Effects of turbulent intensity on nano-PMMA flame propagation behaviors

Abstract

Experiments with different ignition delay times (td) corresponding to different residual turbulent intensities after dispersion were conducted to reveal the effects of turbulent intensity on nano-PMMA flame propagation behaviors. The residual vertical root-mean-square (RMS) turbulent velocities and RMS vorticities measured by a particle image velocimetry system (PIV) at td = 0.9 s, 1.0 s and 1.1 s were 0.22 m/s, 0.16 m/s, and 0.15 m/s and 50.40/s, 38.23/s, and 36.68/s, respectively. It was indicated that the residual turbulent intensity in the combustion space decayed with increasing ignition delay time. One-hundred nanometer PMMA dust flames with a nominal concentration of 450 g/m3 were approximately spherical in shape and propagated continuously after ignition at different ignition delay times. As time proceeded, the flame front of td = 0.9 s was still smooth, whereas the flame fronts of td = 1.0 s and 1.1 s were irregular due to the dominant effects of flame self-instability against the residual turbulence. The average pulsating flame propagation velocities of td = 0.9 s, 1.0 s and 1.1 s were 0.67 m/s, 0.54 m/s and 0.44 m/s, respectively. The pulsating level was enhanced by extending the ignition delay time due to the weaker residual turbulent intensity compared with the turbulence induced by the dust flame self-instability. In addition, the suspended particle size distribution was measured by a Phase Doppler Particle Analyzer (PDPA). It was found that the effective suspended particles existed as agglomerates of fine particles rather than as the primary particles themselves.