Investigation of Horizontal Vent Flow Behavior Coupling with Fire Behavior in a Ceiling Vented Enclosure


Student thesis: Doctoral Thesis

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  • Xiao CHEN


Awarding Institution
Award date22 Jun 2015


Ships are becoming increasingly important in the modern world, especially when they serve civilian and military purposes. Ship cabins commonly have ceiling vented enclosures. Horizontal vent flow is obviously distinct from vertical vent flow because of the characteristic of the horizontal opening. Vent flow through a vertical opening becomes a stable, layered flow, whereas an irregular oscillatory flow occurs in a horizontal opening. Hence, understanding the behavior and mechanism of fire-induced horizontal vent flow is a significant aspect of ship fire safety, as it can provide essential data and serve as theoretical basis for ship fire prevention and ventilation system design.
To investigate the horizontal vent flow behavior coupling with fire behavior, we performed experimental and theoretical studies on fire-inducted vent flow behavior, compartment fire behavior affected by vent flow, and the interaction between fire and vent flow with consideration of several configurations characterized by different pool diameters and ceiling vent sizes. The fire-induced vent flow model was then proposed. The principle conclusions obtained from the experiments under the tested conditions are as follows.
The horizontal vent flow behavior induced by fire, that is, the flow pattern and the oscillatory behavior, was investigated. The flow pattern is an important factor that indicates vented flow regularity. Three vent flow patterns, namely, unidirectional flow, bidirectional flow, and compound flow, were observed during the tests by a video camera and a laser technique. The theories of Cooper and Chow based on saltwater experiments and numerical calculations, respectively, were also introduced to predict the vent flow pattern in the actual fire scenario. The good agreement in the results indicated that these theories could also be used to predict vent flow patterns with a few modifications.
An opening area ratio (area ratio of horizontal opening and fuel) was proposed to distinguish the three flow patterns correlated with the buoyancy number and flow area ratio. The vent flow oscillatory behavior was observed with the smoke visualization measurement. Smoke gas area and smoke temperature were adopted to characterize the vent flow oscillation frequency. The oscillation frequency of the smoke gas area decreased with the increasing horizontal opening, and the smoke temperature oscillation frequency presented a rising trend with the increasing heat release rate. The empirical equations on the oscillation frequency of the smoke gas temperature in the opening were verified.
Fire behavior in a confined compartment with a horizontal opening was studied. The influence of the horizontal opening on the pool fire pulsation behavior was studied with consideration of a number of configurations characterized by different pool diameters and different ceiling vent sizes, which ranged from well-ventilated to under-ventilated conditions. The mean flame height presented a binomial distribution with the increasing horizontal opening size, and the ratio of the mean flame height and diameter correlated well with the horizontal ventilation parameter. The results are consistent with the usual dependence of an unstable frequency on pool diameter. However, the results also reveal an unexpected dependence of unstable frequency on ceiling vent size. A new correlation between the Strouhal number and Froude number was proposed. The compartment effects on the rate of fuel evaporation were incorporated into the proposed correlation.
The influence of the horizontal opening on compartment fire parameters, such as the average mass loss rate values, oxygen concentration value at extinction time, average rate of gas temperature increase, and compartment pressure in a closed and confined compartment, was considered. A dimensionless horizontal ventilation parameter coupling horizontal opening and fuel areas was proposed with the theoretically simplified method based on energy and oxygen conservations to analyze the fire parameters. The comprehensive frequency correlation aimed to reveal the interactions between fire and vent flow. Vent flow frequency could be easily obtained on the basis of the stable product of the flame pulsation frequency and vent flow oscillation frequency under varying horizontal openings, which depended on the pool fire parameters.
A model of fire-induced vent flow through a horizontal opening was proposed on the basis of the multilayer concept. In proposing this model, we considered the interactions between fire and vent flow. The fire-induced flow process and the flow mechanism were discussed with consideration of the opening configuration and flow pattern. The different flow transport processes were presented for the unidirectional and bidirectional flow, along with the flow mechanism based on the force analysis of the outflow and inflow. The fire-induced vent flow model for the unidirectional flow and bidirectional flow was established on the basis of the multilayer zone model. A new method for obtaining the mass flow rate experimentally through the horizontal opening was presented. The mass outflow and inflow rates for the unidirectional and bidirectional flows were obtained with the laser technique and pitot tube. A comparison between the theoretical and experimental results was conducted to verify the feasibility of the vent flow model.