entiresurface. MostEFDtechniquesrely on probes inserted eitherdirectly onto
the surface, intothe flow domain or having a transparent model of the
equipment. An additionaladvantageof havinga validated CFDmodelof flow
in a componentis that a converged solution contains not only datathat were
interesting fromthe outsetof the simulations,but also additionaldatathat can
be used for furtherevaluations of, for example,pressure loss and careful
processing.
The mainreasonfor suggesting CFDas a tool in the questof improving the
hygienicdesignof closedequipmentis the fact that CFDproducesdata overthe
entiredomain, not only at discretepoints as many experimentalmethods.
Furthermore, experimentalmethodsfor measuring wallshearstressand fluid
exchange (the two mostinterestingfor evaluating cleanability)are complexto
use and modifiedequipment (transparent or withspecialmeasuringprobes
inserted)is required.ResultsobtainedfromCFDsimulationsshould, to some
degree,be compared withexperimentalor analytical data.If the purpose of the
CFDsimulationsis to comparea number of designchanges, this is possible
without a validation, as the effect of a designchangeon, e.g.,the wallshear
stressdistribution can thenbe seenrelativelyto the resultsobtainedin the
relateddesigns.
11.3 Computationalfluiddynamics modelsfor optimisinghygiene
Fromthe aboveit is clearthat flowis importantto the cleaning characteristics of
a pieceof closed equipment.Hence,to comparethe hygieniccharacteristics of
twofamiliar pieces of closed equipment or the effectof design changes,
knowledgeof the flowinsidethe equipmentis needed. Flowcan be visualised
experimentally or by modelling using CFD. Each has advantages and
disadvantagesas discussedin Section11.2.3.
In the late 1990s,commercialCFDcodesweremadeavailable for Windows
and Linuxplatform users.This,combined with increased(andcheap) CPU
powerand memory capacityof personalcomputers, has made it feasibleto use
CFDcodeson personalcomputers.Discretisationof the flow domain (creating
the mesh)and the processof creatingand setting-up CFDmodels havebeen
simplified and made more user-friendlyover the past5 to 7 years.Still,a
background in fluid mechanicsis recommended to make modelset-upeasierand
evaluation of results more straightforward. Some of the most popular
commercial codes are CFX (www-waterloo.ansys.com/cfx), Fluent Inc.
(www.fluent.com)and Star-CD(www.cd.co.uk), all availablefor bothUNIX
and Windowsplatforms.
MostCFDsimulations are basedon the samereciperight fromthe creation of
the meshuntila converged solution is obtained.
∑ The flowdomainis dividedinto smallcontrolvolumes(calledthe mesh).
Momentum (velocity) and pressurein eachcontrol volume,basedon the
Improvingthe hygienicdesignof closedequipment 197