The spectral absorptivity of a large diffuse surface is ?_? = 0.9 for A ? = 0.3 for ? > 1?m. The bottom of the surface is well insulated while the top may be exposed to one of two different conditions.

(a) In case (a) the surface is exposed to the sun which provides an irradiation of G_s = 1200 W/m² , and to an airflow for which T? = 300 K. If the surface temperature is T_s = 320 K, what is the convection coefficient associated with the airflow?

(b) In case (b) the surface is shielded from the sun by a large plate and airflow is maintained between the plate and the surface. The plate is diffuse and gray with an emissivity of ?_p = 0.8. If T? = 300 K and the convection coefficient is equivalent to the result obtained in part (a), what is the plate temperature T_p that is needed to maintain the surface at T_s = 320 K?

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h, T h, T. T.7 Case () Case (a)