Global Rainbow Thermometry for average temperature measurement of spray droplets
Global Rainbow Thermometry for average temperature measurement of spray droplets

T. Grosges
Laboratoire d'Optique appliquée
Université Catholique de Louvain
Chemin du cyclotron , 2
B-1348 Louvain-La-Neuve

J.P.A.J van Beeck
von Karman Institute for Fluid Dynamics
Chaussée de Waterloo, 72
B-1640 Rhode Saint Genese


T. Girasole, C. Rozé and G. Grehan
CORIA/LESP, Laboratoire d'Energétique des Systemes et Procédés
Université de Rouen
Site Universitaire du Madrillet - BP 12
76801 Saint Etienne du Rouvray cedex - France


Sinds the end of the 80's, standard rainbow method is used to measure the size and temperature of individual spray droplets, and has many applications in fuel combustion research. Nevertheless, major problems (like temperature gradients inside the droplet, non-sphericity of the droplet,...) affect the temperature measurement. However, the main source of measurement uncertainty, for standard rainbow thermometry, occurs for droplets that are non-spherical. Take a look at Figure 1 to have an idea of the problem. It shows an interaction between the primary and the secondary rainbow formed by non-spherical droplet. It is clear that the position of the rainbow depends not only on the droplet temperature but also on its shape. This include an ambiguity in the temperature measurement.

Fig. 1 A typical rainbow pattern formed by a non-spherical droplet.

Global rainbow thermometry primarily aims at illuminating the non-sphericity effects that exists for the standard rainbow technique since its invention in 1988.

Figure 2 shows a picture of the setup. A CW Argon-ion laser beam illuminates a water spray. The most important remark is that there is no spatial filter like for standard rainbow thermometry. Instead, a transparent piece of paper is installed at the position of the focal plane of the receiving lens. This means that not one but all the droplets crossing the laser beam will contribute to the angular scattered ligth distribution, visible on the transparent screen. This results in the so-called global rainbow pattern that is recorded from the other side of the transparent screen by a digital camera.

Fig. 2 Picture of the setup. One distinguishes the water spray (1), receiving lens (2), transparent screen (3) and video camera (4). The laser beam is added during post-processing of the photograph and does not represent the real thickness.

A typical global rainbow pattern is seen in Figure 3. It is recorded when the water works at stable ambient temperature. Although most probably numerous non-spherical droplets are crossing the laser beam, the position of the global rainbow pattern is not moving. This implies that the pattern is formed by constructive interference of the spherical droplets, because for each of them the geometrical rainbow angle is indentical. Destructive interference occurs for the non-spherical droplets, because their interference fringes are randomly oriented and thus yield a uniform background. Consequently, there is no need for complex selection criteria to look for global rainbow pattern from spherical droplets. The selection is done automatically, just like the rainbow in the sky is a static phenomenon, even though numerous raindrops are not spherical.

Fig. 3 A typical global rainbow pattern in a water spray recorded by a video camera.

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