Q2: A layer of 2.5 in thick firebrick (kb 1.0 Btu/hr ft °F) is placed between two in.- thick steel plates (ks 30 Btu/hr ft °F). The faces of the brick adjacent to the plates are rough, having solid-to-solid contact over only 35 % of the total area, with the average height of asperities being L2-2/32 in. If the surface temperatures of the steel plates are 250° and 800°F, respectively. The conductivity of air ka is 0.02 Btu/hr ft °F, determine the rate of heat flow per unit area. %3D

A large slab of copper is initially at a uniform temperature of 90◦C. Its surfacetemperature is suddenly lowered to 30◦C. Calculate the heat-transfer rate through aplane 7.5 cm from the surface 10 s after the surface temperature is lowered.

4. Polyethylene (HDPE) pipe is used as a strut in a crutch 22 mm assembly. The pipe generally prepared by extrusion followed by external cooling. The attached figure is a plot of variation of density through a PE pipe. a.) What is the % 958- 956 954 1952 bore pipe wall Outside Density (kg m 3)

A 4mx5m wall consists of 3 glass windows of 1.5mx1.0m dimensions. The wall has thickness of 0.125 m and a thermal conductivity of 0.5 W/mK, while the glass windows are 6 mm thick with a thermal conductivity of 1.24 W/mK. The values of Internal and external surface conductance for the wall (including glass) are 8.3 W/m K and 34.4 W/m²K, respectively. The internal and external temperatures are 21°C and -30°C, respectively. Calculate the total heat transfer rate through the wall. What percentage of this heat transfer is through the windows?