Studies On The Flame Necking-in Characteristic And Temperature Profile In Developing Area Of Pool Fires

Flame necking-in is a fundamental behavior at the base of diffusive pool fires caused by the entrained flow approaching the flame induced by buoyancy (density difference), which is also the major generation source of flame periodic oscillatory instability. However, measurements have not been previously reported for the evolution of the flame necking-in dynamic characteristics and how they relate to flame instability behaviors. This paper quantifies the flame necking-in dynamics and instability motions in0.04-0.25m diameter ethanol pool fires with different lip heights (0.3-2cm). Based on direct time-sequence analysis on flame photographs, the necking-in characteristic maximum depth ((?)max), average necking-in velocity (Unecking-in), maximum uprising height ((?)max), average uprising velocity(Uuprising), as well as the vortices shedding instability frequency (f) and characteristic vortices formation lift-time (τ) are quantified to find their evolutions with pool size and lip height along with their associated dominant instability motions. Three different flame instability motions are identified:short life Rayleigh-Taylor (R-T) instability, extended R-T instability and puffing instability. The dominant instability motion is found to transit from extended R-T instability to puffing instability with increase in pool size or lip height. The pumping capacity of large-scale vortices formation (which could be quantified by (?)max,(?)max, Unecking_in, Uuprising) is primarily associated with the extended R-T instability frequency. The frequency (f) of the necking-in extended R-T instability is found to be greater than the puffing frequency. The puffing frequency increases slightly with lip height and decreases with pool diameter, D, following the well-known pool fire square root law (f～D-1/2, or non-dimensionally St～Fr-1/2). Meanwhile the frequency (f) for Extended R-T instability is found to be well correlated by fR-T,extended～(Cfg2/Q)1/3. The characteristic life-times (τ) of the extended R-T instabilities increase with pool diameter while remaining smaller than the puffing life-times that scale by τ～D1/2.Also this paper did experiments on the flame necking-in behavior under the situation of different low atmospheric pressures. It did analysis on the evolution of short life R-T instability, extended R-T instability and puffing with atmospheric pressure and pool size. It is found that the dominate instability transit from extended R-T instability to puffing with the increase of atmospheric pressure and pool diameter. The necking-in characteristic maximum depth ((?)max) and average necking-in velocity (Unecking-in) changes little with atmospheric pressure. Maximum uprising height ((?)max) and average uprising velocity (Uuprising) decrease with atmospheric pressure. And the ratio of maximum depth((?)max) and maximum uprising height ((?)max) and the ratio of average necking-in velocity (Unecking-in) and average uprising velocity (Uuprising) both increase with atmospheric pressure. The frequency of puffing increase slightly with atmospheric pressure and increase with pool diameter. The frequency of extended R-T instability does not show obvious tendency when changing with atmospheric pressure and pool diameter.In addition to the necking-in characteristic, this paper also did study on the initial vertical rising temperature. The initial vertical rising temperature above the fuel surface (gradually rising from fuel boiling point to full developed flame temperature during moving up) at the base of diffusive pool fires is not included in the current classic three-regime (continuous flame, intermittent flame and buoyant plume) fire plume models. The experimental data about this initial vertical temperature rising profile was also few in the previous literatures. In this work, this temperature profiles for pool fires of different sizes are firstly quantified experimentally. It is found that the temperature rising profile as well as the height to arrive the highest temperature is different for different pool diameters. Such temperature rising behavior is physically interpreted due to the gradual mixing of entrained air into the flame to react the unburnt fuel vapor during rising. Then, a Beta-PDF (Probability Distribution Function) method is developed to predict the mixture fraction of the fuel with air, and thus to predict the temperature profile along the centerline of the flame. It is found that the proposed model well predict the measured temperature values.