Advanced

Drop Break-up in High-Pressure Homogenisers

Innings, Fredrik LU (2005)
Abstract (Swedish)
Popular Abstract in Swedish

Den här avhandlingen behandlar hur fettdroppar i mjölk slås sönder i en mjölkhomogenisator. Mekanismerna bakom sönderslagningen har analyserats genom uppmätning av flödesfältet i spalten samt genom att fotografera dropparna, när de slås sönder.



För att möjliggöra mätning och fotografering har två skalmodeller av en homogenisator tagits fram. En fullskalemodell, vilken är en direkt kopia av en homogenisatorspalt, men med fönster för att möjliggöra fotografering. Normala homogeniseringstryck kunde användas och det var möjligt att fotografera droppar ner till 5µm. Ännu noggrannare mätningar kunde utföras, när en uppskalad modell i plexiglas konstruerades, där homogenisatorspalten... (More)
Popular Abstract in Swedish

Den här avhandlingen behandlar hur fettdroppar i mjölk slås sönder i en mjölkhomogenisator. Mekanismerna bakom sönderslagningen har analyserats genom uppmätning av flödesfältet i spalten samt genom att fotografera dropparna, när de slås sönder.



För att möjliggöra mätning och fotografering har två skalmodeller av en homogenisator tagits fram. En fullskalemodell, vilken är en direkt kopia av en homogenisatorspalt, men med fönster för att möjliggöra fotografering. Normala homogeniseringstryck kunde användas och det var möjligt att fotografera droppar ner till 5µm. Ännu noggrannare mätningar kunde utföras, när en uppskalad modell i plexiglas konstruerades, där homogenisatorspalten uppskalades cirka 100 gånger. Uppskalningen gjordes genom att hålla de relevanta dimensionslösa talen konstanta, så att de mekanismer som styr droppsönderbrytningen i en riktig homogenisator också dominerade i modellen.



Mätningarna visade att dropparna inte slogs sönder i spaltens inlopp. Stora droppar dras ut något i inloppet, medan de små dropparna förblir runda. I spaltens inlopp är hastighetsprofilen plan. I en liten pilotskalehomogenisator hinner gränsskikten växa till och den turbulenta hastighetsprofilen är helt utvecklad i spaltens utlopp, medan i en stor produktionshomogenisator så hinner gränsskikten knappt växa till alls och hastighetsprofilen är därför plan även i utloppet. Gränsskikten verkar påverka dropparna mycket lite, men under färden genom spalten hinner små droppar, som har blivit deformerade relaxera tillbaks till sin sfäriska form, medan de stora dropparna förblir deformerade.



Den här studien visar tydligt att droppsönderslagningen sker i den turbulenta jetstrålen, som bildas i spaltens utlopp. Hastighetsmätningarna visar att jetstrålen är mycket fladdrig och att den bryts ner mycket fortare än en jetstråle i fri vätska. Beroende på utseendet av utloppskammaren så kan jetstrålen antingen gå rakt fram eller vidhäfta till någon av 45-gradigt ställda väggarna. Jetstrålen är mycket turbulent med turbulenta intensiteter på 50-100%. Turbulensmätningarna indikerar att de turbulenta strukturer vars storlek är något mindre än spalten är ovanligt intensiva och att de antagligen är viktiga för droppsönderbrytningen. Från fotografier på dropparna som bryts upp, kan man se att de turbulenta virvlar som bryter sönder dropparna varierar i storlek, från lite mindre till mycket större än dropparna själva. De stora virvlarna bryter sönder dropparna genom att de skapar höga hastighetsgradienter vilka drar sönder dropparna. De små virvlarna däremot skapar vätskestötar som deformarar dropparna så att de bryts upp.



Den kritiska fasen i droppsönderbrytningen är den initiala deformationen. När väl droppen är deformerad så att den är 3-5 gånger längre än vad den är tjock, så går resten av processen mycket fort. Den förlängda droppen blir snabbt ännu mer utdragen till en tråd, vilken sedan virvlas runt av turbulensen, för att till slut brytas ner till många små droppar. (Less)
Abstract
The overall aim of this project was to investigate the drop break-up process in milk homogenisers. This was done by measurements and calculations of the flow fields in the gap region and by visualisation of drops being broken up.



To make visualisation and measurements possible, two scale models of a homogeniser gap were developed. The full-scale model was a direct copy of the gap in a production-scale homogeniser, but with optical access. Normal operational homogenisation pressures could be tested, and drops down to 5µm in diameter could be visualised. The second model was scaled-up about 100 times ensuring that the relevant dimensionless groups were kept constant, so that the same factors governed the drop break-up... (More)
The overall aim of this project was to investigate the drop break-up process in milk homogenisers. This was done by measurements and calculations of the flow fields in the gap region and by visualisation of drops being broken up.



To make visualisation and measurements possible, two scale models of a homogeniser gap were developed. The full-scale model was a direct copy of the gap in a production-scale homogeniser, but with optical access. Normal operational homogenisation pressures could be tested, and drops down to 5µm in diameter could be visualised. The second model was scaled-up about 100 times ensuring that the relevant dimensionless groups were kept constant, so that the same factors governed the drop break-up process. The scaled-up model was made of transparent plastic and was used for both velocity field measurements and drop visualisation. From these measurements it was concluded that the drops did not break up in the entrance of the gap. Larger drops were elongated to some extent and smaller ones remained spherical. Not much happens in the gap itself. The velocity profile is very flat throughout the gap in a production-scale homogeniser. In a pilot-scale homogeniser the boundary layers have time to grow and the velocity profile is almost developed at the gap exit. The growing shear layers seem to have a limited effect on the drops. During passage through the gap small drops will have time to relax back to their spherical shape, while large ones will leave the gap with almost the same aspect ratio as when they entered it.



This study shows that drop break-up takes place in the turbulent jet at the gap outlet. The flow velocity measurements show a very unsteady jet breaking down faster than a jet in a free liquid. Depending on the geometry of the chamber at the gap outlet, the jet can attach to either of the 45-degree walls and become a wall jet. The turbulence in the jet is very high, with turbulence intensities of 50-100%. Indications were found that flow structures of the size of, or slightly smaller than, the gap height, have very high intensities. Drop deformation experiments and theoretical analyses show that the eddies breaking up the drops range in size from much larger than, to just smaller than, the drop. The larger eddies deform the drop viscously by the velocity gradient created by the eddy. The smaller eddies deform the drop by fluid inertia.



The critical phase of the drop break-up process is the initial deformation. If the drop is deformed to an aspect ratio of 3-5, the drop is then very quickly elongated into one or more filaments which may be bent, coiled and further deformed before they break up into many small droplets. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Ulrich, Joachim, Martin-Luther-Universität Halle-Wittenberg, Germany
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Food and drink technology, Livsmedelsteknik, plasma, fluiddynamik, Gaser, plasmas, fluid dynamics, POD, Gases, PIV, Eddy, Flow, Jet, Milk, Break-up, Emulsion, Drop, Homogeniser
pages
167 pages
publisher
Food Engineering, Technology and Nutrition
defense location
Room K:A, Centre for Chemistry and Chemical Engineering, Getingev.60, Lund Institute of Technology
defense date
2005-10-21 13:15
ISBN
91-628-6646-X
language
English
LU publication?
yes
id
2b2f0456-b48e-4813-913c-05f937d8eb40 (old id 545470)
date added to LUP
2007-10-13 12:25:38
date last changed
2016-09-19 08:45:15
@misc{2b2f0456-b48e-4813-913c-05f937d8eb40,
  abstract     = {The overall aim of this project was to investigate the drop break-up process in milk homogenisers. This was done by measurements and calculations of the flow fields in the gap region and by visualisation of drops being broken up.<br/><br>
<br/><br>
To make visualisation and measurements possible, two scale models of a homogeniser gap were developed. The full-scale model was a direct copy of the gap in a production-scale homogeniser, but with optical access. Normal operational homogenisation pressures could be tested, and drops down to 5µm in diameter could be visualised. The second model was scaled-up about 100 times ensuring that the relevant dimensionless groups were kept constant, so that the same factors governed the drop break-up process. The scaled-up model was made of transparent plastic and was used for both velocity field measurements and drop visualisation. From these measurements it was concluded that the drops did not break up in the entrance of the gap. Larger drops were elongated to some extent and smaller ones remained spherical. Not much happens in the gap itself. The velocity profile is very flat throughout the gap in a production-scale homogeniser. In a pilot-scale homogeniser the boundary layers have time to grow and the velocity profile is almost developed at the gap exit. The growing shear layers seem to have a limited effect on the drops. During passage through the gap small drops will have time to relax back to their spherical shape, while large ones will leave the gap with almost the same aspect ratio as when they entered it.<br/><br>
<br/><br>
This study shows that drop break-up takes place in the turbulent jet at the gap outlet. The flow velocity measurements show a very unsteady jet breaking down faster than a jet in a free liquid. Depending on the geometry of the chamber at the gap outlet, the jet can attach to either of the 45-degree walls and become a wall jet. The turbulence in the jet is very high, with turbulence intensities of 50-100%. Indications were found that flow structures of the size of, or slightly smaller than, the gap height, have very high intensities. Drop deformation experiments and theoretical analyses show that the eddies breaking up the drops range in size from much larger than, to just smaller than, the drop. The larger eddies deform the drop viscously by the velocity gradient created by the eddy. The smaller eddies deform the drop by fluid inertia.<br/><br>
<br/><br>
The critical phase of the drop break-up process is the initial deformation. If the drop is deformed to an aspect ratio of 3-5, the drop is then very quickly elongated into one or more filaments which may be bent, coiled and further deformed before they break up into many small droplets.},
  author       = {Innings, Fredrik},
  isbn         = {91-628-6646-X},
  keyword      = {Food and drink technology,Livsmedelsteknik,plasma,fluiddynamik,Gaser,plasmas,fluid dynamics,POD,Gases,PIV,Eddy,Flow,Jet,Milk,Break-up,Emulsion,Drop,Homogeniser},
  language     = {eng},
  pages        = {167},
  publisher    = {ARRAY(0x78485f8)},
  title        = {Drop Break-up in High-Pressure Homogenisers},
  year         = {2005},
}