Anonymous
Anonymous asked in 科學及數學其他 - 科學 · 6 years ago

Thermal phys: heat flow

The multi-layer wall system consisting of three plane walls "A", "B", and "C" as shown in the figure, which is transferring heat from fluid I at 100oC to fluid II at 20oC. These fluids are in contact with its left and right surfaces respectively. Estimate the heat flow form fluid I to fluid II through a unit width of the multi-layer wall system by assuming one-dimensional steady-state conduction and negligible radiation. Thermal conductivities of the wall materials are kA=0.6W/mK, kB=1.5W/mK, kC=0.8W/mK.

Ans: 83.96W

What can I solve this? The figure is so complicated....

Please help.

figure: http://postimg.org/image/w98i7wbbd/

Update:

Sorry, I just found the solution. But sttill have questions about it.

total resistance = resistance of I + resistance of A + resistance of BC + resistance of II

= 1/[(15)(8.4)] + (0.8)/[(0.6)(8.4)] + 1/[ (1.5)(3)/(1.2) + (0.8)(5.4)/(1.2) ] + 1/[(8)(8.4)]

=0.3176

Update 2:

total heat transfer = (100-20)/0.3176=251.9W

So do you know why resistance of I and II are also included and what are the values 8.4, 3, and 5.4? Area??

Thank you very much!!

1 Answer

Rating
  • 天同
    Lv 7
    6 years ago
    Favorite Answer

    Thermal conductivity of composite wall B and C

    = (1 x 1.5 + 1.8 x 0.8)/(1+1.8) w/m^2.K = 1.05 w/m.K

    Resistance of composite wall B and C = 1.2/1.05 m^2.K/w = 1.143 m^2K/w

    Resistance of wall A = 0.8/0.6 m^2K/w = 1.333 m^2K/w

    Total resistance of walls A, B and C = (1.333 + 1.143) m^2K/w = 2.476 m^2K/w

    Hence, heat flow by conduction pe unit width

    = (100 - 20)/2.476 w = 32.3 w

    Please check if your given answer is correct.

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