New High Quality Diesel Nozzle L017PBB for Injection Nozzle Diesel Engine Parts

Abstract
To address the issues of large atomized particle size and insufficient turbulence intensity in conventional cylindrical nozzles under high-pressure injection, this paper focuses on the convergent-divergent nozzle design for fuel injectors (referred to as "convergent-divergent nozzles"). Combining spray breakup theory with multiphysics simulation, this paper systematically analyzes the mechanisms by which structural parameters such as throat diameter, divergence angle, and contraction angle influence fuel velocity distribution, turbulence intensity, and spray particle size (SMD). The study shows that by creating a supersonic jet (exit velocity reaching 420 m/s) and strong turbulence (turbulence intensity >25%) in the diverging section, the convergent-divergent nozzle reduces SMD by 28% compared to cylindrical nozzles. Optimal atomization is achieved with a throat diameter of 0.15 mm, a contraction angle of 30°, and a divergence angle of 15°, achieving a minimum SMD of 12 μm. This mechanism provides a new approach for optimizing the nozzle structure of high-pressure common rail injectors.

I. Introduction
In high-pressure common rail systems (rail pressure 150-300 MPa), the jet characteristics at the nozzle outlet directly determine the spray breakup effect. Traditional cylindrical nozzles, due to their constant flow channel cross-section, tend to form a "rigid liquid core" at the outlet. Secondary breakup relies on external aerodynamic forces, resulting in a large atomized particle size (SMD typically 20-25 μm), making it difficult to meet the mixture uniformity requirements of China VII emission standards. The convergent-divergent nozzle (inspired by the Laval nozzle) accelerates the fuel to the speed of sound in the convergent section and then further exceeds the speed of sound in the divergent section. This utilizes the synergistic effect of shock waves and turbulence to enhance liquid core breakup, providing a structural innovation for improving atomization performance.
Current research focuses primarily on the flow characteristics of convergent-divergent nozzles, ignoring the inherent mechanisms that influence spray breakup—such as how the shock wave structure in the divergent section alters the stress state of the liquid core and the interaction between the supersonic jet and air. This paper constructs a "flow field-breakup-particle size" correlation model to quantitatively analyze the influence of key parameters of a converging-diverging nozzle and reveal the core mechanism for optimizing atomized particle size.

II. Structural Characteristics and Flow Field Properties of a Converging-Diverging Nozzle
2.1 Structural Parameter Definition
A converging-diverging nozzle consists of three parts (as shown in Figure 1):

Converging section: From the inlet to the throat (minimum cross-section), it adopts a conical transition with a convergence angle α (15°-45°) to control the fuel acceleration rate;

Throat: The minimum cross-section of the nozzle, with a diameter d_th (0.12-0.18mm), determines the critical flow velocity (the speed of sound);

Diverging section: From the throat to the outlet, it has a divergence angle β (5°-20°), which enables supersonic jet formation and turbulence enhancement. Taking a certain type of high-pressure common rail injector nozzle as a benchmark, the total nozzle length L was set to 1.2 mm, and the outlet diameter d_out was determined by the divergence angle and throat diameter (d_out = d_th + 2L_exp × tanβ, where L_exp is the divergence length).

2.2 Numerical Analysis of Flow Field Characteristics

A convergent-divergent nozzle flow field model was established using ANSYS Fluent:

Fluid model: Compressible fluid equations were used (considering the density variation of the fuel under high pressure, 860 kg/m³ at 180 MPa). The turbulence model used was SST k-ω (to accurately capture the interaction between shock waves and the boundary layer).

Boundary conditions: Inlet pressure 200 MPa, outlet pressure 0.5 MPa (combustion chamber environment), wall roughness Ra 0.02 μm, and a no-slip boundary was used.

Computational mesh: The throat region was refined (mesh size 5 μm) to ensure that shock wave details were captured. Simulation results show the following:
Velocity distribution: The fuel accelerates in the converging section, reaching the speed of sound (approximately 320 m/s) at the throat. The velocity continues to increase after entering the diverging section, reaching 420 m/s (supersonic) at the exit, a 50% increase compared to the cylindrical nozzle (exit velocity of 280 m/s).

Pressure distribution: The pressure at the throat drops to 80 MPa (critical pressure). An expansion wave forms in the diverging section, causing the pressure to drop to 0.5 MPa at the exit. This represents a threefold increase in pressure gradient compared to the cylindrical nozzle.

Turbulence intensity: Due to the increased velocity gradient in the diverging section (420 m/s to 320 m/s), the turbulence intensity reaches 28%, an 87% increase compared to the cylindrical nozzle (15%), providing sufficient energy for primary breakup.

III. The Effect of Converging-Diverging Nozzle Orifice on Atomized Particle Size
3.1 Effect of Key Parameters on SMD
3.1.1 Throat Diameter (d_th)
When d_th increases from 0.12mm to 0.18mm:
SMD shows a trend of first decreasing and then increasing, reaching its minimum (12μm) at d_th = 0.15mm. When d_th is too small (0.12mm), although the flow velocity at the throat reaches the speed of sound, the flow rate is too low (3.2mm³/s), resulting in insufficient jet momentum and weakened secondary breakup. When d_th is too large (0.18mm), it becomes difficult to form a stable sonic flow at the throat (the flow velocity is only 290m/s), and the diverging section cannot achieve supersonic speeds. The turbulence intensity drops to 20%, and the SMD increases to 18μm. The flow rate increases linearly with d_th. At d_th = 0.15mm, the flow rate is 5.8mm³/s, meeting the engine's single injection volume requirement (5-7mm³).

3.1.2 Convergence Angle (α)

When α increases from 15° to 45°:

The SMD is minimized at α = 30° (12μm). When α is too small (15°), the converging section is too long (0.8mm), resulting in gentle fuel acceleration and a throat turbulence intensity of only 22%, leading to insufficient initial fragmentation. When α is too large (45°), the local pressure gradient in the converging section increases sharply (180MPa → 80MPa), causing cavitation (cavitation rate 5%). The cavitation bubble collapses, leading to irregular fragmentation of the liquid core and an increase in SMD to 16μm.

Increasing the converging angle reduces flow resistance at the nozzle inlet, reducing the flow loss rate from 8% to 3%. 3.1.3 Divergence Angle (β)

When β increases from 5° to 20°:

When β = 15°, the SMD is minimized (12μm). When β is too small (5°), the divergence section is too long (1.0mm), the flow velocity increases slowly, and the outlet turbulence intensity is only 23%. When β is too large (20°), a strong shock wave (shock wave intensity 1.8) forms in the divergence section, reducing jet stability and causing droplet aggregation, increasing the SMD to 17μm.

At a divergence angle of 15°, the outlet jet divergence angle reaches 12°, 50% wider than that of a cylindrical nozzle (8°), which improves the spatial uniformity of the spray distribution.​
3.2 Impact Mechanism Analysis

Based on spray breakup theory (KH-RT instability), the converging nozzle reduces atomized particle size through the following three approaches:

Supersonic jet enhances secondary breakup: The outlet velocity reaches 420 m/s, increasing the aerodynamic We number from 25 for cylindrical nozzles to 48 (We > 12 is the threshold for sufficient breakup). This significantly enhances the aerodynamic tearing effect on the liquid core, reducing droplet size by 35%;

Strong turbulence promotes primary breakup: The turbulence intensity in the expansion section is 28%, an 87% increase compared to cylindrical nozzles. Turbulent vortices rapidly break up the liquid core (initial breakup time is shortened from 0.8 ms to 0.3 ms), reducing the initial breakup particle size from 100 μm to 60 μm;

Expansion waves suppress cavitation hazards: The expansion wave in the expansion section causes a gradual pressure drop, keeping the cavitation rate below 2% (the cavitation rate for cylindrical nozzles is 2.5). 8%), avoiding irregular disturbance of droplets caused by cavitation collapse, reducing the SMD standard deviation from 3.5μm to 1.8μm, and improving atomization uniformity by 48%.

IV. Experimental Verification and Performance Comparison
4.1 Spray Characteristics Experiment
On a high-pressure spray test bench (rail pressure 200 MPa, ambient temperature 350°C), a laser diffraction particle size analyzer (Malvern Mastersizer 3000) was used to test the atomization performance of a converging nozzle (dth = 0.15 mm, α = 30°, β = 15°) and a cylindrical nozzle (d = 0.15 mm).

SMD Comparison: The converging nozzle had a SMD of 12 μm, while the cylindrical nozzle had a SMD of 16.7 μm. The atomized particle size of the converging nozzle was 28% lower.

Particle Size Distribution: The converging nozzle had a more concentrated particle size distribution range (8-18 μm) than the cylindrical nozzle (12-25 μm), with a coefficient of variation (CV) of 15% (CV of the cylindrical nozzle was 22%).

Spray Cone Angle: The converging nozzle had a spray cone angle of 12 μm. The 18° nozzle angle is 50% wider than the cylindrical nozzle (12°), increasing spray coverage to 95%.

4.2 Engine Bench Test

Both nozzle types were installed in a 2.0L direct-injection diesel engine for performance testing:

Combustion Efficiency: The converging nozzle improved combustion efficiency from 90.5% to 95.2%, reducing fuel consumption by 4.8%.

Emission Performance: NOx emissions were reduced by 12.3% (due to improved mixture homogeneity and reduced localized high-temperature zones), and particulate matter emissions were reduced by 21.5% (due to a smaller atomized particle size, resulting in more complete combustion).

Power Performance: Maximum power increased by 5.2%, and torque response speed (the time it takes to accelerate from 1000 rpm to 3000 rpm) was shortened by 0.3 seconds.

V. Conclusion
The convergent-divergent nozzle in a fuel injector significantly reduces fuel atomized particle size through the synergistic effect of "supersonic jet flow + strong turbulent disturbance." The core mechanism is: supersonic flow is generated in the throat, while supersonic and turbulent flow are enhanced in the divergent section, significantly increasing the We number and turbulence intensity, providing sufficient energy for spray breakup. The optimal structural parameters are a throat diameter of 0.15mm, a convergence angle of 30°, and a divergence angle of 15°. At this point, the SMD can be minimized to 12μm, a 28% reduction compared to a cylindrical nozzle. The convergent-divergent nozzle atomization mechanism revealed in this study provides a theoretical basis and practical reference for the high-performance design of high-pressure common rail injector nozzles, particularly for engines meeting China VII emission standards and above, which require stringent atomization accuracy.