Temperature measurements on droplets in monodisperse stream using laser-induced fluorescence

Abstract

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

Temperature and chemical composition of droplets by optical measurement techniques: a state-of-the-art review

“…Another alternative to the forward scatter detection system shown in Fig. 18 is to use a single-probe backscatter configuration in combination with LDV measurements (Lavieille et al 2000). Based on the optical setup in Fig.…”

“…Spatially averaged measurements were mainly applied to study lines of monosized and periodically spaced droplets. The droplet temperature was characterized for many liquids in situations including droplet heating in flames (Castanet et al 2005b, c;Lavieille et al 2001), droplet heating in a hot air stream Deprédurand et al 2010), droplet heating in the thermal boundary layer of a hot wall (Castanet et al 2002), droplet heating when colliding with a hot wall (Castanet et al 2009) and droplet cooling in ambient air (Lavieille et al 2000. The same method was used to measure the liquid temperature in a spray of decane droplets convected in an overheated turbulent gas flow (Depredurand et al 2011) and in a high-pressure diesel spray ).…”

“…Lavieille et al (2000) succeeded in using a single band of detection to measure the temperature of ethanol droplets flowing in air at room temperature. Measurements were made possible by the restrictive conditions in which the experiments were conducted.…”

“…Geometrical optics was used in association with ray tracing to describe the propagation of light in terms of rays whose paths are governed by Snell's law and whose intensity changes at the interfaces are calculated according to the Fresnel coefficients (Frackowiak and Tropea 2010a). Paraxial approximation and ray transfer matrix analysis have been also used to describe the internal excitation field (Lavieille et al 2000). Among all the calculations, there is general agreement on the fact that the fluorescence signal from a droplet is not proportional to the droplet volume (Domann et al 2002;Frackowiak and Tropea 2010a).…”

Measurement of the temperature distribution within monodisperse combusting droplets in linear streams using two-color laser-induced fluorescence

“…5b and 5c are off-centered situations at (x =0, y =65, z =0) and ( x =45, y =)40, z =0) respectively. In all these pictures, the local incident intensity distribution in the probe volume was calculated by the model described in Lavieille et al (2000), which takes into consideration the Gaussian distribution of energy in the laser beam and the lensing effect of the droplet interface. The 3D refraction at the droplet interface and that of the laser beam axis and of the collection volume are also included in this model.…”