## 3 Spray Development from Nozzle Orifices
### Determination of the Primary Spray Breakup with the Optical Connectivity Method  
The liquid core of an injector breaks into structures and later into droplets within the spray. For the near field processes near the primary breakup zone the direct measurement is only rarely possible with common visualization techniques. [PERSON] has proposed an illumination of the liquid dense core from inside the nozzle in the early 1990 th, allowing one to get an indication of the primary breakup length [23]. The laser light is guided into the injection nozzle to illuminate the liquid jet from inside. [PERSON] used fluorescent fuels for rather low injection pressures. [PERSON] and his team redeveloped and expanded this technique around 2009. They named it the optical connectivity method (OCM) and applied it to specially designed single hole injectors [24; 25]. In our group, [PERSON] and [PERSON] applied this measurement approach for the first time to a real-size Diesel injection nozzle and applied it also for high pressure injection [26; 27]. The laser light is guided by an optical fiber into the sac volume of the nozzle and part of the light passes through the continuous liquid phase until it exits the nozzle orifice. The liquid dense core acts similar to an elongated optical fiber until it breaks into separate droplets or structures. On the boundary of the liquid phase, the light is emitted. With that, the optical connectivity method allows one to visualize the liquid dense core from inside the nozzle, and its structure becomes visible from outside the spray [26; 28]. The OCM has the disadvantage, that a fiber has to be guided into the injector, which reduces the application flexibility within test cells with high temperatures. A modified OCM setup, the remote optical connectivity method (ROCM), has been proposed by [PERSON] et al. 2015 [29]. Instead of a connected fiber between the laser and the injector, here, the incident laser light is focused on a short fiber core going through the wall of the injector tip. The fiber surface is polished and therefore the fiber acts as a laser incident window, through which the incident light passes into the sac of the nozzle. A part of this light illuminates the dense core at the jet exit from inside the nozzle (Figure 11a). Figure 11: Principle and results from the Remote Optical Connectivity Method (ROCM): (**a**) Principle of ROCM; (**b**) Illuminated dense core signals for three conditions during start of injection (0.4 ms ASOE) and in the steady injection phase (1.2 ms ASOE) for different injection pressure (single shot examples, filtered signal). A single hole injector is applied with one nozzle hole with a length of 800 \(\upmu\)m, a diameter of 120 \(\upmu\)m, a conicity factor of 2 (slightly converging form) and a slightly rounded inlet corner of the nozzle hole with \(r_{\text{inlet}}\) = 20 \(\upmu\)m. Standard diesel fuel is injected for 2000 \(\upmu\)s. Measurement examples are shown in Figure 11b for an early stage during the injection with \(t_{\text{ASOE}}\) = 0.4 ms (\(t_{\text{ASOE}}\) = time after start of energizing of the injector solenoid), and for the quasi-steady state of the injection with \(t_{\text{ASOE}}\) = 1.2 ms for three different injection pressures between 40 and 200 MPa. Shown are exemplary single shot images. It should be noted that the images had to be filtered, as the focusing approach of the ROCM technique leads to background scatter [29]. Clearly seen is the evaluation of the spray, being shorter during the early injection phase. For the increased pressure cases, the spray length decreases, which is in accordance with the expectation. From a statistical evaluation, it was found that the effect of increased injection pressure leads clearly to decreased dense core length, with measured values of 940 \(\upmu\)m, 800 \(\upmu\)m and 740 \(\upmu\)m for injection pressures of 50, 100 and 150 MPa (for details, see [29]).