The Third Body Effect of Carbon Dioxide on N-heptane Ignition Delay Characteristics under O₂/CO₂ Conditions

ABSTRACT

In order to study the impact of CO₂ during n-heptane ignition delay under O₂/CO₂ conditions, this study presents a numerical and experimental study in four cases (40 mol.% O₂/60 mol.% CO₂, 50 mol.% O₂/50 mol.% CO₂, 57 mol.% O₂/43 mol.% CO₂ and 65 mol.% O₂/35 mol.% CO₂). Firstly, a third body effect (TBE) model is presented, which considers the third body efficiency of CO₂. Secondly, the TBE model adds the third body efficiency into a one-step global mechanism and obtains an ignition delay time formula according to the auto-ignition theory. Thirdly, experiments are carried out in a constant volume combustion chamber (CVCC) test system. Furthermore, the experiment and simulation results are compared and discussed. In addition, two parameters (α and β) are defined to study the third body effect and the chemical effect of CO₂ respectively. Finally, chemical kinetic analyses are used to study the CO₂ effect. The results show that the TBE model is in agreement with the experiment and the maximum difference is 8.94% under the 57 mol.% O₂/43 mol.% CO₂ condition. The third body effect is the main effect of CO₂ and it can decrease small molecule intermediates and radicals. Besides, C₃H₆O is the most affected species by the third body effect of CO₂ among four prominent intermediate species. Furthermore, the third body effect of CO₂ inhibits most paths of OH production. H+ O₂→O+ OH (R11) is the most affected pathway whose peak rate of production (ROP) decreases by 51.5%.

KEYWORDS:

• Ignition delay time
• O₂/CO₂ conditions
• CO₂ effect
• n-heptane

Acknowledgments

The authors thank for the financial support provided by the National Natural Science Foundation of China (No. 51976007), Beijing Municipal Natural Science Foundation (No. 3192011) and the BUCEA Postgraduate Innovation Project (No. PG2020009).

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [51976007]; Beijing Municipal Natural Science Foundation [3192011]; BUCEA Postgraduate Innovation Project [2020009].