Understanding the reversal dilution effect of the species sampling in the negative temperature coefficient region

Species sampling during fuel autoignition can provide valuable insights in studying the chemical kinetic mechanism. Various species sampling techniques have been developed for species quantification in different reactors. However, unavoidable dilution in the physical sampling brings uncertainty to the measurements. Particularly in the negative temperature coefficient (NTC) region, the effect of dilution remains unclear. To address this issue and improve measurement accuracy, numerical species sampling during the autoignition of n-butane mixture within the NTC region was studied in a rapid compression machine reactor. Results show that greater fuel consumption and intermediate generation were observed in regions with lower temperature instead of the hot core zone. Consequently, the dilution effect on the sampling results is reversed inside the NTC region. Furthermore, results from two typical sampling methods were compared with the target core profiles to determine their uncertainties. The feasibility of applying dilution ratio calibration on the sampling results of these two methods was also tested, which further indicates the necessity of considering the NTC effect, particularly for intermediates with lower concentrations. Alternatively, increasing the sampling duration of the fast-sampling method effectively reduces the uncertainty of species concentration and enhances the robustness of mechanism validation regardless of the NTC behavior.