ConductionTransfer MechanismsDirect ContactThermal ConductivityHeat Flow RateConductors and InsulatorsSwimming HoleTwo LayersThermal ResistanceR-FactorConductionConductionTransfer MechanismsTransfer MechanismsConductionConduction•Energy flow Energy flow from direct from direct thermal contactthermal contactRadiationRadiation•Energy radiating Energy radiating from an object from an object into into surroundingssurroundingsConvectionConvection•Fluid flow Fluid flow carrying energycarrying energyHeat can be transferred in three ways. Heat can be transferred in three ways. Transfer can include more than one way.Transfer can include more than one way.Direct ContactDirect ContactItems in direct contact Items in direct contact transfer heat.transfer heat.Molecules in hot regions Molecules in hot regions have greater kinetic energy.have greater kinetic energy.•Elastic collisions with cool Elastic collisions with cool moleculesmolecules•Kinetic energy transfer at Kinetic energy transfer at boundaryboundaryThermal ConductivityThermal ConductivityHeat flow within an object is Heat flow within an object is due to transfer by due to transfer by conduction.conduction.Thermal conductivity (Thermal conductivity () ) measures the ability for heat measures the ability for heat to move in a material.to move in a material.•Measured in W / m-KMeasured in W / m-K•High number means high High number means high rate of transferrate of transferMaterialMaterialThermal Cond.Thermal Cond.AirAir0.026 W/m-K0.026 W/m-KStryrofoamStryrofoam0.029 W/m-K0.029 W/m-KWoodWood0.11 W/m-K0.11 W/m-KWaterWater0.61 W/m-K0.61 W/m-KGlassGlass0.8 W/m-K0.8 W/m-KConcreteConcrete1.0 W/m-K1.0 W/m-KSteelSteel46 W/m-K46 W/m-KAluminumAluminum240 W/m-K240 W/m-KCopperCopper400 W/m-K400 W/m-KHeat Flow RateHeat Flow RateThe rate of heat flow The rate of heat flow depends on the temperature depends on the temperature gradient.gradient.•Change in temperature with Change in temperature with distancedistanceDepends on surface area Depends on surface area AA for contact.for contact.xATHT + TxTAtQHConductors and InsulatorsConductors and InsulatorsThermal conductors have Thermal conductors have high values of k. high values of k. •Metals with conducting Metals with conducting electronselectrons•Greater than 10 W/m-KGreater than 10 W/m-KStill air is an excellent Still air is an excellent thermal insulator.thermal insulator.•Materials that trap air are Materials that trap air are good: wood, styrofoamgood: wood, styrofoamVacuum would be the best.Vacuum would be the best.Swimming HoleSwimming HoleA lake with a flat bottom and A lake with a flat bottom and steep sides has a surface steep sides has a surface area 1.5 kmarea 1.5 km22 and is 8.0 m and is 8.0 m deep. The surface is at 30 deep. The surface is at 30 C C and the bottom is at 4 and the bottom is at 4 C.C.What is the rate of heat What is the rate of heat conduction through the lake?conduction through the lake?Convert area to mConvert area to m22..1.5 km1.5 km22 = 1.5 x 10 = 1.5 x 1066 m m22Use the equation for heat Use the equation for heat flow.flow.HH = - = -kAkA((TT//xx))-(0.61 W/m-K)(1.5 x 10-(0.61 W/m-K)(1.5 x 1066 m m22) ) (26 K) / (8.0 m)(26 K) / (8.0 m)HH = -3.0 x 10 = -3.0 x 1066 W. W.Two LayersTwo LayersIf there are two layers in If there are two layers in thermal contact, the rate of thermal contact, the rate of heat flow must be the same heat flow must be the same for both.for both.•Energy doesn’t accumulate Energy doesn’t accumulate in the layer.in the layer.Hx2HT1T211212232xTTAxTTAHT3x1Thermal ResistanceThermal ResistanceFor an arbitrary set of layers For an arbitrary set of layers the intermediate temperature the intermediate temperature is unknown.is unknown.•Define thermal resistanceDefine thermal resistance•For multiple layers R addsFor multiple layers R addsHR2HT1T221RRR T3R1AxRRTHR-FactorR-FactorIn the US, thermal resistance In the US, thermal resistance is measured per unit area.is measured per unit area.•RR = = xx / k / k•Units are ftUnits are ft22 F hr / BTUF hr / BTU•1 BTU = 1055 J1 BTU = 1055 JMaterialMaterialR-factorR-factorGlass (1/8”)Glass (1/8”)11Brick (3½”)Brick (3½”)0.6 – 10.6 – 1Plywood (1/2”)Plywood (1/2”)0.60.6FiberglassFiberglass insulation (1”)insulation
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