Question
Asked Nov 6, 2019
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Air at 20°C and 101 kPa enters a passage between
P4-36
two printed circuit boards inside a desktop computer. One
board contains nine chips each dissipating 2 W and five chips
each dissipating 1.3 W. No heat enters the passage from the
other card. If the exit temperature is 28.5°C, find the volumet
ric flow rate of the air. Neglect kinetic and potential energy
changes
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Air at 20°C and 101 kPa enters a passage between P4-36 two printed circuit boards inside a desktop computer. One board contains nine chips each dissipating 2 W and five chips each dissipating 1.3 W. No heat enters the passage from the other card. If the exit temperature is 28.5°C, find the volumet ric flow rate of the air. Neglect kinetic and potential energy changes

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Expert Answer

Step 1

Follow the assumption:

  1. Specific heat is taken as constant.
  2. The system operates in a steady state.
  3. Frictional heating in the fluid is neglected.
  4. As mentioned, neglect potential and kinetic energy.
  5. No heat is conducted into the card.

 

The given system is an open system. Apply first law for open system.

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6-2a[ar%v@]>2«{a<8<0} dE h,+ dt Here, i istaken for inlet, and istaken for outlet. Neglecting the given condition, the above equation simplifies as, 0

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Step 2

Thus, the above equation can be written in the form of constant specific heat as,

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mc AT mc, () Thus, c, (7- Put the values and solve, [(9x2)+(5x1.3)] W 1000 J |(28.5-20) °C (1.006 kJ/kg°C) 1 kJ = 0.00287 kg/s Here, [Take C 1.006 kJ/kg°C from table A-7 at 300 K ]

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Step 3

Find the value of avera...

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T+T avg 2 20 °C+28.5 °C 2 24.25 °C 297.3 K

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