FLAME AERODYNAMICS COMBUSTION AND FUELS

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Annabel Logan
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1 FLAME AERODYNAMICS

2 IMPORTANCE OF AERODYNAMICS IN COMBUSTION Fuel Heat Combustion chamber, furnace Flue gas Air Heat Flow reactor

3 Oxidizer: Fuel: Flue gas: MEDIA - air: primary, secondary sometimes tetriary - enriched air: [O 2 ] > 21%, oxygen,... - gas (mixing), oil (dispersion), dust (transport) - mixing (recirculation), heat exchange, errosion and corrosion.

4 BASIC OBJECTS OF AERODYNAMICS OF COMBUSTION Basic objects: - burners, - boiler furnaces, - combustion chambers.

5 REQUIREMENTS FOR FURNACE AND BURNER - fuel delivery to the furnace assuring required thermal power, - air delivery to the furnace assuring required stoichiometric ratio λ, - mixing of air and fuel to get proper flame pattern (shape) - keeping fuel in furnace to complete burnout.

6 MIXING IN COMBUSTION PROCESSES 1. A direct contact between fuel and oxygen is necessary for burning. 2. Mixing is a process which secures a contact between fuel and air. 3. Two mixing patterns are distinguished in the combustion processes: a. molecular mixing, b. turbulent mixing.

7 MIXING AND FLAME PATTERN 1. Depending on the flow pattern two types of flames can be distinguished: laminar flames, turbulent flames. 2. In laminar flames mixing has molecular character. 3. In turbulent flames mixing has turbulent (also molecular on small level) character.

8 NEAR BURNER AERODYNAMICS (NBA) NBA influences: - flame pattern, - fuel burnout, - pollutants, - heat transfer.

9 JETS

10 FORMATION OF FREE JET Streamlines orifice

11 STRUCTURE OF FREE JETS (fully separated) orifice transition initial range boundary layer main range

12 VELOCITY PROFILES OF FREE JETS (fully separated) Axial profile of velocity Radial profile of velocity temperature, concentration velocity

13 MIXING PATTERN DUE TO VELOCITY GRADIENT y 0 x Boundary layer

14 MIXING BY JET PENETRATION u 1 u 2 D c h s View from widok z góry the top

15 JETS IN CO-FLOWING PARALLEL STREAMS ANNULAR AND COAXIAL JETS initial range transition range main range orfice y x

16 SWIRLED FLOWS SWIRLED JETS

17 FREE SWIRLED JETS Week swirl Strong swirl 1 parallel strem in the pipe, 2 swirler, 3 swirled stream, 4 recirculation zone

18 STRUCTURE OF SWIRLED JETS radial distance, cm steam lines recirculation distance, cm

19 CONFINED SWIRLED JETS Recirculation zone

20 THE SWIRL NUMBER S S = G φ /(0.5 G xd o ) Swirl number S is a non-dimensional characteristic of rotating flow G x is the axial thrust G x = R uρu2πrdr + R 0 0 P2πrdr G φ is the annular momentum G φ = R 0 ( wr ) ρu2πrdr,

21 RECIRCULATION ZONE OF SWIRLED FLOW swirl generator Swirl number S oil burner Recirculation zone

22 SWIRLER β 2R h 2R

23 TURBULENCE

24 BASIC FUTURES OF TURBULENT FLOWS u = u m + u, T = T m + T

25 GENERATION OF TURBULENCE Boundary layer A wake Free jet

26 LAMINAR AND TURBULENT MIXING laminar turbulent

27 STRUCTURE OF TUBULENT FLAME Stosunek Ratio u kw/sl Re < perfect Reaktor chemical reactor doskonałego wymieszania Re = 1 Da < 1 Da = 1 zone Strefa of dispersed spalania combustion rozproszonego Płomyki folded flamelets pofałdowane Płomyki wrinkled pomarszczone flamelets Stosunek Ratio l/l l/l FF Da > 1 Ka > 1 Ka = 1 Ka < 1 flamelets Strefa płomyków zone

28 SURFACE MECHANISM OF TURBULENT COMBUSTION S T S L S T = A L S L A T A T A L

29 ISLAND MECHANISM OF TURBULENT COMBUSTION u o c = 0 c = 1

30 MECHANISM OF TURBULENT COMBUSTION Fresh mixture Products of reactions Mixing and reactions

31 THE TRANSITION FROM LAMINAR TO TURBULENT FLAME laminar flame trasition range turbulent flame envelope of flame length stream velocity height height stream velocity Variation of flame height and structure with the outlet velocity.

32 CRITICAL REYNOLD NUMBERS FOR TRANSITION FROM LAMINAR TO THE TURBULENT FLOW Re CR Gas Re CR hydrogen: 2000 town gas: CO: 5000 hydrogen + air: town gas + air: propane, acetylene: methane: 3000

33 FLAME STABILIZATION

34 FLAME POSITION Flame front S u Direction of mixture flow α U Direction of flame propagation S u = U cos(α)

35 NECESSARY CONDITION OF FLAME STABILIZATION The condition of flame stabilization in a flow field of nonuniform velocity is that there is a point in the flow field where the flow velocity is equal and opposite to the velocity of the combustion wave. gas velocity profile, v front of flame wall of burner atmosphere

36 METHODS OF FLAME STABILIZATION in boundary layer by a pilot flame by recirculating flow

37 FLAME STABILIZATION BY HOT FLUE GAS Stabilization by hot gases: - pilot flame, - recirculating flows. Recirculation: - outer, - inner.

38 RECIRCULATION ZONE OF CONFINED JETS Zone of recirculation

39 FLAME STABILIZATION BY INNER RECIRCULATION Inner recirculation can be generated by: - bluff bodies, - strong swirl.

40 AERODYNAMICS METHODS OF FLAME STABILZATION Examples of recirculating flows generators.

41 BLUFF BODY STABILIZER swirl layer ignition zone flame flammable mixture flame stabiliser products of combustion mixing zone recirculation flame substrates and products of reaction products of reaction Flow in bluff body recirculation.

42 FLAME STABILISATION ON A FLAME HOLDER

43 STABILITY LIMITS FOR BLUFF BODY STABILIZERS Fuel/air ratio Stable flame Unstable flame Airflow, kg/s COMBUSTION Stability limit AND FUELS

44 STABILITY LIMITS OF SWIRLED PULVERIZED COAL FLAME load kw unstable stable Influence of swirl no. S on the stability limits for pulverized coal flame

CHAPTER 1 INTRODUCTION

CHAPTER 1 INTRODUCTION Study of combustion process in all combustion systems is one of the most important and complex problems. Generally, the main objective is to achieve a stable combustion proves that