Simulation Of Gas Focused Liquid JetsRizwan Zahoor
, 2018, doctoral dissertation
Abstract: The main aim of dissertation is to develop an experimentally verified computational fluid dynamic (CFD) model of micron-sized liquid jet, produced by an injection molded Gas Dynamic Virtual Nozzle (GDVN). In these nozzles, liquid jets are efficiently orientedly transporting mass and momentum. They are produced by intelligently projecting hydrodynamic focusing effect from a high-speed stream of a co-flowing lower density and lower viscosity gas on a stream of liquid from a feeding capillary. Liquid micro-jets are used for delivery of protein crystal samples in a hard X-ray beam in serial femtosecond crystallography experiments. The diffraction patterns of crystals are collected just before their destruction. The samples are hard to crystallize and very precious, so a thorough knowledge of the jet used in delivering them is required. The jet characteristics are analyzed as a function of operating parameters, geometry and material properties.
The physical model is described by mixture formulation and Navier-Stokes equations for transient, Newtonian, two-phase, compressible flow. Multiphase flow problem is solved with finite volume method (FVM), where fluid-fluid interface tracking is obtained with volume of fluid (VOF). The implementation of FVM-VOF CFD model is available in open source codes OpenFOAM and Gerris. They are validated by performing a series of standard interface advection and multiphase flow test cases. Both open source codes are compared for their abilities in solving GDVN flow problem. Due to the compressible nature of the focusing gas flow, OpenFOAM was chosen for GDVN simulations, since Gerris has no compressible flow option.
Constant effective material properties are used in the phases together with ideal gas density constitutive relation. A mixture model of the two-phase system is solved in axisymmetry. The discretization of the nozzle and chamber system uses approximately 300 000 finite volumes. Mesh independent results are obtained with the finite volumes of the size 0.25 µm in the vicinity of the jet and drops. The simulations are compared with experimental results according to the jet thickness and length for distilled water jet and helium focusing gas, discharging into low-pressure environment of 150 Pa. Reynolds numbers of the liquid and gas are in the range 413-3828 and 17-1222, respectively and Weber number in the range 3-353. A reasonably good agreement with experimental and scaling results is found for the range of nozzle operating parameters never tackled before.
Subsequently, a numerical study of effects of nozzle geometry on stability, shape and flow characteristics of micron-sized liquid jets is performed. The jet characteristics are described as a function of (i) capillary-to-orifice distance, (ii) nozzle outlet orifice diameter and (iii) liquid feeding capillary angle. The study is performed for two sets of liquid flow rates while keeping the gas flow rate unchanged. It is observed that for each value of capillary-to-orifice distance and nozzle outlet diameter, there exists a minimum liquid flow rate below which the jet stability cannot be achieved. It is found that the changes in the nozzle outlet diameter have the biggest influence on the jet diameter, length and velocity, while the liquid capillary angle has no observable effect on the jet characteristic. Change in capillary-to-orifice distance does not affect the flow field around micro jet, so the jet stability and shape is found to be affected by the way liquid-gas interacts near meniscus.
The same numerical model is used to additionally analyze the jet performance under the influence of Argon, Carbon dioxide and Nitrogen focusing gases. The study shows that the helium gas at the same mass flow rate provides twice the length of the jet compared to other gases. The jet focused with helium is also much thinner, faster and interestingly shows no considerable temperature drop at the nozzle outlet.
This work for the first time discuss the computational model of an injection molded micron-sized nozzle and produces valuable information for their design.
Keywords: Microfluidics, gas dynamic virtual nozzle, flow focusing, micro-jet, convective instability, absolute instability, compressible multiphase flows, dripping, spurting, jetting, jet thickness, jet length, computational fluid dynamics, finite volume method, volume of fluid method
Published in RUNG: 27.03.2018; Views: 5601; Downloads: 166
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