×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Determination of flow velocity at pump exit

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

RE: Determination of flow velocity at pump exit

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.  

RE: Determination of flow velocity at pump exit

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.

RE: Determination of flow velocity at pump exit

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources