LAUSR

Abstract The flash boiling of fuel sprays can occur in engines when the fuel is injected into the ambient gas which pressure is lower than the saturation vapor pressure of the fuel. Recently, the flash boiling has attracted many engine researchers’ interest due to its superior spray atomization performance. It has been discussed that the formation of the flash boiling sprays is governed by the bubble dynamics in the fuel such as bubble nucleation, growth, and burst. However, the detailed bubble dynamics and its effects on near-nozzle spray dynamics have remained a mystery due to great difficulties in the observation of flow characteristics inside and near the nozzle. The current study investigates the near-nozzle dynamics of flash boiling sprays from a 6-hole gasoline-direct-injection injector using an X-ray imaging technique. Various Rp (ratio of saturated vapor pressure to ambient gas pressure) conditions were applied by altering the ambient pressure under the fixed fuel temperature. The results showed that the flash boiling did not alter significantly the emerging flow velocity at the nozzle exit. The center of flash-boiling sprays did not decelerate in the near-nozzle region while that of the subcooled spray showed a gradual deceleration. The radial velocity of the flash boiling sprays increased at the edge of the spray as Rp increased while that in the spray center was near zero regardless of Rp condition. The radial velocity of the flash boiling spray was solely governed by Rp regardless of injection pressure while that of the subcooled sprays was altered by the injection pressure. Based on the results, the initial formation mechanism of the flash boiling sprays and the associated bubble dynamics were thoroughly discussed.