Research PaperCO concentration decay profile and ceiling jet entrainment in aircraft cargo compartment fires at reduced pressures
Introduction
Early and accurate detection is crucial to determine suitable control in an inflight fire which may cause catastrophic casualties and loss of property [1], especially for the inaccessible areas of the aircraft, such as cargo compartments, where a direct visual inspection is not possible during flight [2]. Multi-sensor detection system has been developed recently to eliminate false alarms and to reduce alarm response time [3], therefore CO concentration profile and entrainment properties of a ceiling jet especially for small fires are particularly critical parameters. It should be noted that the aircraft cargo compartment is a flat long and narrow closed compartment and the atmospheric pressure inside changes from 100 kPa to 70 kPa corresponding to the sea level and cruising altitude respectively. Therefore, to provide theoretical basis in the design of fire detection systems for the aircraft cargo compartment or other similar compartments at high altitude, an investigation of the low pressure effects on CO concentration profile and entrainment of a ceiling jet is practically worthwhile.
Extensive studies have been conducted by former researchers to address the characteristics of ceiling jet, such as heat transfer and temperature profile, in certain thermal engineering research for large industrial or commercial storage facilities and tunnels [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. The classical research came from the work carried out by Alpert [11] who proposed a generalized theory to predict gas velocities, gas temperatures, and the thickness (or depth) of a steady fire-driven ceiling jet flow for a weak plume under an unconfined ceiling when the height of the ceiling above the fuel source is much higher than flame height. Then, Heskestad and Delichatsios [12] extended the application by developing new correlations of non-dimensional velocity and temperature. Additionally, Zhang et al. [13] modified the correlation of the flame extension length and temperature profile of a ceiling jet induced by a buoyant turbulent round jet impingement upon a horizontal plate, focusing on the scenarios that flame heights were comparable to or larger than plate-source height. More recently, Wang et al. [14] conducted fire tests under four pressures with results showing that the maximum ceiling temperature increased and the ceiling temperature decayed faster at reduced ambient pressures, and thus the correlations were modified accordingly by introducing the entrainment coefficient ratio. However, very limited work [15], [16], [17] is reported on CO concentration decay profile of a ceiling jet, which is controlled by the fresh air entrainment into the ceiling jet acting as a dilution effect, while the temperature decay profile is dominantly affected by the heat loss to the ambient. Hu et al. [16], [17] and Tang et al. [15] compared the ceiling temperature profile with CO concentration profile and found that the ceiling temperature decayed much faster than the CO concentration for a ceiling jet and their difference could be quantified by a formula. However, these studies only concern the mass fraction of CO concentration at normal pressure.
In recent years, the low pressure effects on heat transportation and combustion have received some researchers’ attention [19], [20], [21], [22], [23], due to the increasing construction at high altitude. However, most of their work [19], [20], [21] focuses on the low pressure effects on fire flame, and it seems that there are only a few researches [22], [23] about the properties of combustion productions under low pressure conditions. Hu et al. [22] conducted a set of n-heptane fire tests in Tibetan plateau and obtained the radial average extinction coefficient at a given height above the fuel surface was proportional to the pressure. Fang et al. [23] gave the qualitative theoretical analysis about CO concentration in Lhasa and Hefei with results that maximum changes of CO concentration increased under low pressure. From the literature review, we find that the pressure effect on CO concentration decay profile and entrainment of a ceiling jet has not been addressed systematically and further represented by theoretical or empirical models.
Therefore, this paper focuses on the influence of low pressure on CO concentration decay profile and entrainment properties of a ceiling jet driven by weak plume through theoretical analysis and comparison between the experimental ceiling temperature and CO concentration data. Attempts have also been made to develop theoretical and empirical models which predict and describe CO concentration decay profile and entrainment coefficient of a ceiling jet at reduced ambient pressure. The work is of significance to improve the understanding on ceiling jet of the aircraft cargo compartment fires at reduced ambient pressure and provide the basis for fire detection system design.
Section snippets
Theoretical analysis
As shown in Fig. 1, the buoyancy-driven fire smoke flow in an aircraft cargo compartment could spread along the ceiling [24], where the smoke temperature is relatively low [14]. Hence, two assumptions are taken:
- (1)
The oxidization of CO in the smoke flow at this stage is neglected, because CO within the upper smoke layers can be almost oxidized to CO2 if the upper layer temperatures are higher than 900 K (or 627 °C), but could hardly be oxidized to CO2 if the upper layer temperatures are below about
Experiment setup
Fig. 2 shows the full scale simulated aircraft cargo compartment which is a long rectangular prism with curved sidewalls and consists of the compartment, pressure controlling system, forced ventilation controlling system and other auxiliary systems (e.g. illumination and surveillance systems). The compartment is made of 8 mm thick stainless steel with inner dimensions of 467 cm long (L) and 112 cm high (H), and the width of top and bottom is 300 cm and 122 cm respectively, which are very close to
CO concentration decay profile
Fig. 3 shows the volume fraction of CO concentration in the ceiling jet at the measured position with changing pressure. It is confirmed that the low pressure affects CO concentration, where CO concentration at the same position for the same fuel pool size under low pressure is higher than that in normal pressure. This is because the combustion would be relatively insufficient under low pressure, then more CO concentration is produced. Meanwhile, we can see from Fig. 3 the volume fraction of CO
Conclusions
CO concentration and ceiling temperature were measured in a set of simulated aircraft cargo compartment fires at four atmospheric pressures (100 kPa, 90 kPa, 80 kPa, 70 kPa), and were analyzed to investigate low pressure effects on the entrainment properties of a ceiling jet. Major findings include:
- (1)
The CO concentration decay profile is independent of pressure, meanwhile, a theoretical formula which agrees well with the experimental data (Eq. (9); Fig. 5) is derived and a brief empirical correlation
Acknowledgements
This study was supported by Opening Fund of State Key Laboratory of Fire Science (SKLFS) under Grant No. HZ2016-KF08 and Grant No. HZ2013-KF01, Fostering Young Key Scholars Program of Wuhan University of Science & Technology under Grant No. 2016xz007, Key Program of Science Research Project of Educational Commission of Hubei Province under Grant No. D20161105 and Opening Fund of Key Laboratory of Ministry of Public of Building Fire Protection KFKT2014MS01.
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