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E. Demosthenous E. Mastorakos R. S. Cant Division A

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1 Direct Numerical Simulations of premixed and non-premixed dual fuel autoignition
E. Demosthenous E. Mastorakos R. S. Cant Division A Engineering Department

2 Outline Pilot-ignited natural gas engines
Project approach – DNS formulation Pilot-ignited premixed methane flame Effect of the amount of the pilot fuel Pilot ignition of non-premixed methane flame Problem description parameters Results and discussion 1

3 Introduction - 1 Conventional fossil fuels: heavy H/C → increasingly substituted by natural gas Natural gas (CH4 main component) The high octane number of natural gas necessitates the introduction of an energy source in order to achieve ignition. The required energy source is often supplied by the injection of a pilot fuel with different autoignition characteristics such as diesel Applications: Availability Low emissions High efficiency 80 % Adaptability transportation, power generation, marine 2 Karim Lida et al Papagiannakis et al Schlatter et al. 2012 3

4 Introduction - 2 Working principle: Natural gas: Premixed or Non-premixed Challenge of dual fuel combustion regime: Presence of gaseous fuel affects the autoignition of the pilot fuel due to slow methane oxidation Presence of pilot fuel determines the gaseous fuel flame initiation Premixed Non-Premixed Improve our limited understanding → Direct Numerical Simulations 3 Karim Lida et al Papagiannakis et al Schlatter et al. 2012 4

5 DNS: Mathematical Formulation-1
Liquid phase n-heptane droplets → point sources Constant temperature within droplet 4 Neophytou et al Borghesi et al. 2012

6 DNS: Mathematical Formulation-2
Gaseous phase Continuity of mass, momentum, energy, species: Yα (Ns) Gaseous fuel: methane (CH4), Liquid fuel: n-heptane Chemical mechanism: Liu et al. (2004) CNF → complex chemistry 5 Neophytou et al Borghesi et al Liu et al. (2004) Neophytou et al Borghesi et al. 2012

7 Premixed methane Purpose:
Investigate pilot fuel autoignition ←→ amount of the pilot fuel Study the subsequent propagation and initiation of the premixed flame initiation This work will be published in Combustion Flame (accepted 2015) Computations performed with ARCHER 6 7

8 Computational parameters
DNS code: SENGA 2 (University of Cambridge) Case A R=0.55 Case B R=0.16 Air/CH4 Air/CH4 Chemistry: Liu et al. (2004) Droplets + Air/CH4 Droplets + Air/CH4 HRR (n-heptane) n-heptane R = n-heptane HRR (n-heptane) + HRR (methane) 7 E. Demosthenous, G. Borghesi, E. Mastorakos, R. S. Cant. Combust. Flame (accepted 2015)

9 Ignition of n-heptane Case A R=0.55 Iso-surface of T=1250 K Case B
ξ tign=1.035 ms ξ=0: air/CH4 ξ=1: C7H16 Iso-surface of T=1250 K Case B R=0.16 ξ tign=1.148 ms 8 E. Demosthenous, G. Borghesi, E. Mastorakos, R. S. Cant. Combust. Flame (accepted 2015)

10 Premixed flame initiation
Case B t = 1.32 ms t = 1.40 ms t = 1.48 ms t = 1.40 ms t = 1.53 ms C=0 CH4 unreacted C=1 CH4 fully burned Iso-surface of C=0.5 9 E. Demosthenous, G. Borghesi, E. Mastorakos, R. S. Cant. Combust. Flame (accepted 2015)

11 Modes of methane consumption
DNS data Homogeneous reactor calculations 1D Premixed methane flame calculations C = 0: unreacted C = 1 fully burnt φ=0.6 Case A t = 1.47 ms R=0.55 Case B t = 1.40 ms R=0.16 10 E. Demosthenous, G. Borghesi, E. Mastorakos, R. S. Cant. Combust. Flame (accepted 2015)

12 Concluding remarks - 1 DNS: Autoignition of n-heptane sprays in constant-volume methane/air mixture The amount of n-heptane dictates: The ignition mechanism of the premixed flame: Small amounts of n-heptane: Presence of intermediates in the ox. stream Large amounts: of n-heptane: Increased temperatures due to n-heptane combustion Premixed flame behaviour Small amounts of n-heptane: Premixed flame Large amounts: of n-heptane: Autoignition regime 11 E. Demosthenous, G. Borghesi, E. Mastorakos, R. S. Cant. Combust. Flame (accepted 2015) 12

13 Non-premixed methane Purpose:
Investigate pilot fuel autoignition when both n-heptane and methane have inhomogeneous distribution Reveal effect of each fuel towards the other on the ignition mechanism Part of this work has been presented in the MCS-2015, Rhodes, Greece. Computations performed with ARCHER 12 13

14 DNS configuration and conditions
DNS configuration details CH4 Mixing layer distribution Case NP1 & NP2 Case NP3 R=0.1 methane n-heptane droplets air Y0CH4 = 1.0 13

15 Parameter definition-1
Mixture fraction: ξ1=0: air Tair = 1300 K t=0.1 ms t=0.46 ms ξ1=1: fuel Tf = 450 K 1 Definition of n-heptane / methane contribution: H1: enthalpy of a heptane/air mixture H2: enthalpy of a methane/air mixture ξ2=0: methane ξ2=1: n-heptane 1 14

16 Parameter definition-2
Dual fuel combustion involving mixtures of two fuels with different C/H ratio 1 → CH4 2 → C7H16 EH, EC → local elemental mass fraction of H, C EC,1, EC,2 → elemental mass fraction of C in fuel 1 or 2 EH,1, EH,2 → elemental mass fraction of C in fuel 1 or 2 Determination of n-heptane / methane contribution: Determination of fuel/air contribution: ξ1,CH =0 air Tair = 1300 K ξ1,CH =1 fuel Tf = 450 K ξ2,CH =0 methane ξ2,CH =1 n-heptane 1 1 15

17 Pre-ignition phase - 1 n-heptane methane air fuel 16 Case NP1
t=0.320 ms n-heptane methane air fuel 16

18 Pre-ignition phase - 2 Case NP1 n-heptane methane fuel air Physical space z=0.5 L The methane/air mixing layer creates pockets around the droplets facilitating the transport of intermediate species to the vicinity of methane. 17

19 Ignition phase Case NP1 18

20 Ignition phase 19 Case NP1 Case NP2 Case NP3 tign=0.435 ms

21 Favorable conditions for ignition
t=0.400 ms Case NP1 Case NP2 Case NP3 20

22 Post-ignition phase Case NP1 t=0.600 ms 21

23 Concluding remarks - 2 Autoignition of turbulent methane-air mixing layers in the presence of n-heptane spray: Pre-ignition phase: → Intermediate species are primarily produced by the n-heptane droplets surrounded by the hot oxidizer while consumption of methane is minor. → The methane/air mixing layer is distorted due to turbulence creating pockets around the droplets and facilitate transport of intermediate species to the vicinity of methane. Ignition phase: ignition occurs at lean mixtures where the liquid fuel has migrated towards the hot air zone. Subsequently after ignition, consumption of both fuels is rapidly increased and eventually a methane-air flame is established. Ignition and propagation are favored at low Nξ1,CH and finite Νξ2,CH 22

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