Gas Dynamic Virtual Nozzles: computational fluid dynamics and optimization

Giulio Scrimali (University of Trieste)

CHALLENGE - The device, named Gas Dynamic Virtual Nozzle, is mounted on the in-vacuum experimental chamber and uses a coaxial gas stream to focus a liquid flow exiting from a micrometric capillary. Both the gas and the liquid are conveyed through a micrometric sized aperture: the dynamic forces of the gas together with the liquid superficial tension create an elongated meniscus at the liquid channel outlet, at the point of which a liquid jet is emitted, with a diameter smaller than the liquid capillary diameter. The study focused on the numerical fluid dynamics modeling of the device.

SOLUTION - With some geometrical and theoretical simplifications, the solution of the problem was performed with a two-dimensional, axisymmetric, steady-state model. The fluid dynamics calculations were performed with the proprietary CFD suite of ANSYS INC; the effective calculations were done with a specialized bi-dimensional solver of the FLUENT software. It was shown that the steady state model is able to describe the liquid focussing phenomenon. Once the model was proved working correctly, it was used in an optimization process to find the optimal liquid and gas inlet parameters to inject efficiently samples in the case of experiments with FEL light. 

BENEFITS - The optimization process was performed with the specialized software modeFRONTIER, by ESTECO. Experimental measurements were carried out to obtain real data to compare with the numerical simulations results. Further analysis revealed a difference in the expected pressure level in the experimental measurement, induced by the difference in the geometry. Finally, a tridimensional analysis was performed to validate the generality of the results. Some advice to better the device design were derived from the results found, to be used in the next iteration of the designing process. modeFRONTIER capabilities were used through the whole project, from data analysis to job distribution using the GRID Manager, and helped to secure results in short times.