1. Introduction

Most of the combustion process in practical combustors is associated with a strong turbulent environment where local non uniform temperature distribution plays a vital role on flame dynamics. Recently, Tsuchimoto [1a] and Kyozu [1b] et al. examined the effect of non uniform temperature distribution on flame dynamics by imposing an external heat source such as laser ray along the center line of a combustion tube. They observed the flame oscillation during propagation in the combustion tube. Since the complete mechanism of the flame oscillation phenomenon has is not been confirmed yet, the goal of this study is to figure out the mechanism of the flame oscillation phenomenon observed in experiments [1a] and [1b].

2. Numerical Simulation

In this study, a two-dimensional time-dependent system of governing equations for reacting flows, in the absence of the Soret effect, Dufour effect, pressure gradient diffusion, gravity and radiation, is chosen and is discretized by using finite volume method (FVM) on a hexahedral structured grid cells and solved them by adopting Front Flow Red, a multi-scale and multi-physics computational fluid dynamics (CFD) solver under the low Mach number approximation in a two dimensional channel. The combustion chemistry is modeled by the single-step irreversible overall chemical reaction between ethylene and oxygen.

3. Results and discussion

To reach the stated goal, we have divided our investigation into four stages as below:

In the first stage, we have investigated the influence of the motion vortices, evolved from the sudden gas expansion at the larger ignition zone, on the propagation of a laminar premixed flame in a channel [see Fig. 1(a)]. The strong instantaneous movement of these vortices rapidly deformed and enlarged the flame surface area, which gives rise to flame oscillation during the propagation and is shown in Figs. 2 and 3. The results observed in this stage suggest that the suppression of this type of strong vortex evolution is essential for the investigation of the main issue of this dissertation.

In the second stage, we have successfully controlled evolution of the large vortices by reducing the ignition zone over the channel [see Fig. 1(a)]. It is found that the propagation is very stable instead of showing oscillation during propagation and is displayed in Fig. 4. Therefore, the smaller ignition zone makes a suitable platform for the investigation of the main issue.

In the third stage, we have investigated the external heat source induced in-line pre-heating effect, along the center line of a combustion channel with a width of 1 mm [see Fig. 1(b)], on the flame dynamics. The highly deformed flame front is observed due to pre-heating effect and the propagation speed is significantly accelerated compared to non pre-heating effect (see Fig 5). This high deformation in the flame front induces a negative velocity distribution (see Fig. 7) ahead of flame tip (see Fig. 6), which generates a very weak pair vortex in front of flame tip at the side of reactant (see Fig. 8). This phenomenon is seemed to be related to the flame oscillation phenomenon observed in experiment [1a] and [1b] as the interaction between a flame and vortex often generates flame oscillation during the propagation.

In the final stage, we have investigated the influence of flow-induced instability ahead of a flame tip at the side of a reactant due to a very thin pre-heating treatment of width 0.5 mm along the center line of a combustion channel [see Fig. 1(b)]. It is observed that the flame front is extremely deformed due to the narrow pre-heating effect along the center line of the channel. The deformed flame front induces a thin vortex sheet that is attached on the frontal side of the deformed flame front. This thin vortex sheet induces a strong negative velocity ahead of the flame tip, which enhances the generation of a strong pair vortex at the side of the reactant. The pair vortex interacts with the flame tip [see Fig. 9] and rolling up along the flame surface towards upstream during the propagation. The interaction between the flame and vortex shows oscillatory behavior during the propagation in the combustion channel. This flame-vortex interaction would be one of the possible sources of laser induced flame oscillation in experiments [1a] and [1b].

4. Conclusion

In this study, we have obtained a pair vortex ahead of flame tip, due to highly deformed flame front driven negative velocity distribution, induced by an in-line pre-heating treatment. This induced vortex pair not only plays a vital role to explain the instability mechanism but also very useful to develop an efficient flame model to simulate the local non uniform temperature variation in practical devices during turbulent combustion.

Fig. 1(a) Schematic view of the simulation domain and coordinate system.

Fig. 1(b) Schematic view of the installation of pre-heating zone in the calculation domain.

Fig. 2 Temperature distribution in the channel with ignition zone of 25 mm2 for time 0.006 [sec]

Fig. 3 Vorticity distribution in the channel with ignition zone of 25 mm2 for time 0.006 [sec]

Fig. 4 The flame front location versus time along the center line (y = 25 mm) of combustion channel.

Fig. 5 The temperature distribution in the combustion channel: (a) for non pre-heating effect and (b) for PHT of Case 1, (c) for PHT of Case 2, and (d) for PHT of Case 3 at time 0.025 sec with 1 mm pre-heating width ( PHW), ( Here, PHT: stands for pre-heating temperature)

Fig. 6 Instantaneous temperature distribution for PHT : 600 K and PHW : 1mm.

Fig. 7 The detailed negative velocity distribution 1 mm ahead (it is located at the pink-colored vertical line in Fig. 6) of flame tips for pre-heating and non pre-heating effect.

Fig. 8 Visualization of weak pair vortex (by vorticity contour) ahead of flame tip due to negative velocity distribution (shown in Fig. 7)

Fig. 9 Visualization of the strong flame-vortex interaction by the temperature (in solid) and vorticity [1/sec] (in contour) distribution for PHT: 600 K and PHW: 0.5 mm.

本論文は、乱流燃焼に代表されるような様々な燃焼器における非均一な温度場が火炎に与える影響を明らかにするために、特にレーザーにより局所加熱された層流予混合火炎において観察される火炎非定常伝播を対象として数値解析による研究を行い、火炎変形と流動場の関係および火炎振動メカニズムの解明を試みたものである。本論文の構成は以下のように述べられている。

第一章では、層流予混合火炎の火炎振動に関する既研究を概観し、燃焼工学における火炎振動の予測制御の重要性、および、その基礎的課題として本研究が対象するレーザー局所加熱された層流予混合火炎の振動現象の既研究を紹介し、本研究の目的を示した。

第二章では、本研究で適用した層流予混合火炎の数値解析法として、熱流動場および総括化学反応モデルの基礎方程式を示し、導入された近似に関して検討を行った。

第三章では、対照実験を模擬した予混合火炎の着火過程の計算方法について述べ、導入された近似に関して検討を行った。

第四章では、対照実験を模擬した2次元火炎伝播流路における着火過程に生じる渦生成と火炎に及ぼす影響を数値解析し、渦生成による火炎速度増加のメカニズムを示した。また、導入された数値手法の評価を行い近似の有効性を確認した。

第五章では、さらに、第四章で行った解析に対して着火条件の違いが火炎速度に与える影響を系統的に検討し、着火領域すなわち着火エネルギーの大きさが生成する渦強さおよび火炎加速の大きさを支配することを数値的に示した。その結果から、着火時の火炎不安定を抑制して対照実験を模擬した層流予混合火炎を数値的に安定に形成する方法を明らかにした。

第六章では、前節で示された層流予混合火炎の解析方法を用いて、対照実験を模擬した流路中央部に1mm幅でレーザー局所加熱された層流予混合火炎の数値解析を行い、実験で観察される火炎先端形状の変形とそれに伴う火炎速度の増加が数値的に予測できることを示した。また、従来実験においては計測困難であった火炎近傍の速度場を解析し、理論等で推定されていた曲率を持つ火炎先端から前方へ向かうnegative velocity分布および、それに誘起される火炎前方の弱い渦対の存在を数値的に明らかにした。これらの結果から、火炎前方の渦対が火炎振動に影響を与えうることを考察した。

第七章では、対照実験での観察を元に火炎振動が促進される条件としてレーザー局所加熱幅を狭く集中した条件を導入し、対照実験を模擬する火炎先端の非定常変形を予測することに成功した。また、加熱幅を狭く集中することにより火炎先端の形状、negative velocity分布、および、渦強さなどが強調されることを明らかにし、それが火炎不安定に関係付けられることを考察した。これらの数値計算結果を従来の対照実験での観察と比較検討し、強い曲率を持った火炎先端に生じる渦対と火炎との相互干渉が、レーザー局所加熱された層流予混合火炎の火炎振動のメカニズムの一つとして考えられることを新たに提案した。

最後に第八章では本研究の成果を総括した。

以上のように、本論文は、工学上重要な燃焼振動現象の解明において、層流予混合火炎の火炎振動に着目し、数値解析を用いて、特に非均一な温度場から生じる渦が火炎振動メカニズムに果たす役割を明らかにした。これは、機械工学、特に、燃焼工学および流体工学の発展に寄与するところが大であるといえる。

よって本論文は博士(工学)の学位請求論文として合格と認められる。