[aldodwirizaldi]

Hello,
This is my first project on CFX,
I'm trying to make an analysis on pump impeller performance by changing the vanes number as variable (vanes: 5,7,9).


I've tried to refine my mesh and encounter the same problem which the curve isn't look like a parabolic line instead of linear line

here's the expression that I use:
Total Efficiency in to out:
Q * H in to out * g * rho/Shaft Power * 100

head
(massFlowAve(Total Pressure in Stn Frame )@R1 Outlet-massFlowAve(Total Pressure in Stn Frame )@R1 Inlet)/(ave(Density )@R1 Outlet*g)

shaft power
abs(sum(Velocity in Stn Frame u * Force X + Velocity in Stn Frame v * Force Y + Velocity in Stn Frame w * Force Z)@Calc Wall Region) * numComponents

is there any problem with the expression?
I use mass flow rate as input parameter (defult 50kg/s)

here my attached result and some pump performance curve parabolic line that I meant.

thank you

[Jiricbeng]

I am not sure about the shaft power, I woud recommend to try to calculate the power as P= torque * angular velocity.

Assuming "z" is axis of rotation, power is
P = torque_z()@blade * number of blades * angular velocity [rad/s].

[ghorrocks]

Have you considered using the turbomachinery macro in CFD-Post? It has lots of turbo machinery calculations built in.

[kabuforever]

I think the shaft power shoud only be calculated about the rotating axis, not all the three axes. Head shoud include dynamic energy and gravitational energy (no idea whether they can be neglected). Better you use the default template in CFD-POST.

[aldodwirizaldi]

thank you for your all kindly reply and guidance...
I will try your suggestion on the next calculation. I just recalculate my design point several hours ago.

I just recalculate my Design Point cause I make some mistake which my maximum pressure should be 50kg/s.
which I'm plotting the pressure only 1kg/s to 50 kg/s. which make the graph looks climbing to the peak.
I've expand the plot on design point from 20kg/s to 130 kg/s.

the expression that I use is build in except for the Head exp.
I've another question which:
1. I'm quite confused on the difference between "Static Efficiency in to out and Total Efficiency in to out" can you please explain about it?
-Static Eff in to out= Q * (Hp out - H in) * g * rho/Shaft Power * 100
(the result is below 80%)
-total eff in to out= Q * H in to out * g * rho/Shaft Power * 100
(the result is over 80%)
which one to use?
2. and why the result on the Total eff in to out is over 80% which the result on the simulation that I see on the journal also over 80% even 90% (I Attached it beside design point pict)?
3. are my current graph plot good enough?

sorry for asking to many question, I still have much to learn.
hope this experiment could make one good experience for me.

[kabuforever]

your peak total efficiency is definitely too high, almost 100%. can you show me the equations of total efficiency and static efficiency? i'm not sure if a pump can get such a high efficiency.

[aldodwirizaldi]

The equation expression is on my first question above

[kabuforever]

total efficiency is calculated by total pressure, and static efficiency by static pressure. if the fluid is incompressible and the velocity in/out are the same, they should be almost the same too.

[Jiricbeng]

Evaluate the efficiency on the basis of the torque as I propose. Head should be evaluated on the basis of total pressure (in stationary frame).

[aldodwirizaldi]

hello, I still haven't able to find the solution and still weren't able to make the Torque expression.

but I do find a video tutorial by ANSYS, and their efficiency also around 90%. the same cases as I do right now. can anyone explain why the efficiency is so high?

my workflow also look about the same as on the video tutorial by ANSYS here https://www.youtube.com/watch?v=LZGCsU9Sfj0

thank you

[turbo]

Let me clarify "Pump Efficiency in CFD" for you (and also for many people who have misunderstood it).

Below are what you defined,
"head
(massFlowAve(Total Pressure in Stn Frame )@R1 Outlet-massFlowAve(Total Pressure in Stn Frame )@R1 Inlet)/(ave(Density )@R1 Outlet*g)

shaft power
abs(sum(Velocity in Stn Frame u * Force X + Velocity in Stn Frame v * Force Y + Velocity in Stn Frame w * Force Z)@Calc Wall Region) * numComponents"

The bottom part, what you named "shaft power", is however not the real shaft power because your CFD domain excludes the followings :
- Disk friction on the front and back cavities of the impeller
- Leakage flow power loss through the front and rear seals
- Any power loss due to recirculation flows returning back to the impeller
- Bearing windage power losses

Therefore, any well-designed pump impeller will show over 90% efficiency from CFD if all of them are excluded in the analysis, and if the bottom is a pure calculation of (Torque x omega) from CFX-Post. No surprises ! The CFD pump efficiency cannot be compared to test efficiency unless all of external parasitic losses are included in CFD (but so much rare cases).

In other words, what CFX-Post displays "Pump hydraulic efficiency = (rho * g * Q * dH) / (Torque x omega)" is not a true one, only confusing users. Be careful.