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ENGI 2102 Thermo-Fluid Engineering I

Chapter 3 : Work and Heat

G. Mazzanti

Process Engineering and Applied Science

Dalhousie University

Fall 2019

Slides by Michele Hastie, 2016

Outline

 3.1 Forms of Energy

 3.1.1 Internal Energy

 3.1.2 Kinetic Energy

 3.1.3 Potential Energy

 3.1.4 Mechanical Energy

 3.1.5 Nuclear Energy

 3.2 Energy Transfer

 3.3 Mechanical Forms of Work

 3.3.1 Shaft Work

 3.3.2 Spring Work

 3.3.3 Non-Mechanical Forms of Work

 3.4 Temperature and the Zeroth Law of Thermodynamics

 3.5 Problems

Introduction (page 56)

 Kinetic energy – Energy related to the

translational or rotational motion of

the system relative to some reference

frame.

 Potential energy – Energy

related to the position of

the system in a potential

field relative to some

reference position.

 Commonly a gravitational or electromagnetic field  A fluid under pressure can also be considered a form of potential energy

 Internal energy – All energy possessed by a system other than kinetic or

potential energy. This includes energy due to:

 Molecular motion, electromagnetic interactions, interactions between atomic and subatomic constituents

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Introduction (page 57)

 Energy can be transferred between a system and its

surroundings in the form of:

 Heat – energy that flows from a high temperature region to a

low-temperature region.

 Work – energy that enters or leaves a system as a result of any

driving force other than a temperature difference.

gas

Work

gas gas

Heat

 The state of a system is described by its properties at a

particular instant:

 Temperature

 Pressure

 Specific volume

 Density

 Specific internal energy

 Specific heat capacity

 etc.

 For a single-phase, single-species system, the state can be

fixed by specifying two independent properties.

 When there is no tendency for the properties to change

with time, we refer to the system as being in a state of

thermodynamic equilibrium.

Example in equilibrium

Example not in equilibrium

Thermal equilibrium – same

temperature everywhere.

Mechanical equilibrium – forces are

balanced (and also pressure is the same

everywhere except as a result of gravity).

Water

20°C

Surroundings

20°C

Water

20°C

Surroundings

4°C

Gas 200 kPa

Surroundings 101 kPa

F

Gas 200 kPa

Surroundings 101 kPa

 When a system changes from one equilibrium state to another, the

path of successive states that the system goes through is called a process.

 Many real processes can be approximated as an equilibrium process.

 If a process is carried out relatively slowly , it will be quasi-static and

so can be considered in quasi-equilibrium.

Fig. 3.1. Depiction of a process (a process diagram)

 We may choose to hold a particular property constant

throughout a process

 Isothermal – Constant temperature

 Isobaric – Constant pressure

 Isochoric – Constant volume

 If a system undergoes a process that returns to its initial

state, it is said to have undergone a cycle.

3.1.2 Kinetic Energy (KE)

 Energy a system possesses by virtue of its motion relative

to some frame of reference.

 For a rotating body,

^ (3.3) 

  

 =  = Nm= J s

m kg 2

1

2

2 2 KE mu

 

  





 

= =  kg

J

kg

N m

kg m s

N

s

m

s

m

2

2 - 2

2

2

2 2 u ke (3.4)

 



 

 =   =J s

1 kg m 2

1

2

2 2 KE I ^ (3.5)

3.1.3 Potential Energy (PE)

 Energy a system posses by virtue of its position in a

potential field relative to some arbitrary reference point.

 Commonly a gravitational or electromagnetic field.

 Other forms of potential energy:

 Elastic potential energy or “spring energy”.

 Chemical potential energy (from chemical bonds or structural

arrangement of molecules).

(3.6)  

  

 =  m =J s

m PE mgz kg (^2)

 



 

 =  = kg

J m s

m

2

pe gz (3.7)

 Closed systems:

 Open systems:

where:

( )  

E U

E U m u mg z

 = 

 = +  +  J 2

(^1 ) (3.8)

 

  

  =  kg

J e ˆ^ u^ ˆ (3.9)

0 0

 

  



 

  



 

  



= 

kg

J

s

kg

s

J

E  m  e^ ˆ

m =  V =  Auavg  

Rigid (or fixed) boundary

Movable (or deformable) boundary

gas

Real boundary

Imaginary boundary

Example 3.1: A 2500 kg Jeep travelling at 100 km/h slams

into the back of a stationary 1200 kg automobile. After the

collision, the Jeep slows to 50 km/h and the smaller vehicle

has a speed of 80 km/h. Taking both vehicles as the system,

what is the increase in internal energy of the system?

Solution

Assume that there is no heat transferred to or work done

on the surroundings. The total energy of the vehicles at the

initial conditions must equal the total energy of the

final conditions (Δ E = 0).

A U KE

U KE

E U KE PE

 = − 

 + =

 =  + + =

0

0

0

u auto,1 = 0 u jeep, 1 = 100 km/h

u auto,2 = 80 km/h u jeep,2 = 80 km/h

 The final kinetic energy is,

 The change in internal energy must be equal to the

change in the kinetic energy,

f

KE 2 = KE jeep, 2 + KE auto, 2 = 537. 4 kJ

 U = KE 1 − KE 2 = 427. 1 kJ

3.1.4 Mechanical Energy

 Many practical engineering applications

involve fluid flow through pipe systems

and equipment:

 Liquid is pumped up to a higher elevation.

 Liquid is allowed to flow under the force of

gravity to turn turbines to generate

electricity.

 Air is moved around in a building using

fans.

 These examples involve mechanical

forms of energy (pumps, compressors,

fans, turbines).

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