Studies on Fire Smoke Characteristics and Temperature Distribution Law in Ceiling Jet Zone of an Aircraft Cargo Compartment

Abstract

An inflight fire may cause catastrophic losses in casualties and properties if left undetected before it quickly develops into an uncontrollable size. Early and accurate detection is a key point in maintaining aircraft fire safety, particularly for the cargo compartment, which cannot be directly inspected during flight. Current smoke fire detectors in the aircraft cargo compartment can detect fires well, but they also give rise to another problem, that is, a high false alarm rate, which causes huge economic losses and reduces confidence in the fire detection system. Multi-sensor fire detection systems of aircraft cargo compartments have been investigated to minimize false alarms and reduce alarm response time. The characteristics of fire smoke in the ceiling jet zone, including ceiling temperature, smoke density, and gas concentrations beneath the ceiling, must be explored to provide a theoretical basis in designing multi-sensor fire detection system for the aircraft cargo compartment. This thesis investigates the features of aircraft cargo compartments, such as fire locations and low ambient pressures, and their effects on the fire smoke characteristics in the ceiling jet zone.

The influence of horizontal fire locations on the properties of the ceiling jet is examined via a series of pool fires located at the center, adjacent to enclosure walls, and in a corner of a simulated aircraft cargo compartment. Placing the fire off the center increases the mass loss rate (MLR) and the average gas temperature, but delays the fire from reaching its peak and reduces O2 consumption. This finding indicates that the effects of heat feedback from walls on the burning fuel are stronger than the available limited air. Wall restrictions enhance the maximum ceiling temperature, which is predicted with a correlation based on Heskestad’s plume equation and mirror theory with modified coefficients in comparison with Alpert’s and Delichatsios’s equations. The proposed formulas are extended to fires at the early stage by slightly adjusting the coefficient and by replacing the constant heat release rate Q with an appropriate time-dependent variable Q. Ceiling temperature decay profiles do not strongly depend on the fire locations, except higher temperatures of wall and corner fires at the same ceiling position. The modified Heskestad and Delichatsios’s equation agrees with experimental data at the steady stage better than Alpert’s equation, Heskestad and Delichatsios’s equation and exponential model, and gradually become applicable in predicting the ceiling temperature decay profiles at the early stage with an increase in combustion time. Wall restrictions increase the level of smoke layer interface, smoke density, maximum CO concentration, and increase rate of CO concentration, but only slightly affect CO2 concentration and relative humidity (RH).

The ceiling jet characteristics of elevated fires in an aircraft cargo compartment are analyzed, particularly at the early stage. The MLR related to flame behaviors is greatly increased by flame impingement. The maximum ceiling temperatures of the elevated fires meet the three-regime description proposed by McCaffrey and can be determined by the same modified equations. An exponential model is proposed to evaluate the ceiling temperature decay profiles, in consideration of elevation height. Elevating a fire prompts the smoke to be closer to the ceiling and increases CO and CO2 concentrations and O2 consumption at the early stage. RH is increased at the early stage and is then reduced.

The effects of low pressure on the ceiling jet characteristics are clarified. MLR is proportional to atmospheric pressure, as m ∝ A ∙ Px thereby confirming the findings of previous studies. When pressure is reduced, the average gas temperature and O2 consumption are increased, indicating that MLR is determined with the limited air available at low pressure. The maximum ceiling temperature increases with decreasing pressure. Considering the low pressure effect and entrainment coefficient, the entrainment coefficient ratio Cα, which is the ratio of the entrainment at low and normal pressures, is proposed in predicting the maximum ceiling temperature. Ceiling temperature decays faster when ambient pressure is reduced. The classic correlations established by Alpert, Heskestad and Delichatsios for predicting the ceiling temperature decay profiles are modified by introducing the entrainment coefficient ratio Cα and extended to the low pressure condition. The results based on Heskestad and Delichatsios’s method are more accurate than that of Alpert’s method. Low pressure increases the smoke layer interface, but decreases the smoke density proportional to ambient pressure, 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 using the ideal gas assumption.The maximum CO2 concentration, CO2 yield rate and the reduction of RH decrease with a decline in ambient pressure.

The coupling effects of low pressure and fire locations on ceiling jet characteristics in aircraft cargo compartment fires are discussed. The values obtained for MLR, average gas temperature and O2 consumption confirm the above analysis. The coupling effects of fire locations and low pressures significantly increase the maximum ceiling temperature. A uniform correlation for the maximum ceiling temperature is proposed by incorporating the entrainment coefficient ratio Cα 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 correlations considering the entrainment coefficient ratio Cα are still suitable to fires located near the walls. The smoke layer interface, smoke density, the maximum CO concentration and its increase rate, the maximum CO2 concentration and its increase rate and RH follow the same law specified above.

Date of Award	17 Sept 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Siu Ming LO (Supervisor)