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A molecular-beam mass spectrometry study and modeling of ethylene flames

Anuj Bhargava, University of Massachusetts Amherst


Molecular-beam mass spectrometry (MBMS) has been used to measure concentration profiles for stable and radical species in low-pressure, one-dimensional flames. Temperature measurements with ceramic-coated-thermocouples, area expansion ratio measurements with hot-wire-anemometry have been made, along with concentration measurements with MBMS, to characterize ethylene flames fully. The fuel-rich flame had a composition of 19.4 mol % $\rm C\sb2H\sb4,$ 30.6 mol % O$\sb2,$ and 50.0 mol % Ar $(\phi=1.9).$ It was operated at a pressure of 2.67 kPa (20.00 Torr) with an unburned burner gas velocity of 62.5 cm/s (300 K). Mole fraction profiles for 42 species were measured in this flame. The fuel-lean flame had a composition of 8.6 mol % $\rm C\sb2H\sb4,$ 34.5 mol % O$\sb2$ and 56.9 mol % Ar and an equivalence ratio, $\phi,$ of 0.75. This flame had a burner velocity of 30.0 cm/s (300K) and was operated at 4.00 kPa (30.00 Torr). Concentration profiles for 22 species and upper-bound measurements for seven species were made in the fuel-lean flame. Species flux balance calculations were used to obtain rate coefficients for reactions important in the combustion process. C$\rm\sb2H\sb4$ destruction chemistry has been examined extensively and rate coefficient for key reactions have been measured:$$\eqalign{\rm C\sb2H\sb4+H=C\sb2H\sb3+H\sb2&\quad{\it k\/}(\rm cm\sp3mol\sp{-1}s\sp{-1})=\cr &\quad3.70\times10\sp7 \times T\sp{2.17}\rm exp({-}13400/RT)\cr}$$over the temperature range of 1850-2150 K and$$\eqalign{\rm C\sb2H\sb4+OH=C\sb2H\sb3+H\sb2O&\quad{\it k\/}(\rm cm\sp3mol\sp{-1}s\sp{-1})=\cr &\quad5.53\times10\sp5\times T\sp{2.31}\rm exp({-}2900/RT)\cr}$$for temperatures between 1400-1800 K. Rate constants for vinyl decomposition reaction $\rm C\sb2H\sb3=C\sb2H\sb2+H$ were also measured and interpreted in terms of high-pressure and low-pressure limits. This analysis indicated the possibilities of a higher decomposition rate constant for vinyl than has been reported before or a calibration factor that is higher by nine times than the calibration performed by relative ionization cross section method. Formation and destruction chemistry of $\rm C\sb2H\sb2$ and CH$\sb3$CHO species was also analyzed. The data have been compared to model predictions from two different reaction sets and from a reaction set being developed as part of this work. The predictions from the new reaction set compares best with the data for many stable and radical species, but the agreement for C$\sb3$ and heavier species is not very good from any of the three sets. The new set also eliminated a general shift of the predictions from the literature sets which predicted profiles to be 1-3 mm closer to the burner than the experimental data.

Subject Area

Chemical engineering

Recommended Citation

Bhargava, Anuj, "A molecular-beam mass spectrometry study and modeling of ethylene flames" (1997). Doctoral Dissertations Available from Proquest. AAI9809309.