Boundary-Layer Model to Predict Chemically Reacting Flow within Heated, High-Speed, Microtubular Reactors

Chen nozzle experiments are used to study the early-stage unimolecular decomposition of larger molecules and, sometimes, the following chemistry. The nozzle itself is typically a small-diameter (order millimeter), short (order 20–50 mm), heated (order 1700 K) tube (nozzle) that exhausts into a vacuum chamber where a variety of diagnostics may be used to measure gas-phase composition. Under typical operating conditions, the velocities are high and the exhaust flow is near sonic. Quantitatively interpreting the measurements requires a model for flow within the nozzle that is coupled with reaction kinetics simulations. The present model shows that the flow can produce significant radial and axial variations in both the thermodynamic conditions and species concentrations. Thus, plug-flow models may not be appropriate. Results show that using He as a carrier gas produces much more plug-like flow than is the case with Ar as the carrier gas. The boundary-layer model provides a computationally efficient approach to modeling detailed chemical kinetics within Chen nozzles. Results are illustrated using acetaldehyde decomposition kinetics.