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1、Heat transfer Principles of Chemical Engineering-CDUTHeat conductiondQtdSn The physical law describing the phenomenon of heatconduction is Fouriers law,which is expressed as follows:Fouriers Lawtemperature gradientthermal conductivityDifferential heat fluxthe direction of heat flux is opposite to th
2、e direction of temperature gradientPrinciples of Chemical Engineering-CDUTThermal conductivityThermal conductivitydQ dStn Thermal conductivity indicates the thermal conductionproperties of the material.It is one of the important basicphysical properties of matter,and its value is related to thematte
3、rs shape,composition,density,temperature,etc.Principles of Chemical Engineering-CDUTThermal conductivitythermal conductivity W/(m.oC)0.0060.060.070.70.23.0154200.25gasliquidnon-conducting solidmetalinsulation materialPrinciples of Chemical Engineering-CDUTThermal conductivity of solidFor most homoge
4、neous solids,its thermal conductivity isapproximately linear with the temperature,i.e01tFor most metallic materials,is negative;while for mostnon-metallic materials,is positive;for ideal gas，=1/T，1/K。Principles of Chemical Engineering-CDUTThermal conductivity of liquidThermal conductivity of liquidE
5、xcept for water and glycerin,the thermal conductivity ofmost non-metallic liquids decreases with the increase oftemperature;The thermal conductivity of metallic liquids is higher thanthat of ordinary liquids;The thermal conductivity of pure liquid is larger than that ofits solution.Principles of Che
6、mical Engineering-CDUTThermal conductivity of gasThe thermal conductivity of gasThe thermal conductivity of gas increases with the increaseof temperature.In a large pressure range,the change of thermal conductivityof gas with pressure is very small,which can be ignored.The thermal conductivity of ga
7、s is very small,which is unfavorable to heat conduction,so it is often used for heat preservation and insulation.Principles of Chemical Engineering-CDUTOne dimensional steady-state heat conduction of single-layer flat wallAssumptions：1.The thermal conductivitydoes not change with thetemperature,or t
8、he averagevalue can be taken;2.Stable state in onedimension3.Ignore the heat loss.Fifure 3 heat conduction of single-layer flat wallPrinciples of Chemical Engineering-CDUTOne dimensional steady-state heat conduction of single-layer flat walldtQSdx 12()SQttbdifferential equation integral formula Acco
9、rding to Fourier law,the following can be obtained for stable state heat conduction in one dimension on a one-layer flat wall:Integrating the differential equation and processing it:Principles of Chemical Engineering-CDUTOne dimensional steady-state heat conduction of single-layer flat wall12tttQbRS
10、12ttQtqbSRthermal conduction resistancePrinciples of Chemical Engineering-CDUTOne dimensional steady-state heat transfer of multi-layer flat wallFigure 4 heat conduction of three-layer flat wallAssumptions：1.Thethermalconductivitydoesnotchangewithtemperature,so we can useaverage value;2.Stablestatei
11、nonedimension3.The ignore of the heat loss4.Nothermalcontactresistance,that is,differentflat walls in good contactPrinciples of Chemical Engineering-CDUTOne dimensional steady-state heat transfer of multi-layer flat wallThe heat transfer rate through the flat wall section of each layer must be equal
12、 1234QQQQQ233412123123ttttttQSSSbbb233412123123ttttttQbbbSSSorPrinciples of Chemical Engineering-CDUTOne dimensional steady-state heat transfer of multi-layer flat wallSteady state heat conduction rate equation of three-layer flat wall 14312123ttQbbbSSSFor n-layer flat wall,the heat transfer rate eq
13、uation can beexpressed as11niittQbSPrinciples of Chemical Engineering-CDUTThermal contact resistanceDue to the roughness of two contact surfaces,the additional thermalresistance may be caused by the obvious temperature reduction betweenthe interfaces of different materials.Because there are holes be
14、tween the two contact surfaces and theholes are full of air,the thermal contact resistance includes the thermalresistance through the actual contact surface and the thermal resistancethrough the holes(there is radiation heat transfer at high temperature).The thermal contact resistance is related to the contact surfacematerial,surface roughness and the pressure on the contact surface.It canbe measured by experiment.Thermal contact resistancePrinciples of Chemical Engineering-CDUTThermal contact resistanceFigure 5 Effect of contact thermal resistanceThermal contact resistance