Effects of nitrogen and carbon dioxide on hydrogen explosion behaviors near suppression limit

https://doi.org/10.1016/j.jlp.2020.104228Get rights and content

Highlights

  • Flame behavior under buoyancy instability is analyzed.

  • The critical suppression ratio of carbon dioxide and nitrogen is obtained.

  • The suppression mechanism of carbon dioxide and nitrogen is revealed.

Abstract

By varying inert gas content, equivalence ratio and initial pressure, this study is aimed at investigating flame propagation behaviors and explosion pressure characteristics near suppression limit. For carbon dioxide, the weakest flame floating phenomenon is observed at Φ = 1.5 and the buoyant instability is enhanced when the equivalent ratio deviates to the rich and lean sides. For nitrogen, the buoyant instability decreases with increasing equivalent ratio. Both maximum explosion pressure and maximum pressure rise rate increase firstly and then decrease with the increase of equivalence ratio, and they decrease significantly with increasing content of carbon dioxide and nitrogen. For carbon dioxide, the critical suppression ratio of Φ = 0.6, 0.8, 1.0, 1.5 and 2.0 is 7.50, 7.18, 5.74, 3.83, and 2.87. For nitrogen, the critical suppression ratio of Φ = 0.6, 0.8, 1.0, 1.5 and 2.0 is 15.83, 11.87, 9.50, 6.33 and 4.75. Compared to nitrogen, the carbon dioxide is more effective on suppressing hydrogen explosion pressure. The adiabatic flame temperature, thermal diffusivity and mole fraction of active radicals continue to decrease with increasing content of carbon dioxide and nitrogen, which contributes to the decrease of laminar burning velocity.

Introduction

An increasingly prominent problem about environmental pollution has been caused by excessive utilization of fossil fuel. The exploitation of new clean energy is prerequisite to carry out green development concept. As a clean and carbon-free energy carrier, there is a great prospect for hydrogen energy (Holborn et al., 2013; Kalenchuk et al., 2018). However, due to lower ignition energy, wider flammability range and higher diffusivity, the hydrogen explosion accidents happen frequently (Liu et al., 2016; Ng et al., 2007). Due to cleanness and effectiveness, the hydrogen explosion suppression by inert gas is attracting more and more attentions. Benedetto et al. (2009) analyzed the explosion behavior of H2/O2/N2/CO2 mixtures and suggested that the increase of carbon dioxide concentration could decrease maximum explosion pressure and maximum pressure rise rate by decreasing flame temperature and combustion rate. Azatyan et al. (2010) studied the effects of different additives (N2, CO2, H2O) on the laminar burning velocity of hydrogen-air mixtures using numerical simulation and found that the addition of N2, CO2 and H2O could decrease laminar burning velocity of the rich mixtures. Li et al. (2019a) investigated the effects of carbon dioxide on unstable flame propagation of hydrogen-air mixture and pointed out that the destabilization effect of hydrodynamic instability decreases gradually with increasing carbon dioxide addition. Wei et al. (2018) experimentally compared the effects of Ar/N2/CO2 addition on the confined combustion process of hydrogen-air mixture. The results demonstrated that the shock wave was attenuated or vanished with increasing content of the inert gases, and carbon dioxide was more effective on reducing the reaction rates and flame velocity due to its higher specific heat. Li et al. (2018a) obtained the explosion characteristics of hydrogen/oxygen mixture diluted by He, Ar, N2 and CO2 at various equivalence ratios. It was reported that CO2 is the most effective to mitigate unconfined hydrogen cloud explosion due to the fact that the third-body effect of CO2 was relatively stronger. Qiao et al. (2007) theoretically calculated the laminar burning velocity of near-limit hydrogen/air mixtures diluted by He, Ar, N2, and CO2. The results suggested that the suppression effect is in the increasing order of He, Ar, N2, and CO2, which could be attributed to increasing specific heat and decreasing mass and thermal transport properties. Besides, some scholars have studied the suppression effects of inert gas on the laminar burning velocity of H2/CO/air mixtures (Vu et al., 2010; Han et al., 2015; Li et al., 2016; Zhang et al., 2015). Li et al. (2016) found that the H2/CO/CO2/air flame tended to be unstable by increasing content of carbon dioxide. Zhang et al. (2015) suggested that the addition of carbon dioxide and nitrogen could decrease laminar flame speed of H2/CO/air mixtures due to decreasing adiabatic flame temperature and thermal diffusivity.

To sum up, the existing studies about hydrogen explosion suppressed by inert gas are mainly focused on analyzing flame characteristics, explosion pressure and laminar burning velocity. But the effects of inert gas on hydrogen explosion behaviors near explosion suppression limit have not been reported well up to now. In view of this, by changing inert gas type, inert gas content, equivalence ratio and initial pressure, this work firstly analyzes flame behaviors under buoyant instability. Then the maximum explosion pressure and maximum pressure rise rate near explosion suppression limit are analyzed. Finally, the suppression mechanism of carbon dioxide and nitrogen on hydrogen explosion is revealed.

Section snippets

Experimental system

Fig. 1 shows the schematic of experimental apparatus and the related description could be found in our previous works (Li et al., 2018b, 2019b). The experimental system is consisted of high-speed schlieren camera system, pressure testing and data acquisition system, high voltage ignition and time controller system and gas supplying system. The flame morphology is captured by high-speed schlieren system and the operating rate of high-speed camera is 10,000 frame/s. The pressure data is recorded

Flame behaviors near suppression limit

Fig. 2 shows the effects of carbon dioxide and equivalence ratio on flame characteristics. It could be found that the transition from spherical flame to upward floating flame is observed with increasing content of carbon dioxide. For a given equivalence ratio, the expanding flame requires more time to reach same height with increasing content of carbon dioxide. When the content of carbon dioxide is 85%, the upward floating is only observed at Φ = 0.6, the spherical flame could be maintained at

Conclusions

This work firstly analyzes flame behavior under buoyant instability. Secondly, the maximum explosion pressure and maximum pressure rise rate near explosion suppression limit are obtained. Finally, the suppression mechanism of carbon dioxide and nitrogen on hydrogen explosion is revealed. The main results are summarized as follows:

  • (1)

    When the content of carbon dioxide and nitrogen is close to suppression limit, the flame behaviors are mainly controlled by buoyant instability. For carbon dioxide,

Author statement

Caicai Yan: Conceptualization, Methodology, Investigation, Writing - Original Draft. Mingshu Bi: Review & Editing. Yanchao Li: Review & Editing. Wei Gao: Resources, Writing - Review & Editing, Supervision, Data Curation.

Declaration of competing interest

We declare no financial interests.

Acknowledgments

The authors appreciate the financial supported by National Natural Science Foundation of China (No. 51674059, No. 51922025 and No. 51874067), Science and Technology Major Project of Liaoning Province (2019) (2019JH1/10300002) and the Fundamental Research Funds for the Central Universities (DUT20GJ201).

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