Validation of leading point concept in RANS simulations of highly turbulent lean syngas-air flames with well-pronounced diffusional-thermal effects

Salman Verma,Florian Monnier,Andrei N Lipatnikov

doi:10.1016/j.ijhydene.2021.01.022

Abstract

While significant increase in turbulent burning rate in lean premixed flames of hydrogen or hydrogen-containing fuel blends is well documented in various experiments and can be explained by highlighting local diffusional-thermal effects, capabilities of the vast majority of available models of turbulent combustion for predicting this increase have not yet been documented in numerical simulations. To fill this knowledge gap, a well-validated Turbulent Flame Closure (TFC) model of the influence of turbulence on premixed combustion, which, however, does not address the diffusional-thermal effects, is combined with the leading point concept, which highlights strongly perturbed leading flame kernels whose local structure and burning rate are significantly affected by the diffusional-thermal effects. More specifically, within the framework of the leading point concept, local consumption velocity is computed in extremely strained laminar flames by adopting detailed combustion chemistry and, subsequently, the computed velocity is used as an input parameter of the TFC model. The combined model is tested in RANS simulations of highly turbulent, lean syngas-air flames that were experimentally investigated at Georgia Tech. The tests are performed for four different values of the inlet rms turbulent velocities, different turbulence length scales, normal and elevated (up to 10 atm) pressures, various H2/CO ratios ranging from 30/70 to 90/10, and various equivalence ratios ranging from 0.40 to 0.80. All in all, the performed 33 tests indicate that the studied combination of the leading point concept and the TFC model can predict well-pronounced diffusional-thermal effects in lean highly turbulent syngas-air flames, with these results being obtained using the same value of a single constant of the combined model in all cases. In particular, the model well predicts a significant increase in the bulk turbulent consumption velocity when increasing the H2/CO ratio but retaining the same value of the laminar flame speed.

Highlights

Due to unique characteristics of H2-air flames, such as a high laminar burning velocity, a wide range of flammability limits, a low ignition energy, etc. [1], hydrogen is considered to be an additive capable for significantly improving basic characteristics of combustion of fossil fuels [2e10], as well as renewable fuels such as biogas [11e13]
The present paper aims at filling this knowledge gap by performing Reynolds-Averaged Navier-Stokes (RANS) simulations of recent experiments done by Venkateswaran et al [30,31] with highly turbulent lean syngas-air flames
As reviewed elsewhere [65,67], capabilities of the Turbulent Flame Closure (TFC) model [68,69] used in the present work for predicting dependencies of turbulent burning velocity UT on u0 were already documented by various research groups in RANS simulations of different experiments performed under substantially different conditions

Summary

Introduction

Due to unique characteristics of H2-air flames, such as a high laminar burning velocity, a wide range of flammability limits, a low ignition energy, etc. [1], hydrogen is considered to be an additive capable for significantly improving basic characteristics of combustion of fossil fuels [2e10], as well as renewable fuels such as biogas [11e13]. The performed 33 tests indicate that the studied combination of the leading point concept and the TFC model can predict well-pronounced diffusional-thermal effects in lean highly turbulent syngas-air flames, with these results being obtained using the same value of a single constant of the combined model in all cases.

Results

Conclusion