Heat Release Studies by pure Rotational Coherent Anti-Stokes Raman Scattering Spectroscopy in Plasma Assisted Combustion Systems excited by Nanosecond Discharges

Heat release studies of plasma assisted combustion have been performed in fuel-air mixtures excited by nanosecond dielectric barrier discharges initially at room temperature and maintained at low pressure (~40 -- 50 torr). The following topics have been extensively investigated: (i) the applicability of pure O2 broadband Rotational Coherent Anti-Stokes Raman Scattering spectroscopy at very low O2 pressures of ~8 torr or less to obtain rotational temperature, (ii) validation of a proposed low temperature fuel-oxidation kinetics mechanism fully decoupled from NOx chemistry, (iii) characterization of nanosecond pulse discharges in a dielectric barrier discharge cell and a pin-to-pin discharge geometry, and (iv) effect of fuel addition on heat release in a pin-to-pin discharge geometry at low pressure.;For the first topic, the applicability of pure O2 broadband Rotational Coherent Anti-Stokes Raman Scattering (RCARS) Spectroscopy at very low O2 partial pressure of ~ 8 torr or less to obtain rotational temperature has been demonstrated. Very good experimental precisions of ~ +/- 1 to 2 K has been demonstrated for diffuse and volumetric plasmas excited by a repetitively pulsed nanosecond discharge. It is shown that the electron-multiplication feature of an EMCCD camera increases the signal to noise ratio significantly.;For the second topic, the pure O2 RCARS system was applied to the dielectric barrier discharge cell to obtain time-resolved temperature measurements in nanosecond pulse discharges. It was found that a 0-D model predictions for temperature are in very good agreement in the baseline mixture without fuel and the hydrogen containing mixtures. However, the model predicts that the heat release in hydrogen containing mixtures is only weakly dependent on equivalence ratio, which is inconsistent with experimental results. Furthermore, In C2H2 containing mixtures, the model consistently under-predicts the temperature, further delineating the need for more accurate low-temperature plasma/combustion chemistry decoupled from NOx processes for both hydrogen and ethylene fuels.;For the third topic, plasma characterization has been carried out for the mixtures in the aforementioned dielectric barrier discharge cell in addition to air-fuel mixtures in a pin-to-pin discharge geometry. The dielectric barrier discharge couples a very small amount of the energy, ~0.1 mJ/pulse, that is stored in the capacitive load formed during breakdown to the plasma. Good agreement between these energy coupling results and a prediction from a 0-D analytical model was found. On the other hand, the pin-to-pin discharge has a higher energy loading of ~3 mJ/pulse and a model is currently in development.;On the fourth topic, in the pin-to-pin discharge geometry it is demonstrated that a fast heating and a slow heating regime exist in air and air-fuel mixtures and are clearly distinct from each other after the onset of the discharge pulse. It is indicated that air-ethylene mixtures do not exhibit a clear distinction between slow and fast heating. In all cases, with increasing fuel addition, the rate of the heat release increases. Radial temperature profiles were taken for air at three different time points relative to the onset of the pulse. The radius was found to be the same in all three cases, strongly indicating that there is no contraction or expansion of the plasma filament. Preliminary results with a 1-D model still in development show very good agreement, which is promising. It is expected that the model will attribute fast heating primarily to collisional quenching of N2 excited states in air and air-hydrogen containing mixtures. In ethylene mixtures, ethylene oxidation processes are expected to have a larger contribution as experimental results indicate a strong dependence on equivalence ratio. Slow heating is expected to be dominated by V-T transfer from vibrationally excited N2 by collisional quenching of O-atoms, with additional release by fuel-oxidation. (Abstract shortened by UMI.).