Question

III. A composite walled window separates oven cavity from room air. The composite is to consist of two high temperature plastics of thickness L. = 2L. and thermal conductivities k = 0.15 W/(m·K) and k, = 0.08 W/( m K). During the self-cleaning process, the oven w and oven air temperatures are at 400°C, while the room air temperature is at 25°C. The insid convection coefficient, outside convection coefficient, and radiation heat transfer coefficient to be confused with Stefan Boltzmann coefficient) are each at 25 W/(m²K). What is the minimum window thickness needed to ensure that the temperature at the outer surface of the [40] window does not exceed 50°C?

Answer 1

Roomour Window m m Oven cavity two ! ta = 2500 in Itwa to KA I Knobri to: 400°C " KLAKLA Given :- LA=2 LB , window thockness = LA + LB =? KA = 0,15 W/mk , kB = 0008 W/mk to = 400°C, ta = 25°C ho=hr-ha=25 W/m²K outside surface temp. of window = 50°C e tere it is not mentioned that wheather outside of window means the oven side or room arr Side, Il gh re taken as over side, the result come out as negative. There fore we must assume that twa= 50°c ) solumon Let q = Q watt/m2 as the a heat flus. q= hac twa-ta) Coom air side) = 25 (50 - 25) = 625 W/m2 Also q (from oven side to room and soole) is given as, . q = - to - ta ħat het om te onthou the → 625 = 400 – 25 ( 5 * 3) + egy hangt bedre => 012 + LQ ( 131333 +12.5) - 1995 - LB x 251833 = 0.6 -0.12 3) Lg = 048 -0.01858 m = 18158 mm 251833 Minimum thickness required = LATLB = (2x18158)+ 181558 = 55.74 mm (ory, Msnt)