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

Fuel, vol. 430, pp. 141107

Abstract

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.

Authors 12

1. Peter W. Augustin corresponding Aachen Institute of Heat and Mass Transfer

   RWTH Aachen University

   Affiliation as printed

   Institute of Heat and Mass Transfer, RWTH Aachen University, Augustinerbach 6, Aachen, 52062, Germany

2. Marvin Alt Aachen Institute of Heat and Mass Transfer

   RWTH Aachen University

   Affiliation as printed

   Institute of Heat and Mass Transfer, RWTH Aachen University, Augustinerbach 6, Aachen, 52062, Germany

3. Lukas Heine Aachen Institute of Heat and Mass Transfer

   RWTH Aachen University

   Affiliation as printed

   Institute of Heat and Mass Transfer, RWTH Aachen University, Augustinerbach 6, Aachen, 52062, Germany

4. Reinhold Kneer Aachen Institute of Heat and Mass Transfer

   RWTH Aachen University

   Affiliation as printed

   Institute of Heat and Mass Transfer, RWTH Aachen University, Augustinerbach 6, Aachen, 52062, Germany

5. Wilko Rohlfs Aachen Institute of Heat and Mass Transfer

   RWTH Aachen University

   Affiliation as printed

   Institute of Heat and Mass Transfer, RWTH Aachen University, Augustinerbach 6, Aachen, 52062, Germany

6. Fabian Fröde Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology, RWTH Aachen University, Templergraben 64, Aachen, 52062, Germany

7. Heinz G. Pitsch Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology, RWTH Aachen University, Templergraben 64, Aachen, 52062, Germany

8. Henry Axt Aachen Chair for Laser Technology

   RWTH Aachen University

   Affiliation as printed

   Chair for Laser Technology, RWTH Aachen University, Campus-Boulevard 73, Aachen, 52074, Germany

9. Christian Hinke Aachen Chair for Laser Technology

   RWTH Aachen University

   Affiliation as printed

   Chair for Laser Technology, RWTH Aachen University, Campus-Boulevard 73, Aachen, 52074, Germany

10. Martin Kratz Aachen Fraunhofer Institute for Laser Technology ILT Chair for Laser Technology

    RWTH Aachen University · Fraunhofer Institute for Laser Technology

    Affiliation as printed

    Chair for Laser Technology, RWTH Aachen University, Campus-Boulevard 73, Aachen, 52074, Germany

    Fraunhofer Institute for Laser Technology ILT, Steinbachstr. 15, Aachen, 52074, Germany

11. Constantin Häfner Aachen

    RWTH Aachen University · Fraunhofer-Gesellschaft

    Affiliation as printed

    Fraunhofer-Gesellschaft e.V., Hansastraße 27 c, München, 80686, Germany

    RWTH Aachen University, Templergraben 55, Aachen, 52062, Germany

12. Manuel Armin Reddemann corresponding Aachen Institute of Heat and Mass Transfer

    RWTH Aachen University

    Affiliation as printed

    Institute of Heat and Mass Transfer, RWTH Aachen University, Augustinerbach 6, Aachen, 52062, Germany