For the thought experiment, my guess is that the unmoving circular pipe will likely freeze over from the top down (due to ice buoyancy), and any areas of the pipe that are less insulated than others will completely freeze over first.
I’m guessing the pump-driven pipe will probably start freezing more from the outside in, as the ice crystals latch onto the pipe walls where the velocity trends to zero, and ice buoyancy is less dominant. But the pump pressure is going to continue to clear/prevent any complete blockages from forming until the whole length of the pipe is plugged.
That’s only been mentioned like 50 times so far…
Related question: For those like me in areas where it might get very cold for periods in the winter (but not very often), does it make sense to keep the faucets dripping while on a long vacation? I’m thinking of heading down to Miami for a month, and no one else will be in the house. Is shutting the main water line into the house a better option?
I’d agree with you for turbulent flow, but for laminar flow; which this thread is all about; I doubt this is true.
Laminar flow in the pipe (due to the dripping of water) will create a flow (and consequently a thermal) boundary layer which will significantly cut down heat flow to the center of the pipe.
When my wife and I leave our house for more than a day or two, we shut off the water supply and depressurize the pipes by cracking open a faucet. Keeping the faucets flowing may prevent a freeze/burst, but if it doesn’t, the flooding could be catastrophic. OTOH, shutting off the water supply and opening a faucet won’t prevent the pipes from freezing, but it places a hard limit on the total amount of flooding.
It be better to turn the water off and open some faucets to relieve the pressure. Opening hose spigots will typically drain most of the water from the house since they are typically the lowest outlet. Not only will that mean you don’t have to worry about freezing, but you also won’t have to worry about a sink or washer hose bursting.
But one thing that this makes me realize is that dripping water typically only has to delay freezing by a few hours. In the typical situation, the household will use lots of water during the day and freezing is not an issue. You really only have to keep the pipes from freezing in the few hours in the night when no one is using water in the house. But if instead you were on vacation and were just dripping the pipes for days and days, I would think the pipes would eventually be able to freeze up if the temperatures were cold enough the whole time.
Ideally you’d drain down the pipes as much as possible rather than just shutting the main off. You open the lowest faucet (probably the water heater’s drain, or a basement laundry sink), and the highest faucet (say a shower on the 2nd floor) to break the vacuum created, and it’ll suck out pretty fast. However that’ll still leave water in a lot of pipes, but hopefully not enough to cause a problem. For a vacation that may be adequate. When homes are actually winterized, to the point that heat is turned off, they may even blow out pipes with compressed air, like you see with lawn sprinklers, and they also put antifreeze in sink drains and toilet bowls to keep ice from forming and cracking the porcelain, but also to maintain a liquid seal against sewer gasses. Again, probably overkill for a vacation, but definitely better than just letting a faucet drip.
If you drain the water heater (for any reason), also turn off the breaker or fuel to it. Don’t turn power back on until you’re sure it’s filled back up.
Thanks everyone. Quite a bit to think about, considering I’ve been in this house for many years and never did any of these things. Apparently there was a burst pipe due to freezing at some point in the ‘80’s, so all I’ve ever done when it gets below about 15 F is the dripping water. But not for extended periods of time.
I think most people have touched on it. If it is -80 outside, then we are close to the limits of human ingenuity. You better have had some very expensive and insulated stuff installed to keep flowing water.
At 0F? Most well constructed modern homes can do it without any needed intervention. If you live in a trailer in WV, then you should start worrying at about 20 degrees F.
Each of these tricks grant you a small reprieve for all of the reasons stated, but as posters have said, Niagara Falls will freeze solid, so it all depends on your setup.