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36 FIGURE 11 Small-scale experiments (physical modeling) (29). FIGURE 9 Backlayering distance vs. longitudinal air velocity for two heptane pool surfaces [tunnel slope = 0.5 %--Chamoise fire tests (21)]. there are only a few examples of reduced-scale model appli- cations for tunnel design that can be mentioned (30). written as a function of the Richardson number (Ri) and One example is a study on smoke stratification stability on depends on the tunnel characteristics. The Richardson num- a one-third scale model. The Froude scaling enables modeling ber considers the density of the gases in the plume impact of thermal effects and smoke backlayering. The fire is modeled zone under the ceiling. using a heptane pool fire and can be characterized by: SMALL-SCALE TESTING (PHYSICAL MODELING) Theoretical total HRR calculated from the mass con- sumption of heptane. Small-scale experiments can be designed to represent a fire Total HRR computed from the oxygen consumption. in a planned tunnel (see Figure 11). This method is based on Convective HRR with volumetric flow rate estimated similarity laws, which are actually the link between a full- by integration of the velocity profile measured down- scale situation and the modeled one (21). stream of the fire. The objective of such experiments is to represent the phe- nomena that develop during a fire within a tunnel. Compared The difference between the two total HRRs is combustion with full-scale tests, this method allows some savings of time efficiency and radiation fraction. and money and the ability to analyze the phenomena in detail. Such tests are not affected by natural factors such as Researchers can use small-scale models for scientific rea- winds, elevations, and solar radiation, and can be repeated as sons. If some specific behaviors have to be characterized, the many times as necessary. One of its goals is also to be demon- best solution can be to show them using totally controllable strative, because it is possible to visualize smoke. However, methods. Complementary tests may be done with full-scale facilities. The knowledge of the laws obtained with the models is useful in planning full-scale experiments. Small-scale models have been used to characterize the efficiency of ceiling trap doors for smoke extraction or to determine nondimensional laws governing the existence of backlayering. The similarity laws are the fundamental link between the model and the corresponding full-scale situation. If this link is not shown to be strong, the study results cannot be consid- ered as representative of the full-scale situation. Actually, in a more general manner, the validity of the experiments has to be considered as relative to the used similarity law. As a con- sequence, it depends on the small-scale model technique. FIGURE 10 Plabutch Tunnel Fire Test sponsored The situation observed during a fire inside a tunnel appears by Graz University of Technology. as the result of an interaction between two major forces: