Chapter 8 Internal Forced Convection8-49A circuit board is cooled by passing cool helium gas 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. 4Helium is an ideal gas. 5 The pressure of helium in the channel is 1 atm. PropertiesThe properties of helium at the estimated average temperature of 25°C are (Table A-16)669.0PrCJ/kg.5193/sm10233.1CW/m.1565.0kg/m1635.024-3=°=×=°==pCkυρHe 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 need to first find the Reynolds number m003944.0m)0.14+m002.0(2)m00028.0(442===PADch9.127/sm10233.1m)944m/s)(0.003(4Re24=×==−hmDVwhich is less than 2300. Therefore, the flow is laminar and the thermal entry length is m0.20<<m0.01687=m)003944.0)(669.0)(9.127(05.0PrRe05.0=
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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.