چكيده به لاتين
In this thesis, the evaporation of a multi-component fuel droplet is simulated using continuous thermodynamics. This method describes the composition of a fuel using a probability density function with respect to one or two variables. As a first step, the transport equations describing the species diffusion in terms of the distribution parameters are developed for the fuel as a whole and for the mean and second moment of the distribution. Then a continuous thermodynamics form of the energy equation and the physical property correlations in terms of the characterizing variable chosen are developed. Ultimately the transport equations in continuous form are incorporated into a finite volume model for droplet evaporation. The numerical solution of these equations and the equations of conservation of mass and species at the droplet surface gives the droplet vaporization rate, the mixture composition field in the vapor phase surrounding the droplet and the change of the liquid composition with time. In this study, a gamma distribution function, with molecular mass as the characterizing variable is chosen to describe the mixture composition.
After verification and validation of the model, it is used to simulate the evaporation of a Mazut droplet. To do this, the gamma distribution parameters for the Mazut are calculated and then, along with the appropriate correlations for the calculation of the properties, are incorporated into the model. Using this model, the computation time is reduced to less than one minute.
The evaporation simulation results of a droplet of 300 K surface temperature and 100 μm diameter which is subjected to a 700 K environment indicate that 80% of the droplet volume evaporates in the first 0.5 s; which is only about 0.3 of the droplet lifetime. At this moment, the mean molecular mass of the droplet, which is 463 kg/kmol at the start of the evaporation, reaches 750 kg/kmol. This indicates that the light components of the droplet reach their boiling points and evaporate shortly after they are subjected to a hot environment, leaving a heavy droplet with a very high boiling point and therefore a very low evaporation rate behind.
On the other hand, studying the changes of the composition of the gas phase surrounding the droplet shows that after these components enter the gas phase, they diffuse outwards creating intensive gradients of temperature and concentration near the droplet surface.
