Effects of Ambient Condition on Flame Spread over a Thin PMMA Sheet

1. 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 e-mail: takahash@mech.gifu-u.ac.jp Subrata Bhattacharjee Department of Mechanical Engineering, San Diego State University, USA e-mail: subrata@voyager5.sdsu.edu

2. Background • 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. Objective • To measure the spread rate of thin PMMA sheets in normal- and micro-gravity with varying O2 level, pressure and opposed-flow velocity.

3. y Vr=Vg+Vf Lgx Pyrolysis zone Lgy x Lsy t Preheat zone Lsx Environment (e) Gas (g) esr ser gsc gsr tvap tsh tcomb sfc Solid (s) ...(i) ...(ii) Vf Control Volumes in the gas and solid phases at the leading edge

4. Thermal-regime if Vg 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. and The dominant driving force of flame spread is the conduction from the gas phase to the solid phase. ...(iii) Scales of the control volume in the gas phase where ...(iv) Scales of the control volume in the solid phase ...(v) Heat required to preheat the fuel where ...(vi) These expressions are identical to the analytical solutions of de Ris [1] and Delichatsios [4]. Substituting Eqs. (i), (iv), (v) and (vi) into Eq. (iii) and for thermally-thin fuel for thermally-thick fuel

5. The extended simplified theory (EST) (S. Bhattacharjee et al.: Proc. Combust. Inst. 26: 1477-1485) for thermally-thick fuel for thermally-thin fuel In the thermal-regime

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

7. N2 port Igniter (Ni-Cr wire) O2 port Vacuum pump port Vf Manometer port Vf Vg Vg~300mm/sec Fuel holder Fuel holder PMMA ： 30mm x 10mm x 15,50,125mm Side view camera Front view camera CCD camera CCD camera Honeycomb Fuel holder Igniter (Ni-Cr wire) Fan Air O2 Vacuum Air Fuel holder PMMA： 30mm x 10mm x 15,50,125mm Igniter (Ni-Cr wire) Apparatus for normal-gravity experiments Apparatus for micro-gravity experiments conducted with the 4.5sec trop-tower (100meter-drop) of MGLAB in Japan.

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

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

10. Spread rate in quiescent normal- and micro-gravity Spread rate in mG Spread rate in NG O2 level: 21% Pressure: 1atm Vr= Vf+ Vg Ratiative effect due to small Vf. Thermal-regime spread due to large Vf Spread rate in micro-gravity with varying opposed-flow velocity, oxygen level and fuel thickness

11. Temperature diffusion layer Mass diffusion layer The unsteady spread observed when the oxygen level is 50% and the fuel thickness is 125mm Radiative loss Scale of the temperature diffusion layer shrinks. flame 0.00sec (Ignition) 0.918sec 1.836sec 2.584sec 2.720sec 2.754sec 2.822sec 3.400sec Unsteady flame spread in micro-gravity (quiescent condition)

12. 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 50mm), the thermal-regime holds in a relatively wide range, even under a quiescent micro-gravity condition.