Experimental Study on the Ignition and Combustion Mechanisms of a

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Experimental Study on the Ignition and Combustion Mechanisms of a Methane−Air Mixture in a Divided Constant-Volume Combustion Chamber Guoxiu Li,† Na Sun,† Hongguang Zhang,*,‡ Yusong Yu,† Jianhua Liu,† and Xuejiao Han‡ †

School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, P.R. China College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, P.R. China

‡

ABSTRACT: An experimental study on the ignition and combustion mechanisms of a methane−air mixture in a divided constant-volume combustion chamber was conducted in this paper. The effects of the initial pressure, equivalence ratio, and orifice diameter on the mechanism pattern were studied using the pressure curve and rate of pressure rise, and the jet speed and the flame propagation speed in the lower chamber were measured. The ignition and combustion mechanisms of the methane−air mixture in a divided constant-volume combustion chamber can be classified into three patterns: (i) chemical chain ignition and burning, (ii) composite ignition and burning, and (iii) flame front ignition and burning. Reasonable explanations for each pattern were then presented. Characterization parameters were defined so that the mechanism pattern can be easily confirmed under any condition. The appropriate increase in the initial pressure, equivalence ratio, and orifice diameter were helpful in obtaining a higher burning speed. Ultimately, analyses of the jet and flame propagation speeds in the lower chamber proved that pattern (ii) was the most advantageous for increasing the burning speed.

1. INTRODUCTION The development of alternative fuel engines to cope with the depletion of oil reserves and abate air pollution is receiving more attention. Among many potential alternative fuels, natural gas (mainly methane) is one of the most promising fuels and has been already widely used around the world. However, due to its slow burning speed and high ignition energy requirement, natural gas engines still have many disadvantages. One such disadvantage is its relatively low thermal efficiency and large cycle-to-cycle variation, which decreases the engine power output and increases fuel consumption. The key to solve these problems lies in the improvement of the combustion process in the combustion chamber. Lean combustion technology is a common method used to realize fuel saving and low emission in the natural gas engine. However, the burning speed becomes slower because of the lower mixture concentration.1,2 To obtain rapid burning speed, researchers have put forward some methods, such as changing the combustion chamber shape, combining natural gas with hydrogen, and stratified combustion.3−12 The main goal of changing the combustion chamber shape is to enhance air motion and create strong turbulence. Strong turbulence helps increase the flame area, thus enabling rapid burning speed. The combination of natural gas with hydrogen may increase the burning speed, but it also enhances the formation of nitrogen oxide (NOx). Many tests have shown that a higher amount of added hydrogen results in a higher amount of NOx.13−15 Stratified combustion is a good method for realizing lean combustion, with a rich mixture around the spark plug and lean mixture in the cylinder. After the rich mixture is ignited, a higher energy can be created to ignite the lean mixture, producing a more stable flame. Murase and Hanada16 had revealed that the pulsed flame jet (PFJ) has a great potential for enhancing the ignition reliability and burning speed in lean mixtures within the flammability © 2012 American Chemical Society

limit. They have used a compact rapid compression machine to create a high-pressure, high-temperature environment. Combustion is initiated from the orifice of the PFJ igniter, which behaves as a trigger for the autoignition of the fuel in the combustion chamber. In this paper, the experimental setup consists of a constantvolume combustion chamber due to some of its obvious advantages, such as simple structure and convenient operation. Due to these advantages, one of the most important properties of fuellaminar burning speedhas been studied.17−20 The laminar burning speed is not only an important laminar combustion characteristic but also provides the basis for investigation on flame stretching and wrinkling, which enables a deep understanding of the process in spark ignition engines. Turbulent combustion has also been generated in a constantvolume combustion chamber. More importantly, studies have proved that a rapid burning speed can be obtained using a divided combustion chamber which is divided into two parts by an orifice adaptor.21 When the flame comes through the orifice, a high-speed jet that has ignition energy in the order of a standard spark plug is created. Thus, a higher flame propagation speed and more stable burning can be achieved. This study aims not only to obtain experimental phenomenon but also to obtain the ignition and burning mechanism divisions under different initial conditions. The findings are useful in supplying theoretical guidance to obtain rapid burning speed.

2. EXPERIMENTAL SETUP AND PROCEDURES Figure 1 shows the experimental setup. It consists of a constant-volume combustion chamber, as well as systems for inlet and exhaust, heating, ignition, synchronization control, data acquisition, and high-speed Received: January 17, 2012 Revised: July 10, 2012 Published: July 11, 2012 4696

dx.doi.org/10.1021/ef300094u | Energy Fuels 2012, 26, 4696−4702

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Figure 1. Experimental setup. schlieren photography. The combustion chamber is a cube with a side length of 100 mm, which can withstand a pressure of 10 MPa. As shown in the figure, an adaptor is built in; thus, the entire chamber is divided into two parts, namely, the upper chamber and lower chamber. The orifice in the center of the adaptor serves as the passage for mass transfer and heat transfer between the two parts. A jet can be produced when the upper flame propagation reaches the orifice. The top located spark plug is used to ignite the upper mixture. Spark ignition as well as pressure data and schlieren picture acquisition can be carried out simultaneously by the pulse generator DG535. Two quartz windows are located on two sides of the chamber for optical access. A highspeed camera operating at 10 000 frames per second at most can record the flame propagation pictures. A pressure transducer and pressure transmitter are mounted on the chamber body to measure instantaneous and initial pressures, respectively. A premixing tank five times the volume of the entire chamber is used to supply a homogeneous mixture. The proportions of oxygen and nitrogen in air used in this experiment are 20.95% and 79.05%, respectively. For the stoichiometric methane−air mixture combustion, the chemical formula is as follows:

Table 1. Initial Experimental Conditions

(1) The equivalence ratio Φ is expressed as

V0 V

(2)

where V0 = (3.773 + 1) × 2 is the theoretical air volume and V is the actual air volume. Therefore, the premixed mixture can be expressed as

CH4 +

2(O2 + 3.773N2) Φ

pressure P0 (MPa)

equivalence ratio Φ

orifice diameter D (mm)

293

0.1 0.2 0.25

0.7 0.8 0.9 1.0

2 3 6 8

3. RESULTS AND DISCUSSIONS 3.1. Phenomenon and Analysis. In this paper, the ignition and burning mechanism of the lower chamber mixture is classified into three patterns: (i) chemical chain ignition and burning, (ii) composite ignition and burning, and (iii) flame front ignition and burning. (i) Chemical chain ignition and burning Figure 2a shows some schlieren pictures taken when there is no adaptor built in the chamber (Φ = 1.0, P0 = 0.1 MPa). Figure 2b shows some schlieren pictures where there is an adaptor with a 3 mm-diameter orifice built in the chamber (Φ = 1.0, P0 = 0.1 MPa). The window dimensions of all pictures are 100 mm ×100 mm, which are comparable to the side length of the chamber (the schlieren pictures in the following are also 100 mm × 100 mm). This finding suggests that the flame arrives at the lower chamber near 0.016 s. At this time, the instantaneous pressure is lower if the adaptor exists, and for a period of time, it remains lower. It is easy to understand that mass and heat can enter the lower chamber freely as there is no adaptor while only a little of mass and heat can enter when an orifice adaptor exists. At orifice diameter D = 3 mm, the pressure curve has an obvious turning point at about 0.0612 s (called “ignition timing in the lower chamber”) when the pressure starts rising rather rapidly. Around this time in the schlieren pictures, it can be seen that the upper chamber mixture has been almost burned out (This is judged by the analysis of the schlieren picture at t = 0.044 s. In the picture, the upper chamber flame has two statuses: one is smooth around the spark plug, which means the mixture in this area has been burned out; the other is wrinkled

CH4 + 2(O2 + 3.773N2) = CO2 + 2H 2O + 2 × 3.773N2

Φ=

temperature T0 (K)

(3)

where (2(O2 + 3.77N2))/Φ is the actual air volume V according to eq 2. During the experiments, the equivalence ratio is directly controlled by the partial pressures of the reactant species. According to the expected values of Φ, the corresponding V and volume fraction of each gas can be calculated, and the partial pressures can be gained according to Dalton’s law of partial pressure. The initial experimental conditions are listed in Table 1. All combinations of the four parameters were studied. 4697


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Figure 2. Comparison of schlieren pictures at Φ = 1.0, P0 = 0.1 MPa.

pictures taken when there is no adaptor built in the chamber (Φ = 0.8, P0 = 0.25 MPa). Figure 4b shows some schlieren pictures where there is an adaptor with a 3 mm-diameter orifice built in the chamber (Φ = 0.8, P0 = 0.25 MPa). As is shown in Figure 5, with an adaptor built-in, the combustion instantaneous pressure appears lower for a period of time like pattern i, it is due to the resistance of the adaptor. At orifice diameter D = 3 mm, the pressure curve has an obvious turning point at about 0.086 s, which is the “ignition timing in the lower chamber”. During this period of time (before “ignition timing in the lower chamber”), the effects of temperature increase, pressure increase, chemical reaction, and air motion occur, which are helpful in reaching the ignition point of the mixture more rapidly. At 0.086 s, the schlieren picture shows that the flame in the upper chamber has both smooth and wrinkled statuses, which means the upper chamber mixture has not been burned out. In other words, there will be additional mass and heat transfer into the lower chamber when the lower chamber mixture burns. So the ignition and burning of the lower chamber mixture in pattern ii is the result of two aspects: one is the effects of temperature increase, pressure increase, chemical reaction, and air motion, which help reach the ignition point of the lower chamber mixture more rapidly; the other is the additional mass and heat transfer into the lower chamber, igniting the mixture. Unlike pattern i, the ignition timing in the lower chamber in pattern ii can be ascertained easily by schlieren pictures because of a larger density gradient indicated by the thicker area (t = 0.086 s and t = 0.088 s in Figure 4b) at the bottom of the chamber. (iii) Flame front ignition and burning If the orifice in the center of the adaptor is sufficiently large, the resistance of the adaptor can be faint. Therefore, at orifice diameter D = 8 mm, the combustion instantaneous pressure is not lower for a period of time like pattern i and pattern ii, but a little higher than the setup with no adaptor (local amplification graph in Figure 7). Figure 6 shows that the flame arrives at the

Figure 3. Comparison of pressure curves at Φ = 1.0, P0 = 0.1 MPa.

which means the mixture in this area is still burning. Accordingly, the schlieren picture at t = 0.0612 s shows only the smooth status appears in the upper chamber, and a jet exists from the lower chamber to the upper chamber.). In other words, there will be no additional mass and heat transfer into the lower chamber when the lower chamber mixture burns, the ignition and burning of the lower chamber mixture is completely due to the effects of the previous process (before “ignition timing in the lower chamber”).These effects include temperature increase, pressure increase, chemical reaction, and air motion that are helpful in reaching the ignition point of the lower chamber mixture more rapidly. However, the effects in pattern i lead to a smaller density gradient in the lower chamber, making little difference among the four pictures from 0.06 to 0.064 s, consequently rendering difficulty in finding the ignition timing in the lower chamber by schlieren pictures. (ii) Composite ignition and burning Apparently, composite ignition and burning occurs between patterns i and iii. Figure 4a shows some schlieren 4698


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Figure 4. Comparison of schlieren pictures at Φ = 0.8, P0 = 0.25 MPa.



Figure 6. Schlieren pictures at Φ = 0.8, P0 = 0.25 MPa, D = 8 mm.

lower chamber at about 0.024 s when the pressure starts rising quite rapidly. It means the lower chamber mixture is ignited as soon as the upper chamber flame arrives. There is hardly any effect of temperature increase, pressure increase, chemical reaction, and air motion on the lower chamber mixture. The ignition and burning of the lower chamber mixture is completely due to the hot flame front. Thus, the “ignition timing in the lower chamber” of pattern iii appears the earliest among the three mechanism patterns. When the lower chamber mixture begins to burn, only laminar combustion occurs in the upper chamber.

Figure 8. Rate of pressure rise of the three mechanism patterns.

Compared with Figures 3 and 5, the two peak combustion pressures in Figure 7 have no marked difference 4699


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Figure 9. Mechanism pattern division at different initial pressures.

Figure 9 reveals the relationship among the mechanism pattern, equivalence ratio, and orifice diameter at different initial pressures. This relationship suggests that patterns ii and iii are easier to occur with increased equivalence ratio and orifice diameter. With increased initial pressure, the range of pattern i decreases; meanwhile, the range of pattern iii increases. In other words, patterns ii and iii are also more likely to occur with increased initial pressure. Different patterns correspond to different burning speeds. These results indicate that the burning speed can be faster by appropriately increasing the equivalence ratio, orifice diameter, and initial pressure. To generalize the conclusions, the characterization parameters α and β are defined as follows:

between with and without the orifice adaptor. As stated above, if the orifice in the center of the adaptor is sufficiently large, the resistance of the adaptor can be faint. Therefore, when ignition appears in the lower chamber, the lower chamber pressure rises quickly, and the upper chamber pressure rises correspondingly, keeping the pressure of the two partitions balanced as much as possible. Therefore, although pattern iii has the earliest ignition timing in the lower chamber, it is not beneficial in obtaining higher peak combustion pressure. Finally, the rates of pressure rise of the three mechanism patterns are displayed in Figure 8. A major difference can be seen between with and without the orifice adaptor. The result indicates that an orifice adaptor in the chamber can enhance the combustion process, and the extent of the enhancement depends on the mechanism pattern. Figure 8 also shows that pattern i has the slowest pressure rise, whereas pattern ii has the fastest pressure rise. 3.2. Mechanism Pattern Division under Different Initial Conditions. The ignition and burning process of the lower chamber mixture can be classified into different mechanism patterns according to different initial conditions.

α = s1/s2

(4)

β = Φα

(5)

where s1 is the cross-sectional area of the orifice and s2 is the cross-sectional area of the adaptor. In the experiments, β can be easily calculated: 2.198 × 10−4 ≤ β ≤ 0.000 502. Figure 10 shows the mechanism pattern versus β. Numbers 1, 2, and 3 in the Y-axis denote patterns i, ii, and iii, respectively. 4700


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Figure 10. Mechanism pattern versus β.

Figure 11. Variation in the jet speed at different orifice diameters.

Each pattern corresponds to a range of β; therefore, the mechanism pattern can be easily confirmed under any condition. For example, at Φ = 0.8, P0 = 0.2 MPa, and D = 5, then β = 0.00157 can be calculated and assigned to pattern ii according to Figure 10. 3.3. Jet Speed and Flame Propagation Speed in the Lower Chamber. The jet speed plays an important role in the ignition and burning process of the lower chamber mixture. A faster jet speed corresponds to greater mass and heat transfer, and a stronger air motion can be completed. The jet speed can be obtained by measuring the jet length in schlieren pictures. It is defined as follows: u j = dl /dt

moment to the image showing the starting of the jet appearing in the lower chamber is defined as “zero”, and the succeeding three moments are defined according to the actual time interval of each image. The same definition is in Figure 11b. The figure shows that the jet speed is the fastest at D = 6 mm. Figure 9a shows that it is pattern ii at D = 6 mm. The result indicates that pattern ii is the most advantageous to the increase in the burning speed. The same rule can be obtained in Figure 11b (Φ = 0.7, P0 = 0.25 MPa). In the lower chamber, the flame propagation speed can be obtained by calculating the equivalent radius of the flame area in schlieren pictures. It is defined as follows: u f = dr /dt

(6)

(7)

where uf is the flame propagation speed in the lower chamber and r is the equivalent radius of the flame area. Figure 12 shows the variation in the flame propagation speed at different patterns (Φ = 1.0, P0 = 0.1 MPa, D = 3 mm; Φ = 0.8, P0 = 0.25 MPa, D = 3 mm; Φ = 0.8, P0 = 0.25 MPa,

where uj is the jet speed and l is the jet length. Figure 11a shows the variation in the jet speed at different orifice diameters (Φ = 0.8, P0 = 0.1 MPa). The “actual relative time of the image” used in the X-axis is defined as follows: the corresponding 4701


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Pattern ii is the most advantageous to the increase in the burning speed based on the results of the jet speed and flame propagation speed in the lower chamber.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by the National Basic Research Program of China (973 Program) (Grant No. 2011CB707202, Grant No. 2011CB710704), the National High-Tech Research and Development Program of China (863 Program) (Grant No.2009AA05Z206), and the Funding Project for Academic Human Resources Development in Institutions of Higher Learning Under the Jurisdiction of Beijing Municipality (Grant No. PHR201008019). The authors would like to thank Mr. Xiaolei Bai and Dr. Baofeng Yao for their great support during the experimental process.

Figure 12. Variation in the flame propagation speed in the lower chamber at different mechanism patterns.

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D = 8 mm). The actual relative time of the image used in the X-axis is defined as follows: the corresponding moment to the image showing the initiation of the flame appearing in the lower chamber is defined as zero, and the succeeding two moments are defined according to the actual time interval of each image. Pattern ii has the fastest flame propagation speed. This conclusion is consistent with Figure 11; that is, pattern ii is the most advantageous to the increase in the burning speed.

REFERENCES

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4. CONCLUSIONS An experimental study on the ignition and burning mechanisms of the methane−air mixture in a divided combustion chamber was conducted. The main conclusions are summarized as follows: 1. The ignition and burning mechanism of the methane−air mixture in a divided combustion chamber is classified into three patterns: (i) chemical chain ignition and burning, (ii) composite ignition and burning, and (iii) flame front ignition and burning. 2. In pattern i, the ignition and burning of the lower chamber mixture is completely due to the effects of the process, which includes temperature increase, pressure increase, chemical reaction, and air motion. In pattern ii, the ignition and burning of the lower chamber mixture is the result of two aspects: one is the effect of temperature increase, pressure increase, chemical reaction, and air motion; the other is the additional mass and heat transfer into the lower chamber, igniting the mixture. In pattern iii, the ignition and burning of the lower chamber mixture is completely due to the hot flame front. 3. Pattern i is the most disadvantageous; pattern iii has the earliest ignition timing in the lower chamber, but it is not beneficial in obtaining a higher peak combustion pressure. After comprehensive consideration, pattern ii is found to be the most advantageous to the increase in the burning speed. 4. The ignition and burning process of the lower chamber mixture can be classified into different mechanism patterns according to the initial conditions. Different mechanism patterns correspond to different burning speeds. The burning speed can be increased by appropriately increasing the equivalence ratio, orifice diameter, and initial pressure. 5. The characterization parameters α and β are defined to confirm the mechanism pattern under any condition. 4702


Experimental Study on the Ignition and Combustion Mechanisms of a

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