Title: Mechanisms of ConcurrentFlow Flame Spread Over Solid Fuels Under NASA Grants NAG31611 and NCC3671 Th
1Mechanisms of Concurrent-Flow Flame Spread Over
Solid FuelsUnder NASA Grants NAG3-1611 and
NCC3-671(Thanks to CI and Boeing for travel
support)
- Linton K. Honda Paul D. Ronney
- Department of Aerospace
- Mechanical Engineering
- University of Southern California
- Los Angeles, CA 90089-1453 USA
- http//cpl.usc.edu
- Presented at the Twenty Eighth International
Symposium on Combustion - Edinburgh, Scotland, July 31 August 4, 2000
2Motivation
- Flame spread over thin solid fuels - simple
model for spreading flames (e.g. building fires) - Well defined steady properties Spread rate (Sf)
- Indicative of heat release and CO production - Sf depends on properties of fuel, atmosphere and
flow - Opposed flow flame spread - reasonably well
understood - generally steady - Concurrent flow flame spread less understood but
important for modeling fire spread in buildings,
on walls, ...
3Motivation (Continued)
- Models (Fernandez-Pello et al.) predict
inherently unsteady spread due to continually
growing flame length - Unlikely that the flame length (L) can grow
indefinitely due to heat and momentum losses - Some studies suggest steady concurrent flame
spread - Tien (1983) - experiment, thin fuel
- Fernandez-Pello (1984) - experiment, thick fuel
- Ferkul Tien (1994) - computation, thin fuel
4Hypotheses
- I. For narrow beds, flame length grows until
boundary-layer thickness (?) width (W) of
sample, where transverse heat and momentum losses
will limit flame length (L) and spread rate (Sf)
(convectively stabilized flames) - II. For wide fuel beds, radiative losses from
the fuel bed will limit L and Sf when radiative
loss heat generation rate because loss L1
whereas heat transfer to fuel bed Ln (n (radiatively stabilized flames)
5Hypothesis I - cartoon
- Boundary layer thickness (d)
- sample width
- NO.
Boundary layer thickness sample width YES !!!
6Model Flame Length (buoyant convection)
- Boundary-layer thickness (?) Nusselt (NuL)
- d LCGrL-c and NuL DGrLd, where GrLgL3/?g2
- (C, c, D, d from established literature values)
- For narrow sample widths, Hypothesis I states ?
W -
- For large W, Hypothesis II states heat generation
radiative loss from the bed. - where PlWºlg(Tf-Tv)/Wes(Tv4-T84) (Planck number)
-
7Model Flame Spread Rate (thin fuels)
Sf,con is estimated by equating total heat
transfer to fuel bed (Q) NuL?g(Tf-Tv)W to heat
transfer needed to raise the fuel bed to the
vaporization temp. ?s Cp,s ?s (Tv-T8) W
Sf For narrow widths (convectively
stabilized) For wide widths (radiatively
stabilized)
8Model - Transition Between Regimes
- Two types of transitions
- Convectively ? radiatively stabilized when L
predicted by 2 stabilization mechanisms are equal - Laminar ? turbulent at GrL 4 x 108
- (convective)
-
-
- (radiative)
9Model - Regimes
10INTERNAL APPARATUS INTERFACES
Lid
Rack/Vessel Umbilical Cable
O-Ring Seal
Mixing fan
Aluminum Clamps
Window
Thermocouple Rack
Fuel sample
Pressure Vessel
Mixed Gas / Vacuum
Test Rack
Kanthal Wire Ignition system
Vacuum
Oxidant
Partial Pressure Gas
Diluent
Mixing System
Computer
Fuel
11Test conditions
- Fuel sample Kimwipes tissue paper
- Thickness ?s?s 0.0018 g/cm2, single or double
thickness - Atmosphere 21 to 50 O2 with Ar, He, N2, CO2,
SF6 diluent - Pressure 0.25 to 3 atm
- Width 0.1 to 8.0 cm
- Length up to 120 cm
- Narrow widths, low pressures lead to extinction -
use enriched O2 levels to avoid near-extinction
conditions - Enables observation of steady spread at GrW from
300 (0.25 atm O2-He) to 3,000,000,000 (3 atm
O2-SF6) within available flame spread distance (
1.2 m) - Determine Sf,opp from downward spread over very
wide samples
12Video
Large GrW
Small GrW
13Results temperature measurements
- Distance between leading and trailing edge ?
length - Correlation with video when 900oC used for flame
edge
14Results Flame Spread Rate (air, 1 atm)
- Small width Sf W2.8 - similar to CL scaling
(Sf W3, Sfcon/Sfopp GrW1) - Large width Sf W.5 - similar to RT scaling
(SfW0, Sfcon/SfoppGrW-1/3PlW1) - Transition at GrW 30,000 - close to prediction
15Results Flame Spread Rate (dimensionless)
16Results flame spread rate (dimensionless)
- Low Grw - CL (Grw1) relation fits well for all
atmospheres
17Results Flame Spread Rate (dimensionless)
18Results flame spread rate (dimensionless)
- Low Grw - CL (Grw1) relation fits well for all
atmospheres - High Grw
- Bending towards horizontal - transition to
radiatively-stabilized flames - Compares well with radiative stabilization
prediction (W0), asymptotic limit depends on
atmosphere -
19Results Flame Spread Rate (dimensionless)
20Results flame spread rate (dimensionless)
- Low Grw - CL (Grw1) relation fits well for all
atmospheres - High Grw
- Bending towards horizontal - transition to
radiatively-stabilized flames - Compares well with radiative stabilization
prediction (W0), asymptotic limit depends on
atmosphere -
- Transition Grw between 5,000 and 200,000
depending on PlW and thus lg - fits prediction
well
21Results flame length
22Results flame length
- Low Grw follows convective stabilzation /
laminar flow prediction - High Grw follows radiative stabilization /
turbulent flow prediction for most atmospheres
23Relation between L/W Sf,con/Sf,opp
- L ? NuL ? Sf
- Combining predictions for L/W Sf,con/Sf,opp
yields
- (convective stabilization)
- (radiative stabilization)
24Results relation between L/W Sf,con/Sf,opp
25Results relation between Sf,con/Sf,opp L/W
- CL Small Grw consistent, though somewhat high
26Results relation between L/W Sf,con/Sf,opp
27Results relation between Sf,con/Sf,opp L/W
- CL Small Grw consistent, though somewhat high
- CT never close
-
28Results relation between L/W Sf,con/Sf,opp
29Results relation between Sf,con/Sf,opp L/W
- CL Small Grw consistent, though somewhat high
- CT never close
- RT
- Small Grw - similar to CL but Sfcon/Sfopp
predictions dont match - Large Grw good agreement
30Results relation between Sf,con/Sf,opp L/W
- CL Small Grw consistent, though somewhat high
- CT never close
- RT
- Small Grw - similar to CL but Sfcon/Sfopp
predictions dont match - Large Grw good agreement
- Moral only CL at low GrW, RT at high GrW are
consistent with BOTH measured Sf and L data -
31Regimes of upward flame spread
32Conclusions
- Concurrent-flow flame spread is not inherently
unsteady accelerating - transverse
conductive/convective losses and surface
radiative losses limit spread rate over
sufficiently long samples - Predictions of simple model are generally
consistent with experimental results for upward
flame spread over thermally-thin fuels over
107-fold range of GrW. - Future work
- Forced flow
- Thermally-thick fuels
33Backup 1 Flame Length
forced-convection flame spread ?LAReL-a and
NuLBReLb buoyant-convection dominated spread
?LCGrL-c and NuLDGrLd, where GrL?gL3/?g2
Hypothesis I states ?W (forced
convection) (buoyant convection)
Hypothesis II states that Q(NuL?g/L)(Tf-Tv)
equals the radiative loss from the bed
(H)??(Tv4_T84) (forced convection)
(buoyant convection)
34Backup 2 Flame Spread Rate (thin fuels)
Sf,con is estimated by equating Q to the rate of
fuel bed enthalpy increase Substitution for
Each Type Yields - Sf/Sf,opp
35Backup 3 Transition Between Regimes
Convectively-Stabilized Flames (buoyant)
(forced) Radiatively-Stabilized
Flames (buoyant) (forced) Transition
Occurs when Lengths are Equal (buoyant)
(forced)