Analysis and Regimes of Multicomponent Spray Combustion Pavan B. Govindaraju Matthias Ihme Stanford University SIAM Numerical Combustion ‘17 April 3, 2017 Orlando, FL Special thanks to Lucas Esclapez, Thomas Jaravel and Alessandro Stagni and CRECK Modeling Group in POLIMI Sponsors : FAA and NASA Thanks to NERSC for computing resources

Motivation Gas turbine and IC engines use liquid hydrocarbon fuels - mixtures of many compounds Surrogate approach for fuels - palette represents entire fuel Based mostly on gas-phase properties [1] Interaction between evaporation and kinetics not very well understood Fuels are multicomponent - expect preferential evaporation effects Turbulent environments offer further challenges Modifies evaporation, mixing and combustion Objective : Understanding multicomponent spray combustion Role of preferential evaporation Multidimensional gas-phase effects [1] Dooley, CnF, 2010

Role of Preferential Evaporation

Preferential Evaporation : Group Contribution Required for evaporation : Liquid properties of organic compounds These are functions of pressure and temperature! Group contribution approach : Calculate critical properties Pc , Vc , Tc , ω in terms of functional groups [1] Compute properties for fuels at specific conditions using a sequence of corrections [2] (available for download) [1] Constantinou, Gani, AIChE, 1994 [2] Govindaraju, Ihme, IJHMT, 2016

Preferential Evaporation : Intra-Droplet Diffusion Evaporation rates governed predominantly by diffusion coefficients and surface composition 1D model to account for all effects [1] Model representation Mono-dispersed droplet in homogeneous isobaric gas-phase Model simplifications Spherical symmetry No liquid-phase reactions Finite diffusion (Stefan-Maxwell approach) Interface Thermodynamic equilibrium Peng-Robinson EoS for fugacity evaluation r [1] OpenSMOKE++, Cuoci et al., PCI 2014

Preferential Evaporation : 1D Model Verification Fuel of interest : POSF4658 Surrogate 5 compound surrogate of Jet-A POSF 4658 [1] Droplet evaporation Distillation [2] [1] Dooley et al., CnF, 2012 [2] Govindaraju, Ihme, IJHMT, 2016

Preferential Evaporation : Composition Profiles Center vs interface d = 20 µm 𝚽 = 1 The most volatile species (iso-octane) evaporates first Radial composition gradients are present inside the droplet Preferential evaporation affects gas-phase composition Effects on ignition?

Preferential Evaporation : Chemical Mechanism Minimize effect of chemistry - use as detailed as possible POLIMI mechanism 352 species 17848 reactions POSF 4658 mechanism [1] 181 species 4089 reactions 10% Maximum error on Ignition delay time [1] Stagni et al., CnF (2016)

Preferential Evaporation : Two-phase auto-ignition Non-monotonic behavior as a function of droplet size and equivalence ratio Highest reactivity region around d = 20 µm - ɸ = 0.8 Competition among: Evaporation rate Cooling rate of gas-phase Ignition delay time (of evaporating mixture)

Preferential Evaporation : Timescale Analysis Identified three ignition regimes: Ignition is driven by chemical kinetics: non-linear, controlled by preferential evaporation: ignition is controlled by evaporation and fuel-availability: Ideal Stagni et al., PCI (2016)

Preferential Evaporation : Is 1D model necessary? In region (ii), ignition is determined by the composition of the evaporating mixture The boundary between (i) and (ii) is very sensitive to preferential evaporation

Multidimensional Gas-Phase Effects

Gas-Phase Effects : Computational Framework So far looked at preferential diffusion effects on uniform gas-phase Gas phase inhomogeneity influences time of ignition Study using DNS with multicomponent evaporation : 3DA Variable-density, low-Mach number solver [1],[2] QUICK scheme, HYPRE library for Poisson equation Staggered representation : velocity at cell face and scalars at center Gas phase : Mass, momentum, species, energy (standard) Analytical Jacobian - pyJac [3] Domain Setup : Slab Case Box length = 12 mm Borghesi, Mastorakos, Cant CnF (2013) [1] Desjardins, Blanquart, Balarac, Pitsch, JCP, 2008 [2] Shashank, PhD Thesis, 2012 [3] Niemeyer et al., CPC, 2017

Gas-Phase Effects : 0D Model Multicomponent evaporation : preferential effects still shown Because diffusion not set to zero as previously Properties in red from Group Contribution Miller, Harstad, Bellan IJMF 1998 Activity ≈ 1

Gas-Phase Effects : Simulation Cases [1] Droplet Diameter Diffusion Model 10_DIST 10µm Distillation 20_DIST 20µm 30_DIST 30µm 20_DIFF Diffusion Auto-ignition (AI) time evaluated for most-reactive (MR) : ɸ = 2.0 Span small, medium and large droplets, and changing droplet number density 5 compound surrogate for preliminary studies Aromatic compounds lumped for visualization Homogeneous gas phase is not a valid assumption Composition spread depends on droplet diameter NC12H26 Aromatics IC8H18 [1] Esclapez, Govindaraju, Ihme, CTR Annual Briefs, 2016

Gas-Phase Effects : Jet-A Surrogate Results Role of diffusion model Case 20_DIST : Homogeneous droplet Surface identical to core (Distillation-limited) Case 20_DIFF : Frozen droplet (zero diffusion). Surface same as initial(Diffusion-limited) NC12H26 Aromatics IC8H18 Local composition in DIST can have more reactive compounds. IDT can be less than previous study 20_DIST 20_DIFF

Gas-Phase Effects : Jet-A Surrogate Results Role of diameter Case 10_DIST : Small (10 µm) Case 30_DIST : Large (30 µm) Larger compositional variation for large droplets Larger slip velocity More separation between sequential evaporation of compounds Note 30_DIST doesn’t ignite at τAI because IC8H18 is much less reactive than surrogate NC12H26 Aromatics IC8H18 10_DIST 30_DIST

Summary Multicomponent evaporation framework for fuels Role of preferential evaporation in multicomponent combustion Non-monotonic effect of diameter and equivalence ratio Regimes of preferential evaporation Effect of diffusion model on regimes Multidimensional gas-phase effects through DNS State-of-the-art DNS solver : Multicomponent spray combustion Slab case simulations Diffusion model : Volatile and reactive dictate IDT Diameter Small : Marginal, gas phase homogenizes before ignition Medium : Maximum spread in composition Large : More effect of preferential evaporation. But ignition dictated by most volatile species.

Thank you! Questions?