SPED Fluid Dynamics & Flow Analysis 2 — Questions and Answers
Question 1: In a piping system, the Moody friction factor for fully turbulent flow in a rough pipe depends primarily on:
- Reynolds number only
- Relative roughness only (Correct answer)
- Both Reynolds number and relative roughness
- Pipe diameter only
Correct answer: Relative roughness only
At fully turbulent (wholly rough) flow, the friction factor becomes independent of Reynolds number and depends only on relative roughness (ε/D).
Question 2: The hydraulic diameter for a rectangular duct with width W and height H is:
- (W + H) / 2
- 2(W + H)
- 4WH / (2(W + H)) (Correct answer)
- WH / (W + H)
Correct answer: 4WH / (2(W + H))
Hydraulic diameter Dh = 4A/P = 4WH / (2(W+H)), used to apply circular-pipe correlations to non-circular cross sections.
Question 3: Which phenomenon causes pressure oscillations when a valve is rapidly closed in a liquid piping system?
- Cavitation
- Water hammer (Correct answer)
- Flashing
- Surge flooding
Correct answer: Water hammer
Water hammer (hydraulic shock) occurs when rapid valve closure converts kinetic energy of flowing liquid into pressure waves that travel through the system.
Question 4: For two-phase gas-liquid flow, the Lockhart-Martinelli parameter X is used to characterize:
- The ratio of liquid to gas viscosity
- The ratio of frictional pressure drops of liquid and gas phases flowing alone (Correct answer)
- The void fraction of the gas phase
- The superficial velocity ratio
Correct answer: The ratio of frictional pressure drops of liquid and gas phases flowing alone
The Lockhart-Martinelli parameter X² is the ratio of frictional pressure drop of the liquid phase alone to that of the gas phase alone.
Question 5: The continuity equation for steady incompressible flow through a converging nozzle states that:
- Velocity decreases as area decreases
- Mass flow rate increases through the nozzle
- The product of area and velocity is constant (Correct answer)
- Pressure increases as velocity increases
Correct answer: The product of area and velocity is constant
Continuity for incompressible flow requires A₁V₁ = A₂V₂, so as area decreases, velocity must increase proportionally.
Question 6: A pump's NPSH required (NPSHr) is exceeded when the system NPSH available (NPSHa) is lower than NPSHr, resulting in:
- Increased pump efficiency
- Cavitation within the pump (Correct answer)
- Reduced discharge pressure only
- Motor overload
Correct answer: Cavitation within the pump
When NPSHa < NPSHr, the liquid pressure at the pump inlet drops below vapor pressure, causing cavitation—bubble formation and collapse that damages impeller.
Question 7: The Colebrook-White equation is used in piping design to determine:
- Pump head requirements
- The Darcy-Weisbach friction factor for turbulent flow (Correct answer)
- The critical flow velocity in gas lines
- Thermal expansion of pipe walls
Correct answer: The Darcy-Weisbach friction factor for turbulent flow
The Colebrook-White equation implicitly relates friction factor to Reynolds number and relative roughness for turbulent flow in the transition and rough zones.
SPED Fluid Dynamics & Flow Analysis 2
In a piping system, the Moody friction factor for fully turbulent flow in a rough pipe depends primarily on: