Download pdf - ME215 thermo CH2me215.cankaya.edu.tr/uploads/files/ME 215-CH2-thermo.pdf · 2016-10-03 · Microsoft PowerPoint - ME215_thermo_CH2 Author: Ekin Ã zgirgin Created Date: 10/3/2016

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ME 215

Fundamentals of Thermal Systems CH-2

Basic Concepts of Thermodynamics

ÇANKAYA UNIVERSITY

Mechanical Engineering Department

Asst. Prof. Dr. Ekin Özgirgin Yapıcı

What is Thermodynamics?

• Thermodynamics is the science dealing with relationships between heat, work and the properties of materials.

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Definition of a System

• The term system is used to identify the subject of the analysis. The system is whatever we want to study. It may be as simple as a free body or as complex as an entire chemical refinery.

• The mass or region outside the system is called surroundings.

• The real or imaginary surface that separates the system from its surroundings is called the boundary.


system surroundings

boundary

Types of Systems:

A closed system (control mass) is defined when a particular quantity of matter is under study. A closed system always contains the same matter. There can be no transfer of mass across its boundary.

An open system (control volume): With this approach, a region within a prescribed boundary is studied. The region iscalled a control volume. Mass may cross the boundary of a control volume.

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Types of Systems:

If no mass can enter or leave a closed system ans also ifenergy is not allowed to cross the boundary, that system is called an isolated system.

Definition of a Sysem

An open system (control volume)

A closed system (control mass)The thermodynamic relations thatare applicable to closed and opensystems are different. It is importantto recognize the type of systembefore you start analyzing it

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Engineers are interested in studying systems and how they interact with their surroundings. Terms and concepts used to describe systems and how they behave;

Property (extensive property, intensive property)

State

Process

Phase and Pure Substance

Equilibrium

Steady state

Thermodynamic cycle

Describing Systems andProperties

A property is a macroscopic characteristic of a system such as mass, volume, energy, pressure, and temperature to which a numerical value can be assigned at a given time without knowledge of the previous behavior (history) of the system.

Thermodynamic properties can be placed in two general classes:

extensive and intensive.


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extensive

Dependent of the size and extent of the system

Additive over the system

m V E

Intensive

İndependent of the size and extent of the system

Not Additive over the system

T P

b=B/m where b is intensive, B is extensive property


State: The word state refers to the condition of a system as described by its properties. Since there are normally relations among the properties of a system, the state often can be specified by providing the values of a subset of the properties.

If a system exhibits the same values of its properties at two different times, it is in the same state at these times. A system is said to be at steady state if none of its properties changes with time.


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When any of the properties of a system change, the state changes and the system is said to have undergone a process. A process is a transformation from one state to another.

A process occurring at constant temperature is an isothermal process.

A process occurring at constant pressure is an isobaricprocess.


Thermodynamic cycle: is a sequence of processes that begins and ends at the same state. At the conclusion of a cycle all properties have the same values they had at the beginning.

Describing Systems and Properties

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Phase: quantity of matter that is homogeneous throughout in both chemical composition and physical structure.

Homogeneity in physical structure means that the matter is all solid, or all liquid, or all vapor (or equivalently all gas).

When more than one phase is present, the phases are separated by phase boundaries.

A pure substance is one that is uniform and invariable in chemical composition. A pure substance can exist in more than one phase, but its chemical composition must be the same in each phase.

For example: liquid water and water vapor mixture


Equilibrium:In mechanics, equilibrium means a condition of balance maintained by an equality of opposing forces.

Isolate the system from its surroundings and watch for changes in its observable properties. If there are no changes, we conclude that the system was in equilibrium at the moment it was isolated. The system can be said to be at an equilibrium state.


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Units and Dimensions

A unit is any specified amount of a quantity by comparison with which any other quantity of the same kind is measured.

EX: meters, centimeters, kilometers, feet, inches, and miles are all units of length. Seconds, minutes, andhours are alternative time units.

Because physical quantities are related by definitions and laws, a relatively small number of them are usedto measure all others. These may be called primary (or basic) dimensions.

The others may be measured in terms of the primary dimensions and are called secondary.



Four primary dimensions suffice in thermodynamics, fluid mechanics, and heat transfer.

They are mass (M), length (L), time (t), and temperature (T).

Alternativelyt, Force (F) can be used.

Basic Dimension Systems:

MLTθ (mass length time temperature)

FLTθ (force length time temperature)

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Primary Units and Dimensions

MLT

Secondary Units and Dimensions

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Volume(V) m3

Density ():kg/m3

The density, or local mass per unit volume, is an intensive property that may vary from point to point within a system. Spesific volume (): m3/kg (1/density) It is the volume per unit mass. Like density, specific volume is an intensive property and may vary frompoint to point.In certain applications it is convenient to express properties such as a specific volume on a molar basis.kg/kmol

Describing Systems andProperties; secondary dimensions

Sometimes the density of a substance is given relative to the density of a well-known substance. Then it is called specific gravity, or relative density, and is defined as the ratio of the density of a substance to the density of somestandard substance at a specified temperature:

Describing Systems andProperties; secondary dimensions

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Temperatureis a measure of hottness or coldness. Several properties of materials change with

temperatures, so it is very important to measure thetemperature.

Temperature and Zeroth Law

A device for temperature measurement is

the liquid-in-glass thermometer

That is, when a body is brought into contact with another body that is at a different temperature, heat is transferred from the body at higher temperature to the one at lower temperature until both bodies are at same temperature

At that point, the heat transfer stops, and the two bodies are said to have reached thermal equilibrium.

The zeroth law of thermodynamics states that if two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.

Temperature and Zeroth Law

http://www.wiley.com/college/moran/0470495901/animations/ext_int_properties/ext_int_properties.html

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Temperature Scales

Celsius scale (formerly called the centigrade scale)

Fahrenheit scale

Kelvin scale(thermodynamic temperature scale:that is independent of the properties of any substance or substances)

Rankine scale

Temperature Measurement

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Pressure = Normal Force / area (1 N/m2= 1 Pa)

Pressure measurement manometer

barometer

Pressurehttp://www.wiley.com/college/moran/0470495901/animations/ext_int_properties/ext_int_properties.html

The actual pressure at a given position is called the absolute pressure.

the difference between the absolute pressure and the local atmospheric pressure is called the gage pressure.

Pressures below atmospheric pressure are called vacuum pressures

Pressure

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Variation of Pressure with Depth The pressure difference between two points in a

constant density fluid is proportional to the vertical distance z between the points and the density of thefluid.

Liquids are incompressible substances, and thus the variation of density with depth is negligible. This is also the case for gases when the elevation change is small

Variation of Pressure with Depth For fluids whose density changes significantly with elevation, a

relation for the variation of pressure with elevation can be obtained as:

The pressure is the same at all points on a horizontal plane in a given fluid regardless of geometry

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The Manometer Measurement


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Barometer and atmospheric pressure

The atmospheric pressure is measured by a device called a barometer; thus,the atmospheric pressure is often referred to as the barometric pressure.

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Example A closed tank contains compressed air and oil (SGoil 0.90)

as is shown in Fig. A U-tube manometer using mercury(SGHg = 13.6) is connected to the tank as shown. For column heights h1=15 cm, h2=7 cm, and h3=10 cm, determine the pressure reading of the gage.

The pressure at level (1) isequal to the pressure at level (2), since these two points are at the same elevation in a homogeneous fluid at rest