Chapter 13, Problem 13.101P

A double-glazed window consists of two panes of glass, each of thickness $t=6\text{mm}$ . The inside room temperature is $T_i=20°\text{C}$ with $h_i=7.7{\text{W/m}}^{\text{2}}\cdot \text{K}$ , while the outside temperature is $T_o=-10°\text{C}$ with $h_o=25{\text{W/m}}^{\text{2}}\cdot \text{K}$ . The gap between the glass sheets is of thickness $L=5\text{mm}$ and is filled with a gas. The glass surfaces may be treated with a low-emissivity coating to reduce their emissivity from $\epsilon =0.95$ to $\epsilon =0.05$ . Determine the heat flux through the window for case ${\epsilon }_1={\epsilon }_2=0.95$ , case 2: ${\epsilon }_1={\epsilon }_2=0.05$ , and case 3: ${\epsilon }_1=0.05$ , ${\epsilon }_2=0.95$ . Consider either air or argon of thermal conductivity $k_{\text{Ar}}=17.7\times {10}^{-3}\text{W/m}\cdot \text{K}$ to be within 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.