# Effect of Fuel Mass Flow at the End of Injection on Cavitation and Gas Ingestion in the Nozzle
## 2 Experiment  
The visualization experimental platform built for this measurement consisted of three parts--fuel delivery, signal control, and optical imaging system--and it is shown in Figure 1.  
The fuel delivery system (Taian) included two parts: a DB2000-75 high-pressure oil pump bench and a Delphi Multec DCR 1400 high-pressure common-rail fuel pipe. The stable injection conditions were maintained by the fuel delivery system under various working conditions and provided an appropriate fuel flux for the injector. This ensured the diesel engine to operate stably and enabled the injector reliable work.  
Visu98 signal control system was used to control fuel mass flux and injection pressure. It included a pressure sensor which was mounted on the injection system. The revolution speed should not be too high to avoid excessive injection pressure. Considering the bearing capacity of acrylic material, the excessive injection pressure would cause the transparent nozzle, therefore the revolution speed was set to 250 rpm. For the same reason, 30 MPa was selected as the injection pressure in this experiment.  
High-magnification cavitation images were captured by the optical image imaging system, and it consisted of an image acquisition system and a lighting system. The image acquisition system was mainly composed of a long focal distance microscope (QM-100, QUESTAR), a high-speed framing camera (VRI-VEO-710L), and a Phantom camera controller. The lighting system was a prerequisite for the success of the experiment; it consisted of a LED cold-light source and was used for back-lighting technology in this paper. To freeze the flow field and obtain the highest-resolution image possible, the exposure time was set to 1 us, and the pixel resolution was 720 \(\times\) 480. Under this shooting condition, the frame rate could be as high as 20,000 frames per second, i.e., the time interval between each image is 50 us.  
Due to the refractive index of 0# diesel (\(n\) = 1.46-1.51) being close to that of the polymethyl methacrylate (PMMA) (\(n\) = 1.49) [40], the light can pass directly through the fuel and the nozzle without being refracted and appears as a transparent area in the images. Meanwhile, when the light illuminates the interface between the fuel and the cavitation, scattering occurs due to the difference in the refractive index between the two, thereby forming a black area in the images. Based on the above reasons, the spherical part of a real injector was removed, and an optically transparent nozzle made of PMMA (including a small pressure chamber and a straight hole) was used instead of the original nozzle; the diameter of the orifice was 0.3 mm, and the aspect ratio was 5. The experimental nozzle is shown in Figure 2.  
The Phantom camera controller (PCC) was adopted to obtain the fuel cavitation images efficiently. All the experimental equipment was controlled synchronously by the corresponding electronic equipment. In addition, the long focal distance microscope, the high-speed framing camera, and the LED light keep the same line. Because the trans  
Figure 1: The experimental visualization platform. 1. Oil tank; 2. High-pressure oil pump; 3. High-pressure common-rail pipe; 4. Pressure sensor; 5. Fuel injector; 6. Experimental nozzle; 7. Electronic control unit (ECU); 8. High-speed framing camera; 9. Long focal distance microscope; 10. LED light; 11. Computer.  
Figure 2: Experimental nozzle.  
parent nozzle is deformed when overheated, the temperature was kept at a low level in all experiments, and the nozzle was allowed to take a break every three fuel injection cycles. The optimal injection event was obtained after using five transparent nozzles.