This axis includes research on acoustic and hydrodynamic cavitation. Our team is now studying hydrodynamic cavitation at all scales, from low Reynolds numbers in laminar flow in fluidic microsystems to very high Reynolds numbers in turbulent conditions at the macroscopic scale. Cavitation is a unifying activity of the team, since it interacts with the other two scientific axes as well as with the MOST team. Cavitation erosion naturally affects turbomachinery; thermodynamic effects, sonochemistry and acoustic cavitation are linked to the intensification of transfers. Six doctoral students and four postdoctoral fellows participated in this activity (notably thanks to the support of TEC XXI). Local, national and international collaborations have been established or continued with Simap Laboratories, IGE CEA, Insa Lyon, ILM, Institut Pprime, University of Ljubljana, Brussels, Manchester, Michigan, Office of Naval Research (USA).
Cavitation is one of the three areas of research of the Energy team, in line with the activities on turbomachinery and those on the intensification of transfers. Cavitation phenomena reflect changes in the liquid phase – vapour and vapour – liquid following a drop and then a sudden rise in the pressure of a fluid on either side of its saturation value. Acoustic cavitation is controlled by an acoustic wave while hydrodynamic cavitation results from the Bernoulli principle.
In addition to macrodynamic hydrodynamic cavitation, which is historically an internationally recognized LEGI activity, hydrodynamic cavitation “on a chip”, i. e. at the heart of fluidic microsystems, has been added in recent years. Finally, we saw in the previous paragraph that acoustic cavitation created by ultrasonic waves is now applied to certain transfer intensification problems. In addition to the purely mechanical aspect of the implosion of a bubble and its harmful effects (shock wave, erosion), a physical (sonoluminescence, cryogenic fluids), chemical (chemiluminescence, production of hydroxyl radicals, degassing), process engineering (exfoliation of particles, intensification of transfers) approach is now added. For all these aspects, the size and shape of the bubbles before implosion are the important parameters controlling the physical phenomena mentioned above. Thus, our team has broadened the range of topics covered by cavitation and LEGI will be a candidate next year to join the GDR Cavitation recently created with the support of the C.N.R.S.
Research activities on the phenomenon of hydrodynamic cavitation at the macroscopic scale concern erosion[Roy et al. 2015, Deplancke et al. 2015]. This subject is addressed jointly by numerical and experimental methods. The objective of cavitation flow simulations is to develop a wear model capable of predicting damage by cavitation and applicable to hydrofoils, centrifugal pumps and turbomachinery (C. Leclercq, L. Krumenacker, EDF/CETIM). These 3D flow simulations are based on a single bubble scale analysis, taking into account compressibility and phase change. Cavitation erosion was also addressed through fluid/structure interaction (continuation of a collaboration previously initiated with Simap) (S. Joshi, P. Sarkar, CaFE Innovative Training Network). For the simulation of the implosion of a cavitation bubble in the vicinity of a wall, the calculation of the implosion dynamics is now combined with that of the response to the wall (Y. Paquette, TEC21 & coll. Univ. Michigan). This numerical approach was complemented by unpublished experimental studies on the Prevero loop, with the first development tests of pressure sensors based on PVDF films (J.B. Carrat, GE – HydroLike), with acoustic emission measurements and their correlation with the efftrittance of the material subjected to cavitation (M. Ylönen, coll. Tampere Univ. Techno.), as well as the development and testing of new special polymer coatings for protection against erosion (C. Gauthier, ANR Astrid).
Hydrodynamic cavitation “on a chip”, i.e. at the heart of fluidic microsystems with a hydraulic diameter of less than 300 µm, is an experimental approach that allows us to study hydrodynamic cavitation under flow conditions inaccessible at the macroscopic scale, and to associate specific metrology, which is also inoperative on a macroscopic device, with the cavitation flow[Ayela et al. 2015]. Thus, our team was able for the first time to produce a cavitating flow in laminar mode[Mossaz et al. 2017], study the role of aggregates in triggering hydrodynamic cavitation of nanofluids, cavitate different solvents and confirm the anomaly of ethanol which cavitates more easily than conventional models suggest (S. Mossaz, TEC21).
Cavitation is a state change with latent heat exchange and, in collaboration with the ILM, we were able for the first time to highlight the thermal effects associated with these changes on water, thanks to the seeding of thermofluorescent nanoparticles and the use of microreactors in the installation of a confocal microscope. Liquid cooling zones where the vapour is created and heating zones where the bubbles implode have been visualized. We have experimentally found that the position of these zones depends strongly on the flow regime, and these findings have been validated by a first numerical study. Cavitation is a locally violent physical phenomenon and the implosion of bubbles can dissociate water molecules into H° and OH° radicals. The production of OH° hydroxyl radicals can be applied to advanced pollutant oxidation processes. Acoustic cavitation, through its sustained bubble formation mode, creates hydroxyl radicals. But the effectiveness of hydrodynamic cavitation for sonochemistry has always been questioned, due to the lack of concrete quantitative results. A second highlight of our team is that for the first time we were able to quantify the rate of production of OH° radicals in a cavitating flow, using luminol (chemiluminescent compound reacting to the presence of OH°) as a working fluid in microreactors coupled to a photon counter[Podbevsek 2018]. This research continues by coupling and correlating the experimental measurement of sonoluminescence with that of chemiluminescence (L. Perrin, TEC21). On a more applied subject, hydrodynamic cavitation on a chip has made it possible to develop an economical and ecological process of exfoliation of graphite in graphene (X. Qiu) patented[Qiu et al. 2017 & 2019]. This work continues as part of a maturation project funded by SATT Linksium (Grenoble Green Graphenofluid, Dr S. Ponomareva, see document GGG ). Finally, research programs are also underway with the CEA’s Low Temperature Department (cavitation of cryogenic fluids) and the IGE (degassing of liquid from melting ice cores, paleoclimatology).
(F. Ayela, D. Colombet, S. Ferrouillat, E. Goncalves, R. Fortes – Patella, J.P. Franc)