td03_firstlaw

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Department of Engineering Physics, Faculty of Engineering, Gadjah Mada University

(Study Programs of Engineering Physics & Nuclear Engineering)

Jl. Grafika 2, Yogyakarta 55281,(+62 274) 580882, http://www.tf.ugm.ac.id/

The First Law of

Thermodynamics

THD


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FacultyofEngineering

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First Law of Thermodynamics

Energy can neither be created nor destroyed It can change forms

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g

Energy Balances

3

energysystem

totalin theChange

SystemtheLeaving

EnergyTotal

SystemtheEntering

EnergyTotal

EEE outin

How can energy get into or out of a system?

Heat, Work and Energy Transfer with Mass

12 EE


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g

Lets Look at Closed Systems First

There is no mass transfer into a closed system The only way energy can get into or out of a

closed system is by heat transfer or work

4

EWQ outnet,innet,


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What is E equal to?

Since

5

pk EEUE

pk EEUE

Usually we dont need to worry about kineticenergy or potential energy

pkoutin EEUWQ


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How to solve problems

Draw a picture

List the data you know

Identify the goal (What do you want to solve for)

List the equations you know

Draw a Process Diagram

Solve for the unknowns

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Do some homework problems

We have covered all the material you need to doany of the problems in the book, up through 4-55.

Do the homework problems for Wednesday.

Do as many additional problems as you need to

do to feel comfortable

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Now we can move on to Control Volumes

How are control volumes different from closedsystems?

What effect does this have on the energybalance?

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Energy can flow in with the matter

We are interested in rates


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Total Energy of a flowing fluid

9

gzc

ueeue pk

2

2

The fluid possesses an additional form of

energythe flow energy (flow work)

pk eeupvepv

Methalpy

Rememberthis fromlastchapter?

pk eeh


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g

We can rewrite the energy balance

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cvee

eeii

ii Egzc

hmWgzc

hmQ

22

22

This work represents everything but

the flow work

The flow work

is included in

the enthapyterm


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This is getting pretty complicated !

Lets look at a special case

The Steady flow process

o A process during which a fluid flows through a controlvolume steadily

o Steady means no change with time

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0

0

cv

cv

E

m


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g

outin mm

022

22

e

e

eeneti

i

iinet gz

c

hmgz

c

hm WQ

ii

iiee

ee

gzc

hmgzc

hm

netnetWQ

22

22

Steady Equation

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Some common steady flow devices

13Only one in and one out


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Nozzles

Diffusers

Turbines

Compressors

Throttling Valve

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ieie

ie zzgcc

hhmWQ

2

22

Often the change in kinetic energy

of the fluid is small, and the

change in potential energy of the

fluid is small

Single Stream Steady Flow System


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Nozzles and Diffusers

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A nozzle is a device that

increases the velocity of a

fluid at the expense of

pressure

A diffuser is a device that

slows a fluid down


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ieie

ie zzgcc

hhmWQ

2

22

Is there work in this system? NO

Is there heat transfer? Usuallyit can be ignored

Does the fluid change

elevation?NO

What happened to the m?

It divided out

Nozzles and Diffusers


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20

22

ieie cchh

How can you find the mass flow rate in a nozzle?

e

ee

i

ii

v

Ac

v

Acm

In a nozzle, enthalpy is converted into

kinetic energy

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Turbines and Compressors

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A turbine is a device that

produces work at the

expense of temperature

and pressure

A compressor is a device

that increases the pressure

of a fluid by adding workto the system


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ieie

ie zzgcc

hhmWQ

2

22

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Is there work in this system? Yes!


Does the fluid change elevation? NO

Does the kinetic energy change? Usuallyit can be ignored



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Throttling Valve

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A throttling valve reduces

the fluid pressure

For example, the water

that comes into your house

goes through a throttling

valve, so it doesnt have

excessive pressure in your

home.


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ieie

ie zzgcc

hhmWQ

2

22

22

Is there work in this system? NO


Does the fluid change elevation? NO

What happened to the m? It divided out

Does the fluid change velocity? Usuallyit can be ignored

Throttling Valve


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hysics&NuclearEngineering

Throttling Valves

hin

= hout

pin > pout

For gases that are not ideal, the temperaturegoes down in a throttling valve

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Throttling Valves

What happens if the gas is ideal?

For ideal gases

o h = CpT

o But h = 0

o So T = 0o The inlet and outlet temperatures are the same!!!

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Throttling Valve

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For an ideal gas, the temperature does not

change in a throttling valve!!!


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Mixing Chamber

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Mixing two or more fluids

is a common engineering

process

Mixing

Chamber


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Mixing Chamber

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ii

iiee

eegz

chmgz

chm

netnet WQ 22

22

We no longer have only one inlet and one exit stream

Is there any work done? No

Is there any heat transferred?

NoIs there a velocity change? No

Is there an elevation change? No


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Mixing Chamber

Material Balance

28

ie

mm


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Mixing Chamber

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332211 hmhmhm

321 mmm

Energy

Balance

Material

Balance


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Heat Exchanger

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A heat exchanger is a

device where twomoving fluids

exchange heat

without mixing.


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Heat Exchangers

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Your analysis approach will depend on how you

define your system


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Heat Exchangers

Energy balance is the same as a mixing chamber,

buto Two inlets

o Two outlets

Material Balanceo Divide into two separate streams with equal inlet and

outlet flow rates

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Pipe Flow

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A steady flow model

is usually a good

approximation

Q W


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Pipe Flow

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.

Theres work going into

the pump

Theres an elevation

change


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ieie

ie zzgcc

hhmWQ

2

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Pipe Flow

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Is there work in this system? Sometimes

Is there heat transfer? Usually

Does the fluid change elevation? Sometimes

Does the kinetic energy change? Not usually


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For example..

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We need to make some assumptions

Uniform flow

o The system can change with time, but the inletconditions are constant

o Everything in the system is in the same state

o

Fluid exiting the system is at the same state as thesystem

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We need to look at our balances again

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mmmmm systemexitin 12

cvee

eeii

ii EgzchmWgzchmQ

22

22

We arent using the rate form of the

balances here. Why?


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cvee

eeii

ii EgzchmWgzchmQ

2222

pk EEUE

pk EEUE

This is the kinetic energy of

the system

This is the potential

energy of the system

Usually, they both equal 0

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cvee

eeii

ii EgzchmWgzchmQ

2222

Usually both the kinetic energy and potential energy of

thefluidare zero too

1122 umumhmhmWQ eeii

Time 1 and time 2Inlet and Exit conditions,

assuming only a single

inlet and a single exit

stream 40


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Consider a bottle filling problem

What happens to the temperature when you fill anempty tank with air?

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Consider a bottle filling problem

What happens to the temperature when you fill anempty tank with air?

o The air gets hot

o Why?

o It takes energy to push the air into the tank (flow work).That energy is converted into internal energy

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ing

For our bottle filling problem

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1122 umumhmhmWQ eeii

2mmi

2uhi

hi is bigger than ui,so

u2 is bigger than ui

That means thetemperature in the tank

is higher than the inlet

temperature


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What if

How would we handle inlet or exit conditions thatchange with time?

The best we can do at this point is to take theaverage

If we knew more, we could integrate over time

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For example

What happens to the temperature when you use

a bottle of canned air? The bottle gets cold.

Why?

It takes energy to push the air out of the can (flowwork)o That energy comes from the energy of the air that

remains in the can

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ing

But.

The air coming out of the can gets colder withtime

That means the exit conditions are not constant

1122 umumhmhmWQ eeii What conditions should you

2

12 hhhave

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