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112 CHAPTER THIRTEEN EFFECTS OF VARIOUS VENTILATION CONDITIONS, TUNNEL GEOMETRY, AND STRUCTURAL AND NONSTRUCTURAL TUNNEL COMPONENTS ON DESIGN FIRE CHARACTERISTICS--LITERATURE REVIEW INFLUENCE OF VENTILATION was little change in the peak HRRs. However, the Runehamar ON FIRE HEAT RELEASE RATE fire tests showed no significant HRR changes resulting from ventilation [up to air velocities of 2 to 2.5 m/s (394 to Prior publications reported that mechanically ventilating a fire 492 fpm)]. Earlier discussions about a stronger dependence could result in a more rapid fire development (35). The Second were not confirmed by the Runehamar experiments. Benelux Tunnel fire test addressed the influence of ventilation on the FHRR (74). The fire development rate, with a ventila- Ventilation is applied during a fire to keep escape routes tion air velocity of 4 to 6 m/s (787 to 1,181 fpm), appeared to free from smoke and to assist the fire department and others be two times faster than development without ventilation. The in reaching the accident site. In most cases, mechanical ven- peak heat output was about 1.5 times higher. The increase in tilation will lead the fire to burn fully. Thus, the total duration HRR and fire growth rate, a result of increased velocity, is the of the fire will be limited and the structure will not be sub- result of more effective heat transfer from the flames to the fuel jected to a high thermal load concentration. surface. In some cases, it results in a more effective transport of oxygen into the fuel bed, which enhances the mixing of oxy- gen and fuel. Theoretically, a fire on densely packed wood It is understood that there could be a negative effect on cribs may be locally underventilated; however, in forced ven- ventilation as forced ventilation may cause significant flame tilation flow, the transport of oxygen to the underventilated deflection, which leads to the chance that the fire might region enhances the combustion rate. spread to other vehicles and threaten the integrity of the tun- nel structure on a larger surface, assuming the ventilation In most road tunnels, such as the Benelux Tunnel (74), there cooling effect and reduction in radiation at the source are is a large amount of oxygen already available and the availabil- insignificant. ity of oxygen is not increased by the ventilation. At the same time, the ventilation has a cooling effect on the fire environ- As reported, when a powerful ventilation system is sud- ment, whereby the heat can be easily let out to the environment. denly activated during an underventilated fire situation, the effects may be dramatic; the flames may suddenly increase in Ventilation has an influence on the fire development that size and length and the fire may easily spread forward because does not always conform to expectations (75): of the preheated vehicles downstream of the fire. However, this phenomenon cannot be defined as flashover. This situation Owing to increased ventilation, the fire development for may become very hazardous for firefighters and those who are a car can be slowed if the fire is ignited at the front of the still trapped inside the tunnel. Starting a ventilation system car. This is in contrast to the accepted view of supposed when the fire has been going for some time in a tunnel with accelerated development resulting from ventilation. high vehicle density is always very risky. However, as venti- The influence of increased ventilation on the observed lation cannot immediately reach its full operating mode, the fire behavior depends on the ignition location. Note that risk is not that significant. 95% of fires begin in the engine compartment (i.e., at the front). Literature observations were made from the Benelux and Under the influence of a high-ventilation velocity, the fire Runehamar fire tests on the influence of ventilation rate on development accelerates for a covered load at a rate 2 to fire growth rate and are presented in Table 34. 3 times faster, and not by a factor 20 as predicted by some authors. The fire size was 20% to 50% higher as a result Tests have indicated that it may be that the fastest fire of a high-ventilation speed. growth occurs at about 3 m/s airflow velocity. Both higher and lower ventilation rates may result in slower growth fires. The results from model fire tests indicated that if the wood These observations were made on the basis of only a few cribs are densely packed the increase in peak HRR by venti- experiments; more research is needed to confirm (or other- lation can be up to a factor of 1.5. If not densely packed, there wise) the validity of these conclusions (72).