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