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Validation: ASHRAE Elbow Loss Coeffs

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Old   April 13, 2009, 19:37
Default Validation: ASHRAE Elbow Loss Coeffs
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LKF
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I'm curious, has anyone studied / compared CFX results to tabulated ASHRAE elbow loss coeffecients (or something similar)?

I've been studying a 60 inch diameter 90 degree elbow (5 Gore) for air between 2,000 fpm and 6,000 fpm.

Currently I'm seeing a CFX estimated Co that is about 0.65 times the ASHRAE value. Comparison against other reference values suggests similar under-prediction.

Last edited by LKF; April 13, 2009 at 22:00.
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Old   April 13, 2009, 21:19
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Glenn Horrocks
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Hi,

Did you model the full experimental test setup, such as inlet and exhaust pipes? Have you done sensitivity analysis of all important variables?

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Old   April 13, 2009, 21:58
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Hi Glenn,

I've modeled 3 diameters of duct length on either end - mass flow inlet w. zero gradient turbulence (to approximate fully developed flow), opening BC at 0 Prel on outlet. I understand there are possible improvements to the BC's, however in terms of sensitivity, I've considered different inflow velocity profiles such as 1/7 power law and uniform inlet velocity, with negligable change to the results. I could perhaps benefit from extending the outlet (as I've been advised from CFX tech support) and switching to an outflow condition. I've considered wall roughness, but have since reverted to a no slip smooth wall condition since there was little response to a rough wall.

I demonstrated grid indepence at about 300,000 nodes (using symmetry along the center, results are consistent for the whole domain). Y+ has only negligable impact at an average value of Y+ < 80 (that is Y+ in POST, not the solver y+ which is 1/4 of that value). Mesh spacing is about 2" and I've got 12 nodes of inflation that is about 2.5" thick.

I've compared both the standard k-e model and the SST model (with the appropriate mesh correction to get Y+ ~3). In all cases, I've converged to a max residual (not RMS) of 1E-4 (on some cases less than 5E-5). Mostly I've run double precision, but again haven't seen appreciable differences between single and double precision.

I believe I've done a fairly comprehensive analysis . . . and I still observe total pressure loss through the elbow that is around 0.65 times references of elbow loss data that I have access to (I would have expected +/-15% and would even be happy to get +/-20%). I should mention I'm using area based averaging instead of mass flow averages. I know mass flow averages are theoretically correct, but a lot of industry measurements I come across are area averages. Also, the mass flow based losses have even less agreement.

Do you have any experience with comparing this sort of scenario to reference / experimental results?
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