Thermo quiz 1

How do you increase the thermal efficiency of a Carnot heat engine? ---> increase hot temp or decrease cold temp for actual heat engines? -

decrease the sink temperature and increase the reservoir temperature

Describe an imaginary process that violates both the first and the second laws of thermodynamics.

device that operates on a single resvervoir

The net work done during a cycle is the difference between the the work done ___ & the work done ___ the system.

By, on

a device that increases the pressure of a fluid by slowing it down

Diffusers

What is the driving force for (a) heat transfer, (b) electric current, and (c) fluid flow?

(a) Temperature difference. (b) The electric potential difference (voltage). (c) The pressure difference.

Does a heat engine that has a thermal efficiency of 100 percent necessarily violate (a) the first law and (b) the second law of thermodynamics?

(b) the second law of thermodynamics

What is a thermal energy reservoir? Give some examples.

A body with a large thermal capacity that can supply or absorb finite amount of heat without changing temperature. i.e oceans, lakes, rivers

A diffuser is an adiabatic device that decreases the kinetic energy of the fluid by slowing it down. What happens to this lost kinetic energy?

A diffuser is a device that increases the pressure of a fluid by slowing it down. The lost kinetic energy is converted as internal energy.

Describe an imaginary process that satisfies the first law but violates the second law of thermodynamics.

A heat rejection with a value of zero; violates Kelvin Planck statement (2nd law).

What is the difference between saturated vapor and superheated vapor?

A vapour that is about to condense is called saturated vapor. And a vapor that is not about to condense is called superheated vapor.

What is the difference between a refrigerator and a heat pump?

Both the refrigerator and the heat pumps carry the heat from low temperature reservoir or the sink to the high temperature reservoir or source. The objective of a heat pump, however, is to maintain a heated space at a high temperature. A refrigerator absorbs the heat from the evaporator and throws it to the atmosphere which is at high temperature.

A theoretical reversible cycle consisting of isothermal and adiabatic processes

Carnot Cycle

1. Efficiency of an irreversible heat engine is always less than a reversible one. 2. Efficiencies of all reversible heat engines between the same reservoirs are the same

Carnot Principles

It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body. - a refrigerator cannot operate unless its compressor is riven by an external power source; electric motor

Clausius Statement

A can of soft drink at room temperature is put into the refrigerator so that it will cool. Would you model the can of soft drink as a closed system or as an open system?

Closed system is a system which neither exchanges energy nor mass with the surroundings. This system also does not experience any external force as well. Ans: closed system

Devices used for increasing the pressure

Compressor (fan, compressor, pump)

Net mass transfer to/from a control volume during Δt equals net change in total mass within the control volume.

Conservation of mass principle

- Source = a reservoir that supplies energy in the form of heat - Sink = reservoir that absorbs energy in the form of heat

Difference between source and sink

To determine the entropy change for an irreversible process between states 1 and 2, should the integral fr 1 to 2 dQ/T be performed along the actual process path or an imaginary reversible path?

Entropy change should be determined by carrying out this integration along some convenient imaginary internally reversible path between the specified states.

Describe an imaginary process that satisfies the second law but violates the first law of thermodynamics.

Engine that takes energy from higher temperature & rejects it lower energy reservoir at TL.

A property that quantifies the second-law effects and measures the level of molecular disorder or randomness in a system.

Entropy

When a system is adiabatic, what can be said about the entropy change of the substance in the system?

Entropy change never decreses.

A piston-cylinder device contains helium gas. During a reversible, isothermal process, the entropy of the helium will (never, sometimes, always) increase.

Entropy may increase sometimes this is because isothermal process can exchange heat with surroundings.

T/F Someone claims that the absolute pressure in a liquid of constant density doubles when the depth is doubled. Is this true?

False, The absolute pressure in a liquid of constant density does not doubles when the depth is doubled. It's the gauge pressure that gets doubled. Absolute pressure = actual pressure at a given position measured relative to absolute vacuum. Gage pressure = The difference between the absolute pressure and the local atmospheric pressure.

What is flow energy? Do fluids at rest possess any flow energy?

Flow work is the amount of work required to push the entire fluid into or out of the control volume across its boundaries is called flow energy. No, the fluid at rest does not posses any flow work.

What are the different mechanisms for transferring energy to or from a control volume?

For a control volume (open system) the energy can be transferred to or from a system in three forms: 1. Heat 2. work 3. mass flow

Why is it impossible for heat engines to have a thermal efficiency of 100 percent?

For a power plant to operate, the working fluid must exchange heat with the environment & furnace.

Device that converts heat to work, operates on a cycle, and involves a working fluid receive heat from high temp source > converts part of this heat to work > reject the remaining waste heat to low temp sink > cycle

Heat Engine

When is the flow through a control volume steady?

If the properties in a control volume involve no change with time at any fixed position

Is it true that water boils at higher temperature at higher pressure?

True, higher energy is required to separate the molecules that are closely packed.

What are the ordinary and absolute temperature scales in the SI and the English system?

In SI system:- Ordinary temperature - Celsius Absolute temperature - Kelvin English system:- Ordinary temperature - Fahrenheit Absolute temperature - Rankine (R)

States that the total entropy of an isolated system can never decrease over time and will either remain constant in idealized reversible processes or increase in actual processes.

Increase of Entropy Principle

What is the Kelvin-Planck expression of the second law of thermodynamics?

It is impossible for any system to operate on a cycle to receive energy heat from a single reservoir and produce a net amount of work.

Define mass and volume flow rates. How are they related to each other?

Mass flow rate is defined as mass per unit time. Volumetric flow rate is defined as the volume of fluid per unit time. Mass flow rate is directly proportional to the volumetric flow rate.

List the forms of energy that contribute to the internal energy of a system.

Microscopic forms of energy is related to the molecular structure of a system and the degree of molecular activity. Different forms of energy that contribute to the internal energy are sensible energy, latent energy, chemical energy, and nuclear energy.

A piston-cylinder device contains nitrogen gas. During a reversible, adiabatic process, the entropy of the nitrogen will (never, sometimes, always) increase

Never, a reversible adiabatic process the entropy will never increase.

Is an isothermal process necessarily internally reversible? Explain your answer with an example.

No, An isothermal process need not be internally reversible. It can be irreversible also.

The entropy of a hot baked potato decreases as it cools. Is this a violation of the increase of entropy principle? Explain.

No, As the potato cools, the air surrounding gains heat and its entropy increase even more. This makes the total entropy change positive.

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, The inlet and exit flow rates are not equal so that the mass within the control volume will be increasing with time.

Why does a bicyclist pick up speed on a downhill road even when he is not pedaling? Does this violate the conservation of energy principle?

No, it does not violate the conservation of energy principle because one form of energy is converted to another form. But the total energy is conserved.

Is it possible to compress an ideal gas isothermally in an adiabatic piston-cylinder device?

No, it is not possible to compress air at constant temperature in an adiabatic piston cylinder process.

Are the efficiencies of all the work-producing devices, including the hydroelectric power plants, limited by the Kelvin-Planck statement of the second law

No, not limited to efficiencies of all devices that produce work. only holds for heat engines.

Does the cyclic integral of heat have to be zero (i.e., does a system have to reject as much heat as it receives to complete a cycle)?

No, the cyclic integral of work is not zero. For a cycle the system can produce less or more amount of work than it receives. The difference is the network output.

Consider a device with one inlet and one outlet. If the volume flow rates at the inlet and at the outlet are the same, is the flow through this device necessarily steady?

No, the flow through the control volume is not necessarily steady unless density of fluid remains constant. To achieve steady flow, the mass flow rate through the control volume must remain constant.

Can the condenser be taken out of the plant to save the wasted energy?

No, without a heat rejection process, the cycle cannot be completed ---> every heat engine must waste some energy by transferring it to a low temperature reservoir

a device that increases the velocity of a fluid at the expense of pressure* the cross sectional area of a nozzle decreases in the flow direction for subsonic flows and increases for supersonic- opposite for diffusers

Nozzle

What is quality? Does it have any meaning in the superheated vapor region?

Quality(x) is the ratio of mass of vapor to the total mass of the mixture. It has significance for saturated mixtures only.

How are the reduced pressure and reduced temperature defined?

Reduced pressure and temperature are the normalized pressure and temperature with respect to their critical pressure and temperature.

Describes the direction of heat flow and states that heat cannot spontaneously flow from a colder location to a hotter location.

Second Law of Thermodynamics

A piston-cylinder device contains superheated steam. During an actual adiabatic process, the entropy of the steam will (never, sometimes, always) increase.

The entropy always increases as actual processes are never reversible. They are always irreversible.

How do the energies of a flowing fluid and a fluid at rest compare? Name the specific forms of energy associated with each case.

The fluid at rest has total energy: internal, kinetic and potential energies. (3) Flowing fluid additionally has flow energy: internal, kinetic and potential & flow energies (4)

T/F: Temperature and pressure are dependent properties in the saturated mixture region. why if so

True, the phase change process (i.e., either boiling or condensation) occur at certain temperature only if the pressure is held constant

The kinetic energy of a fluid increases as it is accelerated in an adiabatic nozzle. Where does this energy come from?

The nozzle is a device that increases the velocity of the fluid at the expense of pressure. The internal energy of the fluid will be converted into kinetic energy.

Consider an alcohol and a mercury thermometer that read exactly 0°C at the ice point and 100°C at the steam point. The distance between the two points is divided into 100 equal parts in both thermometers. Do you think these thermometers will give exactly the same reading at a temperature of, say, 60°C?

The thermometers will give the same reading

An ideal gas at a given state expands to a fixed final volume first at constant pressure and then at constant temperature. For which case is the work done greater?

The work done is greater for constant pressure process.

Can there be any heat transfer between two bodies that are at the same temperature but at different pressures?

There will be no heat transfer.

How are heat, internal energy, and thermal energy related to each other

Thermal energy is generated and measured by heat of any kind. The sum of all the microscopic forms of energy is called the internal energy of a system

An experimentalist claims to have raised the temperature of a small amount of water to 150°C by transferring heat from high-pressure steam at 120°C. Is this a reasonable claim? Why? Assume no refrigerator or heat pump is used in the process.

Transferring of energy from low temperature to high temperature is not possible and this is not a reasonable claim

Work is entropy free, and sometimes the claim is made that work will not change the entropy of a fluid passing through an adiabatic steady-flow system with a single inlet and outlet. Is this a valid claim?

True, work will not change the entropy of a fluid passing through a reversible adiabatic steady flow system

The fluid flow at any inlet or exit is uniform and steady

Uniform flow process

Under what conditions is the ideal-gas assumption suitable for real gases?

When it is at higher temperatures or low pressures relative to its critical temperature and pressure.

Is a process that is internally reversible and adiabatic necessarily isentropic?

Yes Reason: The entropy of a fixed mass does not change during a process that is internally reversible and adiabatic.

Is a quantity whose cyclic integral is zero necessarily a property?

Yes, a quantity whose cyclic integral is zero depends on the state only and not the process path.

Will the temperature of air rise as it is compressed by an adiabatic compressor?

Yes, the temperature and pressure increases during an adiabatic compression process. Since energy is added to the working fluid which raises the temperature.

Consider the process of baking potatoes in a conventional oven. Can the hot air in the oven be treated as a thermal energy reservoir?

Yes, the temperature of the hot air remains constant within the oven.

The ____ of both the refrigerators and the heat pump decrease as TL (low temp reservoir)

coefficient of performance COP

Which is great cp or cv?

cp is always greater than cv

___ is related to the changes in internal energy and ___ to the changes in enthalpy.

cv, cp

The entropy of the working fluid of the ideal Carnot cycle (increases, decreases, remains the same) during the isothermal heat addition process

entropy will increase during the isothermal heat addition process.

compressed liquids are characterized by

higher pressures & lower temps, specific volumes, internal energies, enthalpies

How do the values of the integral 1 to 2 dQ/T compare for a reversible and irreversible process between the same end states

irrev > rev the entropy change of a closed system during an irreversible process is greater than the integral of dQ/T evaluated for that process.

At ___ pressures, all real gases approach ideal-gas behavior and therefore their specific heats depend on temperature only.

lows

What is mechanical energy? How does it differ from thermal energy? What are the forms of mechanical energy of a fluid stream?

mechanical energy the form that can be converted to mechanical work. Family forms: kinetic and potential

You are trying to understand how a reciprocating air compressor (a piston-cylinder device) works. What system would you use? What type of system is this?

no mass transfer, such type of system is called as closed system.

The boundary work done during a process depends on the ____ followed as well as the end ___

path, states

Is the energy required to heat air from 295 to 305 K the same as the energy required to heat it from 345 to 355 K? Assume the pressure remains constant in both cases.

then the heat required will be higher for 345 to 355 K?

What is a quasi-equilibrium process? What is its importance in engineering?

system remains infinitesimally close to an equilibrium state at all times

What is the principle of corresponding states?

the compressibility factor for all gases is approximately the same at the same reduced pressure and temperature.

Can entropy change of a system be negative?

the entropy change of a system during an internally reversible isothermal process can be positive or negative, depending on the direction of heat transfer. ! entropy generation must be positive !

What is total energy? Identify the different forms of energy that constitute the total energy.

the total energy of the system consists of the kinetic, potential, and internal energies

drive the electric generator in steam, gas, or hydroelectric power plants

turbine