Certainly when water flows through a tube the friction heats it up.
There's a force on the water equal to the area of the tube times the
pressure difference between the inlet and the outlet. If you multiply
that force times the average velocity of the water in the tube, you get
the rate at which work is being done on the water. That's the same as
the pressure drop on the tube times the volume flow rate. Since the
average speed of the water in the tube isn't changing, that power is
going into other forms of energy- the thermal energy of the water, for
the most part.
I did a very crude calculation, easily off by up to a factor of
ten, indicating that for a pressure drop of 1 atmosphere on water in a
1-meter long tube about 1 cm in radius the heating power would be about
0.1 Watt. I left off the fins for simplicity. So this isn't an easy way
to heat the water up by much. On the other hand, if you take a powerful
blender and let it stir a little water around quickly, the same sort of
frictional effects can heat the water up appreciably.
The conversion factor between heat energy and mechanical energy is
4.2 Joules per calorie (where it takes one calorie to heat one gram of
water by one degree Centigrade).
One thing to be careful about here, especially given that the
friction heating is so small, is to watch out for heat transfer in and
out of the tube if the environment has a different temperature. This
can easily account for more of a temperature change in the water than
the friction, but it can be reduced by insulating the tube.
(published on 10/22/2007)