An experimental investigation of droplet dynamics in the dense near-nozzle region of diesel sprays: Velocity, size, and flow characteristics

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The primary breakup in the near-nozzle region has a significant influence on the subsequent spray characteristics. However, due to the high-density nature of the diesel spray near-field region, the distribution laws of droplet velocity and size remain unclear. This study employed high-energy laser PDA to investigate the velocity and droplet size of high-pressure diesel sprays within a constant volume chamber, focusing on the influence of different radial positions within 10 mm downstream of the nozzle exit and varying injection pressures. Results indicate that droplets near the spray axis exhibit a concentrated distribution in terms of average velocity during the injection duration, with velocity fluctuations ranging within less than 30 m/s. When the injection pressure is increased from 40 MPa to 80 MPa, the droplet velocity increases from 270 m/s to 390 m/s. Droplets at this location are dominated by the primary jet generated by the high injection pressure and remain incompletely atomized, with large droplets in the size range of 10–15 µm prevailing. The motion of droplets at the spray edge is primarily influenced by air entrainment, causing further droplet refinement. The average velocity distribution during the injection period spans a broader range, while droplet size is concentrated around 6 µm. Droplets of different sizes exhibit a greater increase in velocity within the high injection pressure range, with relatively low sensitivity to the measurement position. Finally, based on the modified Stokes number calculation, the radial non-uniformity of the near-field spray is quantitatively revealed. For large droplets with diameters exceeding 20 µm at the axis, the Stokes number is generally greater than 1, exhibiting strong inertial characteristics. In contrast, for small droplets below 10 µm at the periphery, the Stokes number is less than 0.1, demonstrating excellent gas-phase following behavior. The findings successfully characterize droplet dynamics in the dense near-nozzle region, providing critical insights for understanding primary breakup, optimizing nozzle design, and improving spray simulations.

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