Study on the propagation characteristics of hydrogen/methane/air premixed flames in variable cross-section ducts
Introduction
The foreseeable depletion of fossil-fuels and the observable deterioration of natural-environment have prompted scholars to conduct extensive research on efficient and clean energy. Among alternative fuels, the combustion characteristics of methane/air mixture have been extensively studied (Ajrash et al., 2017; Mitu et al., 2017; Shen et al., 2016), making natural gas is widely used in various applications, and the use of natural gas in engines is practical (Papagiannakis et al., 2010; Tang et al., 2014). However, natural gas combustion still has some shortcomings, such as a slow flame propagation rate and a low thermal efficiency of the engine (Schefer et al., 2002; Di Sarli, 2014). Through detailed analysis and discussion of the advantages (Sun and Li, 2015) and combustion performance (Sun et al., 2012) of hydrogen / methane fuel, it is proved that hydrogenation in natural gas is an effective measure to solve these problems. However, the mixed fuel has the characteristics of low ignition energy, high flow flame velocity, high reactivity and high combustion temperature (Huang et al., 2006; Hu et al., 2014). Obviously, these properties make the mixed fuel more dangerous than the single methane fuel. In production and daily life, hydrogen/methane mixtures need to be transported through complex pipelines, and there are serious risks in the process of transportation.
Introducing hydrogen into existing natural gas pipelines is the most economical way of transportation (Witkowski et al., 2018). Lowesmith et al. (2011a,b), designed large-scale experiments and simulations under the Naturalhy project funded by the European Commission (EC) to assess the feasibility and impact of introducing hydrogen into natural gas pipeline systems. Prior to the widespread use of methane and hydrogen mixed fuels as alternative fuels priority must be given to related safety issues. So in the past few years, scholars have contributed to the study of the explosion characteristics of hydrogen/methane mixture. Zhang et al. (2016) measured the detonation unit sizes of methane/hydrogen/oxygen mixtures with different compositions. Di Sarli et al. (2012); Di Sarli and Di Benedetto (2013) found that the interaction between the flame and eddy is the key to flame propagation, and the dynamic interaction between the flame and eddy in a hydrogen/methane/air mixture was deeply characterized. Their research provides a theoretical basis for the propagation of premixed flame. The maximum explosion pressure, the rising rate of explosion pressure and the flame propagation velocity are generally used as indexes to evaluate the explosion risk. Ma et al. (2014); Shen et al. (2017) and Zhang et al. (2019) studied the explosion overpressure of premixed gas explosion. In the experiment, the explosion overpressure increased with the increase of hydrogen content. Reyes and Tinaut (2017); Boushaki et al. (2012); Chen (2009); Li et al. (2017b) and Sun (2019); Fairweather et al. (2009); Li et al. (2017a) studied the laminar combustion speed and turbulent combustion speed of methane/hydrogen/air mixtures. With increasing hydrogen content, the laminar combustion speed and turbulent combustion speed of premixed gas significantly increase. The comparison of their results shows that turbulence can significantly increase the risk of explosion. Therefore, keeping the environment calm is one of the important methods to reduce the risk of accidental explosion. The initial pressure(Cammarota et al., 2009; Salzano et al., 2012), ignition position (Zheng et al., 2017c; Yu et al., 2019), premixed gas concentration (Yu et al., 2015a; Riahi et al., 2016) and obstacles(Wang et al., 2018; Porowski and Teodorczyk, 2013; Zheng et al., 2017a) all have important effects on the explosion characteristics of mixed gas. In addition, Yu et al. (2015b) and Zheng et al. (2018a) also studied the structural changes of flame propagation in pipelines of different scales. These can clearly understand the dynamic structure of flame propagation in the pipeline, which plays a vital role in the analysis of explosive characteristics.
Considering the complex working environment in reality, the transportation pipeline will design different geometric structures according to the specific terrain, such as bifurcated pipes (Zhu et al., 2017), curved pipes (Emami et al., 2013; Xiao et al., 2015, 2014), linked vessels (Zhang et al., 2017) and variable cross-section pipelines. In the study of variable cross-section pipelines, Zheng and Wang (2009) numerically simulated the gas explosion in a variable cross-section pipeline by using a high-precision weighted essentially nonoscillatory (WENO) scheme, discussed the influence of the variable cross-section pipeline on the flame propagation in a gas explosion, and obtained the conclusion that the intensity of the gas explosion increases in the variable cross-section pipeline compared to a constant cross-section pipeline. Fan and Lu (2008) numerically simulated the detonation process in a variable cross-section combustor for hydrogen-air reaction flow. The results showed that the area change leads to complex wave phenomenon, and the area change and wave reflection lead to the appearance of extreme parameter value. Kuznetsov et al. (2002) studied the effect of tube cross-section variations on flame acceleration, and the results showed that compared to cases with a constant diameter tube, the critical mixture composition for strong flame acceleration was shifted towards less energetic mixtures under the condition of a variable cross-section tube. The change of pipeline structure increases the risk of gas explosion. Therefore, selecting the appropriate pipeline connection method when necessary can reduce the risk degree and avoid unnecessary losses.
At present, the research on the propagation characteristics of premixed hydrogen/ methane / air flame in variable cross-section ducts is relatively new and rare in the current literature. Therefore, from the perspective of safety, studying the propagation characteristics of premixed hydrogen/methane/air flame in variable-section ducts, which is of great significance to understand the hazards of mixed energy and reasonably design variable-section pipelines.
Section snippets
Experimental setup
The ducts used in the experiment consist of three kinds of tubes with cross-sections of 70 × 70 mm2(Small section duct), 100 × 100 mm2(Medium section duct), and 140 × 140 mm2(Large section duct) and lengths of 500 mm. A total of four kinds of variable cross-section ducts with total lengths of 1000 mm are studied, as shown in Table 1. The left side, the ignition end of the duct, is closed by a plexiglass plate, and the right side, called the downstream end, is completely opened and covered with PVC film
Effect of duct structure and hydrogen content on flame structure and flame front position
The above four kinds of variable cross-section ducts are divided into two types: configurations SM and M–S, in which the larger cross-section area is twice the smaller cross-section area, and configurations SL and L–S, in which the larger cross-section area is four times the smaller cross-section area. The left ends of the composite ducts are used as the ignition ends, and the effect of the sudden expansion or reduction of the duct cross-section area on the propagation process of the premixed
Conclusions
The propagation of hydrogen/methane/air premixed flames in different variable cross-section ducts was experimentally studied using a high-speed camera and pressure sensors. The effects of the gas composition and abrupt cross-section variation on the propagation characteristics of premixed flames were analyzed. The main conclusions are summarized as follows:
- (1)
When the abrupt change rate of the cross-section area is the same, the total propagate time for the flame to propagates from the larger
Declaration of Competing Interest
The authors declare that there are no conflicts of interest.
Acknowledgments
This work was supported by the National Key R&D Program of China (Nos. 2018YFC0808100), the Natural Science Foundation of State Key Laboratory of Fire Science (HZ2013-KF07), the National Natural Science Foundation of China (Nos. 51674104) and the National Natural Science Foundation of China (Nos. 51974107).
References (55)
- et al.
Deflagration of premixed methane–air in a large scale detonation tube
Process. Saf. Environ. Prot.
(2017) - et al.
Effects of hydrogen and steam addition on laminar burning velocity of methane–air premixed flame: experimental and numerical analysis
Int. J. Hydrogen Energy
(2012) - et al.
Flame acceleration in the early stages of burning in tubes
Combust. Flame
(2007) - et al.
Combined effects of initial pressure and turbulence on explosions of hydrogen-enriched methane/air mixtures
J. Loss Prev. Process Ind.
(2009) Effects of hydrogen addition on the propagation of spherical methane/air flames: a computational study
Int. J. Hydrogen Energy
(2009)- et al.
On the “tulip flame” phenomenon
Combust. Flame
(1996) - et al.
On the mechanisms of pressure generation in vented explosions
Combust. Flame
(1986) - et al.
Effects of non-equidiffusion on unsteady propagation of hydrogen-enriched methane/air premixed flames
Int. J. Hydrogen Energy
(2013) - et al.
Time-Resolved Particle Image Velocimetry of dynamic interactions between hydrogen-enriched methane/air premixed flames and toroidal vortex structures
Int. J. Hydrogen Energy
(2012) - et al.
Experimental study on premixed hydrogen/air and hydrogen–methane/air mixtures explosion in 90 degree bend pipeline
Int. J. Hydrogen Energy
(2013)