平均换热系数公式及设置.doc
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一)平均换热系数公式
平均换热系数的计算公式为:
,其中R为某一径向坐标,αR为该坐标处的局部换热系数。
平均换热系数
局部换热系数
图2.15下集管圆形射流壁面射流速度径向变化曲线
图2.16下集管圆形射流换热系数径向分布曲线
下集管圆形射流换热特性的影响因素包括表面射流出口速度v0、射流的出口直径D、射流出口距钢板表面的距离H等。
下集管射流的换热系数随射流出口距钢板表面的距离H的增大而减小,其他因素对换热特性的影响与上集管射流相同。
下面还有设置平均平均换热系数
二)模型设置问题如图
WallFunc.HeatTran.Coef.定义
WallFunc.HeatTran.Coef.isdefinedbytheequation
(30.4-44)
whereisthespecificheat,istheturbulencekineticenergyatpoint,andisdefinedinEquation12.10-5.
Thelaw-of-the-wallimplementedinFLUENThasthefollowingcompositeform:
(12.10-5)
whereiscomputedbyusingtheformulagivenbyJayatilleke[165]:
(12.10-6)
and
=
turbulentkineticenergyatpointP
=
densityoffluid
=
specificheatoffluid
=
wallheatflux
=
temperatureatthecelladjacenttowall
=
temperatureatthewall
=
molecularPrandtlnumber()
=
turbulentPrandtlnumber(0.85atthewall)
=
VanDriestconstant(=26)
=
meanvelocitymagnitudeat
Notethat,forthepressure-basedsolver,theterms
and
willbeincludedinEquation12.10-5onlyforcompressibleflowcalculations.
Thenon-dimensionalthermalsublayerthickness,,inEquation12.10-5iscomputedasthevalueatwhichthelinearlawandthelogarithmiclawintersect,giventhemolecularPrandtlnumberofthefluidbeingmodeled.
Theprocedureofapplyingthelaw-of-the-wallfortemperatureisasfollows.Oncethephysicalpropertiesofthefluidbeingmodeledarespecified,itsmolecularPrandtlnumberiscomputed.Then,giventhemolecularPrandtlnumber,thethermalsublayerthickness,,iscomputedfromtheintersectionofthelinearandlogarithmicprofiles,andstored.
Duringtheiteration,dependingonthevalueatthenear-wallcell,eitherthelinearorthelogarithmicprofileinEquation12.10-5isappliedtocomputethewalltemperatureorheatflux(dependingonthetypeofthethermalboundaryconditions).
ThefunctionforgivenbyequationEquation12.10-6isrelevantforthesmoothwalls.Fortheroughwalls,however,thisfunctionismodifiedasfollows:
(12.10-7)
whereisthewallfunctionconstantmodifiedfortheroughwalls,definedby.Tofindadescriptionoftheroughnessfunction,youmayrefertoEquation7.13-3inSection7.13.1.
SurfaceHeatTransferCoef.(intheWallFluxes...category),asdefinedinFLUENT,isgivenbytheequation
(30.4-33)
whereisthecombinedconvectiveandradiativeheatflux,isthewalltemperature,andisthereferencetemperaturedefinedintheReferenceValuespanel.Pleasenotethatisaconstantvaluethatshouldberepresentativeoftheproblem.Itsunitquantityistheheat-transfer-coefficient.