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Abstract: A high-pressure, double-chamber, fan-stirred, large-scale explosion facility is proposed for measurements of laminar and turbulent burning velocities, S L and S T, of centrally-ignited hydrogen and carbon monoxide syngas/air mixtures over an initial pressure range of p = 0.1–1.0 MPa. Results show that lean syngas laminar flames at elevated pressure are highly unstable resulting in cellular structures all over the expanding flame front surface, where S L∼p −0.15 having a relatively modest decrease with pressure as compared to lean methane flames where S L∼p −0.50. Contrarily, as to lean syngas turbulent flames, values of S T increase with increasing pressure (S T∼p 0.15) at a fixed r.m.s. turbulent fluctuating velocity (u′ ≈ 1.4 m/s). Moreover, it is also shown that increasing u′/S L is still a way much more effective in increasing values of S T/S L than increasing pressure. Finally, discussions are offered and area for further studies identified. [Copyright &y& Elsevier]

Abstract: Recently, Shy and his co-workers reported a turbulent ignition transition based on measurements of minimum ignition energies (MIE) of lean premixed turbulent methane combustion in a centrally-ignited, fan-stirred cruciform burner capable of generating intense isotropic turbulence. Using the same methodology, this paper presents new complete MIE data sets for stoichiometric and rich cases at three different equivalence ratios ϕ =1.0, 1.2 and 1.3, each covering a wide range of a turbulent Karlovitz number (Ka) indicating a time ratio between chemical reaction and turbulence. Thus, ignition transition in premixed turbulent combustion depending on both Ka and ϕ can be identified for the first time. It is found that there are two distinct modes on ignition in randomly stirred methane–air mixtures (ignition transition) separated by a critical Ka where values of Ka c ≈8–26 depending on ϕ with the minimum Ka c occurring near ϕ =1. For Ka < Ka c, MIE increases gradually with Ka, flame kernel formation is similar to laminar ignition remaining a torus, and 2D laser tomography images of subsequent outwardly-propagating turbulent flames show sharp fronts. For Ka > Ka c, MIE increases abruptly with Ka, flame kernel is disrupted, and subsequent randomly-propagating turbulent flames reveal distributed-like fronts. Moreover, we introduce a reaction zone Péclet number (P RZ) indicating the diffusivity ratio between turbulence and chemical reaction, such that the aforementioned very scattering MIE data depending on Ka and ϕ can be collapsed into a single curve having two drastically different increasing slopes with P RZ which are separated by a critical P RZ ≈4.5 showing ignition transition. Finally, a physical model is proposed to explain these results. [Copyright &y& Elsevier]

Abstract: Minimum ignition energy (MIE) of lean methane–air mixtures is quantitatively measured using a high-power pulse generator which can vary ignition energies of a spark-electrode in the central position of a large fan-stirred cruciform burner. The burner equipped with a pair of counter-rotating fans and perforated plates can be used to generate isotropic turbulence having a very wide range of turbulent intensities (u′) up to 8m/s with negligible mean velocities. Observations of ignition, flame kernel development, and subsequent flame propagation in the central uniform region of the burner are recorded by a CMOS high-speed camera (5000frames/s), showing distributed-like flames of very dispersive and fragmental structures with filiform edges for the first time. A complete MIE data set of lean methane–air mixtures at the equivalence ratio ϕ =0.6 as a function of u′/S L is obtained, where S L is the laminar burning velocity. It is found that there is a transition on values of MIE due to different modes of combustion. Before the transition, MIE only increases gradually with u′/S L. Across the transition when u′/S L >24 corresponding to the commonly defined turbulent Karlovitz number Ka =(u′/S L)2(Re T)−0.5 >8, MIE increases abruptly, where Re T is the turbulent Reynolds number based on the integral length scale of turbulence. This transitional value of Ka is much greater than the Klimov–Williams criterion (Ka =1). Since values of MIE under different levels of turbulence should be relevant to the size of the reaction zone at least in the beginning of turbulent combustion, MIE∼ δ 3 based on an order-of-magnitude criterion where δ is the reaction zone thickness. It is thus concluded that this new experimental finding proves the existence of both thin and broken reaction zones regimes proposed by Peters for a new regime diagram of premixed turbulent combustion. [Copyright &y& Elsevier]

Abstract: Using hydrogen or carbon dioxide as an additive, we investigate the bending effect of turbulent burning velocities () over a wide range of turbulent intensities () up to 40 for lean premixed methane combustion at various equivalence ratios (ϕ), where is the laminar burning velocity. Experiments are carried out in a cruciform burner, in which a sizable downward-propagating premixed CH4/diluent/air flame interacts with intense isotropic turbulence in the central region without influences of ignition and unwanted turbulence from walls. Simultaneous measurements using the pressure transducer and pairs of ion-probe sensors at various positions of the burner show that effects of gas velocities and pressure rise due to turbulent combustion on of lean CH4/H2/air flames can be neglected, confirming the accuracy of the data. Results with increasing hydrogen additions (, 20, and 30% in volume) show that the bending of vs plots is diminished when compared to data with , revealing that high reactivity and diffusivity of hydrogen additives help the reaction zone remaining thin even at high . In contrast, the bending effect is strongly promoted when CO2 is added due to radiation heat losses. This leads to lower values of at fixed and ϕ, where the slope n can change signs from positive to negative at sufficiently large , suggesting that the reaction zone is no longer thin. All data with various δ can be well approximated by a general correlation , covering both corrugated flamelet and distributed regimes with very small data scatter, where Da is the turbulent Damköhler number. These results are useful in better understanding how turbulence and diluents can influence the canonical structures of turbulent premixed flames and thus turbulent burning rates. [Copyright &y& Elsevier]