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In a subsonic convergent duct the:
  • A
    temperature increases, velocity increases and pressure decreases
  • B
    pressure increases, velocity decreases and temperature increases
  • C
    pressure increases, velocity decreases and temperature decreases
  • D
    velocity increases, pressure decreases and temperature decreases

Conservation of Energy (Bernoulli’s Theorem)

Total Pressure = Dynamic Pressure (½ρV2) + Static Pressure = Constant

The total energy of an airstream comes in two forms: potential energy (static pressure) and kinetic energy (dynamic pressure).
The total pressure of the airstream is the sum of the static pressure and the dynamic pressure and must remain constant, according to the law of conservation of energy.
Thus, an increase in one form of pressure, must result in a decrease in the other.
Dynamic pressure (q) is expressed by:
q= 1/2* ρ* V2
Assuming the total pressure to be constant, the static pressure and the dynamic pressure vary inversely: If one increases the other decreases and vice-versa.


In Conclusion:
  • In a convergent duct, flow speed increases, increasing dynamic pressure (= velocity). If dynamic pressure increases, static pressure and static temperature decrease.
  • In a divergent duct, flow speed decreases, decreasing dynamic pressure (=velocity). If dynamic pressure decreases, static pressure and static temperature increase.

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