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Experimental study and modeling of single- and two-phase flow in singular geometries and safety relief valves

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VKI PHDT 2012-06, Vasilios Kourakos, Experimental study and modeling of
single- and two-phase flow in singular geometries and safety relief valves, ISBN 978-2-87516-035-5, 353 pgs

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Experimental study and modeling of single- and two-phase flow in singular geometries and safety relief valves
By Vasilios Kourakos

PhD Thesis from the von Karman Institute/Université Libre de Bruxelles, October 2011, 978-2-87516-035-5, 353 pgs


This research project was carried out at the von Karman Institute for Fluid Dynamics (VKI), in Belgium, in collaboration and with the funding of Centre Technique des Industries Mécaniques (CETIM) in France.

The flow of a mixture of two fluids in pipes can be frequently encountered in  nuclear, chemical or mechanical engineering, where gas-liquid reactors, boilers, condensers, evaporators  and combustion systems can be used.  The presence of section  changes or more generally geometrical singularities in pipes may affect significantly the behavior of two-phase  flow and subsequently the resulting pressure drop and mass flow rate. Therefore, it is an important subject of investigation in particular when the application concerns industrial safety valves.

This thesis is intended to provide a thorough research on two-phase  (air- water) flow phenomena under various circumstances. The project is split in the following steps. At first, experiments are carried out in simple geometries such as smooth and sudden divergence and convergence singularities. Two experimental facilities are built;  one in smaller scale in von Karman Institute  and one in larger scale in CETIM.  During the first part of the study,  relatively simple geometrical discontinuities are investigated.  The characterization and modeling of contraction and expansion  nozzles (sudden and smooth change of section) is carried out.  The pressure evolution is measured and pressure drop correlations are deduced. Flow visualization is also performed with a high-speed camera; the different flow patterns are identified and flow regime maps are established for a specific configuration. A dual optical probe is used to determine the void fraction, bubble size and velocity upstream and downstream the singularities.

In the second part of the project, a more complex device, i.e. a Safety Relief Valve (SRV), mainly used in nuclear and chemistry industry, is thoroughly studied.  A transparent  model of a specific type of safety valve is built  and investigated in terms of pressure evolution.  Additionally, flow rate measurements for several volumetric qualities and valve openings are carried out for air, water and two-phase mixtures. Full optical access allowed identification of the structure of the flow. The results are compared with  measurements performed at the original industrial  valve.  Flowforce analysis is performed revealing that compressible and incompressible flow- forces in SRV are  inversed above a certain value of valve  lift.   This value varies with  critical  pressure  ratio, therefore is directly linked to the posi- tion at which chocked flow occurs during air valve operation. In two-phase flow, for volumetric quality of air ?=20%, pure compressible flow behavior, in terms of flowforce, is remarked at full lift.  Numerical simulations with commercial CFD code are carried out for air and water in axisymmetric 2D model of the valve in order to verify experimental findings.

The subject of modeling the discharge through a throttling device in two- phase flow is an important industrial problem. The proper design and sizing of this apparatus is a crucial issue which would prevent its wrong function or accidental operation failure that could cause a hazardous situation.  So far reliability of existing models predicting the pressure drop and flow discharge in two-phase flow through the valve for various flow conditions is question- able. Nowadays, a common practice is widely adopted (standard ISO 4126-10 (2010), API RP 520 (2000)); the Homogeneous Equilibrium Method with the so-called ?-method, although it still needs further validation. Addition- ally,  based on  ?-methodology,  Homogeneous Non-Equilibrium  model has been proposed by Diener and Schmidt (2004) (HNE-DS), introducing a boiling delay coefficient. The accuracy of the aforementioned models is checked against experimental data both for transparent model and industrial SRV. The HNE-DS methodology is proved to be the most precise among the others. Finally,  after application of HNE-DS method for air-water flow with cavitation, it is concluded that the behavior of flashing liquid is simulated in such case. Hence, for the specific tested conditions, this type of flow can be modeled with  modified method of Diener and Schmidt (CF-HNE-DS) although further validation of this observation is required.

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