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Title:Simulation Of Gas Focused Liquid Jets
Authors:ID Zahoor, Rizwan (Copyright holder)
ID Zahoor, Rizwan, University of Nova Gorica (Author)
ID Šarler, Božidar, Laboratory for Fluid Dynamics and Thermodynamics, Faculty of Mechanical Engineering, University of Ljubljana; Laboratory for Simulation of Materials and Processes, Institute of Metals and Technology (Mentor) More about this mentor... New window
Files:.pdf Thesis-Rizwan_2018.pdf (11,47 MB)
MD5: 1880FF32A351E8E6745574A30EEED12F
 
Language:English
Work type:Not categorized
Typology:2.08 - Doctoral Dissertation
Organization:UNG - University of Nova Gorica
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
Place of publishing:Nova Gorica
Publisher:University of Nova Gorica
Year of publishing:2018
Number of pages:194
PID:20.500.12556/RUNG-3881-2059b0ec-a5b6-374e-dca7-68c90aadd52e New window
COBISS.SI-ID:5131003 New window
NUK URN:URN:SI:UNG:REP:DTG5VYEK
Publication date in RUNG:27.03.2018
Views:7773
Downloads:181
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Secondary language

Language:Slovenian
Title:Simulacija plinsko fokusiranih kapljevitih curkov Povzetek
Abstract:Glavni namen disertacije je razviti eksperimentalno verifician model računalniške dinamike tekočin (RDT) mikronskega kapljevitega curka, proizvedenega s pomočjo brizgane plinsko dinamične virtualne šobe (PDVŠ). V takšnih šobah kapljeviti curki učinkovito usmerjeno prenašajo maso in gibalno količino. Proizvedeni so s pomočjo inteligentnega projiciranja hidrodinamskega osredotočnega efekta sotočnega plina z veliko hitrostjo in nižjo viskoznostjo in gostoto na tok kapljevine iz dovodne kapilare. Kapljeviti mikro-curki se uporabljajo za dostavo kristalnih proteinskih vzorcev v žarek trde rentgenske svetlobe v serijskih femtosekundnih kristalografskih eksperimentih. Tu so uklonjeni rentgenski žarki na proteinskih kristalih v mikro curku posneti tik preden pride do njihovega uničenja. Vzorci so dragoceni in jih je težko izdelati, zato je potrebno temeljito poznavanje delovanja mikro curkov, ki so uporabljeni za njihovo dostavo. Karakteristike curka so analizirane kot funkcije procesnih parametrov, geometrije ter snovnih lastnosti. Fizikalni model je opisan s formulacijo mešanice in Navier-Stokesovimi enačbami za Newtonske, dvo-fazne, neustaljene, stisljive tokove. Problem večfaznega toka je rešen z metodo končnih volumnov (MKV), kjer je stik med tekočinama opisan z metodo prostornin tekočin (MPT). Implementacija MKV-MPT RDT modela je na voljo v prostokodnih programih OpenFOAM in Gerris. Pravilno delovanje programov je potrjeno s serijo standardnih testnih primerov za advekcijski in večfazni tok. Oba programa sta med seboj primerjana glede na možnosti pravilnega popisa PDVŠ. Zaradi stisljive narave usmerjevalnega plina je bil za simulacije PDVŠ izbran program OpenFOAM, ker Gerris nima te možnosti. Konstante efektivne snovne lastnosti so uporabljene v fazah, skupaj z idealnim plinskim zakonom. Model mešanice dvofaznega sistema je rešen v osni simetriji. Diskretizacija šobe in komore zahteva približno 300 000 končnih volumnov. Mrežno neodvisni rezultati so doseženi s kontrolnimi volumni velikost 0,25 µm v bližini curka in kapljic. Rezultati simulacij so primerjani z eksperimentalnimi rezultati glede na debelino in dolžino curka destilirane vode, usmerjenega s helijem, ki se stekata v nizkotlačno komoro pod pritiskom 150 Pa. Reynoldsovo število kapljevine je v območju 418-3828, plina pa 17-1222, medtem ko se Webrovo število giblje v območju 3-353. Opaženo je dokaj dobro ujemanje z eksperimentalnimi podatki ter velikostno analizo za območje nikoli prej obravnavanih parametrov delovanja šob. Nato je bila izvedena numerična študija vpliva različnih geometrij šobe na stabilnost, obliko in karakteristiko toka mikronskega kapljevitega curka. Značilnosti curka so opisane kot funkcija (i) razdalje med kapilaro in odprtino, (ii) premera izhodne odprtine šobe, (iii) kota dovodne kapilare kapljevine. Opravljena je bila raziskava za dve različni vrednosti volumskega pretoka tekočine, pri čemer je masni pretok plina ostal nespremenjen. Opaženo je bilo, da za vsako vrednost razdalje od kapilare do odprtine šobe ter za vsako vrednost premera odprtine šobe, obstaja minimalna vrednost volumskega pretoka kapljevine, izpod katere ni mogoče doseči stabilnega curka. Ugotovitve kažejo, da spremembe premera izhodne odprtine šobe najbolj vplivajo na premer, dolžino in hitrost curka, medtem ko sprememba kota dovodne kapilare nima opaznega vpliva na značilnosti curka. Sprememba dolžine razdalje med dovodno kapilaro ter izhodno odprtino šobe ne vpliva na tokovno polje okoli mirko-curka, zatorej sta stabilnost in oblika odvisni le od interakcije med tekočino in plinom v bližini meniskusa. Enak numerični model je uporabljem za dodatno analizo delovanja curka zaradi vpliva usmerjevalnih plinov argon, ogljikov dioksid in dušik. Študija kaže, da lahko z uporabo helijevega plina dosežemo dvojno dolžino curka v primerjavi s preostalimi plini, seveda pri istem masnem pretoku. Curek usmerjen s helijem je tudi tanjši, hitrejši in zanimivo, ne kaže znatnega padca temperature na izhodu iz šobe. To delo prvič obravnava računalniški model brizganih mikronskih šob in podaja pomembne informacije za njihovo nadaljnje projektiranje.
Keywords:Mikrofluidika, plinska dinamična virtualna šoba, usmerjanje toka, mikro-curek, konvekcijska nestabilnost, absolutna nestabilnost, stisljivi večfazni tokovi, kapljanje, pljuvanje, brizganje, debelina curka, dolžina curka, računalniška dinamika tekočin, metoda končnih volumnov, metoda prostornine tekočine


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