Flow control to mitigate cavitation erosion

par Kunpeng Long

Projet de thèse en Génie énergétique

Sous la direction de Olivier Coutier-Delgosha.

Thèses en préparation à Paris, HESAM , dans le cadre de École doctorale Sciences des métiers de l'ingénieur (Paris) , en partenariat avec LML - Laboratoire de Mécanique de Lille (laboratoire) depuis le 26-10-2017 .


  • Résumé

    Cavitation erosion is one major limitation in the lifetime of various hydraulic systems operating at high speed and/or low pressure. Although the small-scale mechanisms of cavitation erosion are still discussed by the scientific community, it is a consensus that the basic phenomenon consists of repeated loads at a solid surface, due to multiple implosions of cavitation bubbles, which result in high speed jets towards the surface combined with pressure waves of large magnitude. Cavitation erosion is especially intense in configurations of cloud cavitation, which is an unstable behavior where the cavitation area fluctuates periodically with large scale vapor shedding. The location of the cloud collapse downstream from the cavitation area is where erosion is mainly observed. Erosion consists of local plastic deformations of the solid surface without mass loss (also called “pits” in the literature), which eventually results in mass loss increasing exponentially in time. The general goal related to industrial concerns is to reduce the rate of cavitation erosion by improving the resistance of surface coating and /or reducing the aggressiveness of cavitation. This second approach is the one that will be investigated in the present project. It has been observed in previous studies that several parameters like surface roughness and small obstacles located in the area of cloud shedding have a significant influence on the flow instability. The general trend is a disorganization of the large-scale shedding, together with a reduction of the magnitude of the pressure waves generated by the cloud collapses. But the actual impact of these simple devices on the rate of cavitation erosion has not been investigated, yet, and no attempt of flow control in order to reduce cavitation has been reported, to the best of our knowledge. The objective of the present study is to propose and test some strategies of flow control in conditions of cloud cavitation, in order to reduce erosion. The recent work of the M. Dular and co-workers, showing simultaneously the cavitating flow and the erosion on a thin aluminium foil located on the solid wall, is the basis of the present work. The same device will be used to test several perturbations and/or actuators and see instantaneously the effects on the rate of pits on the foil. The primary motivation of the work is to identify some simple devices that would enable to reduce significantly the erosion development. Previous studies have shown that increasing the surface roughness or adding some grooves in the area of the cloud detachment results in a decrease of the intensity of pressure fluctuations, combined with a disorganization of the periodical cloud sheddings. Such surface treatments will thus be tested, first, and additional devices will be applied to obtain a reduction of cavitation erosion. The mechanisms that are responsible for the modification of cavitation will be investigated, for example with LIF PIV, i.e. Particle Image Velocimetry with fluorescent particles. Recent works have enabled to obtain two components of both the liquid and vapor velocity fields with this technique. In the present project, it will be extended to 3 components by the use of stereo PIV, in order to investigate the 3D mechanisms of cloud cavitation and the effects induced by the flow control.

  • Titre traduit

    Flow control to mitigate cavitation erosion


  • Résumé

    Cavitation erosion is one major limitation in the lifetime of various hydraulic systems operating at high speed and/or low pressure. Although the small-scale mechanisms of cavitation erosion are still discussed by the scientific community, it is a consensus that the basic phenomenon consists of repeated loads at a solid surface, due to multiple implosions of cavitation bubbles, which result in high speed jets towards the surface combined with pressure waves of large magnitude. Cavitation erosion is especially intense in configurations of cloud cavitation, which is an unstable behavior where the cavitation area fluctuates periodically with large scale vapor shedding. The location of the cloud collapse downstream from the cavitation area is where erosion is mainly observed. Erosion consists of local plastic deformations of the solid surface without mass loss (also called “pits” in the literature), which eventually results in mass loss increasing exponentially in time. The general goal related to industrial concerns is to reduce the rate of cavitation erosion by improving the resistance of surface coating and /or reducing the aggressiveness of cavitation. This second approach is the one that will be investigated in the present project. It has been observed in previous studies that several parameters like surface roughness and small obstacles located in the area of cloud shedding have a significant influence on the flow instability. The general trend is a disorganization of the large-scale shedding, together with a reduction of the magnitude of the pressure waves generated by the cloud collapses. But the actual impact of these simple devices on the rate of cavitation erosion has not been investigated, yet, and no attempt of flow control in order to reduce cavitation has been reported, to the best of our knowledge. The objective of the present study is to propose and test some strategies of flow control in conditions of cloud cavitation, in order to reduce erosion. The recent work of the M. Dular and co-workers, showing simultaneously the cavitating flow and the erosion on a thin aluminium foil located on the solid wall, is the basis of the present work. The same device will be used to test several perturbations and/or actuators and see instantaneously the effects on the rate of pits on the foil. The primary motivation of the work is to identify some simple devices that would enable to reduce significantly the erosion development. Previous studies have shown that increasing the surface roughness or adding some grooves in the area of the cloud detachment results in a decrease of the intensity of pressure fluctuations, combined with a disorganization of the periodical cloud sheddings. Such surface treatments will thus be tested, first, and additional devices will be applied to obtain a reduction of cavitation erosion. The mechanisms that are responsible for the modification of cavitation will be investigated, for example with LIF PIV, i.e. Particle Image Velocimetry with fluorescent particles. Recent works have enabled to obtain two components of both the liquid and vapor velocity fields with this technique. In the present project, it will be extended to 3 components by the use of stereo PIV, in order to investigate the 3D mechanisms of cloud cavitation and the effects induced by the flow control.