hw1-011-ts - whole window b Horrified at the energy you...

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C HEMICAL E NGINEERING 120B Homework 1, 2010 1. Read section 1 of the 120B notes. 2. Describe some interesting heat transfer activity you did over the winter break. Be creative, and try to say something non-trivial about how this activity worked (involving conduction, convection and/or radiation) that you did not know before this class started. 3. Consider a 1m by 1m window, which consists of window glass (thermal conductivity k = 1 W/m K). The window is 1 cm thick. a. On a cold Santa Barbara night, the outside temperature is 41 F, or about 5 C, whereas you keep your house at 70 F, or about 20C. Assuming the window to behave as a flat, planar slab for heat conduction, calculate the temperature profile T ( x ) and the heat flux ( q , per unit area). What is the total heat loss per unit time (in watts) through the
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Unformatted text preview: whole window? b. Horrified at the energy you are wasting, you install a double-paned window. It has the same amount of glass, but now there are two 5 mm-thick slabs, with a 1 mm air gap ( k = . 002W/mK) sandwiched between them. Now compute the temperature profile T ( x ), the heat flux q per unit area, and the total heat loss per unit time. c. Sketch the temperature profiles for the single-pane and double-pane windows. d. Why, in this case, does the order of the materials matter? In other words, what would happen if you put the air layer ‘outside’ the two panes of glass? If it were a layer of, e.g. plexiglass, could you rearrange the layers? 4. Which is colder – bathroom floor tiles or the bathmat? Which feels colder, the tiles or the bathmat? Why?...
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