1. Conduction

2. 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.

3. 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

4. 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

5. 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. xx A T H T +  T

6. 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.

7. 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.

8. 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 x2x2 H T1T1 T2T2 T3T3 x1x1

9. 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

10. 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