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Research Papers: Internal Combustion Engines

Investigation of Nozzle Flow and Cavitation Characteristics in a Diesel Injector

[+] Author and Article Information
S. Som1

Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 West Taylor Street, Chicago, IL 60607-7022ssom1@uic.edu

S. K. Aggarwal

Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 West Taylor Street, Chicago, IL 60607-7022

E. M. El-Hannouny, D. E. Longman

Center for Transportation Research, Argonne National Laboratory, Argonne, IL 60439

1

Corresponding author.

J. Eng. Gas Turbines Power 132(4), 042802 (Jan 15, 2010) (12 pages) doi:10.1115/1.3203146 History: Received November 13, 2008; Revised April 06, 2009; Published January 15, 2010; Online January 15, 2010

Cavitation and turbulence inside a diesel injector play a critical role in primary spray breakup and development processes. The study of cavitation in realistic injectors is challenging, both theoretically and experimentally, since the associated two-phase flow field is turbulent and highly complex, characterized by large pressure gradients and small orifice geometries. We report herein a computational investigation of the internal nozzle flow and cavitation characteristics in a diesel injector. A mixture based model in FLUENT V6.2 software is employed for simulations. In addition, a new criterion for cavitation inception based on the total stress is implemented, and its effectiveness in predicting cavitation is evaluated. Results indicate that under realistic diesel engine conditions, cavitation patterns inside the orifice are influenced by the new cavitation criterion. Simulations are validated using the available two-phase nozzle flow data and the rate of injection measurements at various injection pressures (800–1600 bar) from the present study. The computational model is then used to characterize the effects of important injector parameters on the internal nozzle flow and cavitation behavior, as well as on flow properties at the nozzle exit. The parameters include injection pressure, needle lift position, and fuel type. The propensity of cavitation for different on-fleet diesel fuels is compared with that for n-dodecane, a diesel fuel surrogate. Results indicate that the cavitation characteristics of n-dodecane are significantly different from those of the other three fuels investigated. The effect of needle movement on cavitation is investigated by performing simulations at different needle lift positions. Cavitation patterns are seen to shift dramatically as the needle lift position is changed during an injection event. The region of significant cavitation shifts from top of the orifice to bottom of the orifice as the needle position is changed from fully open (0.275 mm) to nearly closed (0.1 mm), and this behavior can be attributed to the effect of needle position on flow patterns upstream of the orifice. The results demonstrate the capability of the cavitation model to predict cavitating nozzle flows in realistic diesel injectors and provide boundary conditions, in terms of vapor fraction, velocity, and turbulence parameters at the nozzle exit, which can be coupled with the primary breakup simulation.

Copyright © 2010 by Argonne National Laboratory
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Figures

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Figure 1

Schematic of six-hole full-production minisac nozzle. Only two holes are seen in this cross-sectional slice. Nozzle and needle region are identified along with the computational zone used in simulations. The orifice diameter is 169 μm with an included angle of 126 deg.

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Figure 2

(a) Predicted (for two different grid densities) and measured (data from Winklhofer ) mass flow rates plotted versus the pressure difference (ΔP) (b) predicted (for two different grid densities) and measured velocity profiles at a location 53 μm from the nozzle inlet. Simulations are performed at a fixed injection pressure of 100 bar and different back pressures. Grid 1: 90×40; Grid 2: 140×60.

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Figure 3

Comparison between the predicted and measured (data from Winklhofer ) vapor fraction contours for three different back pressures and a fixed injection pressure of 100 bar. In simulations the red color indicates the region of high vapor fraction (significant cavitation) while dark blue indicates the region of zero vapor fraction (no cavitation).

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Figure 4

Rate of injection profiles at different rail pressures

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Figure 5

Grid Generated for cavitation simulations

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Figure 6

Predicted (for two different grid sizes) and measured discharge coefficients for different rail pressures. Correlation from Sarre (41) is also shown. Simulations were performed for Viscor/cerium blend with the base nozzle dimensions.

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Figure 7

K contours computed for injection pressure of 100 bar and back pressure of 1 bar using the different cavitation inception criteria for the nozzle orifice described in Fig. 5. Only the nozzle orifice and sac regions are shown.

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Figure 8

K contours computed for injection pressure of 1367 bar and back pressure of 1 bar using the different cavitation inception criteria for the nozzle orifice described in Fig. 5. Only the nozzle orifice and sac regions are shown.

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Figure 9

(a) Discharge coefficient and initial amplitude parameter plotted versus the Reynolds number for different flow regimes, (b) discharge (Cd), and area contraction (Ca) coefficients plotted versus the Reynolds number in the turbulent and cavitation flow regimes. Simulations were performed at full needle open position for European diesel No. 2 fuel, base nozzle dimensions, and a fixed back pressure of 1 bar.

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Figure 10

Cavitation (vapor fraction) contours for different injection pressures used in the context of Fig. 9, and a fixed back pressure of 1 bar. Simulations were performed with base nozzle dimensions for European diesel No. 2 fuel.

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Figure 11

(a) Discharge coefficient and (b) initial amplitude parameter plotted versus Re for different fuels at full needle open position (0.275 mm) with base nozzle dimensions. Simulations were performed by varying the injection pressure at a fixed back pressure of 1 bar.

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Figure 12

Discharge coefficient plotted versus the cavitation number for different fuels at full needle open position (0.275 mm) with base nozzle dimensions. Simulations were performed by varying the injection pressure and a fixed back pressure of 1 bar.

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Figure 13

Vapor fraction contours (top three) for n-dodecane (a), European diesel No. 2 (b), Viscor/cerium blend (c), and pressure contours (bottom three) for n-dodecane (d), European diesel No. 2 (e), and Viscor/cerium blend (f) at Pin=1000 bar, Pb=1 bar at full needle open position (0.275 mm)

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Figure 14

Vapor fraction contours (top five) at different needle lift positions: (a) 0.275 mm (fully open), (b) 0.2 mm, (c) 0.15 mm, (d) 0.1 mm, and (e) 0.05 mm. Velocity vectors (bottom four) at different needle lift positions: (f) 0.275 mm (fully open), (g) 0.15 mm, (h) 0.1 mm, and (i) 0.05 mm. Simulations were performed with base nozzle and Viscor/cerium liquid blend at Pb=30 bar.

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Figure 15

Discharge coefficient and initial amplitude parameter plotted versus needle lift position, as discussed in context of Fig. 1, for two peak injection pressures. Simulations were performed for the Viscor/cerium blend with the base nozzle orifice dimensions.

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