Problem set _1 - DOC by liwenting


									                                  CHE312: Problem set #6
1-8 Copy the program heat6.exe from the class distribution folder to your flash or H drive.
You can also download the program from the website:

Run the program and choose any problem 1-8. Solve the problem with the data provided by
the program, copy the problem statement to Word. The program will check your answer and
provide an answer code when you click on “Check”. Copy the answer code and paste them
after the problem statement. Your work should look like this:

7) Spheres A and B are initially at 800 K, and they are simultaneously quenched in large
constant temperature baths, each having a temperature of 320 K. The following parameters
are associated with each of the spheres and their cooling processes. Sphere A: diameter 300
mm, density 1600 kg/m3, specific heat 400 J/kg*K, thermal conductivity 170 W/m*K,
convection coefficient 5 W/m2*K. Sphere B: diameter 30 mm, density 400 kg/m3, specific
heat 1,600 J/kg*K, thermal conductivity 1.70 W/m*K, convection coefficient 50 W/m2*K.
Calculate the time required for the surface of sphere A reach 415 K.

Problem 7: Correct, Code =4313336481


Calculate the time required for the surface of each sphere to reach 415oK.

                                             T  T         t
Sphere A: lumped capacitance method:                 = exp   
                                             Ti  T        

                                  Vc        roc       (1600)(0.15)(400)
The thermal time constant  =           =           =                     = 6400 s
                                  hAs       3h                (3)(5)

             T  T                415  320
t =   ln           =  (6400) ln           = 10,367 s
             Ti  T               800  320

Box the answer(s)!

9.1     A thermopane window consists of two pieces of glass 7 mm thick that enclose an air
        space 7 mm thick. The window separates room air at 20OC from outside ambient air at
         10oC. The convection coefficient associated with the inner (room-side) surface is 10
        W/m2K. Glass: kg = 1.4 W/ mK, air: ka = 0.0245 W/ mK. The window is 0.8 m long
        by 0.5 m wide. Neglect radiation, and assume the air enclosed between the panes to
        be stagnant. Compute and plot the effect of ho on the heat loss for 10  ho  100
        W/m2K. Repeat this calculation for a triple-pane construction in which a third pane
        and a second air space of equivalent thickness are added.

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