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    5 2018 12 08!06 16 16 PM

    Uploaded by

    Mikhoy Riveral
    The document contains 6 examples of heat transfer problems involving objects cooling down from an initial temperature to a lower temperature over time due to convection. The examples involve calculating the time required for the temperature at the center or depth of various objects like aluminum spheres, copper spheres, brass plates, concrete walls, stainless steel plates and aluminum bars to reach a specified temperature when exposed to convection environments at different temperatures. The objects differ in terms of their material, initial temperature, size, and the convection heat transfer coefficients of the environments.

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    5 2018 12 08!06 16 16 PM

    Uploaded by

    Mikhoy Riveral
    8 views5 pages
    The document contains 6 examples of heat transfer problems involving objects cooling down from an initial temperature to a lower temperature over time due to convection. The examples involve calculating the time required for the temperature at the center or depth of various objects like aluminum spheres, copper spheres, brass plates, concrete walls, stainless steel plates and aluminum bars to reach a specified temperature when exposed to convection environments at different temperatures. The objects differ in terms of their material, initial temperature, size, and the convection heat transfer coefficients of the environments.

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    The document contains 6 examples of heat transfer problems involving objects cooling down from an initial temperature to a lower temperature over time due to convection. The examples involve calculating the time required for the temperature at the center or depth of various objects like aluminum spheres, copper spheres, brass plates, concrete walls, stainless steel plates and aluminum bars to reach a specified temperature when exposed to convection environments at different temperatures. The objects differ in terms of their material, initial temperature, size, and the convection heat transfer coefficients of the environments.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    8 views5 pages

    5 2018 12 08!06 16 16 PM

    Uploaded by

    Mikhoy Riveral
    The document contains 6 examples of heat transfer problems involving objects cooling down from an initial temperature to a lower temperature over time due to convection. The examples involve calculating the time required for the temperature at the center or depth of various objects like aluminum spheres, copper spheres, brass plates, concrete walls, stainless steel plates and aluminum bars to reach a specified temperature when exposed to convection environments at different temperatures. The objects differ in terms of their material, initial temperature, size, and the convection heat transfer coefficients of the environments.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    of 5

    EX.

    A piece of aluminum weighing 6 kg and initially at a temperature of


    300◦C is suddenly immersed in a fluid at 20◦C. The convection heat-
    transfer coefficient is 58W/m2 . ◦C. Taking the aluminum as a sphere
    having the same weight as that given, estimate the time required to cool
    the aluminum to 90◦C, using the lumped-capacity method of analysis.

    EX.2

    A 4 cm diameter copper sphere is initially at a uniform temperature of


    220◦C. It is suddenly exposed to a convection environment at 30 ◦C with
    h=20 W/m2 ・ ◦C. Calculate the time necessary for the center of the
    sphere to reach a temperature of 80◦C.
    EX.3

    In a production facility, large brass plates of 4 cm thickness that are


    initially at a uniform temperature of 20°C are heated by passing them
    through an oven that is maintained at 500°C. The plates remain in the
    oven for a period of 7 min. Taking the combined convection and radiation
    heat transfer coefficient to be h = 120 W/m2 · °C, determine the surface
    temperature of the plates when they come out of the oven.
    EX.4
    CLASS WORK
    Buildings of various constructions exhibit different responses to thermal
    changes in climate conditions. Consider a 10-cm-thick wall of normal
    weight structural concrete(c=0.9 kJ/kg .◦C) suddenly exposed to a “blue
    norther” at−10◦Cwith a convection coefficient of 65 W/m2 . ◦C. The wall
    is initially at 15◦C. Estimate the time required for the wall temperature to
    drop to 5◦C. State the assumptions.
    EX.5

    A very thick plate of stainless steel (18% Cr, 8% Ni) at a uniform


    temperature of 300◦C has its surface temperature suddenly lowered to
    100◦C. Calculate the time required for the temperature at a depth of 3 cm
    to attain a value of 200◦C.

    SOLUTION:

    H.W
    An aluminum bar has a diameter of 11 cm and is initially uniform in
    temperature at 300◦C. If it is suddenly exposed to a convection
    environment at 50◦C with h=1200 W/m2 .◦C, how long does it take the
    center temperature to cool to 80 ◦C? Also calculate the heat loss per unit
    length.

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