Thermodynamics 1 - Basic Concepts of Thermodynamics

Thermodynamics I 1. 2. 3. 4. 5. 6. 7.

Introduction Basic Concepts of Thermodynamics Energy, Energy Transfer, and General Energy Analysis Properties of Pure Substances Energy Analysis of Closed Systems Energy and Mass Analysis of Control Volumes The Second Law of Thermodynamics Entropy Applications Examples

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Overview – Basic Concepts of Thermodynamics 1-1 1-2 1-3 1-4 1-5 1-6 1-7 1-8 1-9 1-10 1-11 1-12 1-13

Thermodynamics and Energy Dimensions and Units Closed and Open Systems/Control Volumes Properties of a System Density and Specific Gravity State and Equilibrium Processes and Cycles Forms of Energy Energy and Environment Temperature Pressure The Manometer Barometer and the Atmospheric Pressure

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Thermodynamics and Energy • Thermodynamics is the science that primarily deals with energy. • The name Thermodynamics comes from the Greek words thermē (heat) and dynamis (power). • Classical thermodynamics: A macroscopic approach to the study of thermodynamics that does not require a knowledge of the behavior of individual particles. It provides a direct and easy way to the solution of engineering problems and it is used in this text. • Statistical thermodynamics: A microscopic approach, based on the average behavior of large groups of individual particles.

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Conservation of Energy • Energy can change from one form to the other. • Energy cannot be created or destroyed.

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Laws of Thermodynamics • Zeroth Law of Thermodynamics: If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. • First Law of Thermodynamics: – Expression of the conservation of energy. – Asserts that energy is a thermodynamic property. – The energy in the whole universe is constant. – W=Q • Second Law of Thermodynamics: States that energy has quality as well as quantity and processes occur in the direction of decreasing quality. • Third Law of Thermodynamics: At the absolute zero of temperature the entropy of a perfect crystal of a substance is zero. Thermodynamics 1

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Laws of Thermodynamics Second Law of Thermodynamics: • Whenever a temperature difference exists, motive power can be produced (Carnot). • It is impossible for a self-acting machine, by any external agency, to convey heat from a body at a low temperature to one at a higher temperature (Clausius). • The entropy of the universe goes to infinity (Clausius). • We cannot transfer heat into work merely by cooling a body already below the temperature of the coolest surrounding objects (Kelvin). • It is impossible to construct a system which will operate in a cycle, extract heat from a reservoir, and do an equivalent amount of work on the surroundings (Planck). • It is impossible for a heat engine to produce net work in a complete cycle if it exchanges heat only with bodies at a single fixed temperature (Kelvin-Planck). Thermodynamics 1

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Dimensions and Units • Dimensions are used to characterise physical quantities. • Units are arbitrary magnitudes assigned to dimensions. • Fundamental or Primary Dimensions are basic dimensions such as mass m, time t, or temperature T. • Derived or Secondary Dimensions, such as energy E or volume V, are derived from primary dimensions. Thermodynamics 1

• Seven primary dimensions and their SI units are:

• Standard prefixes in SI units

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Dimensions and Units The SI unit prefixes are used in all branches of engineering. • Force:

F = ma [ N ] • Weight

W

mg = [ N ] ( g 9.81m / s 2 )

• Specific Weight

w

ρ= g [ N / m3 ] ( g 9.81m / s 2 )

• Work

Work = Fs [ Nm] Thermodynamics 1

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Dimensional Homogeneity • All engineering equations must be dimensionally homogeneous. • In other words every term in an equation must have the same unit. • Dimensionally inhomogeneous equations are definitely wrong, but a dimensionally homogeneous equations is not necessarily right. • Example 1: E = 25kJ + 7 kJ / kg • Example 2: A tank (volume = 2 m3) is filled with oil (density = 850 kg/m3). What is the mass m of the oil?

Thus,

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V = 2m 3 ρ = 850kg / m 3 m = ρV = (850kg / m 3 )(2m 3 ) = 1700kg

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Systems That portion of the universe selected for study. • May be simple or complex • May have one or many parts • May contain one or more components • May have one or more phases • May be closed or open • May have limits or a surface, in general, boundaries, which can be fixed or moveable.

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System Boundaries • Rigid Boundary: – Prevents system from changing shape or size. • Adiabatic Boundary: – Prevents system from changing unless boundary is changed (Greek a-dia-bainein = not-through-to go) • Isolating Boundary: – Both adiabatic and rigid. • Diathermal Boundary: – Permits flow-through of heat. • Permeable Boundary: – Permits the passage of matter, otherwise impermeable.

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Closed Systems Mass cannot cross the boundaries of a closed system but energy can.

dECV dmCM 0 ≠ 0; = dt dt Thermodynamics 1

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Control Volumes - Open Systems

dECV dmCV ≠ 0; ≠0 dt dt Thermodynamics 1

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Control Volumes - Open Systems

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Systems - Examples

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Properties of a System • Any characteristic of a system in equilibrium is called a property. • Properties of a system can be either intensive or extensive. • Intensive Properties: – System size independent – ‘cannot be added up’ – Eg temperature, pressure • Extensive Properties: – System size dependent – ‘can be added up’ – e.g. volume, mass • Specific properties: – Extensive properties per unit mass Thermodynamics 1

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DENSITY AND SPECIFIC GRAVITY • Volume V is the space that a matter with the mass m fills. • Density is mass per unit volume • Specific volume is volume per unit mass.

Density

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Specific gravity: The ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4°C).

Specific volume

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State and Equilibrium • State Consider a system that is not undergoing any change. The properties can be measured or calculated throughout the entire system. This gives us a set of properties that completely describe the condition or state of the system. At a given state all of the properties are known; changing one property changes the state. • Equilibrium A system is said to be in thermodynamic equilibrium if it maintains thermal (uniform temperature), mechanical (uniform pressure), phase (the mass of two phases, e.g. ice and liquid water, in equilibrium) and chemical equilibrium. Thermodynamics 1 Chapter 1

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State Postulate • A system is described by its properties. • Once a sufficient number of properties are known, the state is specified and all other properties are known. • The number of properties required to fix the state of a simple, homogeneous system is given by the state postulate: The thermodynamic state of a simple compressible system is completely specified by two independent intensive properties.

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Processes and Cycles • Any change from one state to another is called a process.

• During a quasi-equilibrium or quasi-static process the system remains practically in equilibrium at all times.

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Processes and Cycles • In some processes one thermodynamic property is held constant. • Process Property held constant – isobaric pressure – isothermal temperature – isochoric volume System – isentropic entropy Boundary

F Water

Constant Pressure Process

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Processes and Cycles • A process (or a series of connected processes) with identical end states is called a cycle. 2 P

Process B

1 Process A

V

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Carnot Cycle

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Diesel Cycle

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Heat and Work • Heat is energy transfer that appears at the boundary when a system changes its state due to a difference in temperature between the system and its surroundings. • Work is energy transfer that appears at the boundary when a system changes its state due to the movement of a part or the whole of the boundary under the action of a force.

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Temperature • All temperature scales are based on some easily reproducible states such as the freezing and boiling points of water: the ice point and the steam point. • Ice point: A mixture of ice and water that is in equilibrium with air saturated with vapor at 1 atm pressure (0°C or 32°F). • Steam point: A mixture of liquid water and water vapor (with no air) in equilibrium at 1 atm pressure (100°C or 212°F). • The temperature scales used in the SI system today is the Celsius scale. • The absolute temperature scale in the SI is the Kelvin scale, which is related to the Celsius scale by • The magnitudes of each division of 1 K and 1 0C are identical. Thermodynamics 1

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Absolute, Gage and Vacuum Pressures Pgage = Pabs − Patm Pvac = Patm − Pabs

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Pressure • Pressure is the force exerted by a fluid per unit area. • The counterpart of pressure in solids is stress. • Unit:

1Pa = 1N/m 2 1bar = 105 Pa=0.1MPa=100kPa = 1atm 101,325Pa = 1.01325bars • The pressure inside a fluid increases with depth.

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The Manometer It is commonly used to measure small and moderate pressure differences. A manometer contains one or more fluids such as mercury, water, alcohol, or oil. Measuring the pressure drop across a flow section or a flow device by a differential manometer.

The basic manometer.

In stacked-up fluid layers, the pressure change across a fluid layer of density ρ and height h is ρgh. Thermodynamics 1

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Pressure - Example A manometer is used to measure the pressure inside a tank. The fluid has a specific gravity ρS of 0.85, and the manometer column height is 55 cm. If the local atmospheric pressure is 96 kPa, determine the absolute pressure within the tank. We assume that the gravitational acceleration is 9.81 m/s2.

ρ = ( ρ s )( ρ H O ) = (0.85)(1000kg / m 3 ) = 850kg / m 3

ρS

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mg ρ hAg P Patm + ∆= P Patm + = Patm + = = A A   1kPa kg m 96.00kPa + (850.00 3 )(9.81 2 )(0.55m)  = m s  1000 N2 m  = 100.61 kPa Thermodynamics Chapter 1

  =   30

Pressure – Basic Barometer

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