Flow visualization and f-PIV of a two-phase cavitating flow through a safety valve at initial subcooling conditions

Créé le : 30/06/2015

Pinho J., Rambaud P., Chabane S., Buchlin JM., Caridi G. - 01/07/2014 - RC - 17th international symposium on applications of laser techniques to fluid mechanic - Lisbonne - juillet 2014…

The goal of this study is to characterize a challenging two-phase cavitating flow through a safety relief valve (SRV), at initial high subcooling conditions. It is known that at these conditions, the mass flux tends to be reduced due to the fluid compressibility and may cause fluttering, or chattering of the valve, and leading to potential hazardous situations. A transparent model of an API 1’’ . G3’’ SRV has been tested mimicking different operating conditions when the valve is kept open with disk lift in the interval from 3.0 to 8.0mm. Measurements of flow rate, temperature and pressure were performed in the associated different cavitation regime. Moreover, as the transparent model made of PMMA allows optical access, precise flow diagnostics such as high speed visualization and fluorescent particle image velocimetry (f-PIV) were performed. Experimental results confirm that cavitation has major influence on the flow characteristics through a SRV in liquid service. 

In a first configuration, high speed flow visualization is applied with a continuous laser, to observe qualitatively the flow pattern and the inception of liquid vaporization. Particle tracking results suggest that vapor bubbles are formed in the core of vortices detached from the shear layers attached to the valve. These rotational structures promote low pressure regions allowing the liquid to vaporize. In the second configuration, a pulsed laser with wavelength of 532nm is used together with two CCD cameras. The seeding of fluorescent particles allows separating optically both the liquid and vapor phases with a high pass and notch filter, respectively. Results of PIV post-processing confirm the existence of a submerged jet just downstream the geometrical smaller flow path cross section. This jet is characterized by two non-symmetric shear layers at its sides. Under cavitation conditions, PIV results confirm that vapor bubbles are formed preferentially inside the jet shear layers. Finally, the well-known phenomenon of mass flow limitation associated to large pressure drop through the valve is reproduced. The interaction between cavitation and flow topology is highlighted and it is believed that an understanding of the mass flow limitation driven by the cavitation is proposed for the first time. It is noticed that vapor bubbles tend to spread and accelerate the jet downstream the valve critical section. Furthermore, the vapor content is responsible for the local blockage of the flow and consequent mass flow limitation. 

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Saïd Chabane
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