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Unformatted text preview: ween surface and
bath
– h is the convection coefficient, units: W/K/m2
– still air has h 2–5 W/K/m2
• higher when T is higher: selfdriven convective cells
• note that h = 5.67 is equivalent to R = 1 • Our hut from before:
– h 5 W/K/m2
– T = 30° C (if the skin is at the hot temperature)
– A 12 m 2
– P (5)(30)(12) = 1800 W – gentle breeze may have h 5 – 10 W/K/m2
– forced air may be several times larger (h 1 0–50) Winter 2008 9 Winter 2008 10 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Radiative Heat Exchange Radiative Examples
• Standing unclothed in room with 273° C walls
273
walls • The StephanBoltzmann law tells us:
– P = A ( Th 4 Tc 4 )
– The StephanBoltzmann constant, = 5.67 108 W/m2/K4
– in thermal equilibrium (Th = Tc), there is radiative balance, and P = 0
– the emissivity ranges from 0 (shiny) to 1 (black)
– “black” in the thermal infrared band (
10 µm) might not be intuitive
•
•
•
• your skin is nearly black (
0.8)
0.8–1.0)
plastics/organic stuff is nearly black (
even white paint is black in the thermal infrared
metals are almost the only exception – for small T around T, P 4 A T3 T = ( 4 T3) ·A· T
– which looks like convection, with h = 4 T3
– for room temperature, h 5.7 W/K/m2, so similar in magnitude to
convection
Winter 2008 Lecture 4 11 – and assume emissivity is 0.8 for skin
– A 1 m2
– T = 310 K
– P (0.8)(1)(5.67 108)(3104) = 419 W (burr) • Now bring walls to 20° C
– T = 17 ° C
– P (0.8)(1)(5.67 108)(3104 2934) = 84 W
– pretty similar to convection example
– note that we brought our cold surface to 94.5% the absolute
temperature of the warm surface, and only reduced the
radiation by a factor of 5 (not a factor of 18): the fourth
power makes this highly nonlinear Winter 2008 12 3 Thermal Considerations 01/17/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Combined Problems TwoLayer insulation
• Let’s take our icefishing hut and add insulation
Le...
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 Winter '08
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 Energy, Thermal Energy

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