高温反応のコンピュータシミュレーション

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タイトル別名
  • コウオン ハンノウ ノ コンピュータ シミュレーション
  • Computer Simulation on High Temperature Reactions

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高温での炭化水素と窒素よりHCNを生成する過程を電算機シミュレーションした。適切な素反応群と反応速度定数とを用いればよいのであるが, 実験値は僅かであるのに対し, 計算に用いられる反応式, 速度定数は多数で任意性が極めて高く, 何らかの規制を加えないと, 極言すればどうにでも実験値と符合させることが可能である。このうち反応式は原系から生成系に至ると考えられる反応を一旦網羅し, 感度解析を行って不要なものを削除すればよいのであるが, この際, 反応式が素反応であるか, 逆反応が平衡定数から精確に算出可能であるかが大きな問題である。素反応性を確認する方式が確立されていないし, 多数の文献でも多くの矛盾が見られるので, 怪しい反応を取り込むことを避けるのが望ましい。また, 素反応として, 平衡定数より逆反応の速度定数を求める場合には生成熱の精確さが重要なので, これにも検討を加えた。一方速度定数についてはBaulchの推薦値があるものはこれを優先第一順位とし, それが見あたらない時にはMiller, その他の推薦値を, さらにそれも見られない場合には文献に頻出し, 多数の人が採択している速度値を用いることにした。しかし, CH_3+CH_3→C_2H_4+H_2の反応についてはWarnatz説とHidaka説とでは大差があり, その影響も極めて大きいので, ここでは両者を比較検討した結果, Hidaka説に従うこととした。これらの検討の結果, 可逆性に疑問のある場合には正方向のみの反応として, 簡素化を徹底し表8の機構を提案した。

To explain the experimental results on the HCN formation in the shock-heated CN_4+N_2+Ar and C_2H_2+N_2+Ar mixtures, we carry out the computer simulation based on the various reaction schemes. In the course of this study we checked 76 reactions, and finally simplified to the reaction mechanism including 44 reactions presented in Table 1. We use rate constants recommended by Baulch as preference, however, some of them are replaced by the constants recommended by Miller. The adaptability of these two proposals is compared. Moreover, since there still remains the reactions that do not refer by these two authors. In that case the constants refereed by the other authors and widely used in the various papers are selected. 1) In order to simplify reaction mechanism, we define the relative contribution of reactions as follows. Increment or decrement ΔN_<ij> of a molecular species i by the elementary reaction j, and the sum of Σ_i|ΔN_<ij>|_<abs> given by the whole reactions related to the change of that species i are calculated at an arbitrary reaction time. If the ratio ΔN_<ij>/Σ_i|ΔN_<ij>|_<abs> at any time was small relative to the other reaction k≠j, the reaction j is ranked to the minor importance. Accordingly we added the supplementary computation to confirm the effect caused by the exclusion of this reaction i from the reaction mechanism. We confirmed the non-effectiveness of this reaction i. 2) The usual sensitivity analysis was performed to discriminate the sensible reactions. The sensitivity of reaction i is defined in the present paper simply as Si=Yp/Yo. Yo is the HCN yield computed with the rate constants without perturbation. Yp is the yield of HCN computed with the rate coefficient multiplied by a factor of 2 for a given reaction i. 3) Experimental values are summarized to the HCN yield [HCN] at 2800 and 2500K for 1 msec reaction time. The standard deviations from the experimental values of [HCN]_<2800>, and [HCN]_<2800>/[HCN]_<2500> obtained from both of CH_4+N_2+Ar mixtures and C_2H_2+N_2+Ar mixtures are compared to the calculated values using the set of rate coefficients of sensible reactions. It is impossible to obtain the perfect aggreement among these values so far as we used the coefficients originally recommended by Baulch or Miller. There-fore, we modified the rate coefficients by multiplying the set of adjusting factorsβ to the sensible reactions. We aimed to decrease the standard deviation less than 10% by this adjustment and we obtained the more reliable mechanism. Since the reversibility of reactions represented in Table 1 is not definite, we surveryed articles refereed to this subject. However, we couldn't find the definite conclusion. Especially, on the reactions D17 and C9,CH_3+CH_3=C_2H_4+N_2 and C_2+N_2=CN+CN, both opinions opposed sharply. So in the present paper we compute both cases. In one of which, the reverse reactions were assumed to be given by rate constants calculated from the equilibrium constants and in the other the reverse reactions excluded from the reaction mechanism. The validity on the forward rate coefficient of D17 is the other important point in the present computation. Warnatz's constant is more than 100 times greater than Hidaka's constant, and the adoption of the latter is the same as to eliminate this reaction from the mechanism. The reaction C68. C_2H+N_2=HCN+CN, is the another reaction that is not certain whether it is reversible or irreversible. However, calculated results are independent on the removal of the reverse reaction kr of C68,In addition to this, even if we adopt the rate constant pointed out by Hidaka, that means the rejection of D17 from the mechanism, there is no significant influence. Therefore, it is possible to make a new mechanism without the ambiguity on the reversibility of reactions. We exclude the reaction C9 besides the aforementioned selection on the reactions D17 and C68 as shown in Table 8

資料番号: SA0166442000

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