准确模拟高速流场中的热化学非平衡问题一直都是各种流场数值计算的难点。DSMC 作为一种有效的流场模拟方法，从统计学的角度而言，作为统计母体的模拟分子数是影响计算准确性的关键。Bird 等[Bird，1990]针对不涉及热化学非平衡的流场提出了一个统一的模拟分子数选取规则。本文在热浴条件下使用 DSMC 模拟了空气中三种常见氮氧气体的离解反应，对涉及热化学非平衡的流场中模拟分子数对 DSMC 精度的影响进行了分析。结果表明：与 Bird 针对普通流场所提出的统一模拟分子数选取规则不同，在不同的流场物理条件（本文主要为温度）下，得到正确结果的模拟分子数阈值与温度成正相关（最小离解度与最大离解度模拟分子数选择下限相差10倍），超出阈值后增加模拟分子数对流场精度并无明显提高，这一结果与 Parker 的分子能量传递理论相符。从保证计算精度和平衡计算效率的角度，本文的结论对 DSMC 非平衡流场模拟具有参考价值。%To date,the simulation of the nonequlibrium phenomenon including chemical reacting in high-speed flows is still a challenging work.Based on the strategy of statistical mechanics,DSMC (Direct Simula-ting Monte Carlo)is believed to be an effective method in revealing the physic underlying the nonequlibrium phenomenon.Bird et al.pointed out that the number of molecules is an extremely important parameter in DSMC simulation,and an empirical value,approximately 20~30 in each calculation cell,was suggested in a general DSMC simulation without any thermal or chemical nonequilibrium phenomena.In this paper,a new factor considering the thermal and chemical nonequilibrium effects is proposed and examined in the DSMC simulation of the nitrogen-oxygen dissociation reaction flows under a hot bath environment.Results show that the number of molecules could significantly affect the accuracy in all of the simulations,and it is highly related with the specific physical problems.It suggests that the number of molecules in each calculation cell should be proportional to the local temperature and limited by a certain value in order to obtain a sufficiently accuracy results.On the other hand,the independence on the molecule number is identified with continues increasing the molecule number,which agrees with the theoretical analysis.Finally,with respect to the bal-ance between the calculation cost and efficiency,several examined values in the present study are valuable for further references and are able to be regarded as canonical parameters in DSMC simulation.
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