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Homework Help: Heat Transfer

  1. Oct 27, 2009 #1
    1. The problem statement, all variables and given/known data

    An electrical wire 1/8" in diameter is covered with insulation 3/32" thick. Six watss of electrical energy are dissipated as heat by the resistance of the wire per foot length. The heat is given up to the surrounding air at 60 degrees farneheit. Estimate the outside surface temperature of the wire insulation.

    3. The attempt at a solution

    Assuming laminar flow..

    ho = .25(dTs/0.3125)^0.25
    ho = .334 dTs^.25

    Qdot = .0006 kW x 3413 Btu/hr = 20.5 Btu/hr
    (I'm not sure if this is the correct way to solve for heat loss, it feels like I am leaving out the insulation?)

    Qdot = ho x Ao x dTs

    20.5 = .334(dTs^.25) x (0.082) x dTs

    Solving for dTs I get 199.2 F.

    dTs = Ts - To
    Ts = 259.2 F

    For all I know this could be correct, but it seems awfully hot for an insulated electrical wire.
    Alot of problems simmilar to this I solved using Newton's Method. But without a k value for the insulation I am not compeltely sure on how to approach the problem.

  2. jcsd
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