## 4 Comprehensive Discussion on the Spray Processes near the Nozzle  
The flow and atomization characterization of high-pressure injection systems inside and near the injector nozzle is challenging, due to very small dimensions and the optical dense and strongly transient nature. Up to now, many of the transient processes inside the injector and in the primary breakup region have been assumed, based on indirect evidence. In order to reach a comprehensive experimental based knowledge, several partly new and mostly optical measurement approaches are described in this work. Within the boundary that the optical access is limited to single, double or triple-hole injectors, we are able now to come to a more comprehensive discussion on the processes of high-pressure sprays. Firstly, investigations about phenomenon inside the nozzle were focused. The X-ray and neutron transmission imaging techniques were applied to obtain the detailed nozzle geometry for defining the 3D geometry of an injector for the numerical simulation of the 3D flow inside the nozzle. The needle lift and needle tumbling were measured from the sequential images by X-ray transmission imaging techniques. Together with earlier reported measurements of strong cavitation inside the injector, and in the injection holes during the first 300 us of the lifting process [22; 23], we can state that the needle geometry and its movement has a significant influence on the transient spray processes, leading eventually also to an asymmetric spray pattern. The results of the 3D flow simulations show the flow pattern being highly affected by the detailed geometry [19; 20]. This emphasizes the importance of detailed measurements on the nozzle geometry. For that, experiments with transparent nozzles have been designed, with a special focus on detailed geometry, where the injection pressure can especially be increased up to 180 MPa (1800 bar). As an example, the measured cavitation structures with 60 MPa are discussed in Chapter 2.3. These measurements, as well as the detailed geometry data of these injectors, will be provided for the community for the development and validation of numerical simulation models via the Mendeley data base [17]. As an example, the experimental results were compared with own numerical simulations based on a standard literature model, discussed in Chapter 2.3. The starting positions of the relatively large and stable cavitation clouds on the wall can be predicted well. However, the size of the cavitating area, unsteady string cavitation or film cavitation are not yet predicted with these models. Detailed measurements on high pressure conditions are needed here to allow validation data for the model development and test. Further enhanced geometric setups for such transparent measurements have to be evaluated, e.g., with three holes, where the redirection effects of the flow inside the injector are regarded as well. For the future, it would also be of value if the details of the turbulence production induced by the collapse of cavitation bubble in the nozzle flow could be measured quantitatively. Figure 19: Droplet velocity and size results from L2F. Radial profiles at three downstream positions; (a) Droplet velocity; (b) Droplet size. Secondly, investigations on the spray development near the nozzle have been conducted. One valuable approach is the application of the optical connectivity method (OCM) for the direct determination of the length of the primary break-up of the liquid core. In earlier publications, we were able to show that this length decreases with increasing injection pressure [26; 28], which agrees well with the theoretical expectation. Typically, the time dependent OCM signal starts later than the injection, with a delay of about 300 \(\upmu\)s, and it stops before the end of injection. Both fit very well to the observations in transparent injectors of strong cavitation behavior inside the whole nozzle during the opening and closing process of the injector needle. For full cavitation, the light for the OCM signal is disrupted.