Water flows through two different pipes A and B of the same circular c...
Given:
- Length of pipe A (LA) = 1.0 m
- Length of pipe B (LB) = 2.0 m
- Flow in both pipes is laminar and fully developed
- Frictional head loss across the length of the pipes is the same
To find:
The ratio of volume flow rates QB/QA
Solution:
1. Introduction:
In fluid mechanics, the volume flow rate is defined as the volume of fluid passing through a given cross-section per unit of time. It is given by the equation Q = A⋅v, where Q is the volume flow rate, A is the cross-sectional area, and v is the velocity of fluid.
2. Understanding the problem:
We have two pipes, A and B, with different lengths but the same cross-sectional area. The frictional head loss across the lengths of both pipes is the same. We need to find the ratio of the volume flow rates QB/QA.
3. Relationship between frictional head loss and flow rate:
In laminar flow, the frictional head loss across a pipe is given by the Darcy-Weisbach equation:
Δh = f⋅(L/D)⋅(v^2/2g)
where Δh is the head loss, f is the Darcy friction factor, L is the length of the pipe, D is the diameter of the pipe, v is the velocity of fluid, and g is the acceleration due to gravity.
Since the frictional head loss is the same for both pipes A and B, we can write:
fA⋅(LA/DA)⋅(vA^2/2g) = fB⋅(LB/DB)⋅(vB^2/2g)
4. Relationship between velocity and flow rate:
The cross-sectional area of both pipes A and B is the same. Therefore, we can write:
QA = A⋅vA
and
QB = A⋅vB
5. Relationship between lengths and velocities:
Since the flow in both pipes is fully developed, we can assume that the velocity profiles are fully developed across the cross-sections of the pipes. In fully developed flow, the velocity profile is parabolic, and the maximum velocity occurs at the center of the pipe.
In a fully developed laminar flow, the velocity is inversely proportional to the length of the pipe. Therefore, we can write:
vA/vB = LB/LA = 2/1 = 2
6. Finding the ratio of volume flow rates:
Substituting the values of vA/vB = 2 in the equation QB = A⋅vB, we get:
QB = 2⋅QA
Therefore, the ratio of volume flow rates QB/QA = 2/1 = 0.5.
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