Computational Fluid Dynamics Applied to Waste-to-Energy Processes: A Hands-On Approach

204 Pages·2020·28.851 MB·English

by  Valter Bruno Reis e Silva, João Cardoso

COMPUTATIONAL
FLUID DYNAMICS
APPLIED TO
WASTE-TO-ENERGY
PROCESSES
COMPUTATIONAL
FLUID DYNAMICS
APPLIED TO
WASTE-TO-ENERGY
PROCESSES
A Hands-On Approach
VALTER BRUNO REIS E. SILVA
Renewable Energy, Polytechnic Instituteof Portalegre,
Portalegre, Portugal
JOÃO CARDOSO
Instituto Superior T(cid:1)ecnico, Universidade de Lisboa, Lisboa, Portugal
Polytechnic Institute of Portalegre, Portalegre,Portugal
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To my wife, who made me to accept this challenge and is the cornerstone of my life.
Dr. Valter Bruno Reis E. Silva
Preface
Computational fluid dynamics is not a simple subject! The equations governing the
behavior of fluids are the result of over a century of hard and intense work. To make
matters worse, complex mathematical tools are required to handle with fluid behavior
peculiarities. Consequently, students and even professionals are often overwhelmed in
their attempt to connect their mathematical knowledge to practical applications. It is
not surprising to find many students looking for help in different internet forums and
complainingaboutsparseormissinginformation.Theyarelookingforthebetterstrategy
toimplementineachparticularcaseoraskinghowtheybuildcustomizedcodes.When
fluiddynamicscoursesaretaughtinuniversities,theirstudyplansareessentiallyfocused
ontheoreticalapproachesmeaningthattheadequateproficiencyandconfidencetodeal
with computational approaches is sometimes neglected.
The major question is how students and professionals become good users in such a
complex software. Generally, they face only two valuable options: self-learning based
onatrial-errormethodthatisverydemandingandtimeconsumingorenrollinginaspe-
cialized course that could be very expensive and lengthy. Under such circumstances, a
book with simplified theoretical background and worked examples with the necessary
analysis of each step comprising problem formulation is a tool required for many. The
bookcombinesanadequatelevelofmathematicalbackgroundwithreal-worldexamples
includingcombustorsandgasifiers.Byworkingthroughexamplesandtakingcomputer
screenshots,applyingstep-by-stepguidelines,andwithsomecustomizationcapabilities,
readers will learn to move beyond button pushing and start thinking as professionals.
Theuseofworkedexamplesincomplexprocessessuchascombustionorgasification
involving fluid dynamics, heat and mass transfer, and complex chemistry sets allow the
readerscomprehensiveknowledgethatcanbeusedinsomanyotherrealproblems.More
specifically,thisisalsoarelevantcontributionforstudentsandprofessionalsengagedwith
thermochemicalconversionprocessesinresearchandindustrialenvironments.Inabroad
spectrum,thisbookcanbeusedbyalltheindividualsinterestedinmultiphaseproblems,
fluid mechanics, simulation, and hydrodynamics.
This book is organized into several chapters balancing theory and application
including step-by-step tutorials and all the skills needed to perform real-life simulation
calculations.
xi
Acknowledgments
I am indebted to my entire research team for their efforts in all stages of the book and
valuable suggestions during the whole process. Special thanks to Mr. Joa˜o Cardoso,
whom I invited to be coauthor of this book, for his relentless effort in supporting me
bydesigningmostpicturesofthisbookandplayingaveryimportantroleinthetutorial
chapters, making them more accessible to all users.
IalsooweadebtofgratitudetoMs.RaquelZanolandMs.JoanneCollettfortheir
outstanding support and help in bringing out the book in the present form.
Finally,myspecialthankstotheElsevierteamfortheirconcernandvaluablesupport
to make this book possible.
xiii

CHAPTER 1
Introduction and overview of using
computational fluid dynamics tools
1. Introduction
Waste-to-energy(WtE)istheprocessofconvertingwasteintoelectricityandheat,orthe
path of turning waste into a fuel source [1]. Rapidly increasing urbanization and world
populationgrowthmeansignificantlylargeramountsofwasteandagreaterdemandfor
different sources of energy. WtE strategies are being set to tackle both problems simul-
taneously,byhelpingtodisposeofthewastewhilegeneratinganimportantcontribution
on the steep energy demand. Waste holds a large potential as a source of renewable
energyand greenhouse gases emissionreductionbut its useis advancingat aslow pace,
andbeyondseveralconcertedstrategiesneededbyprivateandpublicinstitutionstocon-
vincethepublicopinionandovercomepoliticalbarriers,jumpsintechnicalfeaturesare
still required [2].
WtEcompaniesarestrugglingwithmajorchallenges:intensifiedgeneralizedcompe-
tition, the event of disruptive technologies, strict government regulations, and some
immature technological features. Such issues outline the complexity and the risks for
even the best well-prepared players. Beyond the impact of using effective business
models,wellsucceedingcompaniesmustembracenewapproachestoimprovethepro-
cessdesign,minimizetechnologicaluncertainties,andoptimizetheprocessoutputswith
a reduced number of failed attempts. The ultimate goal of a WtE solution preconizes a
reduced environmental footprint combined with a large energy efficiency process and
minimum by-products [3].
Reliablequantitativeanalysisrequiresaveryexpensivelarge-scaleexperimentationwith
workdevelopedonlaboratorylevelgeneratingresultsoftenfarfromthereality[4].Overthe
last decades, with the increasing computational power and numerical solvers efficiency,
computationalfluiddynamics(CFD)isbroadlyusedtodesign,optimize,andpredict the
physical-chemical phenomena regarding several processes and more recently has been
introducedtoWtEsystems[5].CFDcomprisesasetofelaboratemathematicalmodelsgov-
ernedbypartialdifferentialequationsrepresentingconservationlawsformass,momentum,
andenergy,alongsidewiththeoreticalandempiricalcorrelations.CFDshowsclearbenefits
byallowingfasttestingofnewdesignconceptsandconfigurations,byprovidinginforma-
tionevenincaseswhereexperimentalactivitiesarehardtoaccomplish,andbyimproving
ComputationalFluidDynamicsAppliedtoWaste-to-EnergyProcesses ©2020ElsevierInc.
3
https://doi.org/10.1016/B978-0-12-817540-8.00001-7 Allrightsreserved.
4 Computationalfluiddynamicsappliedtowaste-to-energyprocesses
the understanding of the whole systemleadingtounexpectedbreakthroughs. Therefore,
CFDsimulationisastrategicassettouseinalargemajorityoftheengineeringprocesses[6].
IntheparticularcaseofWtEsystems,theuseofCFDwouldrequiresignificantefforts
butnotnecessarilynewtechnologybreakthroughs.Thenextlinesputinevidencesome
relevant examples. Efficient feeding strategies are possibleby testing and evaluating dif-
ferenthydrodynamicfeaturestopreventoperationalfailuresandreduceunnecessarypro-
cedures[7].Boilerefficiencyinwasteplantsisjustabout30%,allowingagreatmarginfor
improvement [8]. The easy change of relevant parameters contributes to a thorough
understandingofhowtheycorrelatewitheachotherandhowtheengineercanoptimize
thefullprocess,providingtothecustomermoreeffectiveandcheapersolutions[3].The
useofvirtualreactorsthatdifferonlyslightlyfromexperimentaldatawillallowconsid-
erablesavesandaquickresponsetothemarketdemands.Industrialcasestudiesshowthat
thetestingtimecanbereduceduptohalfayear[9],andsimulationsofnewstandardized
WtE plants will contribute to pushing the costs down [8].
However, CFD is not a simple subject! The equations governing the behavior of
fluidsaretheresultofoveracenturyofhardandintensework.Tomakemattersworse,
complex mathematical tools are required to handle fluid behavior peculiarities. Conse-
quently, students and even professionals are often overwhelmed in their attempt to
connect their mathematical knowledge to practical applications. Furthermore, and as
in any other computer approach, the use of CFD comprises a set of disadvantages and
limitations that the user should be aware of and conscientious to reduce their impact
[10]. Any CFD solution relies upon physical models and their predictions can only be
as accurate as the models on which they are based. The same line of reasoning applies
totheboundaryconditionsbecausetheiraccuracyisonlyasgoodastheinitialconditions
included in the numerical model. Finally, computer solutions always imply round-off
(finite word size available on the computer) and truncation errors (numerical model
approximation).
The best way to gain proficiency is to understand how the CFD workflow can be
broken down into manageable pieces, allowing the user to integrate the several steps
involved and following through the entire process from A to Z. There are eight basic
steps to implement any CFD attempt: (1) Modeling goals definition; (2) Domain iden-
tification; (3) Solid model geometry; (4) Mesh generation; (5) Configure physics; (6)
Solver settings; (7) Compute solution; and (8) Model revision and improvements.
One common mistake when performing the modeling process is to fail in some of
these steps leading to time-consuming and unnecessary procedures. In some cases, the
usercantakeadvantageofanalyticalapproachesandgetgoodinsightsforsimplegeom-
etrieswhereintensivecomputationisnotnecessary.Beforecommittingtothesimulation
procedure,theusermustfirstattemptanoverallstrategyconcerningwhatitisintendedto
achieve.