Flickering candle flames and their collective behavior

Oscillation and collective behavior of diffusion flames is a fascinating phenomena. Considering candle bundles with different sizes in variable oxygen concentration, the flickering dynamics of the flames are experimentally and theoretically investigated. Trends for the flickering frequency as a function of the candle number in the bundle and oxygen concentration is revealed for various topologies of the candles packing. The collective behavior of the flames as a function of their separation distance is studied by measuring an appropriate synchronization order parameter and through the common oscillation frequency. In agreement with previous results we find a discontinuous phase transition between an in-phase synchronized state at small separation distance and a counter-phase synchronized state at larger separation distances. A previously used dynamical model is modified in order to accommodate our experimental findings.

A vortex-dynamical scaling theory for flickering buoyant diffusion flames

The flickering of buoyant diffusion flames is associated with the periodic shedding of toroidal vortices that are formed under gravity-induced shearing at the flame surface. Numerous experimental investigations have confirmed the scaling,$f\propto D^{-1/2}$, where$f$is the flickering frequency and$D$is the diameter of the fuel inlet. However, the connection between the toroidal vortex dynamics and the scaling has not been clearly understood. By incorporating the finding of Gharibet al.(J. Fluid Mech., vol. 360, 1998, pp. 121–140) that the detachment of a continuously growing vortex ring is inevitable and can be dictated by a universal constant that is essentially a non-dimensional circulation of the vortex, we theoretically established the connection between the periodicity of the toroidal vortices and the flickering of a buoyant diffusion flame with small Froude number. The scaling theory for flickering frequency was validated by the existing experimental data of pool flames and jet diffusion flames.

Numerical and Experimental Study on Negative Buoyance Induced Vortices in N-Butane Jet Flames

Near nozzle flow field in flickering n-butane diffusion jet flames was investigated with a special focus on transient flow patterns of negative buoyance induced vortices. The flow structures were obtained through Mie scattering imaging with seed particles in a fuel stream using continuous-wave (CW) Argon-ion laser. Velocity fields were also quantified with particle mage velocimetry (PIV) system having kHz repetition rate. The results showed that the dynamic motion of negative buoyance induced vortices near the nozzle exit was coupled strongly with a flame flickering instability. Typically during the flame flickering, the negative buoyant vortices oscillated at the flickering frequency. The vortices were distorted by the flickering motion and exhibited complicated transient vortical patterns, such as tilting and stretching. Numerical simulations were also implemented based on an open source C++ package, LaminarSMOKE, for further validations.

On diversity within operators’ EEG responses to LED-produced alternate stimulus in SSVEP BCI

This work is an attempt to identify causes of the widely observed fact that performance of Brain-Computer Interface systems based on Steady-State Visual Evoked Potentials varies between different users. The efficient LED-produced alternate stimulus systems are taken into account. The effect of stimulus color and flickering frequency on measured SSVEP response at first and second harmonics is investigated for 10 women and 11 men. The experimental setup is described, measurement procedure, signal processing and analysis algorithms are outlined. The results are presented and discussed. One of the early conclusions drawn from this extensive research is that the promising strategy of SSVEP-based BCI system optimization for best performance can be through stimulus adjustment to each individual user.

Fuel mixing effect on the flickering of jet diffusion flames

An experimental study has been performed for a buoyant jet diffusion flame, which was observed to oscillate at different frequencies spatially. The flame dynamics and structure were visualized by a commercial digital camera and a high-speed camera. Mixed fuel of methane and propane at certain proportion was found to generate very different vortex shedding behaviours. As a result, the flickering frequency of a methane/propane 1:1 mixture can be half of that of typically observed for pure methane or propane flame. The distance between adjacent flame puffs or the size of vortical structures in the reacting flow field, which can be modified by the fuel composition, was identified to be the key factor that affects the flickering frequencies. Repeated tests confirmed that mixed fuel at certain proportions can have a significant effect on the flame flickering frequency through the modification of vortex structure and dynamics.