A 10 cm x 25 cm printed circuit board whose components are not allowed to come into direct contact with air for reliability reasons is to be cooled by passing cool air through a 25 cm long channel of rectangular cross section 2.5 mm x 90 mm drilled into the board. The heat generated by the electronic components is conducted across the thin layer of the board to the channel, where it is removed by air that enters the channel at 20 °C. The heat flux at the top surface of the channel can be considered to be uniform. If the velocity of the air at the inlet of the channel is not to exceed 5 m/s and the surface temperature of the channel is to remain under 55 °C, determine the maximum total power of the electronic components that can safely be mounted on this circuit board.
Use iteration to find the temperature/heat transfer/Q
Homework Answers
Ans)
Here, Ti = inlet temperature,
Ts = surface temperature,
Properties of air at temp 37.5 0 C . Properties of air at 1 atm pressure .
Density , 
= 1.1545
kg/m3
thermal conductivity of air, k= 0.026065 W/m-k
Specific heat Cp = 1007 J/kg-K
kinematic viscosity, 
= 1.6315 
10-5 m2/s
Prandtl number , Pr =0.7275
Q= m Cp (Ts - Ti)
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