- Email: [email protected]
Experimental observations of turbulent flame propagation effected by flame acceleration in the end gas of closed combustion chamber
Fuel 180 (2016) 157–163
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Experimental observations of turbulent flame propagation effected by flame acceleration in the end gas of closed combustion chamber Haiqiao Wei ⇑, Dongzhi Gao, Lei Zhou, Jiaying Pan, Kang Tao, Zigang Pei State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China
h i g h l i g h t s Laminar flame shows the trend of initial increase and consequence decrease. Accelerating turbulent flame is suddenly formed as flame across orifice plate. Flame velocity across orifice plate shows the ‘‘M” shape. Evolution of turbulent flame in the end gas can be described with three stages.
a r t i c l e
i n f o
Article history: Received 14 December 2015 Received in revised form 11 March 2016 Accepted 9 April 2016
Keywords: Flame acceleration Compression front Turbulent flame Closed combustion chamber
a b s t r a c t In present study, a new designed constant volume combustion bomb (CVCB) equipped with an orifice plate and visualized by high speed schlieren photography has been employed to study the turbulent flame propagation effected by flame acceleration in the end gas region. The orifice plate is employed to achieve flame acceleration and obtain different turbulent flames at different equivalence ratios. We investigate the flame propagation speed, the formation mechanism of compression front, the influence of flame acceleration on the end-gas as well as the flame structure changes. The results show that the laminar flame can be accelerated and transferred to a wrinkled turbulent flame through the orifice plate significantly and there forms a clear compression front ahead of the turbulent flame. The flame propagation speed without orifice plate shows the trend of initial increase and consequence decrease in the confined space. Moreover, the turbulent flame propagation speed shows the significantly different trend and demonstrates the ‘‘M” shape evolution including the self-acceleration, unstable and deceleration process. Finally, the evolution of turbulent flame in the end gas of the confined chamber was described in detail with three stages: compression front formation, flame front distortion and reverse propagation. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Recently energy crisis and environmental pollution have been urging engine manufactures to realize higher thermal efficiency and lower emissions to meet stringent laws [1]. With this background, many energy-saving technologies have been put forward and as one of the most potential technologies, engine downsizing with supercharging has been followed with interest due to its great advantages in light weight and compactness [2,3]. However, relatively high temperature and pressure conditions around the top dead center (TDC) make it critical for subsequence ignition and combustion processes, which increases the possibilities of uncontrollable end-gas auto-ignition occurrence, and finally lead to
⇑ Corresponding author at: 92 Weijin Road, Nankai District, Tianjin, China. E-mail address: [email protected] (H. Wei). http://dx.doi.org/10.1016/j.fuel.2016.04.044 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
engine knock or super-knock [4]. Therefore, it is becoming a major topic to reveal the mechanism of end-gas auto-ignition and knock for modern engines [5,6]. The mechanism for end-gas auto-ignition is rather complex and generally it is considered to be caused by the compression of flame expansion and piston movement [7–10], non-uniformities in temperature and concentration of mixture [11], and turbulence dissipation [12]. Essentially, end-gas auto-ignition occurrence during engine knock is always combined with flame acceleration and pressure wave propagation [13]. Recent study by Wei et al. [14] suggested that the pressure waves originated from propagating flame front could drive end-gas auto-ignition, resulting in pressure oscillations with large amplitude, under downsized SI engine combustion conditions fueled with hydrogen. Another experimental investigation [15] based on 1-D constant volume bomb was developed to study the fundamental combustion process of flame propagation and end-gas auto-ignition at super-knock relevant conditions, which
158
H. Wei et al. / Fuel 180 (2016) 157–163
suggested that flame propagation induced auto-ignition could lead to the occurrence of super knock and detonations. These research basically provide the evidence that flame propagation has a certain influence on the end-gas combustion in the closed chamber. However, most relevant investigations have been carried out based on hydrogen fuels which are definitely different from large hydrocarbon fuels in terms of combustion chemistry. In addition, the combustion regimes studied by previous work fails to include turbulent effects that definitely exist in practical engines [16]. Due to the highly complexities of combustion processes in engines, the utilization of constant volume combustion bomb (CVCB) with physically clear configurations has drawn much attention to study engine relevant combustion issues [15]. In general, the laminar flame cannot yield the obvious compression front and consequently induce the severe burning in the end gas. Thus, a new designed CVCB equipped with orifice plate and visualized by high speed schlieren photography has been developed to study the turbulent flame propagation effected by flame acceleration in the end gas region. In present study, an orifice plate is installed in the closed combustion chamber to achieve flame acceleration and obtain different turbulent flame identified by flame propagation speeds based on the evolutions of flame front development through high-speed schlieren photography. The effect of confinement on propagating flame in closed combustion chamber and the process of flame propagation in the end gas of the confined chamber with orifice plate are investigated as a preliminary study. The paper is organized as follows: the experimental setup and conditions are briefly discussed in Section 2. Section 3 illustrates the speed of flame propagation in closed combustion chamber and the process of flame propagation in the end gas of confined chamber with three stages. Finally, major conclusions from this work are drawn in the last section.
2. Experimental 2.1. Experimental system This experiment has carried out in a new designed CVCB equipped with a high-speed schlieren photography system, as shown in Fig. 1a. The combustion chamber is a closed cylindrical cavity with an inner diameter of 100 mm and volume of 2.32 L. There are two windows respectively mounted on the front and back walls of the CVCB. The windows are made of high-quality quartz glass, which can provide optical accesses with a thickness of 100 mm and 50 mm, respectively. Especially, the front window is racetrack-shaped of 230 mm in length and 80 mm in width and the back window is a circular of 80 mm in diameter. Fuel–air mixture is initially premixed and ignited by a spark plug located on the top of the chamber 40 mm away from the right wall. Pressure transducers (Kistler 6113B) are located on the upper left of combustion chamber to measure the state parameters of end-gas zone. On the bottom of the CVCB, there is a heating resistor with a power of 1000 W to heat the premixed mixture in combustion chamber, and the temperature could be controlled with a temperature controller within ±3 K. To generated turbulent combustion flame, a replaceable orifice plate made of 3 mm thick stainless steel plate has been used. There are many through-holes with a diameter of 2 mm uniformly distributed on it, as shown in Fig. 1b. For the schlieren system, it consists of a light source, a schlieren knife edge, two focusing lenses, a collimator (1.5 m focal length) and a schlieren head (1.5 m focal length). The schlieren system is arranged in a standard Z configuration. High-speed video camera (Photron FASTCAM SA5) has been employed. In addition, the spark igniter, pressure recorder and high-speed digital video camera are triggered simultaneously by the synchronization controller.
2.2. Experimental procedure Before igniting, the fuel of gasoline with Octane Number of 93 and air mixture is initially premixed for 5 min and to the target conditions of initial temperature T0 = 365 K and initial pressure P0 = 0.5 MPa. At the beginning, experiments are performed in different equivalence ratios (0.9, 1.0, 1.11, and 1.25) without the orifice plate. Laminar flame propagation images are obtained from high-speed video camera with 4000 frame/s, and flame front structure and end-gas combustion images are obtained from high-speed schlieren photography by 10,000 frame/s. And next, an orifice plate is installed in the middle of the combustion chamber and turbulent flame experiments under the same conditions are again carried out in order to make comparisons to that of laminar situations. The spark plug igniter is synchronized with camera, so that the flame propagation history and flame speeds can be accurately obtained based on the evolution of flame development images. The experimental conditions are shown in Table 1. In this study, the uncertainty measure for turbulent flame velocity is no more than 2 m/s.
3. Results and discussion 3.1. Flame morphology Fig. 2 shows a time-sequence of laminar and turbulent flame propagation images captured by high speed digital video camera (Photron Fastcam SA5, 4000 frame/s) directly from the front window of CVCB. The two experiments have been performed under the same initial conditions (equivalence ratio u = 1, initial pressure P0 = 0.5 MPa, initial temperature T0 = 365 K). Group 1 shows the flame propagation without orifice plate while group 2 displays the turbulent flame propagation with orifice plate in CVCB. Spark plug discharges at t = 1.5 ms as shown in Fig. 2, but the flame core did not occur immediately. The flame core is formed after a period of time about 4.5 ms and the flame propagates outwardly following it. For the flame without orifice plate in this study, the speed of flame propagation is slow and flame front is smooth. Meanwhile, the speed of the laminar flames at different equivalences ratios are approximate 2–4 m/s which is in the same order of magnitude with the stretched laminar flame speed (2.5 m/s) studied by Baloo et al. [21]. When the flame passes through the orifice plate, its surface is obviously winkled and the propagation speed is greater than the laminar flame and we called it turbulent flame. Comparing the two group pictures above, both of the flame propagation are similar in the early stage of flame development (before 19.5 ms). The flame front is smooth without orifice plate while the laminar flame is transformed into turbulent flame after the orifice plate and the propagation speed is greater than the former. Under the same conditions shown in Fig. 2, the overall time of combustion done completely the whole chamber is 39.5 ms and 48.5 ms respectively. The image in Fig. 2(I) is the premixed laminar flame and the image in Fig. 2(i) is the accelerated turbulent flame passing through orifice plate are obtained at the same time t = 35.5 ms. As can be seen, the surface of laminar flame front without orifice is smooth while it is obviously winkled flame when the flame passes through the orifice plate. It can be found that the flame across the orifice plate can lead to flame acceleration, at same time it distorts the flame front and increase the burning rate, which will be qualitatively discussed later. Thus, the effect of small pores on orifice plate equals to the perturbation of many small eddies on flame front. Meanwhile, the flame across the orifice plate can cause a serial of flame instabilities, such as Kelvin–Helmholtz and Rayleigh–Taylor instabilities, and lead to a rapid increase of the flame surface and form a turbulent flame eventually, which results in the further increase
H. Wei et al. / Fuel 180 (2016) 157–163
159
(a) Experimental setup.
(b) Orifice plate. Fig. 1. Schematic diagram of (a) the experimental setup and (b) orifice plate.
3.2. Flame propagation speed
Table 1 Experimental conditions. Parameters
Value
Combustion chamber volume/V Initial temperature/T0 Initial pressure/P0 Octane number/RON Equivalence ratio/u
2.32 L 365 K 0.5 MPa 93 0.9, 1.0, 1.11, 1.25
of the flame velocity. On the other hand, with the increase of the flame speed, the flame is no longer stable, instability grow and wrinkle the flame. So the wrinkling of the flame surface is accompanied by an increases in flame speed, and the cells on the flame surface are observed to subdivide into smaller cells as the flame continues to propagate outward [17,18]. This feedback loop can result in continuous flame acceleration in theory. Noted that the turbulent flame front is brighter than laminar flame as can be seen in Fig. 2(I) and (i) at the same initial conditions, which indicates that the turbulent combustion produces more soot than laminar flame. Because the chemiluminescence intensity of the hydrocarbon fuel flame is related to the number of soot as discussed in the previous studies [19,20].
The definition of flame propagation speed in the closed chamber of CVCB is shown in Fig. 3. The position of spark plug is defined as the origin that locates at the beginning of the X-axis and the Xaxis is set along the axial direction of the combustion chamber. The orifice plate is located in the chamber vertically at 70 mm away from the spark plug. When the normal flame passes through the orifice plate, a speed-up turbulent flame and a twisted flame front would be formed. This observation can be obtained by the comparisons of flame front in the front and back of the orifice plate. In this study, flame propagation speed Vs, is defined as the absolute speed of flame front, and it can be calculated from the shooting time t, and the location of the flame front propagation S, in the combustion chamber. Then we can obtain the correlation by Vs = dS/dt. For validate the measured laminar flame speed in our study, we review the relevant literatures. In this work, the laminar flame speeds are several meters per second (2–4 m/s) for all cases which are in the same order of magnitude with the iso-octane stretched laminar flame speed of 2.5 m/s at equivalence ratio of 1.1, initial temperature of 363 K and initial pressure of 1 bar studied by Baloo, et al. [21] and 2.1 m/s at equivalence ratio of 1.0, initial temperature of 360 K and initial pressure of 5 bar studied by
160
H. Wei et al. / Fuel 180 (2016) 157–163
Group 1 Time/(ms) No orifice plate
Group 2 Group 1 With orifice plate Time/(ms) No orifice plate
Group 2 With orifice plate
(A)
(a)
(G)
(B)
(b)
(H)
(h)
(g) Orifice plate
Spark plug ignition (C)
(c)
(I)
(i)
(D)
(d)
(J)
(j)
(E)
(e)
(K)
(F)
(f)
(L)
Fig. 2. Flame propagation images (Group 1) without orifice plate and (Group 2) with orifice plate under equivalence ratio u = 1, with initial temperature T0 = 365 K and initial pressure P0 = 0.5 MPa.
18 0 16 0 1 40 1 20 10 0
80
60
40
20
Spark plug
0
x/mm y
Oriice plate
Fig. 3. The definition of flame propagation speed.
Mandilas, et al. [22]. Thus the measured laminar flame speed in our study is reasonable and correct. The experimental results of flame speed at different equivalence ratios have been shown in Fig. 4. The solid line shows the laminar flame propagation and the turbulent flame through orifice plate is represented by dashed line. It can be seen that the flame speed is
=0.9, With oriice plate =1.0, With oriice plate =1.11, With oriice plate =1.25, With oriice plate
28
= 0.9, No oriice plate =1.0, No oriice plate = 1.11, No oriice plate =1.25, No oriice plate
24 20 16 12
Oriice plate
8 4 0
20
40
60
80
100
120
140
160
180
Fig. 4. Flame speed versus propagation distance in the CVCB with/without orifice plate under different equivalence ratios with initial temperature T0 = 365 K and initial pressure P0 = 0.5 MPa.
comparatively slow (approximate 2–4 m/s for all cases) at the initial stage of flame development (S = 20–60 mm), and there even a decreasing tendency occurs at the beginning for the cases with orifice plate, which can be attributed to the resistance of the orifice plate. For both the laminar and turbulent flame propagation, the highest flame speed occurs at equivalence ratio, u = 1.11, and it decreases in both lean and rich mixtures. Furthermore, for the laminar flame without orifice plate, flame propagation in the CVCB accelerates at S = 40–80 mm and decelerates at S = 140–180 mm gradually and there is no rapid acceleration and deceleration process in the confined space as expected. However, at the condition with orifice plate indicated by dashed lines, flame speed Vs declines slightly at S = 60–70 mm, which is due to the fact that the orifice plate has a resistance effect on laminar flame before it passes through the orifice plate. At the distance of S = 70 mm, the laminar flame passes through the orifice plate transforming into turbulent flame. There is a sharp flame acceleration at S = 70–80 mm, leading to the formation of robust turbulent flame. It indicates that the gas flow induced by orifice plate plays an important role in accelerating the laminar flame speed. At about S = 80 mm, Vs reaches the maximum value about 26 m/s at the conditions of u = 1.11. As the flame goes away from the orifice plate, the flame speed decreases because of the turbulence intensity generated by the jet reduces and the flame is affected by the quenching of the sidewalls [23,24]. At the location of S = 120–160 mm, the flame begins to become unstable and the flame front is gradually wrinkled as can be seen in the following Section 3.3. The fluctuation of the flame speed in this process has been shown in Fig. 4. This is due to the interaction of flame front with the pressure wave or acoustic wave [25,26], triggered by flame acceleration and reflected by the sidewalls. When the turbulent flame propagates into the end of the closed combustion chamber at the distance of S = 160 mm, the flame speed decreases considerably until to a stationary one. This is mainly because of the confined space limitations. Overall, The laminar flame propagation velocity shows the trend of initial increase and consequence decrease in the confined space. Moreover, the turbulent flame propagation velocity shows the significantly different trend and demonstrates the ‘‘M” shape evolution including the turbulent flame self-acceleration, unstable and deceleration process in the closed chamber.
161
H. Wei et al. / Fuel 180 (2016) 157–163
3.3. Flame propagation in end gas of closed combustion chamber In previous laminar flame experiment, the speed of flame propagation is slow and flame front is smooth. There is no compression front ahead of the flame. So only turbulent flame propagation with orifice plate is further studied in this section. Fig. 5 shows the images of turbulent flame development and compression front formation taken in the end region of combustion chamber using highspeed schlieren method with the frame rate of 10,000 frame/s, under the conditions of equivalence ratio u = 1.11, initial pressure P0 = 0.5 MPa and initial temperature T0 = 365 K. Basically, a turbulent flame propagates from the left side to the right side of the CVCB, as can be seen from the optical window shown in Fig. 5. The flame front become wrinkled and a clear compression front is formed gradually ahead of the flame front according to the time-sequentially images of flame propagation. The formation of compression front can be explained as shown in Fig. 6, as the flame propagates forward, the unburned mixture is compressed by the flame front, and thus makes its temperature and pressure increase. As shown in Section 3.2, the flame propagation in combustion chamber is an acceleration process after orifice plate and thus generates several disturbance waves ahead of the accelerated flame front (t = 28.3–30.1 ms). Due to the compression from acceleration effects, the disturbance wave formed later propagates faster than that formed previously, with cn > cn 1 > cn 2 > . . . > c2 > c1, shown in Fig. 6b. With the propagation of flame front, the disturbance waves in the back gradually catch up with the front and coalesce to it. Finally, the compression front is formed due to the effect of coalescence, which may lead to higher temperature and inhomogeneity in the end mixture of the CVCB. Fig. 7 shows the images that illustrate evolutions of flame surface distortion in end-gas region of the CVCB. According to Fig. 4, the flame propagation speed is about 11 m/s at t = 32.5 ms (S = 170 mm), which is in the deceleration stage but is also higher than that of the laminar flame. Furthermore, the flame front catches up with the compression front and combined with it and there is the serious flame surface distortions. At t = 34.9 ms, the flame propagation speed is further reduced to the speed 3.5 m/s, and there is enough time to preheat the unburned mixture behind the flame. And meanwhile, the flame front is wrinkled severely due to the instabilities and interaction effects with compression front. There
are two effects on the end of the unburned mixture: on one hand, the flame front distortion leads to an increase in flame surface area, which increases the heat transfer area with the end-gas mixture; on the other hand, the flame propagation speed is reduced in the end region and this extends the time to preheat end-gas mixture. Both factors increase the temperature of the end-gas, resulting in a more intense burning in the end region of closed chamber. The entire sequence of frames are shown in Video 1. As discussed above, premixed mixture in the CVCB with orifice plate has a higher flame propagation speed which leads to a compression front formation and has strong impacts on the end-gas mixtures. Thus the temperature in the end-gas increases and subsequently has a severe reverse combustion. As shown in Fig. 8, the end gas begins to burn at the right side of the window close to the end wall of combustion chamber (t = 37.9 ms), and then rapidly grow up in size as time. It can be visualized that the end-gas ignition flame front spreads from right to left in the period of t = 39.7–47.5 ms. It passes through the whole test window at t = 47.5 ms and leads to a higher flame luminous intensity than the forward flame. It should be noted that the end-gas ignition flame shows a wrinkled structure when flame propagates reversely during t = 42.7–47.5 ms, as shown in Fig. 8. This means that the reverse burning flame is more intensive compared to the forward flame. From previous work [3,6,9], there will be dramatic pressure fluctuations and engine knock if end-gas ignition flame speed is large enough. Based on the experimental results in present work, it can be considered as a weak knock occurred in the end zone of the CVCB, without pressure fluctuations. Recently, it is generally agreed that knock originates by the extremely rapid energy release of the end-gas ahead of the propagating turbulent flame, resulting in high local pressures. Essentially, engine knock are always combined with interactions of flame and shock waves and rapid chemical energy release or a process in which some part or all of the charge may be consumed at extremely high rates. The end-gas autoignition is the most acceptable explanation for engine knock. Thus, this study investigated the flame propagation process in the end of the closed combustion chamber as a primary study for the SI engine knock. Fig. 9 shows the relationship of pressure versus burning time with/without orifice plate under the conditions of equivalence ratio u = 1.11, initial pressure P0 = 0.5 MPa and initial temperature
Flame front 28.3 ms Disturbance waves
28.9 ms
29.5 ms
Fig. 5. Visualizations of compression front formation induced by flame acceleration under the conditions of equivalence ratio u = 1.11, initial pressure P0 = 0.5 MPa and initial temperature T0 = 365 K.
162
H. Wei et al. / Fuel 180 (2016) 157–163
Fig. 6. Schematic diagram of compression front formation.
Fig. 7. Evolutions of flame surface distortions under the conditions of equivalence ratio u = 1.11, initial pressure P0 = 0.5 MPa and initial temperature T0 = 365 K.
End-gas ignition 37.9 ms
39.7 ms
41.0 ms
41.5 ms
Fig. 8. High-speed schlieren images of end-gas ignition and reverse propagation in the CVCB with orifice plate under the conditions of equivalence ratio u = 1.11, initial pressure P0 = 0.5 MPa and initial temperature T0 = 365 K.
T0 = 365 K. After spark ignition, the rate of pressure rise and the peak with orifice plate is higher than that of the case without orifice plate under same equivalence ratios. The maximum pressure with orifice plate can reach 3.8 MPa and is 2 bar higher than that without orifice plate at the same condition. This indicates that the accelerated flame after orifice plate can make obvious compression effects on the end-gas mixture. On the other hand, the slope of pressure with orifice plate is larger than that without orifice plate and there is shorter time to reach the peak value due to faster flame propagation after orifice plate. Furthermore, experimental results show that the greater the turbulent flame propagation accelerates, the higher the pressure in the end-gas zone is, and thus greater possibility of severe end-gas ignition is. If reverse flame in the end-gas region propagates fast enough, it is reasonable
to consider that intense pressure fluctuations may happen, such as the severe engine knock in the practice SI engines. 4. Conclusions The flame acceleration mechanism and the effect of confinement on the evolution of propagating flame for gasoline–air mixture in a new designed CVCB with orifice plate are carried out at the initial temperature of 365 K, pressure of 0.5 MPa and different equivalence ratios of 0.9, 1.0, 1.11, 1.25. The compression front ahead of the turbulent flame is visualized using high-speed schlieren photography. The experiment results provide important theoretical references for knock occurring in internal combustion engines.
H. Wei et al. / Fuel 180 (2016) 157–163
4.0
Pmax=3.8 MPa
Pressure (MPa)
3.5 3.0 2.5 2.0 1.5 1.0 0.5
=1.11,No oriice plate =1.11,With oriice plate
Fig. 9. Pressure versus burning time for gasoline–air mixtures in closed chamber of CVCB with/without orifice plate under the conditions of equivalence ratio u = 1.11, initial pressure P0 = 0.5 MPa and initial temperature T0 = 365 K.
(1) The laminar flame propagation shows the trend of initial increase and consequence decrease in the confined space. While, the turbulent flame propagation velocity shows the significantly different trend and demonstrates the ‘‘M” shape evolution including the turbulent flame self-acceleration, unstable and deceleration process in the closed chamber. (2) There is a significant acceleration in flame speed when flame passes through orifice plate. Clear compression front induced by the turbulent flame acceleration can be visualized by high-speed schlieren photography. The evolution of turbulent flame in the end gas of the confined chamber can be described in detail with three stages: compression front formation, flame front distortion and reverse propagation.
Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant Nos. 51476114, 51176138). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2016.04.044. References [1] Rudloff J, Zaccardi JM, Richard S, Anderlohr JM. Analysis of pre-ignition in highly charged SI: emphasis on the auto-ignition mode. Proc Combust Inst 2013;34:2959–67.
163
[2] Bradley D, Kalghatgi GT. Influence of autoignition delay time characteristics of different fuels on pressure waves and knock in reciprocating engines. Combust Flame 2009;156:2307–18. [3] Kawahara N, Tomita E. Visualization of auto-ignition and pressure wave during knocking in a hydrogen spark-ignition engine. Int J Hydrogen Energy 2009;34:3156–63. [4] Wang Z, Qi Y, He X, Wang J, Shuai S, Law CK. Analysis of pre-ignition to superknock: hotspot-induced deflagration to detonation. Fuel 2015;144:222–7. [5] Kawahara N, Tomita E, Sakata Y. Auto-ignited kernels during knocking combustion in a spark-ignition engine. Proc Combust Inst 2007;31:2999–3006. [6] Dahnz C, Spicher U. Irregular combustion in supercharged spark ignition engines pre-ignition and other phenomena. Int J Eng Res 2010;11:485–98. [7] Hajireza S, Mauss F, Sundén B. Hot-spot autoignition in spark ignition engines. Proc Combust Inst 2000;28:1169–75. [8] Bäuerle B, Hoffmann F, Behrendt F, Warnatz J. Detection of hot spots in the end gas of an internal combustion engine using two-dimensional LIF of formaldehyde. Proc Combust Inst 1994;25:135–41. [9] Pan J, Sheppard CGW, Tindall A, Berzins M, Pennington SV, Ware JM. End gas inhomogeneity, autoignition and knock. SAE paper 982616; 1998. [10] Bradley D, Head RA. Engine autoignition: the relationship between octane numbers and autoignition delay times. Combust Flame 2006;147:171–84. [11] Pöschl M, Sattelmayer T. Influence of temperature inhomogeneities on knocking combustion. Combust Flame 2008;153:562–73. [12] Lacaze G, Cuenot B, Poinsot T, Oschwald M. Large eddy simulation of laser ignition and compressible reacting flow in a rocket-like configuration. Combust Flame 2009;156:1166–80. [13] Pan JY, Shu GQ, Wei HQ. Interaction of flame propagation and pressure waves during knocking combustion in spark-ignition engines. Combust Sci Tech 2014;186:192–209. [14] Wei H, Shang Y, Chen C, Gao D, Feng D. One-dimensional numerical study on pressure wave-induced end gas auto-ignition near top dead center of a downsized spark ignition engine. Int J Hydrogen Energy 2015;40:4874–83. [15] Qi Y, He X, Wang Z, Wang J, Zhang H. An experimental investigation of super knock combustion mode using a one-dimensional constant volume bomb. Int J Hydrogen Energy 2015;40:2377–85. [16] Abu-orf GM, Cant RS. A turbulent reaction rate model for premixed turbulent combustion in spark-ignition engines. Combust Flame 2000;122:233–52. [17] Law CK, Jomaas G, Bechtold JK. Cellular instabilities of expanding hydrogen/ propane spherical flames at elevated pressures: theory and experiment. Proc Combust Inst 2005;30:159–67. [18] Bradley D, Cresswell TM, Puttock JS. Flame acceleration due to flame-induced instabilities in large-scale explosions. Combust Flame 2001;124:551–9. [19] Kook S, Pickett LM. Soot volume fraction and morphology of conventional and surrogate jet fuel sprays at 1000-K and 6.7-MPa ambient conditions. Proc Combust Inst 2011;33:2911–8. [20] Qi Y, Wang Z, Wang J, He X. Effects of thermodynamic conditions on the end gas combustion mode associated with engine knock. Combust Flame 2015;162:4119–28. [21] Baloo M, Dariani BM, Akhlaghi M, Chitsza I. Effect of iso-octane/methane blend on laminar burning velocity and flame instability. Fuel 2015;144:264–73. [22] Mandilas C, Ormsby MP, Sheppard CGW, Woolley R. Effects of hydrogen addition on laminar and turbulent premixed methane and iso-octane-air flames. Proc Combust Inst 2007;31:1443–50. [23] Bonhomme A, Selle L, Poinsot T. Curvature and confinement effects for flame speed measurements in laminar spherical and cylindrical flames. Combust Flame 2013;160:1208–14. [24] Bai B, Chen Z, Zhang HW, Chen SY. Flame propagation in a tube with wall quenching of radicals. Combust Flame 2013;160:2810–9. [25] Xiao H, Wang Q, Shen X, Guo S, Sun J. An experimental study of distorted tulip flame formation in a closed duct. Combust Flame 2013;160:1725–8. [26] Ciccarelli G, Johansen CT, Parravani M. The role of shock-flame interactions on flame acceleration in an obstacle laden channel. Combust Flame 2010;157:2125–36.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Experimental observations of turbulent flame propagation effected by flame acceleration in the end gas of closed combustion chamber Haiqiao Wei ⇑, Dongzhi Gao, Lei Zhou, Jiaying Pan, Kang Tao, Zigang Pei State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China
h i g h l i g h t s Laminar flame shows the trend of initial increase and consequence decrease. Accelerating turbulent flame is suddenly formed as flame across orifice plate. Flame velocity across orifice plate shows the ‘‘M” shape. Evolution of turbulent flame in the end gas can be described with three stages.
a r t i c l e
i n f o
Article history: Received 14 December 2015 Received in revised form 11 March 2016 Accepted 9 April 2016
Keywords: Flame acceleration Compression front Turbulent flame Closed combustion chamber
a b s t r a c t In present study, a new designed constant volume combustion bomb (CVCB) equipped with an orifice plate and visualized by high speed schlieren photography has been employed to study the turbulent flame propagation effected by flame acceleration in the end gas region. The orifice plate is employed to achieve flame acceleration and obtain different turbulent flames at different equivalence ratios. We investigate the flame propagation speed, the formation mechanism of compression front, the influence of flame acceleration on the end-gas as well as the flame structure changes. The results show that the laminar flame can be accelerated and transferred to a wrinkled turbulent flame through the orifice plate significantly and there forms a clear compression front ahead of the turbulent flame. The flame propagation speed without orifice plate shows the trend of initial increase and consequence decrease in the confined space. Moreover, the turbulent flame propagation speed shows the significantly different trend and demonstrates the ‘‘M” shape evolution including the self-acceleration, unstable and deceleration process. Finally, the evolution of turbulent flame in the end gas of the confined chamber was described in detail with three stages: compression front formation, flame front distortion and reverse propagation. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Recently energy crisis and environmental pollution have been urging engine manufactures to realize higher thermal efficiency and lower emissions to meet stringent laws [1]. With this background, many energy-saving technologies have been put forward and as one of the most potential technologies, engine downsizing with supercharging has been followed with interest due to its great advantages in light weight and compactness [2,3]. However, relatively high temperature and pressure conditions around the top dead center (TDC) make it critical for subsequence ignition and combustion processes, which increases the possibilities of uncontrollable end-gas auto-ignition occurrence, and finally lead to
⇑ Corresponding author at: 92 Weijin Road, Nankai District, Tianjin, China. E-mail address: [email protected] (H. Wei). http://dx.doi.org/10.1016/j.fuel.2016.04.044 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
engine knock or super-knock [4]. Therefore, it is becoming a major topic to reveal the mechanism of end-gas auto-ignition and knock for modern engines [5,6]. The mechanism for end-gas auto-ignition is rather complex and generally it is considered to be caused by the compression of flame expansion and piston movement [7–10], non-uniformities in temperature and concentration of mixture [11], and turbulence dissipation [12]. Essentially, end-gas auto-ignition occurrence during engine knock is always combined with flame acceleration and pressure wave propagation [13]. Recent study by Wei et al. [14] suggested that the pressure waves originated from propagating flame front could drive end-gas auto-ignition, resulting in pressure oscillations with large amplitude, under downsized SI engine combustion conditions fueled with hydrogen. Another experimental investigation [15] based on 1-D constant volume bomb was developed to study the fundamental combustion process of flame propagation and end-gas auto-ignition at super-knock relevant conditions, which
158
H. Wei et al. / Fuel 180 (2016) 157–163
suggested that flame propagation induced auto-ignition could lead to the occurrence of super knock and detonations. These research basically provide the evidence that flame propagation has a certain influence on the end-gas combustion in the closed chamber. However, most relevant investigations have been carried out based on hydrogen fuels which are definitely different from large hydrocarbon fuels in terms of combustion chemistry. In addition, the combustion regimes studied by previous work fails to include turbulent effects that definitely exist in practical engines [16]. Due to the highly complexities of combustion processes in engines, the utilization of constant volume combustion bomb (CVCB) with physically clear configurations has drawn much attention to study engine relevant combustion issues [15]. In general, the laminar flame cannot yield the obvious compression front and consequently induce the severe burning in the end gas. Thus, a new designed CVCB equipped with orifice plate and visualized by high speed schlieren photography has been developed to study the turbulent flame propagation effected by flame acceleration in the end gas region. In present study, an orifice plate is installed in the closed combustion chamber to achieve flame acceleration and obtain different turbulent flame identified by flame propagation speeds based on the evolutions of flame front development through high-speed schlieren photography. The effect of confinement on propagating flame in closed combustion chamber and the process of flame propagation in the end gas of the confined chamber with orifice plate are investigated as a preliminary study. The paper is organized as follows: the experimental setup and conditions are briefly discussed in Section 2. Section 3 illustrates the speed of flame propagation in closed combustion chamber and the process of flame propagation in the end gas of confined chamber with three stages. Finally, major conclusions from this work are drawn in the last section.
2. Experimental 2.1. Experimental system This experiment has carried out in a new designed CVCB equipped with a high-speed schlieren photography system, as shown in Fig. 1a. The combustion chamber is a closed cylindrical cavity with an inner diameter of 100 mm and volume of 2.32 L. There are two windows respectively mounted on the front and back walls of the CVCB. The windows are made of high-quality quartz glass, which can provide optical accesses with a thickness of 100 mm and 50 mm, respectively. Especially, the front window is racetrack-shaped of 230 mm in length and 80 mm in width and the back window is a circular of 80 mm in diameter. Fuel–air mixture is initially premixed and ignited by a spark plug located on the top of the chamber 40 mm away from the right wall. Pressure transducers (Kistler 6113B) are located on the upper left of combustion chamber to measure the state parameters of end-gas zone. On the bottom of the CVCB, there is a heating resistor with a power of 1000 W to heat the premixed mixture in combustion chamber, and the temperature could be controlled with a temperature controller within ±3 K. To generated turbulent combustion flame, a replaceable orifice plate made of 3 mm thick stainless steel plate has been used. There are many through-holes with a diameter of 2 mm uniformly distributed on it, as shown in Fig. 1b. For the schlieren system, it consists of a light source, a schlieren knife edge, two focusing lenses, a collimator (1.5 m focal length) and a schlieren head (1.5 m focal length). The schlieren system is arranged in a standard Z configuration. High-speed video camera (Photron FASTCAM SA5) has been employed. In addition, the spark igniter, pressure recorder and high-speed digital video camera are triggered simultaneously by the synchronization controller.
2.2. Experimental procedure Before igniting, the fuel of gasoline with Octane Number of 93 and air mixture is initially premixed for 5 min and to the target conditions of initial temperature T0 = 365 K and initial pressure P0 = 0.5 MPa. At the beginning, experiments are performed in different equivalence ratios (0.9, 1.0, 1.11, and 1.25) without the orifice plate. Laminar flame propagation images are obtained from high-speed video camera with 4000 frame/s, and flame front structure and end-gas combustion images are obtained from high-speed schlieren photography by 10,000 frame/s. And next, an orifice plate is installed in the middle of the combustion chamber and turbulent flame experiments under the same conditions are again carried out in order to make comparisons to that of laminar situations. The spark plug igniter is synchronized with camera, so that the flame propagation history and flame speeds can be accurately obtained based on the evolution of flame development images. The experimental conditions are shown in Table 1. In this study, the uncertainty measure for turbulent flame velocity is no more than 2 m/s.
3. Results and discussion 3.1. Flame morphology Fig. 2 shows a time-sequence of laminar and turbulent flame propagation images captured by high speed digital video camera (Photron Fastcam SA5, 4000 frame/s) directly from the front window of CVCB. The two experiments have been performed under the same initial conditions (equivalence ratio u = 1, initial pressure P0 = 0.5 MPa, initial temperature T0 = 365 K). Group 1 shows the flame propagation without orifice plate while group 2 displays the turbulent flame propagation with orifice plate in CVCB. Spark plug discharges at t = 1.5 ms as shown in Fig. 2, but the flame core did not occur immediately. The flame core is formed after a period of time about 4.5 ms and the flame propagates outwardly following it. For the flame without orifice plate in this study, the speed of flame propagation is slow and flame front is smooth. Meanwhile, the speed of the laminar flames at different equivalences ratios are approximate 2–4 m/s which is in the same order of magnitude with the stretched laminar flame speed (2.5 m/s) studied by Baloo et al. [21]. When the flame passes through the orifice plate, its surface is obviously winkled and the propagation speed is greater than the laminar flame and we called it turbulent flame. Comparing the two group pictures above, both of the flame propagation are similar in the early stage of flame development (before 19.5 ms). The flame front is smooth without orifice plate while the laminar flame is transformed into turbulent flame after the orifice plate and the propagation speed is greater than the former. Under the same conditions shown in Fig. 2, the overall time of combustion done completely the whole chamber is 39.5 ms and 48.5 ms respectively. The image in Fig. 2(I) is the premixed laminar flame and the image in Fig. 2(i) is the accelerated turbulent flame passing through orifice plate are obtained at the same time t = 35.5 ms. As can be seen, the surface of laminar flame front without orifice is smooth while it is obviously winkled flame when the flame passes through the orifice plate. It can be found that the flame across the orifice plate can lead to flame acceleration, at same time it distorts the flame front and increase the burning rate, which will be qualitatively discussed later. Thus, the effect of small pores on orifice plate equals to the perturbation of many small eddies on flame front. Meanwhile, the flame across the orifice plate can cause a serial of flame instabilities, such as Kelvin–Helmholtz and Rayleigh–Taylor instabilities, and lead to a rapid increase of the flame surface and form a turbulent flame eventually, which results in the further increase
H. Wei et al. / Fuel 180 (2016) 157–163
159
(a) Experimental setup.
(b) Orifice plate. Fig. 1. Schematic diagram of (a) the experimental setup and (b) orifice plate.
3.2. Flame propagation speed
Table 1 Experimental conditions. Parameters
Value
Combustion chamber volume/V Initial temperature/T0 Initial pressure/P0 Octane number/RON Equivalence ratio/u
2.32 L 365 K 0.5 MPa 93 0.9, 1.0, 1.11, 1.25
of the flame velocity. On the other hand, with the increase of the flame speed, the flame is no longer stable, instability grow and wrinkle the flame. So the wrinkling of the flame surface is accompanied by an increases in flame speed, and the cells on the flame surface are observed to subdivide into smaller cells as the flame continues to propagate outward [17,18]. This feedback loop can result in continuous flame acceleration in theory. Noted that the turbulent flame front is brighter than laminar flame as can be seen in Fig. 2(I) and (i) at the same initial conditions, which indicates that the turbulent combustion produces more soot than laminar flame. Because the chemiluminescence intensity of the hydrocarbon fuel flame is related to the number of soot as discussed in the previous studies [19,20].
The definition of flame propagation speed in the closed chamber of CVCB is shown in Fig. 3. The position of spark plug is defined as the origin that locates at the beginning of the X-axis and the Xaxis is set along the axial direction of the combustion chamber. The orifice plate is located in the chamber vertically at 70 mm away from the spark plug. When the normal flame passes through the orifice plate, a speed-up turbulent flame and a twisted flame front would be formed. This observation can be obtained by the comparisons of flame front in the front and back of the orifice plate. In this study, flame propagation speed Vs, is defined as the absolute speed of flame front, and it can be calculated from the shooting time t, and the location of the flame front propagation S, in the combustion chamber. Then we can obtain the correlation by Vs = dS/dt. For validate the measured laminar flame speed in our study, we review the relevant literatures. In this work, the laminar flame speeds are several meters per second (2–4 m/s) for all cases which are in the same order of magnitude with the iso-octane stretched laminar flame speed of 2.5 m/s at equivalence ratio of 1.1, initial temperature of 363 K and initial pressure of 1 bar studied by Baloo, et al. [21] and 2.1 m/s at equivalence ratio of 1.0, initial temperature of 360 K and initial pressure of 5 bar studied by
160
H. Wei et al. / Fuel 180 (2016) 157–163
Group 1 Time/(ms) No orifice plate
Group 2 Group 1 With orifice plate Time/(ms) No orifice plate
Group 2 With orifice plate
(A)
(a)
(G)
(B)
(b)
(H)
(h)
(g) Orifice plate
Spark plug ignition (C)
(c)
(I)
(i)
(D)
(d)
(J)
(j)
(E)
(e)
(K)
(F)
(f)
(L)
Fig. 2. Flame propagation images (Group 1) without orifice plate and (Group 2) with orifice plate under equivalence ratio u = 1, with initial temperature T0 = 365 K and initial pressure P0 = 0.5 MPa.
18 0 16 0 1 40 1 20 10 0
80
60
40
20
Spark plug
0
x/mm y
Oriice plate
Fig. 3. The definition of flame propagation speed.
Mandilas, et al. [22]. Thus the measured laminar flame speed in our study is reasonable and correct. The experimental results of flame speed at different equivalence ratios have been shown in Fig. 4. The solid line shows the laminar flame propagation and the turbulent flame through orifice plate is represented by dashed line. It can be seen that the flame speed is
=0.9, With oriice plate =1.0, With oriice plate =1.11, With oriice plate =1.25, With oriice plate
28
= 0.9, No oriice plate =1.0, No oriice plate = 1.11, No oriice plate =1.25, No oriice plate
24 20 16 12
Oriice plate
8 4 0
20
40
60
80
100
120
140
160
180
Fig. 4. Flame speed versus propagation distance in the CVCB with/without orifice plate under different equivalence ratios with initial temperature T0 = 365 K and initial pressure P0 = 0.5 MPa.
comparatively slow (approximate 2–4 m/s for all cases) at the initial stage of flame development (S = 20–60 mm), and there even a decreasing tendency occurs at the beginning for the cases with orifice plate, which can be attributed to the resistance of the orifice plate. For both the laminar and turbulent flame propagation, the highest flame speed occurs at equivalence ratio, u = 1.11, and it decreases in both lean and rich mixtures. Furthermore, for the laminar flame without orifice plate, flame propagation in the CVCB accelerates at S = 40–80 mm and decelerates at S = 140–180 mm gradually and there is no rapid acceleration and deceleration process in the confined space as expected. However, at the condition with orifice plate indicated by dashed lines, flame speed Vs declines slightly at S = 60–70 mm, which is due to the fact that the orifice plate has a resistance effect on laminar flame before it passes through the orifice plate. At the distance of S = 70 mm, the laminar flame passes through the orifice plate transforming into turbulent flame. There is a sharp flame acceleration at S = 70–80 mm, leading to the formation of robust turbulent flame. It indicates that the gas flow induced by orifice plate plays an important role in accelerating the laminar flame speed. At about S = 80 mm, Vs reaches the maximum value about 26 m/s at the conditions of u = 1.11. As the flame goes away from the orifice plate, the flame speed decreases because of the turbulence intensity generated by the jet reduces and the flame is affected by the quenching of the sidewalls [23,24]. At the location of S = 120–160 mm, the flame begins to become unstable and the flame front is gradually wrinkled as can be seen in the following Section 3.3. The fluctuation of the flame speed in this process has been shown in Fig. 4. This is due to the interaction of flame front with the pressure wave or acoustic wave [25,26], triggered by flame acceleration and reflected by the sidewalls. When the turbulent flame propagates into the end of the closed combustion chamber at the distance of S = 160 mm, the flame speed decreases considerably until to a stationary one. This is mainly because of the confined space limitations. Overall, The laminar flame propagation velocity shows the trend of initial increase and consequence decrease in the confined space. Moreover, the turbulent flame propagation velocity shows the significantly different trend and demonstrates the ‘‘M” shape evolution including the turbulent flame self-acceleration, unstable and deceleration process in the closed chamber.
161
H. Wei et al. / Fuel 180 (2016) 157–163
3.3. Flame propagation in end gas of closed combustion chamber In previous laminar flame experiment, the speed of flame propagation is slow and flame front is smooth. There is no compression front ahead of the flame. So only turbulent flame propagation with orifice plate is further studied in this section. Fig. 5 shows the images of turbulent flame development and compression front formation taken in the end region of combustion chamber using highspeed schlieren method with the frame rate of 10,000 frame/s, under the conditions of equivalence ratio u = 1.11, initial pressure P0 = 0.5 MPa and initial temperature T0 = 365 K. Basically, a turbulent flame propagates from the left side to the right side of the CVCB, as can be seen from the optical window shown in Fig. 5. The flame front become wrinkled and a clear compression front is formed gradually ahead of the flame front according to the time-sequentially images of flame propagation. The formation of compression front can be explained as shown in Fig. 6, as the flame propagates forward, the unburned mixture is compressed by the flame front, and thus makes its temperature and pressure increase. As shown in Section 3.2, the flame propagation in combustion chamber is an acceleration process after orifice plate and thus generates several disturbance waves ahead of the accelerated flame front (t = 28.3–30.1 ms). Due to the compression from acceleration effects, the disturbance wave formed later propagates faster than that formed previously, with cn > cn 1 > cn 2 > . . . > c2 > c1, shown in Fig. 6b. With the propagation of flame front, the disturbance waves in the back gradually catch up with the front and coalesce to it. Finally, the compression front is formed due to the effect of coalescence, which may lead to higher temperature and inhomogeneity in the end mixture of the CVCB. Fig. 7 shows the images that illustrate evolutions of flame surface distortion in end-gas region of the CVCB. According to Fig. 4, the flame propagation speed is about 11 m/s at t = 32.5 ms (S = 170 mm), which is in the deceleration stage but is also higher than that of the laminar flame. Furthermore, the flame front catches up with the compression front and combined with it and there is the serious flame surface distortions. At t = 34.9 ms, the flame propagation speed is further reduced to the speed 3.5 m/s, and there is enough time to preheat the unburned mixture behind the flame. And meanwhile, the flame front is wrinkled severely due to the instabilities and interaction effects with compression front. There
are two effects on the end of the unburned mixture: on one hand, the flame front distortion leads to an increase in flame surface area, which increases the heat transfer area with the end-gas mixture; on the other hand, the flame propagation speed is reduced in the end region and this extends the time to preheat end-gas mixture. Both factors increase the temperature of the end-gas, resulting in a more intense burning in the end region of closed chamber. The entire sequence of frames are shown in Video 1. As discussed above, premixed mixture in the CVCB with orifice plate has a higher flame propagation speed which leads to a compression front formation and has strong impacts on the end-gas mixtures. Thus the temperature in the end-gas increases and subsequently has a severe reverse combustion. As shown in Fig. 8, the end gas begins to burn at the right side of the window close to the end wall of combustion chamber (t = 37.9 ms), and then rapidly grow up in size as time. It can be visualized that the end-gas ignition flame front spreads from right to left in the period of t = 39.7–47.5 ms. It passes through the whole test window at t = 47.5 ms and leads to a higher flame luminous intensity than the forward flame. It should be noted that the end-gas ignition flame shows a wrinkled structure when flame propagates reversely during t = 42.7–47.5 ms, as shown in Fig. 8. This means that the reverse burning flame is more intensive compared to the forward flame. From previous work [3,6,9], there will be dramatic pressure fluctuations and engine knock if end-gas ignition flame speed is large enough. Based on the experimental results in present work, it can be considered as a weak knock occurred in the end zone of the CVCB, without pressure fluctuations. Recently, it is generally agreed that knock originates by the extremely rapid energy release of the end-gas ahead of the propagating turbulent flame, resulting in high local pressures. Essentially, engine knock are always combined with interactions of flame and shock waves and rapid chemical energy release or a process in which some part or all of the charge may be consumed at extremely high rates. The end-gas autoignition is the most acceptable explanation for engine knock. Thus, this study investigated the flame propagation process in the end of the closed combustion chamber as a primary study for the SI engine knock. Fig. 9 shows the relationship of pressure versus burning time with/without orifice plate under the conditions of equivalence ratio u = 1.11, initial pressure P0 = 0.5 MPa and initial temperature
Flame front 28.3 ms Disturbance waves
28.9 ms
29.5 ms
Fig. 5. Visualizations of compression front formation induced by flame acceleration under the conditions of equivalence ratio u = 1.11, initial pressure P0 = 0.5 MPa and initial temperature T0 = 365 K.
162
H. Wei et al. / Fuel 180 (2016) 157–163
Fig. 6. Schematic diagram of compression front formation.
Fig. 7. Evolutions of flame surface distortions under the conditions of equivalence ratio u = 1.11, initial pressure P0 = 0.5 MPa and initial temperature T0 = 365 K.
End-gas ignition 37.9 ms
39.7 ms
41.0 ms
41.5 ms
Fig. 8. High-speed schlieren images of end-gas ignition and reverse propagation in the CVCB with orifice plate under the conditions of equivalence ratio u = 1.11, initial pressure P0 = 0.5 MPa and initial temperature T0 = 365 K.
T0 = 365 K. After spark ignition, the rate of pressure rise and the peak with orifice plate is higher than that of the case without orifice plate under same equivalence ratios. The maximum pressure with orifice plate can reach 3.8 MPa and is 2 bar higher than that without orifice plate at the same condition. This indicates that the accelerated flame after orifice plate can make obvious compression effects on the end-gas mixture. On the other hand, the slope of pressure with orifice plate is larger than that without orifice plate and there is shorter time to reach the peak value due to faster flame propagation after orifice plate. Furthermore, experimental results show that the greater the turbulent flame propagation accelerates, the higher the pressure in the end-gas zone is, and thus greater possibility of severe end-gas ignition is. If reverse flame in the end-gas region propagates fast enough, it is reasonable
to consider that intense pressure fluctuations may happen, such as the severe engine knock in the practice SI engines. 4. Conclusions The flame acceleration mechanism and the effect of confinement on the evolution of propagating flame for gasoline–air mixture in a new designed CVCB with orifice plate are carried out at the initial temperature of 365 K, pressure of 0.5 MPa and different equivalence ratios of 0.9, 1.0, 1.11, 1.25. The compression front ahead of the turbulent flame is visualized using high-speed schlieren photography. The experiment results provide important theoretical references for knock occurring in internal combustion engines.
H. Wei et al. / Fuel 180 (2016) 157–163
4.0
Pmax=3.8 MPa
Pressure (MPa)
3.5 3.0 2.5 2.0 1.5 1.0 0.5
=1.11,No oriice plate =1.11,With oriice plate
Fig. 9. Pressure versus burning time for gasoline–air mixtures in closed chamber of CVCB with/without orifice plate under the conditions of equivalence ratio u = 1.11, initial pressure P0 = 0.5 MPa and initial temperature T0 = 365 K.
(1) The laminar flame propagation shows the trend of initial increase and consequence decrease in the confined space. While, the turbulent flame propagation velocity shows the significantly different trend and demonstrates the ‘‘M” shape evolution including the turbulent flame self-acceleration, unstable and deceleration process in the closed chamber. (2) There is a significant acceleration in flame speed when flame passes through orifice plate. Clear compression front induced by the turbulent flame acceleration can be visualized by high-speed schlieren photography. The evolution of turbulent flame in the end gas of the confined chamber can be described in detail with three stages: compression front formation, flame front distortion and reverse propagation.
Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant Nos. 51476114, 51176138). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2016.04.044. References [1] Rudloff J, Zaccardi JM, Richard S, Anderlohr JM. Analysis of pre-ignition in highly charged SI: emphasis on the auto-ignition mode. Proc Combust Inst 2013;34:2959–67.
163
[2] Bradley D, Kalghatgi GT. Influence of autoignition delay time characteristics of different fuels on pressure waves and knock in reciprocating engines. Combust Flame 2009;156:2307–18. [3] Kawahara N, Tomita E. Visualization of auto-ignition and pressure wave during knocking in a hydrogen spark-ignition engine. Int J Hydrogen Energy 2009;34:3156–63. [4] Wang Z, Qi Y, He X, Wang J, Shuai S, Law CK. Analysis of pre-ignition to superknock: hotspot-induced deflagration to detonation. Fuel 2015;144:222–7. [5] Kawahara N, Tomita E, Sakata Y. Auto-ignited kernels during knocking combustion in a spark-ignition engine. Proc Combust Inst 2007;31:2999–3006. [6] Dahnz C, Spicher U. Irregular combustion in supercharged spark ignition engines pre-ignition and other phenomena. Int J Eng Res 2010;11:485–98. [7] Hajireza S, Mauss F, Sundén B. Hot-spot autoignition in spark ignition engines. Proc Combust Inst 2000;28:1169–75. [8] Bäuerle B, Hoffmann F, Behrendt F, Warnatz J. Detection of hot spots in the end gas of an internal combustion engine using two-dimensional LIF of formaldehyde. Proc Combust Inst 1994;25:135–41. [9] Pan J, Sheppard CGW, Tindall A, Berzins M, Pennington SV, Ware JM. End gas inhomogeneity, autoignition and knock. SAE paper 982616; 1998. [10] Bradley D, Head RA. Engine autoignition: the relationship between octane numbers and autoignition delay times. Combust Flame 2006;147:171–84. [11] Pöschl M, Sattelmayer T. Influence of temperature inhomogeneities on knocking combustion. Combust Flame 2008;153:562–73. [12] Lacaze G, Cuenot B, Poinsot T, Oschwald M. Large eddy simulation of laser ignition and compressible reacting flow in a rocket-like configuration. Combust Flame 2009;156:1166–80. [13] Pan JY, Shu GQ, Wei HQ. Interaction of flame propagation and pressure waves during knocking combustion in spark-ignition engines. Combust Sci Tech 2014;186:192–209. [14] Wei H, Shang Y, Chen C, Gao D, Feng D. One-dimensional numerical study on pressure wave-induced end gas auto-ignition near top dead center of a downsized spark ignition engine. Int J Hydrogen Energy 2015;40:4874–83. [15] Qi Y, He X, Wang Z, Wang J, Zhang H. An experimental investigation of super knock combustion mode using a one-dimensional constant volume bomb. Int J Hydrogen Energy 2015;40:2377–85. [16] Abu-orf GM, Cant RS. A turbulent reaction rate model for premixed turbulent combustion in spark-ignition engines. Combust Flame 2000;122:233–52. [17] Law CK, Jomaas G, Bechtold JK. Cellular instabilities of expanding hydrogen/ propane spherical flames at elevated pressures: theory and experiment. Proc Combust Inst 2005;30:159–67. [18] Bradley D, Cresswell TM, Puttock JS. Flame acceleration due to flame-induced instabilities in large-scale explosions. Combust Flame 2001;124:551–9. [19] Kook S, Pickett LM. Soot volume fraction and morphology of conventional and surrogate jet fuel sprays at 1000-K and 6.7-MPa ambient conditions. Proc Combust Inst 2011;33:2911–8. [20] Qi Y, Wang Z, Wang J, He X. Effects of thermodynamic conditions on the end gas combustion mode associated with engine knock. Combust Flame 2015;162:4119–28. [21] Baloo M, Dariani BM, Akhlaghi M, Chitsza I. Effect of iso-octane/methane blend on laminar burning velocity and flame instability. Fuel 2015;144:264–73. [22] Mandilas C, Ormsby MP, Sheppard CGW, Woolley R. Effects of hydrogen addition on laminar and turbulent premixed methane and iso-octane-air flames. Proc Combust Inst 2007;31:1443–50. [23] Bonhomme A, Selle L, Poinsot T. Curvature and confinement effects for flame speed measurements in laminar spherical and cylindrical flames. Combust Flame 2013;160:1208–14. [24] Bai B, Chen Z, Zhang HW, Chen SY. Flame propagation in a tube with wall quenching of radicals. Combust Flame 2013;160:2810–9. [25] Xiao H, Wang Q, Shen X, Guo S, Sun J. An experimental study of distorted tulip flame formation in a closed duct. Combust Flame 2013;160:1725–8. [26] Ciccarelli G, Johansen CT, Parravani M. The role of shock-flame interactions on flame acceleration in an obstacle laden channel. Combust Flame 2010;157:2125–36.