MET 220 Exam 2 Book HW Questions

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

A steam power plant operating on the simple ideal Rankine cycle is considered. The total net work produced by the cycle is 700 kJ/kg, and the amount of heat input to the cycle is 3000 kJ/kg. Determine the thermal efficiency of this cycle.

0.23

A steam power plant operating on the simple ideal Rankine cycle is considered. The heat input into the cycle is 2728 kJ/kg, and the heat output is 2018 kJ/kg. Determine the thermal efficiency of this cycle.

0.26

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

Which of the following devices are used in an ideal Rankine cycle?

A boiler A condenser A pump A turbine

A solar-thermal heat engine operates between the temperatures of 27oC and 77oC. Determine the theoretical maximum efficiency of this engine.

14%

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 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

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

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%

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

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

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

Determine the theoretical maximum efficiency for a heat engine operating between the temperatures of 1200 K and 300 K.

75%

Which of the following devices are considered as mixing chambers?

A tub with inlets for both hot and cold water and one outlet A Y-elbow A T-elbow

is the production of more than one useful form of energy from the same energy source.

Cogeneration

Boiler

Constant-pressure heat addition

Condenser

Constant-pressure heat rejection

If the operating pressure of the condenser is increased for a Rankine cycle, how will it affect the cycle's thermal efficiency?

Decrease

During a throttling process, if the flow energy increases due to significant increase of the specific volume, the temperature of a fluid will ______.

Decreases

What does the COP of a refrigerator represent?

Effectiveness

In a typical cogeneration power plant, what is being cogenerated beside heat?

Electricity

Which of the following statements are true?

Entropy generation can be used as a quantitative measure of irreversibilities. The performance of engineering systems is degraded by the presence of irreversibilities. Entropy is a thermodynamic property.

True or false: A heat pump supplies 10 kJ of heat to a house while consuming 3 kJ of net work. Does the heat pump violate the first law of thermodynamics.

False

True or false: A process in which the system can be returned to its initial state is said to be reversible.

False

True or false: Entropy per unit mass, s (kJ/kg⋅K), is an extensive property.

False

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

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

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

Which of the following statements are true?

Fluid friction causes pressure drops in the boiler, the condenser, and the piping between components, and will affect the cycle's thermal efficiency. Two common sources of irreversibilities in actual Rankine cycles are fluid friction and heat loss to the surroundings. The isentropic efficiency for turbine can be defined as actual turbine work divided by isentropic turbine work done.

Which of the following are common sources of irreversibilities for an actual vapor power cycle?

Heat loss Fluid friction

From the following list, select all of the devices to which the Clausius statement of the second law directly applies.

Heat pump Air conditioner

For turbines, which of the following are usually neglected in the energy balance equation? (Select all that apply.)

Heat transfer Potential energy

Which of the following statements are true regarding isentropic processes?

If a system undergoes an isentropic process, its entropy does not change. A system undergoing a reversible adiabatic process is necessarily isentropic. A substance will have the same entropy value at the beginning and end of a process if it underwent an isentropic process.

If the operating pressure of the boiler is increased for a Rankine cycle, how will it affect the cycle's thermal efficiency? Multiple choice question. Increase

Increase

Which of the following ways are often used to increase the efficiency of the Rankine cycle?

Increase the boiler pressure Heat the steam to a higher temperature Decrease the condenser pressure

Pump

Increases pressure for liquids Isentropic compression

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

From the following processes, select the four processes that make up an ideal Rankine cycle.

Isentropic expansion Isentropic compression Constant-pressure heat addition Constant-pressure heat rejection

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 of the following energies are part of the total energy of a flowing fluid?

Kinetic energy Enthalpy Potential energy

For a compressor with an intercooler, which of the following energy changes are usually neglected in the energy balance equation?

Kinetic energy Potential energy

Which of the following statements are true?

Mass flow rate can be defined as the multiples of density, pipe average velocity, and cross section area normal to the velocity. Mass flow rate is defined as the amount of mass flowing through a cross section per unit time. A volume flow rate is defined as the volume of the fluid flowing through a cross-section per unit time.

Which of the following statements are true?

Nozzles and diffusers typically involve no heat transfer. Nozzles and diffusers typically involve no work. Any change in potential energy in nozzles and diffusers can be neglected.

Turbine

Processes Shaft Work Isentropic expansion

Which component sets the high pressure in an actual Rankine cycle?

Pump

From the following list, select the characteristics of a refrigerator. (Select all that apply.)

Reject heat to a high-temperature reservoir Receive heat from a low-temperature reservoir

During a steady-flow process, the fluid properties at an inlet or exit must ______.

Remain Constant

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

A reservoir that absorbs heat from a system is called a thermal energy

Sink

The following statements describe characteristics of entropy for pure substance. Which ones are true?

Specifying the value of entropy per unit mass, together with another intensive property, fixes the system's thermodynamic state. 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.

Which of the following statements are true regarding the following two equations: T ds = du + P dV T ds = dh - v dP

The entropy change during a process can be determined by integrating either of these equations between the initial and the final states. They are applicable for open or closed systems. They are known as the Gibbs equations.

Flow work can be expressed in terms of other properties.

True

Which of the following statements are true regarding throttling valves?

The flow through a throttle valve may be assumed to be adiabatic. For a single-stream, steady flow throttle valve, the inlet and outlet enthalpies can be approximated to be the same. Throttle valves are flow-restricting devices that cause a pressure drop in a fluid.

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 statement accurately defines the coefficient of performance of a refrigerator?

The heat removed from the refrigerated space divided by the work input

Which of the following best explains the difference 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.

Which of the following statements are true regarding a regenerator?

The process of extracting steam from the turbine to heat the feedwater is called regeneration. A regenerator is used to help improve cycle efficiency. A regenerator provides a convenient means of deaerating the feedwater.

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. 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 steady flow process, boundary work is zero.

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

The is the fraction of heat input that is converted to net work output from a heat engine.

Thermal efficiency

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

Select all of the following that are characteristic of all heat engines.

They reject heat to a low temperature sink. They receive heat from a high temperature source. They convert some heat to work.

Which one of the following statements best describes the objective of a heat engine?

Transform heat into work

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

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

Which of the following statements are true?

Unsteady flow systems may involve boundary work. Unsteady-flow or transient flow involves changes within the control volume with time, such as charging or discharging problems. Unlike steady flow processes, unsteady flow processes start and end over some finite time period instead of continuing indefinitely.

Which of the following statements are true regarding compressor work? (Select all that apply.)

Use of an intercooler helps in minimizing compressor work. To minimize compressor work, the irreversibility's need to be minimized as well. Reducing the work input to a compressor requires that the gas be cooled as it is compressed.

Entropy generation Sgen is always ______.

a positive quantity or zero

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 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

The COP of a heat pump is _____ _____ that of a refrigerator when operating at the same conditions.

greater than

The entropy of a system is related to the total number of possible microscopic states of the system. Therefore, if the molecular randomness of a system increases, the system's entropy will _____

increase.

On a T-s diagram, the area under the process curve represents the heat transfer for _____.

internally reversible processes

Entropy change in an internally reversible isothermal process is _______ to the temperature of the system.

inversely proportional

A device that transfers heat from a low-temperature medium to a high-temperature medium is called a _____.

refrigerator heat pump air conditioner

A body with a relatively large thermal energy capacity that can supply or absorb heat without changing temperature is called a thermal energy

reservoir

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

If properties are allowed to change within the control volume, the processes are called flow.

transient

The fluid is the substance to and from which heat is transferred in a heat engine or other cyclic device.

working

According to the Clausius inequality, for irreversible cycles, the value for the cyclic integral ∮δQ/T is always:

≤ 0