Microgravity Forward Smolder Experiments in the Space Shuttle

The Microgravity Smoldering Combustion (MSC) experiment is a study of the smolder characteristics of porous combustible materials in a microgravity environment. The objective of the study is to provide a better understanding of the controlling mechanisms of smolder for the purpose of control and prevention, both in normaland microgravity. Recently, two forward forced flow tests using air as oxidizer were conducted aboard the NASA Space Shuttle (STS-105 and STS-108 missions). A complimentary series of ground-based tests were conducted to determine the effect of gravity on the forward smolder propagation. A comparison of the tests conducted in normaland microgravity indicates that smolder propagation velocities are higher in microgravity than in normal gravity, and that there is a greater tendency for a transition to flame in microgravity than in normal gravity. A one-dimensional numerical model of the microgravity forward smolder propagation is used to predict smolder propagation velocity. This finding has important implications from the point of view of fire safety in a space-based environment, since smolder can often occur in the forward mode and potentially lead to a smolder-initiated fire. INTRODUCTION Smoldering is defined as a non-flaming, selfsustaining, propagating, exothermic, surface reaction, deriving its principal heat from heterogeneous oxidation of the fuel (i.e. direct attack of oxygen on the surface of a solid-phase fuel) [1,2,3]. If the material is sufficiently permeable, smoldering is not confined to its outer surface and can propagate as a reaction wave through the interior of the material. Smoldering is a basic combustion problem that encompasses a number of fundamental processes, including: heat and mass transfer in a porous media, endothermic pyrolysis of the combustible material, ignition, propagation and extinction of heterogeneous exothermic combustion reactions at the solid/gas pore interface [1,2,3] Smoldering presents a serious fire risk because the reaction can propagate slowly in the material interior and go undetected for long periods of time. It typically yields a substantially higher conversion of fuel to toxic compounds than does flaming (albeit more slowly), and may undergo a sudden transition to flaming. Smolder of cable insulation, another common fire hazard, is of particular concern in the space program; to date there have been a few minor incidents of overheated and charred cables and electrical components reported on Space Shuttle flights [4, 5]. Recently, with the establishment of the International Space Station, and the planning of a potential manned mission to 1 41st Aerospace Sciences Meeting and Exhibit 6-9 January 2003, Reno, Nevada AIAA 2003-987 Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Mars, there has been an increased interest in the study of smoldering in microgravity because of the need to preempt the possibility, and/or to minimize the effect of a smolder initiated fire during the operation of these facilities [4, 5]. There are two distinct classifications for onedimensional propagation of a smolder reaction: opposed and forward [1,3]. These are defined according to the direction in which the fuel and oxidizer enter the reaction zone. In forward smolder, the mode reported here, the reaction front propagates in the same direction as the oxidizer flow. This configuration is also referred to as con-current, or diffusion-flame-like smolder, because with the coordinate system anchored at the reaction zone, fuel and oxidizer enter the reaction zone from opposite directions, and generally with different velocities. The heat released by the heterogeneous oxidation (smolder) reaction is transferred ahead of the reaction by conduction, convection, and radiation, heating the unreacted fuel, but not the incoming oxidizer. The resulting increase of the virgin fuel temperature leads to the onset of the smolder reaction, and consequently gives way to its propagation through the fuel. In smolder propagation, the combustion process is generally oxygen deficient, and the propagating reaction leaves behind a char that contains a significant amount of unburned fuel [1,3]. The rate of smolder propagation is dictated primarily by a balance between the rate of heat released by the reaction and the energy required to heat the solid fuel and gaseous oxidizer to the smolder reaction temperature. Increasing the oxidizer flow rate increases the rates of fuel oxidation and heat release, and consequently the rate of smolder propagation. However in opposed flow smolder, the oxidizer velocity reaches a point at which the rate of heat losses to the incoming oxidizer overwhelm the heat released by the reaction and extinction occurs [6]. In the presence of gravity, buoyancy affects both forced convection and diffusion and, consequently, the transport processes controlling smoldering. In the absence of gravity, the transport processes lead to a secondary classification of smoldering into forced convection driven and diffusion driven smolder. In reality, with multidimensional systems, smolder propagation is always some combination of opposed and forward modes with one mode usually dominating. The MSC experiment is part of a study of the smolder characteristics of porous combustible materials in a microgravity environment. The aim of the experiment is to provide a better fundamental understanding of the controlling mechanisms of smoldering combustion under normaland microgravity conditions. This in turn will aid in the prevention and control of smolder-originated fires, both in normalgravity and in a space-based environment. The microgravity experiments are conducted in the NASA Space Shuttle. Recently, two forward smolder tests were conducted aboard the NASA Space Shuttle (STS-105 and STS-108 missions), two forward forced flow tests with air as oxidizer. The MSC data are compared with normal-gravity data to determine the effect of gravity on smolder. The normal gravity experiments were conducted under exactly the same conditions as the microgravity experiments using the MSC chamber. Space-based experiments are necessary as smoldering is a very slow process and consequently its study in a microgravity environment requires extended periods of time that can only be achieved in space.