Two-point high repetition rate measurement of temperature and thermal dissipation in a turbulent non-premixed jet flame
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A high-repetition rate (10 kHz) laser Rayleigh scattering facility was developed and used to study the temperature fluctuations, power spectra, gradients and thermal dissipation rate characteristics of a nonpremixed turbulent jet flame at a Reynolds number of 15,200. The flame studied here is similar to the Turbulent Nonpremixed Flame Workshop simple jet flame (DLR_A flame). The radial temperature gradient was measured by a two-point technique, whereas the axial gradient was inferred from temperature time-series measurements combined with Taylor’s hypothesis. Resolution and noise can greatly affect such measurements, and thus a model is proposed to account for the effects of resolution, noise, filtering and data processing on the measured dissipation. The model clearly shows the interplay between resolution and noise, and that noise will create an apparent dissipation (or bias), which will be more significant at high spatial resolution. Techniques to correct the measured mean dissipation for this bias are discussed for the two-point time-series thermal dissipation measurements reported here. A general technique to estimate the noise level for scalar dissipation measurements is also proposed. The resulting two-point time-resolved measurements in a turbulent flame show that the temperature power spectra along the jet centerline exhibit only a small inertial subrange due to the low local Reynolds number of the flow (Reδ ∼ 2,500), although a larger inertial subrange is present in the spectra at off- centerline locations. Furthermore, the power spectra collapse in the dissipation range when the frequencies are normalized by the Batchelor frequency. Probability density functions of the thermal dissipation are shown to deviate from lognormal in the low-dissipation portion of the distribution when only one component of the gradient is used; however, nearly lognormal distributions are obtained along the centerline when both axial and radial components are included. A procedure is developed for correcting the thermal dissipation for the apparent dissipation introduced by noise. This procedure uses redundant measurements, either temporally or spatially, to quantify the noise contribution on the mean dissipation. This analysis shows that noise has a dominating effect on the dissipation as the apparent dissipation can be as large as five times the actual dissipation on centerline. The corrected dissipation measurements show that the radial profile of the mean thermal dissipation exhibits a peak off centerline at all downstream locations. These results indicate that the underlying turbulence, as inferred from the temperature fluctuations, is in large part similar to that of nonreacting jet flows, provided the Reynolds number is properly modified to account for heat release.