Experimental and numerical study on formation mechanism of premixed hydrogen-air squish flame in wall constrained environment

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Experimental and numerical study on formation mechanism of premixed hydrogen-air squish flame in wall constrained environment Yang Hua a,*, Fushui Liu a,b,**, Xiaoyu Zhang a, Han Wu a, Shuaiyi Li a a b

School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China Beijing Electric Vehicle Collaborative Innovation Center, Beijing 100081, China

article info

abstract

Article history:

The study of hydrogen-air premixed flame propagation behavior and induced accelerated

Received 24 March 2019

combustion in a constrained environment is of great significance for realizing rapid

Received in revised form

controllable combustion and promoting the efficient use of hydrogen energy. In this work,

5 May 2019

the propagation process of hydrogen-air premixed flame and the effect of different initial

Accepted 9 May 2019

conditions on flame shape and propagation velocity under the wall constraint were studied

Available online 31 May 2019

in a constant volume bomb using the schlieren method. Then, the flow field characteristics in the wedge space were studied by CFD (computational fluid dynamics) simulation, and

Keywords:

the formation mechanism of the squish flame was revealed. The experimental results

Hydrogen-air

show that the flame propagation process under wall constraint includes four stages:

Squish flame

laminar flame, cell flame, squish flame and spontaneous combustion. The squish flame

Wall turbulence

tends to appear at lower initial temperature in the lean zone and tends to occur at higher

Flame instability

initial temperatures in the rich zone. The flame propagation speed increases with the in-

Wedge-shaped space

crease of the initial pressure and the initial temperature, but increases first and then decreases with the increase of equivalence ratio. The flame propagation velocity of the rich zone is more sensitive to changes in the initial temperature than the lean zone. The simulation results show that during the flame propagation process, the flow of unburned gas in the wedge-shaped space formed by the flame and the two walls forms a large velocity gradient with the wall surface, which induces strong wall turbulence. The turbulence intensity induced in the wedge-shaped space is higher than that induced by the flame and the single wall surface, and the faster the flame propagation speed, the stronger the wall turbulence generated in the wedge-shaped space. When the flame itself is unstable, the strong wall turbulence in the wedge-shaped space causes the instability or even turbulence of the near-wall flame, which accelerates the rapid propagation of the unstable flame. Therefore, the combination of strong wall turbulence formed in the wedge-shaped space and flame instability results in the formation of squish flame. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. ** Corresponding author. School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China. E-mail addresses: [email protected] (Y. Hua), [email protected] (F. Liu). https://doi.org/10.1016/j.ijhydene.2019.05.069 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Introduction Hydrogen energy is considered as one of the most promising alternative fuels for internal combustion engines due to its abundant reserves, clean and high efficiency [1e3]. Compared with other hydrocarbon fuels, hydrogen has the advantages of low ignition energy, wide ignition range, fast burning speed and high diffusivity [4,5]. The minimum ignition energy of hydrogen is only 0.019 mJ, the ignition limit of hydrogen-air premixed gas is in the range of 4e75% (by volume), and the maximum laminar burning velocity is 8 times than that of methane and gasoline [6,7]. These characteristics make the hydrogen application have a stable ignition performance, strong working condition adaptability and high thermal efficiency when applied to an internal combustion engine [8e10]. Meanwhile, hydrogen is one of the most promising alternative fuels for internal combustion engines due to its low pollution of combustion [11]. Therefore, research on hydrogen-air flame propagation behavior and combustion is of great significance for achieving rapid controllable combustion, promoting efficient use of hydrogen energy and the development of internal combustion engine technology. Much research has been conducted on the flame characteristics of free propagation of hydrogen/air mixtures. Some studies have shown that hydrogen addition can accelerate the propagation of the combustion flames because the laminar burning speed of hydrogen is much higher than that of other fuels [12e16]. Kuznetsov et al. [17] reported that the laminar burning velocity of hydrogen-air premixed mixtures increases first and then decreases with the increase of equivalence ratio at normal temperature and pressure, and reaches the maximum near the equivalence ratio of 1.7. Dowdy et al. [18], Koroll et al. [19], Kwon et al. [20], Bradley et al. [21], Dayma et al. [22] also reported similar conclusions. Hu et al. [23] and Sun et al. [24] studied the effects of temperature and pressure on the laminar burning velocity of hydrogen in different shapes of constant volume combustion vessel. The results show that the laminar burning velocity of hydrogen-air premixed mixtures increases with the increase of initial temperature and decreases with the increase of initial pressure. However, Pareja et al. [25,26] reported that under different equivalence ratios, the influence of pressure on the laminar burning velocity of hydrogen-air premixed flame is inconsistent. As the pressure decreases, the laminar burning velocity of the hydrogen-air premixed gas gradually increases in the lean zone and gradually decreases in the rich zone. When the mixture is extremely rich, the laminar burning velocity does not change significantly with pressure. In addition, flame instability also affects the burning velocity and structure of the flame surface, and is also an important reason for the flame to transition from laminar combustion to turbulent combustion [27,28]. In the field of flame dynamics research, the surface of the flame covered with different sizes of cell structures is a manifestation of flame instability. Bechtold et al. [29] and Addabbo et al. [30] analyzed the development process of the cellular instability state of spherical expansion flame in detail, and studied the effects of two unstable mechanisms of fluid mechanics and unequal diffusion on the flame comprehensively. They clearly defined the critical

dimensions (flame radius when the flame surface is covered with a uniform cell structure) and the critical Peclet number (the ratio of the critical radius of the flame to the thickness of the flame) used to characterize the two instability mechanisms of fluid mechanics and unequal diffusion. The studies of Xie et al. [31] and Hu et al. [32] have shown that hydrogen can exacerbate the tendency of flame instability. Hu et al. [32] also pointed out that as the initial pressure increases, the onset of the cellular instability is advanced and the critical radius is reduced, indicating an increase in flame instability. Liu et al. [33] further studied the critical Peclet number of hydrogen-air premixed spherical expansion flame under different initial conditions. The results have shown that the stability of the premixed hydrogen/air flame increases with increasing equivalence ratio and is insensitive to temperature and pressure. In fact, the free propagation flame is only an ideal combustion far from many practical applications. The combustion process in the cylinder of an internal combustion engine is affected by wall confinement and strong turbulence, and the flame propagation behavior is much more complex than that of a free flame [34,35]. However, the influencing factors of the actual internal combustion engine cylinder are complicated, moreover, it is inevitably affected by the cyclic variability [36]. Therefore, studying the accelerated propagation behavior of flames in a constrained environment in a simple wall constrained space is more conducive to an in-depth understanding of the essential issues of flame propagation and combustion processes [37e39]. Some new flame shape have been discovered as the flame travels in different confined narrow spaces. An interesting flame shape, called the “tulip flame”, was observed in channels with an aspect ratio greater than about 2 and has attracted the interest of many researchers [40e42]. Bychkov et al. [43] performed a direct numerical simulation of combustion equations and found the so-called “tulip flame”. Regarding the formation of “tulip flame”, there are various mechanisms to explain, such as the swirl flow caused by the flame, the instability effect, and the pressure wave flame. Dunn-Rankin et al. [44] proposed the term “squish flow” in the study of two-dimensional simulation of tulip flame. They regarded that the squish flow movement was due to the unburned gas being constrained by the wall of the vessel and the compression of the expanded gas. The expansion and compression of burned gas is similar to the compression of the piston in an internal combustion engine. Therefore, a squish flow movement similar to that in the internal combustion engine cylinder occurs at the near wall surface. Gonzalez et al. [45] simulated the formation mechanism of tulip flame using a two-dimensional compressible reaction flow model. They pointed out that the accelerated flow in the wedge-shaped unburned area formed by the flame front and the wall of the pipe is critical to the formation of the tulip flame. Clanet et al. [46] proposed four flame dynamics stages according to the propagation shape of premixed flame in the pipeline: the hemispherical/spherical flame stage, the finger-point flame stage, contact wall flame stage, and the classic tulip flame stage. Xiao et al. [47,48] studied the development pattern of hydrogen-air premixed flame in pipeline in a wide equivalent ratio range, found the deformed Tulip flame (that is, the flame front will continue to

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sag to the wall to form a secondary tulip flame), and defined it as the fifth stage of flame development in pipeline. They regarded that the acceleration of the flame near the wall is caused by the wedge-shaped squish flow formed by the flame front and the wall. Meanwhile, they pointed out that the squish flow is more likely to occur in the hydrogen-air premixed flame propagation process with high diffusivity and laminar burning velocity. Afterwards, Liu et al. [49] discovered a new phenomenon called “squish flame” in the study of hydrogen-air premixed flame propagation behavior under the condition of wall constraint. They found that under some special conditions, two fast-developing flames with a velocity far greater than the original flame appeared near the near wall during the flame propagation process. These two rapidly developing flames were named as “squish flame”, which can greatly increase the flame propagation speed and has a great influence on the combustion process. However, this kind of flame has only been briefly introduced, and its formation process and mechanism have not been studied in depth. The accelerated propagation and combustion of premixed flame in a constrained environment is a complex and important research topic in the field of combustion, which is of great significance for practical applications [50,51]. However, the above research shows that the present studies on the accelerated propagation of flames mainly focus on the fully developed flame in long pipelines. For obstacle-free objects, the research focuses on the problem of tulip flame and deformation of tulip flame. Currently, there are few study on other phenomena of wall-induced flame acceleration. In particular, the study on the new flame acceleration phenomenon, that is squish flame, is almost blank in the limited space where the long diameter is relatively small. The characteristic, influencing factors and formation mechanism of the squish flame are still unknown. In order to make up for the shortcomings of the current research, this study is based on a cylindrical constant volume combustion bomb, which uses a central overhead ignition to ignite combustible gas to simulate the flame propagation behavior under narrow confined space constraints. The propagation characteristics of squish flame with hydrogen-air premixed flames under wall constraints are studied in depth. Then the flow field distribution of the wedge space is simulated by CFD software with a simple calculation model. The model is simplified reasonably based on the analysis of the characteristics of the squirting flame. Finally, the formation mechanism of the squish flame is revealed by combining flame instability analysis. The research results of this work are helpful to understand the formation process and mechanism of the squish flame, which is of great significance for achieving rapid controllable combustion, promoting the efficient use of hydrogen energy and the development of internal combustion engine technology.

Experimental setup and method Experimental system Fig. 1 shows the schematic diagram of the experimental system, which has been introduced in our previous study [52].

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Fig. 1 e Experimental system schematic.

The system mainly includes: cylindrical constant volume combustion bomb, intake and exhaust system, temperature control system, high speed camera system, ignition system, schlieren system, pressure test systems and synchronous control and data acquisition systems. The size of the combustion chamber is F100 mm
100 mm with a volume of 0.79 L. JGS1 grade quartz glass with a diameter of 120 mm and a thickness of 50 mm is installed on both sides of the bomb. The maximum working pressure of the bomb is 6 MPa, and the maximum design temperature can reach 500  C. During the test, the images of the flame propagation process are recorded using the Phantom V7.3 high-speed camera produced by TRI Company with a maximum shooting speed of 200,000 fps (frame per second). The lens used in the test has a focal length of 105 mm, a maximum aperture of F2.8, an exposure time of 20 ms, and an acquisition frequency of 10 kHz. The ignition system is mainly composed of a host computer system consisting of a controllable electronic control unit, an intelligent ignition coil, and a spark plug for the vehicle. The ignition pulse width is set to 5 ms, which can meet the requirements of stable ignition. In the test, the combustion chamber pressure was collected by the Kistler-type 6115B quartz pressure sensor with a range of 0e200 bar and a sensitivity of 10 pC/bar. Considering the transient characteristics of the combustible gas combustion process and the later data analysis, the sampling frequency is set to 100 KHz. The ignition signal and trigger signal of high-speed camera and pressure collection are controlled by the E-tec integrated system produced by Changzhou Yikong Automotive Electronics Co., Ltd.

Experimental method Before the test, the vacuum pump is first used to evacuate the bomb, so that the vacuum in the bomb is below 0.098 MPa. In the test, Dalton's partial pressure law was used to configure different concentrations of hydrogen-air premixed gas. The concentration of hydrogen used was 99.99%. The air was industrial oxygen and nitrogen in a ratio of 21:79. Then the inlet was closed and let it stand for 3 min to ensure that the two gases are well mixed. The spark plug is installed on the top of the bomb, and the flame is restrained by the wall after ignition. The high-speed camera and optical schlieren method are

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used to record the dynamic change of the flame with time, and the pressure change trend in the bomb is collected. The test collected hydrogen-air premixed flame propagation schlieren images under different equivalence ratios and different initial temperature bars, and the test under each working condition was repeated 5 times.

Numerical model and method Simplification of the calculation model The test was carried out in a constant volume combustion bomb of F100 mm
100 mm. After the combustible gas was ignited at the center of the top of the volume bomb, the flame propagated in both horizontal and vertical directions. According to the size and ignition position, the maximum distance of flame propagation in the horizontal direction is 50 mm, and the maximum distance of flame propagation in the vertical direction is 100 mm. The experimental analysis shows that the flame propagation distance is above 60 mm when the squish flame appears. Thus, it can be inferred that when the squish flame occurs, the flame front propagating in the horizontal direction has already touched the optical window. At this time, the positional relationship between the flame front surface and the volume bomb is shown in Fig. 2(a). It can be seen that the flame front surface, the bomb wall and the optical window form four analogous triangular pyramidal

Fig. 2 e (a) Schematic diagram of analogous triangular pyramidal region. (b) Schematic diagram of simplified model.

regions, as shown in the yellow circle in the Fig. 2(a). The formation of squish flame is closely related to the flow field distribution in the triangular pyramidal region. The triangular pyramidal region in which the squish flame appears is similar to the wedge-shaped region mentioned by Dunn-Rankin et al. [44]. The difference is that the wedge shape proposed by Dunn-Rankin is a two-dimensional region concept, that is, the region formed by the flame front and a constraining plane. The triangular pyramidal region in this study is a three-dimensional space, that is, a region formed by a flame front surface and two constraining wall surfaces connected at a certain angle. This type of structure is collectively referred to as a wedge-shaped space. According to the above analysis, the analogous triangular pyramidal region shown in Fig. 2(a) can be reasonably simplified into a wedge-shaped space formed by a flame front and two vertical planes, as shown in Fig. 2(b). This can save the calculation cost and facilitate the qualitative analysis of the mechanism. By simulating the flow field characteristics of the simplified wedge-shaped space, the flow field variation of the triangular pyramidal region and the formation mechanism of the squish flame can be deeply understood.

Model building and parameter settings AVL FIRE software is used for simulation calculation in this study. In order to form a wedge-shaped space as shown in Fig. 3 during the flame propagation process, a rectangular parallelepiped model of 20
20
100 mm is established, the specific dimensions of which are shown in Fig. 3. The ignition point is set at an intersection of the rectangular parallelepiped, as shown by the red dot in the figure, the two faces adjacent to the ignition point are set as the symmetry plane, and the rest are wall faces. In order to facilitate the later analysis, the corresponding coordinate system is established for the model. The ignition point is the coordinate origin, the ignition point to the end wall is defined as the positive x-axis direction, and the ignition point to the intersection of two side wall surfaces is defined as the positive direction of the y-axis. After the combustible mixture is ignited, the flame develops along the x-axis and the y-axis. During the process of the flame gradually traveling along the y-axis to the side wall surface, a wedge-shaped space is formed between the flame

Fig. 3 e 3D calculation model and size.

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and the two side wall surfaces. The grid size of the calculation model is about 0.57 mm, the total grid number is 249,812, and the time step is set to 0.01 ms, which is equivalent to the grid size of the calibration model, and the time step is the same. The initial conditions of the simulation and the model parameters are shown in Table 1.

Verification and validation of the combustion model Compared with other fuels, laminar combustion of hydrogen is faster and more diffusive. Thus, it has certain special requirements for the choice of combustion model. Liu et al. [53] made a detailed study on the simulation model suitable for hydrogen combustion. They used the simple one-dimensional Rectangle model, two-dimensional Cake model and threedimensional Pyramid model to examine the suitability of the Magnussen model, the Coherent Flamelet Model (CFM), the Transported PDF Model (PDF), and the Turbulent Flame Speed Closure Model (TFSC) for simulating hydrogen combustion from the aspects of pressure, concentration, temperature, geometry and calculation time. The results showed that the flame velocity simulated by the Magnussen model is greatly affected by the initial turbulence and geometric model. The CFM model is not suitable for simulating faster combustion, such as the combustion of hydrogen. The PDF model is based on turbulent combustion and the results depend on the initial turbulent conditions. However, the TFSC model is based on laminar combustion and is suitable for turbulent combustion. It is almost unaffected by the geometry and considers the interaction of turbulence and chemical reaction mechanisms during flame propagation and is a model that is more suitable for hydrogen combustion simulation. This paper mainly studies the flame propagation process and flow field characteristics under wall constraints. The entire combustion process includes laminar combustion and turbulent combustion. Therefore, it is more appropriate to choose the TFSC model. In the AVL FIRE software, there is only one adjustable control parameter CFP (constant flame propagation) in the TFSC model, which represents the multiplier factor of the flame propagation reaction rate. The larger the CFP value, the faster the reaction rate of the mixture and the higher the flame propagation speed. Therefore, the parameter CFP needs to be verified and validated. Laminar burning velocity is an important parameter for premixed gas combustion. Its size is only related to the physicochemical properties of the mixture itself, and is independent of the constrained shape. At present, the laminar burning velocity is generally measured by a centrally ignited

Table 1 e The initial conditions of the simulation and the model parameters. Project parameter Initial temperature (K) Equivalent ratio Initial pressure (MPa) Turbulent kinetic energy (m2/s2) Turbulence length scale (m) Turbulence model Combustion model

Parameter value 300 0.6, 0.8, 1.0 0.1 0.001 0.0005 k-ε TFSC

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free-expansion flame. Therefore, a spherical center ignition model is established to simulate laminar combustion. The grid size is 0.57 mm, the total grid number is 89,600, and the time step is 0.01 ms. The initial parameters for calculation are shown in Table 2. The laminar burning velocity can be calculated from the simulated combustion pressure and combustion process and the mass conservation formula of the flame front. Fig. 4 shows the relationship between CFP and laminar burning velocity. It can be seen that the laminar burning velocity of the hydrogenair premixed gas increases linearly with the value of CFP. The linear relationship between the two can be expressed by formula (1). Thus, when the temperature is 300K, the pressure is 0.1 MPa and the equivalent ratio is 1.0, the hydrogen-air premixed laminar burning velocity of hydrogen-air mixture is 2.17 m/s. Then, by substituting the numerical value into the formula (1), the CFP value is 0.22, which is more suitable for the simulation of hydrogen-air premixed combustion. SL ¼ 9:9046
CFP

(1)

Results and discussion Experimental results Flame propagation process Fig. 5 shows the variation of the hydrogen-air premixed flame propagation image and combustion characteristics with time at an initial temperature of 300 K, an initial pressure of 0.1 MPa, and an equivalence ratio of 0.6. It can be seen that after the hydrogen-air premixed combustible gas is ignited at the top of the volume chamber, the flame propagates in a hemispherical shape along the wall of the volume chamber. In the initial stage of combustion, the combustion pressure is almost consistent with the initial state, the pressure rise rate and pressure fluctuation are almost zero, the flame surface is always kept smooth, and the flame stability is strong, which belongs to a relatively stable laminar flame stage. As the combustion progresses, the combustion pressure and pressure increase rate gradually increase, but there is almost no large pressure fluctuation during the combustion process. The flame surface gradually changes from smooth to wrinkled, and the uniform cellular structure gradually spreads over the whole flame front. Braley et al. [54], Bechtold et al. [29], and Addabbo et al. [30] have studied the cellular flame in detail, and pointed out that under the non-extreme condition, the cellular flame is caused by the mechanisms of thermalmass inequality diffusion and hydrodynamic instability in

Table 2 e Initial parameters for laminar combustion simulation. Project parameter Initial temperature (K) Equivalent ratio Initial pressure (MPa) Turbulent kinetic energy (m2/s2) Turbulence length scale (m) CFP

Parameter value 300 1.0 0.1 0.001 0.0005 0.1, 0.4, 2, 3

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Fig. 6 e Morphological structure of squish flame.

Fig. 4 e Relationship between laminar burning velocity and parameter CFP.

the burned zone and the unburned zone during the combustion process. This stage is called the cellular flame stage. When the flame propagated to 5.6e6.0 ms, the combustion pressure and pressure rise rate in the bomb increased significantly. At this time, two rapidly developing flames appeared

at the two side walls of the volume chamber. The new flame has clear propagation boundary and trajectory. It travels faster and reaches the bottom of the volume chamber before the original flame in a short time. These two newly emerging flames are named “squish flame” in Ref. [49], and their specific shapes are shown in Fig. 6. It can be seen that two squish flames appear on both sides of the flame near the wall, and then propagate along the wall towards the bottom of the bomb. When the squish flame appears, its surface has been severely wrinkled and turbulent. It can be seen that the squish flame is essentially a rapidly

Fig. 5 e Flame schlieren images and combustion characteristics under wall constraints.

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developing turbulent flame formed by flame and wall induction, which propagates much faster than the flame in the central region. Its appearance greatly increases the contact area between the premixed flame and unburned gas, and accelerated the combustion process in the bomb, which is of great significance for the rapid controllable combustion. This stage is also known as the squish flame stage. At 6.0e6.3 ms, the flame propagation gradually approaches the bottom of the bomb, and the temperature and pressure in the bomb reach a certain level. At this time, end mixture is instantaneously compressed and ignited, so that the pressure inside the bomb is drastically increased. Unlike squish flame, compression ignition flames burn very fast and have no clear flame propagation boundaries. Subsequently, several flame beams meet and collide at the bottom of the ammunition, creating a strong turbulent environment. This makes the combustion pressure in the bomb oscillate violently and produces greater pressure fluctuation. The stage from the start of spontaneous combustion to the end of combustion is called the spontaneous combustion stage. In summary, the flame propagation process under the wall constraint includes four stages: laminar flame, cellular flame, squish flame and spontaneous combustion. The combustion process under different initial conditions may not fully experience these four stages. In some cases, the flame propagation may go through one or more stages, but not beyond the four flame forms. Among them, the squish flame is a new kind of accelerated propagating flame. Its appearance has greatly increased the speed of flame propagation, accelerated the combustion process and playing an important role in the combustion process.

Flame propagation shape Fig. 7 shows the schlieren image of the flame propagation process at different initial temperatures with an initial pressure of 0.08 MPa and an equivalence ratio of 0.6, 1.0 and 1.5. At the initial stage of flame propagation, when the flame propagation distance is less than 40 mm, the surface of the flame at each initial temperature and equivalent ratio is smooth. At F ¼ 0.6, the influence of the initial temperature on the flame instability at the end of the flame propagation is gradually reflected, as shown in Fig. 7(a). As the initial temperature increases, the wrinkles on the surface of the flame gradually decrease and tend to be smooth, indicating that the flame stability is gradually enhanced. When the hydrogen-air premixed gas concentration is constant, the thermal-mass diffusion instability is hardly affected by the initial temperature. At this time, the instability of the flame is mainly affected by the hydrodynamic instability. Thus, as the initial temperature increases, the hydrodynamic instability is weakened. When F ¼ 1.0 and 1.5, the flame surface is always kept smooth and the stability of the flame is strong during the propagation process. The initial temperature has no obvious influence on the flame instability, as shown in Fig. 7(b) and (c). This is mainly because the Lewis number [55] in the rich combustion zone is greater than 1.0, the thermal diffusion is greater than the mass diffusion, the heat-mass diffusion makes the flame tend to be stable, and the influence of the initial temperature on the hydrodynamic instability of the flame did not cause the flame to destabilize and the cellular

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structure. Therefore, as the initial temperature increases, the hydrodynamic instability of the flame decreases, but the initial temperature has less effect on flame instability. Under the three equivalent ratios, the influence of initial temperature on squish flame is quite different. At F ¼ 0.6, squish flame occurs during the flame propagation only at the initial temperature of 300K. When the initial temperature is greater than 300K, no squish flame occurs during the combustion process. At F ¼ 1.0, squish flame occurs during the combustion process at all four initial temperatures. At F ¼ 1.5, the squish flame appears only at the initial temperature of 450K, and below this initial temperature, no squish flame appears, which is contrary to the trend at F ¼ 0.6. It can be concluded that the squish flame is more likely to appear near F ¼ 1.0. The influence of the initial temperature on the squish flame is obvious under the working condition far away from F ¼ 1.0, Moreover, the influence of the initial temperature on the occurrence of the squish flame shows an opposite trend under the conditions of rich combustion and lean combustion. In the lean zone, the squish flame tends to occur at lower initial temperatures, while in the rich zone, the squirting flame tends to occur at higher initial temperatures. In summary, as the initial temperature increases, the flame instability is slightly weakened, the squish flame tends to appear at a lower initial temperature in the lean zone, and tends to appear at a higher initial temperature in the rich zone. As the equivalence ratio increases, the flame instability gradually decreases, and the squish flame tends to occur near F ¼ 1.0.

Flame propagation speed Fig. 8 shows the variation of flame propagation velocity with the equivalent ratio under different initial conditions. Under different initial pressures and temperatures, the flame propagation speed increases first and then decreases with the increase of equivalence ratio, reaching the maximum near F ¼ 1.0. According to the research in Ref. [17], the maximum laminar burning velocity of the hydrogen-air premixed flame appears near the equivalence ratio of 1.7, while the maximum flame propagation velocity under the wall constraint appears near the equivalence ratio of 1.0. The reason for this difference is that the flame propagation speed under the wall constraint is not only affected by its own burning speed, but also related to the expansion of the burned and unburned gas, the flame deformation and the wall effect. When F ¼ 1.0, the expansion ratio of the combustible mixture is the largest, and the deformation under the wall constraint is large, which increases the propagation speed of the flame front. The average propagation velocity of the hydrogen-air premixed flame under the wall constraint increases with the increase of the initial pressure and the initial temperature. From the ideal gas equation PV ¼ nRT, it can be seen that under the condition of constant initial temperature and volume, the higher the initial pressure, the more the amount of combustible mixture, the higher the heat from combustion, the higher the temperature inside the bomb, and the higher the flame propagation speed. Compared with the lean burning zone, the increase of the initial temperature has a greater influence on the average flame propagation speed in the rich combustion zone. This is mainly because the laminar burning velocity of

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Fig. 7 e Propagation images of hydrogen-air premixed flame at different initial temperatures and equivalence ratios.

the rich combustion mixture increases greatly with the increase of temperature, while the increase of lean combustion mixture is smaller [56]. In summary, the flame propagation speed increases first and then decreases with the increase of the equivalence ratio, reaching a maximum near F ¼ 1.0. The flame propagation speed increases as the initial pressure and initial temperature increase. The flame propagation velocity in the rich combustion zone is more sensitive to changes in the initial temperature than the lean zone.

Numerical results The experimental results show that the appearance of squish flame is closely related to the hydrodynamic instability of the flame and the flame propagation speed. It is preliminarily concluded that the stronger the hydrodynamic instability of the flame, the higher the flame propagation speed, and the easier the squish flame will be formed. In order to further understand the formation mechanism of squish flame more

deeply, CFD numerical method is used to analyze the flow field characteristics of flame propagation. In addition, in order to study the flow field variation in the wedge-shaped space during the flame propagation, the flow field distribution and change process of section A along the Y axis and section B 58 mm away from the ignition point are analyzed in detail. The positions of section A and section B are shown in Fig. 3.

Flow field distribution characteristics and development process Fig. 9 shows the velocity and turbulent kinetic energy distribution of the section A at different times when the equivalence ratio is 1.0. The black arrow in the figure represents the direction of substance flow. After the premixed gas is ignited, as the flame propagates, the unburned gas is squeezed and has a certain flow velocity. In the early stage of flame development, the velocity gradient decreases gradually along the yaxis due to the gas viscous and boundary layer effect on the confined side wall. At this time, the flow rate of the unburned gas in the central region is larger, while the flow rate of unburned gas near wall surface is small, and the speed

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Fig. 8 e Variation of flame propagation velocity with equivalence ratio under different initial pressure and initial temperature conditions.

difference between the wall is small, resulting in a smaller turbulent intensity on the wall, as shown by t ¼ 0.7 ms. With the development of flame, the front of flame approaches the side wall gradually, and the flow velocity in front of the front increases gradually, which causes both the flow velocity near the wall and the turbulent kinetic energy formed with the wall increase gradually. However, at this time, the flame front is far away from the strong turbulent region on the wall, and is less affected by turbulence, as shown by t ¼ 0.9 ms. At this stage, the flame is gradually elongated to form a fingertip flame due to the influence of flow velocity distribution. Both the burned and unburned gas propagate along the x-axis, and the flame undergoes a short acceleration process. With the advance of the reaction process, the flame front propagates gradually along the positive direction of the y-axis to the angle between the two walls. At this time, a wedgeshaped space is gradually formed between the flame front and the two sides of the wall due to the influence of flame shape, as shown in the t ¼ 1.7e1.9 ms images. At this stage, the unburned gas is further compressed and the pressure is increased, resulting in a gradual decrease in flame propagation speed. In addition, the flow direction of the burned gas changes and starts to propagate along the negative direction of the x-axis, while the flow direction of the non-gas remained unchanged, but its flow velocity gradually decreased, forming

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a velocity gradient gradually increasing along the positive direction of the y-axis. At this time, the flow velocity of the unburned gas in the central region is small, while the flow velocity near the wall in the wedge-shaped space is larger, so a large velocity difference is formed between the wedge and the wall. Therefore, a strong wall turbulence is induced, as shown by the turbulent kinetic energy distribution of t ¼ 1.7e2.3 ms. After the wedge-shaped zone is formed between the flame front and the side wall, the flow velocity of the unburned gas in this zone is greater than that in other unburned areas, as shown by the black circles of t ¼ 1.9 and 2.3 ms, which is also the squish flow motion mentioned by Dunn-Rankin et al. [44]. At t ¼ 1.9 ms, a wedge-shaped space has been formed between the flame front surface and the two side wall surfaces. At this time, the distributions of velocity and the turbulent energy on the section B are shown in Fig. 10. It can be seen that the flow rate of unburned gas in the wedge-shaped space is greater than that in other regions, and there is a distinct highspeed zone. At the same time, a strong turbulence is generated near the wall surface in the wedge-shaped space. The turbulent kinetic energy distribution shows that the turbulent kinetic energy in the angular region between the two sides of the wall is greater than that formed by the single side wall surface and the flame front. This indicates that the wedgeshaped space formed by the flame and two adjacent walls will result in a stronger turbulent motion. From the above flow field analysis, it can be seen that in the process of flame propagation, a larger velocity gradient is formed between the unburned gas and the wall in the wedgeshaped space formed by the flame and the two walls, which will induce strong wall turbulence. Moreover, the induced turbulence intensity in wedge-shaped space is higher than that in flame and single wall. From the analysis of Fig. 9, it can be seen that the wall turbulence mainly depends on the velocity of the near-wall unburned gas flow. The greater the velocity, the stronger the wall turbulence. With the development of combustion process, the flow velocity of unburned gas increases first and then decreases. Therefore, the combustion reaction process has a great influence on the flow velocity of unburned gas and the turbulence intensity on the wall.

Flow field characteristics under different equivalence ratios In order to compare and analyze the flow field characteristics of flame propagation at different concentrations, the distance from ignition point to the front of reaction process is defined as the flame propagation distance. The flow field distribution at the same flame propagation distance (L) is studied when the equivalence ratio is 0.6, 0.8 and 1.0. Fig. 11 shows the comparison of reaction process images with different flame propagation distances under three equivalent ratios of 0.6, 0.8 and 1.0. It can be seen that the flame propagating distances are quite different when the flame forms a wedge space under the three concentrations. As the equivalence ratio increases, the flame propagation distance increases when a wedge space is formed between the flame and the two sides of the wall. This is mainly due to the reduction of equivalence ratio, which makes the flame more unstable. Therefore, the stretching effect of the flow field decreases, and the velocity difference along the x-axis and y-axis

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 8 5 5 9 e1 8 5 7 2

Fig. 9 e Velocity and turbulent kinetic energy distribution of section A at different times.

decreases, resulting in the formation of a wedge-shaped space within a relatively short flame propagation distance. Fig. 12 shows the variation of flame propagation velocity with propagation distance under three equivalent ratios of 0.6, 0.8 and 1.0. It can be seen that the flame propagation velocity at each equivalence ratio shows the same trend with the propagation distance, that is, the propagation velocity increases first and then decreases with the increase of the flame propagation distance. When L < 20 mm, the mixture is just ignited and is in the ignition stage. The difference of flame

propagation velocity under three equivalence ratios is small. Subsequently, the flame undergoes a short acceleration process. When L ¼ 40 mm or so, the flame propagation speed reaches its maximum. With the spread of the flame, the unburned gas is further compressed, and the pressure in the unburned zone increases, which hinders the spread of the flame and greatly reduces the flame propagation speed. Therefore, when L > 70 mm, the flame propagation velocities at all three concentrations are at a low level and fluctuate around 10 m/s.

Fig. 10 e Velocity and Turbulence Energy Distribution of Section B at t ¼ 1.9 ms.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 8 5 5 9 e1 8 5 7 2

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Fig. 11 e Images of reaction process at different flame propagation distances with equivalence ratios of 0.6, 0.8 and 1.0.

From Fig. 11, it can be seen that the flame forms a wedgeshaped space in the range of 20 < L < 70 mm at all three concentrations. In this range, the flame propagation velocity is important for the flow velocity and turbulent energy distributions of unburned gas in the wedge-shaped space. Fig. 12 shows that the flame propagation speed increases with the increase of equivalence ratio in the range of 20 < L < 70 mm. That is to say, at the same propagation distance, the flame propagation speed with equivalence ratio of 1.0 is the fastest, followed by 0.8, and the flame propagation speed with equivalence ratio of 0.6 is the slowest. Fig. 13 shows the velocity distribution at different flame propagation distances when the equivalent ratios are 0.6, 0.8

and 1.0. It can be seen that the velocity near the wall increases with the increase of equivalence ratio at the same flame propagation distance. Combining with Fig. 12, it can be concluded that the larger the flame propagation speed is, the greater the flow velocity of the unburned gas in the wedge space is, and the greater the velocity gradient formed with the wall is, the greater the turbulent kinetic energy induced is. Fig. 14 shows the variation of the maximum turbulent kinetic energy near the wall with the flame propagation distance under the three equivalent ratios of_0.6, 0.8 and 1.0. It can be seen that the maximum turbulent kinetic energy near the wall increases first and then decreases with the increase of flame propagation distance under three equivalent ratios, which is similar to the trend in Fig. 12. When the flame propagation distance L < 20 mm, the combustible mixture has just been ignited, and the flame propagation velocity and wall turbulence kinetic energy are smaller. When L > 60 mm, the combustion is approaching the end. Therefore, these two stages have little influence on flame acceleration. Near-wall flame acceleration mainly occurs at the stage of 20 < L < 60 mm, so turbulent kinetic energy distribution in this range plays an important role in flame acceleration. As shown in Fig. 14, in the range of 20 < L < 60 mm, the maximum turbulent kinetic energy in wedge space increases with the increase of equivalent ratio, that is, the largest turbulent kinetic energy is at the equivalent ratio of 1.0, the second is at 0.8, and the lowest is at 0.6.

Formation mechanism of squish flame formation Fig. 12 e The variation of flame propagation velocity with propagation distance at equivalent ratios of 0.6, 0.8 and 1.0.

In this study, after the premixed gas is ignited, the flame develops along the wall towards the capacitance window and the bottom. A wedge-shaped space is formed after the flame

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 8 5 5 9 e1 8 5 7 2

Fig. 13 e Velocity distribution at different flame propagation distances with equivalent ratios of 0.6, 0.8 and 1.0.

contacts the optical window, and a strong wall turbulence is generated in this region. The flame is unstable under the disturbance of its own instability and wall turbulence. At the same time, the diffusion of turbulence accelerates the transport of substances, and then accelerates the rapid propagation of unstable flame, eventually forming a squish flame. Therefore, it can be concluded that the combination of the strong wall turbulence formed in the wedge-shaped space and flame instability in wedge space leads to the formation of rapidly developing squish flame.

Fig. 14 e The variation of maximum wall turbulent kinetic energy with flame propagation distance at equivalent ratios of 0.6, 0.8 and 1.0.

It can also be explained that the squish flame found in the experiment is more likely to appear near the equivalent ratio of 1.0 (as shown in Fig. 7). When the equivalence is small, squish flame is not easy to occur. The main reason is that the weaker hydrogen-air premixed flame has strong instability. Even without external disturbance, it can develop into a turbulent flame itself. However, because of its small flame propagation speed and the small wall turbulence intensity formed in the wedge space, it cannot promote the rapid development of turbulent flame near the wall to form a squish flame. When the equivalence is large, although the flame propagates faster and the turbulence intensity formed in the wedge-shaped space is larger, the flame itself is more stable, even under external disturbance, it is difficult to destabilize. Therefore, it is also more difficult to form a squish flame. Under medium concentration condition, the flame propagates faster, the wall turbulence in wedge space is stronger, and the flame itself is more unstable. Both of them promote the formation of squish flame at medium concentration. In addition, the effect of initial temperature on the appearance of squish flame shows an opposite trend under rich and lean burning conditions. In the lean-burning zone, the squish flame tends to appear at a lower initial temperature, while in a rich-burning zone, the squish flame tends to appear at higher initial temperatures (as shown in Fig. 7). In the lean-burning zone, as the initial temperature increases, the expansion ratio decreases, the flame thickness increases slightly, but the flame instability decreases slightly. Compared with the lean zone, the initial temperature has a greater influence on the flame propagation speed in the rich-burning

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 8 5 5 9 e1 8 5 7 2

zone. The higher the initial temperature, the greater the flame propagation velocity. The greater the flame propagation speed, the stronger the wall turbulence of the wedge-shaped space and the greater the disturbance to the flame. Therefore, the larger turbulence intensity promotes the formation of the squish flame when the flame itself is unstable.

Conclusions In this work, the propagation process of hydrogen-air premixed flame and the effect of different initial conditions on flame propagation shape and propagation velocity under the wall constraint were studied in a constant volume bomb using the schlieren method. Then, the flow field characteristics in the wedge space were studied by CFD simulation, and the formation mechanism of the squish flame was revealed. The main conclusions are as follows: The flame propagation process under the wall constraint consists of four stages: laminar flame, cell flame, squish flame and spontaneous combustion. The flame instability decreases slightly with the increase of initial temperature. The squish flame tends to appear at lower initial temperature in the leanburning zone and tends to occur at higher initial temperatures in the rich-burning zone. In addition, the flame instability decreases gradually with the increase of equivalence ratio, and the squish flame tends to appear near the equivalence ratio of 1.0. The flame propagation velocity increases first and then decreases with the increase of equivalence ratio, reaching the maximum near the equivalence ratio of 1.0. The flame propagation velocity increases with the increase of initial pressure and temperature. Compared with the lean-burning zone, the flame propagation velocity in the rich-burning zone is more sensitive to the change of initial temperature. The relationship between the parameters CFP and laminar burning velocity in TFSC combustion model is SL ¼ 9.9046*CFP. CFP ¼ 0.22 is more suitable for the simulation of hydrogen-air premixed combustion. During the flame propagation process, the flow of unburned gas in the wedge-shaped space formed by the flame and the two walls form a large velocity gradient with the wall, which induces strong wall turbulence. The turbulence intensity induced in the wedge-shaped space is higher than that induced by the flame and the single wall surface, and the faster the flame propagation speed, the stronger the wall turbulence generated in the wedge-shaped space. When the flame itself is highly unstable, the strong wall turbulence in the wedge space causes the near-wall flame to destabilize or even turbulence, accelerating the rapid propagation of the unstable flame, and finally forms a squish flame. Therefore, the combination of strong wall turbulence formed in the wedge-shaped space and flame instability results in the formation of squish flame.

references

[1] White CM, Steeper RR, Lutz AE. The hydrogen-fueled internal combustion engine: a technical review. Int J Hydrogen Energy 2006;31(10):1292e305.

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[2] Das LM. Hydrogen-fueled internal combustion engines. Compendium Hydrogen Energy 2016:177e217. [3] Baykara SZ. Hydrogen: a brief overview on its sources, production and environmental impact. Int J Hydrogen Energy 2018;43(23):10605e14. [4] Ilbas M, Crayford AP, Yılmaz I, Bowen PJ, Syred N. Laminarburning velocities of hydrogeneair and hydrogenemethaneeair mixtures: an experimental study. Int J Hydrogen Energy 2006;31(12):1768e79. [5] Zhang Y, Huang Z, Wei L, Zhang J, Law CK. Experimental and modeling study on ignition delays of lean mixtures of methane, hydrogen, oxygen, and argon at elevated pressures. Combust Flame 2012;159(3):918e31. [6] Law CK, Kwon OC. Effects of hydrocarbon substitution on atmospheric hydrogeneair flame propagation. Int J Hydrogen Energy 2004;29(8):867e79. [7] Kim W, Sato Y, Johzaki T, Endo T, Shimokuri D, Miyoshi A. Experimental study on self-acceleration in expanding spherical hydrogen-air flames. Int J Hydrogen Energy 2018;43(27):12556e64. [8] Verhelst S. Recent progress in the use of hydrogen as a fuel for internal combustion engines. Int J Hydrogen Energy 2014;39(2):1071e85. [9] Balat M. Potential importance of hydrogen as a future solution to environmental and transportation problems. Int J Hydrogen Energy 2008;33(15):4013e29. [10] Smirnov NN, Nikitin VF. Modeling and simulation of hydrogen combustion in engines. Int J Hydrogen Energy 2014;39(2):1122e36. [11] Karim GA. Hydrogen as a spark ignition engine fuel. Int J Hydrogen Energy 2003;28(5):569e77. [12] Huang Z, Zhang Y, Zeng K, Liu B, Wang Q, Jiang D. Measurements of laminar burning velocities for natural gasehydrogeneair mixtures. Combust Flame 2006;146(1e2):302e11. [13] Hu E, Huang Z, He J, Jin C, Zheng J. Experimental and numerical study on laminar burning characteristics of premixed methaneehydrogeneair flames. Int J Hydrogen Energy 2009;34(11):4876e88. [14] Tang CL, Huang ZH, Law CK. Determination, correlation, and mechanistic interpretation of effects of hydrogen addition on laminar flame speeds of hydrocarboneair mixtures. Proc Combust Inst 2011;33(1):921e8. [15] Liu F, Hua Y, Wu H, Lee C, Li Y. Experimental investigation of polycyclic aromatic hydrocarbons growth characteristics of gasoline mixed with methanol, ethanol, or n-butanol in laminar diffusion flames. Energy Fuel 2018;32(6):6823e33. [16] Liu F, Hua Y, Wu H, Lee C, Wang Z. Experimental and kinetic investigation on soot formation of n-butanol-gasoline blends in laminar coflow diffusion flames. Fuel 2018;213:195e205. [17] Kuznetsov M, Kobelt S, Grune J, Jordan T. Flammability limits and laminar flame speed of hydrogeneair mixtures at subatmospheric pressures. Int J Hydrogen Energy 2012;37(22):17580e8. [18] Dowdy DR, Smith DB, Taylor SC, Williams A. The use of expanding spherical flames to determine burning velocities and stretch effects in hydrogen/air mixtures. Proc Combust Inst 1991;23(1):325e32. [19] Koroll GW, Kumar RK, Bowles EM. Burning velocities of hydrogen-air mixtures. Combust Flame 1993;94(3):330e40. [20] Kwon OC, Faeth GM. Flame/stretch interactions of premixed hydrogen-fueled flames: measurements and predictions. Combust Flame 2001;124(4):590e610. [21] Bradley D, Lawes M, Liu K, et al. Laminar burning velocities of lean hydrogeneair mixtures at pressures up to 1.0 MPa. Combust Flame 2007;149(1):162e72. [22] Dayma G, Halter F, Dagaut P. New insights into the peculiar behavior of laminar burning velocities of hydrogeneair

18572

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30] [31]

[32]

[33]

[34]

[35]

[36]

[37] [38]

[39]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 8 5 5 9 e1 8 5 7 2

flames according to pressure and equivalence ratio. Combust Flame 2014;161(9):2235e41. Hu E, Huang Z, He J, Miao H. Experimental and numerical study on laminar burning velocities and flame instabilities of hydrogeneair mixtures at elevated pressures and temperatures. Int J Hydrogen Energy 2009;34(20):8741e55. Sun ZY, Liu FS, Bao XC, Liu XH. Research on cellular instabilities in outwardly propagating spherical hydrogen-air flames. Int J Hydrogen Energy 2012;37(9):7889e99. Pareja J, Burbano HJ, Ogami Y. Measurements of the laminar burning velocity of hydrogeneair premixed flames. Int J Hydrogen Energy 2010;35(4):1812e8. Pareja J, Burbano HJ, Amell A, Carvajal J. Laminar burning velocities and flame stability analysis of hydrogen/air premixed flames at low pressure. Int J Hydrogen Energy 2011;36(10):6317e24. Kobayashi H, Kawazoe H. Flame instability effects on the smallest wrinkling scale and burning velocity of highpressure turbulent premixed flames. Proc Combust Inst 2000;28(1):375e82. Gu X, Huang Z, Wu S, Li Q. Laminar burning velocities and flame instabilities of butanol isomerseair mixtures. Combust Flame 2010;157(12):2318e25. Bechtold JK, Matalon M. Hydrodynamic and diffusion effects on the stability of spherically expanding flames. Combust Flame 1987;67(1):77e90. Addabbo R, Bechtold JK, Matalon M. Wrinkling of spherically expanding flames. Proc Combust Inst 2002;29(2):1527e35. Xie Y, Wang J, Cai X, Huang Z. Self-acceleration of cellular flames and laminar flame speed of syngas/air mixtures at elevated pressures. Int J Hydrogen Energy 2016;41(40):18250e8. Hu E, Huang Z, He J, et al. Measurements of laminar burning velocities and onset of cellular instabilities of methaneehydrogeneair flames at elevated pressures and temperatures. Int J Hydrogen Energy 2009;34(13):5574e84. Liu F, Bao X, Gu J, Chen R. Onset of cellular instabilities in spherically propagating hydrogen-air premixed laminar flames. Int J Hydrogen Energy 2012;37(15):11458e65. € hm B. Advanced laser diagnostics for an Dreizler A, Bo improved understanding of premixed flame-wall interactions. Proc Combust Inst 2015;35(1):37e64. Hua Y, Liu F, Wu H, Lee C, Wang Z. Experimental evaluation of various gasoline surrogates based on soot formation characteristics. Energy Fuel 2018;32(11):11961e9. Wang Q, Wang B, Yao C, Liu M, Wu T, Wei H, et al. Study on cyclic variability of dual fuel combustion in a methanol fumigated diesel engine. Fuel 2016;164:99e109. Metzener P, Matalon M. Premixed flames in closed cylindrical tubes. Combust Theor Model 2001;5(3):463e83. Almarcha C, Denet B, Quinard J. Premixed flames propagating freely in tubes. Combust Flame 2015;162(4):1225e33. Xiao H, He X, Duan Q, Luo X, Sun J. An investigation of premixed flame propagation in a closed combustion duct with a 90 bend. Appl Energy 2014;134:248e56.

[40] Dunn-Rankin D, Sawyer RF. Tulip flames: changes in shape of premixed flames propagating in closed tubes. Exp Fluid 1998;24(2):130e40. [41] Istratov AG, Kidin NI, Fedorov AV. Cellular and tulip flame configurations. J Appl Mech Tech Phys 2003;44(3):395e9. re B. Tulip flame-the [42] Ponizy B, Claverie A, Veyssie mechanism of flame front inversion. Combust Flame 2014;161(12):3051e62. [43] Bychkov V, Fru G, Eriksson LE, Petchenko A. Flame acceleration in the early stages of burning in tubes. Combust Flame 2007;150(4):263e76. [44] Dunn-Rankin D, Barr PK, Sawyer RF. Numerical and experimental study of “tulip” flame formation in a closed vessel. Proc Combust Inst 1988;21(1):1291e301. [45] Gonzalez M, Borghi R, Saouab A. Interaction of a flame front with its self-generated flow in an enclosure: the “tulip flame” phenomenon. Combust Flame 1992;88(2):201e20. [46] Clanet C, Searby G. On the “tulip flame” phenomenon. Combust Flame 1996;105(1e2):225e38. [47] Xiao H, Wang Q, He X, Sun J, Shen X. Experimental study on the behaviors and shape changes of premixed hydrogeneair flames propagating in horizontal duct. Int J Hydrogen Energy 2011;36(10):6325e36. [48] Xiao H, Wang Q, Shen X, An W, Duan Q, Sun J. An experimental study of premixed hydrogen/air flame propagation in a partially open duct. Int J Hydrogen Energy 2014;39(11):6233e41. [49] Liu X, Fan Z, Liu F, Jiu J, Wang R. An experimental study of combustion characteristics of hydrogeneAir pre-mixture on the wall boundary condition. Int J Hydrogen Energy 2012;37(3):2655e60. [50] Xiao H, Wang Q, He X, Sun J, Yao L. Experimental and numerical study on premixed hydrogen/air flame propagation in a horizontal rectangular closed duct. Int J Hydrogen Energy 2010;35(3):1367e76. [51] Wang J, Huang Z, Tang C, Zheng J. Effect of hydrogen addition on early flame growth of lean burn natural gaseair mixtures. Int J Hydrogen Energy 2010;35(13):7246e52. [52] Liu F, Zhang X, Li Y, Wu H, Hua Y. Characteristics of premixed hydrogen/air squish flame in a confined vessel. J Energy Inst 2018;91(6):1102e12. [53] Liu F. CFD study on hydrogen engine mixture formation and combustion. Ph.D.thesis. Brandenburg Technical University Cottbus; 2004. [54] Bradley D, Sheppart CGW, Woolley R, Greenhalgh DA, Lockett RD. The development and structure of flame instabilities and cellularity at low Markstein numbers in explosions. Combust Flame 2000;122(1):195e209. [55] Lai J, Moody A, Chakraborty N. Turbulent kinetic energy transport in head-on quenching of turbulent premixed flames in the context of Reynolds Averaged Navier Stokes simulations. Fuel 2017;199:456e77. [56] Bao XC, Liu FS. Measurement and calculation of burning velocity of hydrogeneair laminar premixed flames. J Combust Sci Technol 2011;17:407e13.