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Q2 a) A leading manufacturer of household appliances is proposing a self-cleaning oven design that involves use of a composite window separating the oven cavity from the room air (Figure 2a). The composite is to consist of two high-temperature plastics (A and B) of thicknesses LA = 2Lb and thermal conductivities ka = 0.15 W/m K and kb = 0.08 W/m.K. During the self-cleaning process, the oven wall and air temperatures, Tw and Ta, are 400°C, while the room air temperature To is 25°C. The inside convection and radiation heat transfer coefficients hi and hr, as well as the outside convection coefficient ho, are each approximately 25 W/m².K. What is the minimum window thickness, L = LA + LB, needed to ensure a temperature that is 50°C or less at the outer surface of the window? This temperature must not be exceeded for safety reasons.
Composite window, LA = 2LB LA LBY Oven cavity I sit C 75.o 50 °C T, = 400 °C h, = 25 W/m2K Air "B, kg = 0.08 W/mK = m A, ka = 0.15 W/m.K 111 111 T = 400 °C h; = 25 W/m2K Air T = 25 °C h, = 25 W/m2K = Figure 2a