# Fan dilemma

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 February 4, 2008, 14:49 Re: Fan dilemma #2 CycLone Guest   Posts: n/a Hi Jenny, Have you tried applying a total pressure inlet and static pressure outlet? How is the mass flow rate determined in the experiment? Are you sure you are comparing the same pressures? -CycLone

 February 4, 2008, 22:02 Re: Fan dilemma #3 Jenny Guest   Posts: n/a Hi Cyclone, Thank you for your reply. The measurements I have taken in the experiment are the total and static pressure readings from pitot measurements taken 200mm from the outlet. From these values and the calibrated pitot tube I have calculated the dynamic pressure and thus volume flow and mass flows. I'm definitely comparing the right pressures and have had several people (CFX engineers included) looking through my model. I haven't tried the boundary conditions you suggested, so will give this a go. The latest thing I am going to try is modelling a different geometry for the inlet. Currently I have a dome shape in the front, but I seem to be losing too much mass flow in the system, so am just going to extend the duct as my inlet opening and see what affect this might have. The only way I can simulate the correct pressures from the experiment are when I greatly reduce the mass flow in the system at the more loaded condition when the pressure plate is close to the opening. Jenny

 February 7, 2008, 14:58 Re: Fan dilemma #4 CycLone Guest   Posts: n/a Have you compared the dynamic and static pressures at the same location as your pitot tube? When you estimate the mass flow from the pitot readings, do you assume a constant velocity profile? -CycLone

 February 7, 2008, 17:03 Re: Fan dilemma #5 Jenny Guest   Posts: n/a Hi Cyclone, Yes the pressures are being compared at a plane in exactly the same position where the pitot tube measurements were taken. The velocity profile in the experimental results is asymmetric, so I have been using an area weighted average rather than just an arithmetic average. I have tried the numerical model with a dome like shape at the inlet and also just an extension of the tube in case this was affecting the mass flow rate too dramatically. When I used a total pressure inlet and static pressure outlet my static pressure values were in the right ball park, but the mass flow and velocity were too low. When I use a mass flow inlet condition with a static pressure outlet then I get a really wonky pressure reading when the outlet is being choked by the pressure plate, but the velocities are more realistic. However I don't understand how the velocity still doesn't agree with the experimental value when I put in the mass flow value obtained from the experiment. This just shouldn't be possible since mass flow is directly proportional to the dynamic velocity. I've got a CFX engineer currently investigating the problem, but we are all a bit stumped at the moment. Thanks for your reply. Jenny

 February 8, 2008, 13:17 Re: Fan dilemma #6 CycLone Guest   Posts: n/a Which velocity doesn't agree? The problem may be with the experimental evaluation of mass flow rate. A single pitot tube measurement only tells you the velocity at a single point, but you need to integrate the velocity variation over the surface to get the mass flow. Assuming a velocity profile will add some error to this calculation. It also depends on the pitot tube being aligned with the flow and properly calibrated for the experimental Reynolds number. This is not to suggest that the simulation is right, just that experiment has errors too. Also be careful comparing averaged data at a plane with a single point of data, such as your pitot tube measurement; these will not be the same. You should locate a point at the same location in space and probe the pressures at that point. Finally, some degree of error is normally expected. These arise from a number of sources: 1. Modeling errors arise from assumptions about boundary conditions, missing features, simplifications (periodicity, symmetry, defeaturing), fluid properties and physical models (turbulence), 2. Numerical errors arise from grid resolution, grid quality, computer precision, numerical models, etc. 3. Solution errors arising from unconverged systems and computer precision. Most of our discussion thus far has centered around the modeling errors, which are usually responsible for large discrepancies between simulation and test results. One common problem is the test set-up can be quite different from the simulation set-up, although it sounds like your modeling the test set-up, which is good. If you are satisfied that the model closely matches the test, I would start to look at the other two sources of error. CFX is well proven in industry for these applications, so accurate results are attainable. -CycLone

 February 10, 2008, 02:43 Re: Fan dilemma #7 Jenny Guest   Posts: n/a Thanks again for your reply. The velocity at the plane where the pitot point readings were taken doesn't agree with the experimental. I have done some theoretical calcs and the mass flow rates are very similar to what I'm getting experimentally, plus I've gone through the experimental results with a fine tooth comb and recalibrated the pitot tube three times now. So I am as confident as I can be with the experimental data. The pitot readings were not taken at a single point in either the experimental or the numerical work. I took 6 readings across the diameter of the tube in the vertical and horizontal, following the British Standard for fan flow analysis. I've integrated the velocity variation over the plane where the readings were taken to get the mass flow. I'm going to try and run the model without the fan in it and see how this compares in case there is something fundamentally wrong with the setup, otherwise I'm almost out of ideas on how to get the static pressure, velocity and mass flow results for the numerical working ALL agreeing at the same time, with the experimental. If I get the pressure close then the velocity and mass flow are off, and vice versa. Thanks again for your suggestions and advice. Jenny

 February 11, 2008, 13:17 Re: Fan dilemma #8 CycLone Guest   Posts: n/a I wouldn't expect the answers to be exactly the same at all operating points. Assuming you are confident that you are comparing the same data and the boundary conditions and geometry are sufficiently similar, I would look into what is happening at the fan when the simulation deviates from the test results. Also verify that your inlet turbulence level is reasonable. Earlier you mentioned a loss of mass, can you please explain? If you applied a specified mass flow, the inlet flow should match this (note that if there is a pitch change, the mass per full revolution is conserved, the mass per pitch could be different). -CycLone

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