表面积散热.doc

表面积散热可由傅立叶定律来求得Q=s(dt/dx)物体的导热系数（W/m）S传热表面积（m）t过于温度()x导热面上的坐标（m）也可以用Q=st对流换热系数（W/m）对流换热系数又称表面换热系数，物理意义是指单位面积上，流体与壁面之间在单位温差下及单位时间内所能传递的热量。当表面积散热是在热载体与空气之间传导时对流换热系数的准数关系式为Nu=c(GrPr)式中：Nu=l/努谢尔特准数Gr=gl/vt葛拉晓夫准数Pr普兰德数参数查附录2l物体特性尺寸，比如水平管路的管径；垂直管路、板、箱体的高对流换热系数（W/m）空气的导热系数（W/m）v=/空气的运动粘度（m/s）空气的密度（kgm-3）参数查附录2空气的粘度（Pas）参数查附录2=1/Tm流体的体积膨胀系数（1/K）Tm定性温度Tm=(T1+T0)/2+273T1和T0分别代表热源的表面温度和空气温度，t=T1-T0过于温度（）c和n可由GrPr值查附录1的到具体的公式l/=c(GrPr)=c(GrPr)/l=c(gl/vtPr)/l=c(9.8l/(/)(1/(T1+T0)/2+273)(T1-T0)Pr/l表面积散热量Q=st参数查附录1表式Nu=c(GrPr)中的c和n值加热表面形状特征尺寸（GrPr）范围cn水平圆管外径do1041090.531/410910120.131/3垂直管或板高度L1041090.591/410910120.101/3参数查附录2干空气的物理性质温度t/密度/kgm-3比定压热容cp/kJkg-1K-1导热系数/10-2Wm-1K-1粘度/10-5Pas普兰德数Pr-201.3951.0092.2791.620.716-101.3421.0092.3601.670.71201.2931.0092.4421.720.707101.2471.0092.5121.760.705201.2051.0132.5931.810.703301.1651.0132.6751.860.701401.1281.0132.7561.910.699501.0931.0172.8261.960.698601.0601.0172.8962.010.696701.0291.0172.9662.060.694801.0001.0223.0472.110.692900.9721.0223.1282.150.6901000.9461.0223.2102.190.6881200.8981.0263.3382.280.6861400.8541.0263.4892.370.6841600.8151.0263.6402.450.6821800.7791.0343.7802.530.6812000.7461.0343.9312.600.6802500.6741.0434.2682.740.6773000.6151.0474.6052.970.6743500.5661.0554.9083.140.6764000.5241.0685.2103.300.6785000.4561.0725.7453.620.687例:在一室温为20的大房间中，安有直径为0.1m、水平长度为10M的管道，若管道外壁平均温度为180，试求该管道因自然对流的散热量.解：自然对流的对流换热系数=c(GrPr)/l=c(gl/vtPr)/l由t=T1-T0=180-20=160查表可得=0.815kgm-3=2.45*10-5Pas）Pr=0.682=3.640*10-2Wm-1K-1=1/Tm=1/【(T1+T0)/2+273】=1/373Gr=9.8l/(/)(1/(T1+T0)/2+273)(T1-T0)=9.80.10.10.1（2.45*10-50.815）1/373160=9.8*10-30.42（3*10-5）=4.11*10-3（3*10-5）=4.57*106GrPr=4.57*1060.682=3.12*106查表可得c=0.52n=1/4=c(gl/vtPr)/l=3.640*10-20.52(3.12*106)/0.1=0.019(3.12*106)