Evaluation and reconstruction of afterburning reaction kinetics of rocket exhaust plume

Objective Diatomic/polyatomic molecules in rocket exhaust plumes emit specific bands of infrared radiation during high-temperature vibrational transitions, making them crucial radiation sources of concern in the measurement field. Usually, afterburning occurs when rocket exhaust plumes mix with air, releasing a large amount of heat and significantly raising the temperature level and infrared radiation of the plumes. Therefore, afterburning is a crucial step in accurately calculating the reacting flow field parameters and infrared radiation of rocket exhaust plumes. Using computational fluid dynamics (CFD) methods to predict the reaction flow field of rocket exhaust plumes and evaluate the degree of afterburning has become a feasible technical approach. This underscores the importance of constructing an accurate chemical reaction kinetics model to predict afterburning in rocket exhaust plumes. A highly accurate flow field structure is essential, as different chemical reaction dynamics models can lead to significant differences in the composition, content, and distribution of the flow field of rocket exhaust plumes. However, the infrared radiation characteristics of rocket exhaust plumes are extremely sensitive to the flow field temperature, component content and distribution. To improve the accuracy of infrared radiation calculation for rocket exhaust plumes, higher requirements are placed on the chemical reaction dynamics model for accurate flow field parameters of rocket exhaust plumes.

Methods With solid rocket engines as the research focus, the central difference scheme method is employed to solve the three-dimensional Navier-Stokes (N-S) equations with chemical reaction sources. Based on the finite rate chemical reaction model expressed in the Arrhenius formula, a 10-step gas-phase chemical reaction kinetics model is developed for the CO/H₂ reaction system. The gas radiation physical properties parameters are computed using the statistical narrow spectral band model, and the radiation transport equation is solved using the apparent light method. Through fitting of experimental data using the three-parameter Arrhenius formula, a chemical reaction kinetics model is structured to closely match the experimental data.

Results and Discussions  There is little difference in the plume structure calculated by different chemistry models (Fig.4), and a maximum temperature difference of 200 K exists in the area where afterburning occurs (Fig.6). The influence of chemical reaction kinetics on the unstable product CO is most significant (Fig.9(b)). The maximum difference in CO₂ reaches nearly 50% (Fig.9(a)), mainly occurring in high-content regions, and the impact on low mole fraction components varies by 2-3 orders of magnitude (Fig.10). The impact of different chemical reactions on the peak intensity of spectral radiation varies by nearly 40% in the 2.7 μm and 4.3 μm bands (Fig.11), and the difference in integrated spectral band intensity within different bands reaches about 40% (Fig.13). Based on the proposed CO/H₂ reaction system with 9 components and 10 steps, the difference between the calculated spectral intensity of BEM-II plumes' infrared radiation and the measured data is less than 6% (Tab.4).

Conclusions  Different chemical reaction kinetics models have a relatively small impact on the structure of the rocket exhaust plume flow field but have a significant effect on temperature, component generation, and infrared radiation characteristics. For different chemical reaction models, the collision frequency, temperature correlation index and activation energy parameters corresponding to each chemical reaction kinetic equation are different. Under the same operating conditions, the reaction rate and heat release (absorption) between components are different, which affects the temperature, component content, and distribution of the reaction flow field. A chemical reaction kinetics model with small errors was constructed by combining the trend of positive reaction rate curves corresponding to each reaction kinetics equation and experimental data. This study can provide high-fidelity chemical reaction models for accurately predicting the reaction flow field and infrared radiation of rocket exhaust plumes.