The paper investigates droplet interactions in a dense stream using light scattering techniques, focusing on the evolution of drag coefficients and evaporation rates influenced by droplet spacing. Key methodologies included using electrostatic deviatators and standard rainbow thermometry to measure droplet characteristics like diameter, velocity, and temperature. It highlights the challenges of accurately measuring temperature due to gradients within droplets and discusses the impacts of varying environmental conditions on droplet behavior.

1. F0210
Droplet thermal behavior study with light scattering
technique
Atthasit, A., Castanet*1
, G., Biscos, Y., Lemoine*1
, F. and Lavergne, G..
ONERA – Centre de Toulouse
BP 4025, 31055 Toulouse Cedex 4, Toulouse, France
Tel:+33-0562252835 / FAX:+33-0562252583
E-mail: lavergne@onecert.fr
*1 LEMTA-UMR 7563, Avenue de la Forêt de Haye
BP 160, F-54504 Vandoeuvre-les-Nancy Cedex France
Keywords: Visualization, Standard Rainbow Thermometry, Droplet interaction, vaporization
The aim of this paper is to present the results obtained from a basic experiment on droplet interaction in a dense linear droplet
stream. The interaction of individual droplet with one another and with surrounding influences their transport characteristics.
A vibrating orifice generator produces a stream of monosized droplets. The experiments have been performed by using the
electrostatic deviator to obtain the evolution of the main droplet characteristics in a wide range of the spacing parameter
value Co (ratio between droplet spacing to droplet diameter). The basic experiment allows quantifying precisely the evolution
of the drag coefficient and the droplet evaporation rate for different droplet spacings.
Measurements of droplet diameter, droplet velocity and droplet temperature are performed using non-intrusive laser
interferometry. Ethanol has been used as simulation fluid for all the experiments. The droplet stream was investigated in
different environment conditions. To study the droplet evaporation with quasi-steady state momentum transport, the high
temperature droplets are injected in the ambient conditions. The stream was also injected in the thermal boundary layer of a
heated plate for studying the dynamical and thermal behaviors due to evaporation processes. For reacting conditions, a heated
coil ignited the droplet and gave a laminar diffusion flame along the stream.
The relative velocity is equal to the droplet velocity Vg because the experiment takes place in a quiscent environment.
The droplets are observed by shadowgraphy to measure their velocity. The measured spacing Sg is used to determine the
velocity, knowing droplet frequency f(Vg=Sg.f).
The Standard Rainbow Thermometry (SRT) has been investigated (e.g., König et al., 1986). This technique is based on
the interactions between a spherical droplet and a light ray which measures the temperature and size of droplets. In our case,
a laser beam is focused along the axis of the droplet stream. The fringe scattering pattern (Fig.1) can be observed in forward
direction. The angular spacing measured between two interference maxima can determine droplet sizing, without knowing
the refractive index accurately, calculated from optical geometry theory.
The first rainbow observed in the backward direction is strong enough to be easily detected. The position of the first
rainbow is recorded by another linear CCD array. The Airy/Walker theory (e.g. Walker J.D., 1976) explains that the position
of the rainbow is also function of the liquid refractive index and size. The refractive index is linked to the droplet temperature
by using a calibration curve obtained from the refractometry
measurements (e.g.,Atthasit et al.,2001).
The experimental results show the problems of the
accuracy of the temperature measuring technique due to the
temperature gradient within the droplet. As main results, this
paper presents the evolution of droplet drag coefficient and
droplet evaporation rate with the droplet spacing.
References
König G., Anders K., Frohn A., 1986, "A new light-scattering
technique to measure the droplet diameter of periodically
generated moving droplets", J. Aerosol Sci., Vol.17,
No.2, 157.
Walker J.D., 1976, "Rainbows from single drops of water and
other liquids", Am.J.Phys., Vol.44, No.5, 421.
Atthasit A., Biscos Y., Giuliani F., Lavergne G., 2001,
"Mesure de la température des gouttes en évaporation et
en combustion par la technique Arc-en-Ciel", Regeuil
des actes 9 FluVisu, Rouen, 79.
Fig. 1 Light Diffusion Phenomena provided by SRT