A flat-plate solar collector is placed horizontallv on a roof, as shown in Fig 4.15. To determine its efficiencv, it is necessarv to calculate the heatloss from its surface to the environment. The collector is a long strip 0.3m wide. The surface temperature of the collector is 60 C. If a wind at 16’C is blowing over the collector at a velocity of 3 m/s, calculate the following quantities at x-0.3 m and x-xc : (a) boundary layer thickness (b) local friction coefficient (c) average friction coefficient (d) local drag or shearing stress due to friction (e) thickness of thermal boundary layer (f) local convection heat transfer coefficient (g) average convection heat transfer coefficient (h) rate of heat transfer by convection Collector 0.3m Collector surfeo 60 Au 16C Wind FIGURE 4.15 Flat-plate solar collector for Example 4.3. SOLUTION The relevant properties of air at 38Â°C are p 1.13 kg/m3 Cp = 1.005 kJ/kg K H 1.91 x 10-5 kg/m s k = 0.0266 W/m K Pr = 0.72
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