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 Air Stryrofoam Wood Water Glass Concrete Steel Aluminum Copper Thermal Cond. 0.026 W/m-K 0.029 W/m-K 0.11 W/m-K 0.61 W/m-K 0.8 W/m-K 1.0 W/m-K 46 W/m-K 240 W/m-K 400 W/m-K Heat Flow Rate • Change in temperature with distance A H T + DT The rate of heat flow depends on the temperature gradient. T Depends on surface area A for contact. Dx H DQ DT kA Dt 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 T3 T2 H T1 H 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. Dx2 Dx1 H k 2 A T3 T2 T T k1 A 2 1 Dx2 Dx1 Thermal Resistance T3 T2 T1 H H For an arbitrary set of layers the intermediate temperature is unknown. • Define thermal resistance R R2 R1 DT H R Dx kA • For multiple layers R adds R R1 R2 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 Glass (1/8”) Brick (3½”) Plywood (1/2”) Fiberglass insulation (1”) R-factor 1 0.6 – 1 0.6 4 next