Effect of orifice shapes on the detonation transmission in 2H2–O2 mixture
Introduction
Under the general trend of the increasingly serious global energy crisis. Hydrogen, as a next promising energy carrier, is given more attention due to its unique characteristics, such as high efficiency and zero emission [1,2]. However, the flame hazard is likely to occur once hydrogen leakage due to its low ignition energy and broad flammability limit [3,4]. For example, during the Three Mile Island and Fukushima Daiichi Nuclear power Plant accidents, the hydrogen-air mixture is ignited after leakage, producing severe explosion with high overpressure. Of particular concern is the possibility of hydrogen explosion transiting to detonation inducing higher overpressure about 15–20 times initial pressure, which seriously threatens the person and properties safety [[5], [6], [7], [8], [9], [10], [11]]. Therefore, the detonation characteristics must be investigated in detail in hydrogen-air or hydrogen-oxygen mixtures prior to its widely used.
In the past, a round or square tube with obstacle is one of the most common apparatus used to investigate the problem of detonation propagation. The effect of blockage ratio (BR), obstacle spacing and geometry shapes is considered well. Some typical references are listed as follows: Peraldi et al. [12] investigated the process of detonation propagation in detail in a tube filled with obstacle, who suggested that the detonation propagation significantly depends on the intense turbulent shear mixing produced by the obstacles. Kellenberger and Ciccarelli [13] systematically studied the mechanisms of deflagration to detonation transition (DDT) in an obstructed tube, and four various propagation regimes were observed, i.e., fast flame, discontinuous detonation, continuous detonation and limit conditions. Teodorczyk et al. [14] experimentally investigated the effect of BR and obstacle spacing on the flame acceleration in hydrogen-air mixtures, and the results indicated that the detonation wave is easier to be produced in the cases of smaller BR and larger obstacle spacing. Cross and Ciccarelli [15] and Ciccarelli et al. [16] systematically studied the critical condition of detonation propagation in different combustible gaseous mixtures, who suggested that the detonation limit depends on the mixture type and the obstacle features (BR and spacing). Zhang et al. [17] and Sun et al. [18] experimentally studied the effect of large-scale perturbation induced by the orifice plate on the detonation propagation in a round tube, and the attention was mainly paid to the cases of BR > 0.9. Liu et al. [19] systematically explored the effect of the shapes of a single orifice plate on the detonation transmission, including square, triangular, elliptical and circular orifice. The results indicated that the critical condition of detonation propagation among different orifice shapes is consistent with the criterion of d/λ ≈ 13. Mehrjoo et al. [20,21] further explored the effect of irregular geometries on the detonation transmission with the BR values in the range of 0.05–0.25, who suggested that the detonation propagation is independent of the obstacle shape, and it is only a function of its BR. More recently, Sun et al. [22] experimentally investigated the influence of orifice shapes on the detonation propagation in a round tube with repeating orifice plates, and concluded that the effect of orifice shape on the detonation propagation is enhanced with the increases of BR.
Based on these work discussed above, it can be found that most of them consider the influence of obstacle BR and spacing on the detonation propagation, little attention is paid to the effect obstacle shapes on the detonation transmission. In practice, the obstacles with different shapes, including regular and irregular geometries, are more common in industrial safety. It is of great significance to study the mechanism of detonation propagation in a tube with various obstacle geometries. Here, three regular obstacle geometries are considered for the convenience of experimental operation and analysis, i.e., round, square and triangular orifice. The experimental results also can provide some theoretical basis for the optimal design of detonation arrester and further investigating the effect of irregular geometries on the detonation propagation. Moreover, in previous studies considering the obstacle shapes, the BR values of obstacle are smaller, the case of BR ≈ 0.9 is not found despite it's a more practical and important problem in industrial safety [17,18].
The main purpose of this article is to investigate the effect of geometry shapes of a single orifice plate on the detonation transmission. Three different orifice shapes are considered, i.e., round, square and triangular. The BR of orifice plates is fixed at 0.889. The propagation mechanisms of detonation wave and the phenomenon of detonation re-initiation are studied systematically with the help of detonation cellular patterns. Finally, the critical condition of detonation propagation is also analyzed quantitatively.
Section snippets
Experimental apparatus
The apparatus used in this experiment is a square tube with 6-m in length and an inner side length of 300-mm, as shown in Fig. 1a. The first 3-m of the tube is the accelerating section with a perforated plate downstream of the ignitor, which is used to accelerate the formation of a planner detonation wave. The rest section of the tube is the test section. A single orifice plate with various shapes (i.e., circular, triangular and square) was installed between these two sections to investigate
Effect of orifice plate geometries
To investigate the effect of orifice shapes (round, triangular and square) on the detonation transmission, the averaged velocity and detonation cellular pattern are given in this part. Fig. 3 presents the normalized velocity with the theoretical CJ value (VCJ) as a function of distance. Herein, the VCJ calculated by the CHEMKIN package [23] is given for comparison. Three different propagation modes can be observed: (1) when the initial pressure (P0) is smaller than the critical value (Pc), the
Conclusion
In this study, the effect of orifice geometries on the detonation propagation is investigated systematically. Three different orifice shapes are considered, i.e., round, square and triangular. Some interesting results are obtained as below:
- (1)
Three different propagation modes are observed. Firstly, when the initial pressure (P0) is smaller than the critical value (Pc), the steady detonation wave cannot be produced before the orifice plate, afterwards, the detonation wave can be formed by the
Acknowledgements
This work was supported by the National Key R&D Program of China [No. 2016YFC0802101]. The authors also would like to thank Dr. Jin Guo for providing the test site for us in the Fuzhou University.
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