Michal Palacz

CFD simulations of transport phenomena during transcritical flow of real fluid (CO2) within ejector

Purpose – The purpose of this paper is to overview successful approaches to the computational simulation of real fluid (R744 – carbon dioxide (CO2)) flow within an ejector is presented. Important issues such as the ejector geometry and its optimisation, the adapted equations of state and the proposed models of the process, fluid parameters, etc., are examined and critically discussed. Whenever possible, the discussed models are experimentally validated. In the conclusion, some trends in future research are pointed out. Design/methodology/approach – Flow within CO2 ejector is generally transcritical and compressible. Models existing in the literature are shortly described and critically compared. Whenever possible, those models were validated against the experimental data. In a model validation process, the primary and secondary mass flow rates as well as the pressures at the selected points in the mixing section and diffuser were compared, showing a satisfactory agreement between experimental and computat...

IMPURITIES REMOVAL PROCESS FROM THE VACUUM INDUCTION FURNACE CHARGE: A NUMERICAL STUDY

The technology of mental melting in a vacuum induction furnace enables the efficient removal of impurities and provides an opportunity to melt refractory metals, such as titanium. These materials can be applied in cutting edge technologies, such as aviation (turbine blades) and biotechnology (prosthesis and implants). To control metallurgical heat and mass processes within an induction furnace, measurements and a numerical analysis can be conducted. In this paper, numerical approaches are discussed. Simulation requires the development of vacuum induction furnace coupling between fluid dynamics and electromagnetic fields. The proposed numerical domain was modelled as a threedimensional slice with a properly defined periodic boundary condition. To define the analysed electromagnetic problem, a set of Maxwell differential equations was specified. A fluid dynamics sub-model was composed of the mass and momentum conservation equations using the volume of fluid multiphase formulation, two-equation k- turbulence model and species transport to track the inclusion position within the melt. The main purpose of this study was an examination of the impurities removal process via the free surface of the melt within an induction furnace. The coupled computations were performed for five operating conditions, including different power inputs of the inductor. The results indicated a strong influence of the inductor power on the free surface area and therefore on the purification process intensity.

A modified homogeneous relaxation model for CO2 two-phase flow in vapour ejector

In this study, the homogenous relaxation model (HRM) for CO2 flow in a two-phase ejector was modified in order to increase the accuracy of the numerical simulations The two- phase flow model was implemented on the effective computational tool called ejectorPL for fully automated and systematic computations of various ejector shapes and operating conditions. The modification of the HRM was performed by a change of the relaxation time and the constants included in the relaxation time equation based on the experimental result under the operating conditions typical for the supermarket refrigeration system. The modified HRM was compared to the HEM results, which were performed based on the comparison of motive nozzle and suction nozzle mass flow rates.

A comparison of homogeneous equilibrium and relaxation model for CO2 expansion inside the two-phase ejector

In this study, the comparison of the accuracy of the homogeneous equilibrium model (HEM) and homogeneous relaxation model (HRM) is presented. Both models were applied to simulate the CO2 expansion inside the two-phase ejectors. Moreover, the mentioned models were implemented in the robust and efficient computational tool ejectorPL. That tool guarantees the fully automated computational process and the repeatable computations for the various ejector shapes and operating conditions. The simulated motive nozzle mass flow rates were compared to the experimentally measured mass flow rates. That comparison was made for both, HEM and HRM. The results showed the unsatisfying fidelity of the HEM for the operating regimes far from the carbon dioxide critical point. On the other hand, the HRM accuracy for such conditions was slightly higher. The approach presented in this paper, showed the limitation of applicability of both two-phase models for the expansion phenomena inside the ejectors.

Parallel Work of CO2 Ejectors Installed in a Multi-Ejector Module of Refrigeration System

A performance analysis on of fixed ejectors installed in a multi-ejector module in a CO2 refrigeration system is presented in this study. The serial and the parallel work of four fixed-geometry units that compose the multi-ejector pack was carried out. The executed numerical simulations were performed with the use of validated Homogeneous Equilibrium Model (HEM). The computational tool ejectorPL for typical transcritical parameters at the motive nozzle were used in all the tests. A wide range of the operating conditions for supermarket applications in three different European climate zones were taken into consideration. The obtained results present the high and stable performance of all the ejectors in the multi-ejector pack.