The effects of incident electric fields on counterflow diffusion flames

[syoukera]

https://www.sciencedirect.com/science/article/pii/S0010218018301019#! Abstract

Numerical simulations of counterflow laminar diffusion flames impinged by sub-breakdown DC electric fields are performed in this work using multi-component transport and a detailed chemical mechanism for methane–air combustion that includes elementary steps for the conversion of six electrically charged species. The electric field is induced by two electrodes located on the oxidizer and fuel sides and arranged parallel to the mixing layer in a configuration resembling the one recently studied experimentally by Park et al. (2016) [1], which unveiled significant electric-field effects on the aerodynamics. In these simulations, the electric drift of the charged species leads to a bi-directional ionic wind that is axially directed toward both injectors. The major components of the ionic winds are the and ions, which are steered by the electric field into the fuel and oxidizer streams, respectively. At sufficiently high electric fields of the order of a few kilovolts per centimeter, the ionic wind intricately couples with the aerodynamic field of neutral molecules flowing into the burner, in a manner that ultimately leads to non-negligible disturbances of the velocity field. The overall effect of these interactions consists of a decrease in the local strain rate and in the stoichiometric scalar dissipation rate, which increases the burning rate of the diffusion flame. The functional form of the scalar dissipation rate depends on the applied electric field, which may have consequences for the subgrid-scale modeling of these processes. In contrast to electrified one-dimensional premixed flames, here the current ceases to vary monotonically with the voltage as a result of the coupling with the aerodynamic field. Comparisons between the present numerical simulations and the experiments performed by Park et al. (2016) [1] are made that indicate qualitative agreement. Quantitative disagreements related to the saturation intensities and to the strength of the electric disturbances of the velocity field are discussed, and possible sources of these discrepancies are identified.

[syoukera]

アブストラクト

サブブレイクダウン直流段階の作用下における対向流層流拡散火炎の数値シュミレーションを，他成分の輸送係数とメタン空気燃焼の詳細化学反応機構（６の電荷をもった化学種の素反応を含む）を用いて行った．Parkらによって近年おこなわれた実験（電界による空力への大きな影響を明らかにした）を模擬する形状で，電界は酸化剤と燃料側の混合層に平行となるように設置された二つの電極によって誘起される．これらのシュミレーションでは，荷電粒子の電気的ドリフト（両インジェクタへ向かう軸方向の）は双方向のイオン風を引き起こすイオン風の主要な要素は，電界によって燃料と酸化剤の流れにそれぞれシードされたH3O+とO2-イオンである．kV/cm程度の十分に高い電界では，最終的に非常に大きな流動場の変更につながるかたちで，イオン風は中性流子のバーナ内部へと流れる空力場と複雑に絡み合う．これらの相互作用の全体的な影響は（拡散火炎の燃焼率の増加につながる）局所伸長率と両論スカラー消散率の減少で構成される．スカラー消散率の関数形は印加電界に依存する．これは，今回の手法のサブグリッドスケールのモデリングに由来するかもしれない．1次元の予混合火炎と比較して，今回のケースは電圧にたいして単調であり，その結果空力とのカップリングが見られた．Parkらの実験結果と今回の数値シミュレーションの比較は定性的な一致を示した．飽和強度と電界による流れ場の変更変更に関する定量的な食い違いと，その原因の特定について議論する．

[syoukera]

背景

Counterflow laminar diffusion flames represent a cornerstone in subgrid-scale models for nonpremixed turbulent combustion [11], [12]. However, their interactions with electric fields have been the focus of only a very few studies to date.

イシュー

[syoukera]

先行研究，数値計算，火炎伝播への電界印加

Numerical simulations involving electric-field effects on combustion, subject to detailed chemistry and complex transport, are scarce in the literature and have been mostly limited to one-dimensional (1D) premixed flames1 [17], [18], [19], [20], [21], [22]. In these, the electric force induced by the motion of ions solely acts to readjust the hydrodynamic pressure gradient to satisfy mass conservation. Other problems such as two-dimensional laminar jet flames, where the interactions with the underlying flow field are more complex, have been simulated using skeletal mechanisms [23], [24], [25] and flamelet-based models to reduce the computational cost of resolving the reacting layers [26], [27], [28]. In most cases, numerical predictions tend to disagree with experimental measurements of electric intensities and charged-species profiles by factors of 2–3, indicating the relatively early stage at which the predictive capabilities in this research discipline are to date.

[syoukera]

数値解析上の問題点

(a) the multi-scale nature of the electric/aerothermochemical coupling phenomena, including the existence of a large disparity in time scales of the motion of electrons and neutrals; 電子と中性粒子間での運動の大きな時間スケールの差異を含めた，電気・空力熱化学のカップリング現象のマルチスケール性 (b) the complexities associated with the description of the molecular transport of charged species; 荷電粒子の分子輸送の表現に起因する複雑さ (c) the absence of accurate descriptions of ionic chemistry and the overreliance of ionic chemical pathways on the prediction of sub-ppm concentrations of radical precursors. イオンの化学反応の正確な表現の欠如と，サブppmの濃度のラジカル前駆体に対してイオンの化学反応経路を信頼しすぎていること

[syoukera]

Overcurrent

overcurrent occurs, is found that results from the competition between electric drift, charge production, and molecular diffusion of reactants.

[syoukera]

実験結果との比較

[syoukera]

0 kVの結果はそれなりにあっているが，2.4 kVの結果が定性的にも一致しているとはいいがたい．数値計算では元の0 kVの結果をなますような形で，電界の影響は火炎近傍のガスを+ux方向へ加速させた効果．一方で実験結果は火炎近傍0<x<0.2で+uxへの加速が大きい．一方で負極側の-0.4<x<0では-uxへの加速が発生している．x=0に陽イオンのドリフトで火炎が移動したと考えれば，その前後で反対方向（火炎からそれぞれの電極側）にイオン風が発生したと考えれば都合がよい

[syoukera]

荷電・非荷電粒子の軌跡

[syoukera]

二つのよどみ面ができているとの実験結果だったものの，それは荷電粒子のみであらわれるということ？図の右側の荷電を受けていない粒子はよどみ面の内部まで浸透しているように見える

[syoukera]

参考文献

1. この論文の続き，自己相似性 #3
2. 1993年の電界印加の数値解析https://www.sciencedirect.com/science/article/pii/001021809390125M
3. flamelet model in weak electric field https://www.sciencedirect.com/science/article/pii/S004579301730302X