title: Caractérisation quantitative de la pulvérisation de pesticides par imagerie
text: Keywords:
  
  Pie
  
  zo
  
  electric droplet generator, high-speed imaging technique, image processing, spray angle, spray shape, droplet character
  
  ization, spray nozzles 25 S
  
  AMENVATTING
  
  De voorbije jaren werd veel aandacht besteed aan innovaties in gewasbescherming vooral met het oog op het verhogen van opbrengsten op een duurzame manier. Het gebruik van chemische gewasbeschermingsmiddelen (GBM) met een hoge efficiëntie speelt hierin nog steeds een belangrijke rol. Hieraan zijn echter ook belangrijke nadelen verbonden. Tijdens hun toepassing kunnen GBM b.v. verloren gaan en op ongewenste plaatsen terechtkomen met financiële, economische en milieukundige gevolgen. GBM worden meestal toegepast met landbouwspuiten. Deze spuittoestellen gebruiken hydraulische spuitdoppen die het spuitbeeld vormen en de spuitvloeistof omzetten in spuitdruppels met een breed spectrum van druppelgroottes en –snelheden. Kleine druppels driften echter makkelijk weg onder invloed van wind terwijl grote druppels van het gewas kunnen afrollen en de bodem contamineren. Om die redenen worden meer en meer inspanningen geleverd om tot een duurzamer gebruik van GBM te komen en is er steeds meer internationale regelgeving betreffende het gebruik van GBM. Eén van de grootste uitdagingen is om via geschikte spuittechnieken de verliezen naar de omgeving te minimaliseren in combinatie met het maximaliseren van depositie en bio-efficiëntie. Omdat het druppelvormingsproces van hydraulische spuitdoppen complex en moeilijk te kwantificeren of modelleren is, is er nood aan accurate meettechnieken. De recente ontwikkelingen in soften hardware openen mogelijkheden om hogesnelheidscamera’s en beeldverwerking te gebruiken voor landbouwtoepassingen in het algemeen en spuittoepassingen van GBM in het bijzonder. Dit onderzoek focust op de ontwikkeling en toepassing van technieken van snelle beeldacquisitie en –verwerking voor het niet-intrusief opmeten van de micro(druppelgrootte en –snelheid) en macro(spuithoek, spuitbeeld, lengte vloeistofvlies) karakteristieken van spuitnevels van GBM. Na een review over spuittoepassingen van GBM en technieken om spuitnevels te karakteriseren (Hoofdstuk 2), werden in dit proefschrift twee snelle beeldacquisitiesystemen ontwikkeld in Hoofdstuk 3 op basis van single-droplet experimenten met een
  
  27 Samenvatting
  
  snellebeeldcamera en een piëzo-electrische druppelgenerator. 58 combinaties van lenzen, lichtbronnen, diffusoren en sluitertijden werden getest en geëvalueerd op basis van beeldkwaliteitsparameters (signaal-ruisverhouding, entropieratio en contrastratio), lichtstabiliteit en hoeveelheid overbelichting en de meetnauwkeurigheid. Dit resulteerde in twee bruikbare beeldacquisitiesystemen. De hogesnelheidscamera met een macrovideozoomlens op een afstand van 143 mm met een gezichtsveld van 88 mm x 110 mm in combinatie met een halogeenspotlicht en een diffusor werd geselecteerd voor het meten van macrokarakteristieken van spuitnevels (spuithoek, spuitbeeld, lengte vloeistofvlies). De optimale opstelling voor het opmeten van de microkarakteristieken (druppelgrootte en -snelheid) bestond uit een hogesnelheidscamera met 6 μs sluitertijd, een microscopische lens op een afstand van 430 mm, resulterend in een gezichtsveld van 10,5 mm x 8,4 mm, en een Xenon tegenlicht zonder diffusor. Met deze laatste techniek en de nodige ontwikkelde beeldverwerkingsalgoritmen (Hoofdstuk 4) werden in Hoofdstuk 5 metingen uitgevoerd met een druppelgenerator in droplet on demand (DOD) en continue modus. De effecten van instellingen (elektrische spanning, pulsbreedte en pulshoogte) en dopopening (261 μm, 123 μm, 87 μm and 67 μm) op druppelgrootte en –snelheid werden bepaald. De experimenten in DOD-modus toonden aan dat de initiële druppelkarakteristieken bepaald werden door de dubbele pulsbreedte (absorptietijd ta en pulsatietijd tp) en de grootte van de dopopening. Door te spelen met de pulsbreedte was het mogelijk om druppelgrootteen snelheid te beïnvloeden. Algemeen leidden een daling van ta en een stijging van tp tot een toename van de druppelgrootte. In DOD-modus varieerden de druppelgroottes tussen 134,1 μm en 461,5 μm. De kleinste en tegelijk snelste druppels werden gegenereerd met de kleinste dopopening. De druppelsnelheden varieerden van 0,08 m/s tot 1,78 m/s. Daarnaast werd ook een effect van pulshoogte vastgesteld. De verhouding tussen druppelgrootte en grootte van de dopopening varieerde van 1,3 tot 3,5 in DOD-modus. In continue modus werd voor elke dopopening een hoogfrequent signaal (kHz) aangelegd resulterend in een continue druppelproductie. De effecten van frequentie en amplitude op druppelgrootte en –snelheid en de afstand tussen de druppels werden bepaald. Naar analogie met de DOD-modus werden druppelgroottes voornamelijk bepaald door de grootte van de dopopening. Druppelgroottes varieerden van 167,2 μm tot 455,8 μm. Daarnaast resulteerde een grotere dopopening eveneens in hogere 28 Samenvatting druppelsnelheden en was er een duidelijk lineair verband tussen druppelgroottes en snelheden. De verhouding tussen druppelgrootte en grootte van de dopopening varieerde van 1,3 tot 3,9 in continue modus terwijl de druppelsnelheden varieerden 1,84 m/s en 4,66 m/s. Zowel in DODals in continue modus kon een gelijkaardige range van druppelgroottes gegenereerd worden maar in continue modus waren de druppels sneller en dus representatiever voor een echte spuittoepassing. In hoofdstuk 6 werd het beeldacquisitiesysteem voor het opmeten van de microkarakteristieken van spuitnevels gebruikt om de druppelgroottes en –snelheden op te meten voor werkelijke spuitnevels van hydraulische spuitdoppen. In een eerste fase werd een infocuscriterium opgesteld op basis van de grijswaarden-gradiënt om te bepalen of een druppel al dan niet in focus is. Verschillende druppelgroottes werden gegenereerd in continue modus met de druppelgenerator op verschillende afstanden van het focaal vlak met behulp van een micro-positioneersysteem. Voor elke positie en druppelgrootte werden de grijswaarden-gradiënt en de in-focusparameter bepaald. Op basis hiervan werd een kritische in-focusparameter (Infc) gedefinieerd voor elke druppelgrootte en een in-focus criterium dat bepaalt of een druppel al dan niet in focus is op basis van de diameter en de infocusparameter. De focused droplet zone (FDZ) werd gedefinieerd als de zone waarin een druppel met een bepaalde diameter in focus is. Een lineaire correlatie tussen druppelgrootte en FDZ werd vastgesteld. Het ontwikkelde in-focuscriterium werd toegepast op beelden genomen met vijf spuitdoppen op verschillende posities in de spuitnevel: twee standaardspleetdoppen (XR 110 01 & 04 bij 4.00 kPa), een luchtmengdop (AI 110 04 bij 400 kPa) en twee werveldoppen (ATR oranje bij 600 kPa en ATR rood bij 800 kPa). De effecten van doptype, dopgrootte en positie op druppelgroottes en –snelheden werden geëvalueerd. De gemeten druppelgroottes en –snelheden met de beeldverwerkingstechniek toonden aan dat het mogelijk was om microkarakteristieken van spuitnevels op te meten op een niet intrusieve manier. De gemeten druppelgroottes varieerden van 24 μm tot 543 μm afhankelijk van doptype en –grootte. Druppelsnelheden op 0,5 m onder de spuitdop varieerden van 0,5 m/s tot 12 m/s. Druppelgroottekarakteristieken zoals DV0.1, DV0.5, 0.9 en RSF en druppelsnelheidskarakteristieken zoals VVol10, VVol50, VVol90 and VSF werden berekend. 29
  
  Samenvatting
  
  Gelijkaardige effecten van doptype en positie op druppelgroottes en –snelheden werden vastgesteld met de ontwikkelde beeldverwerkingstechniek als met de algemeen aanvaarde Phase Doppler Particle Analyzer (PDPA)--lasertechniek. Er werd een relatief goede correlatie gevonden tussen druppelkarakteristieken gemeten met de nieuw ontwikkelde techniek en de PDPA laser. In vergelijking met de PDPA-laser werden met de beeldverwerkingstechniek algemeen minder zeer kleine druppels opgemeten en in sommige gevallen ook een kleiner aantal grote druppels. Verschillen tussen beide technieken waren onder meer te wijten aan het feit dat de minimale gemeten druppelgrootte met de beeldverwerkingstechniek 24 μm is terwijl met de PDPA nog kleinere druppels kunnen worden opgemeten. Daarenboven was het aantal opgemeten druppels met de beeldverwerkingstechniek klein ten opzichte van de PDPAtechniek waardoor de kans stijgt om enkele van de zeer grote druppels te missen. Het nemen en analyseren van meer beelden kan dit verhelpen. Het verschil in druppelsnelheden tussen beide technieken is te wijten aan het feit dat de PDPA-laser slechts de verticale component van de druppelsnelheid meet, waardoor de werkelijke druppelsnelheid onderschat wordt. Daarnaast konden met de beeldverwerkingstechniek geen druppels sneller dan ongeveer 12 m/s opgemeten worden op basis van FOV en frame rate terwijl met de PDPA sporadisch druppels met hogere snelheden opgemeten werden voornamelijk bij de XR 110 04. In de toekomst kan de beeldverwerkingstechniek verder geoptimaliseerd worden door te meten bij een hogere frame rate met een zelfde nauwkeurigheid. In hoofdstuk 7 werd het beeldacquisitiesysteem voor het opmeten van de macrokarakteristieken van spuitnevels gebruikt om de spuithoek, het spuitbeeld, en de lengte van het vloeistofvlies te bepalen voor dezelfde dop-druk–combinaties als hierboven. Waar mogelijk werden de resultaten vergeleken met die van traditionele meettechnieken zoals de verdeeltafel. De opgemeten spuithoeken bleken groter dan de nominale waarden met uitzondering van de XR 110 01. Voor de werveldoppen (ATR oranje en rood) en de luchtmengdop (AI 110 04) werd een goede overeenkomst gevonden tussen de met de beeldverwerkingstechniek gemeten spuithoeken en met de traditionele meettechniek terwijl voor beide standaardspleetdoppen (XR 110 01 en XR 110 04) een lagere waarde werd gevonden met de beeldverwerking. De kleinste lengte van het vloeistofvlies werd opgemeten voor de XR 110 01 (18.5 mm), gevolgd door beide werveldoppen (27.4 en 31.3 mm), terwijl de grootste waarden opgemeten werden voor de XR 110 04 (38.9 mm) en de AI 30 Samenvatting 110 04 (43.1 mm). De spuitbreedtes op vier hoogtes (5, 10, 15 en 20 cm) werden opgemeten samen met de overeenkomstige spuithoeken en vergeleken met de resultaten van de horizontale verdeeltafel. Voor de spuithoek werd een goede overeenkomst tussen beide meettechnieken gevonden met een relatieve fout van 0.5% voor de XR 110 01, 0,6% voor de XR 110 04 en de AI 110 04 en 2,8% en 5,4% voor respectievelijk de oranje en de rode werveldop. Trefwoorden: Piëzo-electrische druppelgenerator, snelle beeldacquisitie en –verwerking, beeldverwerking, spuithoek, spuitbeeld, druppelkarakterisatie, spuitdoppen
  
  31 TABLE OF CONTENTS
  
  Preface........................................................................................................................................ 1 Résumé....................................................................................................................................... 5 Summary.................................................................................................................................. 21 Samenvatting............................................................................................................................ 27 Table of contents...................................................................................................................... 33 List of figures............................................................................................................................ 39 List of tables............................................................................................................................. 45 List of abbreviations................................................................................................................. 46 1 2 General introduction......................................................................................................... 47 1.1 Introduction............................................................................................................... 47 1.2 Research objectives and thesis outline..................................................................... 49 The spray application process and techniques for spray characterization: A review...... 53
  
  2.1 The use of plant protection products........................................................................
  
  54
  
  2.2 Spray application process..........
  
  ................................
  
  ................
  
  ................................ 55
  
  2.2.1 Spray tank...........
  
  ................................................................................................ 56 2.2.2
  
  Droplet break-up regimes and atomization
  
  ....................................................... 56 2.2.3 Transport to target............................................................................................. 65 2.2.4 
  
  Impact on target
  
  ................................................................................................. 65 2.3 Measuring spray distribution and deposition........................................................... 66 2.3.1 Spray scanner..................................................................................................... 66 2.3.2 Vertical patternator............................................................................................ 67 2.3.3 Spray distribution bench.................................................................................... 68 2.3.4 Measuring spray deposition............................................................................... 68 33 Table of contents
  
  2.4 2.4.1 Optical non-imaging light scattering spray characterization techniques..........
  
  69 2.4.2
  
  High-speed imaging spray analyzers
  
  ..................
  
  ................
  
  ................................ 73
  
  2.5 3 Methods for measuring spray droplet characteristics..............................................
  
  69
  
  Conclusion.................................................................................................................
  
  79 Development of high
  
  speed
  
  image acquisition systems based on single droplet experiments..............
  
  ................
  
  ................................
  
  ................................................................ 81
  
  3.1 Introduction...............................................................................................................
  
  82
  
  3.2 Materials and methods.............................................
  
  ................................................
  
  83 3.2.1 Piezoelectric Droplet Generator................
  
  ........................................................
  
  83 3.2.
  
  2 Image acquisition system
  
  ................................................................................... 85 3.2.3 Image analysis.................................................................................................... 88 Entropy Ratio.................................................................................................................... 90 Contrast Ratio................................................................................................................... 90 Signal-to-Noise Ratio (SNR)............................................................................................... 91 3.2.4 Accuracy of the droplet size measurement....................................................... 92 3.2.5 Statistical analysis............................................................................................... 92 3.3 3.3.1 Image quality parameters.................................................................................. 93 3.3.2 Light stability and overexposure ratio............................................................... 96 3.3.3 Accuracy of the droplet size measurement..................................................... 101 3.4 4 Results and discussion............................................................................................... 93 Conclusion............................................................................................................... 102 Image analysis algorithms for single droplet characterization....................................... 105 4.1 Introduction............................................................................................................. 106 4.2 Droplet characterization with image analysis......................................................... 106 34 4.2.1 Pre-processing the images to enable analysis................................................. 108 4.2.2 Image segmentation......................................................................................... 109 Table of contents 4.2.3 Droplet extraction and size measurement...................................................... 116 4.2.4 Droplet tracking algorithm............................................................................... 118 4.2.5 Results.............................................................................................................. 119 4.3 5 Conclusion...............................................................................................................
  
  120 Droplet generation and characterization using a piezoelectric droplet generator and imaging techniques................................................................................................................
  
  121 5.1 Introduction............................................................................................................. 122 5.2 Materials and methods........................................................................................... 123 5.2.1 Droplet generator set-up................................................................................. 123 5.2.2 Statistics........................................................................................................... 128 5.2.3 Image acquisition system................................................................................. 128 5.2.4 Droplets(s) characterization with image analysis............................................ 129 5.2.5 Validation......................................................................................................... 129 5.3 5.3.1 Droplet on demand generation....................................................................... 130 5.3.2 Continuous mode droplet generation............................................................. 137 5.3.3 Comparison between 
  
  DOD
  
  and c
  
  ontinuous
  
  mode.......................................... 141 5.4 6 Results and discussion............................................................................................. 130 Conclusion............................................................................................................... 142
  
  Micro-spray characterization
  
  from
  
  a
  
  single nozzle.........................
  
  ................................
  
  144 6.1 Introduction............................................................................................................. 145 6.2 Materials and methods........................................................................................... 147
  
  6.2.1 Development of the in-focus drop
  
  let 
  
  criterion
  
  ................................................ 147 6.2.2 Spray droplet characterization using the in-focus droplet criterion............... 160 6.2.3 Image analysis for spray droplet characterization........................................... 162 6.3 Results and discussion............................................................................................. 167 6.3.1 Spray droplet size distribution......................................................................... 167 35 Table of contents 6.3.2 Spray droplet velocity distribution................................................................... 176 6.3.3 Comparison between imaging and PDPA measuring technique..................... 183 6.4 7 Conclusion...............................................................................................................
  
  195 Macro-spray characterization from a single nozzle........................................................
  
  198 7.1 Introduction............................................................................................................. 199 7.2 Materials and methods........................................................................................... 200 7.2.1 Hydrauli
  
  c
  
  spray
  
  nozzle
  
  s.................................................................................... 200 7.2.2
  
  Existing
  
  non-imaging techniques for spray characterization........................... 202 7.2.3 Image acquisition system for spray characterization...................................... 204 7.2.4 Image analysis for spray characterization........................................................ 205 7.3 7.3.1 Flow rate........................................................................................................... 208 7.3.2 Spray angle....................................................................................................... 210 7.3.3 Liquid sheet length........................................................................................... 210 7.3.4 Spray distribution............................................................................................. 212 7.3.5 Spray shape...................................................................................................... 214 7.4 8 Results and discussion............................................................................................. 208 Conclusion............................................................................................................... 219 General conclusions and suggestions for future work................................................... 220
  
  8.1 General conclusions................................................................................................. 220 8.1.1 Development of high speed image acquisition systems based on single droplet experiments.................................................................................................................... 221 8.1.2 Droplet generation and characterization using a piezoelectric droplet generator and imaging techniques..................................
  
  ................................................
  
  ................
  
  222
  
  8.1.3 Micro
  
  -
  
  spray characterization
  
  from
  
  a single nozzle.......................................... 223 8.1.4 Macro
  
  -spray characterization
  
  from 
  
  a single no
  
  zzle
  
  ......................................... 225 8.2 36
  
  Suggestions for future work.................................................................................... 226 Table of contents Appendix A Tables............................................................................................................ 228 Appendix B List of publications........................................................................................ 230 References.............................................................................................................................. 232 37 LIST OF FIGURES
  
  FIGURE 1.1. ORCHARD (LEFT) (WWW.AIRTECSPRAYERS.COM) AND FIELD (RIGHT) CROP SPRAY APPLICATION (ILVO) 48 FIGURE 1.2. SCHEMATIC REPRESENTATION OF THE OUTLINE OF THIS THESIS 51 FIGURE 2.1. DELIVERY CHAIN OF THE SPRAY APPLICATION PROCESS (MATTHEWS, 2000) 56 FIGURE 2.2. DIFFERENT BREAKUP REGIMES REPRESENTED BY THE RELATIONSHIP OF THE VELOCITY V VERSUS THE DROPLET DIAMETER D FOR WATER AS EJECTED LIQUID (LINDEMANN, 2006) 57 FIGURE 2.3. SHEET BREAK-UP BY RIM (LEFT), PERFORATED SHEET (MIDDLE) AND WAVE-SHEET DISINTEGRATION (RIGHT) (MATTHEWS, 2000) FIGURE 2.4. SPRAY PATTERNS FOR DIFFERENT NOZZLE TYPES (SCHICK, 1997) 58 60 FIGURE 2.5. LIQUID SHEET AND DROPLET FORMATION FROM A HOLLOW CONE NOZZLE (WWW.LECHLER.DE) 61 FIGURE 2.6. 94
  
  FIGURE 3.6. IMAGE QUALITY PARAMETERS (ENTROPY RATIO (A), CONTRAST RATIO (B) AND SNR (C)) FOR THE K2/SC LONG-DISTANCE MICROSCOPE LENS FOR DIFFERENT EXPOSURE TIMES AND LIGHTING SYSTEMS. * ** BARS WITH ASTERISK ARE STATISTICALLY DIFFERENT: = P<0.05 (SCHEFFÉ TEST) AND = P<0.05 (SNK TEST) 96
  
  FIGURE 3.7. HISTOGRAMS OF TEN CONSECUTIVE IMAGES FOR FOUR IMAGING SET-UPS: (A) MACRO VIDEO ZOOM LENS X SPOTLIGHT X 120 GRIT DIFFUSER X 15 μS, (B) MACRO VIDEO ZOOM LENS X SPOTLIGHT X 220 GRIT DIFFUSER X 15 μS, (C) K2/SC LONG DISTANCE MICROSCOPE LENS X LED X NO DIFFUSER X 5 μS AND (D) K2/SC LONG DISTANCE MICROSCOPE LENS X XENON X NO DIFFUSER X 10 μS 99 FIGURE 3.8. (A) IMAGE QUALITY PARAMETERS FOR K2/SC LONG-DISTANCE MICROSCOPE LENS, XENON LIGHT, WITHOUT DIFFUSER AT 5, 6, 7, 8, 9 AND 10 ΜS EXPOSURE TIME AND (B) EFFECT OF EXPOSURE TIME ON DROPLET DIAMETER MEASUREMENT 100 FIGURE 3.9. IMAGE HISTOGRAMS USING THE K2/SC LONG-DISTANCE MICROSCOPE LENS AND XENON LIGHT WITH NO DIFFUSER AT 5, 6, 7, 8, 9 AND 10 ΜS 101 FIGURE 4.1. FLOW CHART OF THE IMAGE ANALYSIS ALGORITHM FOR DROPLET(S) CHARACTERIZATION 107 FIGURE 4.2. ORIGINAL IMAGE (A) AND IMAGE AFTER PRE-PROCESSING (B) IN DOD 108 FIGURE 4.3. ORIGINAL IMAGE (A) AND IMAGE AFTER PRE-PROCESSING (B) IN CONTINUOUS MODE 108 FIGURE 4.4. BI-MODAL HISTOGRAM FOR IMAGE IN (A) DOD MODE AND (B) CONTINUOUS MODE BOTH WITH A THRESHOLD VALUE OF 55% 110 FIGURE 4.5. DOD DROPLET IMAGES AFTER THRESHOLDING AT A) 30 %, B) 55% AND 60% OF THE MEAN INTENSITY VALUE 111 FIGURE 4.6. CONTINUOUS MODE DROPLET IMAGES AFTER THRESHOLDING OF A) 30 %, B) 55% AND C)
  
  OF THE OF THE MEAN INTENSITY
  
  
  
  VALUE 112
  
  FIGURE 4.7. CONCEPT APPROACH FOR CALCULATING THE EDGE GRADIENT OF PIXEL I(I,J) FROM IMAGE F USING A 3 X 3 MASK FIGURE 4.8.FILTER MASKS USED BY SOBEL EDGE DETECTOR 114 115 FIGURE 4.9. RESULTS ON DROPLET EDGE DETECTION (B) USING SOBEL EDGE DETECTOR AND (C) CANNY EDGE DETECTOR 116 FIGURE 4.10. RESULT AFTER DROPLET EXTRACTION, SIZING AND LOCATING THE DROPLET CENTER IN DOD MODE (THE DROPLET CENTER IS MARKED WITH A BLUE STAR AND THE DROPLET EDGE IS IN RED) 117 FIGURE 4.11. RESULT AFTER DROPLET EXTRACTION, SIZING AND LOCATING THE DROPLET CENTERS IN CONTINUOUS MODE (THE DROPLET CENTERS ARE MARKED WITH A BLUE STAR AND THE DROPLET EDGES ARE IN RED) 40
  
  118
  
  List of figures
  
  FIGURE 4.12 DROPLET TRACKING RESULT FROM TWO CONSECUTIVE IMAGES IN DOD MODE. 119 FIGURE 4.13. DROPLET VELOCITY DIAGRAM 120 FIGURE 5.1. PIEZOELECTRIC DROPLET GENERATOR: 1-GLASS NOZZLE, 2-PIEZOELECTRIC ELEMENT, 3-CLAMP, 4TUBES 123 FIGURE 5.2. (A) BLOCK DIAGRAM OF THE DOUBLE PRESSURE PULSE FOR DOD MODE AND (B) CONTINUOUS SQUARE SIGNAL FOR CONTINUOUS MODE 125 FIGURE 5.3. (A) EXAMPLES OF SINGLE DROPLET EJECTION IN DOD MODE AND (B) UNIFORMLY SIZED DROPLETS IN CONTINUOUS MODE 125 FIGURE 5.4. SELECTED PULSE WIDTH VALUES FOR THE SINGLE DROPLET CHARACTERIZATION AT VP = ± 4.5V FOR THE 4 DIFFERENT SIZED NOZZLES WHICH RESULTED IN A SINGLE DROPLET EJECTION 126 FIGURE 5.5. IMAGE SEQUENCE SHOWING THE FORMATION OF A 461.5 ΜM DROPLET IN DOD MODE WITH NOZZLE 1 WITH AN EJECTION DROPLET VELOCITY OF 0.59 M/S. THE DIFFERENT FRAMES CORRESPOND WITH TIMES OF 0, 3, 6, 9, 12 MS AFTER THE FIRST FRAME 129 FIGURE 5.6. DROPLET DIAMETER (μM, MEAN ± STD.) FOR DIFFERENT NOZZLES AND PULSE WIDTH COMBINATIONS. DIFFERENT LETTERS INDICATE STATISTICAL DIFFERENCES WITHIN THE SAME NOZZLE (P < 0.05) 132 FIGURE 5.7. MEAN DROPLET DIAMETER RESULTS FOR THE SELECTED PULSE WIDTH VALUES FOR THE 4 NOZZLES WITH VP = ± 4.5 V 133 FIGURE 5.8. DROPLET VELOCITY (M/S, MEAN ± STD.) FOR DIFFERENT NOZZLES AND PULSE WIDTH COMBINATIONS. DIFFERENT LETTERS INDICATE STATISTICAL DIFFERENCES WITHIN THE SAME NOZZLE (P < 0.05) 134 FIGURE 5.9. CORRELATION BETWEEN DROPLET DIAMETER AND VELOCITY IN DOD MODE FOR 4 DIFFERENT NOZZLES AT PULSE AMPLITUDE
  
  ±4.5 V
  
  135 FIGURE 5.10. DROPLET DIAMETER (ΜM, MEAN ± STD.) AND VELOCITY (M/S, MEAN ± STD.) FOR NOZZLE 1 AT DIFFERENT PULSE AMPLITUDES AND TA = 5 MS AND TP = 0.01 MS. DIFFERENT LETTERS INDICATE STATISTICAL DIFFERENCES BETWEEN DATA POINTS WITHIN THE SAME CURVE (P < 0.05) 136 FIGURE 5.11. CORRELATION BETWEEN DROPLET DIAMETER AND VELOCITY IN DOD MODE FOR NOZZLE 1 AT DIFFERENT PULSE AMPLITUDES AND TA = 5 MS AND TP = 0.01 MS 137 FIGURE 5.12. DROPLET DIAMETER (ΜM, MEAN±STD.) IN CONTINUOUS MODE FOR DIFFERENT NOZZLES AND PULSE AMPLITUDES AT A FREQUENCY OF 8.0 KHZ. DIFFERENT LETTERS INDICATE STATISTICAL DIFFERENCES WITHIN THE SAME NOZZLE (P<0.05) 138 FIGURE 5.13. DROPLET VELOCITY (M/S, MEAN±STD.) IN CONTINUOUS MODE FOR DIFFERENT NOZZLES AND PULSE AMPLITUDES AT A FREQUENCY OF 8.0 KHZ. DIFFERENT LETTERS INDICATE STATISTICAL DIFFERENCES WITHIN THE SAME NOZZLE (P<0.05) 138 FIGURE 5.14. CORRELATION BETWEEN DROPLET DIAMETER AND VELOCITY IN CONTINUOUS MODE FOR DIFFERENT NOZZLES AND PULSE AMPLITUDES AT A FREQUENCY OF 8.0 KHZ 139 41
  
  List of figures
  
  FIGURE 5.15. INTER-DROPLET SPACING (ΜM, MEAN ± STD.) IN CONTINUOUS MODE FOR DIFFERENT NOZZLES AND PULSE AMPLITUDES AT A FREQUENCY OF 8.0KHZ. DIFFERENT LETTERS INDICATE STATISTICAL DIFFERENCES WITHIN THE SAME NOZZLE (P<0.05) FIGURE 6.1. IMAGE ACQUISITION SYSTEM FOR ESTABLISHING THE IN-FOCUS DROPLET CRITERION 141 148 FIGURE 6.2. DROPLET IMAGES IN CONTINUOUS MODE USING A NOZZLE WITH 65 μM ORIFICE SIZE AT 5.0 V AND 8.0 KHZ AT DIFFERENT DISTANCES FROM THE LENS: A) 420 MM; B) 423 MM C) 426 MM 149 FIGURE 6.3. FLOW CHART OF THE IMAGE ANALYSIS ALGORITHM FOR ESTABLISHING THE IN-FOCUS DROPLET CRITERION 150 FIGURE 6.4. MEASURED DROPLET DIAMETER AND CORRESPONDING HALO AREA FOR PICTURES TAKEN OF THE 222.9 μM DROPLET DIAMETER AT VARIOUS DISTANCES FROM THE LENS 152 FIGURE 6.5. DETAIL AND GRAY LEVEL INTENSITY PROFILES FROM THE MARKED DROPLETS SHOWN IN FIGURE 6.2 A, B & C, RESPECTIVELY 154 FIGURE 6.6. MEASURED DROPLET DIAMETER AND CORRESPONDING IN-FOCUS PARAMETER FOR PICTURES TAKEN OF THE 222.9 μM (27.1 PIXELS)DROPLET DIAMETER AT VARIOUS DISTANCES FROM THE LENS 155 FIGURE 6.7. ACCEPTABLE MEASURED DROPLET DIAMETERS FOR PICTURES TAKEN OF THE 222.9 μM DROPLET DIAMETER AT VARIOUS DISTANCES FROM THE LENS 156 FIGURE 6.8. CRITICAL IN-FOCUS PARAMETERS AND FDZ FOR PICTURES TAKEN OF THE 222.9 μM DROPLET DIAMETER 157 FIGURE 6.9.
  
  RE
  
  LATION
  
  BET
  
  EEN THE MEASURED DROPLET DIAMETER AND DISTANCE TO THE LENS FOR DROPLET SIZES RANGING FROM 119.3 μM UP TO 497.1 μM 158
  
  FIGURE 6.10. RELATION BETWEEN THE IN-FOCUS PARAMETER AND DISTANCE TO THE LENS FOR DROPLET SIZES RANGING FROM 119.3 μM UP TO 497.1 μM 158 FIGURE 6.11. RELATION BETWEEN THE CRITICAL IN-FOCUS PARAMETER AND MEASURED DROPLET DIAMETER 159 FIGURE 6.12. RELATION BETWEEN FDZ AND MEASURED DROPLET DIAMETER 160 FIGURE 6.13. IMAGE ACQUISITION SYSTEM FOR REAL SPRAY DROPLET CHARACTERIZATION 160 FIGURE 6.14. SPRAY MEASUREMENT POINTS FOR: A) FLAT FAN AND B) HOLLOW CONE NOZZLE 162 FIGURE 6.15. EXAMPLE OF SPRAY DROPLET IMAGE WITH XR110 04 NOZZLE AT 400 KPA IN THE CENTER 163 FIGURE 6.16. SPRAY DROPLET IMAGE SHOWN IN FIGURE 6.15 AFTER IMAGE PRE-PROCESSING 164 FIGURE 6.17. SPRAY DROPLET IMAGE SHOWN IN FIGURE 6.15 AFTER DROPLET LOCALIZATION 165 FIGURE 6.18. SPRAY DROPLET IMAGE SHOWN IN FIGURE 6.15 AFTER APPLYING MORPHOLOGICAL OPERATIONS 165 FIGURE 6.19. EXAMPLES OF SINGLE SPRAY DROPLET IMAGES AFTER CANNY EDGE DETECTION 166
  
  FIGURE 6.20. DROPLET TRACKING PRINCIPLE IN THE CENTER OF THE SPRAY 167 FIGURE 6.21. RELATIVE FREQUENCY OF DROPLET SIZE OCCURRENCE (BLUE) AND CUMULATIVE DROPLET SIZE DISTRIBUTION (RED) FOR THE XR 110 04 AT 400 KPA AT 500 MM BELOW THE NOZZLE IN THE CENTER OF THE SPRAY. THIS DATA IS OBTAINED FROM THE IMAGING SYSTEM 42
  
  168 List of figures
  
  FIGURE 6.22. CUMULATIVE DROPLET SIZE DISTRIBUTIONS FOR THE FIVE NOZZLE PRESSURE COMBINATIONS (XR 110 01, XR 110 04, AI 110 04, ATR ORANGE, ATR RED) AT 0.5 M BELOW THE NOZZLE IN THE CENTER, AT 200 MM AND AT THE EDGE OF THE SPRAY 171 FIGURE 6.23. EFFECT OF THE NOZZLE TYPE AND SIZE ON THE CUMULATIVE DROPLET SIZE DISTRIBUTION AT EACH MEASUREMENT POINT 173 FIGURE 6.24. DROPLET SIZE DISTRIUBUTION PARAMETERS DV0.1, DV0.5 AND DV0.9 (μM, MEAN ± STD.) FOR FIVE NOZZLE PRESSURE COMBINATIONS (TABLE 6.3) AT 0.5 M BELOW THE NOZZLE AT THREE MEASUREMENT POINTS (CENTER, 200MM, EDGE OF THE SPRAY) 175 FIGURE 6.25. RSF (MEAN ± STD.) FOR THE DIFFERENT NOZZLE PRESSURE COMBINATIONS AT DIFFERENT POSITIONS 176 FIGURE 6.26. CUMULATIVE DROPLET VELOCITY DISTRIBUTIONS FOR THE FIVE NOZZLE PRESSURE COMBINATIONS IN THREE MEASUREMENT POINTS (TABLE 6.3) 179 FIGURE 6.27. EFFECT OF THE NOZZLE-PRESSURE COMBINATION ON THE DROPLET VELOCITY DISTRIBUTION AT EACH MEASUREMENT POINT 181 FIGURE 6.28. VOLUMETRIC DROPLET VELOCITY PARAMETERS FOR EVERY NOZZLE PRESSURE COMBINATION FROM TABLE 6.3 AT THREE MEASUREMENT POINTS AT 500 MM BELOW THE NOZZLE
  
  URE 6.29. VELOCITY SPAN FACTOR FOR THE DIFFERENT NOZZLE PRESSURE COMBINATIONS 182 183
  
  FIGURE 6.30. CUMULATIVE DROPLET SIZE DISTRIBUTION RESULTS USING THE IMAGING TECHNIQUE AND PDPA LASER FOR THE FIVE NOZZLE PRESSURE COMBINATIONS IN THREE MEASUREMENT POINT 186 FIGURE 6.31. COMPARISON BETWEEN NMD MEASURED VALUES WITH THE IMAGING TECHNIQUE AND NMD VALUES WITH THE PDPA LASER FOR ALL THE FIVE NOZZLE PRESSURE COMBINATIONS 187 FIGURE 6.32. COMPARISON BETWEEN RSF MEASURED VALUES WITH THE IMAGING TECHNIQUE AND RSF VALUES WITH THE PDPA LASER FOR ALL THE FIVE NOZZLE PRESSURE COMBINATIONS 188 FIGURE 6.33. CUMULATIVE DROPLET VELOCITY DISTRIBUTION RESULTS USING THE IMAGING TECHNIQUE AND PDPA LASER FOR THE FIVE NOZZLE PRESSURE COMBINATIONS IN THREE MEASUREMENT POINT 192 FIGURE 7.1. ALBUZ ATR HOLLOW CONE: ORANGE AND RED NOZZLE 201 FIGURE 7.2. TEEJET EXTENDED RANGE FLAT FAN NOZZLES: XR 110 01 (ORANGE) AND XR 110 04 (RED) 201 FIGURE 7.3. RELATIVE FLOW VALUES (% AND STDEV.) OF THE FIVE NOZZLE PRESSURE COMBINATIONS AT HEIGHTS OF (A) 15 CM, (B) 30
  
  CM
  
  AND (
  
  C
  
  )
  
  50 CM
  
  214
  
  FIGURE 7.12. SPRAY ANGLE AND SPRAY SHAPE ESTIMATION BASED ON SPRAY DISTRIBUTION MEASUREMENTS AT 3 DIFFERENT HEIGHTS OF THE (A) ALBUZ ATR ORANGE NOZZLE AT 600 KPA, (B) ALBUZ ATR RED AT 800 KPA, (C) TEEJET XR 110 01 AT 400 KPA, (D) TEEJET XR 110 04 AT 400 KPA AND (E) TEEJET AI 110 04 AT 400 KPA 217 FIGURE 7.13. SPRAY ANGLE AS WELL AS SPRAY SHAPE OF THE (A) ALBUZ ATR ORANGE NOZZLE AT 600 KPA; (B) ALBUZ ATR RED AT 800 KPA; (C) TEEJET XR 110 01 AT 400 KPA; (D) TEEJET XR 110 04 AT 400 KPA AT 400 KPA AND (E) TEEJET AI 110 04 AT 400 KPA 217 FIGURE 7.14. SPRAY SHAPE ESTIMATION FOR TEEJET XR 110 04 NOZZLE AT 400 KPA UP TO 20 CM BELOW THE NOZZLE 44
  
  218 List of figures
  
  LIST OF TABLES TABLE 2.1.OVERVIEW OF PARAMETERS TO DESCRIBE DROPLET SIZE SPECTRA 64 TABLE 3.1. SUMMARY OF THE TESTED IMAGE ACQUISITION SET-UPS 88 TABLE 3.2. FIRST-ORDER STATISTICAL EQUATIONS 90 TABLE 4.1.DROPLET SIZE MEASUREMENTS IN DOD MODE WITH NOZZLE 2 FOR TA =0.4 MS, TP =50 MS, VP= ± 4.5 V APPLYING DIFFERENT THRESHOLD VALUES 111 TABLE 5.1. SELECTED PULSE WIDTH VALUES FOR THE SINGLE DROPLET CHARACTERIZATION FOR THE 4 NOZZLES AT VP = ±4.5 V WHICH RESULTED IN A SINGLE DROPLET EJECTION 127 TABLE 5.2. EFFECT OF THE PULSE WIDTH VALUES (MS) ON THE DROPLET DIAMETER (ΜM) AND DROPLET VELOCITY (M S) PRODUCED WITH NOZZLES 1 TO 4 (MEAN ± STD) 131 TABLE 5.3. RATIO BETWEEN NOZZLE ORIFICE OPENING AND MINIMAL-MAXIMAL MEAN DROPLET DIAMETERS AND MINIMAL AND MAXIMAL DROPLET DIAMETERS IN DOD AND CONTINUOUS MODE 142 TABLE 6.1 ACTUAL DROPLET DIAMETERS IN CONTINUOUS MODE FOR THE DIFFERENT NOZZLE ORIFICE SIZES AND CONTINUOUS MODE SETTINGS 148 TABLE 6.2 ACTUAL DROPLET DIAMETERS AND THEIR CORRESPONDING INFC AND FDZ 157 TABLE 6.3 MANUFACTURE SPECIFICATIONS OF THE TESTED HYDRAULIC SPRAY NOZZLES 161 TABLE 6.4 COMPARISON BETWEEN DROPLET SIZE DISTRIBUTION PARAMETERS OBTAINED WITH THE IMAGING TECHNIQUES AND PDPA LASER FOR THE FIVE NOZZLE PRESSURE COMBINATIONS 189 TABLE 6.5 COMPARISON BETWEEN DROPLET VELOCITY DISTRIBUTION PARAMETERS GAINED WITH THE IMAGING TECHNIQUES AND PDPA LASER FOR THE FIVE NOZZLE PRESSURE COMBINATIONS TABLE 7.1. PPP are mainly applied as liquid solutions by using different types of sprayers equipped with hydraulic spray nozzles (Figure 1.1). These nozzles atomize the liquid to produce a broad spectrum of droplet sizes (~ 10-1000 μm) and velocities (~ 0-25 m/s) (Nuyttens et al., 2007a; 2009). Droplet size as well as droplet velocity characteristics both influence the efficiency of the spray application. First, small droplets are subject to spray drift which distorts the spray pattern and causes environmental pollution (Nuyttens et al., 2010). Second, large droplets have a greater kinetic energy which increases their ability to penetrate into the canopy but also the chance to rebound or shatter when the droplet impacts the plant surface (Zwertvaegher et al., 2014). 47
  
  Chapter 1 Figure 1.1. Orchard (left) (www.airtecsprayers.com) and field (right) crop spray application (ILVO)
  
  Besides droplet sizes and velocities, other important spray characteristics influencing the efficiency of the application process are droplet directions, the volume distribution pattern, the spray sheet structure and length, the structure of the individual droplets and the 3D spray dimensions (Miller & Ellis, 2000). Spray characteristics depend on nozzle type, nozzle size, liquid properties, spray pressure and sprayer settings (boom height, driving speed, etc.). Hence, the challenge is to reduce spray losses during transport to their target and maximize spray deposition and efficacy and minimize off-target spray deposition by improving the spray application process (Zabkiewicz, 2007) by selecting and using adequate spray equipment and spray solutions at the right conditions (Dorr et al., 2007). Because mechanisms of droplets leaving a hydraulic spray nozzle are very complex and difficult to quantify or model, there is a need for accurate quantification techniques. Although techniques are available to measure some specific spray characteristics, none of them are able to fully characterize a spray application process. Therefore, adjusting sprayers is still based on practical experience and ‘trial and error’ as it is impossible to test any combination of sprayer type and settings, nozzle type, air support, spray pressure, crop characteristics, driving speed etc. on the application efficacy. The use of models could increase the knowledge on droplet transport processes and the effect of sprayer settings and environmental conditions. The validation of this is however difficult and cannot only be performed with traditional measuring techniques (like spray liquid distribution and air velocity measurements).
  
  48
  
  General introduction
  
  Spray characterization techniques can be classified in three broad categories: mechanical, electrical and optical. With mechanical techniques, droplets are collected and analyzed using sampling devices. However, these sampling devices may affect the spray flow behavior and can only be used to evaluate spray deposition and estimate droplet size (Rhodes, 2008). Therefore, the availability of non-intrusive systems for spray characterization is of great importance. Quantitative (optical) non-imaging droplet characterization techniques are available but they are complex, expensive and in most cases limited to small measuring volumes. The limitations of the non-imaging techniques and the recent improvements in digital image processing, sensitivity of imaging systems and cost reductions, have increased the interest in high speed imaging techniques for agricultural applications in general and for fertilizer (Vangeyte, 2013; Cool et al., 2014) and pesticide applications in specific. Imaging analyzers are spatial sampling techniques consisting of a light source, a camera and a computer with image acquisition and processing software. The small droplet size and the high velocity of the ejected spray droplets make it a challenge to use imaging techniques for spray characterization. 1.2 RESEARCH OBJECTIVES AND THESIS OUTLINE
  
  Spray nozzles do not atomize the liquid into droplets of identical size and velocity but into a range of droplets of various sizes and velocities. These micro spray characteristics (droplet size and velocity) as well as the macro spray characteristics (spray angle, liquid sheet length, etc.) influence the efficiency of the spray application. Consequently more information on the micro and macro spray characteristics can lead to a more efficient pesticide usage. Therefore, the general objective of this doctoral research is to develop image acquisition systems for pesticide sprays which are then used to systematically measure different micro and macro spray characteristics. To achieve this main objective, the following sub-objectives are addressed, each corresponding with one of the following chapters: 49 Chapter 1  To review the spray application process and application techniques together with the available measuring techniques for micro and macro spray characteristics (Chapter 2);  To develop and describe image acquisition systems for measuring the micro and macro spray characteristics (Chapter 3);  To develop and describe image analysis and image processing algorithms for single droplet characterization (Chapter 4);  To generate and characterize single droplets using imaging techniques and a piezoelectric droplet generator in two modes: Droplet on Demand (DOD) and continuous mode (Chapter 5);  To develop image processing algorithms for micro-spray characterization and to compare imaging results with Phase Doppler Particle Analyzer results for different agricultural spray nozzles (Chapter 6);  To develop image processing algorithms for macro-spray characterization and to compare these results with the horizontal spray patternator results (Chapter 7). Finally, Chapter 8 highlights the most important conclusions and some guidelines for future research. A schematic overview of the outline of this thesis is presented in Figure 1.2. 50
  
  General introduction Chapter 1 General introduction Chapter 2 The spray application process and techniques for spray characterization: A review 
  
  Chapter 3 Development
  
  of image
  
  acquisition systems for micro
  
  &
  
  macro spray characterization Micro
  
  -
  
  spray characteristics
  
  Macro-spray 
  
  characteristics Chapter 4 Image analysis and
  
  image
  
  processing algorithms for single drop
  
  let
  
  characterization
  
  Chapter
  
  5 Single
  
  droplet
  
  characterization
  
  DOD mode Continuous mode Chapter 7 Macro-spray characterization from
  
  
  
  a single
  
  nozzle
  
  Horizontal patternator Image analysis Chapter 6 Micro-
  
  spray
  
  character
  
  ization
  
  from
  
  
  
  a single
  
  no
  
  zzle PDPA Image
  
  analysis
  
  Chapter 8 Conclusions and Future work
  
  Figure 1.2. Schematic representation of the outline of this thesis
  
  51 52 2
  
  THE SPRAY APPLICATION PROCESS AND TECHNIQUES FOR SPRAY CHARACTERIZATION: A REVIEW
  
  i A review of the use of plant protection products and the spray application process is given in this chapter. Different techniques for measuring the spray distribution and deposition are also described. Furthermore, methods for measuring spray droplet characteristics, like nonimaging and imaging and their advantages and limitations are listed here. i Part of this chapter was adapted from: Hijazi, B., Decourselle, T., Vulgarakis Minov, S., Nuyttens, D., Cointault, F., Pieters, J., and Vangeyte, J. 2012. The use of high speed imaging for applications in precision agriculture
  
  . Book chapter in
  
  "
  
  New
  
  Technologies
  
  : Trends,
  
  Innovations
  
  and Research". ISBN
  
  978-9
  
  53-51-0480-3. 53
  
  Chapter 2 2.1 THE USE OF PLANT PROTECTION PRODUCTS
  
  In the coming years, agriculture will face a 30 percent increase in the global population, intensifying competition for increasingly scarce land, water and energy resources, and the existential threat of climate change. To provide for a population projected to reach 9.3 billion in 2050 estimates are that food production will need to increase from the current 8.4 billion tons to almost 13.5 billion tons a year (FAO, 2014). Plant protection products (PPP) help farmers to grow more food on less land by protecting crops from huge losses due to pests and diseases and raising yields per hectare. These PPP are generally chemicals used to eliminate or control a variety of agricultural pests during the growing season. The most applied PPP are insecticides (to kill insects), herbicides (to kill weeds) and fungicides (to control fungi, mold and mildew). When applying PPP the aim is always to maximize the amount reaching the target and minimize the amount reaching off-target areas. However, the use and disposal of toxic PPP by farmers and the general public provide many possible sources of PPP in the environment. This may lead to transport of PPP through the air or water, into the soil or even into living organisms. It is estimated that about 25% of more than 2.5 million tons of PPP that are yearly applied in the world is used in the EU (Balsari & Marucco, 2011). Pesticide regulations exist to ensure the safe use of pesticides, so that farmers are equipped with the right tools for sustainable productivity so that consumers can be confident about the safety of their food and environments. Therefore, farmers and industry work together within the framework of EU Regulations and Directives to minimize any negative impacts. In 2006, the European Commission adopted the ‘Thematic Strategy on the Sustainable use of Pesticides’ (European Commission, 2006). It was stated that by 2014 each country must adopt the principles of integrated pest management (IPM) and all pesticides application equipment will have to be inspected at least once by 2016 to grant a proper efficient use of any PPP (European Commission, 2009). IPM is an ecosystem strategy for crop production and protection which combines techniques to control pests and diseases and minimizes crop damage and the use of pesticides. IPM uses four approaches for pest management evaluations, decisions and controls (US Environmental Protection Agency, 2013): 54
  
  The spray application process and techniques for spray characterization: A review
  
   Action thresholds are set: the pest population density, at which it makes economic sense to take a control measure, is determined.  Pest monitoring: pests are identified in the field and their population build-up is monitored to allow for sound control decisions.  Pest prevention: if significant pest damage is expected, preventive measures are taken to avoid this damage.  Pest control: when action thresholds are surpassed, despite the applied preventive measures, the pest control method posing the least risk to producers, consumers and the environment is chosen first. If this control method does not work, more risky methods can be applied. Instead of only using synthetic chemical pesticides as a blanket solution, integrated pest management uses a wide array of methods for pest prevention and control. This thesis focuses on spray application techniques which is still the most used method to apply PPP. Spray application is a key component of IPM. Poor spray application can result in failure in what otherwise would be a successful IPM strategy. 2.2 SPRAY APPLICATION PROCESS
  
  The spray application process consists of several sub-processes forming a ‘delivery chain’ (Matthews, 2000) (Figure 2.1). Every part of the chain can influence the spray efficiency. This thesis focuses on the droplet formation and atomization (2.2.2) and transport to the target (2.2.3). The developed image acquisition techniques can also be used to study droplet impact on the target and retention (2.2.4) as done by Zwertvaegher et al. (2014). 55
  
  Chapter 2 Figure 2.1. Delivery chain of the spray application process (Matthews, 2000) 2.2.1 Spray tank
  
  The first step in the spray application process is preparing the spray tank solution. It is very important that the spray solution is mixed sufficiently before starting the application. With an inhomogeneous solution, a good and homogeneous application cannot be reached. The homogeneity of the solution is not only determined by good mixing practice, it also depends on the characteristics of the solvent. Certain products dissolve better in water than others. Compared to water, these adjuvants influence the liquid sheet formation and droplet characteristics (Wright et al., 1982). 2.2.2 Droplet break-up regimes and atomization
  
  The process of droplet ejection is not as simple as taking a fluid chamber with a small hole and pressurizing it enough for fluid to start emerging from the ejection nozzle hole (Lee, 2003). To accomplish monodisperse droplets ejected out of a nozzle, one needs the ability to produce high-speed fluid jets of approximately the diameter of the desired droplets 56 The spray application process and techniques for spray characterization: A review (Lindemann, 2006). Furthermore some terms concerning the droplet formation and ejection are necessary to be defined as they are later used in this thesis. Three breakup regimes can be distinguished as shown in Figure 2.2 of which the first two are found using a piezoelectric droplet generator (Chapters 3 to 5) while the atomization breakup is typical for hydraulic spray nozzles (Chapters 6 and 7).
  
  Figure 2.2. Different breakup regimes represented by the relationship of the velocity v versus the droplet diameter D for water as ejected liquid (Lindemann, 2006)  Droplet-on-demand Breakup (DOD): characterized by ejection of a single droplet with a diameter approximately equal or slightly bigger than the nozzle diameter (Figure 5.3a) (Lindemann, 2006);  Continuous mode: characterized by a continuous stream of uniformly sized droplets (Figure 5.3b). This continuous jet disperses into single droplets after a certain distance from the nozzle due to the source of acoustic energy causing instability and standing waves on the fluid. However, to form uniformly sized droplets, a suitable resonance frequency must be applied to it (Lee, 2003). ν 9.016 r0 Eq. 2.1 d = 1.89 d0 Eq. 2.2 f= 57
  
  Chapter 2 where f is the optimal frequency, v is liquid velocity at the nozzle exit, r0 is the nozzle orifice radius and d0 is the diameter of the nozzle orifice;  Atomization Breakup: is a process in which a liquid sheet is fragmented into a fine spray of many single droplets (Figure 7.7a). This involves emitting the liquid through a small orifice at a high pressure. As a result, a wide spectrum of droplet sizes is generated by atomizers ranging from very narrow (several hundred μm) to wide (over a thousand μm) (Kirk, 2001; Teske et al., 2005). Atomization breakup is typical for the application of plant protection products using hydraulic nozzles as studied in this thesis. Therefore, the atomization break up, the liquid sheet disintegration process is discussed in more detail in 2.2.2.1. Finally, the relation between the spray nozzle and the resulting spray characteristics is described in 2.2.2.2 together with the spray quality (2.2.2.3).
  
  2.2.2.1 Liquid sheet disintegration
  
  When the aerodynamic forces acting on the liquid sheet become larger than the surface tension, the sheet starts to disintegrate (Lefebvre, 1989). Most atomizers demonstrate three mechanisms of sheet disintegration: by perforation, by oscillation and by rim disintegration (Figure 2.3). Figure 2.3. Sheet break-up by rim (left), perforated sheet (middle) and wave-sheet disintegration (right) (Matthews, 2000)
  
  Which kind of disintegration occurs, depends on the velocity of the liquid relative to the surrounding air. A large relative velocity can be caused by fast droplets moving in still air or by droplets with a low velocity moving in high velocity airflow (Lefebvre, 1989). At relatively low liquid velocities, the liquid sheet is perforated leading to growing interruptions in the 58 The spray application process and techniques for spray characterization: A review sheet and separated droplets (disintegration by perforation) (Miller & Ellis, 2000). At higher velocities, wave phenomena occur in the sheet which leads to disintegration by oscillation. At the end of the liquid sheet, a jet-like, torus shaped structure is separated from the sheet and disintegrates into droplets (Figure 2.3) (Bayvel & Orzechowski, 1993). The last kind of sheet break-up is rim disintegration which occurs when both surface tension and viscosity are high (Lefebvre, 1989). Surface tension can act upon the edges of the liquid sheet and contract them into a rim. The moment of sheet break-up determines the size of the droplets. A sheet breaking up early and close to the nozzle is thick and produces large droplets. A sheet breaking up at a greater distance from the nozzle orifice is thinner and produces smaller droplets (Miller & Ellis, 2000). At very high velocities, no liquid sheet is formed. Droplets are formed directly underneath the nozzle outlet and a very fine fog is produced (Bayvel & Orzechowski, 1993). In this thesis, liquid sheet length is measured using imaging techniques for different types of agricultural spray nozzles (Chapter 7). The mechanism of sheet disintegration is also influenced by fluid characteristics, like surface tension and viscosity. Increasing values of both properties lead to a longer sheet and thus smaller droplets (Kirk, 2001).
  
  2.2.2.2 Spray nozzles and their characteristics
  
  Each spray consists of a range of droplet sizes, referred to as the droplet size distribution. For a given formulation, the droplet size spectrum is determined by the nozzle type, the nozzle size and the spray pressure (Nuyttens et al., 2007a). The spray nozzle is the primary link between the plant protection product (PPP) and proper application to the target. The nozzle is designed to meter or regulate liquid flow rate, to form and control droplet size and to disperse and distribute the droplets in a specific pattern (Azimi et al., 1985). The nozzle type not only determines the amount of spray applied but also the uniformity of the applied spray, the coverage obtained on the sprayed surfaces and the amount of drift that might occur (Sumner, 2009). Each nozzle type has specific characteristics and is designed to be used for different applications. Once a system is designed, selecting a nozzle based on its characteristics is 59 Chapter 2 crucial. The most important nozzle parameters are: flow rate, operating pressure and pressure losses, nozzle material, nozzle spray angle, nozzle positioning, spray height, spray width, spray thickness, atomization degree or droplet size, impact, spray pattern, etc. (Lefebvre, 1989). Selecting a nozzle based on the spray pattern and other spray characteristics that are required generally yields good results (Lipp, 2012). Different types of spray nozzles are available as shown in Figure 2.4. Air atomizing nozzles produce the smallest droplet sizes followed by fine spray, hollow cone, flat fan and full cone nozzles (Schick, 1997).
  
  Figure 2.4. Spray patterns for different nozzle types (Schick, 1997)
  
  The most commonly used nozzle types for ground application of PPP are the flat fan and hollow cone nozzles (Sumner, 2009). The nozzle size and corresponding color are defined by the International Organization for Standardization (2005). The higher the ISO number the larger the orifice and the flow rate and the larger the droplets in the spectrum (Lefebvre, 1989). Pressure has an inverse relationship effect on droplet size, thus, increase in pressure will generally reduce the droplet size (Schick, 1997). In this study, hollow cone nozzles as well as standard flat fan nozzles and air inclusion flat fan nozzles are considered (Chapters 6 and 7). a. Hollow cone nozzles: are mainly used on hand-operated sprayers and on orchard sprayers in which spray droplets are projected into the canopy by a blast of air from a fan (Nuyttens, 2007a). They produce a cone-shaped pattern with the spray 60 The spray application process and techniques for spray characterization: A review concentrated in a ring around the outer edge of the pattern (Figure 2.5). This is the most popular nozzle type for orchard and
  
  vine
  
  yard spray applications. Figure 2.5. Liquid sheet and droplet formation from a hollow cone nozzle (www.lechler.de)
  
  b. Flat fan nozzles: are mostly used on field crop sprayers. They can achieve a very good cross distribution under a spray boom with the correct nozzle spacing and spray height. Flat fan nozzles produce a fan-shaped spray pattern (Nuyttens et al., 2007a) i.e. delivering the highest amount of spray volume in the center and lower spray volumes at the boundaries. Therefore, in practice, it is important to create a good spray overlap in order to obtain a homogeneous coverage of the field (Faqiri & Krishnan, 2005) (Figure 2.6). The most commonly used top angle is 110° generally resulting in a uniform spray distribution for a nozzle height and nozzle spacing of 0.5 m.
  
  Figure 2.6. Nozzle overlap for flat fan sprayers
  
  61
  
  Chapter 2 - Standard flat fan nozzles are producing the smallest droplets as compared to the other two types of flat fan nozzles (drift-reducing and air inclusion flat fan nozzles) for the same nozzle size and pressure (Figure 2.7). The small droplets secure a very effective coverage of the surface, but are very drift prone (Nuyttens, 2007b)
  
  Figure 2.7. Standard flat fan nozzles - Air inclusion flat fan nozzles contain a venturi insert.
  
  This
  
  vent
  
  uri induces
  
  air
  
  through
  
  two holes at the side of the nozzle. The
  
  air
  
  is
  
  mixed
  
  with the liquid in the nozzle
  
  
  
  chamber (Figure 2.8
  
  ). Because the liquid leaving the flat fan orifice is a mixture of air and spray liquid, the sheet becomes unstable very quickly and breaks up into large droplets, resulting in a very coarse spray (Nuyttens, 2007a). Therefore these droplets are less drift prone and explode on impact with leaves which reduces the risk of a droplet bouncing off a leaf surface. Figure 2.8. Air inclusion nozzle (Nilars, 2003) 62
  
  The spray application process and techniques for spray characterization: A review 2.2.2.3 Spray quality
  
  The term “Spray quality” is primarily used for describing the droplet size spectra of (agricultural) sprays. The spray characteristics like droplet size, velocity and direction influence the penetration and deposition of droplets. The efficiency of pesticide distribution often depends on droplet size (Hislop, 1987) but hydraulic spray nozzles are not able to produce uniform droplets. A high coverage of the target is usually best achieved with small droplets (Cawood et al., 1995) which are more subject to wind drift (Nuyttens et al., 2011). On the other hand, large droplets increase the risk of run off from target surfaces but have a higher kinetic energy which improves canopy penetration. The droplet size distribution is not homogeneous and depends on the position within the spray (Butler Ellis et al., 1997). Because of the importance of droplet size, the British Crop Protection Council (BCPC) has devised a nozzle classification scheme and also described the entire droplet spectrum generated by hydraulic spray nozzles (Southcombe et al., 1997). The spray classification system divides the quality of sprays into five categories: very coarse (VC), coarse (C), medium (M), fine (F), very fine (VF) and extremely coarse (XC) (Figure 2.9). Figure 2.9. Spray quality standards (Southcombe et al., 1997) A comparison of the percentile volume fractions produced by a nozzle to that of specific standard
  
  ized
  
  reference
  
  nozzles classifies a droplet size spectrum. The D V0.5 or volume median diameter (VMD) is commonly used to characterize the droplet size characteristics of a spray. The VMD is the droplet diameter at which 50% of the spray volume is contained in larger droplets and the other half is contained in smaller droplets (Schick, 1997). Two nozzle- 63 Chapter 2 pressure combinations with the same VMD may actually produce a quite different droplet spectrum. Droplet spectra are normally represented by a frequency histogram or a cumulative volumetric droplet size distribution (Nuyttens et al., 2007a). A summary of the main values that are used to describe the droplet size spectra is presented in Table 2.1.
  
  Table 2.1.Overview of parameters to describe droplet size spectra
  
  Parameter Description Units VMD or Dv0.5 Volume median diameter or diameter for which a volume fraction of 50% is made up of droplets with diameters smaller than this value μm Dv0.1, Dv0.9 Diameter at which a volume fraction of 10, 90 percent is made up of droplets with diameter smaller than this value μm RSF Relative span factor; indicating the uniformity of the droplet size D −Dv0.1 distribution= v0.9 VMD - NMD Number median diameter; droplet diameter for which 50% of the number of droplets is smaller than this value μm Other important drop
  
  let
  
  characteristics are
  
  droplet velocity and direction (trajectory). Increasing the spray liquid pressure to the nozzle results into a finer spray and increases the droplet velocities. As the effect of making a finer spray dominates with initial increases of pressure, the risk of drift tends to increase. Further increase in pressure does not result in a further increase of drift and may even, with some nozzles designs, result in a decrease in drift at high pressure due to the dominance of the droplet velocity effect (Miller & Butler Ellis, 1997).
  
  Sid
  
  ahmed
  
  (1996) formula
  
  ted
  
  a droplet-size/velocity equation based on the energy
  
  balance
  
  equation: d 3 va2 v=( 3 ) da 64 1⁄ 2
  
  Eq. 2.3 The spray application process and techniques for spray characterization: A review Where da and va are the size-class median droplet diameter and velocity and d and v are the representative diameter and velocity. Hence the droplet velocity depends on its diameter at formation.