IR THERMOGRAPHY FLOW VISUALIZATION OF SINGLE AND MULTI-HOLE SPRAY IMPACTING

The Direct Injection (DI) in Spark Ignition (SI) engines offers undoubtedly advantages with respect to thePort Fuel Injection (PFI) offering the flexibility of multi-mode operation and the generation of a stratifiedcharge that results in an overall higher efficiency of the combustion process. The spreading of DISI engineshas given new impetus to the study of fuel spray behavior. Indeed, spray droplets hitting on the surface mayrebound, stick to each other to form a film on the wall, or undergo heating and evaporation. Knowledge ofspray-wall interaction dynamic is a key factor affecting the air-fuel mixture formation and equivalence ratioat spark timing particularly in turn influencing pollutants formation.IR thermography flow and Schlieren visualizations (Figure 1) were applied to two injector configurationsnamely single- and multi-hole. In particular, the current work presents a detailed analysis about the thermalfootprint of both, single-hole and eight-hole injectors, through heated thin foil technique [1]. Theexperimental apparatus for IR measurements is sketched in Figure 2 and includes a INVAR® foil (200 mmwide, 200 mm long and 50 ?m thick). The foil, constituting the target plate, is steadily and uniformly (inspace) heated by Joule effect. The surface temperature distribution is measured by viewing the rear face ofthe foil (i.e. the side opposite to spray jet impingement) through a mirror, as shown in Figure 2. The injectoris located in a jacket for the temperature governor of the nozzle nose while the fluid temperature iscontrolled by a thermostatic system. The single-hole is configured with an exit hole diameter d of 0.200mm, and a ratio L/d = 1 while the injection pressure varied in the range 5.0 - 20.0 MPa. Another set ofexperiments is acquired using a multi-hole injector having the hole diameter of 0.165 mm, a L/d = 1, anda static flow of 15 cc/s at 10.0 MPa. These holes are distributed symmetrically along a circular annulus withrespect to the injector axis and forming a full outer cone angle of 80°. In both cases, characteristicparameters like injection pressure, injection duration, quantity of injected fuel, and ambient/fluidtemperature are varied in order to sum up a synthetic sight of both configurations behavior. Each case isinvestigated over 30 phases. Each phase is obtained through a phase-locked mean over 200 samples. Aspecific time separation between two consecutive fuel injections is applied in order to realize statisticallyindependent experiments from each other.The thermal footprints for the single-hole and the multi-hole cases are acquired. The single-hole device ischaracterized by an axial-symmetric behavior as it could be expected. In addition, the thermal footprintdepends strongly from the nozzle/plate distance and from the injection duration. Extending duration ofexperiments over a specified limit of time, the formation of fuel film on the wall is clear. This simple caseis used as reference for the more complex multi-hole case in order to drive the heat transfer comparisonanalyses between the two configurations. The second hole arrangement provides a clearly visible eight-holepattern. Together with the colder impinging zone of eight injectors it is noticeable the presence of arelatively hot zone in the center of the pattern which could be ascribed to a counter rotating coherent vortexstructure caused by multi-jet interaction.