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Wednesday, November 25, 2015

Simplified Convection Model

With the asymptote established, it was time to work on the lower speeds.  My models were stubbornly predicting too much convection for 0109rpm-20151031T023629.  It turns out that the apparent deficit for rough upward natural convection was nearly equal to the apparent deficit for rough downward convection.  Thus it was likely that I was underestimating the radiative cooling and natural convection from the four sides, which act in both orientations.  Increasing the modeled side surface conductances yields values for the rough surface matching smooth natural convection within the experimental variations.

I managed to coax the wind-tunnel fan to run at 93 r/min for a data run, extending test coverage down to Re=8000.  Where my earlier plot showed points clustering around the L4-norm of the Scaled-Colburn-Analogy-Asymptotes, the new point aligns with the others along the asymptotes themselves, as predicted by my SCAA formula:

  NuSCAA = max(Nu8.9, Nu8.11, NuRS)

The small convection from 0093rpm-20151125T170700 and 0109rpm-20151031T023629 also confirm that mixed convection uses a high degree norm (the highest being the "max" function).  This essentially says that natural and forced convection do not mix; it is either one or the other.  When the Convection Machine gets fitted with a smaller fan, I will be able to test at the transition point.

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