Enhancement of biodiesel droplet interface deformation and breakup under an electric field

Abstract

Biodiesel suffers from inherently high surface tension and viscosity that impede droplet atomization and subsequent air–fuel mixing upon injection. In this study, biodiesel droplet generation under a direct current electric field (BoE < 6.03) at flow rates of 5–20 ml/h was examined via high-speed photography, focusing on the effect of the electric field on the interface deformation and subsequent droplet breakup. The findings indicate that the imposed electric field markedly enhances interface evolution. Specifically, the outward normal Maxwell stress overcomes the restoring effect of surface tension, driving morphological transitions from dripping mode (BoE < 1.58) to a cone-jet mode (1.58 < BoE < 3.2), and ultimately to a multi-jet mode (BoE > 3.2). The high relaxation time ratio Cτ=6.17 leads to capillary-gravity instability breakup at low electric fields, whereas high fields enable dynamic coupling of charge migration and interface deformation, producing droplet breakup at frequencies above 104 Hz. In the cone-jet regime, the droplet diameter D32 is reduced by approximately 33 times compared to the field-free condition. The size distribution evolves from a broad bimodal profile into a sharp monodisperse peak, with the relative span factor converging to a stable range of 0.3–0.5 for BoE > 1.8. Further validation is conducted by comparing the experimental results with the prediction of the scaling laws. It is found that the droplet size distribution fits better with the scaling laws. Overall, electrospray technology shows significant promise for improving biodiesel atomization and provides a viable pathway toward cleaner combustion in engines.