| For a multi-hole diesel injector, there are injection rate diversities among nozzle holes due to the inaccuracies in workmanship and the differences in structure parameters and hydraulic conditions among nozzle holes, which will lead to the non-uniform spatial and temporal distributions of the fuel within the combustion chamber and affect the combustion and emission characteristics of diesel engine. The measuring methods and equipments used for injection rate determination commonly can give the accurate testing result of the total injection rate of a multi-hole diesel injector, but they can’t provide any information about the possible differences in injection rates among nozzle holes. A small number of scholars have ever carried out some relative researches about the measuring methods and equipments used for the measurement of injection rate of each nozzle hole, however, there are few reports about the methods and equipments validated experimentally which could measure the real injection process, have adequate response characteristics and the potential to be applied easily.So far, a lot of scholars have carried out the simulation researches on the internal flow characteristics of diesel injector. However, these studies were mainly focused on the influences of injection conditions, structure parameters of diesel injector, needle valve movement and different fuels on the internal flow characteristics. For a multi-hole diesel injector, the diversities of the internal flow characteristics among nozzle holes do exist due to the inaccuracies in workmanship and the differences in hydraulic conditions among nozzle holes. Nevertheless, few scholars have ever carried out the above related researches. Due to the significant effects of the differences in internal flow characteristics among nozzle holes on injection characteristics of each nozzle hole, fuel atomization, fuel evaporation, fuel-air mixing and combustion process, it is necessary to conduct the above related researches.Based on the mass conservation law, Bernoulli’s equation, the relationship between the mean velocity at the outlet and the discharge coefficient, the relationship between the discharge coefficient and the spray momentum flux, the momentum conservation law and so on, a transient measuring method for injection rate of each nozzle hole based on spray momentum flux is proposed. According to the measuring method proposed, the experimental rig used for the determination of injection rate of each nozzle hole is built. Calibrated piezoelectric force sensors were employed to detect the spray impact forces by means of circular targets screwed directly on the sensor heads. A magnetic stand used for the positioning of the target-sensor assembly was equipped with a distance adjusting screw and an angle adjustment knob, which allow the target-sensor assembly to be moved and to be rotated, respectively. A self-developed data acquisition system is employed to measure and record the related parameters. A clamp-on pressure sensor clamped on the cylinder of sac upstream is used to measure the injection pressure. The reliability and stability of the measuring method proposed are validated experimentally under different operating conditions, meanwhile, the influences of the measurement procedure details on the determination of the injection rate are analyzed. A three-dimensional gas-liquid two-phase model of cavitation flow is developed, and it’s prediction accuracy is validated based on the existing experimental data. Combining the bench experiment and the numerical simulation, the influences of fuel temperature, structure parameters of nozzle hole and injection conditions on the injection rate of each nozzle hole and the discrepancies in injection rates among nozzle holes are studied. At the same time, the influences of the structure parameters of nozzle hole and injection conditions on the internal flow characteristics of each nozzle hole are analyzed.The experimental results show that:using the measuring method proposed, the injection rate of each nozzle hole can be measured accurately. With the distance between the outlet and the target of10mm and the angle between the target and spray axis of90°, the relative errors between the cycle fuel injection quantity obtained based on the measurement of the injection rate of each nozzle hole and the experimental result are less than5%under different operating conditions. When the distance between the outlet and the target, the angle between the target and spray axis are less than12mm and100°, respectively, the injection rate time-histories measured at different positions of the force sensor are close to one another for the same nozzle hole. For the multi-hole diesel injector, the injection start will be delayed, the injection end will be advanced, and the cycle fuel injection quantity will be decreased with the increment of the angle between nozzle hole axis and needle axis. With the increase of fuel temperature, the injection start of each nozzle hole will be delayed, the injection duration will be shortened, meanwhile, injection rate, cycle fuel injection quantity and non-uniform coefficient of cycle fuel injection quantities among nozzle holes will be decreased.The numerical simulation results show that:the fuel flowing into a nozzle hole includes two portions entering it from the sac upstream and the sac bottom, respectively. With the increment of the angle between nozzle hole axis and needle axis, the above two parts will be reduced gradually, and the mean flow velocity at the outlet will be decreased little by little. During the opening phase of the needle, the cavitation effect of each nozzle hole is enhanced gradually. During the closing phase of the needle, the cavitation effect of each nozzle hole is increased slightly, which is particularly apparent for the smaller needle valve lift. With the increment of the injection pressure and the orifice diameter, the cavitation effect of each nozzle hole is enhanced gradually. But with the increment of the injection back pressure, the corner radius at the inlet and the orifice length, the cavitation effect of each nozzle hole is decreased little by little. With the increase of the injection pressure, the corner radius at the inlet and the orifice diameter, the injection rate and the cycle fuel injection quantity of each nozzle hole are increased gradually. But with the increase of the injection back pressure and the orifice length, the injection rate and the cycle fuel injection quantity of each nozzle hole are decreased by degrees. With the increment of the injection pressure, the orifice diameter and the orifice length, the discharge coefficient of each nozzle hole is reduced gradually. But with the increase of the corner radius at the inlet and the injection back pressure, the flow coefficient of each nozzle hole is increased little by little. |
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