The following sections offer a brief overview of techniques used in optical combustion investigations. The methods most relevant to current project are direct imaging of flames, shadowgraphy and Schlieren imaging. Shadowgraphy and Schlieren imaging both utilise backlight illumination as the light source making use of the change in the density gradient in the working section resulting from either sprays or combustion. A brief description of a few other methods is additionally given in Section 2.4.3.4.
2.4.3.1 Direct High Speed Imaging
The simplest technique to follow when studying combustion optically is to use high speed imaging to capture the flame luminosity directly. The method relies upon the flame or combustion to produce enough luminosity for the aperture to capture and has been shown to work well with both gasoline and diesel combustions [177, 178]. When luminosity has been found problematic, a sufficient increase in luminosity levels has been achieved through an addition of selected salts to the fuel that are thought not to affect flame propagation [179]. Winklhofer and Fuchs [180] state that with shutter speeds of 10 µs or faster allow for good imaging of the flame structure under the
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influence of turbulent flow field but make note of the high requirements of the optical aperture because of short exposure times.
2.4.3.2 Shadowgraphy
Shadowgraphy implies that a light source faces the camera directly or often through a diffuser to generate equal light distribution. A beam of light is directed through the working section and is refracted due to density changes that affect the refractive index of the working environment. Schematics of a simple shadowgraphy set-up can be seen in Figure 2.9 [181]. The method does not produce a focused optical image of the object but instead produces a shadow of the investigated phenomena. Its response to the second spatial derivative of the refractive index makes it well suited for experiments with large density gradients. Although sometimes applied to combustion analysis [182], as demonstrated by several researchers, shadowgraphy is mostly used for fuel spray analysis [183, 184, 185].
Figure 2.9: Diagram of Shadowgraph method [181]
2.4.3.3 Schlieren Imaging
Schematics for the Schlieren system can be seen in Figure 2.10 [186] where ε represents the refraction angle of light rays. Schlieren system works on the same principal of variation in refractive index as shadowgraphy but uses twin concave mirrors or lenses to form a collimated beam of light that travels through the working section. Before being reaching the camera again, the light rays are re-focused.
A light source is directed through a condenser lens to create a collated light or alternatively a collated light source is used, is sent to the first concave mirror that reflects it as a parallel beam through the working environment. The second concave
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Figure 2.10: Schlieren system aperture schematics [186]
mirror focuses the beam at which point part of the light is cut off by a knife edge or graded filter. This effectively acts as means to control the contrast in light intensity.
The remaining light is allowed to reach the Schlieren camera. Images respond to the first derivative of refractive index and as a result possess much higher sensitivity to changes in density than shadowgraph images. Furthermore, the Schlieren system produces a 1:1 scale of the studied object and according to Kostiuk and Cheng [187], unlike in shadowgraph images where detail is often reduced, it is emphasised.
2.4.3.4 Other Techniques
Direct, shadowgraph and Schlieren imaging of flames allows for overall characterisation of combustion reactions but often understanding of specific reactions and species present during those reactions is sought after.
In such cases numerous laser techniques can be employed [151]. Laser induced fluorescence (LIF) uses a laser source to excite radicals that then can be captured by an intensified charge couple device (ICCD) camera. If enough energy from the laser is available this can further be converted to a planar laser induced fluorescence technique (PLIF), where a point source is converted to a laser sheet, enabling view of the cross section of the flame. Radicals such as CH, CH2O and OH have been associated with inner flame front, preheat and oxidation/post flame zones, respectively [188]. Dependent upon the wavelength of the laser, different radicals can be excited which enables visualisation of the reaction zones and as a result characterisation of the flame structures under various conditions. Moreover, simultaneous OH and CH2O fluorescence imaging can be used to evaluate the local heat release rate [189].
A series of experiments on diesel combustion in a constant volume combustion vessel were carried out at University of Valladolid [190, 191]. Using 306 nm and 430
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nm band-pass filters on two Hamamatsu 9536 photomultiplier tubes (PMTs) to detect the OH* and CH* radical chemiluminescence, respectively, to measure the auto-ignition time of diesel sprays. The time-resolved signal traces of the OH* and CH*
radical chemiluminescence were compared to that of pressure and mechanical vibrations and good agreement was found.
Several other combustion characterisation methods are used. Hentschell [192]
applied the use of optical fibres within a modified head gasket. About 100 optical fibres were used to form an optical grid covering the cross section of the engine. The flame position could be resolved spatially by using a tomographic reconstruction algorithm. Fibres were also used by Spicher and Velji [193] for flame detection but were built into the walls of the combustion chamber of a single cylinder SI engine.
They bring out the possibility of a three-dimensional resolution to the flame propagation analysis compared to other optical techniques.