Trying to keep up w/ Chem Geek

[chem geek]

Regarding the laminar vs. turbulant flow discussion, I did some calculations on that a while ago when I started looking at my piping situation and I thought it strange that the FAFCO solar panels specification showed a parabolic curve for the head loss vs. flow rate since that implies turbulant flow. I counted around 200 tubes per panels with spacing around 1/4" so at the desired 4 GPM that would be 4/200=0.02 GPM per tube. Even if the inner diameter of each tube were as low as 0.1", the Reynolds number would be 632 which should be laminar flow (which has a Reynolds number less than 2000).

I also ran some calculations to predict head loss and could not get their results. I wrote to them about this asking them if they actually measured the head loss or if they calculated it. They wrote back (which gives them points in my book -- many vendors don't even respond to questions) and said that

The head loss was physically measured on a sample number of collectors matched against theoretical calculations. The contributors to the overall head loss include the header pipe, small tubes, and metering plenum that evenly distributes flow to the small tubes. It appears the metering plenum is the missing component in your calculations.

They were absolutely right. I had not accounted for the metering plenum -- I didn't even know it was there. So their tubes probably aren't that narrow in inner diameter so the flow is most certainly laminar inside those tubes, but getting from the main pipe into these tubes goes through constrictions designed to restrict overall flow rate and to evenly distribute the water and THAT is where the turbulant flow exists and is probably where the bulk of the head loss comes from (thus resulting in the parabolic curve).

Now, interestingly, FAFCO has a dimpled version of their solar panel (see this link) called "Revolution" that causes the water in the small tubes to spiral (slowly). This makes the panel output about 5% more energy due to its higher efficiency at transferring heat from the sun to the water because the water is better "mixed" in the tubes so that it all gets heated thus having a lower temperature difference between the water and the panel itself (remember, as Carl pointed out, that the highest efficiency is achieved when there is a minimal temperature difference since that keeps the panel cooler and minimizes the radiative losses -- a cool, or at least "air temperature" panel is an efficient panel).

Richard