Mikrokanallarda basınç düşümü, cidar kayma gerilmesi ve partikül fokuslanmasının sayısal olarak incelenmesi
Numerical investigation of pressure drop, shear stress and particle focusing in microchannels
Yazar: UFUK DEMİR
Danışman: PROF. DR. HASAN GÜNEŞ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Bu tez çalışmasının amaçlarından ilki, endotel ve immun sistem hücrelerinin farklı fiziksel koşullar altında sergiledikleri davranışları inceleyebilmek için üretilmesi amaçlanan bir mikroakışkan sistemin, üretim aşamasından önce tasarımının yapılarak basınç farklılıkları ve cidar kayma gerilmesi profillerinin Hesaplamalı Akışkanlar Dinamiği (HAD) analizlerinin yapılarak incelenmesidir.
Çalışmanın ikinci amacı ise, hücre/partikül zenginleştirme/ayrıştırma işleminin bir önceki basamağı olarak görülen partikül fokuslanma işlemi üzerinde farklı debi değerleri ve partikül boyutları kullanılarak Hesaplamalı Akışkanlar Dinamiği (HAD) analizlerinin yapılarak incelenmesidir. Bu çalışmanın ilk kısmında elde edilen sayısal sonuçlar, literatürde yer alan deneysel çalışmalar ile karşılaştırılmıştır.
Her iki çalışma içinde sayısal analizler Ansys-Fluent yazılımı kullanılarak gerçekleştirilmiştir. Analizlerde akışkan olarak su (water-liquid) kullanılmıştır.
Tez çalışmasının ilk kısmında analizler iki ve üç boyutlu olarak yapılmıştır. İki boyutlu analizlerde üç farklı mikrokanal geometrisi üzerinde incelemeler yapılırken üç boyutlu analizlerde beş farklı mikrokanal modeli kullanılmıştır. Mikrokanal modelleri belirlenirken detaylı literatür araştırması ve belirlenen kanalın üretilebilirliği faktörleri göz önünde bulundurulmuştur.
Modellerin belirlenmesini müteakiben Ansys-Design Modeler yazılımı kullanılarak mikrokanal geometrileri oluşturulmuş daha sonra sayısal analizler için uygun çözüm ağları oluşturulmuştur. Çözümün ağdan bağımsız hale getirilmesi için farklı çözüm ağlarıyla birçok analiz yapılmıştır. Farklı hacimsel debiler ile birlikte farklı "aspect ratio" (en/boy) ve "expansion ratio" (genişleme oranı) değerleri kullanılarak sürekli rejim şartları altında HAD analizleri gerçekleştirilerek akışkanın mikrokanal giriş ve çıkışı arasındaki basınç farkı değerleri ile cidar kayma gerilmesi değerleri elde edilmiştir.
Ayrıca düz mikrokanal geometrisi için analitik çözüm yapılmış ve sayısal olarak elde edilen basınç farkı ve cidar kayma gerilmesi değerleri analitik sonuçlarla karşılaştırılmıştır.
Ikinci kısımda simetrik kıvrımlı mikrokanal geometrisinde 150 μl/dak. debi değerinden başlanarak 1100 μl/dak. değerine kadar olan hacimsel debi değerlerinde partiküllerin davranışları incelenmiştir. Bu kapsamda 60 adet yarım daireden oluşan simetrik kıvrımlı mikrokanal geometrisi oluşturulmuştur. HAD analizleri için Ansys Fluent yazılımında ayrık faz modeli (DPM-Discrete Phase Model) kullanılmıştır.
Bu bölümde ilk olarak 10 μm çapındaki partiküllerin çeşitli hacimsel debi değerlerindeki davranışları ve fokuslanma çizgileri sayısal olarak incelenmiş, elde edilen sonuçlar deneysel verilerle karşılaştırılmıştır.
Daha sonra 5, 7 ve 10 μm çaplarındaki partiküller kullanılarak partikül çapının fokuslanma üzerindeki etkisi sayısal olarak incelenmiştir. Bu bölümde partikül fokuslanması üzerinde etkin rol oynayan birçok parametre ile ilgili teorik bilgide verilmiştir. Yapılan deneysel çalışmalar sonucunda fokuslanmanın sağlanmasına yönelik elde edilen bazı bulgular sayısal olarak doğrulanmıştır.

Abstract:
In this thesis, numerical investigation of microchannel flows at various geometries has been performed in order to determine the optimum channel geometry for a possible microfluidic device. In addition three-dimesional numerical investigation of particle laden flow in a microchannel was performed using a finite volume based flow solver, Ansys-Fluent.
First aim of this study was design of a microfluidic device which is specific for observing biological responses and behaviours of endothelial and immune system cells which play a crucial role in comprise of atherosclerosis plaques, under different shear stress and pressure drop by changing flow rate.
The second aim was numerical investigation of particle focusing in symmetric curved microchannels. The addition of curvature to a straight microchannel allows an secondary flow known as a Dean flow. That's why curved microchannels were used commonly for passive particle seperation. The effects of flow rate and particle diameter in focusing were investigated in this part.
Several microchannel geometries were created after an extensive literature review. All of the microchannel geometries were designed in Ansys-Design Modeler. To obtain reliable numerical results, intensive mesh independence tests have been performed for all microchannel geometries. The pressure difference between the inlet and outlet boundaries of the microchannel was obtained and compared with each other. Water-liquid was used as fluid in all of the simulations. The flow was assumed to be steady, incompressible, laminar and fluid had constant thermo-physical properties.
For the first part of the study, early two dimesional three different geometries were created, namely straight channel, symetric curved channel and straight channel with one side cavity. Then three dimensional five different geometries were created which are straight microchannel, microchannel with ER, symetric curved microchannel, sharply curved microchannel and straight microchannel with one side expansion respectively.
Momentum equations were solved for both two dimensional and three dimensional straight microchannels analytically. Thus pressure drop and shear stress values obtained both numerically and analytically for this model. The boundary conditions were velocity inlet for 2D, mass flow rate for 3D at the inlet, zero gage pressure at the outlet and no-slip conditions on all walls. The solver algorithm was selected SIMPLE (Semi-Implicit-Pressure-Linked-Equations). The continuity and momentum equations have been discretized using second order upwind schemes.
In order to reveal the effects of channel geometry over pressure drop and shear stress,
different aspect ratios (A: 1,2,3) were used in straight microchannels and different
expansion ratios (E: 3,5,7) were used in microcahannel with ER.
After the analyses, due to the frictional forces cause that the pressure drops, low
pressure region obtained at outlet of the channel when high pressure region obtained
at the inlet.
On the other hand a linear relation obtained between wall shear stress and flow rate
for straight microchannel, channel with ER and microchannel with one side
expansion. For sharply curved microchannel, pressure drop got higher values than
the other geometry types which had same total length due to the 900 bends. In
microchannel with ER and microchannel with one side expansion, two different wall
shear stress profile obtained. It is considered that cells behaviour will be different in
the wide and narrow regions for both microchannels.
For the second part of the study, symetric curved microchannel which formed 60
semicircles was created. Each of the semicircles had 350 μm width and 50 μm
height. In order to achieve mesh independency in the analyses, several numerical
grids were generated with different number of mesh elements.
The boundary conditions were mass flow rate at the inlet, zero gage pressure at the
outlet and no-slip conditions on all walls. The boundary conditions for particles were
escape at the inlet-outlet and reflect condition on all walls. The solver algorithm was
selected SIMPLE (Semi-Implicit-Pressure-Linked-Equations). Two-phase flow has
been analyzed using Discrete Phase Model (DPM) approximation. The continuity
and momentum equations have been discretized using second order upwind schemes.
Second part of the thesis consisted of two main sections, first one effects of different
Reynolds number and the second one effects of different particle size (also different
particle confinement ratio) were investigated numerically.
In order to reveal the effects of Reynolds number in particle focusing several
analyses performed with 10 μm particle and flow rates ranging from 100 to 1200
μl/min..
After the results were examined, dean drag force was found to be inefficient at low
flow rates so inertial lift force dominated the flow ( L D F  F ) and particles started to
move through center to the channel wall. With increased flow rate, Dean drag force
began to emerge and particles started to move their equilibrium positions. In a
specific flow rate range two focusing line obtained but at a certain flow rate single
focusing line achieved ( 1 f R ).
Streak behaviour at high Uavg was investigated also in this section. At high flow rates
Dean drag force got higher values than inertial lift force and caused that the particles
loss the balance and breakdown of focusing occured finally. Experimental results and
numerical results for particle positions in the microchannel were compared each
other on the graphic at the end of this section.
In order to reveal the effects of particle size and particle confinement ratio in particle
focusing several analyses performed with 5, 7 and 10 μm particle and flow rates
ranging from 150 to 1100 μl/min..
After the results were examined, at low flow rates it can be seen that all three
particles exhibit close behaviours. With increased flow rates particles began to move
their own equilibrium positions which were close to the channel walls and finally formed two focusing lines.
Single focus line was formed initially by 10 μm particles and after 7 μm particles. Single focus line could not be obtained for 5 μm particles and breakdown of focusing could not formed for 7 μm particles.
Consequently, some of the findings about particle focusing which were obtained in many experimental studies, were verified with this numerical study.