Conduction.

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Presentation transcript:

Conduction

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

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

Thermal Conductivity 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

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

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.

Swimming Hole 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.

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

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

R-Factor 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