Question

Consider two rectangular surfaces perpendicular to each other with a common edge which is 1.6 m1.6 \mathrm{~m} long. The horizontal surface is 0.8 m0.8 \mathrm{~m} wide and the vertical surface is 1.2 m1.2 \mathrm{~m} high. The horizontal surface has an emissivity of 0.750.75 and is maintained at 400 K400 \mathrm{~K}. The vertical surface is black and is maintained at 550 K550 \mathrm{~K}. The back sides of the surfaces are insulated. The surrounding surfaces are at 290 K290 \mathrm{~K}, and can be considered to have an emissivity of 0.850.85. Find the net rate of radiation heat transfers between the two surfaces, and between the horizontal surface and the surroundings.

Solution

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Given:\textbf{Given:}

  • Two perpendicular rectangles of length: W=1.6W = 1.6 m;

  • The horizontal surface A1A_1 width: L1=0.8L_1 = 0.8 m;

  • The vertical surface A2A_2 height: L2=1.2L_2=1.2 m;

  • The horizontal surface emissivity: ε1=0.75\varepsilon_1=0.75;

  • The horizontal surface temperature: T1=400T_1 = 400 K;

  • The vertical surface emissivity: ε2=1\varepsilon_2 = 1;

  • The vertical surface temperature: T2=550T_2=550 K;

  • The emissivity of the surrounding surfaces: ε3=0.85\varepsilon_3 = 0.85;

  • The temperature of the surrounding surfaces: T3=290T_3=290 K;

Required:\textbf{Required:}

a) The net rate of radiation heat transfer between the two surfaces: Q˙12\dot Q_{12};

b) The net rate of radiation heat transfer between the horizontal surface and the surroundings: Q˙13\dot Q_{13};

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