Ignition by hot jets Dr.-Ing. Detlev Markus

Ignition by hot turbulent jet Investigation of ignition process by hot jets (PTB, Braunschweig, Germany)

Why PDF? 3 LaminarTurbulence

Modeling approaches Direction numerical simulation (DNS) Probability density function method (PDF) Reynolds average simulation (RAS) Large eddy simulation (LES) covariances mean values mean chemical source term

Transport equation Transport in : I.physical space due to fluctuating velocity field; II.velocity space due to mean pressure gradient and mean viscous stress; III.scalar space due to chemical reactions; IV.velocity space due to conditional viscous stresses and pressure fluctuation gradients (conditional acceleration); V.scalar space due to molecular diffusion.

Lagrangian particle method Due to the high dimensionality of transport equation for PDF (Pope 1985): Monte-Carlo technique The joint PDF is represented by an ensamble of stochastic particles. For variable-density flows the mass density function (MDF) is used. MDF discrete MDF

Chemie: Reaction-Diffusion Manifold Fuel/air detailed mechanism  Hydrogen/Air : 9 species, 37 reactions (Maas & Warnatz 1988)  Propane/Air : 63 species, 487 reactions (T. Kathrotia, Dissertation, 2011) Transport model  equal diffusivity and unity Lewis number (although it is not limited in REDIM ) REDIM (Bykov & Maas 2007) REDIM (Bykov & Maas 2007) input Reduced chemistry (2D manifold) Reaction progress variable H 2 O for Hydrogen case CO 2 for Propane case Mixing state (enthalpy) output Premixed Combustible/air mixture (Steinhilber, Dissertation, 2015)

Configuration Grid# of particles per cell non-uniform 100x85320 Nozzle inletCo-flow velocity300 m/s10 m/s Compositionburnt, stoich. H 2 /Air, T=1400 Kfresh, stoich. H 2 /Air, T=300 K diameter1 mm16 mm Boundary condition Discretization 2-dimensionalΦ(Φ H 2 O, h) REDIM table CΦCΦ 2 Mixing model constants

PDF simulation of an ignition event Mean temperature Mean H mass fraction

Interacting processes  Macromixing: turbulent entrainment  Micromixing: dissipation of local composition fluctuations  Ignition:  local ignition: ignition of small fluid packages (i.e. a notional particle)  global ignition: increase of the mean temperature macromixing + micromixing + reaction an example for one computational cell t = 0.4 ms macromixing (no micromixing, no reaction) macromixing + micromixing (no reaction)

Interacting processes: global ignition Outside a certain composition range:  ignition very slow  only possible when mixed with previously ignited particles  accumulation of local ignition events  global ignition after ignition delay time reaction rate (Ghorbani et al.,2015, Proc. Combust. Inst.)

Jet head vs. jet stem Conditions for ignition is most likely in the jet head:  small frequency  ignitable composition fluctuations survive long enough  longest time history (necessary for accumulation of local ignition events) Jet stem same location (Ghorbani et al.,2014c)

Chemical reactions propane case T b = 1550 K T u = 300 K U j = 50 m/s hydrogen case T b = 1400 K T u = 300 K U j = 300 m/s

Chemical time scales propane case T b = 1550 K T u = 300 K

Comparison of time scales mean (turbulent) Mixing frequency maximum reaction rate fluctuations in mixture fraction ( ξ rms ) (governed by mixing) fluctuations in progress variable ( c rms ) (governed by mixing and reactions) Deviation of c rms from ξ rms is due to chemical reactions

Summary and conclusion (1)  PDF-PM algorithm  The algorithm was developed for low-Mach reacting turbulent flow  Stable despite noise inherent in PDF methods  Ignition by hot jet (transient turbulent flow)  Ignition appears first at the jet head vortex  Conditions in jet head vortex favors initiation of ignition  Initial phase of jet evolution is essential for global ignition in the jet head vortex  Accumulation of the local ignitions in the jet head vortex responsible for global ignition  Interaction between macromixing, micromixing and chemical reactions guides delay time and position of the global ignition

Physikalisch-Technische Bundesanstalt Braunschweig and Berlin Bundesallee Braunschweig Dr.-Ing. Detlev Markus Telefon: