Chapter 7 External Forced Convection7-40A hot stainless steel ball is cooled by forced air. The average convection heat transfer coefficient and the cooling time are to be determined. Assumptions1Steady operating conditions exist. 2Radiation effects are negligible. 3Air is an ideal gas with constant properties. 4The outer surface temperature of the ball is uniform at all times. PropertiesThe average surface temperature is (350+250)/2 = 300°C, and the properties of air at 1 atm pressure and the free stream temperature of 30°C are (Table A-15) 7282.0Prkg/m.s10934.2kg/m.s10872.1/sm10608.1CW/m.02588.05C300@,525-=×=×=×=°=−°−∞skμυAir V∞= 6 m/s T∞= 30°C D = 15 cm Ts= 350°C D AnalysisThe Reynolds number is 42510597.5/sm1057.1m)m/s)(0.15(6Re×=×=υ=−∞DVThe Nusselt number corresponding this Reynolds number is determined to be 6.14510934.210872.1)7282.0()10597.5(06.0)10597.5(4.02PrRe06.0Re4.024/1554.03/245.044/14.03/25.0=⎟⎟⎠⎞⎜⎜⎝⎛×××+×+=⎟⎟⎠⎞⎜⎜⎝⎛μμ++==−−∞skhDNuHeat transfer coefficient is
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