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, 2001, 15,247—254
INTERNATIONAL
Agrophysics
Effectof the triticale grain moisturecontenton the spontaneousheatingofgrainandonthe pressureagainst the silowall
E。Kusinska
Department ofEngineeringandMachinery,Universityof Agriculture, Doswiadczalna44,20-236Lublin
ReceivedOctober9,2000;acceptedMay29,2001
theresultsofstudiesonthetemperatureandhorizontalpressureoftriticale grainina model silo. Thestudiesincludedgrainwithaninitial moisturecontent of13,16 and18% grainstoragedurationwas25 showedthat graintemperatureisaffectedby its initialmoisture aninitial moisture contentof18%,ahigherinitial moisturecontent resultsingreaterincreasesinpressure.
Keywords:triticale,moisturecontent, silo,heating
INTRODUCTION
Temperatureandmoisturecontent arethemostimportantfactorsaffectinggrainqualityinthecourseofits effectonstoredgrain,cause water migrationandchangesin itsdistributionwithinthe materialstored。Theaccurateprediction ofmoisturecontent and thetemperatureofgraininstorageis necessaryfor theeffectivecontrolof the process ofventilation,applied toprovide optimum storageconditionsforgrainandtheminimizing ofconditionsforinfestationbyinsects [2]。
Thefreemigration ofwaterdepends on anumberof factors—thekindandqualityof grain instorage,thesizeandshapeofthe grain,its temperature, initialmoisturecontent, and storage,aswellasonthesorptiveand diffusivepropertiesofthegrain. Thosefactors cause the processofwatermigrationtobeunstable. Watertendstomigratefrom warmertocooler areaswithingrainmass. The migration rateisfasteringrainwitha highermoisturecontentthanin drygrain[10].
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EFFECTS OFWATERMIGRATIONIN PLANTMATERIALS
Numerousauthorshavetriedto describethephenomenonofwatermigration。HolmanandCarter[10] studiedtheprocessinoveradozensoybean varietieswith differentbean soybeanvarieties, which [19]conductedexperimentsinvolving measurementsofthewheatgrain that watermigrationgenerallybeginsin the second halfofSeptemberoratthebeginningofOctober。
HellevangandHirning [9]performedastudyon16 varietiesofbeans of varioussizesduringthe periodfromAprilto %inthe upperlayer anda % .
Knowledge ofchangesoccurringinthecourseofcerealgrainstorage is veryimportantfor practicalpurposes. The applicationof asuitablemodel forthecalculationanddeterminationofthe quantitativeand qualitativedistributionofwaterandtemperaturewithinthegrainmassinstorage,thegrainbeingacommercial commodity, can helptheproperstorageof variouscerealsunder avariety ofclimaticconditions。
Anumericalmodelfor thecalculation ofwater migrationwithin grainmassin storagehasbeendevelopedbyKhankari[11]。Hederivednon-linearequations describingthetemperature, moisture and rateof free convection, forthe calculation ofwatermigrationwasusedforthe calculationoftemperatureand moisture distributioningrain, thentested experimentally [13]。Forthetests, Khankariusedacylindricalsilo,10 mhighand 10mindiameter,inwhich hestoredmaizegrain withanaveragemoisturecontentof14%atanaveragetemperatureof
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25°Cforthe periodofone year,beginning fromOctober, withoutventilation。Values ofthermalconductivity calculatedbymeansofthe modelconformed totheresultsofthe experiment. Khankari etal。[12]alsogave thevaluesof theother parametersof diffusion formaize grain。Theyfoundthat ,。,duringthe autumn rainyperiodandearly winter,watermigration islimitedtoareas closetothesilowalls. Theeffectofnaturalconvectiononwater flowappears atthe endofDecember andatthe beginningofJanuary,whentemperaturesreachthemaximumlevels。Therefore,watermigrationrateisthe fastestin the increasedthermalconductivityofgrainhasalimitingeffect onnaturalair convection, andthatwatermigrationtakesplace insilosofallsizes,thoughitbeginsearlier in downto0°C intheautumnpermitsitsmoisture contentto bekeptstablethroughouttheyear.
Loetal. [17]usedChen's andClayton’s equationforthesimulationof radialchangesinthemoisture contentofwheatgrainstoredinaconcrete equationwasbasedonthe assumption thatmoisturecontentchangesareonlyrelatedto temperature.
Thompson[20]andFanet al.[3]wereinvolvedwithmodelingthe [20] developedamodel representingtemperature changesofgraininstorage, itsmoisturecontent,anddry massdistribution。He arrived attheconclusionthatatruebalancebetweentheairand thegrainispossibletomaintain whenthegrainis ventilatedwith ambientairatlowflowrates。
Fanetal.[3] studiedwaterdiffusion in variousvarietiesof thatthecoefficientofwaterdiffusioninwheatgraincanbeexpressedintheformofan oppositetothe exponentialfunctionofabsolutetemperature,andthecoefficientdoes not changeitsvalue forhard wheatwithinthetemperaturerangeof26-54°C。 Theydeterminedthecoefficients ofdiffusion forseveralwheatvarieties withinatemperaturerangefrom26 to98°C。Thevaluesspanned a rangefrom2x10-12to245 x 10-12ms—1,depending onthetemperature andthewheatvarieties。
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Changet al. [2]maintainthattheaveragemoisturecontentofgrainstoredduringtimet +△tis:
Wu=W0+(H0—Hu)Mr (1)
where:Wu—averagemoisturecontent inthe grainlayer,finalorsubsequentsimulationfor△tperiod,kg kg-1 (decimal,。);W0 —moisture content,initialorprior tosimulationfor△tperiod,kg kg—1(decimal,d。b.); H0 -humidity ratioof ambientair,kgkg-1;Hu -humidityratioof airleavingthe grainlayer,kg kg—1;Mr-mass ratio ofinletairtothedrygrainduring△t.
Chang etal. [1]studiedwheat grainwithaninitialmoisture %, storedinsilos6。6mhighand thebasis ofthestudies, theyconcludedthatthe simulationvaluesofthegrainmoisturecontentcoincidedwiththegainmoisturevalues measured duringaperiod of15monthsand thatthe moisturecontentinthelayercloseto the surfacedecreasedby2to %duringthesummermonths,whilein thecentralandbottom partsof the silos,thechanges ingrainmoisture content wereonlyslight.
Modelingoftemperatureand themoisturecontentofricestored insiloswasthesubjectofinterestforFreer [4],and Haughetal.[8]. Haugh etal.[8]concludethatgrain temperatureisthemostimportant parameter in grainstorageand should bemaintainedat10—15°Cirrespective of thebroadrangeofthegrainmoisture ,graintemperatureis themostsignificant,though grainmoisturecontentis alsovery important.
Accordingto Freer etal.[4],theair temperaturearound thesiloshouldbeknowninorder to calculate the temperature differencesbetweenthegrain diurnaltemperatureforthe year,takingintoaccountthelatitude,and forthedetermination ofthemoisture contentofunpolishedrice,as well asofdrymass atwo—dimensionalmodelwhich theyusedto analyzechangesintemperatureandmoisturecontent,the levelof drymass losses,and thelevelofwatercondensation。Intheirstudy theyused initialgraintemperaturesof 10, 20 and 30°C,moisturecontentlevelsof 11,13and15%, andthreecharging times。Inthe testprogramtheyassessedtheinitialtemperature ofgrain,the initialgrain moisture content,and thechargingtime(thetime offilling the silowithgrain). observedatgraintemperature of
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30°Cat15%initialmoisturecontent。Highinitialtemperaturesand moisturecontent levelshad asignificanteffectonwatermigrationtowardsthetopofthesilo, which means thatthetop areaismore conducive to thegrainturningbadandtoincreasedmicrobial activity.
Increasedgraintemperature causesanincreasein thepressureexertedbygrainonthe wallsandbottomofsilos。Theeffectofthepropertiesofthematerialstored(sand, shelled maize,wheat,and sorghum) on lateralpressures inducedthermallywerestudiedby Puri etal.[18].Theresults oftheexperimentsindicatethatthermallyinducedstressin storagetanksdependsonthe bulkdensityof the thermaloverpressure (Pt)as afunctionof temperaturedrop (△T)theyusedlinearequations:
Pt=Cp(△T) (2)
whereCp—thermal pressurecoefficient(kPa℃—1).
Zhangetal.[21,23,25]improvedthemodelfor the predictionof model wasdevelopedusingfiniteelementanalysis. Itwasbasedontheelastoplastic theorydeveloped by Lade[14]andZhangetal.[22].Thetheoreticalresultsweretestedby the authorsexperimentally, by measuringthethermally induced stressin thecasingofacylindermade of aluminumsheeting0。8mmthick(。2m high)and filledwithwheatgrain。Intheirexperimentstheyappliedthreefulltemperaturechangecycleswithin the rangeof32-22℃。Stress valuesweremeasuredatthree levels。 Theauthors studiedthe effectofthe cycleorder andtemperaturechangeonthecoefficientoflateralpressure Cp. The mean,forthethreelevels,increaseinpressure,with droppingtemperature, ℃-1inthefirstcycle, 0。36kPa℃-1inthesecond,℃-1 inthethird,whilewithincreasing temperature the correspondingvalueswere0。38,0。℃-1, respectively。The authorsconcludedthatthe relationshipbetweenthe lateral thermalpressureand thetemperature change was linear,, and7。8%higher thaninthecaseofthedropping temperatureinthe first,secondandthirdcycles,respectively。
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Anothermodelof granular materialinstorage,whichtookintoaccountloadsinducedbysilowalls,aswellasthe silowall-grainand silobottom-graininterfaces,waspresentedby Zhangetal.[25].Thatmodeldidnot ,Lietal。[16]expandedanew versionofthemodelbasedonfiniteelement analysisbyincludingvaluescharacteristicforthe materialstoredwithin the rangeof modelwastestedonwheatgrainwith aninitial moisturecontentof 10%andabulk densityof 825kgm- silowiththe wheatgrainwas subjected tocyclictemperaturechangesbetween 32and22°Cwithanamplitude of10°Cper h。 Aftertheapplicationofan additionalloading of40kPa, thegrainwasletrestfor 2h。Theytested additional grain loadingatvarious depths,whichpermittedthedeterminationofthe silowalldeformation[23]。Thetwo—hourperiodofrestallowedthegraintoattain a stable stateof stress—measuringinstrumentsdid ontheemptysilo showedthatnodeformation occurredintheupperandthelowerpartsofthesilo [15]。
Zhangetal.[24]studied thechangesinthevalueof pressurequotientk inrelation to thedistancefromthesilo axisandthegrain layer depth under the effectofstaticand thermalloads inshallowanddeep silos。Theyanalyzedstressincylindricalgrainsilosusingthemodelofsecond theanalysistotwograin silos:onewithadiameter of3and9mhigh,andanother of9mindiameterand9 mhigh。Both weremadeof corrugatedsheet—metalandfilledwithwheatgrainwith 10%moisturecontentandaninitial bulkdensityof801 kgm-3.。Inbothcases theydeterminedthe main directionof stresswithinthegrainmass andtheratio koflateraltovertical foundthatneitherthelateral northevertical staticpressureswereuniform,butdecreasedinthe direction thermalpressures increasedwiththe increasingdistancefromthesiloaxis, whiletheverticalthermalpressures decreased。 Thelateralpressureincreased morethan theverticalwhenthe graintemperaturedropped to
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30°C。Thelateralpressureincreasecloseto thesilowall wasmuch stron