Effects of electrode spark gap, differential diffusion, and turbulent dissipation on two distinct phenomena: Turbulent facilitated ignition versus minimum ignition energy transition
Introduction
Electric spark ignition has been used for more than 100 years since the invention of internal combustion engines. It is still widely used today in most combustion devices due to its simplicity to initiate combustion at predetermined locations and timings. Despite of numerous studies available in literatures (see Ref. [1] for highlights and references therein), unfortunately we do not yet fully understand the intriguing complications of conventional spark ignition especially in intense turbulent conditions. This is because the traditional view that turbulence renders ignition more difficult was recently challenged. A peculiar phenomenon, Turbulent Facilitated Ignition (TFI), was observed by Wu et al. [2], in which turbulence could facilitate ignition through differential diffusion when the effective Lewis number (Le) of mixtures is sufficiently larger than unity. Further, Shy et al. [3] substantiated the occurrence of TFI by measuring statistically the minimum ignition energy (MIE) of hydrogen/air mixtures at the equivalence ratio ϕ = 0.18 and 5.1 with Le ≈ 0.3 << 1 and Le ≈ 2.3 >> 1 in both quiescence (u′ = 0) and intense isotropic turbulence (u′ = 5.4 m/s). Note that u′ is the r.m.s. turbulent fluctuating velocity and Le is estimated by the ratio of thermal diffusivity and mass diffusivity with the mass diffusivity being that of the deficient reactant and the abundant inert. Theoretically, a reacting mixture with n species has n Lewis-numbers, however, in the present study Le ≈ Lehydrogen (Leoxygen) for lean (rich) H2/air mixtures. In [3], three important points deserve to note. First, TFI only occurs at sufficiently small spark gap between electrodes (dgap = 0.58 mm < 1 mm) and at sufficiently large Le ≈ 2.3 >> 1, where MIEL >> MIET at u′ = 5.4 m/s (the subscripts L and T represent laminar and turbulent conditions). Second, MIEL and MIET curves versus dgap can cross each other at modest dgap = 2 mm, where MIEL << MIET at u′ = 5.4 m/s. Third, there is no TFI for the Le ≈ 0.3 << 1 case even at small dgap = 0.3 mm. Nevertheless, many important questions remain unclear. For instances, what are the reasons for causing the crossover of MIEL and MIET curves when dgap increases from 0.58 mm to 2 mm? What role do heat losses to electrodes play on the occurrence of TFI? How exactly does the effect of differential diffusion on the embryonic spark kernel vary with a change of dgap and/or u′? This paper addresses these questions by experimentally scrutinizing the interacting effects among heat losses to electrodes (the effect of dgap), differential diffusion (the effect of Le), and turbulent dissipation (the effect of turbulence on dissipation of the deposited ignition energy) on values of MIE as well as exploring variations of these effects due to the change of the embryonic spark kernel geometry.
The contrary phenomenon, MIE transition, has been previously found at modest dgap = 1–3 mm ≥ 1 mm for both gaseous methane/air mixtures at ϕ = 0.6–1.3 with Le < 1 and Le > 1 [4], [5], [6], [7], [8] and liquid pre-vaporized iso-octane/air mixture at ϕ = 0.8 with Le ≈ 2.98 >> 1 [1], in which the increasing slope of MIE as a function of u′ changes from a linear increase to an exponential increase when u′ is greater than some critical value (u′c). Similar MIE transition using methane/air and propane/air mixtures with Le ≈ 1 in a pipe flow has earlier been found by Maas and co-workers [9]. The key message in [9] was that the mean convective part of the flow (flow velocity up to 40 m/s) exerts little influence on values of MIET, whereas turbulent fluctuations have the crucial influence on the ignition process and thus values of MIET. Moreover, Renou and co-workers [10] experimented laser-induced spark ignition of lean methane/air mixtures in decaying homogenous wind-tunnel turbulence. Again, similar MIE transition was found, regardless of using different ignition sources (conventional electrode [1], [4], [5], [6], [7], [8], [9] vs. laser [10]) and different turbulent flows (fan-stirred isotropic turbulence [1], [4], [5], [6], [7], [8] vs. decaying homogenous wind-tunnel turbulence [10]). These results as found in [1], [4], [5], [6], [7], [8], [9], [10] suggest that MIE transition should be a general phenomenon. To validate its generality, we test whether MIE transition also exits in both very lean and very rich hydrogen/air mixtures where Le ≈ 0.3 << 1 (ϕ = 0.3) and Le ≈ 2.3 >> 1 (ϕ = 5.1), regardless of Le.
In this paper we report detailed measurements of MIEL and MIET for both Le ≈ 0.3 and Le ≈ 2.3 mixtures at various dgap = 0.3–6.5 mm over a wide range of u′ varying from 0 to 8.3 m/s in a dual-chamber fan-stirred cruciform burner having near-isotropic turbulence. Each MIE datum is statistically determined at 50% ignitability from 20 to 40 trials over a range of well-controlled ignition energies (Eig) with near-square voltage and current waveforms using the logistic regression method. Therefore, more than one thousand ignition experiments are carefully conducted to measure values of MIEL and MIET at various Le, dgap, and u′ aiming to find out the reasons for the occurrence of these two contrary phenomena, TFI versus MIE transition. Moreover, high-speed Schlieren imaging is applied to obtain the curvature radii of the very early spark kernels at different dgap. These positive curvatures as a function of dgap are used to explain how exactly the effect of differential diffusion due to positive stretching (curvature) on MIEL would change with a variation of dgap for both Le >> 1 and Le <<1 cases. It will be showed in due course that the effect of preferential diffusion of heat and mass transfer on MIE is very sensitive to dgap. Finally, the conclusions and future studies are highlighted.
Section snippets
Electric spark ignition experiments
Electric spark ignition experiments are conducted in a large dual-chamber, constant temperature/pressure, fan-stirred explosion facility capable of generating near-isotropic turbulence. Such facility has been used to measure high-temperature, high-pressure burning velocities of expanding turbulent premixed flames, and the reader is directed to Ref. [11] and references therein for detailed information of the facility and associated turbulence properties. For completeness, a simplified sketch of
The Le ≈ 0.3 << 1 case
Figure 2(a) presents MIEL and MIET data plotted against the distance between electrodes (dgap) for the very lean hydrogen/air mixture at ϕ = 0.18 where Le ≈ 0.3 << 1. Note that each of four different dgap (= 0.3, 0.58, 1, 2 mm) in Fig. 2(a) includes three different values of u′ (= 0, 4.2, 5.4 m/s) represented by three different symbols. Also plotted for comparison are previous MIEL and MIET data of the very rich H2/air mixture at ϕ = 5.1 (Le ≈ 2.3 >> 1) at two different dgap = 0.58 mm and 2 mm,
Discussion
Let us look at the laminar case first. Figure 4(a) and (b) shows high-speed Schlieren images of the embryonic spark kernels in quiescence, respectively at Le ≈ 2.3 and Le ≈ 0.3, each case having three different values of dgap (= 0.58 mm, 2 mm, 6.5 mm) obtained at t = 0.04 ms (the first available images using 25,000 frames/s), where the same 2-mm electrodes with sharp ends are applied. As can be seen, the embryonic kernel geometry changes from an elliptic shape with a large positive curvature at
Conclusions and highlights for future studies
The coupling effects of heat losses to electrodes, differential diffusion, and turbulent dissipation on spark ignition in premixed turbulent combustion are experimentally investigated by extensive measurements of values of MIE and initial kernel geometries of H2/air mixtures at ϕ = 0.18 and 5.1 where Le ≈ 0.3 and 2.3 over a broad range of dgap and u′, which determine two contrary phenomena: TFI versus MIE transition.
TFI only occurs at Le ≈ 2.3 >> 1 and at sufficiently small dgap = 0.58 mm <
Acknowledgments
The financial support from the Ministry of Science and Technology, Taiwan, under grants (MOST 106-2923-E-008-004 -MY3, 106-2221-E-008-054 -MY3, 107-3113-E-002-008) is greatly appreciated. S.S.S. wants to thank Dr. Chien-Chia (Soda) Liu for the helpful discussion.
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