Etude de la combustion de gaz de synthèse issus d'un processus de ...

Chapter 3
Light from a point source A is transformed into a parallel beam and let through the zone
to be investigated, E. All the rays which are not deflected converge at the focus of lens
D and are cut off by diaphragm F. refracted rays bypass the diaphragm and are
collected by lens G, which projects them onto screen H. Lens G is placed in such a
way as to produce a sharply defined image of plane E on the screen. Simple
geometrical considerations are not sufficient to determine precisely the changes in
screen illumination due to a given disturbance, since they are greatly influenced by the
diffraction of light on the diaphragm and by the source dimensions. Approximately,
however, the relative illumination at the image plane, ∆I/I, is proportional to the beam
deflection angle θ and the focal length f of lens D, as follows:
∆I
≈ θf
(3.11)
I
tel-00623090, version 1 - 13 Sep 2011
The schlieren image is greatly influenced by the form and size of the light source and
diaphragm. Placing a diaphragm at the lens focus amounts to removing a specific
group of harmonics from the diffraction pattern, this, of course, introduces significant
changes in the schlieren image. Hence all schlieren photography apparatus should
consist of a large choice of diaphragms from which selection can be made
experimentally to obtain the most contrasting picture of a given effect. By using a slit
source of white light and a slit diaphragm color schlieren images can also be achieved.
The dependence of changes in illumination at the screen on the refraction angle
implies that the schlieren image visualizes density gradients of the flame:
∆I
∂Q
≈ (3.12)
I ∂ n
Where n represents a normal to the surface of constant density. Superimposed on this
relationship is a spatial function dependent on the structure of the system and its
arrangement relative to the disturbance. The situation is therefore largely qualitative,
although an appropriate setting of the apparatus (on removing the diaphragm, a sharm
image of the flame should appear on the screen) should ensure fairly faithful
representation of the disturbance pattern. To obtain quantitative estimates of the
density gradients at a given setting of the apparatus, optical calibrations are used with
known refraction angles.
Figure 3.18 shows a scheme of the used schlieren apparatus. The laser source (Laser
Árgon Spectra Physics Series 2000) with a maximum power of 6 W generates a
83