The use of electrical discharges for handling laminar and turbulent flows in free fields is widely recognized, at least on the scale of laboratory experiments. This recognition is intimately linked to the ability of these actuators to promote unsteady periodic forcing favoring in certain situations the non-linear interactions of the turbulent or pseudo-turbulent components of the flow to be controlled. In parallel with experiments on plasma / gas interactions, the phenomena of dissociation and recombination as well as the phenomenon of charge injection have demonstrated their interest in handling fluids in the liquid phase. In theory, the accumulation of charges on a liquid / solid interface makes it possible to obtain a fog of nanometric drops. In practice, the results obtained remain quite far from this objective. The studies that we are carrying out in the context of the INTERACTIVE Labex therefore seek to test the validity of theoretical models by setting up fundamental experiments.
Theme 5 is built around the capacities of electric actuators to produce, or simply promote, multi-scale fluid structures and material displacements at time scales driven by the electrical control signal, this in the gas / interface region. solid or liquid / solid. These include extending the scope of these dumps to blank areas of research through interdisciplinary collaborations within the PPRIME Institute.
More specifically, theme 5 is split into 3 research themes:
Action 5.1.Manipulation of internal flows by injecting charges
Supervisors: : Christophe Louste / Eric Moreau
This action concerns the use of electrofluidodynamic actuators for the control of flows. Three points were targeted by this action: (1) Develop, understand and improve electrical discharge systems on micrometric scales for operating conditions in specific atmosphere in terms of temperature and pressure, conditions related to applications in the field plasma assisted combustion; (2) Extend the dielectric barrier discharge technology to liquid phase flow manipulation; (3) Improve the performance of two-phase systems by installing electrohydrodynamic devices in order to promote atomization by destabilizing the liquid / air interfaces, controlling instabilities in the primary atomization zone.
The fundamental aspects of plasma discharges are approached in this action with particular attention to the characterization of interface phenomena using local measurements (spatial distribution of ionized species, development of plasma diagnostics like CARS / RAMAN, multi energy exchanges -échelles …). New types of actuators are developed to meet the specificities and objectives of the action. Mention may in particular be made of the installation of rampant nanosecond discharges from the surface, or else the manufacture and characterization of an active conductive grid system for controlling turbulence scales by electrical control.
With regard to EHD actuators in the liquid phase, it has been shown that the electrical force is produced inside layers of heterocharge and / or homocharge which develop on the liquid / solid interfaces. As direct observation of these layers is not possible, the work is carried out using an inverse approach. The analysis of product flows allows us to go back to the phenomena that take place on the interface. The work is carried out along three axes: (1) development of measurement methods adapted to the fine study of electroconvective flows, (2) characterization and improvement of the performance of EHD actuators finally (3) realization of a demonstrator intended to demonstrate the performance of EHD systems for liquid flow control applications.
Action 5.2.Electrical methods to promote mixing of fuel for combustion
Supervisors: Nicolas Bénard / Marc Bellenoue
High-pressure injection systems are innovative but temporary solutions for atomization on small scales, improving mixing conditions and overall for obtaining less polluting combustion. A further increase in the pressure levels necessary for these new generation injectors is not industrially realistic. The use of electrical charge phenomena or the use of out-of-balance plasma discharges are two options in the development of new combustion systems. For example, the action of electric charges on a jet or spray allows: a large increase in the number of drops, reduces coalescence, promotes dispersion, and therefore overall improves the quality of the mist produced. The action carried out within the INTERACTIFS program concerns: (1) the study of charge injection for combustion by electro-spray, the presence of electric charges favoring an increase in evaporation rates, an increase in inter spaces -particles and ultimately should lead to a reduction in the production of soot; (2) the use of surface plasma discharge for combustion in order to facilitate the conditions for igniting a mixture by local deposition of energy or to control the flame extension by modifications of the reactive chemical components under the effect of the ionization region.
Action 5.3.Plasma-assisted heat transfers for new cooling systems
Supervisors: Nicolas Bénard / Eva Dorignac
Combustion chamber walls are difficult to withstand flame temperatures and associated thermal radiation without the aid of efficient cooling systems.One technique consists in promoting the development of a thin layer of cold fluid at the wall of these chambers by the use of micro-perforations and micro-injections.The objective of this action is to show the capabilities of surface plasma actuators to contribute to the improvement of this type of system.The primary objective is to keep the cold jets ejected from the micro-perforations in the heat exchange zone between the hot wall and the cold jets thanks to the momentum transfers induced by the discharge region.A second approach consists in maintaining the cold jets in a laminar regime in order to have the best impact on the heat exchange coefficients.