Conference on Modelling Fluid Flow (CMFF’0 9) The 14th International Conference on Fluid Flow Technologies Budapest, Hungary, September 9-12, 2009 N UMERICAL S TUDY O F C AVITATING F LOW I NSIDE A F LUSH V ALVE Annie-Claude BAYEUL-LAIN Ẻ 1 , Sophie SIMONET 2 , Guy CAIGNAERT 3 1 Arts et Metiers PARISTECH, LML, UMR CNRS 8107, 8, boulevard Louis XIV 59046 LILLE Cedex Tel.: +33 20 62 39 04, Fax: +33 20 53 55 93, E-mail: annie-claude.bayeul@ensam.eu 2 Arts et Metiers PARISTECH, LML, UMR CNRS 8107, E-mail: sophie.simonet@ensam.eu 3 Arts et Metiers PARISTECH, LML, UMR CNRS 8107, E-mail: guy.caignaert@ensam.eu ABSTRACT Q ref [ kg/s ] reference mass flow rate R [ m ] micro bubble radius In water supply installations, noise pollution [ m 3 ] V l volume of liquid in a control often occurs. As a basic component of a system, a volume flush valve may frequently be a source of noise and [ m 3 ] V v volume of vapour in a control vibration, of which cavitation can be the problem, volume especially during valve closing or valve opening. [ μ m ] e 1 first prism layer thickness The aim of this paper is to show how a [ μ m ] e t total prism layer thickness numerical industrial code can point out a cavitation p min [ MPa ] minimal absolute static pressure problem even if this code doesn’t use a cavitation p [ MPa ] static pressure model. This approach shows a good agreement with p i [ MPa ] static pressure at valve inlet one using a cavitation model. The numerically p iref [ MPa ] static reference pressure at valve obtained contours of the volume fraction of water inlet vapour show cavitation inception distribution t ref [ s ] reference time behind the poppet of the valve. t [ s ] time Computational Fluid Dynamics (CFD) v max [ m/s ] maximal velocity in narrow zone simulations of cavitating flow through water α l [-] liquid volume fraction hydraulic industrial flush valve were performed α v [-] vapour volume fraction using the Reynolds averaged Navier-Stokes (RANS) equations with a near-wall turbulence 1. INTRODUCTION model. The flow was turbulent, incompressible and The effects of boroug h’s improvement on steady. The flush valve under study is a real one. The structure of this flush valve was simplified due working and living environment cannot be ignored. to symmetry considerations. The model used is These can include air pollution, noise and vibration, three dimensional. Flow field vizualization was contamination of land and water. numerically achieved. One of the most important parameters in The effects of inlet pressure as well as mesh building construction is noise control. Cavitation size and mesh type on cavitation intensity (assessed noise generated by components such as valves in by pressure intensity) in the flush valve were water supply systems has frequently raised serious numerically investigated thanks to two commercial problems. In each country, there are legal codes of codes : Fluent 6.3 and Star CCM+ 3.04.009. practice and today new building sites and major construction projects are well controlled. Taps and Keywords: Cavitation, Noise, Numerical valves can, therefore, be classified on the basis of their acoustic behaviour, in accordance with the simulation, Water supply systems ISO 3822 or NF EN 12541 standards ([1, 2]). In order to determine the sources of noise and NOMENCLATURE the best design methods to minimize noise [-] number of vapour bubbles in a N bub generation, experimental and numerical analyses control volume were carried out in our laboratory. The aim of this N cells [-] number of cells paper is to present some numerical results. N it [-] number of iterations N pl [-] number of prism layer Figures 1 and 2 describe respectively flow rate Q [ kg/s ] mass flow rate (fig. 1) and upstream static pressure (fig. 2)
evolutions during one operating cycle, from with conditions of valve opening or closing opening to closure, in a non dimensional form. Such especially when cavitation can occur. This is the tests results are obtained in an open test rig case when pressure in the liquid drops below the composed of : (i) a vessel with a control of pressure, vapour pressure. Vapour bubbles are formed, and (ii) a steel pipe with a laminar flow element flow rapidly collapse when pressure increases. That meter, (iii) a deformable pipe in order to limit collapse of bubbles is clearly associated with noise pressure surges during the valve closure, (iiii) a pipe generation ([3, 4]). with various pressure transducers upstream of the GAO H., FU X., YANG H. and TSUKIJI T. tested valve. The main aim of such a valve is to ([5]) also showed the importance of predicting deliver a fixed volume of liquid (6 or 9 l, for cavitation in water hydraulic valve and the necessity example) with a high enough momentum. It is clear, of continuing investigation. from these two figures, that the operation of such a In this type of valve, when closing or opening, valve is unsteady and transitory. Nevertheless, the static pressure remains relatively constant before cycle duration (about 10s) allows to consider the and behind the singularity formed by piston and flow inside the valve as a succession of quasi- seat (called narrow zone in the next text : detail (a) steady operating conditions. This is the main in fig. 4) where quite all the part of the head drop assumption that has been made in the present study, occurs. because CFD codes are not really still able to The cavitation model in commercial CFD describe accurately such a transitional behaviour codes, designed for two interpenetrating fluids with a control of the opening and closure of the (generally liquid and vapour phases of the same valve by the flow itself. fluid: water in the case of this paper), describes the formation of bubbles when the local pressure Q/Q ref becomes lower than the vapour pressure. The cavitation model solves a single set of momentum equations shared by the two fluids, a continuity equation for the liquid (primary phase) and a volume fraction equation for the secondary phase. It is assumed that all vapour bubbles in a control 1 volume have the same radius R and a homogenous distribution. This assumption leads to describing the bubble distribution by a single scalar field, the vapour volume fraction α v . 4 3 N R V bub 3 (1) 1 t/t ref v v V V V l v Figure 1. Flow rate in a flush valve during a Assuming that only one liquid phase (volume cycle (non dimensional presentation) V l ) and the corresponding liquid-vapour phase p i /p iref (volume V v ) can occupy a control volume V where cavitation takes place, the mass of produced vapour depends on the vapour density, the anticipated average size (radius R ) and vapour bubbles density. Cavitation model also includes mass transfer 1 between the fluids. Models in commercial codes can predict the inception of cavitation but few can predict the collapse of the bubbles. Cavitation is a complex process influenced by a lot of factors. The formation of bubbles followed by their collapse makes this problem highly unsteady. Consequently we have to choose between steady or unsteady approaches for cavitation. In both cases, 1 t/t ref simulation takes a lot of time. The steady approach is sufficient to confirm that cavitation occurs. Figure 2. Upstream static pressure in a flush This paper shows how a commercial CFD code valve during a cycle (non dimensional can point out a cavitation problem with a classical presentation) model without using a cavitation model. Two commercial codes were used: Fluent 6.3 This work deals with the problem of noise and Star CCM+ 3.04.009. generated by a flush valve in water supply systems
In a first step, a comparison between the two to show if cavitation occurs and this model is models (no cavitation and cavitation with steady sufficient to do so. approach) thanks of code Fluent for a relatively fine mesh is presented. 3. COMPARISON BETWEEN NO In a second step, a comparison between the two CAVITATION MODEL AND CAVITATION codes is proposed. MODEL In a third step, the influence of the mesh (size In this paragraph, the flush valve is studied and type) is analyzed. using first a no cavitation model and then a cavitation model. 2. DESCRIPTION OF THE VALVE The geometry of the flush valve, used for the present study, is shown in figure 3. The main path of water stream is represented by white arrows. (a) Figure 5. Flush valve geometry: fluid zone The first geometric model, shown in figures 6 and 7 is a tetrahedral mesh with 607 210 cells. The grid of the fluid domain has been created in pre- processor GAMBIT. The mesh size depends on the Figure 3. Flush valve geometry (cut ¾ view) position. In order to well represent the flow field in the narrow zone, a refined mesh is used there. Small Figure 4 presents details on position between size lies near narrow zone where it is about 0.1 mm piston and valve seat (a) and the narrow zone for and that size increases up to 1mm far away from the fluid between the piston and the seat. The height narrow zone (at the inlet and at the outlet). So there of this zone is 0.8 mm. Simulations show that are about 8 cells in the passage height of the narrow cavitation can occur in this narrow zone. zone. (a) (a) Figure 4. Flush valve geometry (zoom on narrow zone) The associated fluid zone is presented in figure 5. Only the flow in one half of the geometry is calculated due to the symmetry of the valve and to the symmetry of boundary conditions. The choice Figure 6. Mesh of fluid zone of this hypothesis can be discussed because of possible cavitation, but the main goal of this work is
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