Simulation on internal flow characteristics of nozzle for diesel engine fueled with biomass blend fuel

Abstract: The internal turbulent flow and cavitation flow of the nozzle have effect on the fuel spray and atomization of diesel engine, and especially they have a great impact on the process of primary atomization. Biodiesel is a kind of renewable alternative fuel of diesel. Previous studies have shown that biodiesel has higher density, viscosity and surface tension. Compared with diesel, biodiesel is less likely to generate cavitation flow inside nozzle. Therefore, this paper intends to add ethanol to biodiesel so as to improve cavitation flow of biodiesel in nozzle and advance biodiesel atomization. The geometry of the actual nozzle is very small and visual experimental research on full-size jet nozzle is relatively difficult. Therefore, in this paper, the effects of the fuel properties on the internal flow characteristics of nozzle were studied by computational simulation approaches. A three-dimensional nozzle model was created by GAMBIT, in which, pressure distribution, velocity distribution and cavitation distribution of diesel, biodiesel and biodiesel/ethanol blended fuel in nozzle were simulated with the mixture multiphase model of FLUENT. During the simulation, for validating the mixture multiphase model, turbulence model and cavity model, the comparison between visual experiment and computational simulation results of diesel cavitation area was carried out under different injection pressures. These models were confirmed to be effective. The simulation results show that the pressure drops rapidly at the joint connecting the pressure chamber and nozzle entrance, and the pressure tends to be stable after entering the nozzle, while the pressure increases slightly near nozzle exit. The pressure decline range of biodiesel is the greatest. Compared with diesel, the pressure of biodiesel reduces by 23.91% at the different cross section of nozzle. The pressure decline range is comparable between diesel fuel and biodiesel/ethanol blended fuel. The fuel flow velocity increases rapidly at the entrance of the nozzle, and the flow velocity rises slowly after entering the nozzle, while the fuel flow velocity slightly decreases near the nozzle exit. From the nozzle center to the nozzle periphery, the fuel flow velocity gradually reduces owing to viscous force of the nozzle wall. At the different cross sections of the nozzle, the flow velocity of diesel is the fastest and its flow velocity reaches 229.8 m/s at the nozzle outlet. The flow velocity of biodiesel/ethanol blended fuel is slower than that of diesel and its flow velocity is 223.1 m/s at the nozzle outlet, while the flow velocity of biodiesel is the slowest among the 3 kinds of fuels, and its flow velocity is 214.9 m/s at the nozzle outlet. The cavitation first occurs at the corner of the nozzle entrance, and then it develops to nozzle exit and gradually weakens. At the different sections of nozzle, the gas volume fraction of diesel is the greatest and that of biodiesel is the smallest. The gas volume fraction of biodiesel drops on the average by 11.1% compared with that of diesel and the cavitation of biodiesel is relatively weaker than that of diesel. However, the gas volume fraction of biodiesel/ethanol blended fuel is nearly comparable with that of diesel and there is only 1.8% difference between blended fuel and diesel fuel. Adding ethanol to biodiesel can reduce fuel density, viscosity and surface tension, improve fuel flow characteristics and promote the cavitation of biodiesel in the nozzle. The cavitation in the nozzle can provide initial disturbance for the circular jet spray and promote fuel atomization.