See if you can locate a check valve.
It takes specialized equipment, if you cant; locate a check valve.
If the pipe is small in diameter and enough pipe is visible to you, and you have a propane torch, the following method might work. I have never tried this, incidentally.
Apply high heat to the pipe for several minutes, then feel the pipe some distance from the point where your are heating the pipe (on both sides of the heating point). The pipe should be warm some distance from the heating point, in the direction the water is flowing.
This suggestion is the only logical method that comes to mind, without special equipment.
TexMav
To find the direction of fluid in a closed pipe, apply heat to one point and measure temperature on either side of that point. The side with a higher measured temperature is in the direction of fluid flow.
Huh? what's the friction loss? Head? Need some more info.
Downhill. Duh...
Water is flowing in a pipe. You need find the direction of flow inside the pipe. If you need any tips let me know
