1. APS-DFD Meeting 2016 20th November 2016 Portland, OR
Characterization of transcritical and supercritical droplet vaporization regimes using computations
PAVAN Govindaraju, Daniel Banuti, Peter Ma, Murali Raju, Matthias Ihme Flow Physics and Computational Engineering, Stanford University
Special thanks to : Alessandro Stagni and others at Politecnico di Milano
APS-DFD Meeting th November Portland, OR

2. Importance of trans- and supercritical
Energy
Space
Transport
Defense

3. Diesel Injection : Dodecane - N2
At supercritical conditions, (pcr= 18 bar, Tcr = 658 K) surface tension and mixing dominated injection is observed [Oefelein et al. (2012), Dahms et al. (2013), Manin et al. (2014)]
Direct injection : GDI and Common Rail Injection (over 1000 bar)

4. Fundamental Questions
Given injection conditions, is there an interface?
Important for numerics
Surface tension effects
Useful information for experiments : mixing effects
Accuracy of state-of-the-art reduced models
Hard to generate enough validation through experiments
Theoretical predictions deviate a lot from data
Must rely on simulations
Objective : Develop simulation toolchain capable of predicting interface conditions and evaluating accuracy of reduced models

5. Methodology Focus on interfaces post-breakup
Canonical case : Droplet evaporation
1D multicomponent droplet framework
Prediction of transition line
MD : Dodecane molecules in N2 environment
Comparison with existing theories

6. Multicomponent Droplet Evaporation
Model representation
1D multicomponent droplet in homogeneous isobaric gas-phase
Model simplifications
Finite diffusion (Stefan-Maxwell approach)
Interface
Spherical symmetry
Thermodynamic equilibrium for subcritical Diffusion equation for supercritical (Work in progress)
Peng-Robinson EoS for fugacity evaluation r
OpenSMOKE++ (Cuoci et al.) doi: /j.cpc
Delplanque, Sirignano doi: / (93)80006-G
Harstad, Bellan doi: /S (98)

7. Canonical Problem : Spray-A
n-dodecane droplet in N2 environment
Injection (droplet) temperature = 363 K
Ambient temperature = 900 K
Pressure varied to study presence/absence of interface

8. Mass Fractions v Time P0 = 35 bar
Exceeds pure critical pressures Dodecane (18 bar) Nitrogen (33 bar)
Two-phase
TN2, = 900 K
Tdodecane,0 = 363 K

9. Methodology Focus on interfaces post-breakup
Canonical case : Droplet evaporation
1D multicomponent droplet framework
Prediction of transition line
MD : Dodecane molecules in N2 environment
Comparison with existing theories

10. Prediction of Transition Line
Going back to “is there an interface?”
Transition Line : Function of ambient pressure and temperature showing transition from liquid-like to gas-like
Theoretical approaches
Vapor-Liquid equilibrium
Dahms et al. 2013
Knudsen-number criterion assesses interfacial thickness
Qiu and Reitz 2015
Phase change subsequently changes temperature

11. Transition line : Experimental
Manin et al. (2014)

12. Transition line : Theoretical
Qiu, Reitz (2015)

13. Transition line : Theoretical
Dahms (2016)

14. MD Simulations Simulations performed in constant N (# of particles)
P (Pressure)
System temperature is gradually raised and identify the transition point based on first point in discontinuity of interaction energy.

15. Transition Criterion
The intermolecular interaction energy approaches zero indicating the molecules don’t interact and are in a gas-like regime
Temperature at which the transition from a liquid-like to gas-like regime begins

16. Transition line
MD simulations in good agreement with experimental results for transition

17. Summary
Simulation toolchain to quickly study trans and supercritical interfaces
1D multicomponent droplet evaporation
Molecular dynamics
Results
Interface at pressure higher than pure critical pressures
Pressure and temperature plot (extend to diameters)
Transition line from MD
Theoretical comparisons
Acknowledgements :
NASA and Federal Aviation Administration (FAA)