This paper presents CFD predictions for the evaporation of nearly spherical suspended droplets for ambient temperatures in the range 0.56 up to 1.62 of the critical fuel temperature, under atmospheric pressures. The model solves the Navier-Stokes equations along with the energy conservation equation and the species transport equations; the Volume of Fluid (VOF) methodology has been utilized to capture the liquid-gas interface using an adaptive local grid refinement technique aiming to minimize the computational cost and achieve high resolution at the liquid-gas interface region. A local evaporation rate model independent of the interface shape is further utilized by using the local vapor concentration gradient on the droplet-gas interface and assuming saturation thermodynamic conditions. The model results are compared against experimental data for suspended droplet evaporation at ambient air cross flow including single-and multi-component droplets as well as experiments for non-convective conditions. It is proved that the detailed evaporation process under atmospheric pressure conditions can be accurately predicted for the wide range of ambient temperature conditions investigated.

This is the accepted version of the paper.This version of the publication may differ from the final published version. The Navier-Stokes equations, energy and vapor transport equations coupled with the VOF 14 methodology and a vaporization rate model are numerically solved to predict aerodynamic 15 droplet breakup in a high temperature gas environment. The numerical model accounts for 16 variable properties and uses an adaptive local grid refinement technique on the gas-liquid 17 interface to enhance the accuracy of the computations. The parameters examined include Weber 18 (We) numbers in the range 15 -90 and gas phase temperatures in the range 400 -1000K for a 19 volatile n-heptane droplet. Initially isothermal flow conditions are examined in order to assess 20 the effect of Weber (We) and Reynolds (Re) number. The latter was altered by varying the gas 21 phase properties in the aforementioned temperature range. It is verified that the We number is 22 2 the controlling parameter, while the Re number affects the droplet breakup at low We number 23 conditions. The inclusion of droplet heating and evaporation mechanisms has revealed that 24 heating effects have generally a small impact on the phenomenon due to its short duration 25 except for low We number cases. Droplet deformation enhances heat transfer and droplet 26 Permanent repository linkevaporation. An improved 0-D model is proposed, able to predict the droplet heating and 27 vaporization of highly deformed droplets. 28

Numerical investigation on the evaporation of droplets depositing on heated surfaces at low Weber numbers.

The tendency of today's fuel injection systems to reach injection pressures up to 3000 bar in order to meet forthcoming emission regulations may significantly increase liquid temperatures due to friction heating; this paper identifies numerically the importance of fuel pressurization, phase-change due to cavitation, wall heat transfer and needle valve motion on the fluid heating induced in high pressure Diesel fuel injectors. These parameters affect the nozzle discharge coefficient (Cd), fuel exit temperature, cavitation volume fraction and temperature distribution within the nozzle. Variable fuel properties, being a function of the local pressure and temperature are found necessary in order to simulate accurately the effects of depressurization and heating induced by friction forces. Comparison of CFD predictions against a 0-D thermodynamic model, indicates that although the mean exit temperature increase relative to the initial fuel temperature is proportional to (1-Cd 2) at fixed needle positions, it can significantly deviate from this value when the motion of the needle valve, controlling the opening and closing of the injection process, is taken into consideration.Increasing the inlet pressure from 2000bar, which is the pressure utilized in today's fuel systems to 3000bar, results to significantly increased fluid temperatures above 2 the boiling point of the Diesel fuel components and therefore regions of potential heterogeneous fuel boiling are identified.

A numerical model for the complete thermo-fluid-dynamic and phase-change transport processes of two-component hydrocarbon liquid droplets consisting of n-heptane, n-decane and mixture of the two in various compositions is presented and validated against experimental data. The Navier-Stokes equations are solved numerically together with the VOF methodology for tracking the droplet interface, using an adaptive local grid refinement technique. The energy and concentration equations inside the liquid and the gaseous phases for both liquid species and their vapor components are additionally solved, coupled together with a model predicting the local vaporization rate at the cells forming the interface between the liquid and the surrounding gas. The model is validated against experimental data available for droplets suspended on a small diameter pipe in a hot air environment under convective flow conditions; these refer to droplet's surface temperature and size regression with time. An extended investigation of the flow field is presented along with the temperature and concentration fields. The equilibrium position of droplets is estimated together with the deformation process of the droplet. Finally, extensive parametric studies are presented revealing the nature of multi component droplet evaporation on the details of the flow, the temperature and concentration fields.

1 Highlights  CFD simulation of bi-axial droplet motion in continuous air jet experiment  Comparison against detailed experimental data for droplet breakup  Capturing of droplet breakup regimes for a wide range of Weber numbers  Effect of numerical parameters in predicting droplet breakup  The gas phase recirculation affects the breakup outcome  The pressure interpolation scheme affects the predicted flow field Abstract The present work examines numerically the deformation and breakup of free falling droplets subjected to a continuous cross flow. The model is based on the solution of the Navier-Stokes equations coupled with the Volume of Fluid (VOF) methodology utilized for tracking the droplet-air interface; an adaptive local grid refinement is implemented in order to decrease the required computational cost. Neglecting initially the effect of the vertical droplet motion, a 2D axisymmetric approximation is adopted to shed light on influential numerical parameters. Following that, 3D simulations are ACCEPTED MANUSCRIPT A C C E P T E D M A N U S C R I P T 3 performed which include inertial, surface and gravitational forces. The model performance is assessed by comparing the results against published experimental data for the bag breakup and the sheet thinning breakup regimes. Furthermore, a parametric study reveals the model capabilities for a wider range of Weber numbers.It is proved that the model is capable of capturing qualitatively the breakup process, while the numerical parameters that best predict the experimental data are identified.

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Fuel pressurisation up to 3000 bar, as required by modern Diesel engines, can result in significant variation of the fuel physical properties relative to those at atmospheric pressure and room temperature conditions. The huge acceleration of the fuel as it is pushed through the nozzle hole orifices is known to induce cavitation, which is typically considered as an iso-thermal process. However, discharge of this pressurised liquid fuel through the micro-channel holes can result in severe wall velocity gradients which induce friction and thus heating of the liquid. Simulations assuming variable properties reveal two opposing processes strongly affecting the fuel injection quantity and its temperature. The first one is related to the de-pressurisation of the fuel; the strong pressure and density gradients at the central part of the injection hole induce fuel temperatures even lower than that of the inlet fuel temperature. On the other hand, the strong heating produced by wall friction increases significantly the fuel temperature; local values can exceed the liquid's boiling point and even induce reverse heat transfer from the liquid to the nozzle's metal body. Local values of the thermal conductivity and heat capacity affect the transfer of heat produced at the nozzle surface to the flowing liquid. That creates strong temperature gradients within the flowing liquid which cannot be ignored for accurate predictions of the flow through such nozzles.