Experimental Study And Numerical Analysis On Cone-Jet Electrospray Of Liquid Fuel

With the development of new technologies, more and more attention has been focused on the micro-power systems using micro-scale combustors, the miniaturized power generating device with liquid hydrocarbon fuels becomes competitive with that needs batteries for power. Because the electro-spraying can produce monodisperse liquid droplets easily, it has been adapted in the field of combustion.In order to design reasonably a micro-scale electrospraying combustor, two new micro-scale electro-spraying combustors with or without a ring electrode were designed. The experimental studies on the electro-spraying from the micro-scale combustors were carried out using liquid ethanol as fuel. The photos of electro-spraying were taken by a digital camera, the cone angle and voltage were measured, the comparison was performed between those of nozzle electrode combustor and nozzle-ring electrode combustor. The voltage was measured by a data acquisition monitoring the voltage across a resistor placed between the grid and the ground, the atomized electric current was obtained using the law of ohm. The size and velocity distributions of electro-spraying droplets were measured by a Phase Doppler Anemometer both for the nozzle system(without a ring electrode) and the nozzle-ring system(with a ring electrode). The ethanol-air two-phase flow level set method was used to simulate the flow of ethanol. The results were shown as follows:(1) The critical semi-cone angle measured in both the nozzle system and nozzle-ring system is far less than the Taylor angle of 49.3o, closer to the theoretical calculating value of 34.72 o. Under the same spraying mode, the cone formed in the nozzle-ring electrode combustor is less than the one formed in the nozzle electrode combustor.(2) The capillary electrode voltage could be significantly reduced when adopting the nozzle-ring electrode combustor and choosing an appropriate ring electrode voltage. Comparing with the nozzle electrode combustor, the formation of stable cone-jet were enhanced, and the electric power consumption were reduced in the nozzle-ring electrode combustor.(3) When the flow rate was 1 ml/h, and the applied voltage on the ring electrode Vr was 1 kV, the nozzle voltage Vn was 4.19 kV, the semi-cone angle simulated by Comsol was 37.3o, that measured in the experiments was 38.1o, a good agreement with them was obtained, and the accuracy of the simulation was well verified.(4) The simulation results show that axial and radial electric field intensity will increase gradually in the internal of the cone-jet, reach the maximum at the interface, and then gradually decrease in the region near to the capillary. The circulation of the velocity vector is found outside the jet-flow, and the flow moves along the tangential direction, which promotes the formation of cone jet.(5) The electric field strength for the nozzle-ring system was smaller than that for the nozzle system according to the superposition principle of electric field. Comparing with the nozzle system, a more stable cone-jet mode was obtained in the nozzle-ring system, the nozzle-ring electro-spraying system can produce smaller and more uniform droplets because of the more stable cone-jet mode.(6) Based on the results of non-dimensional analysis, it was found that the atomization current in both the nozzle system and nozzle-ring system obey the scaling law as a function of flow rate of ethanol. The charge density decreases with the diameter of the droplet, its change law in both of the nozzle system and the nozzle-ring system showed good agreement with the Rayleigh limit.