If an aluminum plate 4 mm thick is mounted in a horizontal position, and its bottom surface is well insulated. A special, thin coating is applied to the top surface such that it absorbs 80% of any incident solar radiation, while having an emissivity of 0.25. The density p and specific heat c of aluminum are known to be 2700 kg/m3 and 900 J/kg K, respectively. (a) Consider conditions for which the plate is at a temperature of 25 C and its top surface is suddenly exposed to ambient air at T=20 C and to solar radiation that provides an incident flux of 900 W/m2 . The convection heat transfer coefficient between the surface and the air is h = 20 W/m^2 K. What is the initial rate of change of the plate temperature?

Determine the time of the day and month of the year at which the peak sensible cooling load occurs for a top floor, north-eastern corner room of an office building in durban for the following conditions: floor area: 8 x 8 x 3 m east and north walls: 115 mm face brick (outside), 20 mm air space, 115 mm ordinary brick (inside) with 15 mm plaster roof construction: suspended plasterboard ceiling, 450 mm air space, 150 mm concrete, 75mm screed, waterproofing no heat transfer across other surfaces window area 3x 1,5 m high in north wall only. ordinary glass with venetian blinds. lights and occupants : from 07: 00 to 18: 00 average light density: 25 w/m2 number of occupants : 5 seated, light office work room temperature 24°c