A study of the relation between nozzle geometry,internal flow and sprays characteristics in diesel fuel injection systems

This study examines the influence of geometry on the internal flow and macroscopic behavior of the spray in Diesel nozzles. For this investigation, two bi-orifice nozzles were employed : one cylindrical and one conical. The first step is to use a non-destructive characterization method which is based on the production of silicone moulds so that the precise internal geometry of the two nozzles can be measured. At this stage the nozzles have been characterized dimensionally and therefore the internal flow can be studied using CFD calculations. The results gained from this experiment make it possible also to ascertain the critical cavitation conditions. Once the critical cavitation conditions have been identified, the macroscopic parameters of the spray can be studied in both cavitating and non-cavitating conditions using a test rig pressurized with nitrogen and with the help of a image acquisition system and image processing software. Consequently, research can be carried out to determine the influence that cavitation has on macroscopic spray behavior. From the point of view of the spray macroscopic behavior, the main conclusion of the paper is that cavitation leads to an increment of the spray cone angle. On the other hand, from the point of view of the internal flow, the hole outlet velocity increases when cavitation appears. This phenomenon can be explained by the reduction in the cross section of the liquid phase in the outlet section of the hole.     

Measurement and analysis of injection characteristics among each nozzle hole within a heavy-duty diesel engine

The mass flow rate from each injector nozzle hole of a diesel engine influences the distribution, atomization, and combustion of fuel in the chamber. Thus affecting the power, the fuel economy, and the emission quality of the diesel engine. A spray momentum flux test bench was built and used to measure the injection rate from each nozzle hole of a multi-hole nozzle in this study. Selected force sensors used for data acquisition were one of the integral parts of the set-up. The influence of the force sensors’ installed position (location in the set-up) on measured results, were analyzed and the optimum position that ensures independence of the results, determined. Additionally, the effects of injection pressure, injection pulse width and injection hole diameter on the injection characteristics were also investigated. Furthermore, in this research, the reliability and robustness of Strain sensor and Piezoelectric sensors were analyzed with regards to their response. The analysis showed that, strain sensors have weak dynamic response characteristic compared to piezoelectric sensors also, the measured result obtained from strain sensors fluctuated greatly. Piezoelectric force sensor gave a more reliable and stable measurement, comparatively. The accuracy of the results were affected by the installation position of the sensors. A distance of 16 mm (between nozzle hole exit and sensor surface) was determined to be adequate for the acquisition of reliable experimental data. As the injection pressure gets higher (during injection), the rate of mass flow increased, the average cycle-to-cycle variation coefficient and nozzle-to-nozzle variability coefficient of injection quantity decreased. Hence, improving the consistency of each cycle and the uniformity of each hole. In addition, increasing the injection pulse width decreased the average cycle-to-cycle variation coefficient. Also, nozzle-to-nozzle variability coefficient had minimal or no influence with regards to injection pressure. At 80 MPa, the uniformity of injection from the multi-hole nozzle improved significantly. In summary, the larger the hole diameters, the higher the maximum value of mass flow rate and the fuel injection quantity.     

Experimental study on unsteady characteristics of the transient cavitation flow

The characteristics of unsteady cavitation flow under different cavitation numbers are investigated experimentally. An optical test rig is designed to visualize the cavitation flow and the cavity length is defined based on high-speed camera technology. The development of cavitation flow can be divided into inception and fusion region, expansion region and collapse region. Due to the interaction between the re-entrant jet and the cavity interface, the cavitation bubbles near the solid wall collapse earlier than them in the downstream. The cavitation number is also the important factor determining the cavity length pulsation. Further analysis shows that the smaller the cavitation number, the smaller the frequency of cavity length fluctuations and the larger the mean cavity length. The cavitation intensity is stronger at lower cavitation number. Meanwhile, the cycle collapsing frequency of cavitation is around 220–320 Hz. With the decrease of cavitation number, the amplitude of cavitation pulsation increases while the corresponding collapsing frequency decreases.