This paper presents a novel technique based on laser-induced¯uorescence in liquids, allowing the temperature of 200-lm diameter monodisperse droplets to be measured. The droplets are seeded with an organic dye (rhodamine B), and the temperature dependence of thē uorescence quantum yield is used to determine temperature. The use of LDA optics and a single argon laser source allows to obtain an additional simultaneous velocity measurement. The method appears particularly interesting for the validation of numerical models of evaporating and combusting droplets in the ®eld of design of the combustion chambers of aeronautical and automotive engines, where fuel is injected in droplet form. The measurement technique and data processing are extensively described in the paper. The method is demonstrated on a heated monodisperse droplet stream: the temperature and velocity distribution along the jet were determined.List of symbols C molecular concentration of the tracer (C = 5.10 )6 mol/l)carrier gas velocity w laser beam radiusGreek symbols a volumetric expansion coef®cient / laser beam angle (/ = 5.53°) F 0 injector aperture diameter (F 0 = 100 lm) k laser wavelength (nm) l liquid phase dynamic viscosity q g carrier gas speci®c density q d liquid phase speci®c density Notations subscript i quantities relative to injection conditions subscript 0 quantities relative to reference conditions subscript d quantities relative to droplet
IntroductionIn the ®eld of design of combustion chambers of aeronautical and automotive engines, where fuel is often injected in droplet form, physical and numerical models of evaporating and combusting droplets must be developed. The development of these models should directly improve the ef®ciency of the engines and reduce pollutant emission and noise. Two phases must be described by the models: the droplet's entry into the combustion chamber, including heating and evaporation phases; and the combustion. Fuel sprays are composed of individual fuel droplets ranging from a few microns to a few hundred microns.The general equation of motion of a spherical liquid droplet in a carrier gas may be written as (Wallis 1969; Clift et al. 1978):whereṼ d is the droplet velocity,Ṽ is the carrier gas velocity, q d is the liquid phase speci®c density, and q g is the carrier gas speci®c density,g is the gravitational acceleration, and D d is the droplet diameter. The critical parameter of this equation is the drag coef®cient C d , which depends strongly on temperature and Reynolds number (Chiang et al. 1992). In dense sprays, the drag coef®cient is very sensitive to interaction phenomena in the evaporation and combustion phase (Chiang and Sirignano 1993a, b). Experiments are necessary to develop
Aerothermal properties in a fuel spray is a central problem in the field of the design of the combustion chambers of automotive engines, turbojets or rocket engines. Heat and mass transfer models are necessary in the predictive calculation schemes used by the motorists. Reliable experimental data must be obtained for both the validation and development of new physical models linked to heat transfer and evaporation in sprays, where aerodynamic interactions have a key role. This paper proposes an experimental study of the energetic budget of a monodisperse ethanol droplet stream, injected in the thermal boundary layer of a vertical heated plate. The droplet size reduction is measured using a light scattering technique (interferential method) in order to characterize the evaporation, as the droplet mean temperature is monitored using the two colors laser-induced fluorescence technique. The convection heat transfer coefficient and the Nusselt number are inferred from the overall energetic budget, as a function of the inter-droplet distance, characterizing the interaction regime. The results are compared to physical models combined with numerical simulations available in the literature, for moving, evaporating isolated droplets and for three droplets arrangement in linear stream.
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