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.
Liquid sheet break-up in coflowing shear flow is the mean by which liquids are atomized in practical injectors for gas turbine combustors. The present study explores experimentally the mechanisms of liquid sheet instabilities and spray formation. Experiments are conducted on four airblast geometries. A high-speed video camera associated with an image processing unit was used to study the liquid sheet instabilities. A microphone and a frequency analyzer were used to track the disintegration frequency. Instability amplitude and disintegration length of the liquid sheet were measured. A two-component Phase Doppler Particle Analyzer was used to characterize the resultant spray. The spatial distribution of the particle size is influenced by the swirling flow field. These experimental results will be used to assess models of fuel sheet instabilities and disintegration.
The state of the art in Rainbow Thermometry is presented. Rainbow Thermometry is a technique for measuring size and temperature of transparent droplets. For data inversion a rainbow pattern is employed, which is formed by a single droplet or by constructive interference of laser light scattered by an ensemble of spherical droplets. In the first case, one speaks about Standard Rainbow Thermometry (SRT), investigated since 1988. In the second case, the technique is called Global Rainbow Thermometry (GRT), studied since 1999; here, the non-spherical droplets and liquid ligaments results in a uniform background and thus do not influence the interference pattern, formed by the spherical droplets, from which average size and temperature are derived. This is a large improvement with respect to Standard Rainbow Thermometry, which is strongly influenced by particle shape. Moreover, GRT is applicable for smaller droplets than the standard technique because the global pattern is not spoiled by a ripple structure. Data inversion schemes based on inflection points, minima and maxima are discussed for SRT and GRT. The standard technique is applied to a monodisperse burning droplet stream, where the problems with particle shape do not exist. Global Rainbow Thermometry is applied to a heated water spray, where the standard technique fails. For both applications the accuracy in the temperature measurement was a few degrees Celsius.
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