APPLIED THERMODYNAMICSEL 325 (3+0)

ADIABATIC PROCESSES

If a process is carried out in a system such that thereis no heat transferred into or out of the system (i.e.Q=0) then the process is said to be adiabatic.

Liquefaction of gases is the process by which substances in theirgaseous state are converted to the liquid state. When pressure on a gas isincreased, its molecules closer together, and its temperature is reduced, whichremoves enough energy to make it change from the gaseous to the liquid state.

A combustion engine is an engine which generates mechanical power bycombustion of a fuel. Combustion engines are of two general types

External combustion engine(EC engine) is a heat engine where a working fluid,contained internally, is heated by combustion in anexternal source, through the engine wall or a heatexchanger. The fluid then, by expanding and actingon the mechanism of the engine, produces motionand usable work.

an engine which generates motive power bythe burning of petrol, oil, or other fuel withair inside the engine, the hot gasesproduced being used to drive a piston or doother work as they expand.

Internal combustion engine

isochoric /

Systems

ENERGY:

It is that capacity a body or substance possess whichcan result in the performance of work.

The presence of energy can only be observed by itseffects and these can appear in many different forms.

ENERGY FORMS IN THERMODYNAMICSSYSTEMS:

Kinetic Energy:If the fluid is in motion then it possesses Kinetic Energy. For a unit

mass:

Internal Energy:The energy associated with the disordered, random motion ofmolecules is called Internal Energy. It is separated in scale from themacroscopic ordered energy associated with moving objects; itrefers to the invisible microscopic energy on the atomic andmolecular scale. Internal energy is independent of the path.

Kinetic Energy Potential Energy

The energy of a body or a system with respect tothe motion of the body or of the particles in thesystem.

Potential Energy is the stored energy in anobject or system because of its position orconfiguration.

Kinetic energy of an object is relative to othermoving and stationary objects in its immediateenvironment.

Potential energy is not relative to theenvironment of an object.

Kinetic energy can be transferred from onemoving object to another. Potential energy cannot be transferred.

Flowing water, such as when falling from awaterfall.

Water at the top of a waterfall, before theprecipice.

Joule (J) Joule (J)

Speed/velocity and mass Height or distance and mass

The calorific value can be determined using theheat balance.Heat given by the fuel is equal to the heat gainedby the water.

Mass of fuel × calorific value.

Flow or Displacement Energy:

Any volume of fluid entering or leaving a system mustdisplace an equal volume ahead of itself in order toenter or leave the system.

The displacing mass must do a work on the mass beingdisplaced.

Since the movement of any mass can only be achievedat the expense of work.

This is called flow or displacement workAt entry it is energy received by the system.At exit it is energy lost by the system.

W o r k d o n e f d

F o r c e P A

W P A l

A l v

W P v

Heat Received or rejected:

In any system:If heat is received Q is +ve.If heat is rejected Q is –ve.If heat is neither received nor rejected then Q=0.

External work done:

If the external work is done by the fluid then W ispositive.If the external wok is done on the fluid then W isnegative.If no external work is done on or by the fluid then W=0.

THE CONSERVATION OF ENERGYOne form of energy can be transformed into another.E.g. A battery converts stored chemical energy to electricalenergy

Principle:This states that energy can neither be created nordestroyed; it can only changed in form.In an Equation:

Initial energy + Energy entering = Final energy of + Energy leavingof the system the system the system the system

ENTHALPY

Enthalpy is a measure of the total energy of a system. Itincludes the system's internal energy, as well as its volumeand pressure.

It is denoted by the symbol ‘H’.

Its unit is J (joules).H = U + PV

Types of Thermodynamic systems

Closed systems are able toexchange energy (heat andwork) but not matter withtheir environment.

If a process is carried on a closed system then by the principleof conservation of energy:

Initial energy + Energy entering = Final energy of + Energy leavingof the system the system the system the system

Q is assumed positive means it is transferred into the system.W is taken positive means work done by the system.

is the change in the total energy.

E1=initial total energy of the contained substanceE2=final total energy of the contained substanceQ=heat transferred to or from the substance in the systemW=work transferred to or from the substance in the system

E1+Q=E2+WQ=(E2-E1)+WQ-W=E2-E1

The non-flow energy equation:

For a closed system at rest, the contained energy will be only theinternal energy U.

There is no flow of substance into or out of the system.Process is called a non-flow process

1 2

2 1

2 1

U Q U W

U U Q W

U U U

Open systems mayexchange any form ofenergy as well as matterwith their environment.A boundary allowingmatter exchange is calledpermeable. The oceanwould be an example ofan open system. Eg: Pump, compressor,turbine.

Two flow energy equation:It is an open system in which an equal mass of fluid perunit time is both entering and leaving the system. Alsocalled continuity of mass flow.The form of energy associated with moving fluid enteringthe system will be:Internal Energy = U1

Displacement or flow energy = P1V1

Kinetic energy=KE1

Gravitational potential energy=PE1

As the fluid enters the system, let the total energy of thefluid mass actually in the system = ES1

The form of energy associated with the fluid mass leaving thesystem will be:Internal Energy = U2

Displacement or flow energy = P2V2

Kinetic energy=KE2

Gravitational potential energy=PE2

After passing through the system, as the fluid leaves thesystem, let the total energy of the fluid mass remaining in thesystem = ES2

ES1+U1+P1V1+KE1+PE1+Q = ES2+ U2+P2V2+ KE2+PE2+ W

U+PV=H=EnthalpyES1+H1+KE1+PE1+Q = ES2+ H2+ KE2+PE2+ W

Q - W= (ES2 - ES1) + (H2 - H1) + (KE2 - KE1) +(PE2 - PE1)

The steady – flow energy equation:

Steady flow system, it is considered that the mass flowrate of the fluid or substance through out the system isconstant.Also, the total energy of the fluid mass in the systemremains constant.So,

ES2 =ES1

ES2 - ES1=0

From the open system:Q - W= (ES2 - ES1) + (H2 - H1) + (KE2 - KE1) +(PE2 - PE1)

Q - W= (H2 - H1) + (KE2 - KE1) +(PE2 - PE1)

This is known as the steady –flow energy equationMore:

ES1+U1+P1V1+KE1+PE1+Q = ES1+ U2+P2V2+ KE2+PE2+ WES2 - ES1=0

U1+P1V1+KE1+PE1+Q = U2+P2V2+ KE2+PE2+ WThis is for any mass flow rate.For unit mass through system, specific quantities are used.

U1+P1V1+KE1+PE1+Q = U2+P2V2+ KE2+PE2+ W

Replace the values of KE and PE values .

Continuity of mass flow rate:

For a fluid substance flowing through a steady flow opensystem, the mass flow rate through any section in the systemmust be constant.At any section in the system,

Isolated systems arecompletely isolated from theirenvironment.They do not exchange heat,work or matter with theirenvironment. An example of an isolatedsystem is a completelyinsulated rigid container,such as a completelyinsulated gas cylinder.

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