Effects of Ambient Condition on Flame Spread over a Thin PMMA Sheet Shuhei Takahashi, Takeshi Nagumo and Kazunori Wakai Department of Mechanical and Systems Engineering, Gifu University, JAPAN Subrata Bhattacharjee Department of Mechanical Engineering, San Diego State University, USA

Background To measure the spread rate of thin PMMA sheets in normal- and micro-gravity with varying O 2 level, pressure and opposed-flow velocity. Objective Spread rate over a thermally-thin PMMA sheet, where the thickness is less than 1mm, has not been investigated extensively. It is predicted that steady flame spread over PMMA in quiescent micro-gravity is achieved if the thickness is sufficiently thin.

x y sfc gsc gsr esr ser L sx L sy  L gx L gy V r =V g +V f Pyrolysis zone Preheat zone Environment (e) Gas (g) Solid (s) Control Volumes in the gas and solid phases at the leading edge VfVf t comb t vap t sh...(i)...(ii)

Thermal-regime if and The dominant driving force of flame spread is the conduction from the gas phase to the solid phase. V g is not too high to cause kinetic effect and not too low to cause radiative effect. Oxygen level is high enough to allow fast reaction. where Scales of the control volume in the gas phase Scales of the control volume in the solid phase Heat required to preheat the fuel...(vi)...(iv)...(v) Substituting Eqs. (i), (iv), (v) and (vi) into Eq. (iii) for thermally-thin fuel and for thermally-thick fuel...(iii) These expressions are identical to the analytical solutions of de Ris [1] and Delichatsios [4].

In the thermal-regime The extended simplified theory (EST) (S. Bhattacharjee et al.: Proc. Combust. Inst. 26: ) for thermally-thin fuel for thermally-thick fuel

The kinetic effect reduces the spread rate in low oxygen level. (low Da effect) The radiative loss reduces the spread rate with low opposed-flow. (high R effect) Blow offRadiative extinction Thermal-region limit Effect of Damköhler number and radiative loss on spread rate (numerical simulation) This line corresponds to the V r of 10cm/sec. Fuel: thick PMMA Fuel: thin ashless filter paper

Front view camera Side view camera Fuel holder O 2 port N 2 port Vacuum pump port Manometer port Apparatus for normal-gravity experiments CCD camera Air Honeycomb Fan PMMA ： 30mm x 10mm x 15,50,125  m Fuel holder Igniter (Ni-Cr wire) Apparatus for micro-gravity experiments conducted with the 4.5sec trop-tower (100meter-drop) of MGLAB in Japan. Igniter (Ni-Cr wire) Fuel holder Vacuum O2O2 CCD camera Air Igniter (Ni-Cr wire) VfVf VfVf Fuel holder VgVg V g ~300mm/sec PMMA ： 30mm x 10mm x 15,50,125  m

for thermally-thin fuel and for thermally-thick fuel Downward spread rate vs. fuel half-thickness in normal-gravity where

Non-dimensional downward spread rate vs. non-dimensional fuel half-thickness

V r = V f + V g Spread rate in micro-gravity with varying opposed-flow velocity, oxygen level and fuel thickness Spread rate in quiescent normal- and micro-gravity O2 level: 21% Pressure: 1atm Spread rate in  G Spread rate in NG Ratiative effect due to small V f. Thermal-regime spread due to large V f

0.00sec (Ignition) 0.918sec1.836sec2.584sec 2.720sec2.754sec2.822sec3.400sec The unsteady spread observed when the oxygen level is 50% and the fuel thickness is 125  m Radiative loss flame Mass diffusion layer Temperature diffusion layer Unsteady flame spread in micro-gravity (quiescent condition) Scale of the temperature diffusion layer shrinks.

Conclusions The prediction, EST, can accurately predict the downward spread rate in the thermal-regime throughout thin and thick regimes. Low oxygen level and low opposed-flow velocity can cause the kinetics effect and the ratiative effect, respectively, to break thermal-regime. If the fuel is very thin (less than 50  m), the thermal-regime holds in a relatively wide range, even under a quiescent micro-gravity condition.