Fire caused by friction with water Friction causes heat and heat causes fire, but could something catch in fire because of friction with a high speed water stream?
If so, what material would it be and how fast would the water speed need to be flowing?
 A: There is a machining process called abrasive water jet machining. Water mixed with abrasive particles is forced as a jet through a very narrow nozzle and used to cut metal, minerals or even diamonds. But I've not witnessed any material catching fire during the process. There can be multiple reasons for this. The water jet heats the metal but also absorbs some heat during the process. Even if the material heats up to a sufficiently high degree, the water jet will start evaporating and absorb the heat and extinguish any spark that may have been produced by cutting off the oxygen supply.
A: Sure. Fire needs three components:

*

*Fuel

*Heat

*Oxygen

Given enough of those and you can get fire. You want heat to come from friction. That's fine. But understand water has a tendency to take away heat and oxygen. Heck with enough force behind it water will take away your fuel. So what you need is a situation where the water can't take away any of them.
Lets borrow an idea from James Prescott Joule.

lon-capa.org - Mechanical Equivalent of Heat
Joules little demonstration of how mechanical energy relates to heat clearly shows that friction can indeed heat water given some source of energy. Granted, it's not a lot. But since we're trying to make this happen it doesn't need to be a lot.

could something catch in fire because of friction with a high speed water stream?

To make this work we have to violate a lot of expectations. Lets put the stream in a container, the fuel outside the container, and lets turn up the ambient heat. Soon both inside and outside are just barely under the temperature that would ignite our fuel.
An important note is that the water stream must stay in the room. If it goes outside it could take our heat away to some other room at some other temperature. It's far easier to do this in a closed system.
Say we choose paper as our fuel. That ignites between 424 and 475 degrees Fahrenheit (218 and 246 degrees Celsius). That seems like a lot but we don't need to start at room temperature any more than we have to start at 0 degrees. So lets heat up everything until the paper is just about to burn. Then lets paddle until it burns.
If you think heating up the room is cheating then fine. Lets make the fuel
Silane. It ignites around room temperature. Pyrophoric chemicals might be fun to watch but please be safe.
Given this set up you can do this to just about any combustible material. It just seems weird because, well, who does this?

For those that feel the water must be touching the fuel consider making the container out of magnesium (or simply coating the containers interior with it). It's reaction is powerful enough to strip the oxygen out of the water itself. Just be advised that magnesium's autoignition temperature is 883 °F (473 °C) so you'll want a container that can take the pressure. Once it ignites you'd better have one heck of a safety valve on the container or you'll find we have just built a bomb.
A: If you move magnesium fast enough (e.g., 11.2 km/s, like a re-entering spacecraft) through water vapor I'd be surprised if it didn't catch fire.
Conceivably such a scenario — a very fast object moving through a medium — could work with liquid water (which will probably turn into steam when it hits the shock wave, so there may still not be direct liquid water contact).
A: Regardless of the heat source, a material will not ignite unless it reaches, at a minimum, its Self-Ignition Temperature (SIT). In addition to reaching this temperature you need to have the proper mixture of fuel and oxygen for ignition to occur. Finally, for self-sustained combustion sufficient heat must be maintained at the surface of a combustible material.  Water, with its large heat capacity, is extremely effective at absorbing heat, which is why, with the principal exception of flammable liquid fires, it is so effective at extinguishing most fires.
Bottom line: Given the large heat capacity of water, in my opinion it is unlikely that the friction of a high speed water stream could result in ignition and self sustained combustion.
Finally, with regard to the reference to "abrasive water jet machining", from what I have read (e.g. see How Waterjet Works), the mechanism for cutting is primarily pressure as opposed to friction heating. But I admittedly haven't researched it thoroughly.
Hope this helps.
A: Ordinary water is a neutron moderator, so we can run water through a nuclear reactor with enriched uranium and get as much heat as we want. One could object that it would not be "friction" with water that causes heat, but at least it is "interaction" with water.
