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ThermodynamicsChapter 1

Some Introductory Comments

國立成功大學工程科學系

The Simple Steam Power Plant


1.1

Figure 1.1 Schematic diagram of a steam power plant.


A schematic diagram of a recentlyinstalled steam power plant is shown inFig1.1. High pressure superheated steamleaves the drum and enters the turbine. Thesteam expands in the turbine and in doing sodoes work, which enables the turbine todrive the electric generator.


The steam, now at low pressure, exitsthe turbine and enters the heat exchanger,where heat is transferred from the steam(causing it to condense) to the coolingwater. The pressure of the condensateleaving the condenser is increased in thepump, enabling it to return to the steamgenerator for reuse.


Figure 1.4 Schematic diagram of a power plant.


When a conventional power plant isviewed as a whole, as shown in Fig1.4,fuel and air enter the power plant andproducts of combustion leave the unit.There is also a transfer of heat to thecooling water, and work is done in theform of the electrical energy leaving thepower plant.


The overall objective of a power plantis to convert the availability of the fuelwork (in the form of electrical energy) inthe most efficient manner, takingc o n s i d e r a t i o n c o s t , s p a c e , a n denvironmental concerns.


The Vapor-Compression Refrigeration Cycle


1.3


Figure 1.6 Schematic diagram of a simple refrigeration cycle.

A simple vapor-compression refrigerationcycle is shown schematically in Fig. 1.6. Therefrigerant enters the compressor as a slightlysuperheated vapor at a low pressure. It thenleaves the compressor and enters thecondenser as a vapor at some elevated pressure,where the refrigerant is condensed as heat istransferred to cooling water or to thesurroundings.


The refrigerant then leaves thecondenser as a high-pressure liquid. Thepressure of the liquid is decreased as itflows through the expansion valve, and isvaporized in the evaporator as heat istransferred from the refrigerated space.This vapor then reenters the compressor.


The Gas Turbine


1.6


Figure 1.11 A turbofan jet engine.

The basic operation of a gas turbine issimilar to that of the steam power plant,except that air is used instead of water.Fresh atmospheric air flows through acompressor that brings it to a highpressure. Energy is then added byspraying fuel into the air and igniting it sothe combustion generates a high-temperature flow.

This high-temperature, high-pressuregas enters a turbine, where it expandsdown to the exhaust pressure, producinga shaft work output and (or) dischargingthe exhaust gases at high velocity in theprocess.

The Chemical Rocket Engine


1.7

Figure 1.12 Simplified schematic diagram of a liquid-propellant rocket engine.


Fig 1.12 shows a simplified schematicdiagram of a liquid-propellant rocket. Theoxidizer and fuel are pumped through theinjector plate into the combustionchamber where combustion takes place athigh pressure.


The high-pressure, high-temperature,products of combustion expand as theyflow through the nozzle, and as a resultthey leave the nozzle with a high velocity.The momentum change associated withthis increase in velocity gives rise to theforward thrust on the vehicle.


Thermodynamics plays a vital rolein the analysis, development, anddesign of all power-producingsystems, refrigerative equipments,including reciprocating internal-combustion engines and gas turbine.


ThermodynamicsChapter 2

Some Concepts and Definitions


Thermodynamics:the science of energy and entropy.

Thermodynamics:the science deals with heat and work andthose properties of substances that bear arelation to heat and work.


Basis of thermodynamics:Experiments –

They have been formalized into the 1st, 2nd, and 3rd laws of thermodynamics.


2.1

Thermodynamics system and the control volume


System:

A thermodynamic system comprises adevice or combination of devicescontaining a quantity of matter that isbeing studied.


Open system (control volume):a properly selected region in space so that it contains the matter and devices inside a control surface.

Closed system (control mass):when a control surface is closed to mass flow,no mass can cross its boundary.

Isolated system:when the energy (e.g. heat or work) is notallowed to cross the boundary.



Figure 2.2 Example of a control volume.

Figure 2.1 Example of a control mass.


Surroundings:

everything external to the control volume, with the separation given by the control surface or boundary.


2.2Macroscopic vs. Microscopic

Point of View


Microscopic:there are molecules for a 25 ofmonatomic gas at atmospheric conditions.Thus, we must deal with equations atleast (3 position coordinates and 3 velocitycomponen t s ) . I t ’ s a qu i t e hope les stask.


Macroscopic▲ statistical approach (kinetic theory or

statistical mechanics)▲ classical thermodynamics▲ Time-averaged influence (gross or average

effects) of many molecules which can bemeasured (or experienced).


Continuum concept of the macroscopic view

This continuum model is valid when thecharacteristic length of the system underconsideration is much larger than the meanfree path (λ) of the molecules. In other words,we are always concerned with volumes that arevery large compared to molecules dimensions.For example, these are molecules of in1 at 1atm, . λ of air at atmospheric condition in ~0.1 μm


2.3

Properties and states of a substance


Phase : a quant i ty of mat te r tha t i shomogeneous throughout.

State: a state is identified or described bycertain observable macroscopic properties e.g.pressure, temperature).

Properties: a property is defined as anyquantity that depend on the state of the systemand is independent of the path (i.e. the priorhistory).


Equilibrium: it implies a state of balance.

Thermal equilibrium: same temperature.

Mechanical equilibrium: same pressure.

Thermodynamic equilibrium: when a system is in eq’m regarding all possible changes of state


Intensive properties:properties independent of the mass.

Extensive properties:properties dependent on the mass.


2.4

Processes and Cycles


Process:the path of the succession of statesthrough which the system passes.


Figure 2.3 Example of a system that may undergo a quasi-equilibrium process.


Q: how to describe the state of a system during a process when equilibrium does not exist?

A: Quasi-equilibrium process: the deviation from thermodynamic equilibrium states is infinitesimal.


For non-equilibrium processes, we are limited to the system before the process occurs and after the process is completed and equilibrium is restored.

An isothermal process is a constant –temperature process, an isobaric (constant-pressure) process.


A thermodynamic cycle:a system in a given initial state goes through a number of different change of state on processes and returns to its initial state.


2.5

Basic Units for Mass, Length, Time and Force


They are related by Newton’s second law of motion

F = ma (Metric SI system)

Where basic units are mass: kilogram (kg)length :meter (m)time :second (s)force :Newton (N)1N=1kg


Do not confuse “weight” with “mass”, the former is a concept of a force.

Table 2.1


Example 2.1

What is the weight of a one kg mass at an altitude where the local acceleration of gravity is 9.75 m/ ?

Solution:

Weight is the force acting on the mass, which from Newton’s second law is

F = mg = 1kg9.75 m/ [1N /kg m] = 9.75 N

2.6

Energy


Energy can be stored within a system and can be transferred (as heat or work for example) from one system to another.

Later in § 5.2 E = U (internal energy) + KE + PE


Identify energy from the molecular viewpoint

1. Intermolecular potential energy: forces between molecules.

2. Molecular kinetic energy: translational velocities of individual molecules.

3. Intermolecular energy: associated with the molecular and atomic structure and related force.


Take a diatomic molecule (e.g. ) forexample: there is translational energy asmolecules may travel in 3 directions, there isrotational energy as molecules may rotateabout X and Y axes, and there is vibrationalenergy as molecules vibrate along the Y axis.


Figure 2.4 The coordinate system for a diatomic molecule.


Although this course is on themacroscopic viewpoint, it is helpful tohave microscopic perspective in mindto better understand basic concepts ofthermodynamics.


Figure 2.5 Heat transfer to water.


2.7

Specific Volume and density


specific volume, : volume per unit mass (intensive property)

density, ρ: mass per unit volume (kg/ )

the smallest volume for which the mass can be considered a continuum.



Figure 2.6 The continuum limit for the specific volume.

The continuum assumption simply means thatphysical properties of fluid are distributedcontinuously throughout space. Every point inspace has limit values for properties such asveloci ty, density, pressure, e tc . Thisassumption is valid when


where L: some characteristic length of the flow

field of interest.λ: mean free path (i.e., the average distance

a molecule travels before colliding with another molecule.)

A commonly used volume unit is the liter (L). 1L = (the SI unit for volume is )


2.8

Pressure


pressure, P: the normal component of force per unit area


The SI unit for pressure : 1 Pa (pascal) = 1N/

Two other units: 1 bar = Pa = 0.1 MPa1atm (standard atmosphere) = 101325 Pa


Figure 2.9 The balance of forces on a movable boundary relates to inside gas pressure.


In most eases, we are concerned withabsolute pressure. Most pressure andvacuum gauges, however, read differencebetween the absolute pressure andatmospheric pressure existing at the gauge.This is called “gauge pressure”. Pressuredifferences are usually measured with amanometer.



Figure 2.11 Illustration of terms used in pressure measurement.


Figure 2.12 Examples of pressure measurement using a column of fluid.

Since points A and B are at the same elevation,

For distinguishing between absolute and gaugepressures, the term pascal will refer toabsolute pressure. Any gauge pressure will beindicated as in (2.2)


(2.2)

2.10

The Zeroth Law of Thermodynamics


It states that when two bodies haveequality of temperature with a third body,they in turn have equality of temperaturewith each other. It is the basis oftemperature measurement.


2.11

Temperature Scales


The scale used for measuring temperature in SI units is the Celsius scale, It is based on two fixed, easily duplicated points, the ice point and the steam point. They are designated 0 and 100 on the Celsius scale.

The absolute scale related to the Celsius scale is the Kelvin scale, K.

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