Instantaneous Measurement of Three-Dimensional Gradients in Turbulent Flames and Flows

A new laser-based technique for the measurement of instantaneous three-dimensional species concentration gradients in turbulent combustion and flows is presented, using a comparatively simple experimental setup. Two crossed laser sheets excite selected molecules in the probe volume (Fig. 1). The optical detection system allows simultaneous detection of the emitted laser-induced signal from both laser sheets, employing a single CCD camera. Image post processing enables the determination of gradients along the intersection of the two lightsheets. This information can be used for the validation of computer models. Since direct numerical simulation (DNS) is computationally too costly for extended areas in turbulent processes, the information gained with this method is also used to construct empirical probability density functions (PDF), as they are applied in computer simulation and modelling.

Figure 1: Crossed laser sheets in the probe volume

In addition to the determination of gradients, an approximate reconstruction of 3D spatial intensity information is achieved by means of digital image processing. A diffusion process enables the data transport from the two measured planes into the volume. The visualization of the resulting volumetric dataset gives unique insight into turbulent processes. As first applications of this technique laser induced fluorescence (LIF)-measurements of OH in a bunsenburner methane flame are presented (Fig. 2).

Figure 2: Three-dimensional visualisation of OH in a Bunsen burner methane flame

References:

[1] A. Hoffmann, F. Zimmermann, H. Scharr, S. Krömker, and C. Schulz, "Instantaneous three-dimensional visualization of concentration distributions in turbulent flows with crossed-plane laser-induced fluorescence imaging," Appl. Phys. B 80, 125-131 (2005).
[2] A. Hoffmann, C. Schulz, J. Ruckwied, D. Malcherek, T. Dreier, R. Schießl, and U. Maas, "Three-dimensional measurement of OH-concentration gradients in a turbulent flame by simultaneous laser induced fluorescence and Raman scattering," in European Combustion Meeting (Orléans, France, 2003).