Numerical Optimization of Contraction and Expansion Structures in Laval Nozzles for Dehydration

A Laval nozzle is a pivotal component of supersonic separation technology for dehydration, but its performance is constrained by low separation efficiency due to intractable conflicts between droplet nucleation and growth rates. To address this issue, this work for the first time applied the Foelsch-MOC method to design an expansion section with a first-order differentiable and second-order continuous profile. A numerical model for pure water vapor condensation based on the ideal gas equation of state (EOS) was then established to optimize nozzle structures for droplet growth. Major findings revealed that the droplet radius of the Foelsch-MOC nozzle was 30.5% higher than that of the conical nozzle, while its outlet liquid mass fraction was 5.8% lower. The liquefaction capacity was found to increase first and then decrease with the expansion coefficient, peaking at 0.5. Moreover, increasing the axis shift from 9.3 mm to 25 mm promoted the outlet liquid mass fraction by 1.2% and decreased the droplet radius by 9.3%, thereby improving liquefaction capacity. These results highlight the droplet growth superiority of the novel expansion profile, providing a valuable guideline for designing nozzles that simultaneously ensure liquefaction efficiency and facilitate droplet development.