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Jianfei Xie

Molecular dynamics study of the processes in the vicinity of the n- dodecane vapour-liquid interface. Jianfei Xie The Sir Harry Ricardo Laboratories, School of Computing, Engineering and Mathematics, University of Brighton, UK 13.01.2012, Workshop, University of brighton. Outline

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Jianfei Xie

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  1. Molecular dynamics study of the processes in the vicinity of the n-dodecane vapour-liquid interface Jianfei Xie The Sir Harry Ricardo Laboratories, School of Computing, Engineering and Mathematics, University of Brighton, UK 13.01.2012, Workshop, University of brighton

  2. Outline • Engineering Background (Applications) • Modelling of evaporation of Diesel fuel droplets • How molecular dynamics (MD) is involved • How to use MD technique? • Results and analysis of MD simulations • Summary

  3. Optical Diesel Single Cylinder Engine (Imaged from SHRL) Simulations: The hydrodynamic model is an universal approach to model the heating and evaporation of fuel droplet in Diesel engine-like conditions. (Sazhin, S. S., 2006,Prog. Energy Combust. Sci. 32, 162) Experiments:Effect of injection pressure on the initial stage of fuel injection(Crua,C., Shoba,T., Heikal, M., Gold, M., Higham, C., 2010, SAE 2010-01-2247)

  4. Gas pressure up to 30 bar • Radii of droplet smaller than 5 μm • Kinetic effectcan NOT be ignored in the region close to the droplet surface.(Kryukov, A. P., Lovashov, V. Y., Sazhin, S. S., 2004,Int. J. Heat Mass Transf. 47, 2511) New model is based on the combination of the kinetic and hydrodynamic approaches. (Shishkova, I., Sazhin, S., 2006, J. Comp. Phys. 218, 635)

  5. Kinetic effect on the heating and evaporation of fuel droplet Plots of radii and temperature versus time for a Diesel duel droplet, predicted by the kinetic model considering the heat flux in the kinetic region (1), the kinetic model ignoring the heat flux (2) and the hydrodynamic model (3). The total pressure is 30 bar, the initial droplet temperature is 300 K, the initial droplet radius = 5 μm and the gas temperature = 1,000 K.(Sazhin, S. S., Shishkova, I., 2009,Atomization Spray 19, 473)

  6. How MD get involved? Kinetic model:however, the evaporation coefficient should be specified at the Kinetic boundary condition(KBC), and the velocity distribution function as well. (Shishkova, I., Sazhin, S., 2006, J. Comp. Phys. 218, 635) ~10-9m ~10-6m (>0) α≟ 1 Both are unknown for n-dodecane(C12H26), the closest approximation to Diesel fuel. Challenges: using MD to studytheevaporation/condensation coefficientandvelocity distributionat the liquid-vapour interface.

  7. Molecular dynamics study of the processes in the vicinity of the n-dodecane vapour/liquid interface

  8. MD technique Ensemble (constant - NVT) N: number of molecules, V: volume of system T: temperature (Equations used in each step) Initial conditions: coordinates ri, velocity vx and temperature T Computing force F and potential u (CELLLIST and SHAKE) ------- Lennard-Jones (12-6) potential NO Computing new velocity (Verlet leapfrog) and coordinates (PBC) ------- Newton’s equation of motion equilibrium Computer-based Experiment Sampling for macroscopic variables

  9. Potentials in MD: OPLS OPLS (Optimized Potential for Liquid Simulation ) • Nonbonded interaction (intermolecular and intramolecular) potential • Bond bending (intramolecular) potential • Bond torsion (intramolecular) potential

  10. MD model: united atom model Fig. 1. Schematic presentation of a n-dodecane molecule (a) and its presentation using the united atom model (b). Fig. 2. Schematic of bonds and their interactions in a portion of a n-dodecane chain: bonded potentials Cao, Xie & Sazhin, J. Chem. Phys.134, 164309 (2011)

  11. MD simulation: equilibrium simulation Constant - NVT ensemble Number of molecules: N= 720 (Nx= 5, Ny= 12 and Nz= 12) Simulation box length: Lx х Ly хLz=25.25 nm х 6.48 nm х 6.48 nm Liquid temperature: Tl = 400K, 450K, 500K, and 550K Periodic boundary condition (PBC) used in all directions Fig. 3. Snapshots of the simulation system: initial state (a) and liquid-vapour equilibrium (b) at 500 K.

  12. Results and analysis: density distribution Fig. 4. Number density profiles for liquid temperature at 400K, 450K, 500K and 550K. Fig. 5. Liquid and vapour coexistence at phase equilibrium. Xie, Sazhin & Cao, Phys. Fluids 23, 112104 (2011)

  13. Results and analysis: evaporation/condensation behaviours Xie, Sazhin & Cao, Phys. Fluids 23, 112104 (2011) Fig. 7. Time history of a condensating molecule: (a) trajectory of the molecule and (b) its translational energy. Fig. 6. Time history of an evaporating molecule: (a) trajectory of the molecule and (b) its translational energy.

  14. Results and analysis: molecular behaviours Fig. 8. Schematic presentation of typical evaporation and condensation behaviours of simple molecules (a) and n-dodecane chains (b). Cao, Xie & Sazhin, J. Chem. Phys.134, 164309 (2011)

  15. Results and analysis: condensation coefficient Fig. 9. The values of the condensation coefficient for n-dodecane versus reduced temperature. Xie, Sazhin & Cao, Phys. Fluids 23, 112104 (2011) (in equilibrium)

  16. Results and analysis: velocity distribution Fig. 11. The distribution function of molecules with velocity component normal to the interface Vx in the vapour, interface and liquid phase. Xie, Sazhin & Cao, Phys. Fluids 23, 112104 (2011) Fig. 10. The distribution function of molecules with velocity components tangential to the interface Vy (a) and Vz(b) in the vapour, interface and liquid phase.

  17. Summary • The molecules at the liquid surface need relatively large translational energy to be evaporated. • The vapour molecules with largetranslational energy can easily penetrate into the transition layer and condense in the liquid phase. • The condensation coefficient decreases from 0.95to0.45 when the temperature increases from 400 K to 550 K (KBC). • The velocity distribution is close to isotropic Maxwellian in the liquid phase, interface and vapour phase for the parallel components (KBC). • The velocity distribution is close to bi-Maxwellian in the vapour phase for the normal component (KBC).

  18. (>0), KBCrevised: α≠1 and ≠

  19. Acknowledgment The authors are grateful to EPSRC(grant EP/H001603/1) of the UK and the National Natural Science Foundation(grant 50976052) of China for financial support. Thank you !

  20. Molecular dynamics study of the processes in the vicinity of the n-dodecane vapour-liquid interface Jianfei Xie The Sir Harry Ricardo Laboratories, School of Computing, Engineering and Mathematics, University of Brighton, UK 13.01.2012, Workshop, University of brighton

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