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Conduction

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Conduction

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  1. Conduction

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

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

  4. Heat flow within an object is due to transfer by conduction. Thermal conductivity (k) measures the ability for heat to move in a material. Measured in W / m-K High number means high rate of transfer Material Thermal Cond. Air 0.026 W/m-K Stryrofoam 0.029 W/m-K Wood 0.11 W/m-K Water 0.61 W/m-K Glass 0.8 W/m-K Concrete 1.0 W/m-K Steel 46 W/m-K Aluminum 240 W/m-K Copper 400 W/m-K Thermal Conductivity

  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. A H T + DT T Dx

  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. A lake with a flat bottom and steep sides has a surface area 1.5 km2 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 m2. 1.5 km2 = 1.5 x 106 m2 Use the equation for heat flow. H = -kA(DT/Dx) -(0.61 W/m-K)(1.5 x 106 m2) (26 K) / (8.0 m) H = -3.0 x 106 W. Swimming Hole

  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. T3 T2 T1 H H Dx2 Dx1

  9. Thermal Resistance • For an arbitrary set of layers the intermediate temperature is unknown. • Define thermal resistance • For multiple layers R adds T3 T2 T1 H H R2 R1

  10. In the US, thermal resistance is measured per unit area. R = Dx / k Units are ft2F hr / BTU 1 BTU = 1055 J Material R-factor Glass (1/8”) 1 Brick (3½”) 0.6 – 1 Plywood (1/2”) 0.6 Fiberglass insulation (1”) 4 R-Factor