Question

Two parallel black rectangular surfaces, whose back sides are insulated, are ori- ented parallel to each other with a spacing of 5 m. They measure 10 m by 20 m The surroundings are black at 0 K. The two surfaces are maintained at 200 K and 100 K, respectively. View factors for geometries of two parallel rectangular surfaces are illustrated in Figure 3. Determine the following (i) the net radiant heat transfer between the two surfaces; (ii) the net heat supplied to each surface; (iii) the net heat transfer between each surface and the surroundings 0.7 0.5 0.4 0.3 6 0.2 F, 0.1 0.07 2 0.05 0.04 0.03 YIL - 0.1 0.02 0.01 0.1 0.2 0.3 0.5 1.0 2 345 1020 X/L Figure 3: View factors for geometries of two parallel rectangular surfaces

Answer 1

Given that for two flat plates X/L = 10/20 = 0 from graph provided the view factor will be F_12 = 0.2 (approximately) (i) Net radiant heat transfer be t^n two surfaces phi = sigma Times F_12 Times (T_1^n - t_2^n) ./.1/sigma_1 + 1/sigma_2 - 1 Given that plates are black and insulated hence sigma_1 = sigma_2 = 1 phi = 5.67 Times 10^-8 Times 10 Times 20 Times 0.2 Times (200^4 - 100^4) /1 + 1 - 1 phi = 3402 w (ii) net heat supply to each surface: for surface 1 = Q Times A_1 Times f_21