Absolute Velocity Vs Relative Velocity
What do these terms mean with respect to turbo machines ?
these two velocities are actually attributed to the rotating hub.
Answer:
In the context of turbomachinery, such as turbines, compressors, and pumps, absolute velocity and relative velocity play vital roles in the overall functioning and efficiency of these machines.
The rotating hub or impeller of a turbomachine creates a flow of fluid that can be described using these velocity concepts. Let's elaborate on these terms:
Absolute Velocity (C): This refers to the velocity of the fluid observed from a stationary point in space, or in other words, from the frame of reference of the stationary parts of the machine. It is a measure of how fast and in what direction the fluid is moving from the observer's viewpoint who is not moving with the machine.
Relative Velocity (V): This is the velocity of the fluid observed from the frame of reference moving with the impeller. It is a measure of how fast and in which direction the fluid is moving when you are observing it from the impeller itself, essentially 'riding' on the rotating parts. This is important because it's the relative velocity that determines the work exchange between the fluid and the impeller.
In order to understand the motion of the fluid through a turbomachine, engineers also often speak about another velocity, the tangential or whirl component (U), which is the velocity of the blade at the point of fluid interaction. These three velocities are usually connected through the velocity triangle, which is fundamental in turbomachinery design.
It's important to note that these concepts are essential for designing turbomachinery and understanding the complex fluid dynamics that happen within these machines. By optimizing these velocities, engineers can maximize the efficiency and performance of turbines, compressors, and pumps.
Replies
-
zaveriNO replies yet ?
#-Link-Snipped-# #-Link-Snipped-# #-Link-Snipped-# -
Ramani Aswath
As the name implies these are related to the frame of reference considered. In this case, Absolute velocity will refer to the fluid velocity with reference to the place where the turbine is located. For example the velocity of wind hitting the face when standing under a wind turbine. Relative velocity refers to the velocity of the wind with respect to the blade of the turbine over which the wind is blowing.zaveriNO replies yet ?
#-Link-Snipped-# #-Link-Snipped-# #-Link-Snipped-#
It is important to be clear about the concept of velocity triangles in turbo machinery to understand the design issues clearly. These are applicable to all turbo-machinery with compressible fluids like wind turbines or incompressible fluids like in hydro electric turbines.
Chapter 2 on Relative and Absolute Motion beginning on Page 25 of this e book will give more info:
#-Link-Snipped-# -
ISHAN TOPREIf you are interested, also try reading Hydraulic Machines by Dr. Jagdish Lal.
It is very old book, in MKS units. The velocity diagrams are explained very well. -
G.Din turbomachines, the velocity triangles are related by the vectorial addition;
c= u+w
where, u= blade speed(pi*d*n, 'n' in rps)
w= relative velocity of fluid wrt blade
c= absolute velocity of fluid result of blade and relative motion. -
zaveriG.Dwhere, u= blade speed(pi*d*n, 'n' in rps)
What is "d" ?
Is it diameter of the blade hub ? -
G.Dohh yes, it is dia of hub or tip or any pt between them.
You are reading an archived discussion.