Modeling of the evaporation and combustion of jet fuel droplets under microgravity conditions A. Cuoci, M. Mehl, T. Paratico, T. Faravelli, E. Ranzi* Dipartimento di Chimica, Materiali e Ingegneria Chimica Politecnico di Milano, Italy Abstract In this work, model results of the evaporation of multicomponent alkane mixtures are presented and discussed. The predictions of the burning rates of n-nonane and JP8 isolated droplets are compared with experimental measures. The analyses are performed on the basis of a detailed kinetic scheme and referring to different surrogates of JP8. The physical description of the phenomena is based on one-dimensional, sphero-symmetric, time-dependent model for the evaporation and combustion of liquid fuel droplets under microgravity conditions. Particular attention is devoted to multicomponent liquid phase diffusion, described through Stefan-Maxwell theory. Introduction The combustion of liquid fuels, such as JP8, widely used in commercial and military applications, plays an important role in power generation and in ground, air and sea transportation. The proper characterization of this process is therefore relevant in order to enhance the efficiency of the combustion devices and to reduce the formation of pollutant components. Mathematical modeling of microgravity experiments on fuel droplets represents a convenient approach to the study of evaporation and combustion of liquid fuels, as diffusion and reaction phenomena can be studied in a simplified geometry and without the effects of natural convection. The adoption of a very detailed chemical analysis turns out to be fundamental in describing burning rates and induction times. JP8 is a kerosene derivative composed by a large number of constituents that encompass a wide range of boiling points, sooting tendencies and heats of vaporization (Table 1). The broad multicomponent nature of JP8 and hydrocarbon fractions is not compatible neither with reproducible experiments nor with viable model simulations. Therefore it is convenient to use surrogate mixtures (that is, well defined mixtures of a limited number of components) able to correctly describe the physical and chemical properties of the real fuels. In general, surrogate fuels are defined in order to reproduce density, thermal conductivity, heat capacity, viscosity and volatility of the fuel, and/or also ignition temperature, oxidation and burning rates, sooting tendency, etc.. Of course, a comprehensive surrogate mixture is indeed difficult to define; in the present work the surrogate mixtures already discussed in [1] have been considered. Mathematical model The following assumptions are considered in the present model: 1. the droplet is spherical due to the microgravity conditions; 2. the pressure is constant and the momentum equation is unnecessary; 3. no natural convection phenomena are present; 4. Soret and Dufour effects are neglected; 5. equilibrium conditions at the droplet/gas interface are assumed; 6. supercritical gas absorption in the liquid phase is neglected; 7. convective transport and recirculations within the droplet are neglected; 8. reactions in the liquid phase are absent. The governing equations are the usual conservation equations for species, energy and mass both for the gas and for the liquid phase. In the present work the model proposed by Kazakov et al. [2] is extended and the * Corresponding author: eliseo.ranzi@polimi.it URL: http://www.chem.polimi.it/ H/C ratio 1.91 Specific gravity 60/60 0.81 Boiling range 166-266°C Critical temperature 410°C Freezing point -51°C Critical pressure 23.4 bar Flash point 53°C Table 1. Properties of JP8 fuels