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HeatTransfer-I-Section-7

# The maximum design temperature of the heat sink base

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Unformatted text preview: pplicable to this situa'on. •  In later fundamental problems, we will see this factor of “2” change to something else. It is NOT a universal rela'onship! It arises from integra'on. •  We will consider, isoﬂux and turbulent ﬂow separately. 9 Example 1 •  You are considering an electronics cooling system to cool a number of heat dissipa'ng components. The maximum design temperature of the heat sink base plate is 75 C set by the customer’s criteria. You are considering a heat sink which is 50 cm long and contains 20 ﬁns spaced at 1 cm. Each ﬁn is 1.5 mm in thickness. Another member of your engineering design team informs you that you can save weight (and cost) by using only 50% of the ﬁn material in a sloced ﬁn conﬁgura'on to take advantage of the higher heat transfer co ­eﬃcient that a shorter ﬁn experiences. Keeping this in mind, you consider an alternate design which has 20 rows of 5 sloced ﬁns, each of 5 cm length spaced equally in the ﬂow direc'on and having the same lateral spacing as the original design. (Refer to sketch). In both cases the free stream velocity is U = 10 m/s and temperature is Tf = 25 C. –  Validate your co ­worker’s claim, by calcula'ng the heat transfer rates for each design assuming a perfect ﬁn, i.e. no ﬁn resistance. What is the percent increase/decrease in heat transfer of the sloced heat sink? –  Next, since you strongly believe in the old adage “there is no free lunch”, determine the penalty paid for this savings in material, by considering the increase/decrease in skin fric'on or ﬂuid drag. Is this the only component of drag which results? Would you expect the penalty to be greater or less than that predicted? –  Brieﬂy explain why the heat transfer rate is higher/lower for the sloced case....
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