What is the effect of increased viscosity on flow rate?

What is the effect of increased viscosity on flow rate?

• A. Flow rate decreases
• B. Flow rate increases
• C. Flow rate remains the same
• D. Flow rate oscillates

Answer: (A)

Viscosity is the fluid’s internal friction to flow. When viscosity increases, it resists motion more, so for a given driving pressure (pressure drop) through a vessel, the flow rate drops. In laminar tube flow, the flow rate is proportional to the pressure drop and to the fourth power of the radius, and inversely proportional to viscosity and length: Q ∝ ΔP r^4 /(μ L). So higher viscosity directly lowers Q. If you increased the driving pressure instead, you could maintain the same flow, but with the pressure fixed, the flow rate decreases as viscosity rises. The other outcomes—flow rate increasing, staying the same, or oscillating—don’t result from a simple increase in viscosity alone.

Viscosity is the fluid’s internal friction to flow. When viscosity increases, it resists motion more, so for a given driving pressure (pressure drop) through a vessel, the flow rate drops. In laminar tube flow, the flow rate is proportional to the pressure drop and to the fourth power of the radius, and inversely proportional to viscosity and length: Q ∝ ΔP r^4 /(μ L). So higher viscosity directly lowers Q. If you increased the driving pressure instead, you could maintain the same flow, but with the pressure fixed, the flow rate decreases as viscosity rises. The other outcomes—flow rate increasing, staying the same, or oscillating—don’t result from a simple increase in viscosity alone.