Abstract Simulation of unsteady flow of SF6 gas in a simplified high-voltage circuit breaker model describing the nozzle, contacts and their nearest surrounding is presented. SF6 is considered as viscous, compressible, real gas described by Redlich-Kwong model. Heat transfer is taken into account due to the gas compressibility. The heat source, triggered by the electric arc between the contacts, was out of the scope of the current research, thus it was not included in the simulations presented. Turbulence, caused by the gas viscosity, is described using realizable k-ε model. In the simulation model, one of the contact sides – electrodes, is considered as moving at prescribed velocity. The part of the space ‘swept’ by the moving electrode is considered as the gas with imposed artificially increased viscosity in order to imitate the rigid body behaviour. Thus, no moving parts of the computational mesh are used in the model. The conservation equations of mass, momentum and energy, given in integral form, are solved using a finite-volume method on unstructured computational grids.

Paper accepted: 27.05.2019. Preliminary Notes SUMMARY In this paper, a model based on numerical solution of two ordinary differential equations is used to obtain the water level in the surge tank and the static pressure in the headrace tunnel – the properties of essential importance for the functioning of the water supply system during the turbine shut-off. The model allows a fast and reliable simulation of the hydraulic processes in the headrace tunnel and the surge tank. It was validated by comparing the numerical results with the data available from the experiments conducted under real conditions in a surge tank of the HPP Jablanica. This model is used to analyze of influence of different parameters on variations of water level oscillation in the surge tank and the static pressure in the headrace tunnel.

Two design solutions of the heat exchanger containing a material for latent-heat storage are presented. The devices are exposed to controlled energy-charging and -discharging process by virtue of internal water flow. Temperature variations at several monitoring locations are recorded in order to estimate the performance of energy storage and recovery. The experiment confirms that the heat transfer rate in the heat exchanger with fins is significantly larger than in the case of smooth plain tube, which contributes to better storage performance. The unit with plain tube reveals strong temperature gradient in radial direction. In the setup with finned tube, higher temperatures in the storage material as well as shorter charging and discharging times are achieved. Vertical temperature gradient arising during the heating after the phase change indicates development of convective heat transfer in the liquid phase. Melting of the phase-change material, however, turns out to be inhomogeneous yielding remaining solid parts as well as entrapped air bubbles.