Quick recall · 206 cards
Short MCQ-style retrieval prompts. Tap a card to reveal the answer.
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The expression \( \oint \delta W \leq 0 \) is an outcome of which Law of Thermodynamics?
C · Second
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Which of the following is known as the inequality of Clausius?
A · cyclic integral of dQ/T <= 0
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The cyclic integral of entropy change is
C · zero
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Which of the following clearly defines availability or exergy?
C · Both of the mentioned
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Energy is ____ conserved and exergy is ____ conserved.
B · always, not generally
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The maximum work or exergy cannot be negative. State whether this statement is true or false and explain why.
A · True
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When a closed system is allowed to undergo a spontaneous change from a given state to a dead state, its exergy is destroyed while producing useful work. Is this statement true or false?
A · True
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The exergy of an isolated system can ____
C · never increase
PYQ · 2020
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Under which conditions will a real gas behave most like an ideal gas?
B · B. low pressure and high temperature
PYQ · 2019
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A fixed mass of an ideal gas is trapped in a cylinder of constant volume and its temperature is varied. Which graph shows the variation of the pressure of the gas with temperature in degrees Celsius?
B · B. Straight line with positive slope, intercept on pressure axis
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Under conditions of fixed temperature and amount of gas, Boyle's law requires that:
C · C. Both A and B
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In which of the following cycles is heat added at constant volume?
A · Otto cycle
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In absorption refrigeration cycle, which of the following is used?
C · both refrigerant and absorbent
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In absorption system, compressor in vapour compression cycle is replaced by?
C · absorber-generator assembly
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What is the primary energy source used in Vapour Absorption System (VAS) compared to Vapour Compression System (VCS)?
C · Heat energy instead of mechanical energy
PYQ · 2008
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For air at a given temperature, as the relative humidity is increased isothermally,
B · B. The specific enthalpy increases but the wet bulb temperature remains constant.
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In summer air conditioning, identify the process where moist air is cooled below its dew point temperature resulting in both temperature and specific humidity decrease.
C · C. Cooling and dehumidification
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Fourier law of heat conduction is best represented by
A · Q = -k A dt/dx
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Here are some assumptions that are made for Fourier law. Which of the following is correct?
C · Both a and b
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Regarding one dimensional heat transfer, choose the correct statement.
A · Steady – f(x), Unsteady – f(x, t)
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Which statement is true regarding steady state condition?
B · Heat exchange is constant
In steady-state conduction, temperature distribution does not change with time, so heat transfer rate remains constant. Internal energy and temperature profile are time-independent.[5]
PYQ · 2020
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Heat flow in unsteady-state heat conduction may be expressed in terms of which of the following two dimensionless numbers?
D · Biot and Fourier numbers
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The intensity of mixing of fluid in natural convection is _____ the intensity of mixing of fluid in forced convection.
B · b. less than
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In which of the following cases will the convective heat transfer coefficient be maximum?
1. At the top of the plate
2. At the bottom of the plate
3. At the leading edge of the plate
4. At the trailing edge of the plate
A · a. At the top of the plate
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Which of the following is the correct example of forced convection?
C · c. Air blown over a car radiator by a fan
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Which law states that the wavelength related with a maximum rate of emission depends upon the absolute temperature of the radiating surface?
B · Wien’s law
PYQ · 2022
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The NTU (Number of Transfer Units) effectiveness method for the analysis of heat exchanger is used when:
B · There is insufficient information to calculate the Log-Mean Temperature Difference (LMTD)
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What is the range of heat exchanger effectiveness (ε)?
B · 0 to 1
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For an infinitely long fin, the fin efficiency is given by which of the following expressions?
A · \( \frac{1}{ml} \)
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Increasing the length of a fin has which of the following effects on fin efficiency and fin effectiveness?
C · Efficiency decreases, effectiveness increases
PYQ · 2012
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Which of the following fin configurations would have the highest fin effectiveness?
B · Thin, widely spaced fins
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The working cycle in case of four stroke engine is completed in following number of revolutions of crankshaft
C · 2
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Number of working strokes per minute for a two-stroke cycle engine as compared to a four-stroke cycle engine of same speed is
C · more
PYQ · 2020
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A two-stroke cycle engine ____ as compared to a four-stroke cycle engine of the same size.
D · produces more power
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Mechanical efficiency of an engine can be expressed as _________. (Where, IHP = Indicated Horse Power, BHP = Brake Horse Power)
C · BHP/IHP
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Which of the following best states the Zeroth Law of Thermodynamics?
A · If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
The Zeroth Law states that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other, establishing the concept of temperature.
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Refer to the diagram below showing three systems A, B, and C in thermal contact.
Systems A and B are in thermal equilibrium, and systems B and C are in thermal equilibrium. What can be inferred about systems A and C?
B · Systems A and C are in thermal equilibrium.
According to the Zeroth Law, if A and B are in thermal equilibrium and B and C are in thermal equilibrium, then A and C must also be in thermal equilibrium.
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Which of the following is NOT a direct implication of the Zeroth Law of Thermodynamics?
C · Energy conservation in isolated systems.
Energy conservation is related to the First Law of Thermodynamics, not the Zeroth Law, which deals with thermal equilibrium and temperature.
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Refer to the temperature equilibrium diagram below showing three bodies X, Y, and Z.
If X is in thermal equilibrium with Y, and Y is in thermal equilibrium with Z, what is the temperature relationship among X, Y, and Z?
B · Temperature of X = Temperature of Z = Temperature of Y
According to the Zeroth Law, if X is in thermal equilibrium with Y and Y with Z, then all three have the same temperature.
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Which statement correctly expresses the First Law of Thermodynamics for a closed system?
B · The change in internal energy equals heat added minus work done by the system.
The First Law states that the change in internal energy of a closed system equals the heat added to the system minus the work done by the system.
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In the context of the First Law of Thermodynamics, which of the following represents the correct energy balance equation for a closed system?
B · \( \Delta U = Q - W \)
The First Law for a closed system is \( \Delta U = Q - W \), where \( \Delta U \) is the change in internal energy, \( Q \) is heat added to the system, and \( W \) is work done by the system.
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Refer to the energy flow schematic below of a closed system where heat \( Q = 500\,J \) is added and work \( W = 200\,J \) is done by the system.
What is the change in internal energy \( \Delta U \) of the system?
B · \( +300\,J \)
Using the First Law: \( \Delta U = Q - W = 500 - 200 = 300\,J \).
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Which of the following statements about the First Law of Thermodynamics is correct?
C · It is a statement of conservation of energy.
The First Law is essentially the conservation of energy principle, stating that energy cannot be created or destroyed, only transformed.
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A gas in a piston-cylinder device absorbs 1000 J of heat and does 600 J of work on the surroundings.
What is the change in internal energy of the gas?
B · \( +400\,J \)
Using \( \Delta U = Q - W = 1000 - 600 = 400\,J \).
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Which of the following best describes the Second Law of Thermodynamics?
B · Entropy of an isolated system never decreases.
The Second Law states that entropy of an isolated system always increases or remains constant, never decreases.
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Which of the following is NOT a statement of the Second Law of Thermodynamics?
C · Energy can be created from nothing.
Creation of energy from nothing violates the First Law; the Second Law deals with entropy and irreversibility.
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Refer to the entropy vs temperature graph below for a pure substance.
Which region represents the phase change where entropy increases sharply?
B · Phase change region (melting or boiling)
During phase change, entropy increases sharply due to increased molecular disorder.
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Which of the following statements is true regarding entropy?
C · Entropy measures the disorder or randomness of a system.
Entropy quantifies the degree of disorder or randomness in a system; it generally increases in spontaneous processes.
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A heat engine operates between reservoirs at temperatures \( T_H = 600\,K \) and \( T_C = 300\,K \).
What is the maximum theoretical efficiency of this engine according to the Second Law of Thermodynamics?
A · 50%
Maximum efficiency \( \eta = 1 - \frac{T_C}{T_H} = 1 - \frac{300}{600} = 0.5 = 50\% \).
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Refer to the entropy vs temperature graph below for an ideal gas undergoing an irreversible process.
Which curve correctly represents the entropy change during the process?
C · Entropy increases with temperature and process irreversibility.
In irreversible processes, entropy increases due to irreversibility and temperature rise.
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Which of the following statements correctly describes the Third Law of Thermodynamics?
B · Entropy of a perfect crystal approaches zero as temperature approaches absolute zero.
The Third Law states that the entropy of a perfect crystal approaches zero as temperature approaches absolute zero (0 K).
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Which of the following is a direct consequence of the Third Law of Thermodynamics?
A · It is impossible to reach absolute zero temperature.
The Third Law implies that absolute zero temperature cannot be reached by any finite number of processes.
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Refer to the entropy vs temperature graph below showing the entropy approaching a constant value as temperature approaches zero.
What does this graph illustrate according to the Third Law of Thermodynamics?
B · Entropy approaches zero for a perfect crystal at absolute zero.
The graph shows entropy approaching zero as temperature approaches absolute zero, consistent with the Third Law.
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Assertion (A): The entropy change of the universe is zero for any reversible process.
Reason (R): In a reversible process, the system and surroundings are always in thermodynamic equilibrium.
Choose the correct option:
A · A is true, R is true, and R is the correct explanation of A.
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Which of the following best defines entropy in thermodynamics?
B · A measure of the disorder or randomness in a system
Entropy is a thermodynamic property that quantifies the degree of disorder or randomness in a system.
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Entropy is a state function because:
A · Its value depends only on the initial and final states of the system
Entropy is a state function because its value depends only on the state of the system, not on the path taken to reach that state.
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Which of the following statements correctly describes the physical significance of entropy?
B · Entropy indicates the direction of spontaneous processes
Entropy indicates the direction of spontaneous processes; spontaneous processes increase the entropy of the universe.
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The Clausius inequality is mathematically expressed as:
A · \( \oint \frac{\delta Q}{T} \leq 0 \)
The Clausius inequality states that for any cyclic process, the cyclic integral of \( \frac{\delta Q}{T} \) is less than or equal to zero, with equality holding for reversible cycles.
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Refer to the diagram below showing a cyclic process on a temperature-entropy (T-S) diagram.
Which of the following statements about the Clausius inequality is correct for the process shown?
B · The cyclic integral \( \oint \frac{\delta Q}{T} < 0 \), indicating an irreversible cycle
For an irreversible cycle, the Clausius inequality states that \( \oint \frac{\delta Q}{T} < 0 \). The diagram shows an irreversible process with net entropy generation.
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Which of the following is the correct integral form of the Clausius inequality for any cyclic process?
C · \( \oint \frac{\delta Q}{T} \leq 0 \) with equality for reversible cycles
The Clausius inequality states that \( \oint \frac{\delta Q}{T} \leq 0 \) for any cyclic process, with equality holding only for reversible cycles.
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The Clausius inequality is a mathematical expression of which fundamental thermodynamic law?
C · Second law of thermodynamics
The Clausius inequality is a mathematical formulation of the second law of thermodynamics, expressing the irreversibility of natural processes.
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How does the Clausius inequality relate to the entropy change of the universe during an irreversible process?
C · Entropy change of the universe is positive
For irreversible processes, the Clausius inequality implies that the entropy of the universe increases (positive entropy generation).
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Refer to the process flow diagram below illustrating heat transfer between two reservoirs via a heat engine.
Which statement correctly applies the Clausius inequality to this system?
C · The cyclic integral \( \oint \frac{\delta Q}{T} < 0 \) indicating irreversibility
The Clausius inequality states that for irreversible engines, the cyclic integral of \( \frac{\delta Q}{T} \) is less than zero, indicating entropy generation.
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Which of the following correctly distinguishes reversible and irreversible processes in terms of entropy change?
B · Reversible processes have \( \Delta S_{universe} = 0 \), irreversible have \( \Delta S_{universe} > 0 \)
Reversible processes do not generate entropy, so \( \Delta S_{universe} = 0 \), while irreversible processes increase entropy, so \( \Delta S_{universe} > 0 \).
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Refer to the schematic below showing reversible and irreversible paths between states 1 and 2.
Which statement about entropy change \( \Delta S \) of the system is correct?
C · Entropy change is the same for both reversible and irreversible paths between the same states
Entropy is a state function; its change depends only on initial and final states, so \( \Delta S \) is the same for both reversible and irreversible paths.
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Which of the following statements is TRUE regarding entropy generation in irreversible processes?
C · Entropy generation is positive in irreversible processes
Irreversible processes generate entropy, so entropy generation is always positive.
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Refer to the entropy vs temperature graph below for a closed system undergoing heating and cooling.
Which area under the curve correctly represents the entropy change during heating from \( T_1 \) to \( T_2 \)?
A · Area under the curve between \( T_1 \) and \( T_2 \) on the heating path
Entropy change during heating corresponds to the area under the heating curve between \( T_1 \) and \( T_2 \) on the T-S diagram.
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Which of the following is a practical implication of the Clausius inequality in engineering systems?
B · It provides a criterion for the feasibility of thermodynamic processes
The Clausius inequality provides a criterion to determine whether a process is feasible or not based on entropy generation and irreversibility.
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In which of the following applications is the Clausius inequality most directly used?
B · Analysis of heat engine efficiency
The Clausius inequality is used to analyze the efficiency limits of heat engines and refrigerators by accounting for irreversibility and entropy generation.
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Which of the following best describes the physical meaning of entropy in a thermodynamic system?
B · A measure of the disorder or randomness in the system
Entropy is a measure of the disorder or randomness in a thermodynamic system, reflecting the number of microscopic configurations corresponding to a macroscopic state.
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Entropy can be considered as a state function because:
A · It depends only on the initial and final states of the system
Entropy is a state function because its change depends only on the initial and final states, not on the path taken.
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Which of the following expressions correctly represents the Clausius inequality for a cyclic process?
B · \( \oint \frac{\delta Q}{T} \leq 0 \)
The Clausius inequality states that for any cyclic process, the cyclic integral of \( \frac{\delta Q}{T} \) is less than or equal to zero, with equality holding for reversible cycles.
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Refer to the diagram below showing a thermodynamic cycle on a T-S diagram. Which area corresponds to the net heat transfer in the cycle according to Clausius inequality?
A · Area enclosed by the cycle curve
On a T-S diagram, the area enclosed by the cycle curve represents the net heat transfer during the cycle. Clausius inequality relates heat transfer and entropy changes in such cycles.
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Which statement correctly relates the Clausius inequality to the Second law of thermodynamics?
A · Clausius inequality is a mathematical expression of the Second law
The Clausius inequality is a mathematical formulation of the Second law of thermodynamics, expressing the irreversibility of real processes.
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Which of the following correctly describes the entropy change \( \Delta S \) for an irreversible process between two states?
B · \( \Delta S > \int \frac{\delta Q}{T} \) for any reversible path
For an irreversible process, the entropy change of the system is greater than the integral of \( \frac{\delta Q}{T} \) evaluated along any reversible path connecting the same states.
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Which of the following statements is true for entropy change in a reversible process?
B · Entropy change of the system equals the heat transfer divided by temperature
In a reversible process, the entropy change of the system is equal to the heat transfer divided by the temperature at which the transfer occurs.
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Refer to the process flow diagram below showing a system undergoing reversible and irreversible processes. Which process results in a greater entropy generation?
B · Irreversible process
Irreversible processes generate entropy due to dissipative effects, whereas reversible processes do not generate entropy internally.
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Which inequality correctly represents the entropy change in an irreversible process according to Clausius inequality?
C · \( \Delta S > \int \frac{\delta Q}{T} \)
For irreversible processes, the entropy change \( \Delta S \) is greater than the integral of \( \frac{\delta Q}{T} \) along the actual path, reflecting entropy generation.
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In the context of thermodynamic cycles, Clausius inequality implies that the efficiency of a real heat engine compared to a Carnot engine is:
C · Less than Carnot efficiency
Clausius inequality implies that real engines have irreversible losses, so their efficiency is always less than that of an ideal Carnot engine operating between the same temperatures.
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Refer to the P-V and T-S diagrams below for a Rankine cycle. Which statement about the Clausius inequality application is correct?
A · The cycle satisfies \( \oint \frac{\delta Q}{T} = 0 \) indicating reversibility
An ideal Rankine cycle is reversible, so the Clausius inequality holds as an equality with \( \oint \frac{\delta Q}{T} = 0 \).
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Which of the following best describes the use of Clausius inequality in calculating entropy change for a system undergoing an irreversible process?
B · Use a reversible path between the same initial and final states to calculate entropy change
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Calculate the minimum entropy generation in a heat engine operating between reservoirs at 600 K and 300 K if it absorbs 1200 J of heat from the hot reservoir irreversibly. Which option is correct?
B · 2 J/K
Entropy generation \( S_{gen} = \Delta S_{system} + \Delta S_{surroundings} \). For irreversible heat transfer, \( S_{gen} = \frac{Q}{T_c} - \frac{Q}{T_h} = 1200/300 - 1200/600 = 4 - 2 = 2 \) J/K.
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Refer to the entropy change graph below for a process between states 1 and 2. If the area under the curve represents heat transfer over temperature, what does the difference between \( \Delta S \) and this area indicate?
B · Entropy generation due to irreversibility
The difference between the actual entropy change and the integral of \( \frac{\delta Q}{T} \) represents entropy generated internally due to irreversibility.
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In applying Clausius inequality to an irreversible cycle, which of the following is true about the cyclic integral of \( \frac{\delta Q}{T} \)?
C · It is less than zero
For an irreversible cycle, the Clausius inequality states that \( \oint \frac{\delta Q}{T} < 0 \), indicating net entropy generation.
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A system undergoes a cyclic process where the integral of δQ/T over the cycle is measured as -0.02 kJ/K. Which of the following conclusions is correct?
A · The cycle is irreversible with positive entropy generation.
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A system undergoes a process where the heat transfer is 400 kJ at a boundary temperature of 350 K. The entropy change of the system is 1.3 kJ/K. What is the entropy generation and the nature of the process?
A · Entropy generation is 0.14 kJ/K; process is irreversible.
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A gas undergoes a process where the entropy change is zero, but the heat transfer is non-zero. Which of the following statements is TRUE?
B · The process is reversible and the heat transfer occurs isothermally.
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Which of the following best defines exergy in thermodynamics?
B · The maximum useful work obtainable from a system as it reaches equilibrium with the environment
Exergy represents the maximum useful work that can be extracted from a system as it comes into equilibrium with its surroundings, reflecting the quality or usefulness of energy.
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Exergy differs from energy because it accounts for:
B · The quality and usability of energy relative to the environment
Exergy measures not just the amount of energy but also its potential to do useful work considering the environment, unlike energy which is conserved regardless of quality.
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Refer to the diagram below showing a closed system undergoing a thermodynamic process. Which expression correctly represents the exergy change \( \Delta E_x \) of the system?
C · \( \Delta E_x = \Delta U - T_0 \Delta S \)
For a closed system, exergy change is given by \( \Delta E_x = \Delta U - T_0 \Delta S \), where \( U \) is internal energy, \( T_0 \) is ambient temperature, and \( S \) is entropy.
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In an open system with steady flow, the specific flow exergy \( e_x \) is given by:
A · \( e_x = (h - h_0) - T_0 (s - s_0) + \frac{V^2}{2} + gz \)
The specific flow exergy includes enthalpy difference, entropy difference multiplied by ambient temperature, and kinetic and potential energy terms.
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Which of the following statements about exergy destruction is correct?
B · Exergy destruction represents the irreversibility or lost work potential in a process
Exergy destruction quantifies the loss of useful work potential due to irreversibilities and entropy generation in real processes.
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In the exergy balance for a control volume, the term representing exergy destruction is related to:
C · The entropy generation multiplied by ambient temperature
Exergy destruction is calculated as \( T_0 \times S_{gen} \), where \( S_{gen} \) is entropy generation and \( T_0 \) is ambient temperature.
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Which of the following correctly distinguishes available energy from unavailable energy?
B · Available energy can be converted into useful work, unavailable energy cannot
Available energy (exergy) can be converted into useful work, whereas unavailable energy is the portion of energy that cannot be converted due to entropy and irreversibility.
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Which of the following pairs correctly matches the energy type with its exergy content?
A · Heat at ambient temperature - zero exergy
Heat at ambient temperature has zero exergy because it cannot produce useful work; work has 100% exergy; mass flow has exergy depending on its state; heat at high temperature has positive exergy.
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Refer to the exergy flow chart below of a thermal power plant. Which component is responsible for the largest exergy destruction?
A · Boiler
In thermal power plants, the boiler typically causes the largest exergy destruction due to high temperature gradients and combustion irreversibility.
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Which of the following formulas correctly calculates the exergy associated with heat transfer \( Q \) at temperature \( T \) to the environment at temperature \( T_0 \)?
A · \( E_x = Q \left(1 - \frac{T_0}{T} \right) \)
Exergy of heat transfer is the portion of heat that can be converted to work, given by \( Q (1 - \frac{T_0}{T}) \).
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The second law efficiency (exergy efficiency) of a device is defined as the ratio of:
A · Actual work output to the maximum possible work output
Second law efficiency measures how effectively a device converts available energy (exergy) into useful work compared to the ideal maximum.
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Refer to the thermodynamic process graph below showing a system's exergy change over time. Which region indicates maximum exergy destruction?
A · Region A where entropy increases sharply
Exergy destruction is directly related to entropy generation; a sharp increase in entropy corresponds to high irreversibility and exergy loss.
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Which of the following best defines exergy in thermodynamics?
B · The maximum useful work obtainable from a system as it reaches equilibrium with the environment
Exergy is defined as the maximum useful work that can be extracted from a system as it comes into equilibrium with its surroundings, considering the dead state conditions.
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Why is exergy considered a more useful measure than energy when evaluating system performance?
B · Because exergy accounts for irreversibilities and the quality of energy
Exergy measures the useful work potential and accounts for losses due to irreversibilities, unlike energy which is conserved but does not indicate quality or usability.
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Which statement correctly describes the significance of the dead state in exergy analysis?
B · It is the reference environment state where the system has zero exergy
The dead state represents the environmental reference condition where the system is in equilibrium with surroundings and thus has zero exergy.
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Refer to the diagram below showing a closed system undergoing a thermodynamic process. Which expression correctly represents the exergy change \( \Delta Ex \) of the system?
A · \( \Delta Ex = (U - U_0) + P_0(V - V_0) - T_0(S - S_0) \)
For a closed system, exergy change is given by \( \Delta Ex = (U - U_0) + P_0(V - V_0) - T_0(S - S_0) \), where subscript 0 denotes dead state properties.
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In an open system steady flow process, which of the following terms is NOT included in the exergy balance equation?
D · Exergy associated with gravitational potential energy
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Which of the following expressions correctly represents the exergy destruction \( Ex_d \) in a process?
A · \( Ex_d = T_0 \times \Delta S_{gen} \)
Exergy destruction is directly related to entropy generation and is given by \( Ex_d = T_0 \times \Delta S_{gen} \), where \( T_0 \) is the environment temperature.
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Refer to the exergy flow schematic below for a heat exchanger. Which statement correctly identifies the primary cause of exergy destruction in the heat exchanger?
A · Heat transfer across a finite temperature difference
Exergy destruction in heat exchangers mainly arises due to heat transfer across finite temperature differences, causing irreversibility.
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Which of the following defines exergy efficiency (also called second-law efficiency) of a device?
C · Ratio of useful exergy output to exergy input
Exergy efficiency is defined as the ratio of useful exergy output to exergy input, reflecting how well a device converts available energy into useful work.
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A gas turbine has an exergy efficiency of 40%. Which of the following statements is correct regarding its performance?
B · 40% of the input exergy is destroyed due to irreversibilities
An exergy efficiency of 40% means 60% of the input exergy is destroyed due to irreversibilities and losses in the turbine.
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Refer to the T-S diagram below for a reversible and an actual expansion process in a turbine. Which area represents the exergy destruction?
B · Area between the reversible and actual process curves
Exergy destruction corresponds to the irreversibility in the process, represented by the area between the reversible and actual process curves on the T-S diagram.
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Which of the following best describes thermodynamic availability?
B · The maximum useful work obtainable from a system relative to the environment
Thermodynamic availability is the maximum useful work obtainable from a system as it comes into equilibrium with the environment, synonymous with exergy.
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In exergy analysis, the dead state is assumed to have which of the following properties?
B · Same temperature and pressure as the environment
The dead state is defined as the environmental reference state with which the system exchanges no net energy or matter, having the same temperature and pressure as the surroundings.
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Which of the following correctly describes the main process sequence in an ideal Otto cycle?
A · Isentropic compression, constant volume heat addition, isentropic expansion, constant volume heat rejection
The ideal Otto cycle consists of two isentropic processes (compression and expansion) and two constant volume processes (heat addition and heat rejection).
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Refer to the diagram below showing the P-V diagram of an Otto cycle. Which process corresponds to the highest pressure in the cycle?
B · Constant volume heat addition
In the Otto cycle P-V diagram, the highest pressure occurs at the end of the constant volume heat addition process (point 3), where the volume is minimum and temperature is highest.
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For an Otto cycle with compression ratio \( r = \frac{V_1}{V_2} \) and specific heat ratio \( \gamma \), the thermal efficiency \( \eta \) is given by which of the following expressions?
A · \( 1 - \frac{1}{r^{\gamma -1}} \)
The thermal efficiency of an ideal Otto cycle is \( \eta = 1 - \frac{1}{r^{\gamma -1}} \), where \( r \) is the compression ratio and \( \gamma \) is the ratio of specific heats.
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Which of the following statements correctly describes the main difference between the Diesel cycle and the Otto cycle?
B · Diesel cycle has constant pressure heat addition, Otto cycle has constant volume heat addition
The Diesel cycle adds heat at constant pressure (isobaric), while the Otto cycle adds heat at constant volume (isochoric).
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Refer to the T-S diagram below of a Diesel cycle. Which process corresponds to the constant pressure heat addition?
A · Process 2-3
In the Diesel cycle T-S diagram, process 2-3 represents the constant pressure heat addition where entropy increases at constant pressure.
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The thermal efficiency of a Diesel cycle depends on which of the following parameters?
A · Compression ratio and cutoff ratio
The Diesel cycle efficiency depends on the compression ratio \( r \) and the cutoff ratio \( \rho = \frac{V_3}{V_2} \), which is the volume ratio during heat addition at constant pressure.
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Which of the following best describes the Dual cycle (Mixed cycle)?
A · Heat addition occurs partly at constant volume and partly at constant pressure
The Dual cycle combines features of both Otto and Diesel cycles, with heat addition partly at constant volume and partly at constant pressure.
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Refer to the schematic diagram below of a Dual cycle. Which process represents the constant volume heat addition phase?
A · Process 2-3
In the Dual cycle schematic, process 2-3 is the constant volume heat addition phase before the constant pressure heat addition (3-4).
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Which of the following is a key characteristic of the Brayton cycle used in gas turbines?
A · Heat addition occurs at constant pressure
The Brayton cycle involves heat addition and rejection at constant pressure, with isentropic compression and expansion.
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Refer to the P-V diagram below of an ideal Brayton cycle. Which process represents the isentropic expansion in the turbine?
A · Process 3-4
In the Brayton cycle P-V diagram, process 3-4 corresponds to the isentropic expansion in the turbine where volume increases and pressure decreases.
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The efficiency of an ideal Brayton cycle can be improved by which of the following methods?
A · Increasing the pressure ratio across the compressor
Increasing the pressure ratio across the compressor increases the Brayton cycle efficiency by increasing the work output and reducing specific fuel consumption.
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Which of the following statements best describes the Stirling cycle?
A · It is a closed regenerative cycle with isothermal expansion and compression processes
The Stirling cycle is a closed regenerative cycle consisting of two isothermal processes (expansion and compression) and two constant volume regenerative heat transfer processes.
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Refer to the T-S diagram below of a Stirling cycle. Which processes correspond to the isothermal heat addition and heat rejection respectively?
A · Process 1-2 is heat addition, Process 3-4 is heat rejection
In the Stirling cycle T-S diagram, processes 1-2 and 3-4 are isothermal heat addition and heat rejection respectively.
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Which of the following correctly describes the main components of a basic Rankine cycle?
A · Boiler, turbine, condenser, and pump
The basic Rankine cycle consists of a boiler (to generate steam), turbine (to extract work), condenser (to condense steam), and pump (to pressurize the liquid).
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In the Rankine cycle, the pump is used to:
A · Increase the pressure of the liquid water before entering the boiler
The pump increases the pressure of the liquid water before it enters the boiler, preparing it for heat addition.
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Which of the following statements about the condenser in a Rankine cycle is TRUE?
B · It converts saturated steam into saturated liquid
The condenser converts saturated or wet steam exiting the turbine into saturated liquid by removing latent heat at constant pressure.
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Refer to the T-s diagram below of a basic Rankine cycle. Which process corresponds to the isentropic expansion in the turbine?
A · Process 3-4
In the T-s diagram, the isentropic expansion in the turbine is represented by process 3-4, where entropy remains constant and temperature drops.
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Which parameter increase generally leads to an improvement in the thermal efficiency of the basic Rankine cycle?
A · Increasing boiler pressure
Increasing the boiler pressure raises the average temperature at which heat is added, improving thermal efficiency.
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Which of the following is a limitation of the basic Rankine cycle that reheat cycles aim to overcome?
A · Low turbine outlet temperature causing moisture formation
The basic Rankine cycle often results in wet steam at turbine outlet, which can damage blades; reheat cycles reheat steam to reduce moisture content.
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Calculate the thermal efficiency of a basic Rankine cycle if the net work output is 100 MW and the heat input is 400 MW.
A · 25%
Thermal efficiency \( \eta = \frac{W_{net}}{Q_{in}} = \frac{100}{400} = 0.25 = 25\% \).
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Which of the following best describes the purpose of the reheat process in a Rankine cycle?
A · To increase the average temperature of heat addition and reduce moisture content at turbine exit
Reheating steam after partial expansion increases average heat addition temperature and reduces moisture at turbine outlet, improving efficiency and turbine life.
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In a reheat Rankine cycle, steam is expanded in the turbine in two stages with reheating in between. Which of the following is an expected effect of this modification?
A · Increase in average temperature of heat addition and reduction in moisture content at turbine exhaust
Reheat increases the average temperature of heat addition and reduces moisture content at turbine exhaust, improving efficiency and turbine blade life.
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Refer to the schematic diagram below of a reheat Rankine cycle. Which component is responsible for reheating the steam between turbine stages?
A · Reheater
The reheater heats the steam after partial expansion in the turbine before it enters the next turbine stage.
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Which of the following is a disadvantage of the reheat Rankine cycle compared to the basic Rankine cycle?
A · Increased complexity and higher capital cost
Reheat cycles require additional components like reheaters and multiple turbine stages, increasing complexity and cost.
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In a regenerative Rankine cycle, feedwater heaters are used primarily to:
A · Increase the temperature of the feedwater before entering the boiler using steam extracted from the turbine
Feedwater heaters use steam extracted from the turbine to preheat the feedwater, improving cycle efficiency by reducing fuel consumption.
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Which of the following best describes the effect of regenerative feedwater heating on the Rankine cycle efficiency?
A · It increases efficiency by raising the average temperature of heat addition
Regenerative feedwater heating increases the average temperature at which heat is added, thus improving thermal efficiency.
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Refer to the schematic diagram below of a regenerative Rankine cycle with one open feedwater heater. Which component represents the open feedwater heater?
A · Mixing chamber where extracted steam mixes with feedwater
An open feedwater heater is a mixing chamber where extracted steam from the turbine mixes directly with feedwater to raise its temperature.
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Which of the following is an advantage of regenerative Rankine cycles over basic Rankine cycles?
A · Improved thermal efficiency due to preheating of feedwater
Regenerative cycles improve thermal efficiency by preheating feedwater, reducing fuel consumption and increasing average heat addition temperature.
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Which of the following thermodynamic parameters is most critical in determining the thermal efficiency of a Rankine cycle?
A · Average temperature of heat addition
The average temperature at which heat is added to the cycle strongly influences thermal efficiency according to thermodynamic principles.
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For a Rankine cycle operating between boiler pressure \( P_b \) and condenser pressure \( P_c \), increasing \( P_b \) while keeping \( P_c \) constant generally results in:
A · Higher thermal efficiency and higher turbine work output
Increasing boiler pressure raises the average heat addition temperature, improving efficiency and turbine work output.
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Refer to the efficiency comparison graph below between basic, reheat, and regenerative Rankine cycles. Which cycle shows the highest thermal efficiency at the same boiler pressure?
A · Regenerative Rankine cycle
The regenerative Rankine cycle typically has the highest thermal efficiency due to feedwater heating increasing average heat addition temperature.
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Which of the following practical considerations can limit the maximum boiler pressure in a Rankine cycle?
A · Material strength and safety limitations
Material strength and safety concerns limit the maximum boiler pressure to prevent failure and ensure safe operation.
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Which of the following improvements can reduce moisture content at the turbine exhaust in a Rankine cycle?
A · Using reheat and increasing turbine inlet temperature
Reheating steam and increasing turbine inlet temperature reduce moisture content at turbine exhaust, protecting turbine blades.
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Which of the following is a disadvantage of regenerative Rankine cycles in practical applications?
A · Increased cycle complexity and higher initial cost
Regenerative cycles require additional feedwater heaters and extraction lines, increasing complexity and capital cost.
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Which of the following best defines convection heat transfer?
B · Heat transfer due to bulk fluid motion combined with conduction within the fluid
Convection heat transfer involves heat transfer by the combined effect of conduction within the fluid and bulk fluid motion that carries energy.
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In convection heat transfer, the term 'boundary layer' refers to:
A · The thin region near the solid surface where fluid velocity changes from zero to free stream value
The boundary layer is the thin fluid region adjacent to the surface where velocity changes from zero (due to no-slip condition) to the free stream velocity.
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Which of the following equations represents the convective heat transfer rate \( Q \) from a surface?
B · \( Q = hA (T_s - T_\infty) \)
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Refer to the diagram below showing velocity and temperature profiles in forced convection over a flat plate. Which region corresponds to the thermal boundary layer thickness?
A · Region where velocity reaches free stream value but temperature is still changing
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Which of the following best describes the driving mechanism in natural convection?
B · Buoyancy forces due to density differences caused by temperature gradients
Natural convection is driven by buoyancy forces arising from density differences in the fluid caused by temperature gradients.
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Refer to the schematic diagram below showing forced and natural convection setups. Which setup correctly represents natural convection?
B · Fluid motion caused by temperature-induced density differences near a vertical heated surface
Natural convection occurs due to buoyancy forces from temperature-induced density differences near heated surfaces, without external mechanical forcing.
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The Grashof number \( Gr \) in natural convection is a dimensionless number that represents the ratio of:
B · Buoyancy forces to viscous forces
The Grashof number \( Gr = \frac{g \beta (T_s - T_\infty) L^3}{ u^2} \) represents the ratio of buoyancy to viscous forces in natural convection flows.
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Which empirical correlation is commonly used to estimate the average Nusselt number for laminar forced convection over a flat plate with constant surface temperature?
C · \( Nu_x = 0.664 Re_x^{1/2} Pr^{1/3} \)
For laminar flow over a flat plate with constant surface temperature, the local Nusselt number is given by \( Nu_x = 0.664 Re_x^{1/2} Pr^{1/3} \).
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According to the Stefan-Boltzmann law, the total energy radiated per unit surface area of a blackbody per unit time \( E \) is proportional to which power of its absolute temperature \( T \)?
C · C. \( T^4 \)
Stefan-Boltzmann law states that \( E = \sigma T^4 \), where \( \sigma \) is the Stefan-Boltzmann constant. Thus, energy radiated is proportional to the fourth power of absolute temperature.
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A blackbody at temperature \( 6000\,K \) radiates energy. Using the Stefan-Boltzmann law, calculate the energy radiated per unit area if \( \sigma = 5.67 \times 10^{-8} \ \mathrm{W/m^2K^4} \).
A · A. \( 7.35 \times 10^{7} \ \mathrm{W/m^2} \)
Using \( E = \sigma T^4 = 5.67 \times 10^{-8} \times (6000)^4 = 7.35 \times 10^{7} \ \mathrm{W/m^2} \).
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Wien's displacement law relates the wavelength \( \lambda_{max} \) at which the emission of a blackbody is maximum to its absolute temperature \( T \). What is the correct expression for this relationship?
B · B. \( \lambda_{max} = \frac{b}{T} \)
Wien's displacement law states \( \lambda_{max} T = b \), where \( b \) is Wien's constant, so \( \lambda_{max} = \frac{b}{T} \).
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A blackbody has a peak wavelength of \( 500 \ \mathrm{nm} \). Using Wien's displacement law with \( b = 2.898 \times 10^{-3} \ \mathrm{m \cdot K} \), calculate its temperature.
A · A. \( 5796 \ K \)
Using \( T = \frac{b}{\lambda_{max}} = \frac{2.898 \times 10^{-3}}{500 \times 10^{-9}} = 5796 \ K \).
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Which of the following best describes Kirchhoff's law of thermal radiation?
A · A. Emissivity equals absorptivity at thermal equilibrium for all wavelengths and directions.
Kirchhoff's law states that for a body in thermal equilibrium, emissivity equals absorptivity at every wavelength and direction.
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Refer to the schematic diagram below showing a gray body exchanging radiation with surroundings. If the body has an absorptivity of 0.7, what is its emissivity according to Kirchhoff's law?
B · B. 0.7
Kirchhoff's law states emissivity equals absorptivity at thermal equilibrium, so emissivity = 0.7.
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Which of the following statements about blackbody radiation is TRUE?
B · B. A blackbody absorbs all incident radiation and emits radiation with maximum efficiency.
A blackbody is an ideal absorber and emitter of radiation, absorbing all incident radiation and emitting with maximum efficiency.
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Refer to the blackbody radiation curves shown in the diagram below for temperatures \( T_1 \) and \( T_2 > T_1 \). Which of the following is correct about the peak wavelength and total emissive power?
B · B. \( \lambda_{max, T_2} < \lambda_{max, T_1} \) and total emissive power at \( T_2 \) is greater than at \( T_1 \).
According to Wien's law, peak wavelength decreases with increasing temperature, and Stefan-Boltzmann law states total emissive power increases with temperature.
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A surface has an emissivity of 0.85 and absorptivity of 0.75. Which of the following statements is correct according to radiation heat transfer principles?
B · B. The surface is not in thermal equilibrium; emissivity should equal absorptivity.
Kirchhoff's law requires emissivity to equal absorptivity at thermal equilibrium, so the given values indicate the surface is not in equilibrium.
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In radiation heat transfer applications, which of the following factors does NOT affect the net radiative heat exchange between two surfaces?
C · C. Distance between surfaces
In radiation heat transfer, net exchange depends on emissivities, temperatures, and absorptivities; distance affects convection and conduction but not direct radiation exchange between surfaces.
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Which of the following is NOT a common type of heat exchanger?
D · Piston Heat Exchanger
Piston Heat Exchanger is not a recognized type of heat exchanger. The common types include shell and tube, plate, and condenser heat exchangers.
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In a counter-flow heat exchanger, the fluids flow in:
B · Opposite directions
In a counter-flow heat exchanger, the hot and cold fluids flow in opposite directions to maximize the temperature difference and heat transfer efficiency.
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Identify the heat exchanger configuration shown in the diagram below. Refer to the diagram.
C · Cross-flow heat exchanger
The diagram shows two fluids flowing perpendicular to each other, which is characteristic of a cross-flow heat exchanger.
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The Log Mean Temperature Difference (LMTD) is used to calculate:
B · The average temperature difference between hot and cold fluids in a heat exchanger
LMTD represents the logarithmic average of the temperature difference between the hot and cold fluids over the length of the heat exchanger.
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Refer to the temperature profile graph below for a counter-flow heat exchanger. What is the approximate LMTD value if \( \Delta T_1 = 60^\circ C \) and \( \Delta T_2 = 20^\circ C \)?
C · 45\(^\circ\)C
LMTD is calculated by \( \frac{\Delta T_1 - \Delta T_2}{\ln \left( \frac{\Delta T_1}{\Delta T_2} \right)} = \frac{60 - 20}{\ln(60/20)} = \frac{40}{\ln 3} \approx 45^\circ C \).
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Which of the following expressions correctly defines LMTD for a heat exchanger with temperature differences \( \Delta T_1 \) and \( \Delta T_2 \)?
C · \( \frac{\Delta T_1 - \Delta T_2}{\ln \left( \frac{\Delta T_1}{\Delta T_2} \right)} \)
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The Number of Transfer Units (NTU) is defined as:
B · The product of overall heat transfer coefficient and heat exchanger area divided by minimum heat capacity rate
NTU is defined as \( NTU = \frac{UA}{C_{min}} \), where \( U \) is overall heat transfer coefficient, \( A \) is heat transfer area, and \( C_{min} \) is the minimum heat capacity rate.
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Refer to the NTU-effectiveness chart below for a counter-flow heat exchanger with heat capacity rate ratio \( C_r = 0.5 \). If the NTU is 2, what is the approximate effectiveness \( \varepsilon \)?
C · 0.85
From the NTU-effectiveness chart for \( C_r = 0.5 \), at NTU = 2, the effectiveness \( \varepsilon \) is approximately 0.85.
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For a heat exchanger with \( C_r = 1 \), the effectiveness \( \varepsilon \) is given by which of the following formulas?
D · \( \varepsilon = \frac{NTU}{1 + NTU} \) when \( C_r = 1 \)
When \( C_r = 1 \), the effectiveness simplifies to \( \varepsilon = \frac{NTU}{1 + NTU} \).
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The effectiveness \( \varepsilon \) of a heat exchanger is defined as:
A · The ratio of actual heat transfer rate to the maximum possible heat transfer rate
Effectiveness is the ratio of actual heat transfer to the maximum possible heat transfer, indicating the performance of the heat exchanger.
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Which of the following statements correctly describes the relationship between LMTD, NTU, and effectiveness \( \varepsilon \) for a heat exchanger?
B · Effectiveness can be calculated from NTU and heat capacity rate ratio without knowing LMTD
Effectiveness-NTU method allows calculation of heat exchanger performance using NTU and heat capacity rate ratio without requiring LMTD, which is used in a different approach.
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Which of the following is NOT a common type of heat exchanger configuration?
C · Rotary drum
Rotary drum is not a typical heat exchanger configuration; common types include shell and tube, plate, and double pipe heat exchangers.
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In a counterflow heat exchanger, the fluids flow in:
B · Opposite directions
In counterflow heat exchangers, the hot and cold fluids flow in opposite directions to maximize the temperature difference and heat transfer.
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Which of the following expressions correctly defines the Log Mean Temperature Difference (LMTD) for a heat exchanger?
C · \( LMTD = \frac{\Delta T_1 - \Delta T_2}{\ln\left(\frac{\Delta T_1}{\Delta T_2}\right)} \)
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Which of the following best describes the Number of Transfer Units (NTU) in heat exchanger analysis?
C · Dimensionless parameter representing heat exchanger size and heat transfer coefficient
NTU is a dimensionless parameter defined as \( NTU = \frac{UA}{C_{min}} \), representing the size and heat transfer capability of the heat exchanger relative to the fluid heat capacity.
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A heat exchanger has an effectiveness \( \varepsilon = 0.7 \) and a heat capacity rate ratio \( C_r = 0.3 \). Which of the following statements is TRUE about the heat exchanger's performance?
A · It transfers 70% of the maximum possible heat between fluids
Effectiveness \( \varepsilon \) is defined as the ratio of actual heat transfer to the maximum possible heat transfer, so 0.7 means 70% of maximum heat transfer is achieved.
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Refer to the NTU-effectiveness curve below for a counterflow heat exchanger with \( C_r = 0.2 \). If the effectiveness is measured as 0.85, what is the approximate NTU value?
C · 3.5
From the NTU-effectiveness curve for \( C_r = 0.2 \), an effectiveness of 0.85 corresponds approximately to NTU = 3.5.
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Which of the following correctly describes the relationship between LMTD and NTU methods in heat exchanger analysis?
C · Both methods yield the same heat exchanger performance but use different input parameters
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Which of the following best describes the working principle of a Spark Ignition (SI) engine?
A · Fuel-air mixture is compressed and ignited by a spark plug
SI engines operate by compressing a fuel-air mixture and igniting it with a spark plug, unlike CI engines which rely on compression ignition.
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In an SI engine, what is the primary reason for using a throttle valve in the intake manifold?
B · To regulate the engine speed by controlling air intake
The throttle valve controls the amount of air entering the engine, thereby regulating engine speed and power output.
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Which of the following statements correctly describes the working principle of a Compression Ignition (CI) engine?
B · Fuel is injected into highly compressed hot air and ignites spontaneously
In CI engines, air is compressed to a high temperature and pressure, and fuel is injected into this hot air, causing spontaneous ignition.
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Why do CI engines generally have higher thermal efficiency than SI engines?
A · Because of higher compression ratios and leaner air-fuel mixtures
CI engines operate at higher compression ratios and use lean mixtures, which improves thermal efficiency compared to SI engines.
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Refer to the diagram below showing the two-stroke engine cycle. Which process occurs during the upward stroke of the piston?
A · Compression of the fuel-air mixture and exhaust of burnt gases
During the upward stroke in a two-stroke engine, the piston compresses the fuel-air mixture while simultaneously pushing out the burnt gases through the exhaust port.
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In a two-stroke engine, which of the following is a major disadvantage compared to a four-stroke engine?
C · Higher fuel consumption and emissions due to scavenging losses
Two-stroke engines suffer from scavenging losses where some fresh charge escapes with exhaust gases, leading to higher fuel consumption and emissions.
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Refer to the diagram below of a four-stroke engine cycle. Which stroke is represented by the piston moving downward after the combustion event?
C · Power stroke
The downward movement of the piston immediately after combustion is the power stroke, where the expanding gases push the piston down.
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Which of the following correctly sequences the strokes in a four-stroke SI engine cycle?
A · Intake, Compression, Power, Exhaust
The four-stroke cycle consists of intake, compression, power, and exhaust strokes in that order.
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Which of the following is a fundamental difference between SI and CI engines as shown in the comparison table below?
A · SI engines use spark ignition; CI engines use compression ignition
SI engines rely on spark plugs for ignition, whereas CI engines ignite fuel by compression without spark plugs.
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Which of the following is a significant advantage of a four-stroke engine over a two-stroke engine?
C · Better fuel efficiency and lower emissions
Four-stroke engines have better fuel efficiency and produce lower emissions due to more complete combustion and separate exhaust strokes.
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Refer to the p-v diagram below for an ideal SI engine cycle. Which process corresponds to the combustion phase?
A · Constant volume heat addition
In the ideal Otto cycle (SI engine), combustion is modeled as a constant volume heat addition process.
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Which performance parameter is most directly improved by increasing the compression ratio in an IC engine?
A · Brake thermal efficiency
Increasing compression ratio improves the thermal efficiency of the engine, thus increasing brake thermal efficiency.
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What is the definition of thermal efficiency for a heat engine?
A · Ratio of work output to heat input
Thermal efficiency is defined as the ratio of the net work output of the engine to the heat input supplied to the engine.
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A heat engine absorbs 500 kJ of heat and produces 150 kJ of work. What is its thermal efficiency?
A · 30%
Thermal efficiency \( \eta = \frac{W_{out}}{Q_{in}} = \frac{150}{500} = 0.3 = 30\% \).
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Which of the following correctly defines the Coefficient of Performance (COP) of a refrigeration system?
B · Ratio of heat absorbed from cold reservoir to work input
COP for refrigeration is defined as the ratio of heat absorbed from the cold reservoir to the work input.
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Refer to the diagram below showing a refrigeration cycle. If the work input is 5 kW and the heat absorbed from the refrigerated space is 15 kW, what is the COP of the system?
A · 3.0
COP = \( \frac{Q_L}{W} = \frac{15}{5} = 3.0 \).
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A refrigeration system has a COP of 4. If the work input is 3 kW, what is the rate of heat removal from the refrigerated space?
A · 12 kW
Heat removed \( Q_L = COP \times W = 4 \times 3 = 12 \) kW.
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Specific fuel consumption (SFC) is defined as:
A · Fuel consumed per unit power output per hour
SFC is the amount of fuel consumed to produce one unit of power output per hour.
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If an engine produces 100 kW power and consumes 20 kg of fuel per hour, what is its specific fuel consumption?
A · 0.2 kg/kWh
SFC = Fuel consumption / Power output = 20 kg/hr / 100 kW = 0.2 kg/kWh.
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Exergy efficiency is best described as:
B · Ratio of useful work output to exergy input
Exergy efficiency measures how effectively the available work potential (exergy) is utilized.
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Refer to the exergy flow diagram below for a thermal system. If the exergy input is 100 kW and exergy destruction is 30 kW, what is the exergy efficiency?
A · 70%
Exergy efficiency \( \eta_{ex} = \frac{Exergy\ output}{Exergy\ input} = \frac{100 - 30}{100} = 0.7 = 70\% \).
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The effectiveness of a heat exchanger is defined as the ratio of:
A · Actual heat transfer to maximum possible heat transfer
Effectiveness is the ratio of actual heat transfer to the maximum possible heat transfer between the fluids.
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Which of the following is NOT a common performance parameter of refrigeration and heat pump systems?
B · Thermal efficiency
Thermal efficiency is typically used for heat engines, not refrigeration or heat pump systems.
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Refer to the schematic of a vapor compression refrigeration cycle below. If the refrigeration capacity is 10 kW and the work input is 2.5 kW, what is the COP of the heat pump mode?
A · 5.0
COP of heat pump \( = \frac{Q_H}{W} = \frac{Q_L + W}{W} = \frac{10 + 2.5}{2.5} = 5.0 \).
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Which of the following statements is true regarding the performance parameters of heat pumps?
B · COP of heat pump is the ratio of heat delivered to the hot reservoir to the work input
COP of heat pump is defined as the ratio of heat delivered to the hot reservoir to the work input.
