[AR] Re: Arocket Pump Progress
- From: Peter Fairbrother <zenadsl6186@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Thu, 16 Jul 2015 12:01:42 +0100
On 16/07/15 10:15, Eric Robbins wrote:
Hi Peter,
I'm not sure this is correct...
A Barske pump gets its head in two ways, from centrifugal force increasing
the pressure, and from dynamic pressure recovery of the fast-flowing fluid
slowing down in the tapered volute.
A full mathematical analysis shows that both these effects are equal in an
ideal pump, so I set the designed dynamic pressure of the flow to half the
design pressure, or 3 MPa.
the pressure recovery is done in the diffuser, but all of the net
energy input into the fluid comes from the impeller so the tip speed
needs to supply the full head, all 6 MPa, at 130 m/s.
Imagine there was no exit - the fluid would still have a high static
head pressure at the outside edge. The depth of fluid is only about 17mm
in my design, but the static pressure is large due to the huge
acceleration due to centrifugal effects.
The fluid would also have a high velocity at the edge.
Now in a real Barske pump, the fluid goes round and round a lot more
than it goes from inlet to outlet - in my design it goes around about 5
times between entering and exiting. For most of it's journey round the
rim, there is no exit for the fluid.
So the static pressure at the edge rises, because of the head of fluid
above it in the pump. When exiting, the fluid already has a large static
pressure - and it also has a velocity v.
A little surprisingly, but not entirely coincidentally, when the dynamic
pressure is converted to static pressure in the volute, the extra
pressure recovered is equal in quantity to the preexisting static
pressure at the edge.
Note, the hole in the edge is sized so that the fluid flows out solely
due to it's velocity - there is no static pressure change, the static
pressure does not force the fluid out.
From an energy input POV, the impeller does two things: it forces the
fluid outwards (at static pressure and flow rate due to centrifugal
head); and it also accelerates the fluid to velocity v.
A fuller analysis of this is available in Chapter 11 of Lobanoff and
Ross: Centrifugal Pumps: Design and Application - though they use the
confusing pump designer terminology, not proper physics!!
HTH,
-- Peter Fairbrother
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