The mechanisms of flame quenching through porous media
Gaetano Continillo
Università del Sannio, Benevento, Italy
Seminar given at Pennsylvanya State College, 24 July 2001
Rome Rome
Naples Naples
Benevento
Benevento
Introduction
History of flame-arresting devices
• Sir Humphrey Davy (1814) studied how to prevent mine
explosions ignited by a miner's lamp. He used tubes, concentric circular canals, and other devices to separate the flame from the outside explosive environment. His work was the first to indicate that flames can be quenched when they encounter small
apertures or openings.
• The quenching distance was first defined as a parameter by Holm in 1932.
• Subsequently, a number of experimental investigations have been
carried out to obtain data on the quenching distances of a wide
variety of combustible mixtures.
Flame transmission through an orifice
• largely dependent on boundary conditions
• negligible pressure drop - quenching distance
• quenching distance experiments based on low- speed laminar flames free of gasdynamic effects
S: burning velocity
: thermal diffusivity
Pe SD
D
• Babkin
– extensive studies on flame propagation in packed beds (1991)
– some data available on flame propagation limits in
packed beds (1999), but no correlation to parameters of packed bed
Packed Bed Flame Arrestor
Previous Work
Characteristic length scale??
For the complex geometry of a packed bed, the open channels for flame propagation are non-uniform and only an average dimension can be defined.
Close-Packed Layers in a Bed of Spherical Particles
average channel spacing between spheres: 0.3dsphere
Questions to be answered
• What is the dominant mechanism in flame quenching through porous media?
• Are properties of the solid material important?
• Are geometrical effects important?
• Are multidimensional effects important?
• Are chemical effects important?
Investigation means
• Experiments
• 1D theory and simulation
• 2D theory and simulation
Objectives of the experiments
• To determine flame propagation limits of gaseous hydrocarbon-air mixtures in packed beds of
spherical particles.
• To study quenching limits as a function of packed bed parameters
– sphere material – sphere size
– length of packed bed
Objective of the theory
• To identify the dominant mechanism of flame quenching.
Flame strain vs. wall heat transfer
Experiment: schematic of the apparatus
Air Fuel
Mixing chamber
Flow of combustible mixture
Plexiglas tube 5-cm ID
1.8-m length
Spark ignition (open end) Flow
meter
Configuration of arresting devices
Preliminary Experiments
• Dependence of flammability limits on packing of spheres
– quenching phenomena is a local effect – length scale of importance is channel width
• Dependence of flammability limits on tube diameter
– experiments repeated in a similar tube of 10 cm ID – results in excellent agreement with 5 cm tube – Dsphere/Dtube ≤ 0.25 sufficient for uniform packed bed
Dependence of Quenching Limits on Length of Packed Bed
• 0.5 to 3 tube diameter lengths tested
• flames observed to quench almost immediately upon contact with packed bed
• flame propagation limits independent of length of packed bed
x
screen support
Packed Bed Parameters
• Materials tested
– Brass – Steel – Glass
• Sphere sizes tested
– 9.53 mm – 11.1 mm – 12.7 mm
Increasing conductivity
0 0.5 1 1.5 2 2.5 3
Propane-Air Results
downward upward Brass
Steel Glass
Brass Steel Glass
Brass Steel Glass open tube
12.7 mm sphere size
11.1 mm sphere size
9.53 mm sphere size
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Methane-Air Results
downward upward Brass
Steel Glass
Brass Steel Glass
Brass Steel Glass open tube
11.1 mm sphere size
9.53 mm sphere size 12.7 mm sphere size
• Flame propagation limits correlate to quenching distance of mixture
– 0.3dsphere approximation
• Flame quenches immediately upon entry into quenching element
– independent of bed height
• Natural convection effects negligible in packed bed
– independent of direction of propagation
• Thermal properties of packed bed have negligible influence on quenching limits
Summary of experimental results
• Thermal properties of packed bed have negligible influence on quenching limits
Can we conclude that the
dominant quenching mechanism
is NOT heat transfer to the solid?
Effect of Thermal Properties of Spheres
• Order of magnitude analysis:
– thermal diffusivities of tested materials: ~10-5 to 10-7 m2/s – time scale of passing flame front: ~10-2 sec
» depth of penetration of heat ≈ 10-1 to 10-2 mm
t
solid sphere
Flame @ 1500 K
:thermal diffusivity t:time
• The depth of penetration is large enough that solid temperature cannot increase more than few degrees Celsius during the flame transit time
1-D model vs. multi-D models
The 1-D model
• can incorporate (via correlations) heat transfer to the solid
• could incorporate (via correlations) flame strain effects
• cannot resolve flame structure in terms of flame deformation and strain
Model assumptions:
• The system is one–dimensional, time-dependent
• The fluid phase behaves like a mixture of ideal gases
• All properties, except for the gas density, are constant
• The Mach number is very small --> neglect pressure gradient terms in the energy equation, viscous dissipation, pressure deviations in the state equation
• The motion in the porous medium follows a generalised Darcy’s law
• Dofour and Soret effects are negligible.
1-D Numerical Model Analysis
1D Numerical Model Equations
• Adjust Arrhenius parameters to fit the flammability limits to experiments for the open tube
• Add solid-phase and calculate flammability limits with no further parameter adjustment
Procedure
0 0.5 1 1.5 0
1 2
Tg
Typical propagating flame
1-D Numerical vs. experimental results
• The 1-D model with heat transfer to the solid (no flame strain effects) provide qualitative and almost quantitative agreement with experiment
therefore
• Heat transfer to the solid seems to be the dominant mechanism
Outcome
2D numerical simulations are used to selectively eliminate wall heat transfer and/or wall friction effects.
Model assumptions:
• Two-dimensional, axisymmetric, unsteady
• Transport coefficients are evaluated by means of diffusion kinetic theory
• Dufour and Soret effects are neglected
• Radiation is not considered
• Single step, global reaction mechanism with Arrhenius-type kinetic law
