04_thermal

3 and 7 respectively they are followed closely by

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Unformatted text preview: , Aerogel has the next highest R-value 10, followed by isocyanurate and phenolic foam insulations with, 8.3 and 7, respectively. They are followed closely by polyurethane and polystyrene insulation at roughly R–6 and R–5. Loose cellulose, fiberglass both blown and in batts, and rock wool both blown and in batts all possess an Rvalue of roughly 3. Straw bales perform at about R–1.45. Snow is roughly R–1. • Absolutely still air has an R-value of about 5 but this has little practical use: Spaces of one centimeter or greater will allow air to circulate, convecting heat and greatly reducing the insulating value to roughly R–1 – thermal resistance R-value is t/ – R-value is usually seen in imperial units: ft2·F·hr/Btu – Conversion factor is 5.67: • R-value of 0.025-thick insulation of = 0.02 W/m/K is: R = 5.67 t / = 5 .67 0.025/0.02 = 7.1 – Can insert Home-Depot R=5 insulation into formula: P = 5.67 A· T/R – so for our hut with R = 5: P 5.67 (12)(30)/5 = 408 W – note our earlier insulation example had R = 7.1 instead of 5, in which case P = 288 W (check for yourself!) Winter 2008 Lecture 4 7 Winter 2008 8 2 Thermal Considerations 01/17/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Convective Heat Exchange Convection Examples • Air (or any fluid) can pull away heat by physically transporting it • Standing unclothed in a 20° C light breeze light – h 5 W/K/m2 – T = 17 ° C – A 1 m2 – P (5)(17)(1) = 85 W – really conduction into fluid accompanied by motion of fluid – full, rigorous, treatment beyond scope of this class • General behavior: power convected = P = h· T ·A – A is area, T is temperature difference bet...
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This note was uploaded on 01/30/2014 for the course PHYS 121 taught by Professor Staff during the Winter '08 term at UCSD.

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