driven heat pumps for heating and cooling. – Ed.: Annett Kühn – Berlin: Universitätsverlag der TU Berlin, 2013 ISBN 978-3-7983-2686-6 (print) ISBN 978-3-7983-2596-8 (online) urn:nbn:de:kobv:83-opus4-39458 [http://nbn-resolving.de/urn:nbn:de:kobv:83-opus4-39458]The utilization of ejectors in heat pump systems as compression components, alone or in combination with other equipment, have gained renewed interest as a thermally driven solution for low temperature heat recovery and upgrading and more efficient energy use. This paper summarizes the main findings and trends, in the area of heat driven ejector based machines using low boiling point working fluids. An overall view of such systems is provided by presenting the ejector principles of physics and the latest developments on ejector design, operation and modeling approaches. Aspects related to the analysis of the complex interacting phenomena taking place in these systems for high performance are highlighted. Conventional and improved ejector heat pump cycles of interest employing ejectors alone or boosted combinations are presented and discussed, and their potential applications are indicated. Finally, sample theoretical and experimental results obtained at CanmetENERGY on ejector operation and design are reported

Structure de l’écoulement et de mélange dans un éjecteur supersonique gaz-gaz installé dans un système frigorifique d’une capacité de 10TR

The complexity of the flow in an ejector requires very careful visualization in order to describe the ejector behavior. This paper is based on a numerical simulation of the flow of R245fa through the gas-gas supersonic ejector installed in a real refrigeration system with a nominal capacity of 10 TR. The k-e RNG turbulence model is chosen for two modes of ejector operation, critical and sub-critical. Analysis shows that back pressure significantly affects both induced refrigerant flow and ejector operating regime. The use of the NIST-REFPROP database and a mesh adapted for each simulation allows to give results very close to the physical reality such that for the critical (on design) regime the relative errors do not exceed 4%, on the other hand, in under critical conditions (off design), they are of the order of 14%.La complexité de l’écoulement dans un éjecteur nécessite une visualisation très poussée afin de décrire le comportement de l’éjecteur. Cet article repose sur une simulation de l’écoulement de R245fa dans l’éjecteur supersonique gaz-gaz installé dans un système frigorifique réel d’une capacité nominale de 10 TR. Le modèle de turbulence k-e RNG est choisi pour deux modes de fonctionnement de l’éjecteur, critique et sous-critique. L’analyse montre que la contre-pression affect d’une façon significative, à la fois, le débit de réfrigérant induit et le régime de fonctionnement de l’éjecteur. L’utilisation de la base de données de NIST-REFPROP et un maillage adapté pour chacune de simulation permet de donner des résultats très proche à la réalité physique tel que pour le régime critique les erreurs relatives ne dépassent pas 4%, par contre, en régime sous critique, elles sont de l’ordre de 14%

Structure de l’écoulement et de mélange dans un éjecteur supersonique gaz-gaz installé dans un système frigorifique d’une capacité de 10TR

The complexity of the flow in an ejector requires very careful visualization in order to describe the ejector behavior. This paper is based on a numerical simulation of the flow of R245fa through the gas-gas supersonic ejector installed in a real refrigeration system with a nominal capacity of 10 TR. The k-e RNG turbulence model is chosen for two modes of ejector operation, critical and sub-critical. Analysis shows that back pressure significantly affects both induced refrigerant flow and ejector operating regime. The use of the NIST-REFPROP database and a mesh adapted for each simulation allows to give results very close to the physical reality such that for the critical (on design) regime the relative errors do not exceed 4%, on the other hand, in under critical conditions (off design), they are of the order of 14%.La complexité de l’écoulement dans un éjecteur nécessite une visualisation très poussée afin de décrire le comportement de l’éjecteur. Cet article repose sur une simulation de l’écoulement de R245fa dans l’éjecteur supersonique gaz-gaz installé dans un système frigorifique réel d’une capacité nominale de 10 TR. Le modèle de turbulence k-e RNG est choisi pour deux modes de fonctionnement de l’éjecteur, critique et sous-critique. L’analyse montre que la contre-pression affect d’une façon significative, à la fois, le débit de réfrigérant induit et le régime de fonctionnement de l’éjecteur. L’utilisation de la base de données de NIST-REFPROP et un maillage adapté pour chacune de simulation permet de donner des résultats très proche à la réalité physique tel que pour le régime critique les erreurs relatives ne dépassent pas 4%, par contre, en régime sous critique, elles sont de l’ordre de 14%

Direct expansion ground source heat pump using carbon dioxide as refrigerant: Test facility and theoretical model presentation

In an attempt to address recent challenges on using natural refrigerants and to develop further knowledge and expertise in the field of direct expansion ground source heat pump (DX-GSHP), an experimental transcritical carbon dioxide (CO2) test bench was built at CanmetENERGY Research Laboratory. A previously developed theoretical model of the system was modified and validated against a set of experimental results and adopted to investigate the system performance in a wide operating range. A parametric analysis was also performed using the theoretical model for understanding the system and at exploring the performance improvement actions for future installations. Validation results showed that the model predicts the experimentation very well within the uncertainty of the measurement. Furthermore, parametric analysis showed that improper control of some parameters such as gas cooler CO2 outlet temperature and discharge compressor pressure can degrade the system performance by up to 25% and the heat pump heating capacity by 7.5%

Carbon dioxide evaporation process in direct expansion geothermal boreholes

Ground Heat Exchangers (GHE) play an important role in the performance of Ground Source Heat Pumps (GSHP). The impact is even more significant in direct expansion GSHP (DX-GSHP) systems as the refrigerant used in the heat pump also acts as the heat transfer fluid for the GHE. In this study, several experiments were carried out to investigate the performance of GHEs in a carbon dioxide (CO2) DX-GSHP. The evaporation of CO2 in the GHE was studied under various mass flow rates and number of active boreholes. For this purpose, a transcritical CO2 DX-GSHP test facility was built and fully equipped at CanmetENERGY-Varennes research laboratory. It was found that a partial two-phase flow regime along the GHE decreases the performance compared to the full two-phase flow and it has to be avoided for more efficient DX-GSHP systems

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