An Experimental Study of Flame Spread Through a Free Stratified Fuel/Air Mixture

1
An Experimental Study of Flame Spread Through a
Free Stratified Fuel/Air Mixture

• Fred Hovermann
• Rowan University
• College of Engineering
• Glassboro, NJ
• January 31, 2003

2
OverviewFlame Propagation Through Free Layers

• Background
• Experimental Configurations
• CFD Modeling
• Flow Field Characterization Tests
• Combustion Tests

3
Flame PropagationUniform Fuel Mixtures

• Flame propagation through uniform, pre-mixed fuel
  systems has been widely studied

• Laminar Flame Speed
• Fundamental combustion parameter
• Depends on fuel mixture properties
• Typically 40 cm/s

4
Flame PropagationNon-uniform Mixtures

• Relatively Little Research
• Practical Situations
• Chemical Spills
• Auto/Aircraft Crashes
• Underground Mining Accidents

F O
V
5
Purpose of ResearchFlame Propagation Through
Free Layers

• Understand properties of flame spread through
  layered fuel systems
• Fundamental Research
• Microgravity
• No prior research in 0-g
• Buoyancy affects flow field
• NASA interest fuel leak in microgravity

6
Goals of ThesisFlame Propagation Through Free
Layers

• Develop 2D non-reacting model of proposed
  geometry
• Build the apparatus
• Run combustion tests
• Analyze flame structure, spread rate

7
Project History NASA Layers

• Research began in 1996
• Rowan University joined in 1998
• Experiments
• Porous Plate Fuel Floor Configuration
• Fuel-emitting Airfoil Configuration
• Numerical Modeling
• 2-D transient chemically reacting flow model
  developed
• 2-D cold flow model

8
Porous Plate Configuration

• Fuel pool diffusing through porous bronze plate
• Experiments conducted in 1-g and mg
• Model reproduces experimental observations
  (qualitatively)

9
Prior ResearchFlame Propagation Through Free
Layers

• Phillips (1965)
• Triple flame propagating 4.5x higher than laminar
  flame speed.

10
Prior ResearchFlame Propagation Through Free
Layers

• Hirano, Suzuki, Mashiko, Iwai (1976)
• Numerical model
• Propagation velocity varied along the fuel
  concentration gradient.

11
Free-Layer ConfigurationPurpose

• Eliminate heat transfer and flow effects
• Simulates fuel leak on board a spacecraft
• Possibility of stabilizing a stationary flame

Inside Dimensions 31.00 x 4 x 4.25
12
Free-Layer ConfigurationAirfoil Design

• NACA 0012 Airfoil
• Porous bronze
• Internal cavity
• Heated
• 3 in. chord length
• Fuel fed through airfoil
• Installed in porous plate gallery
• First attempts to ignite unsuccessful

13
Experimental Apparatus

• Straight, Diverging, or Converging
• Instrumentation Ports
• Coanda Air Inducer
• Honeycomb and Screens

Side view schematics of duct
14
Free-Layer ConfigurationExperimental Apparatus
Igniter
Airfoil
Coanda Air Inducer
Thermocouple
Inlet
Heaters
15
Instrumentation
Instrument Type Use
Thermocouple Type K, exposed end, .020 in. sheath Temperature profile scans
Thermocouple Type T, .020 in. sheath Airfoil temperature measurements
Smoke wire Chromel wire, .002in. (soldering flux paste to produce smoke) Flow visualization
Hotwire Anemometer TSI Model 1210, 6 in. stem length Velocity profile scans
Rainbow Schlieren System Filter sizes 900 mm x 50 mm, 1950 mm x 50 mm Fuel concentration visualization
Camera COHU Model 2222-2040 Side view
Camera Panasonic GP-KR222 Top view
Heaters Watlow Firerod cartridge heater (x4), 125 in. in diameter x 2 in. long, 100W/120V rating Heat airfoil
Air inducer Coanda Pull airflow through duct
Rotameter Key instruments model GS8000 Control fuel flow into airfoil
Igniter Kanthal wire, .0142 in Ignite flame
16
CFD ModelingFree-Layer Configuration

• CFD model developed to better understand fuel-air
  mixture in wake of airfoil
• Mixture property contours
• Optimal ignition location
• FLUENT 5/6
• Gambit 2

Contours of Velocity Magnitude 10 cm/s inlet
flow
17
Operating/Modeling Conditions

• Air Stream Temperature 293 K
• Airfoil Surface Temp. 293-338 K
• Stream Velocity 5 to 40 cm/s
• Re350-2800 (based on hydraulic diameter of duct
  )
• Fuel Used Ethanol

18
CFD Modeling Results

• Contour Plots
• Equivalence Ratio (Custom Field Function)
• Mole Fraction
• Velocity Magnitude
• Temperature
• X-Y Plots
• Equivalence Ratio vs. Y-Position (Post
  Processing)
• Surface Integrals
• Flow Rate Mass Fraction C2H5OH

19
X-Y Plot Equivalence Ratio
20
Buoyancy Effects
Ethanol Mole Fraction Contours
10 cm/s Inlet 0-g
10 cm/s Inlet 1-g
21
Cold Flow Testing

• Velocity Scans
• TSI 1210 Hotwire Anemometer
• Temperature Scans
• Type K Exposed End Thermocouple
• Flow Visualization
• Smoke Wire

22
Velocity ProfileInlet
23
Velocity ProfileDownstream of airfoil
24
Temperature ProfileDownstream of airfoil
25
Smoke Tests
37 cm/s inlet velocity
3 in.
26
Combustion Tests

• Fuel Vapor Visualization
• Verify existence of fuel plume
• Ignition Tests
• Flame Structure
• Flame Spread Rate

27
Fuel Vapor Visualization

• Rainbow Schlieren System
• Rainbow Filter
• Refractive Index Gradient

28
Fuel Vapor VisualizationRainbow Schlieren System
29
Ignition Tests

• Difficult to ignite
• Different igniter positions
• Different igniter type
• Range of inlet velocities/airfoil temp
• 26 Total tests
• 17 Successful ignitions
• 10 Spread rates obtained

30
Flame Spread Through Free Layers
Video fields Dt 1/60s
5 cm
31
Flame StructureSide View

• Triple Flame Structure
• Upper/Lower branches
• Trailing flame
• along center

32
Flame StructureSide View
33
Flame StructureTop View
34
Flame Spread Through Free Layers
Video fields Dt 1/60s
5 cm
35
Flame Spread Rate
36
Summary of Spread Rates
Test Port Flowrate (cm/s) Airfoil Internal Temp (ºC) Airfoil Surface Temp (ºC) Spread Rate (cm/s) Relative Spread Rate (cm/s)
10-23 2 1 36.6 70.3 45 148.31 184.91
12-11 1 1 (side) 40 66 45 148.53 188.53
12-11 3 1 40 82.2 61 195.31 235.31
12-11 4 1 40 83.9 65 136.38 (avg.) 176.38
12-12 1 1 40 83.1 66 186.6 226.6
12-12 2 1 30.6 83 65 174.4 205
12-12 4 1 27.6 83.5 67 217 244.6
12-12 7 2 40 80.6 55 142 182
12-17 1 2 36.6 83.2 60 134 (avg.) 170.6
12-19 2 2 40 75 60 160.3 200.3
37
Spread Rates vs. Airfoil Temperature
38
Flame Location/Shape Predictions

• Develop expression for Laminar Flame Speed as
  function of Equivalence Ratio and Temperature
• Resolve flow velocity components normal to
  estimated flame front
• Stationary flame can exist where flow velocity
  balances flame speed

39
Laminar Flame SpeedUniform Mixture
(Gaussian curve fit from SigmaPlot)

• Ethanol Data from Egolfopoulos and Law (24th
  Symposium on Combustion)

40
Flame Location/Shape
41
Summary/Conclusions

• 2D cold flow model developed
• New flow duct designed and constructed
• Flow characterization tests performed
• Cold flow tests agree with Fluent
• It is possible to ignite a flame in free layer
  conditions
• Tests showed mostly linear spread rates
• Average spread rate 164 cm/s
• Spread rate response to temperature/fuel
  concentration changes inconclusive

42
Suggestions for Future Work

• Redesign airfoil
• No internal cavity
• Redesign fuel delivery
• Manifold to distribute fuel evenly
• Widen duct
• Eliminate possible wall/boundary effects

43
Acknowledgments

• Dr. Anthony Marchese
• Thesis Advisor Rowan University
• Dr. Fletcher Miller
• Co-Thesis Advisor NCMR
• John Easton NCMR
• NASA, NCMR, Rowan University

44
(No Transcript)
45
(No Transcript)
46
Hotwire Calibration Curve
47
Hotwire CalibrationFluent Correction
48
Duct Velocity Calibration
49
Fluent Mesh

• Gambit 2.0
• 12000 quadrilateral cells

50
Velocity ScanUpstream of airfoil
51
Velocity ScanZ-direction
52
Velocity Components
VP
UN
V
Flame front
V
U
Centerline of duct
53
Single Stationary Flame Point
54
Prior ResearchFlame Propagation Through Free
Layers

• U.S. Bureau of Mines (1970)
• Flames propagated into regions below lean
  flammability limit.
• Flame speed varied with surface roughness