20 Thermal Engineering Quiz Questions and Answers

Question 1:
What is the first law of thermodynamics?
A. Energy can be created or destroyed.
B. The entropy of an isolated system always increases.
C. The total energy of an isolated system is constant.
D. Heat flows from cold to hot bodies.
Answer: C
Explanation: The first law of thermodynamics states that energy is conserved, meaning the change in internal energy of a system is equal to the heat added minus the work done by the system.

Question 2:
In a heat engine, what does thermal efficiency depend on?
A. The temperature of the surroundings only.
B. The difference between the hot and cold reservoir temperatures.
C. The volume of the working fluid.
D. The pressure of the intake air.
Answer: B
Explanation: Thermal efficiency of a heat engine is determined by the Carnot efficiency formula, which depends on the absolute temperatures of the hot and cold reservoirs.

Question 3:
What is the primary mode of heat transfer in a vacuum?
A. Conduction
B. Convection
C. Radiation
D. Advection
Answer: C
Explanation: Radiation is the only mode of heat transfer that does not require a medium, as it occurs through electromagnetic waves.

Question 4:
Which cycle is most commonly used in steam power plants?
A. Otto cycle
B. Diesel cycle
C. Rankine cycle
D. Brayton cycle
Answer: C
Explanation: The Rankine cycle is used in steam power plants because it efficiently converts heat into work using water as the working fluid in a closed loop.

Question 5:
What is the formula for the rate of heat conduction through a plane wall?
A. Q = mCΔT
B. Q = kAΔT/L
C. Q = hA(Ts – T∞)
D. Q = σA(T1^4 – T2^4)
Answer: B
Explanation: Fourier’s law of heat conduction gives Q = kAΔT/L, where k is thermal conductivity, A is area, ΔT is temperature difference, and L is thickness.

Question 6:
In thermodynamics, what does entropy represent?
A. A measure of disorder or randomness in a system.
B. The total energy of a system.
C. The heat capacity of a substance.
D. The work done by a system.
Answer: A
Explanation: Entropy is a thermodynamic property that quantifies the degree of disorder or unavailable energy in a system, as per the second law of thermodynamics.

Question 7:
What is the critical point in a phase diagram?
A. The point where solid and liquid coexist.
B. The temperature and pressure above which a gas cannot be liquefied.
C. The triple point where all phases coexist.
D. The boiling point at standard pressure.
Answer: B
Explanation: The critical point is the end point of the vapor-pressure curve, beyond which distinct liquid and gas phases do not exist.

Question 8:
Which of the following is a measure of a refrigerant’s performance?
A. Thermal efficiency
B. Coefficient of Performance (COP)
C. Carnot factor
D. Enthalpy ratio
Answer: B
Explanation: COP is defined as the ratio of heat removed to the work input in a refrigeration cycle, indicating how effectively the system cools.

Question 9:
What type of heat exchanger uses a direct contact between fluids?
A. Shell-and-tube heat exchanger
B. Plate heat exchanger
C. Direct contact heat exchanger
D. Regenerative heat exchanger
Answer: C
Explanation: In a direct contact heat exchanger, the two fluids mix directly, which is common in cooling towers or evaporative coolers.

Question 10:
In fluid dynamics, what does Bernoulli’s equation describe?
A. The relationship between pressure, velocity, and elevation in a flowing fluid.
B. The heat transfer in turbulent flow.
C. The compressibility of gases.
D. The viscosity of liquids.
Answer: A
Explanation: Bernoulli’s equation states that for an incompressible, steady flow, the sum of pressure energy, kinetic energy, and potential energy is constant along a streamline.

Question 11:
What is the primary function of a superheater in a boiler?
A. To increase the pressure of steam.
B. To heat steam above its saturation temperature.
C. To condense steam into water.
D. To remove impurities from water.
Answer: B
Explanation: A superheater raises the temperature of saturated steam to superheated steam, improving the efficiency of turbines in power plants.

Question 12:
Which law governs the radiation heat transfer between two black bodies?
A. Stefan-Boltzmann law
B. Fourier’s law
C. Newton’s law of cooling
D. Kirchhoff’s law
Answer: A
Explanation: The Stefan-Boltzmann law states that the total energy radiated per unit surface area of a black body is proportional to the fourth power of its absolute temperature.

Question 13:
What is the isentropic efficiency of a turbine?
A. The ratio of actual work to isentropic work.
B. The ratio of heat input to work output.
C. The maximum possible efficiency.
D. The Carnot efficiency.
Answer: A
Explanation: Isentropic efficiency measures how closely a turbine operates to an ideal reversible adiabatic process, comparing actual and theoretical work outputs.

Question 14:
In a vapor compression refrigeration cycle, what component increases the pressure of the refrigerant?
A. Evaporator
B. Condenser
C. Compressor
D. Expansion valve
Answer: C
Explanation: The compressor raises the pressure and temperature of the low-pressure vapor refrigerant, enabling it to release heat in the condenser.

Question 15:
What is the Reynolds number used for?
A. To determine the type of fluid flow (laminar or turbulent).
B. To calculate heat transfer coefficients.
C. To measure thermal conductivity.
D. To find the specific heat of a substance.
Answer: A
Explanation: Reynolds number (Re) is a dimensionless quantity that predicts the flow regime based on fluid velocity, density, viscosity, and characteristic length.

Question 16:
Which of the following is an example of an open cycle gas turbine?
A. Reciprocating engine
B. Jet engine
C. Steam turbine
D. Nuclear reactor
Answer: B
Explanation: A jet engine operates on an open Brayton cycle, where air is drawn in, compressed, heated, and expelled as exhaust.

Question 17:
What does the term “steady-state heat transfer” mean?
A. Heat transfer that varies with time.
B. Heat transfer that remains constant over time.
C. Heat transfer in transient conditions.
D. Heat transfer due to phase change.
Answer: B
Explanation: In steady-state heat transfer, the temperature at any point in the system does not change with time, only with position.

Question 18:
In psychrometrics, what is the dew point temperature?
A. The temperature at which air becomes saturated.
B. The dry bulb temperature.
C. The wet bulb temperature.
D. The temperature of the surroundings.
Answer: A
Explanation: The dew point is the temperature at which air is saturated with water vapor, and condensation begins.

Question 19:
What is the air-fuel ratio in a typical internal combustion engine?
A. 1:1
B. 10:1
C. 15:1
D. 50:1
Answer: C
Explanation: A stoichiometric air-fuel ratio of approximately 15:1 (by mass) is common for gasoline engines to ensure complete combustion.

Question 20:
Which parameter is used to evaluate the performance of a boiler?
A. Thermal efficiency
B. Volumetric efficiency
C. Mechanical efficiency
D. Electrical efficiency
Answer: A
Explanation: Thermal efficiency of a boiler is the ratio of heat output to heat input, indicating how effectively fuel is converted to useful heat.