CFD Study of Needle Motion Influence on the Spray Conditions of Single-Hole Injectors

This work consists of studying the effect of needle motion of typical single-hole injectors on spray characteristics. Three-dimensional moving mesh simulations have been carried out to calculate the injection process of cylindrical and conical geometries. The CFD analysis includes a numerical model which simulates the effect of cavitation. Results show that the flow within the nozzle and at the exit varies depending on the nozzle geometry and needle position A kind of hysteresis in the development of the flow has also been observed between needle opening and closing. Introduction In direct injection Diesel engines the appearance of cavitation in the injector internal nozzle flow is fairly common, especially in cylindrical nozzles, and is known to affect significantly the air-fuel mixture in the spray [1]. It is therefore important to understand in what way the inception of cavitation inside the nozzle is caused or enhanced by the needle lift and how this affects the spray characteristics. Because of the difficulties related with the determination of the flow characteristics inside a Diesel nozzle, modeling is a useful tool to analyze the injector behavior. However, most of these studies are two-dimensional, simplify the real injector geometry, and neglect needle lift conditions. This present work deals with the 3D modeling of single-hole diesel injectors in which the influence of needle motion on the nozzle exit characteristics is evaluated. The paper has the following structure: The following section briefly presents the CFD approach highlighting the nozzle geometry and model inputs, while the last section discusses the most important observations. Injector flow modeling In order to precisely characterize the nozzles dimensions, a proven silicone mould technique was used [2]. Due to the symmetry, a 90 degree sector was meshed. In the calculations, the cells between the needle and nozzle expand and contract with the corresponding axial needle movement and a range of 15 to 250 lift μm was covered. Preliminary calculations were performed to decide the cell size, guaranteeing the independence of the obtained results. Constant pressure boundary conditions were assigned at inlet and outlet boundary conditions. The simulations were performed with a commercial finite volume based code [3]; its cavitation model is based on bubble growth theory [4] which links the bubble growth rate with the local pressure. The conventional k-ε turbulence model was adopted to account for the effects of turbulence. For both nozzles, identical needle lift curves of simple linear equations were used since measurement of the needle lift curves was difficult. Results and Discussion Two real single-hole injector geometries have been considered, one with a cylindrical nozzle, the other with a conical nozzle. In table 1 the main injector geometrical features are presented. The comparison of injection features was performed at real engine operating condition: injection pressure=141 Mpa, back pressure=1 Mpa. Results of the simulation at the nozzle exit in terms of injection rate, velocity, turbulence and cavitation are shown in Figure 1. Some common flow characteristics have been observed for both nozzles. The CFD results clearly show that the mass flow rate increases greatly with increasing needle lift until a certain value, after which, the mass flow rate seems to be less sensitive to further lift increase. This behavior shows that for high needle lift, the profile of the nozzle outlet has a greater influence on the mass flow rate than the needle movement. Additionally, at low needle lifts (<150 μm), at both ascending and descending conditions, the turbulence and velocity levels have peaks. This is due to the restricted area in the annulus between needle and nozzle body which locally accelerate the liquid, increasing the turbulence level from this location downstream. However, in comparing the two nozzles, it is noted that the quantity of injected fuel is less for the cylindrical nozzle, due to the appearance of cavitation. This has the effect of decreasing the exit area and consequently in* Corresponding author: [email protected] ILASS – Europe 2010 CFD Study of Needle Motion Influence on the Spray Conditions of Single-Hole Injectors creasing the exit velocity. An apparent increase of turbulence level at the nozzle exit is seen due to the appearance of cavitation patterns inside the nozzle. Note that this effect is stronger as the needle opens. Concerning the cavitation pattern, it is seen that with low needle lift it is more unstable due to the increased flow turbulence caused by the shortened flow passage. Also, at low lifts, peaks of vapor volume fraction are observed, which can be explained by the fact that the cavitation cloud grows and exits the nozzle. A kind of hysteresis in the development of the cavitation has also been observed between needle opening and closing. References [1] Soteriou C., Andrews R. and Smith M., Direct Injection Diesel Sprays and the Effect of Cavitation and Hydraulic Flip on Atomization, SAE paper 950080 (1995). [2] Payri, R., García, J.M., Salvador F.J., and Gimeno J., Fuel 84:551-561 (2005). [3] STAR-CD Methodology, version 4.06, CD adapco, 2008. [4] Rayleigh L., Philosophical Magazine 34:94-98 (1917). Table 1. Technical data of injector Cylindrical Conical Average diameter in the entrance of the orifice (μm) 157 176 Average diameter in the middle of the orifice (μm) 163 170 Average diameter in the outlet of the orifice (μm) 163 165 Orifice nozzle length (μm) 100