Lower limit of weak flame in a heated channel

Transcription

1 Lower limit of weak flame in a heated channel Yosuke Tsuboi, Takeshi Yokomori*, Kaoru Maruta IFS, Tohoku University * Keio University

2 Background No.2 For highly efficient combustion, HiCOT, HCCI, etc - Controlling initial state of combustion important - Exergy (Availability) efficiencies of combustion process significantly improved General characteristics of combustion with heat recirculation studied in the context of microcombustion (Maruta et al. PCI 30, 2005) 1-D straight meso-scale channel with temperature gradient - S-shaped curve (Normal and weak flames identified) - Flames with repetitive extinction and ignition (FREI) identified

3 Background No.3 1-D straight meso-scale channel with temperature gradient covered by: -Dynamics of flame in narrow tube examined (Jackson, Buckmaster, Lu, Kyritsis, Massa, PCI31, 2007) - DNS study, Hydrogen/air flames confirmed details of FREI Flames in high velocity region highlighted (Pizza, Frouzakis, Mantzaras, Tomboulides, Boulouchos CNF152, 2008)

4 Objectives No.4 Further understandings of weak flame characteristics required - Weak flame limit? - Relation with ignition and weak flame?

5 Overviews of our completed studies Overall picture of flame responses understood by linear stability analysis V Flow Stable v Unstable v v v Stable x S-shaped flame responses (Maruta et al. CESW40, 2004: PCI 30, 2005) No.5

6 Overviews of our completed studies No.6 V Flow Stable Extinction points of FREI v Stable Ignition points of FREI x Unstable middle branch confirmed to correspond flames with repetitive ignition and extinction (FREI) (Minaev et al., CTM 2007)

7 Experimental apparatus No.7 Wall temperature profile established by an external heat source Fuel/Air U cm/s flame Meso-scale channel Mixture Visualization Quartz glass tube d=2.0mm, L=300mm Channel : Methane / Air Digital still camera with CH filter

8 Experimental results ~ Flame responses ~ Mean flow velocity (cm/s) φ = 1.0 Normal flame FREI Estimated points of ignition Extinction Flow T w Ignition Wall Temperature, T w (K) (1) Normal (2) FREI (3) Normal V=50cm/s V=20cm/s Location (mm) V=0.2cm/s High and low flow velocity regions Middle velocity region Normal flame FREI S-shaped flame responses No.8

9 Experiments ~ Higher velocity region ~ Mean flow velocity (cm/s) φ = 1.0 Normal flame FREI Estimated points of ignition Extinction (2) FREI (1) Normal Flow T w Ignition Wall Temperature, T w (K) (1) Normal (2) FREI (3) Normal V=50cm/s V=20cm/s 0 (3) Normal Location (mm) V=0.2cm/s Self-ignition at downstream side stabilized at upstream location Flame position shifts toward upstream with the decrease of flow velocity No.9

10 Experiments ~ Moderate velocity region ~ Mean flow velocity (cm/s) φ = 1.0 Normal flame FREI Estimated points of ignition Extinction (2) FREI (1) Normal Flow T w Ignition Wall Temperature, T w (K) (1) Normal (2) FREI (3) Normal V=50cm/s V=20cm/s (3) Normal Location (mm) V=0.2cm/s Ignition in downstream Flame kernel moves toward the upstream Flame extinction due to low temperature of the wall No.10

11 Experiments ~ Lower velocity region ~ Mean flow velocity (cm/s) φ = 1.0 Normal flame FREI Estimated points of ignition (2) FREI (1) Normal Wall Temperature, T w (K) (1) Normal (2) FREI (3) Normal V=50cm/s V=20cm/s 0 (3) Normal Location (mm) V=0.2cm/s (Exposure time : 1200 s) V m < 0.2 cm/s No luminous flame Lowest burning velocity? Flame is stabilized near the ignition position with very small heat release No.11

12 Experiments ~ Gas and wall temperature differences ~ Temperature difference (K) T g - T w φ = 1.2 φ = 1.0 φ = 0.85 φ = Mean flow velocity (cm/s) V = 0.2 cm/sec, T w = 1225 K T g - T w < 2 K Small temperature increase Flame position of weak flame close to an ignition point Lowest flame temperature corresponds to ignition temperature Universal ignition temperature can be identified No.12

13 Experiments ~ Gas and wall temperature differences ~ Temperature difference (K) φ = 1.2 φ = 1.0 φ = 0.85 φ = Mean flow velocity (cm/s) Mechanism of lower limit of weak flame? No.13

14 Computations Calculation code 1-D steady state flame code ( Kee et al., 1989 ) Energy equation K K dt 1 d dt A dt A A 4 Nu M& λ λa + ρyv c + & ω hw ( T T) = 0 dx c dx dx c dx c c d k k pk k k k 2 w p p k= 1 p k= 1 p Chemistry GRI-mech 3.0 Assumption - Fixed wall temperature - Nu = 4 (Constant) Temperature(K) Convective heat transfer Location (cm) Wall temperature profile No.14

15 No.15 Computations ~ Flame responses ~ (1) Normal (2) FREI (3) Normal (No flame) Experimental flame responses Mean flow velocity (cm/s) Flame position (φ =1.0) Wall temperature Wall temperature (K) Location (cm) Two stable and two unstable solutions agreed with experimental regimes Flame responses exhibit ε-shape Limit in lower stable branch

16 Computations ~ Gas and wall temperature difference ~ No.16 Temperature difference (K) T g -T w Mean flow velocity (cm/s) Difference becomes smaller with the decrease of flow velocity T g - T w < 2 K V = 0.1 cm/s,t w = 1230 K There exists the lowest burning velocity

17 Computations ~ Gas and wall temperature difference ~ No.17 Temperature difference (K) T g -T w Mean flow velocity (cm/s) Mechanism of lower limit of weak flame? Our assumption: limit induced by mass dissipation

18 No.18 Computations ~ Discussion ~ Mass transport of light radical (OH) Convective mass flux Diffusive mass flux ρy OH V m ρy OH V OH V m : Mean flow velocity V OH : Diffusion velocity Flux (g/cm 2 s) 2.0x x x x x10-4 OH flux (V m =134cm/s) Diffusion Convection -1.0x Location (cm) V m =134 cm/s Convection > Diffusion Flux (g/cm 2 s) 4.0x x x x x x x10-6 OH flux (V m =1.82cm/s) Diffusion Convection -1.2x Location (cm) V m =1.82 cm/s Convection < Diffusion

19 Computations ~ Discussion ~ No.19 Total mass fluxes Flux (g/cm 2 s) 2.0x x x x10-4 OH flux (V m =134cm/s) Convection + Diffusion Flux (g/cm 2 s) 6.0x x x x x x x Location (cm) V m =134 cm/s -8.0x x10-6 OH flux (V m =1.82cm/s) Convection + Diffusion -1.2x Location (cm) V m =1.82 cm/s Mass dissipation in low velocity region significantly larger than that in higher velocity region: radicals diffuse out Lower limit of weak flame induced by diffusion effects

20 Conclusions No.20 Existence of lower limit of weak flame confirmed experimentally even if heat loss compensated by an external heating. Lowest flame temperature corresponded to ignition temperature. Computed flame responses exhibited ε-shape, which implies existence of lower limit of weak flame in a lower stable branch. Based on computations, mechanism of lower limit of weak flame is supposed to be mass dissipation loss due to mass diffusion in low velocity region.