ENGR 130 MIDTERM 2
From the following list, select the characteristics of a refrigerator. (Select all that apply.)
-Receive heat from a low-temperature reservoir -Reject heat to a high-temperature reservoir
A heat pump with a COP of 4.0 supplies heat to a building at a rate of 100 kW. Determine the power input to the heat pump.
25 kW Reason: The COP of a heat pump is the heat output divided by the work input. So the work input is the heat output divided by the COP, or 100 kW/4.0 = 25 kW.
If a nozzle has an isentropic efficiency of 90% and its isentropic velocity at nozzle exit is 1000 m/s, determine the actual nozzle velocity at the exit.
948.68 m/s
Mixing chamber
A device where two fluids exchange heat when mixed
In a single-stream, steady flow system, the mass flow rate can be defined as the product of ___________, __________, and ___________.
Density, velocity, and area
What does the COP of a refrigerator represent?
Effectiveness
For a closed system undergoing a reversible and adiabatic process, work is done to the system. Which of the following statements are true?
Entropy change of the system is zero. Exergy destruction is zero. Exergy change of the system is equal to total energy transfer.
For a steady flow open system undergoing a reversible and adiabatic process, work is done to the system. Which of the following statements are true?
Exergy destruction is zero. Exergy change of the system is equal to zero. Entropy change of the system is zero. Entropy generation is zero.
For a closed system undergoing a reversible process, no work is done but heat is transferred to the system. Which of the following statements are true?
Exergy destruction is zero. Exergy change of the system is zero. Exergy change of the system is equal to total energy transfer.
Which of the following statements reflects the concept of exergy? (Check that all apply.)
Exergy of a system depends on the condition of the environment. Exergy represents the upper limit on the amount of work a device can deliver. Exergy is a property of the system-environment combination.
Which of the following devices requires work input?
Fan Pump Compressor
A cup of hot coffee sits in a cool, well-insulated room. Heat transfer between the coffee and the air in the room causes the coffee to lose 1 kJ of energy while the air gains 2 kJ. Which law of thermodynamics was violated?
First law
From the following list, select all of the devices to which the Clausius statement of the second law directly applies.
Heat pump Air conditioner
Which one of the following irreversibilities describes the transfer of energy from a high-temperature body to a low-temperature body?
Heat transfer
Pump
Increases pressure for liquids
Compressor
Increases pressure of gases and vapors
Fan
Increases the pressure of a gas slightly and is mainly used to move a gas
Please select the most appropriate answer for the blank: Entropy change is defined only along the path of a(n) ___________ process path.
Internally reversible
∫21 P dV
Moving boundary work for closed systems.
For which one of the cases below is the exergy transfer not equal to the work itself?
Piston is pushed by heated air in a cylinder.
Which of the following variables influence the exergy change of a flow stream? (Select all that apply.)
Potential energy Enthalpy Kinetic energy Entropy
In a gas turbine, what is the source of exergy?
The gas flowing into the turbine
Which statement accurately defines the coefficient of performance of a heat pump?
The heat discharged to the heated space divided by the work input
Which one of the following statements is true?
The second-law efficiency is defined as the ratio of the actual thermal efficiency to the maximum possible thermal efficiency under the same conditions
For a pump, what is the source of exergy?
The shaft work input
Entropy generation Sgen is always ______.
a positive quantity or zero
During a throttling process, if the flow energy increases due to significant increase of the specific volume, the temperature of a fluid will ______.
decrease
True or false: The efficiency of an irreversible heat engine is always more than the efficiency of a reversible one operating between the same two reservoirs.
false
True or false: A process in which the system can be returned to its initial state is said to be reversible.
false Reason: For a process to be reversible, it must be possible to return both the system and the surroundings to their initial state.
In an open system, when mass flows across control volume boundaries, some work is required to push the mass into or out of the control volume, and this is known as ______ work.
flow
The COP of a heat pump is _____ _____ that of a refrigerator when operating at the same conditions.
greater than
For a reversible system, the entropy change of the system is equal to entropy transferred with _____.
heat
On a T-s diagram, the area under the process curve represents the heat transfer for _____.
internally reversible processes
Turbine
produces shaft work
The working fluid in a refrigerator is called the _____.
refrigerant
A refrigerant is the working fluid in a ________.
refrigerator
During a steady-flow process, the fluid properties at an inlet or exit must ______.
remain constant
In a steady flow process, the change of total energy of the control volume must ______.
remain zero
A reservoir that supplies heat to a system is called a _____.
source or heat source
The exergy balance relationship can be used to determine the reversible work when ________.
the exergy destruction is zero
If properties are allowed to change within the control volume, the processes are called ____ flow.
unsteady
In the energy balance equation for heat exchangers, which of the following are often neglected? (Select all that apply.)
Δpe w Δke Heat transfer with entire exchanger as the control volume
A refrigerator is designed to remove 3 kW from the cold space at -10oC while it rejects heat to the kitchen at 25oC. The rate of the reversible work is:
-0.4 kW
A closed system gives out 6 kJ of energy in the form of mechanical work from a spring. Determine the change of exergy in the system.
-6 kJ
From the following list, select the characteristics of all heat engines. (Check all that apply.)
-Reject heat to a low-temperature sink -Receive heat from a high-temperature source
A rigid tank containing an ideal gas undergoes a process where its temperature doubles. If its cv,avg is 0.7 kJ/kg-K, determine its entropy change using the constant-specific-heat assumption.
0.485 kJ/kg-K
Heat exchanger
A device where two fluids exchange heat without mixing
Often, lakes are used as sinks for the waste heat from power plants. Occasionally, this process raises the temperature of the lake significantly, affecting plant and animal life in the lake. This irresponsible management of waste energy is called ______ ______.
thermal pollution
The ______ law of thermodynamics states that the entropy of a pure crystalline substance at absolute zero temperature is zero since there is no uncertainty about the state of the molecules at that instant.
third
The total energy of a flowing fluid at rest consists of internal, kinetic, and potential energies.
true
True or false: A heat engine that takes in 100 kW of heat and produces 100 kW of net work output is a perpetual motion machine of the second kind.
true
Volume, temperature, and pressure are not conserved during a process.
true
true or false: the Clausius inequality is valid for all thermodynamic cycles.
true
Through which of the following forms can exergy be transferred from or to a system? (Check all that apply.)
work heat transfer mass flow
In a steady flow process, energy can be transferred by ______.
work, mass, and heat
If a pump has an isentropic efficiency of 0.9, and its isentropic work is 180 kJ. Determine its actual work.
200 kJ
Air is compressed in a car engine from 300 K and 100 kPa in a reversible and adiabatic manner. If the compression ratio v1/v2 of this engine is 10, determine the final temperature of the air using the variable-specific-heats assumption.
730 K
When is the reversible work the same as the isentropic work?
For a reversible, adiabatic process
Which one of the following statements accurately defines the thermal efficiency of a heat engine?
The net work output divided by the heat input
True or false: A Carnot refrigerator or a Carnot heat pump is a refrigerator or a heat pump that operates on the reversed Carnot cycle.
True
True or false: Flow work can be expressed in terms of other properties.
True
True or false: In a single-stream, steady flow system, the mass flow rates for the inlet and outlet must be the same.
True
True or false: The Gibbs equation implies that the only type of work a simple compressible system may have as it undergoes an internally reversible process is the boundary work.
True
Nozzle matches Choice, A device that increases the velocity of a fluid at the expense of pressure A device that increases the velocity of a fluid at the expense of pressure Diffuser matches Choice, A device that increases the pressure of a fluid by slowing it down A device that increases the pressure of a fluid by slowing it down
hi
Entropy change in an internally reversible isothermal process is _______ to the temperature of the system.
inversely proportional
a parameter that expresses quantitatively how efficiently an actual device approximates an idealized one, which is a measure of the deviation of actual processes from the corresponding idealized ones.
isentropic/adiabatic efficiency
which one of the following is the kelvin-planck statement of the second law?
it is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.
For the worst case, a heat pump will act as a resistance heater. In this case, the COP of the heat pump ___________.
may drop below unity
Does the amount of mass entering a control volume have to be equal to the amount of mass leaving during an unsteady-flow process?
no
A process in which both the system and the surroundings can be returned to their initial state at the end of the process is called _____.
reversible or reversible process
Waste heat from a power plant is rejected to the atmosphere. In this case, the atmosphere is considered a _____.
sink
A furnace during natural gas is used to supply heat to a steam power plant. The furnace is an example of a ______.
source
The _____ fluid is the substance to and from which heat is transferred in a heat engine or other cyclic device.
working
For a well-insulated mixing chamber, which terms in the energy balance equation can be left out?
Δke w Δpe q
Refrigerator
ηII = COP/COPrev
Work-consuming devices
ηII = Wrev/Wu
Work-producing devices
ηII = Wu/Wrev
Heat engines
ηII = ηth/ηth,rev
The Clausius inequality can be expressed as:
∮δQ/T ≤ 0
Which of the following statements are true?
-Entropy is a thermodynamic property. -The performance of engineering systems is degraded by the presence of irreversibilities. -Entropy generation can be used as a quantitative measure of irreversibilities.
Which of the following energies are part of the total energy of a flowing fluid?
Enthalpy Kinetic energy Potential energy
True or false: Entropy per unit mass, s (kJ/kg⋅K), is an extensive property.
False
Impossible process
Sgen < 0
A device that transfers heat from a low-temperature medium to a high-temperature medium is called a _____.
air conditioner heat pump refrigerator
According to the Clausius inequality, for irreversible cycles, the value for the cyclic integral ∮δQ/T is always:
≤ 0 Reason: According to the Clausius inequality, for irreversible cycles, the value for the cyclic integral ∮δQ/T is always less than or equal to zero.
A system is said to be in the dead state when it is in equilibrium with the environment it is in. Which of the following are features of the dead state? (Check all that apply.)
-A system does not interact with its environment. A system is at the same temperature as its environment. -A system is at the same pressure as its environment. A system has no kinetic or potential energy relative to the environment.
Which of the following statements are true regarding throttling valves?
-For a single-stream, steady flow throttle valve, the inlet and outlet enthalpies can be approximated to be the same. -The flow through a throttle valve may be assumed to be adiabatic. -Throttle valves are flow-restricting devices that cause a pressure drop in a fluid.
Which of the following statements are true? (Select all that apply.)
-For monatomic ideal gases, the constant-specific-heat assumption introduces no error. -When the specific heats are independent of temperature, the constant-specific-heat assumption produces exact answers. -The accuracy of using the constant-specific-heat assumption is usually sufficient if the temperature range is not greater than a few hundred degrees.
The following statements describe characteristics of entropy for pure substance. Which ones are true?
-Once the state of the system is fixed, the value of entropy per unit mass s is fixed. -For a closed system, the entropy change of a specified mass can be determined by this simple equation: ΔS = m(s2 - s1) -Entropy is a thermodynamic property. -Specifying the value of entropy per unit mass, together with another intensive property, fixes the system's thermodynamic state.
Which of the following statements are true? (Select all that apply.)
-The rate of change of mass within a control volume minus the net flow rate in and plus the net flow rate out of the control surface must equal to zero. -In a steady flow process, the total rate of mass entering a control volume is equal to the total rate of mass leaving it.
Which of the following statements are true regarding the following two equations: T ds = du + P dV T ds = dh - v dP
-They are applicable for open or closed systems. -They are known as the Gibbs equations. -The entropy change during a process can be determined by integrating either of these equations between the initial and the final states.
Which of the following statements are true regarding compressor work? (Select all that apply.)
-To minimize compressor work, the irreversibilities need to be minimized as well. -Reducing the work input to a compressor requires that the gas be cooled as it is compressed. -Use of an intercooler helps in minimizing compressor work.
Which of the following statements are true?
-Unlike steady flow processes, unsteady flow processes start and end over some finite time period instead of continuing indefinitely. -Unsteady-flow or transient flow involves changes within the control volume with time, such as charging or discharging problems. -Unsteady flow systems may involve boundary work.
Which of the following statements are true? (Select all that apply.)
-the steady-flow process is a process during which a fluid flows through a control volume and does not change with time-during a steady flow process, boundary work is zero-the steady flow implies that the fluid properties can change from point to point, but at any point, they remain constant during the entire process
During a cryogenic application, liquid methane undergoes an isentropic process at 110 K. Assuming that liquid methane is incompressible and its cavg is 3.48 kJ/kg⋅K. Determine temperature change of the methane during this process.
0 K
Air is flowing in a pipe with 0.2 m in diameter at an average velocity of 0.1 m/s. The density of air is 1.754 kg/m3. What is the mass flow rate?
0.0055 kg/s Reason: m⋅ = ρVavgAc, Ac = πD^2/4
If the actual work is 0.85 MJ for a compressor, and the isentropic compressor work is 0.8 MJ, what is the isentropic compressor efficiency?
0.94
A solar-thermal heat engine operates between the temperatures of 27oC and 77oC. Determine the theoretical maximum efficiency of this engine.
14%
During a one-second period, air is added into a rigid tank. The volume of the tank is 3 m3 and the initial density of air is 1.2 kg/m3; at the end of the charging process, the density of air reaches 6.3 kg/m3. What is the mass flow rate of air that is entering the tank?
15.3 kg/s Reason: m⋅in - m⋅out = dmcv /dt, m⋅out = 0, dmcv = mfinal - minitial = ρfinalVcv - ρinitialVcv
A piston-cylinder device contains a saturated mixture at 100°C. During an isobaric process, 600 kJ of heat is transferred to the surrounding air at 27°C. As a result, part of the water vapor in the cylinder condenses. Determine the entropy change of the surrounding.
2 kJ/K
A refrigerator accepts heat at a rate of 10 kW and it consumes power (work) at a rate of 5 kW. Determine the COP of the refrigerator.
2.00
A nozzle is considered to be a steady flow device where a single stream flows in and out. Steam with a density of 1.91 kg/m³ with an average velocity of 122 m/s flows into a nozzle. Its inlet area of the nozzle is 104 × 10^−4 m^2. Determine its mass flow rate at the nozzle outlet.
2.42 kg/s Reason: m∙in =m∙out =ρVavgA
A hydrokinetic turbine with a 3 m diameter rotor is attached to a bridge abutment. The river current is flowing steadily at 2 m/s. Assuming the density of water is 1000 kg/m3, what is the maximum power that can be generated by the hydrokinetic turbine?
28.27 kW
A heat pump is used to heat a house. It receives 10 kW of heat from the ambient air, and it accepts 5 kW of work. Determine the COP of the heat pump.
3.00 Reason: The COP of a heat pump is the heat output divided by the work input. The heat output is the sum of the heat and work inputs, or 10 kW + 5 kW = 15 kW. So, the COP is 15 kW/5 kW = 3.0.
A refrigerator with a COP of 3.0 removes heat from the refrigerated space at a rate of 10 kW. Determine the rate of power input.
3.33 kW Reason: The COP of a refrigerator is the heat input divided by the work input. So, the work input is the heat input divided by the COP, or 10 kW/3.0 = 3.33 kW.
A heat engine accepts heat from a source at a rate of 500 kW, and it rejects heat to a sink at a rate of 300 kW. The remainder of the heat is converted to net work output. Determine the thermal efficiency of the engine.
40% Reason: The thermal efficiency is the net work output divided by the heat input. The net work output is the heat input minus the heat output. So, the net work output is 500 kW - 300 kW = 200 kW and the thermal efficiency is 200 kW/500 kW = 40%.
A heat engine with a thermal efficiency of 50% produces 200 kJ of net work output. Determine the heat input.
400 kJ Reason: The thermal efficiency is the net work output divided by the heat input. So, the heat input is the net work output divided by the thermal efficiency, or 200 kJ/0.50 = 400 kJ.
The volumetric flow rate of water flowing through a pipe is 5 L/s, and the density of water is 1 kg/L. What is the mass flow rate?
5 kg/s Reason: Mass flow rate is equal to the product of density and volumetric flow rate.
A heat pump with a COP of 4.0 supplies heat to a building at a rate of 100 kW. Determine the heat input to the heat pump.
75 kW Reason: The COP of a heat pump is the heat output divided by the work input. So the work input is the heat output divided by the COP, or 100 kW/4.0 = 25 kW. The heat input is the heat output minus the work input, or 100 kW - 25 kW = 75 kW.
Determine the theoretical maximum efficiency for a heat engine operating between the temperatures of 1200 K and 300 K.
75% Reason: The Carnot cycle represents the theoretical maximum efficiency for a heat engine operating between two reservoirs. The Carnot efficiency is equal to 1 - TLTH or 1 - 300 K1200 K = 0.75 or 75%.
Air is compressed in a car engine from 300 K and 100 kPa in a reversible and adiabatic manner. If the compression ratio v1/v2 of this engine is 10, determine the final temperature of the air using the constant-specific-heats assumption. Assume k=1.4 for air.
754 K
Which of the following devices are considered as mixing chambers?
A T-elbow A Y-elbow A tub with inlets for both hot and cold water and one outlet
The process is called an isentropic process if the entropy of a fixed mass does not change during a process that is ______ and internally _______ .
Blank 1: adiabatic Blank 2: reversible
Exergy destroyed represents the lost work potential and is also called the ____ or lost work.
Blank 1: irreversibility
Entropy can be viewed as a measure of molecular _____. (Use one word to fill the blank.)
Blank 1: randomness or disorder
For a steady flow open system that undergoes a reversible process, no work is done but heat is transferred to the system. Which of the following statements are true?
Entropy change of the system is zero. Exergy destruction is zero. Entropy generation is zero. Exergy change of the system is equal to zero.
True or false: A heat pump supplies 10 kJ of heat to a house while consuming 3 kJ of net work. If QL = 0 this violates the first law of thermodynamics. True false question.
False
True or false: A reversible adiabatic process is necessarily isentropic. At the same time, an isentropic process is also necessarily reversible adiabatic.
False
True or false: A thermal energy reservoir must be very large, like oceans, lakes, rivers, or the atmosphere.
False Reason: Depending on the amount of heat supplied or absorbed, smaller systems may behave like thermal energy reservoirs. Additionally, two-phase systems are often considered thermal energy reservoirs.
Which one of the following irreversibilities describes the force that opposes the relative motion between two bodies in contact with one another?
Friction
For the following mechanisms, select those that are used to transfer entropy.
Heat transfer Mass flow
What statements are true regarding isentropic processes?
If a system undergoes an isentropic process, its entropy does not changeA substance will have the same entropy value at the beginning and end of a process if it underwent an isentropic processA system undergoing a reversible adiabatic process is necessarily isentropic.
Which one of the following is the Kelvin-Planck statement of the second law?
It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.
Which one of the following provides the Clausius statement of the second law?
It is impossible to construct a device that operates in a cycle and involves no interactions other than the transfer of heat from low to high temperature.
A heat engine that accepts heat from and rejects heat to the same thermal energy reservoir is an example of a device that violates which of the following?
Kelvin-Planck statement of the second law
Which system can do more useful work, 0.1 kg of CO2 at 225oC and 200 kPa or 0.1 kg of N2 at 225oC and 200 kPa?
N2 can do more useful work than CO2.
A cup of hot coffee sits in a cool, well-insulated room. Heat transfer between the coffee and air in the room causes the coffee to lose 1 kJ of energy while the air gains 1 kJ. Which law of thermodynamics was violated?
Neither
Which of the following statements are true?
Nozzles and diffusers typically involve no work. Nozzles and diffusers typically involve no heat transfer. Any change in potential energy in nozzles and diffusers can be neglected.
Rank the following bodies of water from the BEST (at the top) to the WORST examples of a thermal energy reservoir.
Ocean Large lake Small pond
What is a device that violates either law of thermodynamics called?
Perpetual motion machine
Which of the following forms of energy are 100% available?
Potential energy Kinetic energy
−∫21vdP
Reversible work relation for internally reversible steady-flow, single-stream systems
A cup of hot coffee sits in a cool, well-insulated room. Heat transfer between the coffee and air in the room causes the coffee to gain 1 kJ of energy while the air loses 1 kJ. Which law of thermodynamics was violated?
Second law
Reversible process
Sgen = 0
Irreversible process
Sgen > 0
Two closed systems A and B are at the same pressure as the environment. System A has the same temperature as the environment, and system B has a higher temperature than the environment. Which one of the following statements is correct?
System B has higher exergy than system A.
Two closed systems A and B are at the same pressure as the environment. The system A has the same temperature as the environment, and the system B has a lower temperature than the environment. Which of the following statements is correct?
System B has higher exergy than system A.
Heat transfer occurred on a system at temperature T. The environment is at temperature T0. Under what situation does the exergy of the system increase?
T > T0, and when the system is accompanied by heat transfer to the system.
Which statement accurately defines the coefficient of performance of a refrigerator?
The heat removed from the refrigerated space divided by the work input
For a given amount of energy at some specified state, which of followings can describe the concept of exergy?
The possible work we can extract The available energy The availability The useful work potential
A nuclear reactor is used to provide heat to a steam power plant. Within the heat engine, steam is generated in the boiler, the steam turns a turbine to produce power, and the steam is condensed by rejecting heat to the atmosphere before being pumped to the boiler again. Which substance is considered the working fluid in this heat engine?
The water going through the boiler, turbine, and condenser
Select all of the following that are characteristic of all heat engines.
They reject heat to a low temperature sink. They convert some heat to work. They receive heat from a high temperature source.
True or false: The entropy change of a system can be negative, but entropy generation cannot
True
True or false: The isentropic process of an incompressible substance is also an isothermal process.
True
True or false: The total energy for a flowing fluid consists of an additional term when compared with the total energy for a nonflowing fluid. True false question.
True
The increase of entropy principle implies that the entropy generation during any process cannot _____.
decrease
According to the decrease of exergy principle, the exergy of an isolated system during a process always ______.
decreases
For the worst case, a heat pump will act as a resistance heater. In this case, the COP of the heat pump is _____.
equal to 1
The highest coefficient of performance that a heat pump operating between two reservoirs can have is ______.
equal to that of a Carnot heat pump
The Carnot cycle is composed of four reversible processes. If another heat engine cycle composed of six reversible processes operated between the same two reservoirs, its thermal efficiency would be _____.
equal to that of the Carnot cycle
On a T-s diagram, the area under the process curve represents ______ for internally reversible processes.
heat transfer
which of the following best explians the diffrence between heat pumps and refrigerators
the objective of a heat pump is to keep a heated space warm, while the objective of a refrigerator is to keep a refrigerated space cool.
The ______ ______ is the fraction of heat input that is converted to net work output from a heat engine.
thermal efficiency
