A novel transparent direct injection nozzle concept is used to simultaneously visualize nozzle-internal flows and external primary breakup with transmitted light microscopy. Fused silica enables manufacturing of precise 3D internal nozzle geometries with a few micrometer accuracy based on the selective laser etching. To avoid interaction of the metal injector needle with brittle fused silica, a functional separation between nozzle body and nozzle tip is realized. That way, original nozzle body withstands occurring forces (injection pressure, needle forces) while the transparent nozzle tip provides optical access.
Simultaneous visualization of nozzle-internal flows and external breakup is obtained by combining an optimized outer nozzle geometry with a compensating glass that equalizes the optical path length between image plane of the microscope and the object plane (inside and outside the nozzle).

Recorded images show different internal nozzle phenomena, as for example: i) local liquid-gas interfaces (cavitation structures and entrained air bubbles) indicated by low intensity regions, ii) microscopic flow phenomena (local temperature fluctuations) indicated by schlieren structures, iii) the temporal variation of fuel temperature (presumably by cavitation at needle seat), indicated by the relative refractive index variation between fuel and fused silica. In addition, the relevance of those nozzle internal phenomena for subsequent jet breakup is discussed. For instance jet surface waves that are visible during injection start are connected to boundary-layer instabilities, which are subsequently disturbed by vortex shedding inside the nozzle hole.