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

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caused by inevitable variations of the fuel temperature and/or composition, that could simultaneously affect the static flow rate and engender major alterations of the nozzle exit velocity characteristics, are of serious concern. There is a need for development of a fundamental knowledge of the high-pressure, hollow-cone sprays of the GDi injectors, with respect to the spray atomization characteristics and its dependence on the valve-group design and the fuel system operation parameters. An integral element of this activity is the development, validation and application of computational methods to assist analyses of the relationship between the fluid dynamics of the injector valve-group and the associated spray atomization characteristics. Therefore, joint optical / laser diagnostics experimental and advanced computational activities have been undertaken at the Delphi Customer Technology Center Luxembourg to address the dual objectives for development/validation of experimental/analytical methods and establishment of a broad knowledge of the GDi hollow-cone spray atomization characteristics necessary to meet the customer-specific requirements for a robust and stable spray formation [5 , 6]. The present study is concerned with experimental and computational investigation of the flow in a large-scale model of a GDi outward-opening valve-group that incorporates a swirler component upstream of the conical nozzle. The incorporation of the swiler serves two purposes: it enables accurate static-flow setting (and reduces the fuel metering sensitivity to variations of pintle stroke) and imports tangential momentum onto the flow, thereby improving the spray penetration and atomization characteristics. However, it poses concerns with respect to possible cavitation inception, within the swirler passages and the conical nozzle, and the consequent influences on the spray structure. These could engender circumferential non-uniformity of the liquid sheet thickness and nozzle-exit velocity, with notable effects on the spray primary breakup and atomization characteristics. A second topic of interest is whether a transition of cavitation to full/partial hydraulic flip will occur within the conical nozzle, for the fuel pressure ratios comparable to the GDi fuel system nominal levels. The cavitation phenomena in the fuel injection nozzles and its effect on the spray atomization characteristics has been investigated for the cylindrical nozzles - as the most common industrial injector nozzle geometry - in relation to the full-cone sprays of diesel injectors [7 - 11] and other industrial fields of application [12]. In general a direct correlation between cavitation inception in the nozzle-hole and enhancement of the liquid-jet atomization has been observed. More recently, the transformation of cavitation to hydraulic flip has become the focus of attention, due to its causing the irregular “flipping” of the spray trajectory [13]. The hydraulic flip phenomena is closely – and predominantly - associated with the cavitation in the nozzle of high-pressure full-cone liquid jets. It is referred to as the condition whereby the cavitation bubble exceeds the “super-critical cavitation” condition , extends beyond the nozzle domain and merges with the ambient. Consequently, the ambient air (which normally is at higher pressure than the vapour saturation pressure) is drawn into the cavitation domain and forms a “cushion” between the liquid core and the nozzle walls. This has remarkable influence on the structure and breakup of the issuing liquid jet, through (a) inhibition of the liquid-phase near-wall turbulence production and (b) enhancement of the liquid-air interface instability dynamics. Also, dependent on the nozzle geometry and entrance flow irregularity, there is high probability of asymmetric or partial hydraulic flips which engender sudden change of the spray geometry and trajectory [13]. The experimental investigations of the flow in the largescale models of the GDi outward-opening injector show evidence of presence of a gaseous-phase within the nozzle domain (i.e. upstream of the nozzle exit) which is often interpreted as flow cavitation. However, recent CFD Large-Eddy Simulations (LES) have revealed occurrence of “ingestion” (or entrainment) of ambient air into the injector nozzle domain, with substantial influence on the nozzle-exit structure of the conical-sheet liquid-jet and its near-field breakup pattern [5]. In the context of the present study, partial hydraulic flip is adopted broadly for an entrainment of the ambient air into the nozzle domain, that causes formation of an “air-cushion” that separates the liquid flow from the conical nozzle walls. The large-scale experimental investigation is complemented by the CFD multi-fluid Reynoldsaveraged Navier-Stokes simulations of the two- and three-fluid flow within the injector valve group and its neighboring downstream ambient. The objective is to investigate the capability, the qualitative and the quantitative accuracy of current CFD method – as available in commercial CFD codes - for prediction of the cavitation inception, the cavitation geometry and the cavitation effect on the valve-group hydrodynamic parameters. For this purpose, specifically a valve group design with experimental evidence of multiple cavitation regions has been selected, because of the specific interest to assess the capability to accurately predict distinct, multiple cavitation regions within the simulation domain (in particular to verify that inception of cavitation at a flow upstream location does not affect the prediction of its inception and geometric features at a downstream location). EXPERIMENTAL INVESTIGATIONS The large scale model of a gasoline direct injection (GDi) injector’s outward-opening valve-group – with alternative swirler geometries - is constructed and tested in the Physical Modelling Group at the Delphi Technical Centre
in Gillingham. The experimental methodologies applied are photographic imaging, in conjunction with measurements of the pressure and flow rate. The focus of the study is to obtain basic flow information regarding the condition of flow within the injector – with the aid of imaging and pressure measurement at several locations - and the spray. This information is intended to assist development of the valve-group design and support validation of a current state-of-the-art CFD multi-phase simulation method. The large-scale modelling uses the principles of geometric and dynamic similarity for steady state flow conditions. In this respect, the two most relevant nondimensional groups are the Reynolds number and cavitation number, to match the model operation condition with that of the actual-size counterpart. The Reynolds number similarity takes account of the different properties of the actual-size fluid (i.e. n-Heptane) and the large-scale working fluid (i.e. ISO4113 calibration fluid). Table 1 presents the correspondence between the largescale model tests and the actual-size injector operation. Actual size Model (x35) Liquid Properties Working Fluid n - Heptane ISO 4113 calibration fluid Viscosity [mm2/s] 0.575 2.75 Density [kg/m3] 680 817 Vapour Pressure [bar] 0.0413 0.002 System Pressure [MPa] 5 0.112 10 0.224 15 0.337 20 0.449 Ambient Pressure [MPa] 0.4 0.009 Table 1. Correspondence of the Large-scale model tests and actual-size injector operation EXPERIMENTAL FLOW RIG - Figure 1 shows the flow rig and the large-scale model in situ. The static pressures at the tappings are measured with a Scanivalve unit using a calibrated transducer, and processed / stored via a personal computer. The flow rate is continuously monitored with a vane-type flow meter. In order to create the correct flow cavitation number, the GDi valve -group model discharges into a vacuum chamber. LARGE-SCALE VALVE-GROUP MODEL - The model is shown in Figure 2. It is modular in construction to allow design changes to be easily implemented. The main housing and the swirler are manufactured from acrylic material to allow viewing of the cavitations regions in the fluid flow. The pintle can be adjusted to be concentric or eccentric in the guide bore. A leakage path orifice is fitted into the swirler plate to model leakage flow through the pintle - guide clearance annulus. Figure 1. The in-situ assembled model and the test apparatus Figure 2 : construction of the large-scale model and arrangement of the pressure tappings (the pintle is shown seated)
Page 1: Copyright © 2009 SAE International
Page 5 and 6: • Formation of a cavitation ring,
Page 7 and 8: Fuel System Pressure = 5 MPa - Figu
Page 9 and 10: channel. However, the predicted flo

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

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