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Case study: Simulation of lifted Diesel spray flames with Dacolt PSR+PDF

Introduction

The Dacolt PSR+PDF model [1] is a mixture fraction / progress variable approach with presumed Probability Density Function (PDF) modeling of turbulence – chemistry interactions. Tabkin® is used to generate the CFD look-up tables required for the Dacolt PSR+PDF model. Only a minimal number of additional transport equations are solved during the CFD simulation, yielding good and scalable CPU performance.

Test case

A test case of a lifted n-heptane spray flame from the Engine Combustion Network [2] is used to validate the Dacolt PSR+PDF implementation in ANSYS® FLUENT® and demonstrate the use of Tabkin for generating the required CFD look-up tables. A variation of oxygen concentration is simulated, mimicking the effect of EGR on the ignition and flame development processes.

CFD model set-up

The computational domain is a rectangular box of 50x50x108 mm, discretized with 1.1 million hexahedral cells. The unsteady, two-phase, reactive RANS simulations were run for 2 ms of physical time with a time step of 1.0e-6 s.

The CFD look-up table is generated using Tabkin; a detailed reaction mechanism for n-heptane oxidation is used [3]. The simulation time of the CFD simulation was 41 hours on 6 Intel® Xeon® E5620 CPU cores.

CFD Results

Figure 1 illustrates a typical ignition sequence for the 21 vol-% O2 case. The ignition process is similar for all cases:

  • Ignition happens at the tip of the vapor jet, beyond the final point of flame stabilization
  • The ignition kernel grows and consumes previously formed fuel/air mixture very rapidly
  • A quasi-stable jet flame establishes beyond the liquid penetration length

This overall behavior corresponds qualitatively to the chemiluminescence movies available in the ECN database for these cases.

In Figure 2, the simulated and measured flame lift-off lengths are compared, showing good agreement. The observed experimental trend of increasing flame lift-off with decreasing ambient oxygen concentration is closely followed by the simulation results. Figure 3 shows the comparison for ignition delay time, showing again good agreement.

Conclusions

The auto-ignition, development and stabilization of a lifted n-heptane spray flame are successfully simulated with the Dacolt PSR+PDF combusion model, for a range of ambient oxygen concentrations. Detailed, tabulated chemistry is used yielding very good results at a low CPU cost.

References

  1. F.A. Tap and P. Schapotschnikow, SAE Technical Paper Series 2012-01-0152.
  2. http://www.sandia.gov/ecn
  3. Curran, H. J., Gaffuri, P., Pitz, P., and Westbrook,C. K., Combust. Flame 114:149-177 (1998).

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