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terminology - axial flow vs centrifugal 3

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electricpete

Electrical
May 4, 2001
16,774
OK... this is just a question of semantics.

Two types of pumps are radial flow and axial flow.

Do we refer to them collectively as "centrifugal" pumps, or does that term only apply to radial flow.

From the physics definition of centrifugal, I would expect the term only to apply to radial flow pumps.

But I often hear people broadly group pumps into positive displacement and centrifugal.... and they lump axial along with radial in the centrifugal category.
 
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Electricpete!

In both axial and radial cases fluid will be thrown out by centrifugal force only, but it depends upon in which direction the fluid is thrown. Thus giving the two names.

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.
 
thanks quark. Maybe I have a misconception. When you say thrown "out" I assume you mean the radial direction. Is there any part of an axial flow pump where the flow is "out" or "radial"?
 
electricpete!

No. In axial flow, like radial flow fluid is sucked at (near) the eye of the impeller and leaves the blade tip parallel to the axis of the shaft. Negative pressure is being created because of the centrifugal force.

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.
 
Not that I want to get drawn into a long debate, or that I consider myself a pump expert but....centrifugal force? I agree that there is an inertial effect that results from rotation that creates the impression of a force. However, all forces require contact expect the body forces of gravity, magnetism, and the nuclear forces. Anyhow I'm not some kind of weird physics nut, but the term 'centrifugal force' is one of my pet peeves.
 
Electricpete - another two cents on the matter. Classifying pumps into positive displacement versus non-positive dispacement is much more useful in the pumping world because it will help you understand operation and the surrounding components needed. For the non-positive displacement pumps mentioned here, an axial pump moves water parallel to its axis (shaft). This would be a propeller-type pump as in a main circ pump on a ship. Water is thrown from the blade tips in a manner that is overall parallel with its shaft. A radial pump would throw water purpendicular to it's shaft, such as in a centrifugal pump. These pumps use the concept of centrifugal force from spinning the water mass within their shell so that a pressure gradient exists from the center (the impeller eye) to the outermost portion of the casing. Because there is an inlet in the center portion and an outlet in the volute, water moves by means of centrifugal force.

With respect to electric draw based on the type of non-positive displacment pump, performance varies significantly based on the pump design. If I were just a motor (not caring what's attached), my gut would tell me that I would have to work harder to spin under a no-flow condition because I'm trying to move water where there is no place to move it. Manufacturer's curves can tell otherwise, but my bag is not electrical. I would like if someone with first-hand, experimental experience would post if they have information using and clamp-on ammeter and taking readings while varying pump flow...
 
CB - I have first-hand experience measuring current at varying pump flows. There is no doubt that a standard radial flow pump draws minimum current at minimum flow. I'm not sure exactly what you're after but it's not the subject of this thread (please post another thread if you want to discuss it more)

In my mind the key question of this thread is now: Is there really radial flow in a pure axial flow pump?

Based on my limited experience, I would say no. I have seen a pure axial flow pump (power plant circ water pump) whose impeller is shaped like a standard household fan (propellor shaped). It sits in a pipe of constant cross section. There is no reason for me to suspect that there is any radial flow. If so then there must also be radial flow in an axial flow fan?

Apparently quark believes there is some radial flow in an axial flow pump. That leaves me believing two possibilities:
#1 - He has better understanding of axial flow pump than I do (certainly possible... I'm an electric guy).
#2 - He is thinking of a mixed flow pump which has some axial and some radial flow characteristics.

CB - from what I could tell from your comments you believe (like I do) that the flow in an axial flow pump is purely axial.
 
From Hydraulic Institute Standards, 14th Ed, 1983, Page 10

Radial flow. ...In pumps of this type the liquid enter the impeller at the hub and flows radially to the periphery.

Mixed flow.This type of pump has a single inlet impeller with the flow entering axially and discharging in an axial and radial direction.

Axial flow. A pump of this type, sometimes called a propeller pump, has a single impeller with the flow entering axially and discharging nearly axially.
 
per A.J. Stepanoff's "Centrifugal and Axial Flow Pumps"

"Centrifugal pumps comprise a very wide class of pumps in which pumping of liquids or generation of pressure is effected by a rotary motion of one or several impellers. In the early stage of centrifugal pump development, pumping was ascribed to centrifugal forces. Later this class of pumps was extended to include axial flow pumps, and the conception of the centrifugal action of the impeller was inadequate to explain the operation of axial flow pumps. However, treatment of axial flow pumps as a class by themselves was not justified, because hydraulically they represent one extreme of a continuous series of pump types. This continuity applies to both theoretical treatment and design methods. Some intermediate types are called mixed-flow pumps. In these, the flow through the impeller has both radial and axial components and the impeller resembles a ship propeller."



Regards,

Gunnar
 
Useful flow in radial pumps/fans is discharged radially and in axial pumps axially. Both can have useless axial/radial flows usually called secondary flows which may become really useless(reversing)recirculating flows which cause efficiency losses and vibration producing hydraulic instabilities. If the blade tips of either kind are without continuous shrouds then there can be tangential tip leakage flows over the blade tips from channel to channel which can really screw things up royally. Secondary flows are more likely in the 3-dimensional flowfields in mixed and axial flow impellers than in the (nearly) 2-dimensional flowfields of low specific speed radial flow impellers. Low to high specific speeds characterize impeller shapes called radial, mixed (or Francis vane), and axial. Vanstoja
 
Thanks to all who responded, whether we agreed on everything or not.

I believe gunnarhole has given a satisfying explanation for me. The terminology is based on the historical evolution of these pumps which started as radial flow. "Centrifugal" does not accurately describe the operation of an axial flow pump, but that's what we call it for historical reasons.
 
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