Question

3. (15 pts) A 0.5 m x 1 m double paned window is made of two silica glass panes 1 cm thick, with air in between. Under steady conditions, linear (constant gradient) temperature profiles are found in both silica panes, as well as the air. a) If the room (and the inner surface of the inner glass pane) is at a temperature of 25°C, while outdoors it is -25°C with a convection coefficient of 18 W m2 k-4, what is the rate of heat loss through the window? b) What is the rate of heat loss if the room temperature is decreased to 20°C? c) Finally, what is the rate of heat loss for a room temperature of 25° C, but with one of the panes (and the air) removed? cm 1 cm Outdoors T = -25°C Room Temperature, 25°C or 20°C 0.5 mm h = 18 W m2K1 Air, k = 0.024 W m2 silica glass, k = 0.90 W mk1

Answer 1

A 2 Area of å - Rate of a) AT = T: - To = 25°C-(-25°C) = Soc window 2 0.5X1 = 0.5m2. heat lon there through window Q - AT Li L2 L3 1 K, AI K₂A2 K₃ A₃ ha so + 0.01 0.0005 0.01 1 + + it 0-9Koos 0.024 XO.S 0.9 XO'S 18 X Ons soxORS 0.0111 +0.0208 to 0111 +0.055 255.1 Watts. 25 0.098 b) Q = [20-4-25)] xo.s 000111 +0.0208 +0.0111+ 0.055 - 22.5 = 229.59 Watts 00098 c) One pane and air been removed :.Q [25-(-25)] 50x o-5 LI 습 0.01 I KAI ha 0.9 18 25 on olll to-OSS 25 0.0666 7375.37 Watts So, the heat loss rate has been increased the resistances has been less. as now