Plasma Formation and Flame Kernel Propagation

Though laser ignition has been researched since 1960’s, recent studies have brought new insights into the processes associated with laser ignition [70, 79-83] and the picture that emerges can be summarized as follows:

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(i) When a high power laser pulse is focused in a gaseous medium, high field gradients are introduced at the focal volume, which result in stripping of the electrons from the local molecules via a multi-photon ionization process. A similar result is attained with cascade ionization if one seed free electron exists because of impurities in the medium. These free electrons, in turn, absorb laser energy via inverse bremsstrahlung absorption, accelerate, and result in the release of more electrons when they collide with other gas molecules. This results in a cascade breakdown leading to the electron concentration increasing almost exponentially with time.

(ii) The resulting plasma kernel is opaque and absorbs the remaining incoming photons.

A tear-drop shaped plasma kernel forms that extends towards the incoming laser. As the resulting high-temperatures ( 50,000 K) and high-pressures ( 100 atm.) are localized close to the focal spot position, a blast wave develops and propagates into the surrounding gas. This shock wave, though tear drop shaped initially, expands into an elongated ellipsoid before forming a spherical wave as it expands into the surrounding gas. In parallel, the plasma core collapses and fluid elements move from the front of the plasma kernel in a direction opposite to laser propagation. This results in a roll-up leading to an expanding toroid.

(iii) Following the shock wave propagation, energy is transferred to the surrounding fuel-air mixture primarily through advection. In the immediate vicinity where temperatures reach 2000-3000 K, combustion reactions commence and a flame front develops. The energy released in this thin flame region helps sustain an expanding flame that results in “successful ignition.”

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The next few paragraphs deliver a literature review on LI plasma formations and flame propagation to the reader in order to understand the reasons leading to an efficient ignition system.

Chen et al. [84] investigated the spatial and temporal profiles of LI plasma formation in atmospheric pressure. The Nd:YAG laser was operated at the fundamental wavelength with a pulse energy in the order of 100 mJ and 6.5 ns pulse width to develop a plasma in air. The 6 mm laser beam diameter expanded to 2 cm before focusing it with 10 cm focal length lens. The plasma emission was captured with the help of neutral density filter and CCD camera. In general, the laser plasma is smaller by volume, has a spatial symmetry along the beam axis, and quicker than the spark ignition plasma. The uncertainty of absorption (developing plasma) depends on the laser power, if the power is near by the threshold value, the uncertainty increases and the difference in initiation time can be as high as half of the pulse width. Namely, the probability of plasma formation decreases nearby the threshold laser intensity. On the other hand, higher energy reaches the threshold value earlier which increases the absorption time and hence provides a significant reduction in the uncertainty of plasma formation [84].

Several pulses of energy (16-80 mJ) were used to evaluate the absorbed energy within the plasma, the maximum absorption was detected for a pulse energy of 45 mJ. Laser pulses lower or higher than 45 mJ reduced the absorbance because of shorter initiation time, and plasma saturation respectively. The optimum pulse energy was found to be approximately three times the threshold value (16 mJ). Chen et al. [84] attributed the lower degree of plasma saturation to the expansion in size. Also, if the pulse energy is higher than the threshold value, plasma might form before the focal point and expands opposite the laser beam.

In another study, Yalcin et al. [85] studied the effect of ambient condition on plasma properties such as electron density and temperature by using a focused laser in atmospheric air. It is believed

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that the local thermodynamic equilibrium can be achieved in the plasma because the electron density and temperature are high enough to generate collision rate much faster than the radiative rate. A new approach based on Saha and Boltzmann equations was developed and used to estimate the electron density and temperature [85]. Nd:YAG laser at 532 nm, pulse width (10-13 ns), and a pulse energy in the range of 40-150 mJ was focused by 10 cm lens to create the plasma. Also, a spectrometer was employed to analyze the light emission.

They found that the change of plasma temperature and electron density was within 5% by changing the incident laser energy from 4 to 150 mJ, which is within the uncertainty of the experimental setup. However, the expansion velocity increases as the laser energy increase which results in a longer plasma at the same temperature [85]. This study suggests that by increasing the laser energy, the plasma becomes bigger instead of denser or hotter. A similar observation was seen in Strozzi et al. [31] experiments; the absorbed energy tend to stabilize at high laser pulse where saturation occurs (around 150 mJ). Any further increase in pulse energy will increase the plasma size, not the temperature or the density.

In Almansour et al. laser ignition tests [86, 87], an elongated ellipsoid shockwave was observed by the high-speed camera. Figure 2.4 presents two frames of CH4/air mixture diluted with carbon dioxide at atmospheric pressure and room temperature. The pulse energy was just above the required to create a plasma in air. The reader is advised to read Appendix A for more details about the experimental setup and Appendix B for effect of CO2 diluted mixture on LBV measurements.

Due to the fact that the flame speed was captured at 20000 fps, the shockwave was seen in only two frames 50 µs apart. One notices that the shape of the shockwave transformed from elongated ellipsoid in the first frame to a perfect sphere after 50 µs (see the black arrows). Also, it is clear that shockwave has a slightly bigger end on the left side, this observation attributed to the fact that

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the laser was coming from the left side, where higher absorption occurs as suggested by Endo et al. study [81].

t = 0 t = 50 µs

Figure 2.4. Shockwave formation in methane mixture diluted with carbon dioxide at atmospheric pressure, room temperature, and an equivalence ratio 0.8. Nd:YAG laser at 1064 nm is incident from left side.

Since the behavior of plasma and shockwave formations is not dependent on the mixture composition, further experiments were carried out by using atmospheric air. The high-speed camera setting was altered to a smaller observable window focused on the plasma location.

Because of that, the high-speed camera was capable of recording at 67000 fps. The result is shown in Figure 2.5 where the shockwave appears in three frames. It is clear that the elongated ellipsoid shockwave still exists after 14.9 µs, however, after approximately 30 µs small sections of a circular shockwave were observed (see the white arrows). Unfortunately, plasma formation could not be captured even with the 67000 fps; a faster camera is required to study that phenomenon. However, the author believes that plasma forms as tear drop shaped, this explains the shape of the shockwave in the early stage before developing to a perfect sphere.

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t = 0 t = 14.9 µs t = 29.8 µs

Figure 2.5. Shockwave formation in air at atmospheric pressure and room temperature.

Nd:YAG laser at 1064 nm is incident from left side.

Regarding the flame shape, a toroidal flame kernel and propagation has been observed in several laser ignition studies [18, 21, 28, 88, 89]. Two-stage toroidal flame propagation, in the first stage, the flame propagates in a faster rate toward the incoming laser. After developing the toroidal shape, the flame propagates at the same speed in all directions. Figure 2.6 illustrates flame kernel formation and propagation of methane mixture along the first 900 µs of combustion process (frames are 100 µs apart). Several explanations of this toroidal behavior were proposed in the literature; like preheating process by the focused laser beam (ionized gas absorb more laser energy) [28, 81], and the initial flow field due to the radiation laser beam [81, 89, 90]. This flame shape enhances the combustion process by introducing unburned gases to the initial flame kernel (black arrow in third frame of Figure 2.6).

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Figure 2.6. Flame kernel formation and propagation of methane mixture diluted with carbon dioxide at atmospheric pressure, room temperature, and an equivalence ratio 0.8. Nd:YAG laser at 1064 nm is incident from top, a toroidal flame is shown (frames are 100 µs apart).

Bradley et al. [89] examined the flame propagation of a LI system under quiescent and turbulent environment. Nd:YAG laser at the fundamental wavelength was used to generate pulse energy of 200 mJ for 15 ns to ignite the iso-octane mixture. An elongated ellipsoid shape was noticed for the plasma with the major axis along the laser beam. They attributed the shape to the exponential decay of the absorbed energy with respect to the focal point. Flame propagates after several

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hundred µs with a generation of a third lobe which explained by the effect of gas dynamics [89].

Endo et al. [81] gave a similar justification for the flame shape, except that a cylindrical plasma was assumed in their analysis. Additionally, Morsy and Chung [88] simulated the initial flame kernel development numerically using a lean methane mixture. A cylindrical plasma was assumed with a symmetric absorption in the direction of laser beam. The kernel shape was attributed to the vortical motion generated by the interaction of pressure field and flow (hydrodynamic effects). It is worth noting that the gas dynamics effects occur in both reacting and non-reacting mixtures.

Recently, Endo et al. [81] compared the ignition ability of laser and spark ignition in a quiescent lean-fuel propane/air mixture at atmospheric conditions. Several laser configurations as well as spark ignition system were employed to examine ignition probability, pressure trace, and flame kernel size. One of the laser configurations was with a dummy electrode to provide a setup that has conduction losses such as those exist in SI system. Following a rigorous analysis, Endo et al.

[81] concluded that improved ignitability of a laser-induced spark is not only due to the avoidance of heat loss to the electrodes, but also due to the initial flame kernel enhancement as its apparent energy is augmented by the rapid heat release from the combustible mixture drawn into the kernel by a non-spherical inward flow which is created by the laser-induced spark.