Low Temperature Oxidation of n-Propylcyclohexane in a Pressurized Flow Reactor
Authors:   , Corrubia Julius Anthony , Cernansky Nicholas Peter , Miller David Leslie 
Published: July 2021

To increase efficiency and power and to reduce emissions, propulsion system design has turned to utilizing simulations coupling chemical kinetic mechanisms with computational fluid dynamics (CFD) codes. The chemical kinetic information required for developing and validating chemical kinetic mechanisms is gained primarily through experiments, although recent advances in theoretical chemical kinetics and quantum chemistry have provided a new synergy between theory and experiment. The Pressurized Flow Reactor (PFR) facility at Drexel University was used to study the low temperature oxidation of n-Propylcyclohexane (n-PCH). This work complements previous investigations at Drexel for n-Butylcyclohexane (n-BCH) oxidation in the PFR (Natelson et al., 2011; Natelson, 2010) and examines the effect of alkyl side chain length on the reactivity of n-PCH (C9H18) and n-BCH (C10H20). Both molecules are alkylated cycloalkanes with nearly identical chemical structures, except that n-BCH contains an additional carbon on its side chain, and they are possible surrogate compounds used for chemical kinetic mechanism development of real fuels. Currently, there exist semi-detailed and detailed chemical mechanisms for n-BCH low temperature oxidation; however, low temperature oxidation experimental studies and mechanisms for n-PCH are scarce. The experimental conditions studied were: temperature of 550-850 K; pressure of 8.0 atm; residence time of 120 ms. The initial hydrocarbon mole fractions were: 824 ppm for n-PCH and 1082 ppm for n-BCH. The stable intermediates were extracted from the PFR and then identified and quantified using a Gas Chromatograph / Mass Spectrometer / Flame Ionization Detector (GC/MS/FID) system. Carbon monoxide (CO) levels were measured online to monitor fuel reactivity and map the Negative Temperature Coefficient (NTC) regime. NTC start, as indicated by maximum CO and minimum O2 mole fractions, was at approximately 690 K for n-PCH and 670 K for n-BCH. Linear alkene production peaked in the NTC regime at approximately 700 K and 715 K for n-PCH and n-BCH, respectively. Other stable intermediates that peaked in the NTC regime for both n-PCH and n-BCH included cycloalkenes, aldehydes, and ketones. Analysis of the relative yields of the stable intermediates suggested similarities and differences in the chemical pathways that control the low temperature oxidation of n-PCH and n-BCH. A semi-detailed chemical kinetic mechanism to simulate the low temperature oxidation n-PCH experimental results was proposed and evaluated in the current work. The previous n-BCH work (Natelson et al., 2011; Natelson, 2010) was an essential resource for development of the n-PCH low temperature oxidation chemical kinetic mechanism since similar intermediate species were measured comparing n-PCH and n-BCH oxidation, thus suggesting the chemical kinetics for the low temperature oxidation of alkylated cycloalkanes are mechanistically similar. The semi-detailed n-PCH mechanism predicted the low temperature increasing reactivity regime, followed by maximum reactivity at NTC start and decreasing reactivity in the NTC regime. Overall, as compared to the experimental results the semi-detailed n-PCH mechanism predicted very well O2 consumption; overpredicted n-PCH consumption, CO production, key alkenes (i.e., ethene and propene), and cycloalkenes (i.e., cyclohexene); and underpredicted CO₂ production, and aldehydes (i.e., formaldehyde). Lastly, the low temperature oxidation semi-detailed n PCH mechanism of the current work was compared to a detailed n-PCH mechanism developed with King Abdullah University of Science and Technology (KAUST).