Accepted correlations for the Nusselt number include the Sieder and Tate equation:
Nu = 0.023 Re^0.8 Pr^(1/3) (ub / uw)^0.14
And since the Reynolds number follows:
Re = D V ro / u
the Nusselt number (and therefore the inside heat transfer coefficient) varies by the velocity to the 0.8 power.
After converting from Nusselt to inside coefficient, I obtained the following data:
Velocity hi Re
3 m/s 12 W/m2-K 5,200
5 19 8,800
8 28 14,000
12 38 21,000
20 56 35,000
The Rule of Thumb for the outside heat transfer coefficient ranges from about 30 to 50 W/m2-K (quiet to 5 m/s wind) when considering heat transfer from a pipeline to the environment (see Chris Haslego's article, http://www.cheresour...with-insulation).
Now, my question: Why are the calculated inside heat transfer coefficients so much lower than the Rule of Thumb for outside coefficient? Does this make sense? If the flow rate drops into the laminar range, the calculations (using correlations for laminar conditions) give much lower results.
Calculations were done at a bulk temperature of 135 C, atmospheric pressure, 50 mm inside diameter. See attached Excel spreadsheet.
Secondary question: I can calculate the wall temperature, which for this case is around 40C (room air temperature is at 30C). I can also compute a radiation factor (see Chris's article) using the wall temperature. But is this really valid indoors, not knowing anything about the various surfaces that are within sight of the pipe? It does make a difference -- the radiation heat transfer is about 20% of the convective transfer -- but I'm leaning toward neglecting it because I think the calculation is overly optimistic.