Experimental investigation of turbulent flame-shock wave interactions based on abnormal combustion in internal combustion engine
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摘要: 爆震、超级爆震等非正常燃烧现象是限制小型强化点燃式发动机热效率进一步提升的突出瓶颈。爆震或超级爆震发生时总会伴随着湍流火焰-冲击波的相互作用，因此对湍流火焰-冲击波的相互作用的研究是揭示其机理的关键。本文通过在可视化定容燃烧弹内安装孔板实现火焰过孔板加速并产生冲击波，并通过改变初始热力学条件和孔板的参数，来实现不同强度的湍流火焰和冲击波及其相互作用过程。基于该燃烧装置开展了火焰加速、冲击波的形成以及湍流火焰-冲击波相互作用导致不同燃烧模式的研究。根据燃烧室末端火焰传播和压力振荡情况，总结出5种燃烧模式，其中发生自燃的燃烧模式的压力振荡幅值均超过4.5MPa，是未发生自燃时的4~40倍。因此，湍流火焰-冲击波相互作用对燃烧压力振荡具有重要影响。Abstract: Abnormal combustion phenomena like knock or super-knock are inherent constraint limiting the performance and efficiency of downsized spark ignition (SI) engines.Essentially, engine knock or super-knock is always accompanied by the interactions of turbulent flames and shock waves, as well as rapid chemical energy release.Thus, it is of great significance to investigate the interactions of turbulent flame and shock waves which are the key to reveal the mechanism of knock and super-knock.The major objective of the present work is to experimentally investigate the process of flame acceleration, shock wave formation and interactions of turbulent flame and shock wave in a newly designed constant volume combustion bomb (CVCB) mounted with a perforated plate.In the CVCB, the perforated plate is used to achieve flame acceleration and produce turbulent flame and shock wave.High-speed Schlieren photography was employed to capture the interactions of turbulent flame and shock wave.Hydrogen-air mixture was chosen as the test fuel due to its fast flame propagation velocity and easiness to form obvious shock wave ahead of the flame front.Interactions of turbulent flame and shock wave at different levels could be obtained by changing the initial thermodynamic conditions (including initial pressure and equivalence ratio) and parameters of the perforated plate (including hole size and porosity).Flame acceleration, formation of shock wave and flame-shock wave interactions are discussed in this paper.Depending on the interactions of turbulent flame and shock wave, five combustion modes are obtained by experiments, such as normal combustion, periodically decelerating combustion, oscillating combustion, flame-front autoiginiton and end-gas autoiginiton.The maximum amplitude of the pressure oscillation at combustion models with autoiginiton exceeded 4.5MPa, 4~40 times greater than those without ignition. Therefore, autoiginiton caused by the interactions of turbulent flame and shock wave is the root cause of the intense pressure oscillation in the combustion chamber.
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Key words:
- perforated plate /
- flame propagation /
- shock wave /
- pressure oscillation /
- combustion mode
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图 1 实现火焰-冲击波相互作用的可视化定容燃烧弹(a)观测火焰加速过程；(b)观测末端火焰-冲击波相互作用和自燃
Figure 1. Schematic diagram of the experimental setup for the observation of (a) flame acceleration; (b) flame-shock wave interactions
图 3 过孔板加速过程(a)高速纹影系列图像；(b)火焰传播速度历程示意图
Figure 3. (a)Chronological schlieren images of flame acceleration passing through the perforated plate; (b) evolution of the flame tip velocity
图 4 过孔板加速过程中火焰和气流发展高速纹影系列
Figure 4. Chronological schlieren images of flame acceleration and flow when passing through the perforated plates
图 5 冲击波形成(a)示意图；(b)高速纹影系列图片
Figure 5. (a) Schematic and (b) Chronological schlieren images of formation and enhancement of shock waves
图 6 Mode 4下火焰、冲击波传播纹影系列图片
Figure 6. Chronological schlieren images of flame and shock wave at Mode 4
图 7 Mode 5下火焰、冲击波传播纹影系列图片
Figure 7. Chronological schlieren images of flame and shock wave at Mode 5
图 8 燃烧室末端5种燃烧模式下的火焰传播速度曲线
Figure 8. Evolution of flame tip velocity at different combustion modes
图 9 燃烧室末端5种燃烧模式下(a)燃烧压力；(b)高通滤波后的燃烧压力
Figure 9. Evolution of (a) original and (b) filtered pressure at different combustion modes
表 1 不同燃烧模式的实验条件
Table 1. Experimental conditions at different combustion modes
Combustion modes Equivalence ratio Initial pressure/105Pa Initial temperature/K Hole size/mm Porosity/% Mode 1 1 1 373±2 1.5 12 Mode 2 1.5 1.5 12 Mode 3 2.5 3 12 Mode 4 4 1.5 18 Mode 5 2 1.5 12 
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表 2 不同燃烧模式下的火焰、冲击波传播速度和燃烧压力
Table 2. Flame tip velocity, shock wave velocity and pressure at different combustion modes
Combustion modes Flame tip velocity/(m·s-1) Shock wave velocity/(m·s-1) Auto-ignition Peak pressure/MPa Maximum amplitude of pressure oscillation/MPa Character of combustion modes Mode 1 20~220 Nothing No 0.5 0.11 未受压力波扰动的正常火焰传播：正常燃烧表现为燃烧室中没有产生压力波的弱湍流燃烧现象，燃烧室末端火焰传播速度下降，逐渐燃烧掉全部未燃混合气。 Mode 2 Average 250
Amplitude 78Nothing No 1.2 0.53 声波引起的火焰周期性减速传播：燃烧室中没有产生可见的冲击波，但是声波的反射在燃烧室内产生了往复传播的流场，对火焰锋面产生影响，使之出现周期性的减速传播。 Mode 3 Average 50
Amp1itude 25~204Forward 500
Backward 400No 1.5 0.92 冲击波引起的往复火焰传播：在燃烧室中可以清晰地看到冲击波的来回反射，火焰锋面和反射冲击波相互作用导致火焰前锋出现来回振荡的现象，此时火焰前锋出现明显的后退现象。 Mode 4 200~600 Forward 500
Backward 200~400Near the flame front 5.8 4.58 火焰-冲击波相互作用导致火焰前锋自燃并加速传播：火焰和冲击波同向并相互作用，二者之间相互促进，逐渐由缓燃转变为爆燃，导致急速燃烧并产生剧烈的压力振荡。 Mode 5 Main flame 370
Secondary flame 750Main shock 500
Secondary shock 780In the end gas 4.7 3.45 强冲击波导致末端气体自燃：末端气体自燃是由于火焰加速产生冲击波在燃烧室末端反射引起的。加速火焰足够强时，初级火焰和初级冲击波之间形成次级火焰，次级火焰产生次级冲击波传播速度达到780m/s，其传播到燃烧室的末端，使得末端气体发生自燃。自燃火焰锋面传播速度达到1700m/s，并伴随有强烈的压力振荡现象。
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