Need help simulating a centrifugal pump
Hi Frederic!
First of all thank you for the links!
I have tried setting up my simulation like recommended in the links sent by you. I extended my intake and outlet volumes to a length 5x bigger than the diameter, i have set the pump impeller to accelerate gradually to the nominal rpm using the table, all non zero boundary condition also increase gradually with the rpm using Picewise Linear method, i refined my mesh and also calculated my timesteps. The problem still persists.
Also as you can see from this second photo, the flow enters a stationarry diffuser right after exiting the impeller. The diffuser should further increase the pressure of the flow as it is composed of divergent channels trough which the fluid flows, but in the simulation the pressure drops trough the divergent channels. The pressure then proceeds to decrease gradually towards the outlet untill it reaches 60 bars again right at the outlet, but it is supposed to stay almost constant after the diffuser, maybe decrease a little because of losses due to viscosity and pipe geometry, but certainly not by this much.
This third picture is of the pressure distribution in the intake pipe at timestep 10, at which the impeller has 20% of maximum rpm, but still the intake pressure is almost 60 bars which leads me to think all this has to do with me not setting up my boundary conditions properly.
As boundary condition i set an intake pressure of 3 bars because that is the pressure the fluid will have in the reservoir above the pump and an outlet mass flow rate i expect from my pump to deliver from my calculations.
Do you have any idea of what might be the problem or have any other recommendations i can try?
Hello Fred!
Sure, i have attached to this message the simulation cfz file where you can see my setup. Here is also the last convergence plot which i did not give time to converge properly because i was getting the same odd results as all the times before so i stopped it, but maybe it helps you get an idea as it is.
While running my last simulation i might have discovered what is wrong with my simulation but not how to solve the problem. Let me explain:
Maybe you can take a look at my boundary conditions and see what i did wrong there, if that is to be the problem?
Thank you!
Hello Fred!
I disabled "Intelligent Solution Control" and also set up the boundary conditions that you recommended. I have also made sure the rotating region volume does not touch the casing and actually stops about halfway between the blade tips and the casing wall. Unfortunately there is no improvement.
Do you think the problem could be from the Materials Property tab?
Hello Andrei,
I run your scenario on my end, and i still have huge pressure like you experienced.
The methane velocity inside the rotating region is really high : around 200m/s in both scenario with 0 and 3 bar pressure at the outlet, respectively.
I check the speed of sound in the methane fluid for the prescribed environment (T=100degK ; 3Bar) i found this :
My understanding for a friday afternoon is that temperature will have an impact on the result. So yeah i think you spot the right think here, you should enable heat transfert !
Good Job
Fred
PS : the rotation seems to be setted in the wrong direction according the the orientation of the vanes. Can you confirm, THX
Hello Fred!
Yes indeed, my impeller should be rotating counter-clockwise, i missed the fact that in the simulation it rotates in the wrong direction, haha 🙂 .
To adress your remark about the high fluid speed in the impeller: yes indeed, the impeller is designed to rotate at 285500 RPM. It has such a high RPM because, perhaps you already observed, it has a diameter of just 16mm. This methane pump is designed for a small liquid fuel rocket. I study Aerospace Engineering and me and some friends want to see if it is possible to make a functional liquid fuel rocket at such a small scale (yes, i know we are masochistic!). So, since the centrifugal pump raises the pressure of the fluid trough centrifugal force, we have to compensate such small impeller diameter with a high RPM so we will have a high enough peripheral speed at the blade to compress the fluid to the value we need :).
I will try to simulate the pump with heat transfer, but before i do that i have a few questions:
1. If i run heat transfer simulation, do i need to unsupress the pump casing and impeller so to allow the program to calculate heat loss trough the walls of the casing?
2. If i do unsupress them, will i need to assign rotating region material property to the solid impeller aswell as the volume around it?
3. Should i also select compressible flow option?
4. Is Autoforced Convection and Radiation necessary? Should i select them?
5. For a heat transfer simulation, are there other result quantities i should select beside these?
By the way, thank you very much for the help so far, i really appreciate it!
Hello Andrei,
Thank for the specification, seems really interesting project.
In my last post i made a mistake about the heat transfer. You don't need to activate the heat transfer in the physic tab. The software will automatically use the density of the prescribed at the corresponding temperature. (100degK)
Regarding this last point, i think you made an error relatively to the density of liquid methane :
This is the way you setted the physical propreties of the liquid methane
In other hand this is how it is defined on the autodesk database :
I check the density dependancy according to the pressure applied at 100degK and there's no significant change :
- 3 bar = 439.09 kg/m3
- 2 bar = 439.01 kg/m3
- 1 bar = 438.94 kg/m3
My interpretation of this is you can use a constant value for the moment and use the methane material in the autodesk database. Right now you have a huge error on the density value.
For the compressibility option, i think you should check the magnitude of your velocity field with the new density values and compute the Mach number to know if compressibility is relevant to your analysis.
Hope it helps
Fred
Hello Fred!
So i just ran the simulation on the CFZ you sent on your last message and i have to say that it is the biggest progress i have made this far into getting this simulation done!
My impeller seems to compress the fluid to the compression ratio it was designed! But the problem is that it now thinks that the inlet pressure is 80 bars, so instead of compressing it from 3 bars to 5.4, it compresses it from 80 bars to 140-150. Again the compression ratio is more than perfect but the pressure values are off. Also, the flow still looses pressure while flowing trough the divergent channels.
Any idea why this happens? I will also try to experiment with various boundary conditions to see if anything helps.
Thank you!
