An interior wall of a cold furnace, initially at 0°C, is suddenly exposed to a radiant flux of 15 kW/m2 when the furnace is brought on line. The outer surface of the wall is exposed to ambient air at 20°C through a heat transfer coefficient of 10 W/(m2 K). The wall is 20 cm thick and is made of expanded perlite (k = 0.10 W/(mK), α = 0.03 \ 10–5 m2/s) sandwiched between two sheets of oxidized steel. Determine how long after startup will the inner (hot) sheet metal surface get hot enough so that reradiation becomes significant.
GIVEN
Furnace wall suddenly exposed to radiant heat flux
ASSUMPTIONS
(a) Reradiation becomes significant when the reradiated flux from the exposed wall exceeds 10% of the incident radiant flux.
(b) The oxidized surface of the exposed wall is black.