Away from the recirculation zone, LMIN and LMODE: Decreased with Tair, and, increased with Reair

Were found not to be sensitive to Uinj/Uair

Everything else being the same, LMIN increased relative to the pure (no bluff-body) co-flow experiments that were associated with lower uair'

When LMIN reaches the re-circulation zone, the highly intermittent ‘Spot-Wake Interactions’ behaviour replaces the ‘Random Spots’

0 10 20 30 40 50 600

50

100

150

Time (s)

Min

imum

Aut

oign

itio

n L

engt

h (m

m) Spot-Wake Interaction

Random SpotsFig. 5

Experiments on the Autoignition of EthyleneExperiments on the Autoignition of EthyleneInjected Concentrically into Confined Annular Jets Injected Concentrically into Confined Annular Jets

of Hot Airof Hot Air

Interest in the effect of in-homogeneities and turbulence on autoignition is both fundamental and practical (critical in HCCI engines, important in diesel/CI engines and LPP gas turbines, to be avoided in SI engines, storage of flammables, et.c.) DNS of turbulent mixing layers: marginal propensity for earlier autoignition as the turbulence intensity (uair') is increased Non-premixed counter-flow experiments: higher air temperature necessary for autoignition as uair' is increased

Non-premixed co-flow experiments: autoignition delayed as uair' is increased Engine (e.g. HCCI) research: earlier autoignition as ‘mixing is enhanced’ So: What is the ‘effect of turbulence’ on the autoignition of ‘in-homogeneous flows’?

Objectives

Results

Experimental Methods

Motivation

Non-premixed co-flow experiments in this apparatus with H2 and C2H2 injected into pure confined co-flows (as in Fig. 1, but w/out the bluff body), showed that as Uair (and hence uair') and/or Uinj/Uair were increased:

The mean autoignition length increased non-linearly, so that, The mean residence time until autoignition was delayed

Thus: Investigate a case for which uair' increases independently of Uair and Uinj/Uair

In a practically relevant mixing configuration (akin to LPP premix ducts) Provide well-characterized data in a turbulent reacting flow in which the chemical and fluid-mechanical processes interact on the same scales that can serve as a challenging test-bed for the validation of advanced turbulence combustion models

Conclusions Further Work

Fig. 1. Apparatus Schematic. Mixing patterns for illustration.Fig. 2 (above). ‘Instantaneous’ (1ms exposure) OH*

(310±10nm) chemiluminescence of autoignition, from left-to-right:

1st pair: Tair= 1059K, Tinj= 822K, Uair= 17.8m/s, Uinj/Uair= 2.5.

2nd pair: Tair= 1051K, Tinj= 848K, Uair= 13.2m/s, Uinj/Uair= 3.1.

Air was heated up to Tair of 1100K and flowed upwards through a 3.0mm grid, around a bluff-body with Uair up to 40m/s and into a well-insulated, fully transparent quartz tube The tube was open-ended, so experiments were done at atmospheric pressure Two tube/bluff-body sizes were used, but the blockage ratio, (DBL/DIN)2, was kept equal to 0.17 The grid ensured turbulent flow for all conditions; the macroscale Reair, based on the annular hydraulic diameter (DIN-DBL) and Uair was 1400 – 3600 The fuel was N2-diluted C2H4, with mass fraction of C2H4 in C2H4/N2 equal to 0.74 Fuel was injected continuously and concentrically into the annular air jet behind the bluff-body with Uinj= 10 – 80m/s, Uinj/Uair= 1.1 – 4.4 and Tinj in the range 710 – 900K Autoignition occurred in the form of repeated ‘spotty’ flashes accompanied by a ‘popping’ sound

Christos Nicolaos MarkidesChristos Nicolaos Markides** and Epaminondas Mastorakos and Epaminondas MastorakosHopkinson Laboratory, Department of EngineeringHopkinson Laboratory, Department of Engineering

University of CambridgeUniversity of Cambridge

Fig. 3 (left). Average, RMS and PDF post-processed images, calculated from 200 images taken during constant conditions: Tair= 1066K, Tinj= 745K,

Uair= 19.2m/s and Uinj/Uair= 3.2. Also, RMS and Abel transform of RMS for: Tair= 1091K, Tinj= 832K, Uair= 38.2m/s and Uinj/Uair= 1.5.

Fig. 4 (above). LMIN as a function of Tair for various Reair and Uinj/Uair in ‘Random Spots’ and ‘Spot-Wake Interactions’ regimes. Showing

re-circulation region extending from the injector to 1 – 3 DBL downstream.

Fig. 5 (right). LMIN time series in ‘Random Spots’ and ‘Spot-Wake Interactions’. Note the high intermittency occurring approximately every

5 – 10 s, during which the autoignition location shifts abruptly to very short LMIN.

Further evidence has been obtained, by comparison with homogeneous and more weakly turbulent flows, that turbulent mixing (through uair') inhibits autoignition Turbulent mixing, even in this simple flow, can lead to extreme, possibly dangerous autoignition behaviour (here termed ‘Spot-Wake Interactions’) if the turbulence is strong enough (as it is in HCCI, Diesel/CI, LPP, SI)

LIGN measured optically in the continuous-behaviour ‘Random Spots’ and ‘Spot-Wake

Interactions’ regimes For each run, i.e. set of Tair, Tinj, Uair and Uinj conditions, 200 ‘instantaneous’ images, like those in Fig. 2, were used to compile three processed images: Average, RMS and PDF, as shown in Fig. 3 The minimum length from the RMS (LMIN) and most probable from the PDF image (LMODE), were correlated with the conditions, as shown in Fig.4

Average RMS PDF RMS and Abel TransformProcessed results from all instantaneous images of a

single run

Fig. 3

1040 1060 1080 11000

50

100

150

200

Tair

(K)

LM

IN (m

m)

Re=1410,Uinj

/Uair

=3.1

Re=1860,Uinj

/Uair

=2.5

Re=1860,Uinj

/Uair

=2.9

Re=2600,Uinj

/Uair

=1.74

Re=2600,Uinj

/Uair

=1.74

Re=2600,Uinj

/Uair

=2.3

Re=2600,Uinj

/Uair

=2.3

Re=2600,Uinj

/Uair

=2.3

Re=2600,Uinj

/Uair

=3.2

Re=2600,Uinj

/Uair

=3.2

Re=3510,Uinj

/Uair

=2.1

Re=3550,Uinj

/Uair

=1.5

1 – 3DBL

1 – 3DBL

Fig. 4

The discrepancy with the DNS can only be clarified if a link can be made between the variables: Reair, Uair, Uinj/Uair and uair', and, the mixture fraction (ξ) and scalar dissipation rate (χ) in the flows in which these experiments were performed (Fig. 1) Preliminary results from acetone PLIF measurements of these variables suggest that the delaying effect can be explained in terms ξ and χ, but on a problem-specific basis

C2H4

AIR

Vacu

um

-Insu

late

dQ

uartz T

ube

Tair

N2Electrical Heaters

Grid

Auto

ignit

ion L

ength

(LIG

N)

Tinj

Insulation

Autoignition

Injector

Uinj

Uair

Bluff Body

Fig. 1

(*): cnm24@cam.ac.uk, http://www2.eng.cam.ac.uk/~cnm24/ & http://www.eng.cam.ac.uk/~em257/

Fig. 2