SE Mech Thermo II Chapter 2 Fuels and Combustion

7/29/2019 SE Mech Thermo II Chapter 2 Fuels and Combustion

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Fuels and Combustion

S. Y. B. Tech. Mech. Engg.

Fuels & Combustion

ME0207 SEM-IV Applied ThermodynamicsII

Applied ThermodynamicsII

S.Y. B. Tech.

ME0207 SEMIV

Mechanical Engineering


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S. Y. B. Tech. Mech. Engg.

Outline

Types of Fuels, Higher Calorific value and Lower Calorific value,

Calorimeters to measure the calorific values Bomb and

Boys calorimeters.

Calorific value at constant pressure and constant volume.

Combustion reactions- Mass and Volume basis,

Stoichiometric Air : Fuel ratio,

Exhaust Gas Analysis Orsat Apparatus and Gas Chromatography.

Actual A/F ratio, Excess Air supplied.

Gravimetric analysis and volumetric analysis.

ME0207 SEM-IV Applied ThermodynamicsII


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S. Y. B. Tech. Mech. Engg.ME0207 SEM-IV Applied ThermodynamicsII

Introduction

Fuel : A Combustible Substance, containing Carbonas a mail constituent, which, on

proper burn ing, releases large amount of heat, that can be used for domestic

and industr ial purposes.

Combustion : The atoms of Carbon, Hydrogen, etc.combine with atoms ofOxygen,

and L iberate Heat at a Rapid Rate, due to Rearrangement Of Valence

Electrons, i.e. forming New Compounds.

HeatProductsOxygenFuels


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Classification of Fuels

Fuels

Primary Fuels Secondary Fuels

Natural Fuels Derived Fuels

LiquidSolid Gaseous

Crude

Oil

Wood,

Coal,

Lignite

Natural

Gas

LiquidSolid Gaseous

Petrol,

Kerosene,

Diesel

Coke,

Charcoal

Coal Gas,

Water Gas,

Bio Gas


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Liquid Fuels

Usage

Used extensively in Industrial Applications.

Examples

Furnace Oil

Light Diesel Oil

Petrol

Kerosene

Ethanol

LSHS (Low Sulphur Heavy Stock)


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Liquid FuelsProperties

A. Density : Ratio of the Fuel Mass to its Volume @ 15 C,

Unit :( kg / m3 )

Useful for determining Fuel Quantity And Quality.

B. Sp. Gravity : Ratio of the Weight of Oil Volume to the Weight of

Equal Water Volume@ given temperature.

Specific Gravity of Water = 1!!

Measured by using Hydrometer.

Fuel oil type LDO

(Light Diesel Oil)

Furnace oil LSHS

(Low Sulphur Heavy Stock)

Specific

Gravity

0.85-0.87 0.89-0.95 0.88-0.98


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C. Viscosity : Measure ofFuels Internal Resistance to Flow.

Most important Characteristic for Storage and Use.

asTemperature .

D. Flash Point : Lowest Temperature at which a fuel can be heated so that the

Vapour Gives off Flashes when an Open Flame passes over it.

Flash point ofFurnace Oil = 66oC

E. Pour Point : Lowest Temperature at which a fuel can Flow.

Indication of temperature at which Fuel can be Pumped.


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F. Sp. Heat : kJ needed to raise temperature of1 kg of Oil by 1 C.

Unit : ( kJ / kg. C )

Indication of the Energy Required to Heat thegiven Quantity

of Oil to a Desired Temperature.

G. Calorific Value : Total Quantity of Heat Liberated, when a Unit Mass of Fuel

Burns Completely.

Unit :( kJ / kg ).Solid & Liquid Fuels.

( kJ / m3 ).Gaseous Fuels

Useful for determining Fuel Quantity And Quality.


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H igher Calori fi c Value (HCV) :

Total Amount of Heat produced when Unit Mass of Fuel is Burnt

Completely, and the Products of Combustion are Cooled to Ambient

Temperature.

i.e. Vapour Content in the Products of Combustion are Fully Condensed.

IfC Carbon %, H Hydrogen %, O Oxygen %, S Sulphur % :

kg

kJS

OHCHCV 9160

8000,43,1000,35

100

1


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Lower Calor if ic Value (LCV) :

Total Amount of Heat produced when Unit Mass of Fuel is Burnt

Completely, and the Products of Combustion are Permitted to Escape.

kg

kJHHCVLCV 2460

100

9

IfC Carbon %, H Hydrogen %, O Oxygen %, S Sulphur % :


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H. Sulphur Content : Depends Largely on Source of Crude Oil and Less on

the Refining Process.

Furnace Oil : 24 % Sulphur

Sulphuric Acid Corrosion! !

I. Ash Content : Inorganic Materials in Fuels.

Typically, 0.030.07 %.

Corrosion ofBurner Tips and Materials / Equipments

Damage@ High Temperatures.


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J. Carbon Residue : Tendency of Oil to Deposit a Carbonaceous Solid Residue

on a Hot Surface.

Residual Oil :> 1 % Carbon Residue

K. Water Content : Normally Low inFurnace Oil( < 1 % @ Refinery).

Available in Free / Emulsified Form

Can Damage Furnace Surface andImpact Flame.


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PropertiesFuel Oils

Furnace Oil L.S.H.S L.D.ODensity (Approx. g/cc at 15 C) 0.89-0.95 0.88-0.98 0.85-0.87

Flash Point (C) 66 93 66

Pour Point (C) 20 72 18

HCV (MJ/kg) 43.95 44.34 44.8

Sediment, % Wt. Max. 0.25 0.25 0.1

Sulphur Total, % Wt. Max. < 4.0 < 0.5 < 1.8

Water Content, % Vol. Max. 1.0 1.0 0.25

Ash, % Wt. Max. 0.1 0.1 0.02

Typical Specifications of Fuel Oils :

(Adapted From Thermax India Ltd.)



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Solid Fuels

Solid Fuels

Anthracite Bituminous Lignite

Hard,

Geologically the Oldest

Soft Coal,

Geologically the Youngest

SubClassif ication : 1. SemiAnthracite,

2. SemiBituminous,

3. SubBituminous, etc.


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Solid FuelsProperties

Physical Properties : Calorific Values (HCV / LCV).

Moisture Content.

Volatile Matter.

Ash Content.

Chemical Properties : Chemical Contents of :

Carbon,

Hydrogen,

Oxygen,

Sulphur.


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Higher Calorific Values :

Parameter Lignite

(Dry Basis)

Indian

Coal

Indonesian

Coal

South African

Coal

HCV (MJ/kg) 18.84 16.74 21.0 25.1

A. Moisture Content : % of Moisture in Fuel( < 0.510 % ).

Heating Valueof Fuel.

Results in Weight Loss from Heated and then Cooled

Powdered Raw Coal.

B. Fixed Carbon : Fixed Carbon = 100( Moisture + Volatile Matter + Ash )

Carbon + (H2, O2, S,N2 Residues).

Responsible for Heat Generation during Combustion.


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C. Volatile Matter : Methane (CH3), Hydrocarbons (HC), Hydrogen (H2),

Carbon Monoxide (CO), etc.

Typically 2535 %.

Easy Ignition with Volatile Matter. Results in Weight Loss from Heated and then Cooled

Crushed Coal.


D. Ash Content : Impuritythat will NOT Burn.

Typically 540 %.

Important for Design of Furnaces.

Ash Residue after Combustion.


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1. Proximate Analysis : Determines only Fixed Carbon, Volatile Matter,

Moisture and Ash.

Useful to Find HCV.

Simple Analysis Equipment.

2. Ultimate Analysis : Determines all ofCoal Component :

i.e. C,H2, O2, S, and others.

Useful for Furnace Design

(Flame Temp + Flue Duct Design)

Laboratory Analysis.

Solid FuelsAnalysis


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Solid FuelsComposition

Fuel Moisture in

Dried Sample

(ppm)

C

(%)

H

(%)

N

(%)

O

(%)

Calorific Value

(MJ / kg)

Wood 25 50 6 0.5 43.5 16.7 18.8

Peat 25 57 5.7 2 35.3 17.3 22.6

Lignite 20 67 5 1.5 20.6 27.2 29.7

Bituminous 4 83 5 2 10 33.5 35.6

Anthracite 1.5 93 3 0.7 3 36.2 36.5


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Gaseous Fuels

Advantages of Gaseous Fuels :

Least amount of Handling.

Simplest Burners Systems.

Least Maintenance.

Environmental Benefits : Lowest Emissions.

Gaseous Fuels

Naturally Found From Solid Fuel From Petroleum

Natural Gas,

Methane, etc.

LPG, Refinery Gas,

Oil Gasification

From Coal,

From Biomass,

From Industrial Wastes

From ChemicalFermentation

Processes


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Gaseous FuelsProperties

LPG : Propane + Butane +Unsaturates,

Lighter C2andHeavier C5Fractions

LPG Vapour is Denser than Air! !

Safety Issues : Leaking Gases can flow to Long Distances.

LNG : Methane (CH4)of95 %.

Remaining 5 % (Ethane + Propane + Butane + Pentane + Nitrogen +

Carbon Dioxide + other gases.

High Calorific Value.

No requirement for Storage Facility.

No Sulphur Content.

Readily mixes with Air without Smoke / Soot.


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Fuel Gas RelativeDensity

Higher HeatingValue

(MJ / Nm3)

Air / FuelRatio

(m3/ m3 )

FlameTemp.

(oC)

Flamespeed

(m / sec)

Natural Gas 0.6 39.14 10 1954 0.290

Propane 1.52 93.0 25 1967 0.460

Butane 1.96 119.3 32 1973 0.870

Fuel Oil Coal Natural GasCarbon 84 41.11 74

Hydrogen 12 2.76 25

Sulphur 3 0.41 -Oxygen 1 9.89 Trace

Nitrogen Trace 1.22 0.75

Ash Trace 38.63 -

Water Trace 5.98 -

Gaseous FuelsProperties & Comparison


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Liquid Fuels V/s. Solid Fuels

Advantages ofLiquid Fuels over Solid Fuels :

1. Higher Calorific Values.

2. Economy in Space.

3. Cleanliness of the Surrounding.

4. Easy Control in Combustion.

5. Elimination of Wear & Tear of Grate.

6. Easy Handling and Supply.

7. Easy Starting and Stopping.


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Gaseous Fuels V/s. Solid Fuels

Advantages ofGaseous Fuels over Solid Fuels :

1. Easy Production and Distribution.

2. Remote and Easy Control on Combustion.

3. Easy in Smoke and Ash Disposal.

4. Cleanliness.

5. Gasification helps for Reuse of Low Grade Solid Fuels.

6. Complete Combustion without Pollution is possible.


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Calorimeters

Calorimeter : Device used for Calorimetry,

i.e. Science ofMeasuring The Heat ofChemical reaction (or Physical

Changes) as well as Heat Capacity.

Calorimeter: Calor(Latin) meaningHeat!!

Simple Calorimeter : Thermometer attached to an Insulated Container.

Enthalpy Change,Hper mole of A

in a Reaction between A and B

Initial and Final Temperatures,T.

TCmQEnergy p ,

molesofNo

QmoleperH

.


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Bomb Calorimeter

The BOMB inside is a Steel

Vessel, capable of withstanding

High Pr. ( @ 100 atm ) of the inside

gas.

It should also capable to withstand

the Explosive Forces of the

Burning Reagents after Bombing.

BOMB Rigid Vessel.

Const. Vol. Calorimeter! !


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Principle :

A Known Mass of Fuel is Burnt.

The Quantity of Heat Liberated is

Absorbed in Water.

Thus, Quantity of Heat Produced

per Unit Mass is calculated.

Bomb Calorimeter


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Bomb Calorimeter

Heat Balance :

Heat given by the Fuel due to Combustion

+ Heat given by the Combustion of the

Fuse Wire

= Heat absorbed by the Water + Bomb

Calorimeter.

TCmmCVmHCVm Pcffwf )()()(

C i


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Bomb Calorimeter

TCmmCVmHCVm Pcffwf )()()(

FuelofMassmf

WireFuseofMassmfw

rCalorimeteinWaterofMassmw

rCalorimeteofEquivalentWatermc

RiseeTemperaturWaterT

FuelofValueCalorificHigherHCV

WireFuseofValueCalorificCV

f

fwPcf

m

CVmTCmmHCV

)()(

F l d C b i


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Bomb Calorimeter

Water Equivalent of Calorimeter :

Burning a Fuel ofKnown Calorific Value.

Standard Fuels :

1. Benzoic Acid (CV = 111.2 MJ / kg)

2. Naphthalene (CV = 170.32 MJ / kg)

To avoid Radiation Losses;

Tis Restricted to 3 C.

F l d C b ti


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Boys Gas Calorimeter

Used to measure CV of Gaseous Fuel.

Gaseous Fuel + Air mixture flows down from

the Top of the Container .

Gas Supply with

Flow Measurement

Cooling Coil

Condensate Flow

Cooling Water InCooling Water Out

Burner

Container

Air

Steamin the exhaust,due to the Combustion

of H2, Condenses over the Cooling Coils.

Outer Casing of the Container is Insulated,

to avoid Radiation Losses.

F l d C b ti


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].[

3

g' mconsumed,GasofVol.V ].[kg,circulatedWaterofMassmw

].[. C,circulatedWaterofRiseTempTw

][ waterofcm

,AtmosphereaboveGasofr.Phw

][ waterofcmPr.Barometerhb

].[ Csupplied,GasofTemp.Tg'

760

6.1315.273'

w

b

g

gg

hh

TVV

.@ NTP

F l d C b ti


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760

6.1315.273'

w

b

g

gg

hh

TVV

g

wpw

V

TCmHCV

wpwg TCmHCVV

Energy Balance :

F els and Comb stion


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Principles of Combustion

Combustion : Rapid Oxidation of a Fuel!!

Carbon : (a) CO2

(b) CO

Complete Combustion : Total Oxidation of Fuel

i.e. Adequate Supply of Oxygen required.

Air :20.9% Oxygen, 79% Nitrogen + others (by vol.)

Nitrogen : (a) Combustion Efficiency.

(b) Forms NOx@ High Temperatures.

Heat Production



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3 Tsfor Combustion Optimization :

1T) Temperature

2T) Turbulence

3T) Time


Water Vapor :

1. ByProduct of burning Fuel having H2.

2. Heat from the Flue Gases.

Oxygen :Key To Combustion!!



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Stoichiometric Air : Fuel Ratio :Perfect Amount ofOxygen

for Complete Combustion of Fuel!!

Rich Mixture :Excess Fuel!!

Lean Mixture :Excess Air (or Oxygen)!! O2in Exhaust!!

CO in Exhaust!!



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Combustion

C, H2, S + O Combustion + Heat

22 COOC

A. Carbon Carbon Dioxide :

)44()32()12( kgkgkg .Mole. Wt. basis

2COofkgOofkg3

8Cofkg1

3

112

CO2OC 22

B. Carbon Carbon Monoxide :


COofkgOofkg3

4Cofkg1

3

72



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Combustion

222 CO2OCO

C. Carbon Monoxide Carbon Dioxide :


2COofkgOofkg7

4

COofkg1 7

112

O2HOH 2222

D. Hydrogen Water / Steam :


OHOofkgH 222 ofkg98ofkg1



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Combustion

22 OSOS

E. Sulphur Sulphur Dioxide :


222 ofkg21ofkg1 SOOofkgH



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Combustion of Hydrocarbon Fuels

OHqCOpOyHCx nn 22222

.where x, y, p, & qare Const.

OHnCOnOn

HC nn 22222 12

13

Solving for x, y, p,& qin terms ofn :

OHCOOHC 2222210 11102

130

e.g. for n= 10 :

OHCOOHC 2222210 2220312



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Stoichiometric Combustion

Oxygenper kg of Fuel,

for Complete Combustion :

S

OHC

88

2

8

100

1

Air contains 23 %by mass ofOxygen.

kg of Air required, per kg of Fuel,

for Complete Combustion : 23

100

88

2

8

100

1

S

OHC

S

OHC

88

2

8

23

1



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Orsat Apparatus

Used for Volumetric Analysis ofDry Flue Gases.

3 Flasks for the Selective

Absorption ofCO2, O2 and CO.

May have 4th Flasks with

Calcium Chloride / Silica Gel to

make the Incoming Flue Gases

Completely Dr y.

% N2 = 100(% CO2+ % O2 + % CO)



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Orsat Apparatus

F lask A Solution :NaOH+ KOH

1 part of KOH

+ 2 parts of Water, by wt.

CO2 Absorption.

F lask B Solution :Pyrogalic Acid

5 gm Pyrogalic Acid in 15 cc Water

+ 120 gm KOH in 80 cc Water.

O2 Absorption.

F lask C Solution :Cuprous Chlor ide

CuO dissolvedin 20 times by wt.

ofConc. HCLtill Colourless.

CO Absorption.



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Orsat Apparatus

Initially, all 3 Valves, x, y& zClosed.

3 Way Valve is Open. Aspirator Bottle moved

on to fill the Eudiometer till ZERO Markw.r.t.

Outside Water Level.Valves

Flue Gas

Inlet

3Way Valve

Aspirator

Bottle

Eudiometer

3 Way Valve is Closed. Valve xfor Flask AOpened. Aspirator Bottle moved up-and-down

for Complete Absorption ofCO2 in the Flask.

Aspirator Bottle lowered for Flue Gases to come

back to Eudiometer. Valve xfor Flask AClosed.

Aspirator Bottle set such that Eudiometer Mark

matcheswith Water Level.

hforEudiometer Mark % CO2.



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ue s d Co bus o


Orsat Apparatus

Process repeated in Flask B for O2and Flask Cfor CO.

Remainder % N2.

Valves

Flue Gas

Inlet

3Way Valve

Aspirator

Bottle

Eudiometer

Order for Selective Absorption : 1. CO2.

2. O2.

3. CO

Order for Selective Absorption

type of Absorber implemented.

Eudiometer Capacity = 100 cc

Water Bath : To maintain Temp. Const.



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Orsat Apparatus



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Example 1A coal sample has a following composition by mass :

C = 90 %, H = 3 %, S = 1 %, O = 2 %, N = 2 %, and the remaining is ash.Find the HCV and LCV of the fuel.

kg

kJ

SO

HCHCV

1.35524

6.915.3932500,31

191608

23000,43,190000,35

100

1

91608

000,43,1000,35100

1

.ANS



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Example 1.cntd.

kg

kJ

HHCVLCV

9.34859

24603100

91.35524

2460100

9

.ANS



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Example 2The following observations were made during the test for finding the lower calorific value

of a solid fuel with the help of Bomb Calorimeter :Mass of fuel in crucible = 0.78 gm.

Mass of fuse wire = 0.02 gm.

CV of fuse wire = 6500 kJ / kg.

Mass of water in the calorimeter = 1.88 kg.

Water equivalent of calorimeter = 0.37 kg.

Temp. rise = 3 C.Assume the fuel contains 90 % Carbon, 4 % Hydrogen, the rest being ash.

kg

kJHCV

HCV

HCV

TCmmCVmHCVm Pcffwf

98.36066

325.2187.4100013078.0

3187.437.088.16500

1000

02.0

1000

78.0

)()()(

.ANS



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kg

kJ

HHCVLCV

38.35181

24604100

998.36066

2460100

9

.ANS

Example 2.cntd.



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Example 3The following observations were made during the test for finding the calorific value of a

gaseous fuel with the help of Boys Gas Calorimeter :Gas burnt = 60 lit.

Gas Pressure = 4 cm of water above atm.

Barometer reading = 750 mm of Hg.

Temp. of gas = 30 C.

Water circulated through the calorimeter = 20 kg.

Temp. rise for water = 10 C.Condensate collected during the test = 60 gms.

Find the HCV and LCV of the fuel at NTP.

3

'

05356.0

.56.53

760

6.13

40750

)3015.273(

15.27360760

6.1315.273

m

lit

hh

TVV

w

b

g

gg



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kg

NTP@gasofmperformedCondensate3

12.1

05356.01

100060

Example 3.cntd.

NTPm

kJ

V

TCm

HCVg

wpw

@15634

0535.0

10418720

3.ANS

kg

kJ

vapourby waterawaycarriedHeatHCVLCV

8.12878

246012.115634

.ANS



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Thank You !

Documents

SE Mech Thermo II Chapter 2 Fuels and Combustion