Numerical study on vaporization and burning of multi-component stationary fuel droplets and single-component convective fuel droplets both with complex configuration and variable properties.

摘要

The studies of droplets and sprays have important applications in many combustors. In the study of stationary droplets, an analysis is presented for multicomponent-liquid-fuel vaporization in a general geometrical situation for non-unitary Lewis number. Variable transport properties and only Stefan flow are considered. The problem becomes a one-dimensional problem for the quasi-steady gas-phase scalar properties by using a mass-flux potential function. Transient heat and mass diffusion in the liquid interiors is considered with special attention given to the fast- and slow-diffusion limits. Eight droplets in a cubic array are considered in the calculations with a blended liquid mixture of heptane, octane, and decane. Comparisons are made amongst the results for the various liquid-diffusion models: transient behavior, the fast vaporization limit, and the slow vaporization limit.;In the studies of convecting droplets, the transient burning of fuel droplets in a hot gas stream is numerically investigated by solving the Navier-Stokes, energy and species equations. Droplet surface regression, deceleration of the stream flow due to the drag on the droplet, internal circulation inside the droplet, variable properties, non-uniform surface temperature, and the effect of surface tension are considered. The array configurations examined in these studies include an isolated droplet, single-layer droplet arrays (infinite periodic arrays, semi-infinite periodic arrays with one row or two rows of droplets, and finite arrays with nine droplets with centers in a plane), and infinite periodic arrays with two layers of droplets. The transient flame shape, surface temperature, burning rate, and dimensionless numbers are studied for different array configurations, initial droplet spacing (for multi-droplet cases), initial droplet radius, initial Reynolds number, initial Damkohler number, and ambient conditions including ambient pressure, temperature and oxygen concentration. Particularly, the critical parameters for the determination of the initial flame shapes, the flame transition (from a wake flame to an envelope flame) time and its influence on the burning rate are determined. Both overall one-step chemical kinetics and four-step reduced chemical mechanism are used to account for the gas-phase combustion in the calculation for an isolated droplet, in order to study the differences in the results introduced from the improved chemical kinetics. In the multi-droplet calculation, only one-step chemical kinetics is used.