lecture thermodynamics chp1

Course materials provided for

personal study only.

ENS2259/ENS5259Thermodynamics

Dr Yasir [email protected]

This Lecture

�Coverage in this lecture will be derived from the textbook

•Cengel, Y. A., Turner, R. H. & Cimbala, J. M. (2008). Fundamentals of thermal-fluid sciences (3rd ed). New York: McGraw-Hill Companies, Inc.

•INTRODUCTION AND OVERVIEW (CHP 1)

•1.1 Introduction to Thermal-Fluid Sciences

•1.2 Thermodynamics

•1.3 Heat Transfer

•1.4 Fluid Mechanics

ENS2259 Thermodynamics Edith Cowan University

School of Engineering | Joondalup | Sem 2 | 2010GoTo

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•1.4 Fluid Mechanics

•1.5 Importance of Dimensions and Units

•1.6 Problem-Solving Technique

�This lecture in a nutshell

•What sciences make up thermofluids?

•The basic approaches for …

•Thermodynamics

•Heat Transfer

••Heat Transfer

•Fluids

•What the fundamental units are?

•How to effectively structure our problem solving (methodology)?

Thermofluid Sciences

�Introduction

•Thermal-Fluid sciences (thermofluids) are physical

sciences that study energy• transfer (e.g., through walls)

• transport (e.g., with fluids)

•conversion (e.g., through devices and processes)

•Modern life relies on many thermal-fluid systems



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•Modern life relies on many thermal-fluid systems•power plants

• Internal Combustion (IC) engines

•air conditioners / refrigerators

•A closer look at once such system (e.g., a radiator)

reveals that …

•Thermodynamics identifies the magnitude of heat

loss (how many kJ)

•heat transfer is used to size the radiator core

• fluids to size coolant pumps and cooling fans

Fig 1-1. Cengel, Y. A., Turner, R. H. & Cimbala, J. M.

(2008). Fundamentals of Thermal-Fluid Sciences (3rd

ed). New York: McGraw-Hill Companies, Inc.

Lets take a closer look at the three

ingredients of ingredients of

Thermofluids !

Thermal-Fluid Sciences

�Thermodynamics

•“therme”, Greek (heat)

•“dynamics”, Greek (power)

•term “thermodynamics” first used by Lord Kelvin (1849).

•thermodynamics looks at energy and transformation



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•energy transformation and its manifestation through engineering devices and systems.

•Conservation of Energy Principle

•energy changes form but its total magnitude is conserved (one cannot create or destroy energy).





�Thermodynamics

•Conservation of Energy Principle

•energy changes form but its total magnitude is

conserved (cannot create or destroy energy).

EEE outin ∆=−



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•First Law of Thermodynamics

•Expresses the conservation of energy principle.

•Energy is a thermodynamic quantity.

•Second Law of Thermodynamics

•Energy also has a quality (energy transfer

happens in the direction of decreasing quality).

•“Quality” here related to our ability to harness

energy for useful purposes.Fig 1-4 & 1.5. Cengel, Y. A., Turner, R. H. & Cimbala, J.

M. (2008). Fundamentals of Thermal-Fluid Sciences

(3rd ed). New York: McGraw-Hill Companies, Inc.


�Thermodynamics

•Classical Thermodynamics

•a macroscopic approach to studying

thermodynamics

•for example, in the case of a pressure

vessel, this approach does not look at

the behaviour of individual particles in

order to study overall pressure.



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order to study overall pressure.

•a pressure gauge can be used to infer

this behaviour

•easier, more straightforward, analyses

possible, good for engineering analysis.

•Statistical Thermodynamics

•investigates the average behaviour of

particles

•More complicated.


�Heat Transfer

•Energy transfer occurs from higher temperature mediums to those at lower temperature.

•When does this energy transfer cease?

•



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•when temperatures become the same.

•What form does this energy take?

•‘heat’


�Heat Transfer



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�Heat Transfer

•“heat” is the transfer of energy induced by a temperature differential (fluid flow is induced by a pressure differential)

•“heat transfer” looks at the rates



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•“heat transfer” looks at the ratesover which this type of energy exchange occur.

•thermodynamics will identify the magnitude of energy that will be exchanged. Heat transfer will identify the period needed for that exchange to occur to, or from, the system under investigation. Fig 1-6. Cengel, Y. A., Turner, R. H. & Cimbala, J. M.




�Fluid Mechanics (different branches)

•Stationary and moving bodies

•Fluid Statics: fluids at rest.

•Fluid Dynamics: fluids under motion.

•Hydrodynamics

•Studies moving fluids when they are

incompressible.

•



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incompressible.

•water, low speed gases.

•Hydraulics: liquid flows when applied

to pipes and open channels.

•Gas Dynamics

•High speed gas flows through nozzles.

•Aerodynamics: air flows

•around aeroplanes and cars at low as

well as high speeds.


�Fluid-Mechanics

•Jet flows: laminar CO2 (right)

•Schlieren based flow visualisations.

•Jet flows: turbulent air (left)

•Laser based flow visualisations.



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Work done at the Univ of Tasmania (Al-Abdeli, 2006)Work done at the Univ of Sydney (Al-Abdeli, 2003)


�Fluid Mechanics

•From statics

•Stress: force over unit area

•Normal component: Pressure

•Tangential component: Shear

Stress

•Shear Stresses and Pressures are



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•Shear Stresses and Pressures are

relevant to fluid as well.

•Fluid at rest

•exerts pressure

•shear stress is zero

•Fluid in motion

•Starts to develop shear Fig 1-9. Cengel, Y. A., Turner, R. H. & Cimbala, J. M.




�Dimensions and Units

•Fundamental dimensions

•Note kelvin does not have the degree

symbol (°)

•Capitalisation (Yes or No?)

•Not capitalised … if the unit is



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•used as a “word”

•Capitalised … if the unit is

abbreviated AND derived from a

proper name

•Kelvin … (K)

•No period ‘ . ‘ is used with unit

abbreviations (unless it falls at the

end of the sentence obviously). Table 1-1. Cengel, Y. A., Turner, R. H. & Cimbala, J. M.





•Secondary or derived dimensions

•Velocity (m/s)

•Energy (J)

•Volume (m3)

•Units are used to express dimensions

•



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•Different systems of units

•SI

•Decimal basis for units

•Easy

•More common

•English

•Move to phase out (most places)Table 1-2. Cengel, Y. A., Turner, R. H. & Cimbala, J. M.





•Newton: Unit of force

•1N … “force required to accelerate a mass of 1kg at a

rate of 1m/s2)”

•Specific weight:

•

onAcceleratiMassForce ×=



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••“weight per unit volume”

•g=9.807 m/s2 (sea level, 45°latitude)

•Mass and weight are not the same

•Weight measured at the top of a mountain differs to that

at sea level

•Body mass does not change.

gργ =




mgW =



•Dimensional Homogeneity

•Got to ensure terms in a thermodynamic equation have the same dimensions.

Example 1-1

Q: Where is the error in this step of calculation/equation?

kgkJkJE /725 +=



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Q: Where is the error in this step of calculation/equation?

A: You cannot add the two terms on the right (units are different)

A: The second term has not been multiplied by the unit mass.

Thermal-Fluid Science

�Suggested Problem Solving Methods

•A systematic approach is needed. The following are some recommended steps:

(1) Problem statement. Show …

-Given important information

-Quantities needed



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(2) Schematic. Provide a representative sketch

-Energy and mass interactions

-Convey relevant information on sketch

(3) State any assumptions and approximations.-If any assumptions seem weak (or questionable) assumptions, then explain

or justify them

Thermal-Fluid Science

�Suggested Problem Solving Methods

(4) Apply the relevant physical laws.-Identify regions of interest in the schematic (see step 2, above).

(5) List/calculate properties.-Indicate sources of data.



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-Indicate sources of data.

(6) Calculate-Round numbers to something reasonable.

(7) Checks.-always good to check calculations and revise assumption.

-add some comment on the result to clarify.


�Significant Digits

•Your calculations can

only be (at best) the

same accuracy as the

data give.

•Do the rounding only at



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•Do the rounding only at

the last step.

Table 1-26. Cengel, Y. A., Turner, R. H. & Cimbala, J. M.



lecture thermodynamics chp1

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