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

Velocity through a water turbine

Velocity through a water turbine

Velocity through a water turbine

Hi all

I have a question about calculating power from a water turbine.  If I have a rotor with an inlet area A1 and an inlet velocity V1, and an outlet area A2 and velocity V2, if the rotor tapers down, A2 is smaller than A1, therefore to satisfy the mass flow rate through the closed rotor, V2 (axial) must be greater than V1 (axial).  But... for a water turbine I understand that V2 needs to be smaller than V1 as work is done and hence the dynamic pressure must decrease, not increase.  I'm calculating the power at the inlet of the rotor and the outlet of the rotor using Power = the flow rate (Area x velocity) multiplied by the dynamic pressure (0.5 x area x density x velocity2 ).  My issue is if I use axial velocity, which satisfies the mass flow rate, I get negative power as work is done on the liquid to increase the velocity.  If I use the total velocity component (using CFD) in this equation, I get what seems a realistic power but the mass flow rate in is much greater than the mass flow out.  Please help me!  I am utterly confused!

Also, as I mentioned, the turbine I'm working on has a decreasing cross sectional area through the rotor... is it just me, or is this silly?  Aren't you better with an increasing area through the rotor?  I'd be interested in anyone's thoughts on this.

Thank you,


RE: Velocity through a water turbine

Hi katmech

Check your units if you have:-

Area x velocity) multiplied by the dynamic pressure (0.5 x area x density x velocity2 ).  

Power should be N*m/s from your units above unless I am making a mistake I can't get N*m/s.

formula I was looking at was :-

mass flowrate *(V-v)*v     where V,v are the initial and    
                           final velocities of the jet
                           respectively normal to the vane

thats the power output formula for jets on to a rotor with flat plates.


RE: Velocity through a water turbine

Sorry desertfox, my mistake, the dynamic pressure is 0.5 x density x velocity^2, it doesn't include the area.  The units work out to be Watts with this dynamic pressure multiplied by the flow rate (area x velocity).  The text books I have, and previous reports use this equation for the water turbine I'm looking at, but they just say to use the velocity, and don't specify if this is the velocity parallel to the turbine axis, or the total velocity component of the water exiting the turbine.  Obviously as the water has just past through the turbine, it has a large angular component to the velocity so the total velocity is very different from the parallel velocity component.  So, which one should be used in this power calculation?  Any ideas?  Thanks!


RE: Velocity through a water turbine

hi katmech

Its difficult to say of the description you give any chance of uploading a file of what your looking at.


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! Already a Member? Login


Research Report - How Engineers are Using Remote Access
Remote access enables engineers to work from anywhere provided they have an internet connection. We surveyed our audience of engineers, designers and product managers to learn how they use remote access within their organizations. We wanted to know which industries have adopted remote access, which software they are using, and what features matter most. Download Now
eBook - Managing the Context of Product Complexity Using the Digital Twin
Keeping track of changes to complex products is difficult—think Aerospace & Defense equipment, new generations of commercial aircraft, and software-based automobiles. A new way to managing the digital context of the physical product is required and the answer is the Digital Twin. This ebook explores the opportunity available for Operations and Maintenance for the Digital Twin. Download Now
White Paper - Trends in Industrial Filtration
Substantial progress has been made in filtration technologies in recent years. New filter media materials, designs and processes have led to filters that are more efficient, reliable, compact and longer lasting. This white paper will discuss the various trends that are impacting operational responsibilities of MROs today and the resources that are available for staying up-to-date on the latest filtration solutions. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close