Engineering Certification Engineering Certification Thermodynamics & Fluid Mechanics 2 — Questions and Answers
Question 1: In a heat exchanger, what does the term 'LMTD' stand for?
- Linear Mean Temperature Difference
- Log Mean Temperature Difference (Correct answer)
- Liquid Mass Transfer Degree
- Latent Mean Thermal Design
Correct answer: Log Mean Temperature Difference
LMTD (Log Mean Temperature Difference) is the logarithmic average of the temperature differences between hot and cold fluids at each end of a heat exchanger.
Question 2: What is the thermal efficiency of a Carnot heat engine operating between temperatures T_H and T_C?
- η = T_C / T_H
- η = 1 - (T_C / T_H) (Correct answer)
- η = (T_H - T_C) / T_C
- η = T_H / (T_H + T_C)
Correct answer: η = 1 - (T_C / T_H)
The Carnot efficiency η = 1 - T_C/T_H (using absolute temperatures in Kelvin) represents the maximum possible efficiency for any heat engine operating between those temperatures.
Question 3: What is the gauge pressure of a fluid if its absolute pressure is 200 kPa and atmospheric pressure is 101.3 kPa?
- 301.3 kPa
- 98.7 kPa (Correct answer)
- 200 kPa
- 101.3 kPa
Correct answer: 98.7 kPa
Gauge pressure = Absolute pressure − Atmospheric pressure = 200 − 101.3 = 98.7 kPa.
Question 4: Which dimensionless number characterizes the ratio of inertial forces to viscous forces in a fluid?
- Prandtl number
- Nusselt number
- Reynolds number (Correct answer)
- Mach number
Correct answer: Reynolds number
The Reynolds number Re = ρvL/μ is the ratio of inertial forces to viscous forces and determines the flow regime in fluid systems.
Question 5: In thermodynamics, what is enthalpy (H) defined as?
- H = U - PV
- H = U + PV (Correct answer)
- H = TS - PV
- H = Q/T
Correct answer: H = U + PV
Enthalpy is defined as H = U + PV, where U is internal energy, P is pressure, and V is volume; it accounts for flow work in open systems.
Question 6: What is cavitation in fluid systems?
- Build-up of mineral deposits inside pipes
- Formation and collapse of vapor bubbles due to local pressure drop below vapor pressure (Correct answer)
- Turbulent mixing of two different fluids
- Increase in fluid viscosity at low temperatures
Correct answer: Formation and collapse of vapor bubbles due to local pressure drop below vapor pressure
Cavitation occurs when fluid pressure drops below the vapor pressure, causing vapor bubbles to form and then violently collapse, damaging pump impellers and pipe walls.
Engineering Certification Engineering Certification Thermodynamics & Fluid Mechanics 2
In a heat exchanger, what does the term 'LMTD' stand for?