Alternating Currents Enhanced PAHs Formation in Nonpremixed Jet Flames
In laminar nonpremixed propane jet flames, it was found that alternating current (AC) electric fields applied to a jet nozzle could decrease flame yellow luminosity significantly. Planar laser-induced fluorescence (PLIF) showed polycyclic aromatic hydrocarbons (PAHs) started to form near the nozzle exit, and the concentration of PAHs increased around 30 times with the AC electric fields. The variations in PAHs were much sensitive to the applied voltage than the frequency of AC. Elastic scattering of an UV laser indicated that a toroidal vortex was formed near the nozzle with AC. Due to the recirculating nature of a vortex, generated heat from combustion was accumulated inside the vortex, thus it provided a favorable reaction environment to PAHs to form. Through a particle image velocimetry (PIV), we found that reduction in effective cross-section of fuel jet due to the existence of the toroidal vortex resulted in the increase of local fuel velocity near nozzle exit. Consequent reduced flow residence time acted negatively for soot to form, thus decreased in flame luminosity can be explained. The onset conditions for the vortex were measured with different fuel velocities. The critical conditions can be correlated with the applied voltage and frequency in the form of V a×f = constant. Since the coefficient a was around 2, it confirms stronger dependence of applied voltage rather than AC frequency.