Measuring density fluctuations in a flame using laser vibrometry

Due to the rising interest in this technique, here is a brief article about the basics and potential of laser vibrometry for combustion diagnostics. 

A lightweight and straightforward optical measurement technique designed to monitor in a very refined way the turbulence in a flame is described here. Fabrice Giuliani from CBOne was pioneer on the application of laser vibrometry toward flame stability analysis, often in association with Thomas Leitgeb, Stefan Koeberl and Jakob Woisetschlaeger. A more complete description of Giuliani's works on laser vibrometry can be found in his habilitation memorandum at TU Graz.

Advanced Combustion Management
Fabrice Giuliani
Habilitation memorandum, TU Graz, 2010 

The plot above describes the passage of a cold front of air through a pulsating flame at 25Hz forced pulsation. The background pictures are schlieren visualisations of the pulsating flame in question, and the coloured isosurfaces are laser-vibrometry measurements worked out to highlight the dense zone in the air, hence the cold front. This measurement technique is therefore an asset to describe in a refined way the dynamics of pulsating combustion, or of combustion instabilities.

The results produced above are from 2005. It took some time to convince the combustion community about the relevance of this methodology. It took several international conferences (among which ASME Turbo Expo 2006 and ISABE 2007) before the first journal publication that makes use of this material was published in 2010 (Journal of Engineering for Gas Turbine and Power).


The laser vibrometer

A laser vibrometer is a compact portable device designed to detect and measure vibrations of surfaces. It is adequate for instance to monitor the steadiness of the rotating pool of a machine.

Based on the technology available at TU Graz, a Max Zender interferometer is used. It analyses the interference pattern built by two superposing laser beams issued from the same source. One beam has a fixed path, the second one aims at the surface of interest and is being reflected by the latter. In time, if the interference pattern does not vary, the surface is still. If it varies, the surface moves, or vibrates.

The device used is a Polytec OVD 353 (Polytec, Waldbronn, Germany). In order to detect vibration amplitudes greater than the laser wavelength, and to distinguish a forward from a backward motion, a Bragg cell is used to bring a carrier wave of 40MHz on the reference beam. Steady state is then represented by a fixed frequency corresponding to the modulated interference at 40MHz. Any surface motion will provoke a Doppler effect on this carrier wave (compressive: the surface moves towards the instrument hence the frequency goes above 40MHz, or expansive: the surface moves away from the instrument and the frequency falls below 40MHz). A frequency demodulation allows to get back to both vibration frequency and motion amplitude of the object surface.


Density fluctuations in a flame

If the target point of the laser vibrometer is a fixed surface as a mirror, we are back to the still situation under non-reactive conditions. But if a flame is there, and that the object beam goes through the flame, then the interference pattern variation will depend on the changes in refraction index. And the refraction index is function of the density of the medium, with a factor called the Gladstone-Dale constant.

Therefore, it is possible with a laser vibrometer to come back to the change of density in a flame, integrated in line-of sight.

If the flame has a cylindrical geometry, then an Abel inversion allows to come back to the local change of density in the flame.

Laser vibrometry is recommended for a first investigation on an instable flame. The method is low-demanding and it delivers maps of turbulence in a straightforward manner, for an acceptable amount of data (less data than with ultra high speed cameras for instance). The qualitative results are easy to produce.

Currently, the research goes towards a precise quantification of the density fluctuation. The next steps are the extraction of the unsteady heat release, and based on an hypothesis of constant pressure, LV might directly measure the smallest temperature changes in the flame. It is an exciting field of research with great expectations.


References from author

Giuliani, F., Wagner, B., Woisetschläger, J., and Heitmeir, F., 2006, “Laser Vibrometry for Real-Time Combustion Stability Diagnostic,” ASME Turbo Expo Conference and Exhibition, Paper No. GT2006-90413

S. Köberl, F. Fontaneto, F. Giuliani, J. Woisetschläger, "Frequency-resolved interferometric measurement of local density fluctuations for turbulent combustion analysis", Measurement Science and Technology, vol. 21 (2010), No. 035302. 

Giuliani, F., Leitgeb, T., Lang, A., and Woisetschlaeger, J., 2010, “Mapping the Density Fluctuations in a Pulsed Air-Methane Flame Using Laser-Vibrometry,” Journal of Engineering for Gas Turbines and Power, vol. 132, No. GTP-031603.

Leitgeb, T., Schuller, T., Durox, D., Giuliani, F., Koeberl, S., and Woisetschlaeger, J., 2013. “Interferometric determination of heat release rate in a pulsated flame”. Combustion and Flame, vol. 160, band 3, pp. 589–600.