Numerical simulations of gas-focused micro jets

2017

Stable and reliable micro jets are important for many applications. Double flow focused micro jets are a novelty with an important advantage of significantly reduced sample consumption. Numerical simulations of double flow focused micro jets are a highly complex task. They represents a great computational challenge due to the multiphase nature of the problem, strong coupling between the gas and the two liquids and the sub-micron size cells needed. Simulations were performed with the open source computational fluid dynamics toolbox called OpenFOAM. Two multiphase solvers were used, one of which was modified in order to properly describe the interface between the focusing liquid and the gas. In this study two different incompressible physical models were considered and compared. A model with no mixing of the two fluids (multiphaseInterFoam solver) and a model where the diffusion of the two fluids is permitted (modified interMixingFoam solver). The results of simulations for the two di...

International Journal of Hydromechatronics, 2018

Liquid micro-jets, produced from gas dynamic virtual nozzles (GDVNs), are used as sample carriers for interaction with X-ray beam in serial femtosecond crystallography (SFX). A numerical investigation of the effect of the focusing gas type on the liquid micro-jet properties (its length and thickness) is presented. The study complements our previous research on the influence of operating conditions and the nozzle geometry on GDVN performance. The influence of helium, argon, carbon dioxide and nitrogen gases (at a fixed mass flow rate of 1.6 × 10 4 mg/min) on focusing pure water jet (flow rate of 33 µl/min) is analysed. An experimentally validated numerical model, based on laminar two-phase Newtonian compressible flow with ideal gas assumption, finite volume method and volume of fluid interface tracking, is used. Helium is found to be the most suitable gas among the tested ones for producing thin, long and fast jets. The study provides a basis for the focusing gas selection in SFX experiments.

2019

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). Nozzlesof this type are usedinserial femtosecond crystallographyexperimentsto deliver samplesintoX-ray beam. Thefollowing performance criteria are desirable: the jet to be longer than 100 μm to avoid nozzle shadowing, the diameter as small as possibleto minimize the background signal,and the jet velocityas high as possible to avoid sample'sdouble X-ray exposure.Previouscomprehensive numerical investigation has been extended to includenumerical analysis of the tip jet velocities. These simulations were then comparedwith 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 com...

Microfluidics and Nanofluidics, 2018

We present the development of an experimentally validated computational fluid dynamics model for liquid micro jets. Such jets are produced by focusing hydrodynamic momentum from a co-flowing sheath of gas on a liquid stream in a nozzle. The numerical model based on laminar two-phase, Newtonian, compressible Navier-Stokes equations is solved with finite volume method, where the phase interface is treated by the volume of fluid approach. A mixture model of the two-phase system is solved in axisymmetry using ~ 300,000 finite volumes, while ensuring mesh independence with the finite volumes of the size 0.25 µm in the vicinity of the jet and drops. The numerical model is evaluated by comparing jet diameters and jet lengths obtained experimentally and from scaling analysis. They are not affected by the strong temperature and viscosity changes in the focusing gas while expanding at nozzle outlet. A range of gas and liquid-operating parameters is investigated numerically to understand their influence on the jet performance. The study is performed for gas and liquid Reynolds numbers in the range 17-1222 and 110-215, and Weber numbers in the range 3-320, respectively. A reasonably good agreement between experimental and scaling results is found for the range of operating parameters never tackled before. This study provides a basis for further computational designs as well as adjustments of the operating conditions for specific liquids and gases.

Materials

The purpose of this work is to determine, based on the computational model, whether a mixture of a binary liquid is capable of producing longer, thinner and faster gas-focused micro-jets, compared to the mono-constituent liquids of its components. Mixtures of water with two different alcohols, water + ethanol and water + 2-propanol, are considered. The numerical study of pre-mixed liquids is performed in the double flow focusing nozzle geometry used in sample delivery in serial femtosecond crystallography experiments. The study reveals that an optimal mixture for maximizing the jet length exists both in a water + ethanol and in a water + 2-propanol system. Additionally, the use of 2-propanol instead of ethanol results in a 34% jet length increase, while the jet diameters and velocities are similar for both mixtures. Pure ethanol and pure 2-propanol are the optimum liquids to achieve the smallest diameter and the fastest jets. However, the overall aim is to find a mixture with the lo...

Materials, 2021

Liquid micro-jets are crucial for sample delivery of protein crystals and other macromolecular samples in serial femtosecond crystallography. When combined with MHz repetition rate sources, such as the European X-ray free-electron laser (EuXFEL) facility, it is important that the diffraction patterns are collected before the samples are damaged. This requires extremely thin and very fast jets. In this paper we first explore numerically the influence of different nozzle orifice designs on jet parameters and finally compare our simulations with the experimental data obtained for one particular design. A gas dynamic virtual nozzle (GDVN) model, based on a mixture formulation of Newtonian, compressible, two-phase flow, is numerically solved with the finite volume method and volume of fluid approach to deal with the moving boundary between the gas and liquid phases. The goal is to maximize the jet velocity and its length while minimizing the jet thickness. The design studies incorporate ...

International Journal of Multiphase Flow, 2019

Particle-laden flows in plane, axisymmetric and 3D supersonic micronozzles are investigated numerically using a one-way coupled Eulerian/Lagrangian approach. The carrier gas flow is calculated by solving the Navier-Stokes equations. Rarefaction effects are taken into account by imposing the velocity slip and temperature jump boundary conditions on the nozzle walls. The parameters of the flow around particles are varied in a wide range including hydrodynamic, transitional and free-molecular regimes. It is shown that a collimated beam of particles can be produced using the effect of aerodynamic focusing due to converging flow streamlines in the subsonic part of the nozzle. The collimation is preserved in the supersonic part where the flow is divergent because the rapid drop in the gas density decreases significantly the force acting on the particle. An interesting and unexpected feature of aerodynamic focusing is that the beam collimation is observed in two different ranges of particle sizes. In the first range, for relatively large particles, the collimated beam consists only of particles seeded close to the nozzle axis. In the second range, for smaller particles, the beam includes also a great portion of peripheral particles. The numerical simulation also shows that aerodynamic focusing in a supersonic, convergent-divergent, nozzle enables one to increase significantly the velocity of the collimated beams compared to previously reported results for convergent subsonic nozzles. It may be helpful for technological applications where the aerodynamic scheme of particle focusing can be used (microthrusters, needle-free drug injection, microfabrication, etc.).

Modelling and Simulation in Engineering, 2011

A synthetic jet results from periodic oscillations of a membrane in a cavity. Jet is formed when fluid is alternately sucked into and ejected from a small cavity by the motion of membrane bounding the cavity. A novel moving mesh algorithm to simulate the formation of jet is presented. The governing equations are transformed into the curvilinear coordinate system in which the grid velocities evaluated are then fed into the computation of the flow in the cavity domain thus allowing the conservation equations of mass and momentum to be solved within the stationary computational domain. Numerical solution generated using this moving mesh approach is compared with an experimental result measuring the instantaneous velocity fields obtained by μPIV measurements in the vicinity of synthetic jet orifice 241 μm in diameter issuing into confined geometry. Comparisons between experimental and numerical results on the streamwise component of velocity profiles at the orifice exit and along the centerline of the pulsating jet in microchannel as well as the location of vortex core indicate that there is good agreement, thereby demonstrating that the moving mesh algorithm developed is valid.

International Journal of Multiphase Flow, 2005

The instability of a focused liquid jet is studied by semi-analytical methods and by methods of computational fluid dynamics. The semi-analytical approach relies on earlier work on the instability of an extending liquid thread and is based on the Stokes flow regime and small-amplitude perturbations. The evolution of different excitation modes is evaluated and compared. Through hydrodynamic focusing and the

Progress in Energy and Combustion Science, 2010

This review attempts to summarize the physical models and advanced methods used in computational studies of gasÀliquid two-phase jet flows encountered in atomization and spray processes. In traditional computational fluid dynamics (CFD) based on Reynolds-averaged NavierÀStokes (RANS) approach, physical modelling of atomization and sprays is an essential part of the two-phase flow computation. In more advanced CFD such as direct numerical simulation (DNS) and large-eddy simulation (LES), physical modelling of atomization and sprays is still inevitable. For multiphase flows, there is no model-free DNS since the interactions between different phases need to be modelled. DNS of multiphase flows based on the one-fluid formalism coupled with interface tracking algorithms seems to be a promising way forward, due to the advantageous lower costs compared with a multi-fluid approach. In LES of gasÀliquid two-phase jet flows, subgrid-scale (SGS) models for complex multiphase flows are very immature. There is a lack of well-established SGS models to account for the interactions between the different phases. In this paper, physical modelling of atomization and sprays in the context of CFD is reviewed with modelling assumptions and limitations discussed. In addition, numerical methods used in advanced CFD of atomization and sprays are discussed, including high-order numerical schemes. Other relevant issues of modelling and simulation of atomization and sprays such as nozzle internal flow, dense spray, and multiscale modelling are also briefly reviewed.