A double-glazed window consists of two panes of glass, each of thickness t= 6 mm. The inside room temperature is Ti = 293 K with hi = 7.7 W/(m^2)*K, while the outside temperature To = 263 K with ho = 25 W/(m^2)*K. The gap between the glass sheets is of thickness := 5 mm and is filled with a gas. The glass surfaces may be treated with a low-emissivity coating to reduce their emissivity from 0.95 to 0.05. Determine the heat flux through the window for
case 1 : emissivity1=emissivity2=0.95
case 2 : emissivity1=emissivity2=0.05
case 3 : emissivity1=0.095 emissivity2=0.05
Consider either air or argon of thermal conductivity k(Ar) = 17.7 x 10^-3 W/m*K to be withing the gap. Radiation heat transfer occurring at the external surfaces of the two glass sheets is negligible, as is free convection between the glass sheets.