Non-ideality of flow tube experiments for reaction kinetics

Improving molecular beam diagnostic methods as well as new optical diagnostic approaches have led to a resurgence of experimental studies using flow tubes for investigating detailed reaction systems. Historically, flow tubes have been used for direct elementary reaction studies, but many recent works have applied the technique to observe the complex reaction kinetics of pyrolysis or oxidation of hydrocarbons and hydrocarbon oxygenates. Some of these studies have also considered conditions which involve multiregime (low temperature/negative temperature coefficient/hot ignition) chemical kinetic behaviors. Important aspects of flow tube design, operation, and the chemistry to be studied are both the experimental coupling of fluid dynamics, chemical reaction characteristic, and diagnostic (spatial or line of site) methods, as well as numerical interpretation of such experiments. Conventional plug flow interpretation of the measurements may not reasonably apply for a particular reactor design, flow field, and chemical kinetic system causing interpretive uncertainties both in the measurements and computational predictive modeling. Considerations of earlier treatises on these issues are further illuminated through solving the species, mass, momentum and energy conservation at isothermal wall conditions for flow tube conditions typical of several of the recently reported experiments. The predictions are compared against flow tube data available in the literature. Predictions indicate that significant axial and radial gradients in species concentration and temperature can exist for the reported flow tube conditions and chemistry studied. The local axial and radical gradients are more prominent at lower temperature and their region of influence diminishes with increasing temperature. These changes are not reflected well in experiments that only apply diagnostics using a fixed characteristic reaction time and varying flow parameter initial conditions. Predictions from the more complex flow physics model are found to be in better agreement with the experimental measurements and show that simple plug flow interpretations can be misleading.