eng208_lecture_1p1.pdf

1. Basic Concepts of Thermodynamics

Thermodynamics|ENG208 Dr. Nawaf Aljuwayhel

Part 1

Textbook Thermodynamics: An Engineering Approach 7th Edition, Cengel & Boles

Thermodynamics|ENG208 2

Thermo dynamics

- Conversion of heat into power - The science of energy - The study of energy and its transformation

Conservation of Energy Principle: During an

interaction, energy can change from one form to another but the total amount of energy remains constant (conserved). Energy cannot be created or destroyed.

Example 1: A rock falling off a cliff picks up speed as a result of its potential energy being converted to kinetic energy (Figure 1).

Heat (Q) Power

W

Figure 1.1: Energy is conserved

1. Basic Concepts of Thermodynamics

1-1 Thermodynamics and Energy

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1. Basic Concepts of Thermodynamics 1-1 Thermodynamics and Energy

The change in the energy content (∆E) of a body or any system is equal to the difference between the input energy (Ein) and the output energy (Eout).

Example 2: Human body (Figure 2) Food + No exercise Weight gain

Figure 1.2: Conservation of energy principle for human body

∆E = Ein - Eout

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1st Law of Thermodynamics: During an interaction, energy can change from one form to another but the total amount of energy remains constant

Conservation of Energy Principle

2nd Law of Thermodynamics: Energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy.

Ex. 3 A cup of hot Coffee (Figure 3); the hot coffee will cool down but a cold coffee will not heat up by it self

Figure 1.3: Heat flows in the direction of decreasing temperature

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A substance consists of a large number of particles called molecules and its properties depend on the behavior of these molecules.

Ex. 4 The pressure of a gas in a container is a result of momentum transfer between the molecules and the walls of the container).

Figure 4: Gas molecules http://wps.prenhall.com

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Kuwait University Thermodynamics|ENG208 6


Application Areas of Thermodynamics

Figure 1.5: Some application areas of thermodynamics


1. Basic Concepts of Thermodynamics 1-2 Importance of Dimensions and Units

A physical quantity can be characterized by dimensions. The magnitude assigned to the dimensions are called units.

Dimensions

Units

Fundamental or primary dimensions Ex. mass m, length L, time t, and temperature T

Secondary or derived dimensions Ex. velocity V, energy E, and volume V

English (or USCS) units

SI (or international) units



Dimension SI units English Units Conversion factors

Mass kilogram (kg) pound mass (lbm) 1 lbm = 0.454 kg

Length meter (m) foot (ft) 1 ft = 0.3048 m

Time second (s) second (s) ----

Temperature Celsius (°C)

kelvin (K) Fahrenheit (°F)

rankine (R) T(°F) = 1.8 T(°C) + 32 T(R) = 1.8 T(K)

Example of some primary dimensions in SI and English units and the conversion factors

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Force (F): is a secondary dimension and defined by Newton’s second law as the force required to accelerate a mass of 1 kg at a rate of 1 m/s2:

F = (mass) (acceleration) = ma (1-1) 1 Newton (N) = 1 kg · m/s2 1 lbf = 32.174 lbm · ft/s2

Weight (W): is a gravitational force applied to a body and give as: W = mg (N) (1-2)

g is the local gravitational acceleration (9.807 m/s2 or 32.174 fl/s2 at sea level).

The mass of a body remains the same regardless its location. The weight of a body changes with the magnitude of g. On the top of a mountain or the surface of the moon, a body will weigh

less what it normally weighs on earth at sea level.



Specific weight (w or γ): is the weight of a unit volume of a substance and determined from:

γ = w = ρg (N/m3) (1-3) where ρ is the density

Work (W): is a form of energy and can be defined as force times distance and has the unit “newton-meter” (N · m) which is called a joule (J) or Btu (British thermal unit).

Power (Ẁ): is the work done per unit time. It has a unit of kJ/s or kW.



Dimension SI units English Units Conversion factors

Force Newton (N) pound force (lbf) 1 lbf = 4.45 N

Work Joule (J) British Thermal Units (Btu) 1 Btu = 1055.06 J

Power Watt (W) Btu/h 3.41214 Btu/h = 1W

Example of some secondary dimensions in SI and English units and the conversion factors



Dimensional Homogeneity In engineering, every term in an equation must have the

same unit ⇒ dimensionally homogeneous.

Example 5: E = 25 kJ + 7 kJ/kg Analysis 1. Two different units. 2. E is the total energy and must have the unit of kJ. 3. Last term has a unit of energy per unit mass multiply by m.

- Units can be used to check formulas; they can even be used to derive formulas.

Example 6: A Tank with a volume of 2 m3 and filled with oil of ρ = 850 kg/m3, determine the mass inside the tank. Analysis (Figure 6) m = ρV = (850 kg/m3) (2m3) = 1700 kg

OIL V= 2 m3

ρ = 850 kg/m3 m= ?

Figure 1.6


1. Basic Concepts of Thermodynamics 1-3 Systems and Control Volumes

Thermodynamic system: is defined as a quantity of matter or a region in space chosen for study

Surroundings: The mass and region outside the system . Boundary: A real or imaginary surface that separate the system from

the surroundings and shared by both of them. The boundary can be fixed of movable.

Figure 1.7: System, surroundings and boundary

Figure 1.8: A boundary maybe real, imaginary, fixed or movable.



System Types

Closed System or Control Mass Mass can not cross the boundary (m = constant). Energy may cross the boundary in form of work or heat. If as a special case, even energy is not allowed to cross the boudary isolated systems.

Both energy and mass may cross the system’s boundary, i.e. the control surface

Open System or Control Volume

Figure 1.9: Closed system Figure 1.10: Open system



Exercise 1: In the figures below, specify the system’s type (i.e. open or close system).

Figure 1.11a: A nozzle. Figure 1.11b: A piston cylinder device with one inlet.

Figure 1.11c


1. Basic Concepts of Thermodynamics 1-4 Properties of a System

A system is to characterized by a property

Vm

=ρ

Independent properties Ex. Mass m, pressure P, volume V, temperature T

Dependent properties (Properties that are defined in term of other properties) Ex. Density ρ

• Density: mass per unit volume

(kg/m3) (1-4)

Incompressible substances (solids and liquids): (1-5a)

Compressible substances (gases): (1-5b) ),( PTρρ =

)T(ρρ =



Properties

Intensive properties those that are independent of the size of a system (T, P, ρ,…)

Extensive properties those whose values depend on the size of the system (m, V, E, U…)

; ;....V Ev em m

= =

• Extensive properties per unit mass are called specific properties. Examples are specific volume v and specific total energy e



How to determine whether a property is intensive or extensive?

Figure 1.12: Criteria to differentiate intensive and extensive properties


1. Basic Concepts of Thermodynamics 1-5 Density and Specific Gravity

Specific Gravity or Relative Density (ρs ): The ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4ºC, ρwater = 1000 kg/m3). ρs = ρ / ρwater (dimensionless) (1-6) Ex. Water: ρs = 1.0

Ice: ρs = 0.92 Sea water: ρs = 1.025 If ρs < 1 ⇒ the substance is lighter than water and thus will

float on water.

Specific volume: volume per unit mass and it is the reciprocal of the density.

(m3/kg) (1-7) ρ1

mVv ==


1. Basic Concepts of Thermodynamics 1-6 State and Equilibrium

State: When a system is not undergoing any change, then all of its properties can be measured or calculated throughout the entire system. Thus, the condition of the system or the state, can be described completely with the knowledge of this set of properties (V, T, P, …).

At a give state, all properties have a fixed value

Figure 1.13: A system at two different states



Thermal equilibrium: Uniform temperature throughout the entire system (Figure 1.14).

Mechanical equilibrium: No pressure change at any point of the system with time.

Phase equilibrium: For a system that involves two phases, phase equilibrium is reached when the mass of each phase reaches an equilibrium level and stay there.

Chemical equilibrium: When the chemical composition of a system does not change with time, that is, no chemical reaction occur.

⇒ A system will not be in equilibrium unless all the relevant equilibrium criteria are satisfied.

Figure 1.14



The State Postulate The state of a simple compressible system is completely

specified by two independent, intensive properties. Simple compressible system ⇒ has no electrical, magnetic,

gravitational, motion, and surface tension effects. Two properties are independent if one property can be

varied while the other one is held constant. The state postulate requires that the two properties specified

be independent to fix the state.

Example 7: T and v are always independent

Example 8: T and P are independent properties for single-phase system. However, they are dependent for multiphase system (WHY??? We will learn it in chapter 3)


1. Basic Concepts of Thermodynamics 1-7 Processes and Cycles

Process: Is any change that a system undergoes from one equilibrium state to another.

Path: Series of states through which a system passes during a process.

A process is to be described by: Initial state Final state Path Interaction with surrounding Figure 1.15: The P-V diagram of

a compression process.



Quasi-Equilibrium Process It is significantly slow process that allows the system to

adjust itself internally so that properties in one part of the system do not change any faster than those at the other part.

It is a process that proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all time.

It is an ideal process. But many actual processes is modeled as quasi-equilibrium process with negligible error.

Example 9: A gas piston-cylinder device (Figure 1.16).

Figure 1.16: Quasi-equilibrium and non quasi-equilibrium compression processes

(a) Slow compression (b) Fast compression



Process diagrams are plotted by employing thermodynamic properties as coordinate, they are very useful in visualizing the process.

T, P, V (or v) are properties that are commonly used as coordinates of the process diagram.

Example 1: The P-V diagram of a compressible process of a gas.

Figure 1.15: The P-V diagram of a compression process.



The prefix iso- is often used to designate a process for which a particular property remains constant. Isothermal process: T remains constant. Isobaric process: P remains constant. Isochoric (or isometric) process: v remains constant.

A system is said to have undergo a cycle if it returns to its initial state at the end of the process ⇒ both initial and final states are identical.



Steady: no change with time.

Uniform: no change with location over a specified region.

Steady-flow device: an engineering device operate for long period of time under the same condition, i.e. turbines, pumps, …



Steady Flow Process: A process during which a fluid flows through a control volume

steadily. In such a process, the fluid properties can change from point to

point within the C.V., but at any fixed point they remain the same during the entire process, Figure 17.

Therefore, V, m, E of the C.V. remain constant during a steady-flow process.

300°C

200°C

150°C

Control volume

250°C

150°C

Mass In

Mass Out

300°C

200°C

150°C

Control volume

250°C

150°C

Mass In

Mass Out

Figure 1.17: Steady flow process.

At t1 At t2

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