Cavitation and Atomization in Nozzles : Numerical and Experimental study – CANNEx
Positive and negative effects of cavitation on atomization and on nozzle integrity : real scale experiments on transparent nozzles and advanced numerical simulation of cavitating flows
High-pressure flows in nozzle orifices are likely to cause cavitation, i.e. a phase change of a fraction of the liquid fuel to vapour. Controlling this phenomenon is essential to manage atomization, combustion and thus fuel consumption and pollutant emissions. CANNEx programme aims to enhance understanding of cavitation occurrence in injector nozzles through the use of advanced numerical simulation and of advanced experimental methods.
How to control inception and development of cavitation in realistic injection nozzle in order to manage combustion, fuel consumption and pollutant emissions?
Cavitation is a key phenomenon which must be understood to increase today’s car engine efficiency. Cavitation is a transient phenomenon occurring in region of high velocity gradient and/or low local pressure in nozzle flows. Depending on the type of cavitation, atomization can be enhanced or attenuated on the one hand, and the injection device operating can be altered or not on the other hand. The determination of the conditions for the onset of cavitation in real-size nozzles and the correlations between cavitation in the nozzle and atomization in the emanating jet is not yet fully documented. The CANNEx project aims to achieve a global analysis of the flow inside and outside the nozzle to be able to characterize the impact of the flow conditions on the development of cavitation and to correlate the properties of the cavitation to those of the atomized spray. One of the major difficulties arises from the realization of realistic-scale transparent-nozzle experiments in order to obtain experimental results relevant for engine applications. This is of primary importance for the findings of this programme to be implemented in the targeted application. Another difficulty lies on the optical density of the liquid fuel flow in the vicinity of the nozzle orifice. Numerical simulations of such dense flows need the use of advanced direct numerical simulations. Experimental characterization of these flows might be obtained by employing state-of-the-art optical diagnostic methods.
The study will mainly focus on the vicinity of the nozzle, i.e. the flow structure at the nozzle inlet, the cavitating flow in the nozzle and the two-phase flow in the near field of the nozzle exit. These regions of the flow will be explored experimentally and numerically. The partners of the programme have recognized experiences in conducting experiment on transparent nozzles and in developing advanced optical techniques for spray applications. High-quality visualizations of the flow in the nozzle under high pressure conditions will be obtained by combination of transparent nozzles and index matching methods to explore the entire fuel flow inside the nozzle. Dense spray analysis will be done with the use of femtosecond and ballistic imaging visualization techniques. These advanced and complementary measurement techniques will be applied to a realistic nozzle model. The most ambitious aspect of the project is probably to combine these leading techniques together with advanced numerical simulations on a controlled and repeatable configuration.
Direct numerical simulation will be based on academic ARCHER code developed over the past decade to accurately describe the primary breakup of pure liquid jets in a gaseous medium.
ARCHER was recently adapted to take into account the internal flow using immersed boundary method, with the objective to investigate interactions between external and internal flows. Cavitation inception will be implemented through a stochastic approach. The spatial and temporal evolution of the two-phase flow, including cavitation cavities, will be simulated also.
Two similar experimental setups have been developed at CORIA and LMFA with technical support from DELPHI. The main objective of this first step was to obtain two experimental bench as similar as possible in order to facilitate the development and the use of advanced diagnostic techniques.
Enhancing our current knowledge about cavitation phenomenon and improving our ability to control this phenomenon in the context of high injection-pressure applications is what should be the main result of CANNEx.
H. Purwar, K. Lounnaci, S. Idlahcen, C. Rozé, J.B. Blaisot, L. Méès, M. Michard « Effect of Cavitation on Velocity in the Near-field of the Diesel Nozzle », ICLASS 2015, Taiwan.
The phenomenon of cavitation occurs in high-pressure fuel injectors, extending from their starting point around the nozzle orifice inlet to the exit, where it influences the formation of the emerging spray. The improved spray development is believed to lead to a more complete combustion process, lower fuel consumption, and reduced exhaust gas and particulate emissions. Cavitating holes have proved also their efficiency to limit the risk of coking, i.e. particulates deposit into the injection holes. However, cavitation can decrease the injection process efficiency (discharge coefficient) by limiting the flow rate. It can enhance or limit atomization, depending on the cavitation regime. In addition, imploding cavitation bubbles inside the orifice can cause irreversible damages to the nozzle by material erosion, thus decreasing the life duration and hydraulic performances of the injector. Clearly, an optimum cavitation regime is suitable and it is important to understand the sources and amount of cavitation for more efficient nozzle designs in both Diesel and gasoline applications, including petroleum-based fuels and biofuels. The objective of CANNEx is to deliver new perspectives on cavitation phenomenon to the scientific community, in order to enhance our current knowledge and our ability to control this phenomenon in the context of high injection-pressure applications. To do so, a well-controlled flow will be studied from inside the nozzle to the spray, by using advanced optical diagnostics and efficient numerical tools. Simultaneous visualizations of cavitation inside a transparent nozzle and the liquid jet at nozzle outlet will be performed. The spray will be characterized by using femtosecond ballistic imaging techniques in the near field and an image based drop size technique in the far field. DNS computations incorporating improved cavitation models will be developed and applied to simulate the flow from the nozzle to the emanating jet. Computation and experimental results will be confronted to each other to identify: 1- the relations between cavitation and spray formation , 2- the involved mechanisms. Application to innovative nozzle design is also under the scope of the project as far as these designs will be achievable for transparent nozzles.
Monsieur Jean-Bernard BLAISOT (Complexe de Recherche Interprofessionnel en Aérothermochimie)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Delphi Delphi France SAS
LMFA Laboratoire de Mécanique des Fluides et d'Acoustique
UMR 6614 - CORIA Complexe de Recherche Interprofessionnel en Aérothermochimie
Help of the ANR 869,564 euros
Beginning and duration of the scientific project: December 2013 - 48 Months