Which flow tube length had the greatest flow rate?

Which flow tube length had the greatest flow rate?

• A. 5 mm
• B. 15 mm
• C. 20 mm
• D. 10 mm

Answer: (D)

The main idea is how flow responds to resistance. For a fixed pressure difference, flow through a tube decreases as the tube’s resistance increases, and resistance grows with length. But in real tubes, very short lengths have entrance effects: the flow isn’t fully developed yet, so energy losses at the entry reduce the actual flow. As a result, there can be an intermediate length where the flow is fully developed enough to maximize the rate, while not being long enough to incur large friction losses along the entire length. In this case, the 10 mm tube length hits that balance best, giving the greatest measured flow rate. The 5 mm length likely suffers from insufficient development or higher entrance losses, while the longer lengths introduce more friction losses that reduce flow.

The main idea is how flow responds to resistance. For a fixed pressure difference, flow through a tube decreases as the tube’s resistance increases, and resistance grows with length. But in real tubes, very short lengths have entrance effects: the flow isn’t fully developed yet, so energy losses at the entry reduce the actual flow. As a result, there can be an intermediate length where the flow is fully developed enough to maximize the rate, while not being long enough to incur large friction losses along the entire length. In this case, the 10 mm tube length hits that balance best, giving the greatest measured flow rate. The 5 mm length likely suffers from insufficient development or higher entrance losses, while the longer lengths introduce more friction losses that reduce flow.