Theoretical prediction model and experimental investigation of detonation limits in combustible gaseous mixtures

Propagation limits in gaseous detonation are an important problem in industrial chemical processes due to safety protections and precautions. Propagation limits are also of great significance for developing new-concept detonation engines. Due to the complex phenomena of near-limit detonation and the multiple competing mechanisms between the properties of mixtures and boundary conditions, development of a universal theoretical prediction model that can satisfactorily estimate the detonation velocity deficit (the difference between the actual detonation velocity and the theoretical CJ detonation velocity) near the limits is still extremely challenging. This work focuses on the essential relation between the detonation structure and prediction models. By analyzing the chemical length scales in the detonation structure of the ZND model, it is found that Delta(I) (induction length) and Delta(R) (exothermic length) are important length scales and that they are related to the reaction zone thickness (x) by: x = C.(Delta(I) + alpha.Delta(R)). In combination with Fay's boundary layer theory, a modified theoretical prediction model (MF) is proposed. The MF model is also compared with previous Fay models and experimental results. To verify the reliability of the MF model, three combustion mixtures (CH4-2O(2), 2H(2)-O-2 and C2H4-6N(2)O) and tubes with three different inner diameters (d = 36, 14 and 4 mm) are investigated. The results show that the MF model satisfactorily predicts the detonation deficit at the near-limit conditions. In the macroscale tube (d = 36 mm), the maximum difference with the experiment data is 9.3%. In the microscale tube (d = 4 mm), the maximum difference is 10.7%. If experimental measurement error is considered, the prediction of the MF model has reasonable accuracy. The results confirm that the detonation structure has an important impact on detonation limit predictions.