Numerical and Experimental Study of Polydispersed Acetone Spray Dispersion and Evaporation in Turbulent Flow

摘要

Characteristics of acetone spray in a turbulent flow were numerically predicted and compared to experimental measurements. The focus was on the effect of polydispersity on the dispersion and evaporation of a relatively volatile fuel that featured a wide range of Stokes numbers in a turbulent two phase flow. Droplets were generated using an ultrasonic atomizer. It produced a relatively uniform velocity distribution with a moderate carrier to fuel velocities ratio. The simulations were performed in the framework of Reynolds Averaging Navier Stokes equations along with the Eulerian-Lagrangian approach where 12 different classes of the dispersed phase were tracked. Droplets differed in diameter, mean and rms velocities, and numbers density. The transport equations of the carrier phase were formulated in an Eulerian reference frame that included terms which accounted for the exchange of mass, momentum, energy and turbulence quantities with the spray, i.e. fully two way coupling. The phase transition was modeled by the Langmuir-Knudsen law that accounted for non equilibrium effects based on a consistent determination of the molar mass fraction on the droplet surfaces. Effects of turbulence modulation on the vaporization processes were resolved by a thermodynamically consistent model that determined the turbulence intensity at the droplet location, which affected the vapor concentration gradient near the droplet surfaces. For the droplet diffusion, the Markov sequence model was improved by adding a correction drift term to the fluid fluctuation velocity at the parcel position along the droplet trajectory. This correction term aimed at accounting for the non-homogeneity effects in the turbulent flow. The different sub-models for the prediction of multiphase flow characteristics were applied to a 3D configuration that consisted of a spray nozzle mounted in a 4 m/s coflowing air stream. A number of carrier phase jet velocities were used, thus denoting a variation of the fuel to air mass loading. Radial profiles of the axial and radial velocities and its corresponding rms fluctuations of the acetone spray were predicted and compared to the experimental measurements. Spray mass flux, which determined the degree of evaporation, was plotted at different axial location from the nozzle exit plane. The study aimed at assessing the combination of different models applied to a mono-component spray for the prediction of two-phase flow and at investigating what should be improved for the case of real fuel (eg. Kerosene) for industrial configurations.