p3.5-10 - The efficiency is defined as the ratio of the...

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3.5-10 You are in charge of a processing line that freezes D = 3 inch diameter spheres of dough as they emerge from a mixing chamber. The dough has conductivity k = 0.5 W/m-K and thermal diffusivity α = 1.4x10 -7 m 2 /s. The dough balls leave the mixing chamber at a uniform temperature, T ini = 35ºC. The balls are exposed to very cold air at T = -50ºC with heat transfer coefficient h = 20 W/m 2 -K. a.) Is a lumped capacitance model appropriate for this problem? b.) Assume that your answer to (a) is no. Use the sphere_T function in EES to plot the center and surface temperature of the dough as a function of time. c.) The processing can be terminated when the center temperature reaches the target temperature of T freeze = -5ºC. Use EES to determine the time required for the processing. d.) What is the efficiency of the freezing process?
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Unformatted text preview: The efficiency is defined as the ratio of the minimum possible amount of energy that must be removed from the dough ball to bring it to a uniform temperature of T freeze to the actual amount of heat removed from dough ball at the time identified in (c). You may want to use the function sphere_Q in EES to do this problem. e.) You would like to improve the processing time and are looking at blowers that will increase the heat transfer coefficient by increasing the velocity of the air. Plot the processing time (i.e., the answer to (c)) as a function of the heat transfer coefficient. At high values of h your plot should asymptotically approach a constant value; explain this behavior. f.) Overlay on your plot from (e) the efficiency as a function of h . Explain the shape of your plot....
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This note was uploaded on 08/11/2008 for the course ME 364 taught by Professor Rothamer during the Summer '08 term at Wisconsin.

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