ISBN 9783843922890

978-3-8439-2289-0, Reihe Thermodynamik

Peter Keller
Numerical Simulation of Multicomponent Fuel Evaporation

147 Seiten, Dissertation Technische Universität Bergakademie Freiberg (Sachsen) (2015), Softcover, A5

Zusammenfassung / Abstract

In this work the differential evaporation behavior of multicomponent fuel sprays is investigated and characterized. It is shown, how the chosen thermodynamic models affect the differential evaporation and hence the mixture segregation. After these effects are analyzed in detail for single droplets, it is demonstrated that these findings can also be used to characterize the differential evaporation behavior of 3D sprays.

Different models describing the vapor-liquid equilibrium at the droplet surface are discussed: equations of state, activity coefficient models and the ideal Raoult’s law. The influence of the chosen thermodynamic models is investigated for different binary and ternary mixtures of alkanes and alcohols. While liquid mixtures of structurally similar components such as mixtures of alcohols with alcohols or of n-alkanes with iso-alkanes behave almost ideally, mixtures of structurally dissimilar components are known to form azeotropes with strong non-ideal behavior. The two activity coefficient models Non-Random, Two-Liquid (NRTL) and Universal Quasichemical Functional Group Activity Coefficients (UNIFAC) are suitable to describe the non-idealities of the liquid mixture and applied for the single-droplet and spray simulations.

The differential evaporation behavior of the investigated mixtures is characterized by a combination of the differential evaporation factor (DEF) and the separation factor. While the separation factor predicts the mixture segregation by its initial composition and the corresponding fugacities (corrected partial pressures), the DEF gives an information about the degree of segregation using the current evaporation rates and hence traces the evaporation history.

The influence of intra-droplet thermal and mass diffusion is studied by applying two different droplet models: the zero-dimensional rapid-mixing model (RMM) and the one-dimensional diffusion-limit model (DLM). With the DLM temporal and spatial gradients of the components, the temperature and the substance properties are included, which allows to resolve the change in droplet composition and temperature distribution during the evaporation. The DLM results and the effect on the differential evaporation are compared with those of the RMM, which assumes that the droplets are homogeneous, with infinitely fast intra-droplet diffusion. The binary and ternary mixture spray simulations are compared with experiments concerning the vapor and liquid penetration depths.