Atomization and Sprays Journal of the International Institutes for Liquid Atomization and Spray Systems

ISSN for PRINT: 1045-5110

For Online Access

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1993, Volume3

Issue 2

126 pages

Issue price - $75.00

Constantine M. Megaridis
Department of Mechanical Engineering, The University of Illinois at Chicago, Chicago, Illinois 60607, USA

A model investigating the heat and fluid flow fundamentals of liquid metal droplet cooling during spray deposition processes is presented. The droplet configuration studied involves a laminar, axisymmetric gaseous flow around a spherical, superheated, all-liquid, pure metal droplet, which initially has no internal motion and a uniform temperature before it is injected with zero velocity into a cool, uniform gas stream. This flow configuration is identical to one where the droplet is injected into a quiescent gas with a specified velocity. A detailed solution approach is adopted in both gas and liquid phases, even though the temperatures within typical liquid metal droplets are expected to be almost uniform. However, the establishment of temperature differences of even a few degrees in the droplet interior would create very high temperature gradients, given the small sizes of the droplets. High temperature gradients, in turn, have a strong influence on solidification, with a decisive effect on the material properties of the solidified substance. The model, which accounts for variable thermophysical properties in the gas phase, transient droplet cooling with internal liquid circulation, and droplet deceleration with respect to the free flow due to drag, produces time-varying spatially resolved data for the entire flow field in the vicinity of the droplet. These results provide information on the fundamental processes governing the energy and momentum exchange between the droplet and the gaseous stream. The laminar flow simulations for a liquid aluminum droplet at atmospheric pressure show that temperature gradients of the order of 25,000 K/m are maintained in the droplet interior throughout the droplet flight. These gradients are very significant for the subsequent onset of solidification, which has not been modeled in this study. In addition, the relatively high convective cooling rates achieved (> 10⁵ K/s) are enhanced by reduced ambient pressures. The model predictions for the droplet drag coefficient and Nusselt numbers are compared with the values obtained through widely used correlations for the evaluation of these quantities in more populous metal sprays.

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