Determination of flow velocity at pump exit
Determination of flow velocity at pump exit
(OP)
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
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





RE: Determination of flow velocity at pump exit
RE: Determination of flow velocity at pump exit
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.
RE: Determination of flow velocity at pump exit