Validation of a computational model for a coupled liquid and gas flow in micro-nozzles

The work presents verification of a numerical model for micro-jet focusing, where a coupled liquid and gas flow occurs in a gas dynamic virtual nozzle (GDVN). Nozzles of this type are used in serial femtosecond crystallography experiments to deliver samples into X-ray beam. The following performance criteria are desirable: the jet to be longer than 100 µm to avoid nozzle shadowing, the diameter as small as possible to minimize the background signal, and the jet velocity as high as possible to avoid sample’s double X-ray exposure. Previous comprehensive numerical investigation has been extended to include numerical analysis of the tip jet velocities. These simulations were then compared with the experimental data. The coupled numerical model of a 3D printed GDVN considers a laminar two-phase, Newtonian, compressible flow, which is solved based on the finite volume method discretization and interface tracking with volume of fluid (VOF). The numerical solution is calculated with OpenFOAM based compressible interFoam solver, which uses algebraic formulation of VOF. In experimental setup for model validation a 3D printed GDVN was inserted in a vacuum chamber with two windows used for illumination and visualization. Once the jet was stabilized its velocity was estimated based on a distance a droplet traveled between two consecutive illumination pulses with a known time delay. The experimental and computational study was performed for a constant liquid flow rate of 14 l/min and the gas mass flow rate in the range from 5 mg/min to 25 mg/min. The coupled numerical model reasonably predicts the jet speed and shape as a function of the gas flow