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Date of Award
A new methodology for quantifying intermediate combustion species has been developed and validated through the oxidation of two prominent n-alkanes: n-heptane and n-pentane. The methodology, called filtered natural emission of species (FNES), utilizes the natural emissions of different combustion species to make quantitative measurements on the state and composition of the gas inside the combustion chamber. In this work, FNES was utilized to observe the multi-stage heat release of the fuels in a rapid compression machine. This multi-stage heat release occurs at elevated pressures (over 20 bar) and low combustion temperatures (under 800 K for n-heptane and under 700 K for n-pentane). The concentrations of CO, CO2, and H2O were measured throughout combustion. Data from literature was used to validate the measurements made, when applicable, and both 0-D and 3-D numerical models were used as further validation of the results.Measured peak CO concentrations were in agreement with the results found in the literature. However, the results exhibited an elevated, plateaued concentration after the peak concentration that was observed at every test condition. This behavior was not seen in the literature nor the 0-D models. The 3-D models were able to explain this phenomenon as the result of a secondary ignition in the combustion chamber, caused by delayed reactions in the low-temperature region near the wall of the combustion vessel. This emphasized the need for 3-D simulations to further explain empirical data. CO2 concentrations were initially not able to be observed for the duration of the oxidation, as the radiative intensity was too great for the experimental equipment. The use of a secondary broadband filter reduced the intensity to a level that allowed for quantification. The numerical models accurately simulated the initial concentrations but over-predicted the time necessary to reach a steady-state value. Furthermore, the measured concentrations were approximately half of what was expected by the numerical models. This is likely due to the transmissivity effects of the boundary layer, as exemplified by the measurements’ extreme sensitivity to the ambient conditions. Initial H2O measurements could only capture the final, steady-state concentration of the species, due to the low transmissivity of the optical access in the spectral range of the filter. The use of an alternative filter resolved the issue but contained spectral overlap of species with C-H bond emissions. This overlap resulted in an elevated initial concentration with the new filter. When the ambient environment was accounted for, the steady-state concentration of the measured data was almost double the simulated values. The species expected at the timing of the third heat release (OH and HO2) do not emit in the spectral ranges of the filters, and therefore could not be responsible for the increased concentration. While there was a disparity in the gas temperature between the numerical models, the difference was not large enough to explain the heightened concentration. Further inquiry into the calculated gas temperature, camera calibration, and spectral database is necessary to resolve this issue.
Piehl, Joshua, "Species Concentration Measurements Using The Filtered Natural Emission Of Species" (2022). Wayne State University Dissertations. 3579.