Measurements and statistics of mixture fraction and scalar dissipation rates in turbulent non-premixed jet flames

Abstract

The present study adds to the limited database of mixture fraction and scalar dissipation rate measurements within turbulent flames. One noteworthy aspect is that the current measurements are two-dimensional images, which provide the physical structure of the scalar dissipation rate field. Two turbulent carbon monoxide flames are studied at two different jet-exit velocities, where the latter flame is near blowoff. The effects of increasing levels of local extinction (by increasing the jet velocities) on the structure and statistics of the mixture fraction fluctuation and scalar dissipation rate fields are investigated in detail. In general, the topology of the scalar dissipation rate layers is similar to that found previously in turbulent non-reacting flows, with some notable exceptions. For the lower jet-velocity case, the dissipation layers were somewhat smooth, isolated, and preferentially aligned in the axial direction, while for the higher jet-velocity case (near global extinction), the scalar dissipation rate field was characterized by an increased number of shorter, thinner, and highly wrinkled dissipation layers. It appears that as the flame nears blowoff, the scalar dissipation rate field exhibits an increasing level of isotropy, resembling turbulent non-reacting flows. In addition to providing new insights on the physical nature of the dissipation fields, the two-dimensional statistics of the mixture fraction and scalar dissipation rate are examined in detail, in the form of conditional averaging and through the construction of probability density functions. It is found, that although the two flames have apparent differences in physical structure, the corresponding statistical properties of the scalar dissipation rate were similar. Probability density functions (pdfs) of the scalar dissipation rate exhibited large departures from strict log-normality and the pdfs of the scalar dissipation rate were found to be significantly different if only the values of the scalar dissipation conditioned on the stoichiometric contour were considered compared to the entire flowfield. This has implications for modeling assumptions that seek to describe the entire turbulent flowfield by a single pdf. The nature of the scalar dissipation rate in the current lower-Reynolds number turbulent flames, as revealed through the statistics, is compared to previous, well-known data sets obtained in the Sandia series of piloted jet flames as well as other piloted non-premixed jet flames at higher Reynolds numbers. The present flame conditions are significantly different from previous work, yet the general structure, statistical trends, and general effects of increasing levels of local extinction agree favorably with the previous experimental data.