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area and mass flow averaging giving different results with rough pipes |
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October 21, 2018, 14:11 |
area and mass flow averaging giving different results with rough pipes
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#1 |
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Dear all
I am studying rather basic problems of heat transfer though pipes with different sand roughness (ks) sizes. In my models I have a circular pipe of Ø11mm, a mass flow of helium of 30g/s entering at 300°C at 8 MPa and an homogeneous heat flux in the pipe of 0.5MW/m². I did a parametric analysis with different roughness sizes, ranging from smooth channel to a ks=400µm. Taking the most extreme cases of smooth and rough pipe with 400µm, I wanted to know the flow velocity at the outlet of the pipe. For this I did smooth pipe massFlowAve(Velocity)@outlet = 51.30m/s areaAve(Velocity)@outlet = 49.66m/s rough pipe 400µm massFlowAve(Velocity)@outlet = 51.74m/s areaAve(Velocity)@outlet = 49.63m/s Which evaluation is more accurate? the one with the areaAve sounds the one to take, as it matches the value that one obtains by doing a simple calculation taking into account the density, area and mass flow of the fluid at the outlet. However, I have read in the CFX theory manual that the boundaries are virtually moved the half of the roughness height in channels with rough walls for the near-wall treatment. Therefore I am not sure if the roughness makes an influence on the flow speed due to this virtual shift of the wall, or if this shift is only used as an internal operation to make the calculations for the near-wall treatment. thanks! |
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October 21, 2018, 16:28 |
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#2 |
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Gert-Jan
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If you use the mass flow average, the variable that you average is weighted by the massflow through each element. So if you take the mass flow average of velocity, you are weighing the velocity with velocity*density. If density is constant, you are weighing velocity with velocity. Doesn't sound right. Therefore use area average.
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October 21, 2018, 17:55 |
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#3 |
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Glenn Horrocks
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Gert-Jan, that logic appears flawed. If the density is constant then there is no weighting and the mass flow average is the same as the area average. They are only different if the density is not constant, and in this case you should use massflow average to weight it with variable density.
In short, do you want the integral of density*normal velocity or the integral of the normal velocity? The first one requires a massflow integral, the second area integrals. But the first one is what you want most of the time.
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October 22, 2018, 08:51 |
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#4 |
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Gert-Jan
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Maybe I'm wrong, but I have performed a calculation with plain water. I get different results for the areaAve and massFlowAve of Velocity at the outlet. So, the results are not the same.
The massFlowAve weighs each variable by the massflow per element (see CFX-help). In my case, the constant Density cancels out, so only weighing by Velocity remains. Therefore, weighing Velocity by Velocity, results in a (unwanted) bias to higher velocities.......... For this reason, I use the areaAve of the velocity for an average velocity. For averaged temperature, pressures, or whatsoever, I always take massFlowAve-results. Last edited by Gert-Jan; October 23, 2018 at 14:30. |
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October 23, 2018, 20:22 |
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#5 | ||
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Dear Gert-Jan and Glenn,
thanks for your answers. To your points: Quote:
Quote:
In any case, a couple of questions that arose from this conversation also:
thanks again Last edited by Freeman; October 24, 2018 at 02:39. |
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October 23, 2018, 20:40 |
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#6 |
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Perhaps we are missing the point of massFlowAve(quantity)@locator.
massFlowAve(quantity) = massFlowInt(quantity)@locator / massFlow()@locator It is a convenient quantity to refer to something more important, the total amount of an advected quantity, i.e. quantity = 1 --> total mass flow quantity = velocity --> total linear momentum flow quantity = total enthalpy --> total energy flow quantity = mass fraction --> total mass flow of species k I so far never seen any reason to use massFlowAve/Int unless I am working with conserved quantities. For example, I would not use massFlowAve/Int(Pressure)@locator. Not sure what that means in practice. However, areaAve/Int(Pressure)@locator is the force at the location. Total Pressure is a different issue since the interpretation, via Bernoulli, is similar to energy flow (incompressible flow of course). |
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October 24, 2018, 03:41 |
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#7 | ||
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Gert-Jan
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Quote:
Quote:
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October 24, 2018, 07:37 |
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#8 | ||
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Glenn Horrocks
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Quote:
Quote:
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October 24, 2018, 07:37 |
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#9 | ||
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Glenn Horrocks
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Quote:
Quote:
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October 26, 2018, 18:29 |
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#10 | |
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Quote:
Just FYI, I recently got to know from ANSYS support that since version 18 CFX has implemented a blending function bridging the viscous sublayer and the log-law without the need for virtual wall shifts and similar "tricks". This allows (and even forces) the user to model the boundary layer with full resolution down to Y+~1, regardless of the roughness height. For this, one must enable the beta feature "Blending for near wall treatment (Beta)" in the fluid model. This feature will appear only if the wall is defined as "smooth" or "high roughness (icing)". I did a couple of checks with high roughness again and now the Y+ came back indeed to typical values even if he first thickness lays below the roughness height. I observed that the heat transfer is slightly enhanced with respect to a calculation without this blending function and results correlate better with the available correlations for fully rough regime. |
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