Large Eddy Simulation of Atomizing Sprays - Lund University

130 Pages·2016·6.52 MB·English

Large Eddy Simulation of Atomizing Sprays
Grosshans, Holger
2013
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Citation for published version (APA):
Grosshans, H. (2013). Large Eddy Simulation of Atomizing Sprays. [Doctoral Thesis (compilation), Fluid
Mechanics].
Total number of authors:
1
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LUND UNIVERSITY
PO Box 117
221 00 Lund
[phone]
Large Eddy Simulation
of
Atomizing Sprays
Doctoral Thesis
Holger Großhans
Division of Fluid Mechanics
Department of Energy Sciences
Lund, Sweden, 2013
.
ISBN 978-91-7473-463-8
ISSN 0282-1990
ISRN LUTMDN/TMHP-13/1090 - SE
(cid:13)c Holger Grosshans, March 2013
Division of Fluid Mechanics
Department of Energy Sciences
Faculty of Engineering
Lund University
Box 118
S-221 00 LUND
Sweden
Typeset in LATEX
Printed by Media-Tryck, Lund, March 2013.
Popul¨arvetenskaplig Sammanfattning
V¨atska-gas tv˚afasstr¨omning f¨orekommer ofta i det vardagliga livet,
i naturen, exempelvis fallande regndroppar eller v˚agor i havet, samt in-
dustriellaapplikationer,t.ex.br¨ansleinjektionidieselmotorerellergastur-
biner. F¨or m˚anga av dessa till¨ampningar ¨ar samspelet mellan v¨atska och
gas avg¨orande f¨or en effektiv drift. I detta arbete studeras v¨atska-gas
interaktion numeriskt, speciellt f¨or atomiserande fl¨oden. Large Eddy Si-
mulations (LES) utf¨ors f¨or att erh˚alla tidsberoende resultat. v¨atskestr˚al
uppbrott och bildandet av sm˚a sf¨ariska droppar simuleras med hj¨alp
av Volume of Fluid (VOF) metoden. Utvecklingen av v¨atskesprejer ¨ar
uppl¨osningen i mindre droppar, evaporation och blandningen med den
omgivande gasen simuleras med hj¨alp av Lagrangian Particle Tracking
(LPT)metoden.Dennaavhandlingfokuserarp˚aa)attf¨orb¨attraf¨orst˚aelsen
av den underliggande fysiken och b) att f¨orb¨attra de numeriska modelle-
ringsteknikerf¨oratomiserandetv˚afasstr¨omningar.Betr¨affandefysikenav
tv˚afasstr¨omningar, s˚a studeras hur variationer av v¨atskans egenskaper,
som densitet, viskositet eller ytsp¨anning p˚averkar den prim¨ara uppbryt-
ning av v¨atskestr˚alen, sprej utveckling och v¨atska-gas blandning. D¨ar
visas hur viskositet och ytsp¨anning stabiliserar v¨atske strukturer, me-
dan h¨ogre v¨atskedensitet leder till snabbare likvid brytning. A¨ven pulse-
rande injektion och dess effekt p˚a prim¨ar uppbrott och sprejutveckling
studeras. Indragning ¨ar indikerad vilken utbreder sig snabbare ¨an sj¨alva
str˚alen och ¨okar v¨atska-gas blandning. Betr¨affande numerisk modelle-
ring av tv˚afasstr¨omningar, s˚a utvecklats en ny metod f¨or att koppla en
VOF simulering av str˚alens uppbrytning med en LPT simulering av en
finf¨ordelad sprej. Metoden bygger p˚a en statistisk koppling som visar sig
vara korrekt och f¨orb¨attra betydligt ber¨akningseffektivitet j¨amf¨ort med
de existerande VOF-LPT kopplingsmetoderna. En ny metod f¨oresl˚as f¨or
att korrelera sprej resultat som erh˚allits fr˚an m¨atningar till numeriskt
erh˚allna sprej data. Ist¨allet f¨or att enbart j¨amf¨ora globala sprej paramet-
rar, j¨amf¨ors det kompletta f¨altet av en variabel, dvs. ljusets extinktions-
koefficient.
iii

Abstract
Liquid-gas two-phase flows occur frequently in daily life, in nature
e.g. falling rain drops or waves in the sea, and in industrial applications,
e.g. fuel injection in Diesel engines or gas turbines. For many of these
applicationstheinteractionbetweenliquidandgasiscrucialfortheireffi-
cientoperation. Inthisworkliquid-gasinteractionisstudiednumerically,
specifically for atomizing flows. Large Eddy Simulations (LES) are per-
formed to obtain time-dependent results. The disintegration of a liquid
jet and the formation of small spherical droplets is simulated using the
Volume of Fluid (VOF) method. The development of liquid sprays, the
breakup into smaller droplets, the evaporation and mixing with the sur-
rounding gas is simulated using the Lagrangian Particle Tracking (LPT)
method. This thesis focuses a) on improving the understanding of the
underlying physics and b) improving the numerical modelling techniques
of atomizing two-phase flows.
Concerning physics of two-phase flows, it is studied how variations
of the liquid properties, such as density, viscosity or surface tension in-
fluence the primary breakup of the liquid jet, the spray development
and the liquid-gas mixing. It is shown how viscosity and surface tension
stabilise the liquid structures, while higher liquid density leads to faster
liquid breakup. Also pulsed injection and its effect on primary breakup
and the spray development is studied. The formation of an entrainment
waveisindicatedwhichpropagatesfasterthanthejetitselfandenhances
liquid-gas mixing.
Concerning numerical modelling of two-phase flows, a novel method
is developed to couple a VOF simulation of the jet atomization with a
LPT simulation of a dispersed spray. The method is based on a statis-
tical coupling which shows to be accurate and to improve significantly
the computational efficiency compared to existing VOF-LPT coupling
methods. Also, a novel approach to correlate spray data obtained from
measurements to numerically obtained spray data is proposed. Instead
of only global spray parameters, a complete field of a variable, the light
extinction coefficient, is compared.
v

CONTENTS
Nomenclature xi
1 Introduction 1
1.1 Context and Motivation . . . . . . . . . . . . . . . . . . . 1
1.2 Research Objectives . . . . . . . . . . . . . . . . . . . . . 6
1.3 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 Physical Foundations of Fluid Flows 9
2.1 Laminar and Turbulent Single Phase Flows . . . . . . . . 9
2.1.1 Laminar or Turbulent Flow . . . . . . . . . . . . . 9
2.1.2 Energy Cascade and Kolmogorov Hypotheses . . . 10
2.1.3 Scales of Turbulence . . . . . . . . . . . . . . . . . 12
2.2 Multiphase Flow Types and Definitions . . . . . . . . . . 13
2.2.1 Multiphase Flow Types . . . . . . . . . . . . . . . 13
2.2.2 Instability Mechanisms. . . . . . . . . . . . . . . . 15
2.2.3 Dimensionless Numbers . . . . . . . . . . . . . . . 15
2.3 Liquid Jets and Sprays . . . . . . . . . . . . . . . . . . . . 16
2.3.1 Liquid Jet Regimes . . . . . . . . . . . . . . . . . . 16
2.3.2 Flow and Droplet Timescales . . . . . . . . . . . . 18
2.3.3 Breakup of a Liquid Jet . . . . . . . . . . . . . . . 19
2.3.4 Breakup of Droplets . . . . . . . . . . . . . . . . . 21
vii
CONTENTS
2.3.5 Evaporation and Mixing . . . . . . . . . . . . . . . 22
2.3.6 Pulsed Liquid Injection . . . . . . . . . . . . . . . 23
3 Equations of Single-phase Flows 25
3.1 Navier-Stokes Equations . . . . . . . . . . . . . . . . . . . 25
3.2 Turbulence Modelling . . . . . . . . . . . . . . . . . . . . 27
4 Equations of Liquid Jets 33
4.1 Governing Equations . . . . . . . . . . . . . . . . . . . . . 33
4.2 Surface Tension Modelling . . . . . . . . . . . . . . . . . . 35
4.3 Motion of the Phase Interface . . . . . . . . . . . . . . . . 36
5 Equations of Dispersed Flows 41
5.1 Continuous Phase. . . . . . . . . . . . . . . . . . . . . . . 41
5.2 Dispersed Phase . . . . . . . . . . . . . . . . . . . . . . . 42
5.2.1 Momentum Exchange . . . . . . . . . . . . . . . . 43
5.2.2 Secondary Breakup Models . . . . . . . . . . . . . 45
5.2.3 Droplet Evaporation . . . . . . . . . . . . . . . . . 48
6 Numerics 51
6.1 Discretization Schemes . . . . . . . . . . . . . . . . . . . . 51
6.1.1 The Volume of Fluid Approach . . . . . . . . . . . 51
6.1.2 The Lagrangian Particle Tracking Approach . . . . 52
6.2 Solver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
6.3 Numerical Accuracy . . . . . . . . . . . . . . . . . . . . . 54
7 Coupling VOF to LPT 59
7.1 The Statistical Coupling Approach (SCA) . . . . . . . . . 62
8 Set-up and Results 65
8.1 Volume of Fluid Simulations . . . . . . . . . . . . . . . . . 65
8.2 Lagrangian Particle Tracking Simulations . . . . . . . . . 69
8.3 VOF-LPT Coupling . . . . . . . . . . . . . . . . . . . . . 73
8.4 Pulsed Liquid Injection . . . . . . . . . . . . . . . . . . . 78
8.5 Correlation with Experimental Data . . . . . . . . . . . . 83
8.6 Correlation with OpenFOAM . . . . . . . . . . . . . . . . 90
viii
CONTENTS
9 Conclusions and Future Work 95
10 Summary of the Publications 97
Bibliography 105
ix