The purpose of this study was to extend the scope of application of existing fire field models to reduced gravity (and hence buoyancy) scenarios.
Space station Alpha (SSA) is a joint USA, European, Russian and Japanese endeavour aimed at establishing a permanent presence in low earth orbit within the next 10 years. The main components of the SSA will consist of a cluster of modules, known as Attached Pressurised Modules (APM), and a series of interconnecting nodes and airlocks. The APMs will consist of a crew module for long-duration human habitation, a supply module and three laboratory/workshop modules. The European component, contributed by the European Space Agency (ESA) will be the laboratory module COLUMBUS.
A major concern in manned spaceflight is the possibility of an undetected fire developing within instrumentation or experimental racks and the influence of the Environmental Control and Life Support System (ECLSS) upon the transport of the fire signature. Fire is one of the most serious threats to the manned exploitation of space. While the space industries record concerning spacecraft fires is good, terrestrial experience in similar high risk environments (eg, aircraft, tunnels, submarines) suggests that unexpected fires do occur. When dealing with fires in inhabited spacecraft, the realm of the 'unexpected' takes on even greater proportions as the fire safety community is faced with a variety of engineering and scientific complications beyond the realms of every day experience.
FIGURE: The European component of Space Station ALPHA, the APM Columbus.
The early detection of fire greatly enhances the chances of controlling the outbreak and hence, limiting the resulting losses. Detecting fire requires sensors capable of distinguishing the fire signature - through measurements of heat, smoke, chemical species or radiation - from the normal background environmental conditions.
As it is impractical to completely instrument the spacecraft environment, a limited number of optimally positioned detectors is required. This task is made difficult under microgravity conditions, as the transport of the fire signature is expected to be very different to that found in normal gravity environments. Fire field modelling offers a means of analysing this situation and suggesting fire detector locations most likely to intercept and detect the signature.
The models consisted of a series of transient and steady-state two dimensional simulations - performed using the PHOENICS code - describing the habitable environment of the Columbus APM under reduced gravity conditions. A number of scenarios were investigated. These involved simulating the effects of various cabin ventilation configurations and their interaction with a prescribed heat load produced by the on-board experimental packages and a small 'smouldering' fire source.
The major findings to emerge from this work are that flow and thermal fields are significantly different under reduced gravity compared to normal gravity conditions. In prescribed fire scenarios, the observed differences in flow behaviour has a profound effect on the transportation of components of the fire signature and hence will impact on the successful early detection of fire.
E R Galea and M K Patel. ESA Columbus: Mathematical Modelling of Fire in Microgravity, Findings, Conclusions and Recommendations. July 1991, ESTEC Contract 8711/90/NL/IW.