MULTICOMPONENT FUEL DROPLET VAPORIZATION AT LARGE REYNOLDS NUMBERS

An experimental apparatus and associated measurement techniques have been developed to study the vaporization of free, isolated, single-component and multicomponent fuel droplets subjected to large relative gas-droplet velocities. The experiment, which consists of a monodisperse stream of droplets coaxially injected into a low turbulence level, high speed, heated air flow is used to measure the temporal variation of the droplet's composition, diameter, geometry, and downstream position. Both single-component n-hexane droplets and bicomponent droplets consisting of n-heptane and n-dodecane, vaporizing at Reynolds numbers and vaporization rate coefficients up to 300 and 1 x 10('-2) cm('2)/s, respectively, have been investigated. A semi-empirical model, that includes unsteady droplet heating, and incorporates the solution to the quiescent vaporization problem, a convective correction, and a drag coefficient is used to predict and interpret the experimental results. Particular emphasis is placed on the applicability of results previously determined using suspended droplets vaporizing at low ambient temperatures, the effect of averaging properties, the effect of droplet non-sphericity, and whether internal mixing, demonstrated to significantly affect the compositional history of the droplet, is occurring within the droplet.;A semi-empirical model, that includes unsteady droplet heating is shown to accurately predict the temporal variation of droplet diameter and trajectory for single-component droplets. The model assumes a spatially uniform temperature within the droplet and involves the solution to the quiescent problem, the convective correction of Ranz and Marshall, and a drag coefficient analogous to that determined by Yuen and Chen. In solving the quiescent problem and using the correlation of Ranz and Marshall, all properties are averaged by the 1/3 rule. The drag coefficient is given by the standard drag curve for solid spheres, with the Reynolds number also determined by the 1/3 rule. Thus, the Reynolds number that governs droplet vaporization and drag is an average Reynolds number based upon reference properties. Since this reference Reynolds number can be as much as 6 times larger than the Reynolds number based upon free stream properties, the importance of the high Reynolds number problem is magnified over that previously predicted. In addition, the model is used to demonstrate that the effect of droplet asymmetry on the droplet's trajectory, composition, and vaporization rate is negligible.;Droplet composition measurements indicate that internal mixing exists within a droplet subjected to large relative gas-droplet velocities. For multicomponent droplets an assumption that no internal mixing is occurring within the droplet and that the only internal mechanism of mass transfer is diffusion, significantly underpredicts the compositional change of the droplet. However, the assumption of a spatially uniform composition within the droplet significantly overpredicts the actual compositional change experienced by the droplet. Thus, results appear to support some intermediate model, such as that of Lara-Urbaneja and Sirignano, which includes internal circulation induced by shear on the droplet surface but yet permits internal gradients.;In this dissertation, the accuracy of the experimental techniques used to measure droplet composition, diameter, geometry, and trajectory is established. The accuracy of the droplet composition measurements exceeds +4% -2%, with the major contribution to the error not being due to the phase discriminating probe, but rather the gas chromatograph analysis. The standard deviation of the droplet diameter measurements is less than 1.5% and the standard deviation of the droplet position measurements is less than 4.6%, 60cm downstream of the injection point.