Modélisation de l'écoulement diphasique dans les injecteurs Diesel

prove that atomization is influenced by several factorscaused by the upstream flow (i.e. in the injector)and operating conditions (P inj , total injectedmass). A good review of these factors has been doneby Reitz and Bracco [26]. The most vivid exampleis the Mokaddem vizualisation of the liquid phase inthe combustion chamber [27] (see figure 7). At lowneedle lift, the five issuing jets are different fromone hole to another, whereas at full injection thefive sprays are identical.Figure 7: Temporal sequence of the liquid phaseevolution by M ie scattering method on the liquiddroplets. The crank angle is given after the startof injection. P inj = 65 M P a, Inj = 356.6, ρ l = 20kg/m 3 , Q = 40 mm 3 /injection [27].Using Flash radiography, Warken and Krehl [28]have shown that an intact liquid core exists at thenozzle exit. But optical access to the internal structureof the jet is hardly possible, because of theopacity of the jet immediately downstream of thenozzle. It seems more convenient to talk about adense core. In his experiments, Chaves [7], Badocket al. [29] and Fath et al. [30] have seen gazeousstructures in the core, due to cavitation in the nozzle.V apour bubbles disappear rapidly in the densecore, typically one or two nozzle diameters downstream.This dense region is the one that interestsus with regards to primary breakup. In fact, atomizationneeds a perturbation on the jet surfaceto happen. This perturbation takes place immediatelyat the nozzle exit. The possible causes of theperturbation are listed below.Schweitzer [31] has considered that the turbulenceof the jet is of primary importance in atomization,and that the aerodynamical interactiononly improves atomization. At the present time, thecontrary is generally admitted since several studiestend to show that turbulence does only give to theliquid jet an irregular surface aspect [32], raising thedrag and consequently the friction due to the ambiantgas. Mac Carthy experiments [33] show thatby increasing L 0 /D 0 ratio (see table 1) and keepingthe same Reynolds and Ohnesorge numbers, atomizationis improved. This is only due to turbulencein the nozzle and shows that this factor tends toimprove atomization.Considering that cavitation has a direct effecton atomization is not a good interpretation of theprocess. Paradoxically enough, the collapse of thegazeous structures in the dense spray is a catastrophicprocess [34, 4, 35, 23, 36, 25], but the onlyknown visualization that shows a jet disintegrationdue to a collapsing bubble has been made by Eifler[8]. The collapse of the bubbles (as Winklhofershowed [37]) and the bubble burst at the surface[38] certainly produce surface perturbation, but nodirect atomization is done by this way. The maineffects of cavitation that influence the spray behaviourare rather the raise of the exit velocity dueto the fact that liquid passes through a reduced effectivearea, and the velocity profile perturbationsdue to the transient and intermittent characteristicof cavitation in the nozzle. The attached cavityhas a pseudo-cyclical behaviour, as it can be seenin [39, 40, 41].The velocity perturbations have been studied byChaves et al. [42] in terms of upstream pressurefluctuations. This study is based on the fact thata water-hammer effect is produced due to the needlemovements. Those pressure fluctuations cause atransient behaviour of the spray : as velocity fluctuate,some fast fluid elements can catch up with theslower downstream elements, producing a sort ofsplashing. A bulge is formed,Figure 8: Dieselspray structure.P inj = 80M P a ,T ch = 800K,D 0 = 0.2mm.and this is seen in Chaves vizualisations.For the moment, thecauses of the velocity fluctuationsare not clearly understood: It can whether be the pressurefluctuations, or whether thecavitation fluctuation that producethose structures. In anycase, those factors are closelyconnected. Bruneaux vizualisationsvoir gilles et mieux exploiterla figure. Demandera son GSM la permissionshow the same type of structuresin engine conditions (see figure8).As atomization is producedby aerodynamical interactions,it seems obvious that the exitjet velocity is the main factorinfluencing the spray behaviour.We can deduce fromthe above-mentioned experimentalstudies that the major effectsof the internal flow on the sprayare the velocity fluctuations dueto cavitation and the pressurewaves.