Contact

Thomas Schäfer
Experimental Thermal Fluid Dynamics
thomas.schaeferAthzdr.de
Phone: +49 351 260 - 2481
Fax: +49 351 260 - 2383

Prof. Dr. Uwe Hampel
Head
u.hampel@hzdr.de
Phone: +49 351 260 - 2772
Fax: 12772, 2383

Safety related analysis of the behavior of valves, centrifugal pumps and intake geometries under accident conditions (SAVE)

Project

The joint research project SAVE deals with the analysis and prediction of the behavior of safety related components in light water reactors under critical operation conditions. One issue is the potential gas entrainment into emergency cooling pumps. This may happen in cases of suction from shallow water reservoirs, where hollow vortices may form at the intake. For that existing and new models for hollow vortex formation and critical coverage are being analyzed with special consideration of scale effects. For the latter different experimental geometries are being investigated. HZDR contributes to the analysis and the visualization of the gas-liquid distributions inside centrifugal pumps and valves under gas entrainment conditions. For that tomographic measuring are being used.

Background

During a loss-of-coolant accident (LOCA) in a nuclear reactor emergency core cooling systems come into operation. They provide fresh coolant from large reservoirs, e.g. condensation chambers or reactor sump, to the damaged hydraulic circuit by means of centrifugal pumps. When there is only shallow coverage of the pump inlet port by water, a hollow vortex may develop, which leads to gas entrainment into the pump. This causes loss of pump power up to complete pump failure and damage. Both to detect and handle such critical situations but also to find pump designs which are robust against such, it is necessary to understand and model the hydraulic conditions inside the pump.

Gas-liquid distribution inside the impeller
Gas-liquid distribution inside the impeller of a centrifugal pump disclosed by ultrafast X-ray tomography

Results

We investigated the two-phase flow conditions in a centrifugal pump under gas entrainment using angle-synchronized gamma ray tomography and ultrafast X-ray tomography. Thereby we analyzed the total hydraulic performance as well as the internal flow conditions for varying gas entrainment rates, inlet flow patterns and pump orientations. High-resolution gamma-ray computed tomography (HireCT) has been applied in time-averaged rotation-synchronized scanning mode to obtain highly accurate long-term averaged gas holdup distributions inside the operating pump impeller. The structures of gas accumulation inside the impeller chambers and the impact of a balancing hole on the gas phase accumulation inside the corresponding impeller chamber could be disclosed. Ultrafast X-ray computed tomography (ROFEX) has been used to study the transient details of the flow. For technical reasons the pump had to be modified slightly in geometry (pump outlet) and impeller material (polyamide). The high temporal resolution of the tomographic images offer the opportunity to investigate the transient flow development. It was found, that entrained gas is accumulated initially in the inlet area inside the chambers. Subsequently, with increasing gas entrainment the gas accumulations are increasing and extending, especially on the pressure side of the blades. This leads to the formation of larger gas agglomerations, which strongly reduce the effective cross-sectional flow area inside the impeller chambers. This hinders the entry of the liquid into the impeller chambers and hampers the energy transmission from impeller wheel to the liquid by compressible gas. Intensive gas-liquid phase interactions have been found especially in the wake of the gas pockets. Here, detachment of larger gas bubbles takes place due to strong shear forces.




Partners

Further Links

http://www.tuhh.de/save

References

Acknowledgement

The Project was founded by the German Federal Ministry of Education and Research (BMBF) under the funding code 02NUK023A-D.


Contact

Thomas Schäfer
Experimental Thermal Fluid Dynamics
thomas.schaeferAthzdr.de
Phone: +49 351 260 - 2481
Fax: +49 351 260 - 2383

Prof. Dr. Uwe Hampel
Head
u.hampel@hzdr.de
Phone: +49 351 260 - 2772
Fax: 12772, 2383