I've had a couple questions above the sizing of a pipe, but I'm still a little confused and can't find an easy answer.
I'll use this image to get a representation of what I'm looking to have answered.
Sizes shouldn't matter, but if they were; we'll assume the small pipes are both 1/2" and the big pipe is just 3/4". What effect does it have on the water flow to increase the pipe size (into the center larger pipe)? And then what effect does it have on the flow to decrease the size?
The shape doesn't have to be a U like this; I just figured it'd knock out two birds with one stone. It simply represents an increase and then a decrease. You can treat them separately in the answers or as a whole if it does matter.
My general thoughts, from what I have been able to read through online, is that the pressure might slightly increase when going into the larger pipe or decrease when going down. Simply because of friction though – smaller pipe has more friction towards the water than the larger pipe. If this were one piece of plumbing, would the pressure out at the right be relatively the same as the pressure in at the left?
The other thing I see if the speed of the water. Increasing the pipe size slows down the water flow and decreasing the pipe size speeds it up? Again though, if it were one piece, would this balance out and the speed out would be like the speed in?
Best Answer
You should see the same water pressure on both sides of the bigger section of pipe.
I wouldn't expect the bigger pipe to really make any pertinent difference.
The water will flow more slowly in the bigger pipe, but the pressure will increase (Bernoulli's law, the same thing that makes an airplane wing fly, but applied to fluid dynamics). The water will fill and pressurize the bigger pipe, and the greater pressure in the section of big pipe will force the water into the small pipe on the far side at the same velocity that the water entered on the near side. Again due to the phenomenon described by Bernoulli's law, faster-moving water in the smaller pipe will exert less pressure on the pipe. That isn't to say that there is less energy in the smaller section of pipe. Part of that energy is accounted for in the greater momentum of the fluid. So the water coming out the end of that pipe will be moving with as much force as the water that went into the other end.