Away from the recirculation zone, L MIN and L MODE :

1. Fig. 1 Autoignition Vacuum-Insulated Quartz Tube Autoignition Length (LIGN) Uinj Uair Injector Bluff Body Tair Fig. 2 Grid Insulation AIR Tinj Electrical Heaters N2 C2H4 Fig. 3 Fig. 5 Fig. 4 1 – 3 DBL 1 – 3 DBL Average RMS PDF RMS and Abel Transform Processed results from all instantaneous images of a single run Experiments on the Autoignition of EthyleneInjected Concentrically into Confined Annular Jets of Hot Air Christos Nicolaos Markides* and Epaminondas MastorakosHopkinson Laboratory, Department of EngineeringUniversity of Cambridge Motivation Objectives • 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’? • 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 Experimental Methods • 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 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. Results • LIGNmeasuredopticallyinthecontinuous-behaviour ‘RandomSpots’and‘Spot-WakeInteractions’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 • 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’ 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. Conclusions Further Work • 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) • 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 (*): cnm24@cam.ac.uk, http://www2.eng.cam.ac.uk/~cnm24/ & http://www.eng.cam.ac.uk/~em257/