Selective laser-induced etching enabled study on geometry and surface structure effects on liquid ammonia flash-boiling in micro-scale nozzles

This study examines how nozzle geometry and surface structure govern flash-boiling of liquid ammonia for steady state injection conditions for a 100 µm nozzle diameter. Conventionally manufactured stainless-steel nozzles are compared with optically transparent fused silica glass nozzles fabricated by Selective Laser-induced Etching (SLE) using nominally identical CAD geometries and length-to-diameter ratios 𝐿∕𝐷 = 2.5, 5, and 10. The manufactured internal geometries are quantified by µCT and optical microscopy, which reveal nanoscale surface features most likely relevant to heterogeneous nucleation. High-speed shadowgraphy across injection pressures ranging from 0.86MPa to 1.45MPa and superheating degrees of 𝑅 = 3 to 12 provides visualization of internal two-phase flow and external spray morphology as a function of boundary conditions. Steel nozzles exhibit substantial deviations from nominal geometry and pronounced inlet roughness, leading to asymmetric sprays and significant nozzle-to-nozzle variability. SLE nozzles reproduce the target geometry and generate axisymmetric sprays with reduced dispersion in cone-angle, but somewhat attenuated flashing, consistent with smoother internal surfaces and more controlled nucleation. Transparent SLE nozzles reveal previously inaccessible internal phenomena, including (i) pulsating atomization driven by pre-existing bubbles in the feed system and (ii) cavitation-enhanced flash-boiling via thin vapor ligaments and Rayleigh-type bubble chains in the orifice. The results demonstrate that flash-boiling of liquid ammonia is controlled by a tightly coupled interplay of superheat, geometry, surface structure, and manufacturing route. This establishes SLE-fabricated fused silica nozzles as a platform for mechanistic studies and design of flash-assisted LNH3 injection systems for low-emission ammonia combustion.