Cavitation and Hydraulic Flip in the Outward-Opening GDi ... - Delphi

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Figure 10: Three-fluid (liquid, vapour, air) simulation results of the stationary flow field ( fuel pressure = 5 MPa, ambient pressure = 0.4 MPa) Figure 11: Three-fluid simulation results of transient developments of cavitation and partial hydraulic flip ( fuel pressure = 5 MPa , ambient pressure = 0.4 MPa) as underscored previously, the imaging data does not permit unambiguous conclusions with regards to the cavitation inception location (on the seat and/or pintle surfaces) and geometry. Three-Dimensional Flow Fields : Three-Fluid Calculations – The following provides three-fluid simulation results to underscore the influence of inclusion of the ambient into the calculations. Fuel System Pressure = 5 MPa - Figure 10 presents a description of the flow, through the field plots of the pressure, liquid-phase velocity and the vapour-phase and air volume-fractions. The general liquid-phase flow structure, and the cavitation within the swirler component (upstream of the conical-nozzle region), are almost identical to the two-fluid model calculations (within the accuracy of the solution method), as evidenced by the features of the super-critical cavitation in the swirler
channel. However, the predicted flow within the conical nozzle is markedly different from the two-fluid simulations, as illustrated by the results in Figures 10 and 11. The iso-surface plots of the fuel vapour and air in Figure 11 reveal a complex transient flow within the conical nozzle, with a fast time-scale. They portray the following sequence: (a) formation of a cavitation ring, on the seat edge, at the nozzle entrance, (b) the growth of the cavitation ring at the nozzle seat edge into a cavitation annulus that extends to the nozzle exit, (c) concurrent flow acceleration and formation of cavitation on the pintle surface that extends to the nozzle exit, (d) air entrainment into the cavitation bubbles and formation of a partial hydraulic flip – extending almost the full nozzle length – on the pintle surface (concurrent with the flow re-attachment on the seat surface). The entrapped air, collocated with the cavitation bubble at the seat-edge in Figure 11 is a remnant of this transient process. The three-fluid simulations of the injector valve-group provide interesting and significant results that indicate concurrent formation of cavitation and partial hydraulic flip in the injector valve group. As noted earlier, the imaging data do not permit differentiation between cavitation and partial hydraulic flip, so experimental confirmation of this results is not readily possible. However, a careful examination of the imaging data for the 5 MPa fuel pressure in figure 3 provides indications in support of the predicted phenomena in Figure 10 (specifically, the irregular cavitation ring at the conical nozzle entrance that appears distinct from the patches of intense two-phase interactions downstream of the pressure tappings, suggesting presence of separate liquid-gas phase interface regions on the pintle and the nozzle seat). Overall, the three-fluid simulation are more physically valid than the two-fluid calculations with respect to the cavitation – partial hydraulic flip phenomenon in the nozzle (as evidenced by the imaging data) and provision of an insight into the potential flow complexities. It must be remarked that the methodology, application range and accuracy of the cavitation simulation method, the nozzle geometry and the flow boundary conditions are of critical importance with regards the predicted flow structure. It is worth mentioning that although the air-entrainment phenomenon is similar to that observed in the LES simulations of the flow within the GDi conical nozzles, the RANS methodology is incapable of predicting the associated non-uniformity of flow separation and liquid – air interface instabilities that are responsible for the particular “jet string” primary breakup structure of the conical-sheet liquid-jets [5]. This is due combined absence of high-resolution simulation of the turbulent large-scale motions (a feature of the Reynolds-averaged treatment of the turbulence in the RANS methodology) and accurate numerical treatment of the air – liquid interface (e.g. via the VOF interface capturing/tracking method [5]). CONCLUSIONS The experimental imaging data reveal inception of a super-critical cavitation bubble in the swirler channels, independent of the fuel system pressure in the range 5 – 20 MPa. They provide evidence of a complex two-phase flow, with multiple cavitation zones, within the conical nozzle region. The computational method provides good quantitative accuracy in prediction of the static pressure variation in the valve-group, the flow rate and the effect of flow cavitation. Hence, it offers a design optimization tool for the valve-group with regards the hydrodynamic flow parameters. In the present analyses, the two-fluid and the three-fluid calculation schemes provide similar level of predictive accuracy for the flow rate - pressure drop relationship: this can be explained as a consequence of the over-riding effect of the swirler geometry - and the associated super-critical cavitation - on the flow rate. The predictions of the cavitation structure within the swirler channels and its insensitivity to the fuel system pressure are in excellent agreement with the data. With respect to the complex flow structure in the conical nozzle, the experimental data is not sufficiently unambiguous to ascertain the accuracy of the simulations. Nevertheless, the imaging data provides sufficient evidence in support of the three-fluid simulations which point to the simultaneous presence / conversion of flow cavitation, separation and partial hydraulic flip within the conical nozzle. In this respect, the simulations provide insight into the potential flow complexity and assist interpretation of the imaging data. REFERENCES 1. Lückert, P., Waltner, A., Rau, E., Vent, G., Schaupp, U (2006). The New V6 Gasoline Engine with Direct Injection by Mercedes Benz, MTZ, 67. 2. Schwarz, Ch., Missy, S., Steyer, H., Durst, B. Schünemann, E., Kern, W., Witt, A. (2007). The New BMW Four and Six-Cylinder Spark-Ignition Engines with Stratified Combustion, MTZ, 68. 3. Varble, D.; Xu, M.; Van Brocklin, P.,Development of a Unique Outwardly-Opening Direct Injection Gasoline Injector for Stratified-Charge Combustion, presented at 7th Aachen Colloquium Automobile & Engine Technology, 1998. 4. Marchi, A., Nouri, J. M., Yan, Y., Arcoumanis, C., Internal Flow and Spray Characteristics of Pintle- Type Outwards Opening Piezo Injectors for Gasoline Direct-Injection Engines, presented at SAE World Congress, 2007-01-1406, 2007.
Page 1 and 2: Copyright © 2009 SAE International
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