Chapter 8 Internal Forced Convection8-48A circuit board is cooled by passing cool air through a channel drilled into the board. The maximum total power of the electronic components is to be determined. Assumptions1Steady operating conditions exist. 2The heat flux at the top surface of the channel is uniform, and heat transfer through other surfaces is negligible. 3The inner surfaces of the channel are smooth. 4Air is an ideal gas with constant properties. 5 The pressure of air in the channel is 1 atm. PropertiesThe properties of air at 1 atm and estimated average temperature of 25°C are (Table A-15)7296.0PrCJ/kg.1007/sm10562.1CW/m.02551.0kg/m184.125-3=°=×=°==pCkυρAir 15°C 4 m/s TeElectronic components, 50°C L= 20 cmAir channel 0.2 cm ×14 cm AnalysisThe cross-sectional and heat transfer surface areas are 22m028.0)m2.0)(m14.0(m00028.0)m14.0)(m002.0(====scAATo determine heat transfer coefficient, we first need to find the Reynolds number, m003944.0m)0.14+m002.0(2)m00028.0(442===PADch1010/sm10562.1m)944m/s)(0.003(4Re25=×==−hmDVwhich is less than 2300. Therefore, the flow is laminar and the thermal entry length is m0.20<m0.1453=m)003944.0)(7296.0)(1010(05.0PrRe05.0==htDLTherefore, we have developing flow through most of the channel. However, we take the conservative
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This note was uploaded on 01/22/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.