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Conduction

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Transfer Mechanisms Conduction Energy flow from direct thermal contactEnergy flow from direct thermal contact Radiation Energy radiating from an object into surroundings Convection Fluid flow carrying energyFluid flow carrying energy Heat can be transferred in three ways. Transfer can include more than one way.

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Direct Contact Items in direct contact transfer heat. Molecules in hot regions have greater kinetic energy. Elastic collisions with cool moleculesElastic collisions with cool molecules Kinetic energy transfer at boundaryKinetic energy transfer at boundary

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Thermal Conductivity Heat flow within an object is due to transfer by conduction. Thermal conductivity ( ) measures the ability for heat to move in a material. Measured in W / m-KMeasured in W / m-K High number means high rate of transferHigh number means high rate of transfer MaterialThermal Cond. Air0.026 W/m-K Stryrofoam0.029 W/m-K Wood0.11 W/m-K Water0.61 W/m-K Glass0.8 W/m-K Concrete1.0 W/m-K Steel46 W/m-K Aluminum240 W/m-K Copper400 W/m-K

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Heat Flow Rate The rate of heat flow depends on the temperature gradient. Change in temperature with distance Depends on surface area A for contact. xx A T H T + T

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Conductors and Insulators Thermal conductors have high values of k. Metals with conducting electrons Greater than 10 W/m-K Still air is an excellent thermal insulator. Materials that trap air are good: wood, styrofoam Vacuum would be the best.

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Swimming Hole A lake with a flat bottom and steep sides has a surface area 1.5 km 2 and is 8.0 m deep. The surface is at 30 C and the bottom is at 4 C. What is the rate of heat conduction through the lake? Convert area to m 2. 1.5 km 2 = 1.5 x 10 6 m 2 Use the equation for heat flow. H = -kA( T/ x) -(0.61 W/m-K)(1.5 x 10 6 m 2 ) (26 K) / (8.0 m) H = -3.0 x 10 6 W.

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Two Layers If there are two layers in thermal contact, the rate of heat flow must be the same for both. Energy doesn’t accumulate in the layer. H x2x2 H T1T1 T2T2 T3T3 x1x1

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Thermal Resistance For an arbitrary set of layers the intermediate temperature is unknown. Define thermal resistance For multiple layers R adds H R2R2 H T1T1 T2T2 T3T3 R1R1

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R-Factor In the US, thermal resistance is measured per unit area. R = x / kR = x / k Units are ft 2 F hr / BTUUnits are ft 2 F hr / BTU 1 BTU = 1055 J1 BTU = 1055 J MaterialR-factor Glass (1/8”)1 Brick (3½”)0.6 – 1 Plywood (1/2”)0.6 Fiberglass insulation (1”)4 next

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