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Combustion and Flame 000 (2015) 1–6

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S on fl ompu ver t e, wh ced fl 1 m fl t f g c b fl f i t k t a p p p t t h 0he critical flow velocity for radiative ext pread in a microgravity environment: A omputational, and theoretical results ubrata Bhattacharjeea,∗, Aslihan Simseka, Sandra Ol

Mechanical Engineering, San Diego State University, San Diego, California 92182, USA

NASA Glenn Research Center, 21000 Brookpark Rd, Cleveland, Ohio, 44135, USA r t i c l e i n f o rticle history: eceived 28 July 2015 evised 16 October 2015 ccepted 18 October 2015 vailable online xxx a b s t r a c t

The effect of opposing flow theory, a comprehensive c

Station. While spread rate o in the microgravity regim absence of buoyancy indueywords: lame spread icrogravity lammability xtinction velocity adiative extinction pace station

Under certain conditions, this c combines scaling arguments w extinction. Results from the rece seem to confirm the prediction v © 2 . Introduction

Opposed-flow flame spread over thermally thin fuels is one the ost fundamental topics in the study of fire spread. The physics of ame spread is considerably simplified in this configuration because he flame spreads steadily and the fuel can be assumed to be uniormly heated across its thickness. Moreover, in the thermal regime, as-phase and pyrolysis chemistry can be considered infinitely fast ompared to the residence time tres ≈ Lg/Vg ≈ αg/V2g , the time spent y the oxidizer as it passes through the length Lg ≈ αg/Vg of the ame leading edge, producing a simplified closed-form expression or the thermal limit [1,2] of the flame spread rate. The spread rate s independent of flow velocity Vg, inversely proportional to the fuel hickness, and directly proportional to a non-dimensional coefficient nown as the de Ris coefficient F = (Tf − Tv)/(Tv − T∞), where Tf is he adiabatic flame temperature, Tv is the fuel vaporization temperture, and T∞ is the ambient and virgin fuel temperature. As the oposed flow velocity is increased, the residence time being inversely roportional to the square of velocity, finite-rate kinetics in the gas hase becomes important leading ultimately to the blow-off extincion. This kinetic regime has been experimentally [3] and computaionally [4] studied and the spread rate, normalized by its thermal ∗ Corresponding author. Fax: +1 619 594 3599.

E-mail address: prof.bhattacharjee@gmail.com (S. Bhattacharjee). l b s c v m t h v fl fl a q [ w m o

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T s ttp://dx.doi.org/10.1016/j.combustflame.2015.10.023 010-2180/© 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Please cite this article as: S. Bhattacharjee et al., The critical flow in a microgravity environment: A comparison of experimental, compu http://dx.doi.org/10.1016/j.combustflame.2015.10.023and Flame r.com/locate/combustflame tion in opposed-flow flame parison of experimental, b, Paul Ferkulb ame spread rate over thin solid fuel is investigated with the help of scaling tational model, and experiments conducted aboard the International Space hin fuels is independent of the opposing flow velocity in the thermal regime, ere the opposing flow can be very mild or even completely absent in the ow, the spread rate is known to decrease as the opposed flow is reduced.an lead to flame extinguishment at a low enough flow velocity. This paper ith computational results to predict a critical flow velocity for such flame ntly conducted limited number of space based tests, presented in this paper, alidating the closed-form formula for the critical extinction velocity. 015 The Combustion Institute. Published by Elsevier Inc. All rights reserved. imit, has been correlated to the non-dimensional Damkohler numer, the ratio of the chemical and residence time. The downward pread in a quiescent 1-g environment can be considered a special ase of opposed-flow flame spread with buoyancy induced flow proiding the opposing flow velocity [5].

In a microgravity environment, the opposing flow can be very ild and even completely absent in the perfectly quiescent situaion of zero gravity. Numerical [6,7] and experimental [8,9] studies ave established the radiative regime in the mild opposed-flow enironment in which the flame spread rate decreases as the opposedow velocity is reduced leading to flame extinguishment [10] if the ow velocity is sufficiently low. This result is also dependent on other mbient conditions as steady spread over thin fuels in a perfectly uiescent environment has been established at higher oxygen levels 11]. The critical velocity, defined as the opposed-flow velocity below hich steady spread rate is not observed in a microgravity environent, has been shown to depend on oxygen level, but its dependence n other parameters such as fuel thickness are still not well known. here is no closed-form formula, verified by experimental results that an be used to predict the critical velocity.

In this work, recent experimental work for flame spread over thin heets of PMMA performed in the International Space Station is reorted. The experimental results are analyzed to determine the critial velocity for different fuel thicknesses and ambient oxygen levels. he results are compared with predictions from a simplified analyis and computational results from a two-dimensional steady-state velocity for radiative extinction in opposed-flow flame spread tational, and theoretical results, Combustion and Flame (2015), 2 S. Bhattacharjee et al. / Combustion and Flame 000 (2015) 1–6

ARTICLE IN PRESS

JID: CNF [m5G;November 17, 2015;21:41]

Nomenclature c specific heat at constant pressure, kJ/kg·K

F de Ris flame coefficient

L length scale, m

T∞ ambient temperature, K

Tv fuel vaporization temperature, K

Tf adiabatic flame temperature, K t time, s

Vg velocity of the oxidizer, m/s