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Case study: auto-ignition analysis with Dacolt PSR

Introduction

Dacolt has conducted a small, systematic investigation of the auto-ignition process of a n-heptane / air mixture. It demonstrates the use of Dacolt PSR for studying auto-ignition behavior in Diesel engines, for example.

The case study consists in the following steps:

  1. Set-up and executing Dacolt PSR simulations for different initial conditions of equivalence ratio, temperature and pressure
  2. Automated post-processing of the Dacolt PSR results, to determine the first and second ignition delays
  3. Analysis of the resulting ignition data
  4. Export of the ignition data to a CFD look-up table, to include in CFD simulations (optional)

These steps are described in the following. The resulting datasets can be viewed directly on our website:

Simulations with Dacolt PSR

In the first step, the reaction kinetics are computed for a number of combinations of equivalence ratio, temperature and pressure. The reaction mechanism by Curran et al. is used. This is done with Dacolt PSR in database mode, using the input script shown in Figure 1.

Writing such script and starting a simulation consumes less than 5 minutes of user time. Calculation of the dataset (153 points) is very quick as Dacolt PSR makes use of scalable Cloud Computing technologies. The user is notified by email once the simulation is completed and the data is ready for visualisation or download.

In the computed dataset, temperature and species mole fractions are stored as a function of time, as shown in Figures 2 and 3. You can also browse the complete dataset of Dacolt PSR results.

Data post-processing

In the second step, the temporal information is reduced to the two ignition delay times, as shown in Figure 4. This ignition data analysis is done using an add-on post-processing module, which is automatically started after completion of the Dacolt PSR simulations. Upon completion of the post-processing procedure, the resulting dataset of ignition delays is made available for visualisation or download.

It is common practice to plot delays as a function of temperature, as presented in Figure 5. You can also browse the complete resulting ignition delays dataset.

Data analysis

The delay times in Figure 5 show abnormal behaviour in the range 850-1000 K. After reaching a minimum around 900-950 K, they increase with increasing temperature. This is in sharp contrast to the general intuition that chemical reactions run faster at higher temperature.

A comprehensive explanation for this phenomenon, known as the cool flame or Negative Temperature Coefficient (NTC) zone, can be found in the work of Curran et al. [1]. In the cool flame zone the transition happens from a low-temperature reaction pathway to a high-temperature reaction pathway. Outside the cool flame zone both delays coincide, as can be seen in Figure 5 for the high-temperature range.

Brief examination of the complete ignition delay dataset yields the following observations:

  1. With increasing pressure, the delay times become smaller. At the same time, the transition temperature increases, so that the cool flame persists for higher temperatures.
  2. With increasing equivalence ratio, the delay times become smaller. The transition temperature remains unchanged.

Export data to CFD look-up table

As a last step, relevant data from the ignition dataset can be exported to a CFD look-up table using available in- and output modules. For example, ignition data can be exported in appropriate format for the TKI [2] model for auto-ignition, which is used in the ECFM-3Z [3] and ECFM-CLEH [4] combustion models.

Such a CFD look-up table is then read at the start of the CFD simulation and combustion chemistry data is interpolated during the iterative solution procedure. Figure 6 illustrates this process.

References

  1. Curran, H. J., Gaffuri, P., Pitz, P., and Westbrook,C. K., Combust. Flame 114:149-177 (1998).
  2. Colin, O., Pires da Cruz, A. and Jay, S., Proc. Combust. Inst. 30:2649-2656 (2005).
  3. O. Colin et al., Oil Gas Sci. Technol. 58 (1) (2003) 47-62.
  4. G. Subramanian et al., SAE Technical Paper Series 2007-01-0154.

Further links

Contact us for more information or feedback on this case study.