Chapter 1. Basic Concepts

Thermodynamic System

Properties of A System

State and Equilibrium

Process and Cycles

1.1 Thermodynamic System Definition A quantity of matter or a region in space chosen for study. For example:

-Cylinder-piston equipment Steam turbine

Surroundings The mass or region outside the system

2Boundary The real or imaginary surface that separates the system from its surroundings.

(3) The mass or energy exchange between system and its surroundings must cross the boundaries.

2. System, Surroundings and boundary

(4) Characteristics of Boundary

Boundaries are selected subjectively.

Boundaries can be fixed or movable.

Boundaries can be real or imaginary.

boiler(turbine)(generator)(pump)(condenser)(reheater)

fixed movablereal imaginary

3. Types of systemsEnergy can cross the boundary, in the form of heat and work.

(1)

A Closed system (a control mass ) consists of a fixed amount of mass, and no mass can cross its boundary. That is, no mass enters or leave a closed system. such as, Piston-cylinder device (- A. Closed system and Open system

An Open system (or a control volume is a properly selected region in space. Both mass and energy can cross the boundary of a control volume.

such as, A Water heater, a turbine and a compressor, etc

Adiabatic system is that no heat cross the boundary or heat is negligible compared with work cross the boundary Isolated system is a special case that no mass and energy cross the boundary.B. Adiabatic system and Isolated system

1 1+2 1+2+3 1+2+3+4

2

Even System) (Uneven System)(multi substance system) (Single substance system) (Multiple Phase) (Single Phase)

Simple compressible systemMoving Boundary WorkCompression WorkExpansion Work

1.2State and State Properties State: it reveals the physical condition of a system.

Consider a system not undergoing any change. At this point, all the properties can be measured or calculated throughout the entire system. It can be described by a set of properties

1, Properties are used to depict any characteristic of a system.

such as Pressure P temperature T volume Vmass m internal energy Uenthalpy entropy viscosity thermal conductivity

2. Property

2Characteristics of State Properties A. Properties of a state are determined by the state. If the state is specified, its properties are fixed, or vise versa.B. The magnitude of the change in property is independent of the path (route), but just depend on the initial and final states.

1 2abpoint function

_994085962.unknown

C. Properties are functions of state, there exist differential for properties

After a series of change, the state returns to the original one, then the changes in its properties are 0

z =z (x , y)dzTotal differentials

p T v

(1) Density and Specific VolumeDensity is defined as mass per unit volume.

3. Basic State Properties

vSpecific volume is the reciprocal of density and is defined as volume per unit mass.

Definition: It is defined as the force exerted by a fluid vertically on a surface of unit area.

(for solid is stress: person stand on foot) Unit of pressure It has the unit of Newtons per square meter N/m2) 1 Pa=1 N/m2 1 kPa= 103 Pa 1 MPa= 106 PaSI(2) Pressure

1 bar =105 Pa=100 kPa =0.1 MPa Standard atmosphere 1 atm= 101325 Pa=101.325 kPa Engineer atmosphere 1 at=1 kgf/cm2 =9.807 N/cm2 =9.807*104 Pa Height of liquid column 1 atm=760 mm Hg 1 at =10 m H2O Other units 1 mmHg=1gh =133.3 Pa

Absolute pressure is the actual pressure at a given position P .

Relative pressure indicates the difference between the absolute pressure and the local atmospheric pressure . Absolute pressure and relative pressure

Gage pressureis denoted as Pg Vacuum pressure Pressures below atmosphere pressure. It is the pressure difference between atmospheric and system pressure when system pressure is lower than atmospheric And it is denoted as PVAC or H. Gage Pressure and Vacuum Pressure=-=-

p > pb pg p < pb pvpbpeppvprelative pressureabsolute pressureGage pressureVacuum pressure

Attentions

Variation of pressure with Depths Pressure is a scalar quantity. At any point in a fluid, Pressure is the same in all directions.Pressure in a fluid increases linearly with depth.

Pressure is the same at all points on a horizontal plane in a given continuous fluid at rest. .

p Pressure Measuring U-tube manometerBourdon Tube

Environmental pressure

Atmospheric pressure 1atm = 760mmHg1mmHg = gh = 133.322Pa

A.The Manometer Manometers measure a pressure difference by balancing the weight of a fluid column between the two pressures of interest.

Pressure in a continuous static fluid is the same at any horizontal level so, For the left hand arm For the right hand armExample 1: U-tube manometer using multiple fluid column (U

It is a mechanical pressure measurement device and consists of a hollow metal tube bent like a hook whose end is closed and connected to a dial indicator needle.

C-TypeB. Bourdon tube

As shown in the following figureit is known that pb=101325Pa, the height difference is H=300mm for murcury liquid. The gauge pressure of B is 0.2543MPa,Then what is the pressure for side A, and what is the value of Pg,A?pb=101325PaU H=300mmB0.2543MPaApAApgAExercise.1 (1

Attentions () pb is the pressure of environment in which the gauge is located ()

R.W. Fowler in 1931B T 0.5 m w 2

(3) Temperature

The Zeroth Law of ThermodynamicsIf two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.Two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact. By replacing the third body with a thermometer (, the zeroth law can be restated as.

Equality of temperature is a necessary and sufficient condition for thermal equilibrium, i.e. no transfer of heat.

1931 T 18401850 E 18541855 S 1906 S

Temperature measurement Scale Temperature scaleReference state

Temperature scale Kelvin scale Celsius scale Fahrenheit scale (German, G. Fahrenheit, 1686-1736) Rankine scale (W. Rankine, 1820-1872)Temperature 273.16 K 0.01 The temperature interval on both scales is the same.

K FR100373.150.01273.160273.15-17.80-273.15212671.6737.8100032-459.670459.67491.67559.67

1: Intensive properties are those independent of the size of a system, such as temperature, pressure and density.

5.Intensive and Extensive Properties

:Extensive properties are those whose values depend on the size or extent of the system (2) m V U Hsuch as mass, volume, internal energy and enthalpy

:

Specific properties is extensive properties per unit mass.

Velocity Kinetic EnergyHeight Potential EnergyTemperature Internal Energy

(1) Definition

A system in equilibrium experiences no changes with time when it is isolated from its surroundings.

1.Equilibrium State 1.3 Equilibrium State and State Postulate

Monatomic gasesPermanent diatomic gasesPolyatomic gases

In an equilibrium state there are no unbalance potentials (or driving forces)

B. A system is not in thermodynamic equilibrium unless the condition of all the relevant types of equilibrium are satisfied.

(2)How to fulfill thermodynamic equilibrium?

(Mechanical equilibrium ) If there is no change in pressure at any point of the system with time.

Temperature differentialUnbalanced potentials Thermal equilibrium If the temperature is the same throughout the entire system. Pressure differentialUnbalanced potentials

Monatomic gasesPermanent diatomic gasesPolyatomic gases

Phase equilibrium If a system involves two phases, when the mass of each phase reaches an equilibrium level and stays there ,it is in phase equilibrium.

(Chemical equilibrium If no chemical reaction occur, that is, the chemical composition does not change with time, a system is in chemical

In an equilibrium state there are no unbalanced potentials

(3) Equilibrium & Steady ++

(4) Equilibrium & Even

The equilibrium state of a system can be described by a set of properties. However, specify a certain number of properties is sufficient to fix a state.

2. The State Postulate

The state of a simple comressible system is completely specified by two independent properties. (2) The State Postulate N= = =+= n+1

A simple compressible system is a system in the absence of electrical magnetic, gravitational motion and surface tension effects.Two independent properties: If one property can be varied while the other one is held constant. (eg.P and T are independent properties for single phase systems, but are dependent properties for multiphase systems.)

N = ?

Not all properties are independent of each other, if two independent properties are known, then other properties of the same state can be determined.

3. Equation of State p,v,T)The relationship between properties is called Equation of State (E.O.S)

Equation of stateN = 2 F(p,v,T)=0

P=f(T,v)

T=f(P,v)

v=f(P,T)

diagram N=2pv1 2 3 p-vT-s21

1.4 Process and Cycles

1.Quasi-static or Quasi-equilibrium process

1Process and path Any change that a system undergoes from one equilibrium state to another is called a process. : The series of states through which a system passes during a process is called the path of the process.

(2)Quasi-static or Quasi-equilibrium process

Processp1 = p0+pTp0T1 = T0p2 = p0T2 = T0pv12..

Quasi-static processp1 = p0+pTp0T1 = T0pv12...

When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times, it is called a quasi-static or quasi-equilibrium process.

Is it applicable?

>> Relaxation timeA quasi-equilibrium process can be viewed as a sufficiently slow process.

2000/ 2/0.15/=20002 0.15/60=10 m/s350 m/s>>

3ppfpA = pA +fA pdlmkgmkgW = pA dl =pdV1kgw =pdvdl p Moving Boundary Workpp

pp2mkgW =pdV1kgw =pdv1

indicator (p-V) diagrampV.12.pp21mkgW =pdV1kgw =pdvW

pV.122 p-V 3 Path function4dV>0 dV

Friction Lossf pp21WW

Friction Loss f 0pp21WW WW

2. Reversible process 1Definition A process that can reversed without leaving any trace on the surroundings. That is, both the system and the surroundings are returned to their initial states at the end of the reverse process.

Both the system and the surroundings are returned to their initial states at the end of the reverse process.

The net heat and net work exchange between the system and the surroundings is zero for the combined original and reverse process.

For example, suppose we have a thermally insulated cylinder that holds an ideal gas. The gas is contained by a thermally insulated massless piston with a stack of many small weights on top of it. Initially the system is in mechanical and thermal equilibrium. Consider the following three processes:A. All of the weights are removed from the piston instantaneously and the gas expands until its volume is increased by a factor of four (a free expansion).B. Half of the weight is removed from the piston instantaneously, the system is allowed to double in volume, and then the remaining half of the weight is instantaneously removed from the piston and the gas is allowed to expand until its volume is again doubled.C. Each small weight is removed from the piston one at a time, so that the pressure inside the cylinder is always in equilibrium with the weight on top of the piston. When the last weight is removed, the volume has increased by a factor of four.

(2) How to fulfill reversible process?

The temperature and pressure difference between the working fluid and its surroundings should be infinitely small.

No internal or mechanical friction is allowed. ---------no dissipative effect. This requires that the working fluid goes through a continuous series of equilibrium states. A reversible process should satisfy the following criteria:

+ = ( irreversibilityDissipative effect

Dissipative effect

The factors that cause a process to be irreversible are called irreversibilities. Such as heat transfer across a finite temperature difference Temperature difference is the driving force of heat transfer. However, a heat transfer process becomes less and less irreversible as the temperature difference between the two bodies approaches zerofriction, The more friction force involved, the more irreversibility the process is..Friction can occur between two solid bodies, and also between solid and a fluid, even between the layers of a fluid moving at different velocities. 3Irreversibilities

Heat transferT1T2T1>T2Q (p1p2p1>p2Frequently encountered irreversibilitiesThrottler

Frequently encountered irreversibilitiesUnrestrained expansionMixing process

electric resistance inelastic deformation of solid

chemical reactions.

Quasi-equilibrium process is an internally reversible processwhile reversible process is a totally reversible process.

Dissipative effects are not allowed in reversible process. 4 The distinction between quasi-equilibrium and reversible process

Reversible process must be quasi-equilibrium process. Howeverquasi-equilibrium process is not definitely reversible process. In brief, reversible process is a quasi-equilibrium process without dissipative process.

5)

1-5 Work1. Definition of Work of MechanicsThe product of a force and the distance through which this force acts But

2. I Work is done by a system if the sole effect on the surroundings could be the raising of a weight.

Work is an energy interaction between a system and its surroundings, if the energy crossing the boundary of a closed system is not heat, it must be work.

1-6 Heat and EntropyHeat is defined as the form of energy that is transferred between two systems (or its surroundings) by virtue of a temperature difference. 1.

2.

p T dV , dv dS , ds

Entropyreversible [kJ/kg.K]ds: qrevT [kJ/K]

1 3 2 Q > 0 dS > 0 Q < 0 dS < 04

pVWTSQ ()

The work input to a system during a reversible processis: W= Marked area on the P-V diagram. The heat supplied to a system during a reversiblprocess is: Q= Marked area on the T-s diagram. 3 . Work and heat in reversible process

1-5 Cycle(Definition) A system is said to have undergone a cycle if it returns to its initial state at the end of the process

Cycle and process

(1)pVTS (Clockwise cycle)2112

p T v s

1

2

3

4

5

6

7

8

Clockwise cycle ()The cycle proceeds clockwise () 1-2-3expand and do work on the surrounding 3-4-1, compressed and work is done on the system (P-v1-2-3 3-4-1)

The net work is positive and heat is converted to work.(wnet=>0),T-s:5-6-7absorb heat from the high temperature reservoir( qq ) 7-8-5reject heat to low temperature reservoir (qq) The net heat flow ()is qnet=qq-qq, and wnet=qn

pVTS (Counter-clockwise Cycle)2112

WT1Q1Q2T2 Thermal Efficiency

WT0Q1Q2T2 Coefficient of Performance

Refrigerator A refrigerator is a heat engine in which work is done on a refrigerant substance in order to collect energy from a cold region and exhaust it in a higher temperature region, thereby further cooling the cold region.

Coefficient of performance (COP):

Heat Pump A heat pump is a device which applies external work to extract an amount of heat QC from a cold reservoir and delivers heat QH to a hot reservoir. A heat pump is subject to the same limitations from the second law of thermodynamics as any other heat engine and therefore a maximum efficiency can be calculated from the Carnot cycle. Heat Pumps are usually characterized by a coefficient of performance which is the number of units of energy delivered to the hot reservoir per unit work input.

Summary System Equilibrium State Quasi-static process Reversible process Work, heat and Entropy p-VT-S p-V and T-S Diagram Cycle

Reading and Review

Book of English version ()Sections 1.3~1.6 on Page 8~14( 8~14 1.3~1.6Sections 1.10~1.12 on Page 25~37 (25~37 1.10~1.12 section 5.7 on Page265~269. 265~269 5.7 Book in (Chinese version) Chapter 1

End of Chapter 1