Consider a subsonic engine inlet (i.e., a diffuser) with an inlet area of 2300 i

Question

Consider a subsonic engine inlet (i.e., a diffuser) with an inlet area of 2300 in2, and an
exit/inlet area ratio of 1.44. Air enters the diffuser flowing at 10 lbm/s with a static pressure
of 5 psia and a velocity of 120 m/s. The gases exit the diffuser at a state pressure of 5.4 psia
and with a velocity of 78 m/s. You should assume the ambient external pressure on the wall
of the diffuser is 5.5 psia.
a.) For these conditions, what is the force (in units of lbf and N) transmitted to the structure
holding the diffuser? Provide both direction and magnitude of the force, and use a
picture to help present your answer.
b.) Based solely on your results, comment on whether the force acting on the diffuser would
help speed up or slow down a vehicle that used this diffuser as part of a jet propulsion
system.

Explanation / Answer

Ai=2300 in2 , Ao=Ai*1.44=3312 in2

m=10 lbm/s

Pi=5 psia , Po=5.4 psia , Pa=5.5 psia

Vi=120 m/s , Vo=78 m/s

a) Force =m(Vo-Vi) = -190.5 N = -42.82 lbf (towards the inlet)

b) since force is negative it will slow down the system.

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