Christian Pellone

Flow Analysis Inside a Pelton Turbine Bucket

The aim of this work is to provide a detailed experimental and numerical analysis of the flow in a fixed bucket of a Pelton turbine. The head, jet incidence, and flow rate have been varied to cover a wide range of the turbine functioning points. The experimental analysis provides measurements of pressure and torque as well as flow visualization. The numerical analysis is performed with the FLUENT code using the two-phase flow volume of fluid method. The results present a good consistency with experimental data. In particular, the pressure distribution is very well predicted for the whole range of the studied parameters. A detailed analysis of torque and thrust allows evaluating the losses due to the edge and the cutout of the bucket. These results give insight into the benefit we can expect of steady flow calculations through the optimization process of the design of Pelton turbines.

Analysis of thermal effects in a cavitating inducer using Rayleigh equation

A simple model based on the resolution of Rayleigh equation is used to analyze thermal effects in cavitation. Two different assumptions are considered for the modeling of heat transfer toward the liquid/vapor interface. One is based upon a convective type approach using a convection heat transfer coefficient or the equivalent Nusselt number. The other one is based upon the resolution of the heat diffusion equation in the liquid surrounding the bubble. This conductive-type approach requires one to specify the eddy thermal diffusivity or the equivalent Peclet number. Both models are applied to a cavitating inducer. The basic pressure distribution on the blades is determined from a potential flow computation in a two-dimensional cascade of flat plates. The sheet cavity, which develops from the leading edge, is approximated by the envelope of a hemispherical bubble traveling on the suction side of the blade. Cavity shape and temperature distribution predicted by both models are compared. The evolutions of cavity length with the cavitation number for cold water (without thermal effects) and for Refrigerant 114 at two different temperatures is compared to experimental data

Hydrodynamic cavitation in microsystems. I. Experiments with deionized water and nanofluids

An experimental study of hydrodynamic cavitation downstream microdiaphragms and microventuris is presented. Deionized water and nanofluids have been characterized within silicon–Pyrex micromachined devices with hydraulic diameters ranging from 51 μm to 104 μm. The input pressure could reach up to 10 bars, and the flow rate was below 1 liter per hour. The output pressure of the devices was fixed at values ranging from 0.3 bar to 2 bars, so that it was possible to study the evolution of the cavitation number as a function of the Reynolds number in the orifice of the diaphragms or in the throat of the venturis. A delay on the onset of cavitation has been recorded for all the devices when they are fed with deionized water, because of the metastability of the liquid and because of the lack of roughness of the walls. For the first time, hydrodynamic cavitation of nanofluids (nanoparticles dispersed into the liquid) has been considered. The presence of nano-aggregates in the liquid does not exhibit any noticeabl...

Hydrodynamic cavitation in microsystems. II. Simulations and optical observations

Numerical calculations in the single liquid phase and optical observations in the two-phase cavitating flow regime have been performed on microdiaphragms and microventuris fed with deionized water. Simulations have confirmed the influence of the shape of the shrinkage upon the contraction of the jet, and so on the localisation of possible cavitating area downstream. Observations of cavitating flow patterns through hybrid silicon–pyrex microdevices have been performed either via a laser excitation with a pulse duration of 6 ns, or with the help of a high-speed camera. Recorded snapshots and movies are presented. Concerning microdiaphragms, it is confirmed that very high shear rates downstream the diaphragms are the cause of bubbly flows. Concerning microventuris, a gaseous cavity forms on a boundary downstream the throat. As a consequence of a microsystem instability, the cavity displays a high frequency pulsation. Low values Strouhal numbers are associated to such a sheet cavitation. Moreover, when the in...

Numerical and experimental analysis of a darrieus-type cross flow water turbine in bare and shrouded configurations

The aim of this paper is to present the results of the analysis of a Darrieus-type cross flow water turbine in bare and shrouded configurations. Numerical results are compared to experimental data and differences found in values are also highlighted. The benefit of the introduction of a channelling device, which generates an efficiency increment factor varying from 2 to 5, depending on the configuration, is discussed.

3D RANS modeling of a cross-flow water turbine

This paper presents several aspects of the URANS numerical modeling of a three-straight-bladed cross flow turbine. In a first part, a 2D analysis is presented as a reference configuration. The near wall grid is refined until the solution stabilizes. Calculations are performed for three tip speed ratios to cover the three classical regions: primary effects, transition, and secondary effects. Three-dimensional modeling uses 3D grids obtained by translating the 2D one in the span direction. A simplified version considers only the 3 straight blades though the complete version takes all the geometric parts into account: blades, arms, shaft, and hub. The mean and instantaneous power coefficients, obtained with the 2D and 3D grids, are compared to those given by the hydrodynamic LEGI tunnel on a small-scale model. Experimental uncertainties are also carefully quantified for these quantities. It is shown that the 3D modeling improves significantly the power predictions. The main loss regions are the blade tips and the spoke-arm attachments where horseshoe vortices take place. The power distribution along the span is strongly affected by these two zones. The analysis of the vorticity field highlights large 3D vortex structures shed by the blades and convicted inside the turbine.

Modeling Fluid-Structure Interaction in Cavitation Erosion: Preliminary Results

This paper is devoted to cavitation erosion modeling. It presents some recent numerical developments made in the code initially developed in collaboration with E.Johnsen and collaborators at University of Michigan [1] in order to account for fluid-structure interaction. The considered test case is that of a single air bubble collapsing near a wall due to an incident shock wave in the surrounding water. In our investigation, we focus on the code implementation and optimization and the bubble implosion mechanism. The paper is focused on the various events occurring during bubble collapse and the computation of the time evolution of the pressure distribution. The influence of the amplitude of the incident wave and the distance of the bubble to the wall are investigated.

Modeling the Unsteady Cavitating Flow in a Cross-Flow Water Turbine

The noncavitating and cavitating flows over a cross-flow water turbine are simulated by using an unsteady Navier-Stokes formulation. For the cavitating flow case, a homogeneous mixture with a varying density is considered and one additional transport equation is explicitly solved in time for the liquid volume fraction. The instantaneous rate of vapor production and absorption appearing as a source term is governed by a hydrodynamic model based on a simplified bubble dynamic equation. The spatial discretization is achieved by a 2D multiblock technique consisting of fixed and rotating blocks, which were especially adapted for Darrieus geometry. Several test cases corresponding to experiments performed on fixed and rotating blades are selected to compare the numerical results with experimental data. Finally, a calculation of a monobladed cavitating cross-flow turbine is presented. The effect of cavitation on the dynamic stall phenomenon and on the turbine performance is analyzed. In particular, it is shown that cavitation earlier reveals the stall phenomenon on the blades and magnifies the size of the shedding vortex structures in the turbine. [DOI: 10.1115/1.4001966].

Cavitating Behaviour Analysis of Darrieus-Type Cross Flow Water Turbines

The aim of this paper is to study the cavitating behaviour of bare Darrieus-type turbines. For that, the RANS code CAVKA, has been used. Under non-cavitating conditions, the power coefficient and the thrusts calculated with CAVKA are compared to experimental values obtained in the LEGI hydrodynamic tunnel. Under cavitating conditions, for several cavitation numbers, the numerical power coefficients and vapour structures are compared to experimental ones. Different blade profiles and camber lines are also studied for non-cavitating and cavitating conditions.

Effets thermiques en cavitation

ABSTRACT A simple model based on the resolution of Rayleigh equation for a single spherical bubble is used to analyse thermal effects in cavitation. Two different assumptions are considered for modelling heat transfer through the liquid-vapour interface. One is based upon a convective type approach, the other one upon the resolution of the heat diffusion equation in the liquid surrounding the bubble. Both models are applied to a cavitating inducer and cavity shapes and temperature distributions are compared. The evolution of cavity length with the cavitation number for cold water (without thermal effects) and for refrigerant R114 at two different temperatures are compared to experimental data.