Does the flow rate increase when 2 pipes of equal diameter are combined
Answers
You need to set some boundary conditions for this to make sense. Diameter is not a parameter that can actually be changed in any given system. If you change it you have a new system. Anyway, the usual case is we have a known pressure drop. That would be the upstream pressure, given by say a pump or a large reservoir of fluid, say by the hydraulic pressure at a depth, or perhaps from a tank, possibly with a cap of pressurized and regulated gas on top, or from a flow governed by a pressure regulator. So the upstream pressure is fixed. Now there is a flow without any change in the upstream pressure. Depending on the arrangement, there may be a fixed pressure drop, say because the pipe is discharging to the atmosphere. Knowing this pressure drop and the pipe diameter (and friction coefficient and viscosity etc), you can then calculate the flow by various formulas. Since the flow and velocity will be different, the friction factor will also be different (depending on the flow regime, i.e, whether laminar flow or turbulent flow). So it is an iterative process to find what flowrate gives exactly the pressure drop you have available, and which is fixed, but only one such flowrate can have the correct (actually available) pressure drop. So if friction, which determines the pressure drop for a given flow, is not being considered, you will get the wrong answer to this problem.
You can now ask suppose I have the same upstream pressure (and therefore also pressure drop) and fluid but a larger pipe diameter, but am changing nothing else. You can again calculate the flow rate. After doing a few of these you can ascertain the dependence on pipe diameter. The answers that some have given presuppose a flow regime (which in any case doesn’t matter to them) and fix the flowrate and therefore the velocity but ignore that there would need to be a different pressure upstream to achieve this result. So depending on the presumed boundary conditions you would get a different result. In any case the fixed flowrate assumption would require an engineer to calculate the pressure drop needed at some point except if a purely hypothetical case is being considered.
Generally, the pressure drop is calculated from friction factor x velocity^2, length divided by diameter. Friction factor itself is dependent on the Reynold’s number, which is diameter x density x velocity divided by viscosity and is dimensionless when proper units are used. A secondary parameter is pipe roughness. Charts are used to read it off, although for laminar flow there are closed form solutions. If you have the flow but not the velocity it is obviously trivial to calculate it, and some of the other answers have spelled that out, e.g., see Item 2. in Srinivas answer.
In any case this is more complicated than it seems at first blush.