# Flow rate in split bottleneck

Water flows from top to bottom.

First there are two pipes, one with $$10$$ LPM max flow rate, and another with $$20$$ LPM max flow rate.
They both go to another pipe, which is $$20$$ LPM max flow rate.

The total flow rate will be $$20$$ LPM, because of the lower pipe bottleneck.
What would be the flow rate in the two upper pipes?

Picture of this problem:

I thought it may be $$10$$ LPM each or $$13.333333$$ in the fatter, $$6.6666667$$ in the slimmer. Both make sense to me.

I would also like to know which field does this question belong to? I mean more specific than just fluid mechanics. What theoretical material should I read? Any specific topics / equations?

• You claim the flow through the lower, single pipe is 20 LPM, yet you don't know that. Without detailed knowledge of pipe diameters and pipe lengths (and assuming smooth pipes) this cannot be solved. – Gert Nov 22 '15 at 20:05
• let's say that upper fat pipe is twice the area of the upper slimmer pipe, and same area as the lower pipe. is that solvable now? – user1952686 Nov 22 '15 at 20:08
• Using Darcy-Weisbach, I get: $\frac{Q_1}{Q_2}=\bigg(\frac{A_1}{A_2}\bigg )^{5/2}$ for the volumetric flow rates in the upper pipes ($A$ are the cross-sections of the pipes) but without lengths and friction coefficients it's as far as this goes. Total flow rate would of course be $Q=Q_1+Q_2$. – Gert Nov 22 '15 at 21:13
• The continuity equation requires that the flow rates from the fat and thin pipe exiting the upper tank equal the flow rate of the pipe entering the lower tank. In addition, the pressure drop in each of the top pipes must be equal. Finally, as Gert pointed out, you need quite a bit more information before you can set up and solve this problem. – David White Oct 9 '16 at 15:53