Determination of flow velocity at pump exit

[geg1633]

Hello,
I'm trying to determine the flow velocity at the exit of an axial flow jet pump used in jetskis. The jet skis have a thrust of 900 lbs, and 80 mm exit diameter, 160 HP. Flow at the exit has a pressure of approx 120-130 psi.

I first evaluated the velocity by computing the head associated with the pressure, but then I was wondering if i could also use the thrust to compute velocity. Flow rate is a function of velocity and area, so if Thrust is a function of flow rate, I would be able to determine velocity, right? So what is the relationship between thrust and velocity?

Thanks!
Greg

 

[CRG]

Are you sure about the pressure of 120 - 130 psi? Was this measured at the pump with unrestricted flow or dead headed? Seems like a lot of pressure drop for the contraction and exit losses; however, I have not ran the numbers. Any good fluid mechanics book should answer your questions.

 

[ApC2Kp]

geg1633,
The pressure seems too high, but checking thrust
Thrust= force= pressure x area
= 120 psi x PI*0.25*(80mm= 3.15")^2
= 935 lbs. ,
so thrust seems to be accounted for. But 900lb thrust would seem to take a jetski vertical! Also, the 160 HP does not fit a typical jetski.
Suggest checking conservation of impulse - momentum.
F*t= 0.5* mass* (velocity)^2 ,
try 500 gpm flow
(935)*(1 sec)= 0.5*(500/60*8.34/32.2 mass)* (vel. ft/sec)^2
vel= sqrt(866)= 29.5 ft/sec
So the thrust and velocity seem to be consistent.
However, the flow and pressure head calculate to a required horsepower of 35 to 70 HP depending on efficiency 100-50%, which is a more reasonable fit on a jetski.

 

[CRG]

I think that the problem has several components: Friction losses, a static head that equates to force over area and a dynamic head that equates to velocity squared over 2g. To mathematically model this I would draw a vectored force diagram and get out my fluid dynamics text.