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-   -   Optimizing an ejectorpump in ANSYS. HELP!! (http://www.cfd-online.com/Forums/main/1410-optimizing-ejectorpump-ansys-help.html)

 Erik Walfridsson October 16, 1999 11:36

Optimizing an ejectorpump in ANSYS. HELP!!

Hi I´m studying at the University of Kalmar. Sweden. I´m working on optimizing an ejector pump. I think the only way to do this is with computer simulation. I´m using ANSYS ED 5.4 It simply doesn´t work and NO ONE at the University knows the CFD part of this software.

First, to verify that my bounadary constraints and everything else is correct, I´ve created a model with the same dimensions as the real ejector they are using right now. If I´ve done everthing right my model should give the same flow as the real ejector does, right?

Well, it doesn´t. The flow in the computer becomes 2-3 times greater then the real flow. I´ve tried everything!(allmost) What is wrong?

I´m using an axisymmetric model with the FLOTRAN Fluid 142-element. the fluid is water. incompressible turbulent flow. If anyone could give me just a small tip som that i could solve this problem I would be VERY thankful. If someone wants, maybe I could email you the model(554 KB, compressed). It´s a little bit difficult to give a total description of the problem in text.

Thanks in advance / Erik Walfrdsson, Sweden

Re: Optimizing an ejectorpump in ANSYS. HELP!!

Hi Erik; Below are some point which may help: 1-Are you solving 3-D? If Not then you may switch to Fluid 141 with choosing axisymmetric at the begining. 2-Check the turbulemce parameters well such as the turbulence intensity for example 10%, the turbulence inlet scale factor,and the turbulence ration (for ex. 100000). 3-Check the relaxation factors (for example relax,visc,0.1,and relax,evis,0.1), and stability parameters such as (stab, mome,1.0 ,stab, pres,1,stab,turb,1,and stab,visc,0.1. 4-Make sure that the solution you got is the converged solution. AT the converged solution, there is no significant decrease in the normalized ressidual and all the variables are monotonically decreasing. 5-Check in the filename.pfl for the mass balance which should be almost equal. FLOTRAN will give you mass in and mass out.

 John C. Chien October 19, 1999 10:52

Re: Optimizing an ejectorpump in ANSYS. HELP!!

(1). I am not trying to answer your question related to the code. ( the last impression I had on the code was the solution did not converge after three months on the computer. a semi-joke.) (2). In general,there are two ways to simulate the internal flow problems. (3). The first one is to specify the inlet velocity profiles completely. The other is to specify the inlet total pressure and exit static pressure. This one is good for compressible flows. (4). So, for incompressible flows, it is easier to specify the inlet velocity profiles. And, if you specify the velocity profile at the inlet, your mass conservation problem should be solved. In other words, you run the ejectorpump problem as if it is a two stream mixing problem. You specify the velocity profile at the primary flow and the secondary flow. (5). This will change your problem into a two pumps flow problem. Then the next thing to do is to adjust the secondary flow velocity, run another calculation, until your ejectorpump condition is achieved, that is the pressure conditions meet your operating conditions. (6). This is assuming that the code is in working conditiion. To find out that, you need to do some benchmark case testings on the code first.

 clifford bradford October 20, 1999 23:35

Re: Optimizing an ejectorpump in ANSYS. HELP!!

i also had many problems using Ansys for fluid analysis. i think your grid must be structured. ie you have to use mapped mesh. i could not do this for my problem so i could not get properly converged results. you haven't said how your results are converging. if they aren't then the range of problems is wide. if they are then you probably have bad boundary conditions. bad turbulence parameters should not give you the magnitude of problems you describe. try running laminar and see. all this is just my speculation. my best suggestion is to contact Ansys and see if they will help. i know student versions don't come with support but if you plead (beg, grovel etc) they might help. or better yet find someone who has a full liscence and put the problem to ansys thru them. now that i think of it perhaps because you are limited in the number of elements in the student version you may be suffering from inadequate mesh size. however you can overcome this so i don't know what to say

 John C. Chien October 22, 1999 10:26

Re: Optimizing an ejectorpump in ANSYS. HELP!!