FEDSM2003-45542 MECHANISTIC MODELS FOR DROPLET FORMATION AND BREAKUP

ABSTRACT Mechanistic models are created to predict the smallest droplet or bubble of a low concentration, inviscid, dispersed fluid that can be formed by shear from the continuous fluid (or droplet impact on a wall) and the largest droplet/bubble that can survive in a shear flow. Weber number criteria are developed for both the smallest and largest droplet/bubbles based on energy and force balances. The droplet deformation is predicted from an energy balance between available energy from drag on the droplet and energy required to deform the droplet. Different droplet deformation geometries are incorporated into the deformation model and compared against available data from the literature. The oblate spheroid deformation was found to provide the better prediction of deformation, terminal velocity, and largest droplet than a sphere, disk or spherical segment. The droplet deformation and breakup models are believed to be applicable to breakup of droplets, bubbles, and dual dispersions in a wide variety of conditions. These relatively simple models compare favorably against experimental data for low viscosity dispersed liquids and have been successfully used to predict liquid carry-over and gas carry-under from various gas/liquid separators. These models have been incorporated into simulation software used to design predict performance of compact separation equipment 1,2

Benchmarking of truss spar vortex induced motions derived from CFD with experiments

ABSTRACT Floating spar platforms are widely used in the Gulf of Mexico for oil production. The spar is a bluff, vertical cylinder which is subject to Vortex Induced Motions (VIM) when current velocities exceed a few knots. All spars to date have been constructed with helical strakes to mitigate VIM in order to reduce the loads on the risers and moorings. Model tests have indicated that the effectiveness of these strakes is influenced greatly by details of their design, by appurtenances placed on the outside of the hull and by current direction. At this time there is limited full scale data to validate the model test results and little understanding of the mechanisms at work in strake performance. The authors have been investigating the use of CFD as a means for predicting full scale VIM performance and for facilitating the design of spars for reduced VIM. This paper reports on the results of a study to benchmark the CFD results for a truss spar with a set of model experiments carried out in a towing tank. The focus is on the effect of current direction, reduced velocity and strake pitch on the VIM response. The tests were carried out on a 1:40 scale model of an actual truss spar design, and all computations were carried out at model scale. Future study will consider the effect of external appurtenances on the hull and scale-up to full scale Reynolds' numbers on the results