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• Flame morphologic characteristic of vertical downward jet fires quantified. • Three regions of a downward jet fire distinguished. • A physical model considering momentum, buoyancy and entrainment developed. • CFD simulation showing the inner structure of flame conducted. • The characteristic length scale for flame geometrical characteristics proposed. This work explored the flame geometrical characteristics of vertical downward turbulent jet fires in still air, whose flame buoyancy was opposite to its initial momentum. Experiments were carried out under varied fuel supply rates with propane as fuel. Three circular nozzles having diameters of 3, 5 and 7 mm were employed. CFD simulation to show the combustion structure inside the flame was conducted in the work, contributing to proposing a new physical model. The inner flow field and average mixture fraction of a downward jet fire have been revealed and the CFD result was validated by experimental data. A downward jet fire can be distinguished in three regions: the downward core flow region where the initial flame momentum is opposed by buoyancy, the flow reversal dome region where the momentum reverses direction, and the upward combusting flow region, which surrounds the core, dominated by buoyancy. Three basic flame morphologic lengths of the downward jet fires were measured, namely the flame downward length below the nozzle, the flame width, and the total flame height. The inherent differences between downward, upward and horizontal jet, caused by the momentum direction, were discussed by comparing the flame sizes of these three types of jet flames. A physical model considering the initial downward volumetric momentum, the flame buoyancy and the air entrainment volume until complete combustion occurs was raised to depict the flame geometric characteristics derived from three characteristic length scales, namely a momentum-buoyancy competition length scale, a flow-rate length scale linked with the source diameter and a total combustion buoyancy length scale. All measured experimental flame lengths were normalized using dimensionless groups (such as L m L q and S m ̇ f / ρ ∞ g ′ L m L m 2 ) derived by these three length scales. The geometric properties of downward jet fires were shown to be well modelled by the proposed non-dimensional groups which were different from previously used length scales in the literature. [ABSTRACT FROM AUTHOR]

• Flame morphologic characteristics under various atmospheric pressures quantified. • Effect of sub-atmospheric pressure on horizontal jet flame behavior revealed. • The characteristic length scale for flame morphology proposed. • Correlations of flame horizontal projection length and vertical height established. • An integral model for the conservation of mass and momentum developed. This paper investigated the flame morphology including horizontal projection length and vertical height of horizontal turbulent jets under various sub-atmospheric pressures, for which both data and model are few. Experiments are conducted for various nozzle diameters in the range of 3 mm to 7 mm. Propane is used as the fuel in this study. The sub-atmospheric pressures, created by a reduced pressure chamber, are varied from 40 kPa to 100 kPa. A digital video camera is used to measure the flame morphologic characteristics. The experimental results show that the flame horizontal projection length increases with the increasing of heat release rate or the decreasing of chamber pressure. Meanwhile, the flame vertical height first increases then decreases with the increasing of heat release rate or the decreasing of chamber pressure, which shows a non-monotonic behavior. A characteristic length scale is proposed by dimensional analysis considering jet initial momentum, flame buoyancy and ambient air density to illustrate the controlling mechanisms in the flow development. And the characteristic air entrainment flow rate ρ ∞ L m 2 g ′ L m is used to normalize the stoichiometric mass flow rate at the flame tip, which determines the total length of the trajectory. Non-dimensional formulas are developed based on the derived length scale and characteristic air entrainment flow rate to describe the flame horizontal projection length and vertical height of horizontal jets under various sub-atmospheric pressures. Finally, an integral model considering air entrainment mechanism, top hat profiles and strong plume theory for the conservation of mass and momentum in horizontal and vertical directions is developed, which shows to describe the flame morphologic characteristics well. [ABSTRACT FROM AUTHOR]

This paper investigates the temperature evolution inside a compartment (i.e. , room) as well as the external flame height ejected through the opening (i.e. , window) of fire compartment, which is associated with complex thermal heat/mass transfer and fluid dynamics. The scale effect on this problem is revealed and quantified employing the multi-scale compartments (inner scale ranged from 0.34 to 0.76 m). The fire heat release rate (Global Equivalence Ratio, GRE), opening sizes and compartment scales were varied with 468 experimental conditions. Models for predicting the upper-part temperature evolution with heat release rate (Global Equivalence Ratio, GRE), opening dimensions and compartment scales are established base on the heat balance analysis and the derived non-dimensional quantities. The external flame height shows a considerable difference for various compartment scales, due to the change of temperature inside the compartment hence buoyancy flux of the out-flow through the opening. A general model characterizing the external flame height of various compartment scales is proposed with the newly defined dimensionless excess heat release rate accounting for opening geometry length scale and the upper-part temperature inside the compartment, which shows a good agreement with the experimental results. This study provides important and new contributions over previous knowledge and has potential practical applications generally concerning the scale effect and for the validation of numerical models. [Display omitted] [ABSTRACT FROM AUTHOR]