Studies On Fire Smoke Characteristics And Temperature Distribution Law In Ceiling Jet Zone Of An Aircraft Cargo Compartment

An inflight fire may cause catastrophic loss on casualties and property if it is not detected before developing into an uncontrollable size. Early and accurate detection is a key point of aircraft fire safety, especially for the cargo compartment, where a direct visual inspection is impossible during flight. Current smoke fire detectors in the aircraft cargo compartment do well in detecting fires, but also bring another problem, i.e. high false alarm rate causing huge economic loss and confidence loss in fire detection system. In order to minimize false alarms and also reduce alarm response time, multi-sensor fire detection systems have been under investigation for aircraft cargo compartments. Aim to provide theoretical basic on the design of multi-sensor fire detection systems for aircraft cargo compartment, it is practically worthwhile to study the fire smoke characteristics in ceiling jet zone including ceiling temperature, smoke density and gas concentrations beneath the ceiling. Therefore, this thesis investigates the features of aircraft cargo compartments, such as fire locations and low pressures, effects on the fire smoke characteristics in ceiling jet zone.The influence of horizontal fire locations on the ceiling jet properties has been examined through conducting a series of pool fires located at the center, adjacent to enclosure walls and in a corner in a simulated aircraft cargo compartment. Placing the fire off the center increases the mass loss rate, the average gas temperature, while delays it reaching the peak and reduces the O2 consumption, which indicates the effects of heat feedback from walls on the fuel burning are stronger than limited air available. Wall restrictions enhance the maximum ceiling temperature which is predicted by a correlation based on Heskestad plume equation and the mirror theory with modified coefficients in comparison with Alpert equation and Delichatsios equation. The proposed formulas are extended to fires at the early stage by adjusting the coefficient slightly and replacing the constant heat release rate Q by an appropriate time-dependent Q. Ceiling temperature decay profiles show weak dependence on the fire locations, except higher temperatures of wall and corner fires at the same ceiling position. The modified Heskestad and Delichatsios equation agrees with experimental data at steady stage better than Alpert equation, and Heskestad and Delichatsios equation and exponential model gradually become applicable to the prediction of ceiling temperature decay profiles at the early stage with the increase of combustion time. Wall restrictions raise the level of smoke layer interface, the smoke density, the maximum CO concentration, the increase rate of CO concentration, but affect CO2 concentration and relative humidity little.Ceiling jet characteristics of elevated fires in an aircraft cargo compartment are studied particularly at the early stage. The mass loss rate related to flame behaviors is increased greatly by flame impingement. Maximum ceiling temperatures of elevated fires meet the three-regime description proposed by McCaffrey and can be calculated by similar modified equations. An exponential model is proposed to evaluate the ceiling temperature decay profile considering the elevation height. Elevating a fire prompts smoke closer to the ceiling, increases CO/CO2 concentrations and O2 consumption at the early stage. Relative humidity increases at the early stage and then drops.Low pressure effects on the ceiling jet characteristics are clarified. The mass loss rate is proportional to the atmospheric pressure, as m∝A·Px, confirming previous studies. Reducing pressure increases the average gas temperature and O2 consumption, indicating the mass loss rate is determined by the limited air available in low pressure. The maximum ceiling temperature increases with decreasing pressure. Considering the low pressure effect and entrainment coefficient, the air entrainment ratio Ca which is the ratio of the entrainment in low pressure and that in normal pressure, is proposed in predicting the maximum ceiling temperature. The ceiling temperature decays faster as ambient pressure reduces. The classic correlations established by Alpert, Heskestad and Delichatsios for the ceiling temperature decay profile are modified by introducing the air entrainment ratio Ca and extended to the low pressure condition. The results based on Heskestad and Delichatsios method are more accurate than that of Alpert method. Low pressure raises the smoke layer interface, but decreases the smoke density proportional to the ambient pressure, as K∝Px2. The maximum CO concentration increases due to low pressure effects, and CO yield rate increases by a minus exponential factor agreeing with the theoretical analysis of Heskestad theory by using the ideal gas assumption. The maximum CO2 concentration, CO2 yield rate and the reduction of relative humidity decrease with decreasing ambient pressure.Ceiling jet characteristics under complex aircraft cargo compartment circumstances including the low pressure and fire locations, and ventilation are discussed. The mass loss rate, average gas temperature and O2 consumption confirm the analysis above. The coupling effects of fire locations and low pressures lead to a higher rise in maximum ceiling temperature. A uniform correlation for the maximum ceiling temperature is proposed by introducing air entrainment ratio Ca and the modified coefficient β in mirror model and expands the application range. Ceiling temperatures decay at similar rate even with various fire locations in the same low pressure, thereby the proposed correlation considering the air entrainment ratio Ca are still suitable to fires located near walls. The smoke layer interface, smoke density, the maximum CO concentration and its yield rate, the maximum CO2 concentration and its yield rate and the relative humidity follow the similar law above.