Chapter 7 External Forced Convection7-29A fan blows air parallel to the passages between the fins of a heat sink attached to a transformer. The minimum free-stream velocity that the fan should provide to avoid overheating is to be determined. Assumptions1Steady operating conditions exist. 2The critical Reynolds number is Recr= 5×105. 3 The fins and the base plate are nearly isothermal (fin efficiency is equal to 1) 4Air is an ideal gas with constant properties. 5 The local atmospheric pressure is 1 atm. PropertiesThe properties of air at the film temperature of (Ts+ T∞)/2 = (60+25)/2 = 42.5°C are (Table A-15) 7248.0Pr/sm10726.1CW/m.02681.025-=×=°=υkAnalysisWe first need to determine radiation heat transfer rate. Note that we will use the base area and we assume the temperature of the surrounding surfaces are at the same temperature with the air (Tsurr=°25 C ) Air V∞T∞= 25°C L= 10 cmTs= 60°C 20 W W1.4]K)27325(K)27360)[(C.W/m1067.5(m)]m)(0.0621.0)[(90.0()(442844=+−+°×=−=−surrssradTTAQσε&The heat transfer rate by convection will be 1.4 W less than total rate of heat transfer from the transformer.
This is the end of the preview. Sign up
access the rest of the document.