[AR] Re: Arocket Pump Progress
- From: Peter Fairbrother <zenadsl6186@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Thu, 16 Jul 2015 08:21:51 +0100
On 16/07/15 02:39, Monroe L. King Jr. wrote:
What does this mean? "BOTE, but sanity-checked: a full analysis takes a
lot of work. This should be reasonably OK."
It's roughly designed, only to back-of-the-envelope standards, not a
full all-out design; but I did check it for mistakes, and it should be
suitable for a preliminary effort.
I would be pleased if anyone else would like to sanity-check my figures
and reasoning.
Once built and performance is measured then it can be tweaked to give
better performance, but it should give reasonable performance as-is.
Here's how I designed it:
Input -> Mass Flow = 0.89 lb/s = 404g/s
Input -> density 0.711
flow = 568 ml/s
Input -> DP = 870 psi = 6MPa
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.
At a density of 0.771 (gasoline) an output flow speed of 91.85 m/s is
required to give the necessary 3 MPa dynamic pressure.
tip speed v = 91.85 m/s
from this we can calculate the outlet aperture effective area =
flow/velocity
= 0.000568÷91.85 = 0.000006184 m^2 = 6.2 mm^2 -> 2.8 mm dia
in practice this needs to be a little larger, say 3.0-3.2 mm, as edge
effects will slow the flow down.
Input -> N = 50 kRPM = 833 rps
and knowing v and N we can calculate the required diameter =
129.9÷833÷pi = 0.035m
A 35mm diameter gives a swept area = 962 mm^2.
We want the fluid to go around about five times between entrance and
exit, so we consider:
desired volume at 6 turns: 568,000x6/833 =4091 mm^3
which divided by the swept area gives a blade height of 4.253mm
desired volume at 5 turns: 3409 mm^3 blade height = 3.544mm
Now some of the volume will be filled with blades, and we want at least
3.2mm at the edge so that the exit hole will fit, so choose blade edge
height of 3.5mm.
From looking at other pumps, both physical and theoretical, and knowing
the general shape needed, I chose a blade center height of 4.2 mm (a SWAG).
The blade free hole diameter is another SWAG, based on a similar pump I
designed in detail some years ago with a 45 psi input pressure (sorry,
can't remember the NPSHR).
> The one Peter F. has thrown out there looks to me to be a bit too
> small
You might well be right - on reflection, you could increase the diameter
from 35mm to 38mm. That would correspond to an entirely reasonable flow
coefficient of 0.8. No need to change the other parameters.
The pictures and diagrams Andrew and Troy have posted should give you
enough ideas to fill in the other details - although I do not understand
the reason for extending of the flat base disk beyond the blades, and my
design does _not_ have that feature.
-- Peter Fairbrother
-------- Original Message --------
Subject: [AR] Re: Arocket Pump Progress
From: Peter Fairbrother <zenadsl6186@xxxxxxxxx>
Date: Wed, July 15, 2015 10:22 am
To: arocket@xxxxxxxxxxxxx
On 15/07/15 10:44, Monroe L. King Jr. wrote:
Mass Flow = 0.89 lb/s
DP = 870 psi
N = 50 kRPM
density 0.771
BOTE, but sanity-checked: a full analysis takes a lot of work. This
should be reasonably OK.
Diameter 35mm
6 blades.
Blade height 4.2mm at theoretical center, 3.4mm at edge
Center blade-free hole 11 mm diameter
Blade width 2.2mm
Output hole should in theory be 2.8mm diameter - but try 3.0 - 3.2mm
actual. A rectangular hole is good too, about 3.2 tall and 2.2 wide.
Output taper 1 in 10 diameter, ie 1 in 3.16 area, at least 35 mm long.
Edges where taper meets casing interior should be sharp-ish.
-- Peter Fairbrother
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