Multi-Dimensional Modeling:
Fuel Spray Modeling

Fig 1. (top) Cavitation induced, (middle) turbulence induced, and (bottom) aerodynamically induced, near-nozzle primary breakup mechanisms.

In a diesel engine, liquid fuel is injected into the combustion chamber near the end of the compression stroke. Following injection, the fuel undergoes atomization and vaporization processes, followed by fuel-air mixing, ignition and combustion processes. It is well known that the primary breakup of the fuel influences downstream processes such as mixing, ignition combustion, etc.

Primary fuel atomization is induced by aerodynamics in the near-nozzle region, as well as cavitation and turbulence from the injector nozzle. The breakup models that are currently used in diesel engine simulations generally consider aerodynamically induced breakup using the Kelvin-Helmholtz (KH) instability model, but do not account for in-nozzle flow effects.

Argonne transportation engineers, in collaboration with the University of Illinois at Chicago, have developed and incorporated the Kelvin-Helmholtz-Aerodynamics Cavitation Turbulence (KH-ACT) model into CONVERGE code. This improved primary breakup model considers cavitation and turbulence effects along with aerodynamically induced breakup.

Primary Breakup Model

Length and time scales are calculated (Fig. 1):

• Cavitation induced breakup: Based on bubble collapse and burst times
• Turbulence induced breakup : Based on k-ε model
• Aerodynamically induced breakup: KH and Rayleigh Taylor (RT) instability
• Dominant ratio of length/time scale causes breakup
• Extensive model validation against X-ray data (link to APS work) at Argonne

The spray simulations using the KH-ACT model are coupled with inner nozzle flow computations. This presents a novel tool that captures the influence of in-nozzle flow and fuel properties on spray, combustion and emission processes. (Fig. 2)

Fig 2. Implementation of primary breakup model

Spray Validation Against X-ray Data

The X-ray radiography data provides near-nozzle fuel distribution which is probably impossible to obtain with any other technique. This near-nozzle data is used for validation of the KH-ACT primary breakup model. Simulations with KH model are also presented for comparison purposes. The projected density plots from data are Gaussian in nature, both near the nozzle (0.3mm) and far afield (7mm). Simulations capture the Gaussian mass distributions from X-ray data well. Spray mass in the core (peak of the Gaussian curve)  and dispersion (tail of the plot) are accurately captured by only the KH-ACT model. The KH model under-predicts spray dispersion and consequently, overpredicts spray mass in the core. (Fig. 3)

The KH-ACT primary breakup model has been extensively validated against other X-ray radiography data from Argonne's Advance Photon Source and optical data from other national laboratories.

Collaborators in this work include University of Illinois at Chicago, Caterpillar, Inc. and Convergent Science, Inc.

Fig 3. KH vs. KH-ACT model in predicting Argonne's x-ray radiography data

Funding

This work is supported by the U.S. Department of Energy’s Vehicle Technologies Program under Gurpreet Singh.

Publications

1. S. Som, S.K. Aggarwal, "Effect of Primary breakup modeling on spray and combustion characteristics of compression ignition engines," Combustion and Flame 157: 1179-1193, 2010.
2. S. Som, A.I. Ramirez, S.K. Aggarwal, A.L. Kastengren, E. El-Hannouny, D.E. Longman, C.F. Powell, P.K. Senecal, “Development and Validation of a Primary breakup model for Diesel Engine Applications,” 2009-01-0838, SAE 2009 World Congress, Detroit, April 2009.
3. A.I. Ramirez, S. Som, S.K. Aggarwal, A.L. Kastengren, E. El-Hannouny, D.E. Longman, C.F. Powell, “Quantitative X-Ray Measurements of High-Pressure Fuel Sprays from a Production Heavy Duty Diesel Injector,” Experiments in Fluids 47: 119-134, 2009.
4. S. Som, S.K. Aggarwal, "Assessment of Atomization models for Diesel Engine Simulations," Atomization and Sprays 19(9): 885-903, 2009.
5. A.I. Ramirez, S. Som, S.K. Aggarwal, A.L. Kastengren, E. El-Hannouny, D.E. Longman, C.F. Powell, “Characterizing Spray Behavior Differences between Common Rail and Unit Injection Systems Using X-Ray Radiography,” 2009-01-0846, SAE 2009 World Congress, Detroit, April 2009.

July 2011