Polytropic processes help

8/8/2019 Polytropic processes help

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Recommended Book

Recommended book

Introduction to Thermal and Fluid Engineering

Kaminski and Jensen John Wiley and Son 2005

ISBN 0-471-26873-9


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Polytropic processes help!

Dr Andy E Young


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The control volume (CV)

System based around a control-volume

Closed systems No mass transfer, only energy transfer

Open systems Mass and energy transfer

AProcess occurs inside the

system whenever

Energy is transfers

Property of the system changes

CV


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Work done [J] by the system = +ve

on the system = -ve

Heat [J]

Added to the system = +ve Lost from the system = -ve

Most cycles have multiple heat

and work exchanges across

system boundaries

Net quantities are calculated

A process with no heat transfer is called Adiabatic

Work and Heat: Conventions

Open orClosed system?


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Workdone from measured data.

Example: Expansion of CO2

P-V data was measured and plotted

Calculate workdone between 1 and 3.

What about ideal

processes?

Universal Gas Constant

Molar Mass for a substance


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Open Systems

Variable Mass: flow

Work and Heat exchange

Examples IC Combustion

Heat added

Mass exchanged

Work done exchanged

Electronics cooling

-Q -M

+Q

+M-Q

-Q -M

-Q

-M

-W

-W

+W

+M

+M +U

Challenge:

Label Q, M, U, and W, +/-


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Closed Systems

Change in Energy = (Energy Entering) (Energy Leaving)

Types of Energy stored inside a system:

Kinetic, KE

Potential, PE

Internal, U

What is this Internal

Energy, U?


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Internal Energy: U[J]

The rider starts and ends at rest (KE=0)

Challenge:

What are the exchanges in heat and work from state 1 to 2?

What changes in internal energy will occur?

Heat:

Convection to air

Work:

Drag on air

Drag of tyres on pavement

Internal:

Bicycle Mechanism

Bio-reactions

Blood pressure! Exchanges ofheat and work

occur at the boundary of the

system.

Internal energy is stored

inside, as heat and

pressure.


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Enthalpy:H [J]

At constant pressure, cp[J/kg.K]

Heat, Q, added at constant pressure, P

Temperature rises by (T

Volume changes by (V

P

V

1 2T

V

1

2

(T

(V


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Specific Heats, cp and cv [J/kg.K]

Recall that for an ideal gas:

H is purely a function of T


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Polytropic Processes:

Definition

A process in which the P-V relationship

follows the general law:

Possible to calculate

workdone by identifyingn forbasic processes


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Basic Types (1) Isochoric process:

Constant volume, Zero work

Heat transferred to the system is stored

as internal energy

Isothermal process:

Constant temperature,

Zero change in internal energy

Energy transfer must equal to work.

P

1

2

V

(P

P

V

1

2

(V


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Basic Types (2)

Isobaric process:

Constant pressure, Work done causes a change in volume.

Adiabatic process:

Constant heat, Work done without heat transfer.

Internal energy converted to work.

External work converted to internal

energy.

P

V

1 2

(V

P

V

1

2

(V

A closer look at Adiabatic

Compression and expansion

(engine cycles)


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Adiabatic process

Constant heat

Work done without heat transfer: Q=0

Internal energy converted to work: -(U = W

Polytropic processes help

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