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Simulations of Transport Phenomena and Porous Structures Using Dissipative Particle Dynamics

Johansson, Erik LU (2015)
Abstract (Swedish)
Popular Abstract in Swedish

Det här arbetet handlar om simuleringar av porösa material och transportfenomen vid en mesoskopisk längdskala, det vill säga en längdskala större än en enskild atom men mindre än att alla atomer beter sig som en kontinuerlig enhet. De porösa material som har studerats är membran som används i bränsleceller. Masstransport har även studerats i membranet. Värme- och masstransport har studerats i raka kanaler, först i kanaler helt fyllda med vätska och sedan även i kanaler med droppvis flöde. I bägge fallen har även stelning, det vill säga fasövergång från flytande till fast form, studerats.



Vad är då syftet med denna studie? Många av våra nya tekniker förlitar sig på fysikaliska... (More)
Popular Abstract in Swedish

Det här arbetet handlar om simuleringar av porösa material och transportfenomen vid en mesoskopisk längdskala, det vill säga en längdskala större än en enskild atom men mindre än att alla atomer beter sig som en kontinuerlig enhet. De porösa material som har studerats är membran som används i bränsleceller. Masstransport har även studerats i membranet. Värme- och masstransport har studerats i raka kanaler, först i kanaler helt fyllda med vätska och sedan även i kanaler med droppvis flöde. I bägge fallen har även stelning, det vill säga fasövergång från flytande till fast form, studerats.



Vad är då syftet med denna studie? Många av våra nya tekniker förlitar sig på fysikaliska processer som sker vid väldigt små längskalor. I en bränslecell transporteras protoner i porösa membran där porerna endast är några få nanometer i diameter. De kemiska reaktionerna i bränslecellen är beroende av att syre, protoner och elektroner reagerar med varandra på en aktiv yta av nanometerstorlek. I bägge fallen är flöden av storleksordningen nanoliter per sekund av stor betydelse för hela bränslecellens prestanda. Ett flöde beter sig väldigt annorlunda i kanaler och porer av denna ringa storlek än vad de skulle i en större apparat. Att förstå hur dessa flödesprocesser fungerar i detalj samt hur de aktiva porösa materialen är konstruerade kan hjälpa oss att bygga effektivare och billigare batterier och bränsleceller. Detta är motiveringen till arbetet.



Målsättningen med detta arbete är att ernå fördjupad förståelse för transportfenomen och porösa strukturer vid en mesoskopisk längdskala. Att utföra experiment är väldigt kostsamt och därför har datorsimuleringar använts som tillvägagångssätt. Mer detaljerat så har målsättningen varit att rekonstruera den mesoskopiska strukturen i det ledande materialet som används som membran i en bränslecell och utvärdera dess transportegenskaper som exempelvis diffusion, samt att utvärdera värme- och masstransport i raka kanaler där extra fokus har lagts på värmeöverföring i inloppet till kanalerna och stelning.



Hela detta arbete bygger på datorsimuleringar och simuleringsmetoden dissipativ partikeldynamik (DPD) har använts för samtliga studier. Studien visar att den presenterade modellen över de porösa membranen överensstämmer väl med tidigare studier både med avseende på masstransport och strukturella parametrar. Studierna av mesoskopiskt flöde i raka kanaler förutspår en effektivare värmeöverföring i kanalernas inlopp jämfört med motsvarande makroskopiska kanaler, samt att stelning kommer att bero på temperaturskillnad mellan inloppet och de kalla väggarna, medans flödeshastigheten kommer att ha en ringa betydelse. (Less)
Abstract
The topic of this work is simulations of porous materials and transport phenomena at a mesoscopic length scale, i.e., a length scale larger than an individual atom but smaller than the continuum. The porous material studied is a membrane used in fuel cells. Mass transfer has also been studied in the membrane. Heat and mass transfer has been studied in parallel plate channels, first in saturated channels, then the study is expanded to also cover droplet flow. In both cases, solidification, i.e. phase change from liquid to solid, has been studied.



So what is the purpose of this study? Many of our merging technologies rely on physical processes that occur at small length scales. In a fuel cell membrane, protons are... (More)
The topic of this work is simulations of porous materials and transport phenomena at a mesoscopic length scale, i.e., a length scale larger than an individual atom but smaller than the continuum. The porous material studied is a membrane used in fuel cells. Mass transfer has also been studied in the membrane. Heat and mass transfer has been studied in parallel plate channels, first in saturated channels, then the study is expanded to also cover droplet flow. In both cases, solidification, i.e. phase change from liquid to solid, has been studied.



So what is the purpose of this study? Many of our merging technologies rely on physical processes that occur at small length scales. In a fuel cell membrane, protons are transported in pores with a diameter of only a few nanometres. The chemical reactions in the fuel cell are dependent on oxygen, protons and electrons reacting with each other on a nanometre-sized active surface. In both cases, fluid flows are of pivotal importance to the performance of the entire fuel cell, and these flows are creeping at a flow rate on the scale of nanoliters per second! A flow of this magnitude will behave quite differently than one in a larger application. To understand how these fluid processes work in detail and how the active porous materials are constructed can help us to construct cheaper and more effective batteries and fuel cells. This is the motivation for the work carried out in this thesis.



The aim of this work is to achieve a deeper understanding of transport phenomena and porous materials at a mesoscopic length scale. To conduct experiments is costly, and therefore computer simulations have been chosen as the approach for this work. To be more specific, the aim has been to reconstruct the mesoscopic structure of a fuel cell membrane and evaluate its structural and transport properties, and to evaluate heat and mass transfer in a parallel plate channel where specific care has been taken to describe the heat transfer in the inlet of the channel and simultaneous fluid flow and solidification.



This work is based on computer simulations using the technique dissipative particle dynamics (DPD). The study shows that the model of the porous structures agrees with previous studies, both regarding mass transport and structural parameters. The studies of mesoscopic flow in parallel plate channels predict a more effective heat transfer in the inlet of the channel as compared to a comparable macroscopic channel, and that solidification will depend on the temperature difference between the inlet and the cold walls and that the impact of flow velocity will be negligible. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Abu-Nada, Eiyad, Khalifa University of Science, Technology and Research, Abu Dhabi, United Arab Emirates
organization
publishing date
type
Thesis
publication status
published
subject
defense location
M:B, M-Building, Ole Römers Väg 1, Lund University, Faculty of Engineering, LTH
defense date
2015-12-10 10:15
ISSN
0282-1990
ISBN
978-91-7623-557-7
language
English
LU publication?
yes
id
1913702c-779b-4f66-98e4-3f467ca4bd0e (old id 8171194)
date added to LUP
2015-11-16 13:54:43
date last changed
2016-09-19 08:45:01
@phdthesis{1913702c-779b-4f66-98e4-3f467ca4bd0e,
  abstract     = {The topic of this work is simulations of porous materials and transport phenomena at a mesoscopic length scale, i.e., a length scale larger than an individual atom but smaller than the continuum. The porous material studied is a membrane used in fuel cells. Mass transfer has also been studied in the membrane. Heat and mass transfer has been studied in parallel plate channels, first in saturated channels, then the study is expanded to also cover droplet flow. In both cases, solidification, i.e. phase change from liquid to solid, has been studied.<br/><br>
<br/><br>
So what is the purpose of this study? Many of our merging technologies rely on physical processes that occur at small length scales. In a fuel cell membrane, protons are transported in pores with a diameter of only a few nanometres. The chemical reactions in the fuel cell are dependent on oxygen, protons and electrons reacting with each other on a nanometre-sized active surface. In both cases, fluid flows are of pivotal importance to the performance of the entire fuel cell, and these flows are creeping at a flow rate on the scale of nanoliters per second! A flow of this magnitude will behave quite differently than one in a larger application. To understand how these fluid processes work in detail and how the active porous materials are constructed can help us to construct cheaper and more effective batteries and fuel cells. This is the motivation for the work carried out in this thesis.<br/><br>
<br/><br>
The aim of this work is to achieve a deeper understanding of transport phenomena and porous materials at a mesoscopic length scale. To conduct experiments is costly, and therefore computer simulations have been chosen as the approach for this work. To be more specific, the aim has been to reconstruct the mesoscopic structure of a fuel cell membrane and evaluate its structural and transport properties, and to evaluate heat and mass transfer in a parallel plate channel where specific care has been taken to describe the heat transfer in the inlet of the channel and simultaneous fluid flow and solidification.<br/><br>
<br/><br>
This work is based on computer simulations using the technique dissipative particle dynamics (DPD). The study shows that the model of the porous structures agrees with previous studies, both regarding mass transport and structural parameters. The studies of mesoscopic flow in parallel plate channels predict a more effective heat transfer in the inlet of the channel as compared to a comparable macroscopic channel, and that solidification will depend on the temperature difference between the inlet and the cold walls and that the impact of flow velocity will be negligible.},
  author       = {Johansson, Erik},
  isbn         = {978-91-7623-557-7},
  issn         = {0282-1990},
  language     = {eng},
  school       = {Lund University},
  title        = {Simulations of Transport Phenomena and Porous Structures Using Dissipative Particle Dynamics},
  year         = {2015},
}