From inherent correlation to constrained measurement: Model-assisted calibration in MBMS experiments

Synchrotron-based molecular-beam mass spectrometry (MBMS) can provide detailed species-resolved information to help develop, validate and optimize combustion kinetic models. While quantification of stable species can be achieved within 30% uncertainty, the measured mole fractions of reactive intermediates often have large systematic errors, mainly due to the large uncertainties associated with estimated photoionization cross sections. These measurements are therefore less effective in improving the model accuracy, and it remains a challenge to make full use of those data for important reactive intermediates with relatively large uncertainties. In the present work, we propose a model-assisted calibration method to reduce the uncertainty of the measurements for those reactive species in the MBMS experiments. The method takes advantage of the inherent correlation of the systematic uncertainty in the MBMS measurements and uses the accurate model predictions to calibrate the correlated experimental data. By global uncertainty analysis, the kinetic model for the methanol/O2 flame was analyzed to select the optimal experimental conditions for which the model prediction of the hydroxymethyl radical (CH2OH) has the smallest uncertainty. Then the correlation factor for the systematic uncertainty is determined by analyzing the new measurement and the model prediction under the designed condition. The correlation factor determined has been successfully used to calibrate the peak mole fraction of the CH2OH radical in a laminar premixed methanol flame, reported earlier.