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Guanghua Wang, Noel T. Clemens and Philip L. Varghese

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Introduction

The objective of this study is to make high-quality, high-repetition rate (10 kHz), two-point laser Rayleigh temperature measurements in a weakly co-flowing turbulent nonpremixed jet flame at a Reynolds number of 15,200, with high signal-to-noise ratio (~50 in room air) and where the finest scales of turbulence are spatially and temporally resolved. These two-point temperature data were used to obtain temperature power spectra and detailed statistics of the thermal dissipation rate. The flame studied here is similar to the TNF simple jet flame (DLR_A) which is used as a benchmark flame for the Turbulent Nonpremixed Flames (TNF) Workshop.

Experimental setup (Research Facility)

The core element of this experiment is the high-repetition rate diode-pumped Nd:YAG laser (Corona, Coherent Inc.). The laser is continuously pumped but is acousto-optically Q-switched to produce 130 ns (FWHM) pulses at 10 kHz. The light is intra-cavity frequency doubled and produces 75 W of average power at 532 nm. This corresponds to about 7.5 mJ per pulse at 10 kHz.

The laser beam is focused into the test section by using a 300 mm focal length lens. The beam diameter was measured to be about 0.3 mm. An external photodiode (DET210, Thorlabs Inc.) is used to correct for variations in the laser pulse energy on a shot-by-shot basis.

Rayleigh Scattering Collection Optics

Rayleigh scattered light is collected using custom-designed optics that consist of a pair of 150 mm diameter plano-convex lenses, one 50.8 mm diameter meniscus lens and one 50.8 mm diameter double-convex lens. The lens system was designed with ZEMAX and produced an aberration-limited blur-spot of less than 34 mm. The working f# was 2.4 and the magnification was 0.685.

Two-point Measurement Optics

Two-point measurements were made by imaging the scattered light onto a broadband hybrid cube beam splitter, which reflected and transmitted the split signal onto two different PMTs (R636-10, HAMAMATSU). An 8-channel high voltage power supply (8-channle high voltage power supply, EMCO) controlled by an analog D/A card (PCI-6703, National Instrument) is used to provide high negative voltages for the PMTs. Two 200 mm slits were placed in front of the PMTs to define the spatial resolution (i.e., length of the beam imaged). The slit width in the image plane corresponded to 300 mm in the object plane. These two slits were arranged such that the separation of probe volumes in the flow was 300 mm. The PMT and photodiode outputs were read by gated integrators (SR250, Stanford Research Systems) operated with gate widths of 300 ns. The integrated signals were synchronously sampled by a 12-bit A/D converter (AT-MIO-16E-1, National Instrument) at 10 kHz. The whole experimental system is synchronized by two digital pulse/delay generators (DG535, Stanford Research Systems).

Co-flow Jet Flow Facility

The flow being studied is a weakly co-flowing turbulent nonpremixed jet flame. The coflow air is filtered to remove particles larger than 0.2 micron and then passed through a flow conditioning section. The coflow velocity is 0.45 m/s. The fuel issues from a long tube with inside diameter d = 7.75 mm. The test section has a 0.75 m x 0.75 m cross section. The whole jet flow facility is mounted on a traverse that is driven by stepper motors to provide positioning in the radial and axial directions.

Turbulent Jet Flame

The fuel composition used in this study is 22.1% CH4, 33.2% H2, 44.7% N2 (by volume), which gives a stoichiometric mixture fraction of 0.167. The source Reynolds number is 15,200 and the measurements are taken at downstream locations from x/d = 40 to 80. Here, x and r are the axial and radial coordinates, respectively. The visible flame length is at about x/d = 84 and the stoichiometric flame length, estimated based on data in the TNF database, is at about x/d = 60.

Mean and RMS Temperature

This figure shows measured mean and rms radial temperature profiles at three downstream stations (Re = 15,200, x/d = 40, 60, 80). For comparison, the temperature profiles from the TNF database are also shown and it is seen that the current measurements agree very well with the database.

Temperature Power Spectra

Temperature power spectra are shown in the above figure. The spectra are shown along the centerline of the jet flame at the five axial stations with the range x/d=40-80. The frequency is normalized by the convective Batchelor frequency, and the power spectral density is normalized by T_rms²/f_B. The spectra have also been corrected to remove the contribution from shot-noise. This procedure requires that the measurement cut-off frequency be higher than the Batchelor frequency so that the power of the noise fluctuations can be determined. Since this was not the case for all of the measurement locations, the correction was not applied in all cases.

Thermal Dissipation

PDFs of the normalized thermal dissipation rate, which were computed from measurements made along the jet centerline and at three axial stations, are shown in the above figure. The PDFs were computed by using the radial term only, and the radial and axial terms. It is seen that the PDFs that used the radial term only, do not exhibit a log-normal distribution, which would appear as an inverted parabola on a log-log plot. This observation is consistent for all three axial stations. The high dissipation values are apparently log-normally distributed but not the low dissipation values. However, when the axial component is included, the PDFs approach the lognormal distribution. These observations are consistent with previous measurements of scalar dissipation in nonreacting jets and mixture fraction dissipation in jet flames. In nonreacting flows the 1-D dissipation exhibits a slope of ½ in the low-dissipation portion of the PDF when plotted in log-log coordinates, which is similar to what is seen here. The power law dependence of the low-dissipation portion of the PDF is a direct result of the 1‑D gradient overestimating the total gradient vector magnitude. It is interesting that including the axial gradient term improves the log-normality of the PDF as well as it does, considering that the axial term is not fully resolved at the upstream stations. It is likely that the low-dissipation end of the distribution is in fact better resolved because the associated structures are much larger than the Batchelor scale.

Ongoing work

We are currently working on simultaneous two-point time series and 2-D imaging of temperature and thermal dissipation rate in the turbulent nonpremixed jet flames.

Publications

• G.H. Wang, N.T. Clemens and P.L. Varghese, "High-Repetition Rate, Two-Point Temperature Measurements in Nonpremixed Turbulent Jet Flames", Proceedings of the Central States Section Meeting, The Combustion Institute, Austin, TX, Mar. 22-23, 2004.

• G.H. Wang, N.T. Clemens and P.L. Varghese, "High-Repetition Rate Measurements of Temperature and Thermal Dissipation in a Nonpremixed Turbulent Jet Flame", Proceedings of the Combustion Institute, Vol. 30, to be published, 2004