The Effect of Conjugate Heat Transfer on Soot Formation Modeling at Elevated Pressures

Abstract

Previous attempts to model elevated-pressure coflow laminar flames have been hindered by neglecting the preheating within the burner, caused by downward heat transfer to the burner. In the present work, the computational domain is extended below the exit plane of the fuel tube to account for the flame preheating effect. Conjugate heat transfer (CHT) is implemented by the harmonic mean method to model the heat transfer between the fluid streams and solid fuel tube. This extension of the domain allows for solutions to a high pressure ethane/air data set from 2 to 33 atm, which is an improvement over previous attempts where only pressures below 15 atm could be modeled. The extended model more accurately predicts centerline soot formation than the truncated model due to capturing fuel pyrolysis that occurs below the exit plane of the fuel tube. This increased fuel pyrolysis increases the contribution by polycyclic aromatic hydrocarbon (PAH) condensation to computed soot volume fractions. For pressures above 20 atm, the extended model predicts that PAH condensation is the dominant mechanism on the wings—a location where surface growth was thought to be heavily dominant regardless of flame characteristics. It is determined that the choice of fuel tube wall treatment does significantly affect numerical predictions, with the best experimental agreement obtained using a CHT model as oppose to an isothermal or adiabatic walls. The extended model with CHT is the only model that displays quantitative accuracy and is a significant advancement over previous qualitatively accurate models.

[Supplementary materials are available for this article. Go to the publisher's online edition of Combustion Science and Technology for the following free supplemental resource: Numerical details.]

Keywords:

• Conjugate heat transfer
• Elevated pressure
• Ethane
• Laminar diffusion flame
• Soot model

ACKNOWLEDGMENTS

The authors acknowledge the Natural Sciences and Engineering Research Council of Canada for financial support. The authors would like to thank Professor Ömer L. Gülder for providing the experimental dataset with which the numerical computations are compared, and Dr. Nadezhda Slavinskaya and Prof. Uwe Riedel for providing the chemical reaction mechanism, thermodynamic data, and transport data for ethane combustion and PAH formation. Computations were performed on the GPC supercomputer at the SciNet HPC Consortium. SciNet is funded by the Canada Foundation for Innovation under the auspices of Compute Canada; the Government of Ontario; Ontario Research Fund—Research Excellence; and the University of Toronto.

Notes

Note: Values in boldface are outside of experimental uncertainty.