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INSTRUCTIONAL MATERIALS FORMEEN 20042 Basic ThermodynamicsCompiled by:Engr. Edsel G. Anyayahan, PMEFOREWORDThis instructional material is intended for the course: MEEN 20042 Basic Thermodynamics. The content of this instructional material was taken from the book: Thermodynamics an Engineering Approa...

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INSTRUCTIONAL MATERIALS FOR MEEN 20042 Basic Thermodynamics

Compiled by: Engr. Edsel G. Anyayahan, PME

MEEN20042 Basic Thermodynamics – Instructional Manual

FOREWORD

This instructional material is intended for the course: MEEN 20042 Basic Thermodynamics. The content of this instructional material was taken from the book: Thermodynamics an Engineering Approach by Micheal A. Boles and Yunus A. Cengel (2004) and other reference book for Thermodynamics. This is intended to be used by Bachelor of Science in Electrical Engineering (BSEE) students at Polytechnic University of the Philippines – Sto. Tomas Branch, Sto. Tomas, Batangas.

Engr. Edsel G. Anyayahan ,PME Faculty PUP Sto.Tomas Branch

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MEEN20042 Basic Thermodynamics – Instructional Manual

TABLE OF CONTENTS

Cover Page Foreword Table of Contents

Lesson 1: Basic Principles

4

ASSESSMENT ACTIVITY: Basic Principles

15

Lesson 2: Heat Quantities

15

ASSESSMENT ACTIVITY: Heat Quantities

22

Lesson 3:Elasticity

22

Lesson 4:Thermal Elongation

25

ASSESSMENT ACTIVITY: Elasticity and Thermal Elongation

27

Lesson 5:Fluid at Rest

27

Lesson 6:Law of Conservation of Mass

33

ASSESSMENT ACTIVITY: Fluids and Law of Conservation of Mass

38

Lesson 7:Heat Transfer

38

ASSESSMENT ACTIVITY: Heat Transfer

46

Lesson 8:Ideal Gas Law

46

ASSESSMENT ACTIVITY: Ideal Gas Law

60

Lesson 9:First Law of Thermodynamics

61

Lesson 10: Second Law of Thermodynamics

69

ASSESSMENT ACTIVITY: First and Second Law of Thermodynamics

79

References

80

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MEEN20042 Basic Thermodynamics – Instructional Manual

Lesson 1: Basic Principles of Thermodynamics Thermodynamics can be defined as the science of energy. Although everybody has a feeling of what energy is it is difficult to give a precise definition for it. Energy can be viewed as the ability to cause changes. The name thermodynamics stems from the Greek words therme (heat) and dynamis (power), which is most descriptive of the early efforts to convert heat into power. Today the same name is broadly interpreted to include all aspects of energy and energy transformations, including power generation, refrigeration, and relationships among the properties of matter (Boles and Cengel, 2004). Laws of Thermodynamics First law of thermodynamics Law of Conservation of Energy. “Energy cannot be created or destroyed dur process; it can only change from one form to another” (Boles and Cengel, 2004).

Figure 1.1 Example of Conversion of Energy

Second law of thermodynamics According to Boles and Cengel (2004) second law of thermodynamics (increase of entropy principle) is expressed as the entropy of an isolated system during a process always increases or, in the limiting case of a reversible process, remains constant. In other words, the entropy of an isolated system never decreases. It also asserts that energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy.

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MEEN20042 Basic Thermodynamics – Instructional Manual

Figure 1.2 Example of “No Such Thing as a 100% Efficient Engine” Two classical statements of the second law •

Kelvin–Planck statement It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.no heat engine can have a thermal efficiency of 100 percent, or as for a power plant to operate, the working fluid must exchange heat with the environment as well as the furnace(Capote and Mandawe, 2014). •

Clausius statement It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a highertemperature body (Capote and Mandawe, 2014).

Third law of Thermodynamics The total entropy of pure substances approaches zero as the absolute thermodynamics temperature approaches zero(Boles and Cengel, 2004). Note: According to Boles and Cengel( 2004) the third body is usually a thermometer. 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. By replacing the third body with a thermometer, the zeroth law can be restated as two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact (Boles and Cengel, 2004).

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MEEN20042 Basic Thermodynamics – Instructional Manual

Systems and Control Volumes System is defined as a quantity of matter or a region in space chosen for study/observe. The system is separated from its surrounding by a boundary (Boles and Cengel, 2004). Surroundings are the mass or region outside the system (Boles and Cengel, 2004). Boundary is the real or imaginary surface that separates the system from its surroundings (Boles and Cengel, 2004).

Figure1.3

Basic Barometer

Note: According to Boles and Cengel (2004) the boundary of a system can be fixed or movable. Note that the boundary is the contact surface shared by both the system and the surroundings. Mathematically speaking, the boundary has zero thickness, and thus it can neither contain any mass nor occupy any volume in space.

Type of System Open system/Control volume According to Boles and Cengel (2004) it usually encloses a device that involves mass flow such as a compressor, turbine, or nozzle. Both mass and energy can cross the boundary of a control volume. A large number of engineering problems involve mass flow in and out ofa system and, therefore, are modelled as control volumes. A water heater, a car radiator, a turbine, and a compressor all involve mass flow and should be analyzed as control volumes (open systems) instead of as control masses (closed systems) (Boles and Cengel, 2004).

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MEEN20042 Basic Thermodynamics – Instructional Manual

Control volume can involve fixed, moving, real, and imaginary boundaries (Boles and Cengel, 2004).

Figure1.4

Open System

A control volume with real and imaginary boundaries.

Figure 1.4 Nussle A control volume with fixed and moving boundaries.

Figure 1.5 Piston and Cylinder

•

Control surface = boundaries of a control volume, they can be real or imaginary (Boles and Cengel, 2004).

•

A control volume can also involve heat and work interactions just as a closed system, in addition to mass interaction (Boles and Cengel, 2004).

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MEEN20042 Basic Thermodynamics – Instructional Manual

Closed system/ Control mass Closed system no mass can enter or leave a closed System But energy, in the form of heat or work, can cross the boundary; and the volume of a closed system does not have to be fixed (Boles and Cengel, 2004).

Examples •

Mixtures of water and steam in a closed vessel

•

Gas expanding in a piston-engine

Figure 1.6 Closed System

Isolated system Neither mass nor energy can cross the selected boundary (Boles and Cengel, 2004). “as a special case, even energy is not allowed to cross the boundary”

Example •

Coffee in a closed, well Insulated Thermos Bottle

Figure 1.7 Isolated System

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MEEN20042 Basic Thermodynamics – Instructional Manual

Properties of a System Any characteristic of a system is called a property (Boles and Cengel, 2004).

Property

Intensive

Extensive

Are those that are independent of the mass of a system.

Are those whosevalues depend on the size or extent of the system.

•

Temperature

•

Pressure

•

Density

• • • • •

Mass Volume Momentum Enthalpy Energy

State and Equilibrium State Is the condition of a system not undergoing any change gives a set of properties that completely describes the condition of that system. At this point, all the properties can be measured or calculated throughout the entire system (Boles and Cengel, 2004).

Figure 1.8 A System at Two Different States

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MEEN20042 Basic Thermodynamics – Instructional Manual

Equilibrium Implies a state of balance. In an equilibrium state there are no unbalanced potentials (or driving forces) within the system. A system in equilibrium experiences no changes when it is isolated from its surroundings (Boles and Cengel, 2004). Thermodynamic equilibrium Is a condition of a system in which all the relevant types of equilibrium are satisfied (Boles and Cengel, 2004). ✓ Thermal Equilibrium ✓ Mechanical Equilibrium

Thermodynamic Equilibrium

✓ Phase Equilibrium ✓

Chemical Equilibrium

Thermal Equilibrium Means that the temperature is the same throughout the entire system (Boles and Cengel, 2004).

Figure 1.9 A Closed System Reaching Thermal Equilibrium

Mechanical equilibrium Is related to pressure, and a system is in mechanical equilibrium if there is no change in pressure at any point of the system with time (Boles and Cengel, 2004). For example, the higher pressure at a bottom layer is balanced by the extra weight it must carry, and, therefore, there is no imbalance of forces (Boles and Cengel, 2004). Phase Equilibrium Is the condition that the two phases of a pure substance are in equilibrium when each phase has the same value of specific Gibbs function. Also, at the triple point (the state at 10

MEEN20042 Basic Thermodynamics – Instructional Manual

which all three phases coexist in equilibrium), the specific Gibbs function of each one of the three phases is equal (Boles and Cengel, 2004). Chemical Equilibrium Is established in a system when its chemical composition does not change with time (Boles and Cengel, 2004).

Processes and Cycles Process Is any change that a system undergoes from one equilibrium state to another. To describe a process completely, one should specify the initial and final states of the process, as well as the path it follows, and the interactions with the surroundings (Boles and Cengel, 2004). Process should specify the initial and final states, as well as the path it follows, and the interactions with the surroundings (Boles and Cengel, 2004). Path of a process is the series of states through which a system passes during a process.

Figure 1.10 A Process between States 1 and 2 and the Process Path

Process diagrams plotted by employing thermodynamic properties as coordinates are very useful in visualizing the processes. Some common properties that are used as coordinates are temperature T, pressure P, and volume V (or specific volume v). Figure 1.11 shows the P-V diagram of a compression process of a gas (Boles and Cengel, 2004).

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MEEN20042 Basic Thermodynamics – Instructional Manual

Figure 1.11The P-V Diagram of a Compression Process

Quasi-static, or Quasi-Equilibrium Process -

-

According to Boles and Cengel (2004) is a process which proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times. A quasi-equilibrium process can be viewed as a sufficiently 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 other parts (Boles and Cengel, 2004).

Non quasi-equilibrium Process -

Is the reciprocal of quasi-equilibrium (Boles and Cengel, 2004). When a gas in a piston-cylinder device is compressed suddenly, the molecules near the face of the piston will not have enough time to escape and they will have to pile up in a small region in front of the piston, thus creating a highpressure region there. Because of this pressure difference, the system can no longer be said to be in equilibrium, and this makes the entire process nonquasiequilibrium (Boles and Cengel, 2004).

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MEEN20042 Basic Thermodynamics – Instructional Manual

Figure 1.12 Quasi-Equilibrium and Nonquasi-Equilibrium Compression Processes Note that the process path indicates a series of equilibrium states through which the system passes during a process and has significance for quasi equilibrium processes only. For nonquasi-equilibrium processes, we are not able to characterize the entire system by a single state, and thus we cannot speak of a process path for a system as a whole. A nonquasi-equilibrium process is denoted by a dashed line between the initial and final states instead of a solid line (Boles and Cengel, 2004).

Prefix-iso Is often used to designate a process for which a particular property remains constant (Boles and Cengel, 2004). -

Isothermal Constant Temperature Isobaric Constant Pressure Isometric Constant Volume

Cycle Is a process or series of processes, that allows a system to undergo state changes and returns the system to the initial state at the end of the process. That is, for a cycle the initial and final states are identical (Boles and Cengel, 2004).

The Steady-Flow Process According to Boles and Cenge (2004), the terms steady and uniform are used frequently in engineering, and thus it is important to have a clear understanding of their meanings. 13

MEEN20042 Basic Thermodynamics – Instructional Manual

The term steady implies no change with time. The opposite of steady is unsteady, or transient. The term uniform, however, implies no change with location over a specified region (Boles and Cengel, 2004). Steady-flow process is a process during which a fluid flows through a control volume steadily according to Boles and Cengel (2004).That is, the fluid properties can change from point to point within the control volume, but at any point, they remain constant during the entire process. During a steady-flow process, no intensive or extensive properties within the control volume change with time. See Fig. 1.13 -1.14

Figure 1.13 During a Steady-Flow Process, Fluid Properties within the Control Volume may Change with Position but Not with Time.

Figure 1.14Under Steady-Flow Conditions, The Mass and Energy Contents of a Control Volume Remain Constant.

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MEEN20042 Basic Thermodynamics – Instructional Manual

ASSESSMENT ACTIVITY: Basic Principles of Thermodynamics A. Answer the following problem 1. Identify which of the following are extensive properties and which are intensive properties: (a) a 10-𝑚3 volume, (b)30 J of kinetic energy, (c) a pressure of 90 kPa, (d)a stress of 1000 kPa, (e) a mass of 75 kg, and (f) a velocity of 60 m/s. Convert all extensive properties to intensive properties assuming m = 75 kg (show your solution). 2. Draw a sketch of the following situations identifying the system or control volume, and the boundary of the system or the control surface. (a)The combustion gases in a cylinder during the power stroke, (6) the combustion gases in a cylinder during the exhaust stroke, (c) a balloon exhausting air, (d)an automobile tire being heated while driving, and (e) a pressure cooker during operation. B. Research 1. A large fraction of the thermal energy generated in the engine of a car is rejected to the air by the radiator through the circulating water. Should the radiator be analyzed as a closed system or as an open system? Explain.

2. For a system to be in thermodynamic equilibrium, do the temperature and the pressure have to be the same everywhere? 3. Is the state of the air in an isolated room completely specified by the temperature and the pressure? Explain. Lesson2. Heat Quantities This lesson introduces heat energy by looking at specific heat and latent heat of solids and gases. This provides the base knowledge required for much ordinary estimation of heat energy quantities in heating and cooling, such as is involved in many industrial processes, and in the production of steam from ice and water. The special case of the specific heats of gases is covered, which is important in later lessons, and introduction is made in relating heat energy to power (Boles and Cengel, 2004).

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MEEN20042 Basic Thermodynamics – Instructional Manual

Specific Heat The specific heat of a substance is the heat energy required to raise the temperature of unit mass of the substance by one degree. In terms of the quantities involved, the specific heat of a substance is the heat energy required to raise the temperature of 1 kg of the materials by 1 ℃ (or K, since they have the same interval on the temperature scale). The units of specific heat are therefore J/kg-K (Boles and Cengel, 2004). Difference substances have different specific heats, for instance copper is 390 J/kg-K and the cast iron is 500J/kg-K. in practice this means that if you wish to increase the temperature of a lump of iron it would required more heat energy to do it than if it was a lump of copper of the same material (Boles and Cengel, 2004).

Alternatively, you could say the iron ‘soaks up’ more that energy for a given rise in temperature. • Remember that heat energy is measured in joules or in kilojoules (1000 joules)(Boles and Cengel, 2004). • The only difference between the Kelvin and the centigrade temperature scales is where they start from. Kelvin starts at -273 (absolute zero) and centigrade starts at 0. A degree change is the same for each (Boles and Cengel, 2004). The equation for calculation heat energy required to heat a solid is therefore the mass to be heated multiplied by the specific heat of the substance, c, available in tables, multiplied by the number of degree rise in temperature, ∆𝑇 (Boles and Cengel, 2004).

Putting in the units

𝑄 = 𝑚𝑐∆T

𝑘𝐽 = 𝑘𝑔 (

𝑘𝐽 ) (𝐾) 𝑘𝑔 − 𝐾

Note that on the right-hand side, the kg and K terms cancel to leave kJ. It is useful to do a units check on all formulas you use (Boles and Cengel, 2004).

Example

1. The boiler in a canteen contains 6 kg of water at20 ℃. How much heat energy is required to raise the temperature o f the water to100 ℃? Specific heat of water = 4190 J/kg-K (Capote and Mandawe, 2014). 𝑄 = 𝑚𝐶 ∆𝑇

𝑄 = (6)(4190)(100 − 20)

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MEEN20042 Basic Thermodynamics – Instructional Manual

𝑄 = 2011200 𝐽 = 2011.2 𝑘𝐽

2. How many kilograms of copper can be raised from 15 ℃ to 60℃ by the absorption of 80 kJ of heat energy? Specific heat of copper = 390 kJ/kg-K (Capote and Mandawe, 2014). 𝑄 = 𝑚𝐶 ∆𝑇

80000 𝐽 = 𝑚 (390 𝑚=

𝑘𝐽 ) (60 − 15)℃ 𝑘𝑔 − 𝐾

80000 (390)(45)

𝑚 = 4.56 𝑘𝑔

Power It is not always useful to know only how much energy is needed to raise the temperature of a body. For instance, if you are boiling a kettle, you are more interested in how long it will be before you can make the tea. The quantity of the energy needed has to be related to the power available, in this case the rating of the heating element of the kettle, and if you have a typical kettle of, say, 2kW, it means that in 1 second it provides 2000 joules of heat energy (Boles and Cengel, 2004). Remember that power is the rate at which that energy is delivered, i.e. work, or heat energy delivered by time taken (Boles and Cengel, 2004).
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