Dynamic behaviors of burning droplet suspended on NiCr-wire with applied AC electric field

Abstract

Effect of applied AC electric field on burning n-decane droplet suspended on NiCr wire (0.5 mm in diameter) was investigated experimentally by varying the AC frequency f_AC (10−1000 Hz) and voltage V_AC (1−5 kV). Three initial diameters of n-decane droplets were tested. Due to the difference in droplet surface temperature between the edge region and the central bottom region of the suspended droplet, caused by the heat transfer through the wire from the flame, an internal circulation is generated inside the droplet during burning, which enhances the evaporation rate, subsequently increases the flame width and height. When an AC is applied, the dynamic behaviors of droplet and flame are significantly influenced. Depending on V_AC and f_AC, six regimes can be classified: dripping (DP) of droplet (Regime I) for large droplets due to decreased surface tension from heat transfer through the wire; vertical oscillation (OS) of the droplet due to a vertical dielectrophoretic force (II); lateral spread (LS) and OS of droplet (III) caused by a horizontal dielectrophoretic force between wire sections with and without the droplet; LS and a flame-spitting (FS) phenomenon due to excessive LS (IV); LS and formation of flame vortex (FV) near flame edges due to induced magnetic field (V); and the simultaneous occurrence of LS, FS, and FV (VI). The behavior of vertical oscillation was examined from the solution of the linearly-perturbed Rayleigh-Plesset equation, where the OS amplitude of the droplet increases with V_AC and decreases with f_AC. The mechanism of lateral spread was investigated by analyzing the onset conditions, specifically in terms of V_AC and f_AC. For burning droplets, it was observed that the vertical oscillatory motion is enhanced due to the imbalance in surface tension between the upper and lower surfaces of the droplet and the internal flow becomes faster due to the reduced droplet viscosity. The internal flow response time decreases with f_AC, leading to an increase in oscillation amplitude with increasing V_AC and/or f_AC. The mechanism of the induced magnetic field was tested through the onset conditions having a constant critical magnetic field intensity. The normalized droplet lifetime is characterized by their related physical parameters, including the gradient of radial electric field intensity at the central surface of droplet, f_AC, droplet diameter, and flame width and height. The droplet lifetime shows a strong correlation with the combination of these parameters in both the OS and LS cases.