A kinetic model of droplet heating and evaporation: effects of inelastic collisions and a non-unity evaporation coefficient
A kinetic model of droplet heating and evaporation: effects of inelastic collisions and a non-unity evaporation coefficient
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DOI：
10.1016/j.ijheatmasstransfer.2012.09.046
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发表时间：
2013
影响因子：
5.2
通讯作者：
S. Sazhin;Jianfei Xie;I. N. Shishkova;A. Elwardany;M. Heikal
中科院分区：
文献类型：
--
作者：
S. Sazhin;Jianfei Xie;I. N. Shishkova;A. Elwardany;M. Heikal
先前开发的动力学模型的液滴加热和蒸发成高压空气的一般化，考虑到在动力学区域中的分子之间的非弹性碰撞，一个非统一的蒸发系数和液滴内的温度梯度的综合影响。它指出，对于典型的柴油机条件下的参数，在动力学区域中的热通量是在该区域的外边界处的蒸汽温度的线性函数，但实际上并不依赖于在该边界处的蒸汽密度的所有模型，包括和不包括非弹性碰撞的影响，包括和不包括非统一的蒸发系数的影响。对于任何给定的温度在动力学区域的外边界处的热通量的值示出为减少与分子的内部自由度的数目增加。对于这些自由度中的少数自由度，这种下降率是强的，但是当这些自由度的数量超过20时，这种下降率可以忽略不计。这使我们能够限制分析的前20个任意选择的自由度的正十二烷分子时，考虑非弹性碰撞的影响。对于上述所有模型，在同一位置，该边界处的质量通量几乎随蒸汽密度的增加而线性减小。对于动力学区域外边界处的任何给定蒸汽密度，考虑内部自由度的贡献的模型的质量通量值小于忽略这些自由度的模型。结果表明，非弹性碰撞的影响导致更强的增加预测的液滴蒸发时间在柴油发动机的条件下相对于流体动力学模型，相比，由动力学模型预测的类似的增加只考虑弹性碰撞。对于1500 K的气体温度，非单位蒸发系数的影响是显著的。严格的动力学模型的应用，考虑到非弹性碰撞和非统一的蒸发系数的影响，并考虑到液滴内的温度梯度的模型，建议当准确预测的值的液滴表面温度和蒸发时间在柴油发动机的条件是必不可少的。
The previously developed kinetic model for droplet heating and evaporation into a high pressure air is generalised to take into account the combined effects of inelastic collisions between molecules in the kinetic region, a non-unity evaporation coefficient and temperature gradient inside droplets. It is pointed out that for the parameters typical for Diesel engine-like conditions, the heat flux in the kinetic region is a linear function of the vapour temperature at the outer boundary of this region, but practically does not depend on vapour density at this boundary for all models, including and not including the effects of inelastic collisions, and including and not including the effects of a non-unity evaporation coefficient. For any given temperature at the outer boundary of the kinetic region the values of the heat flux are shown to decrease with increasing numbers of internal degrees of freedom of the molecules. The rate of this decrease is strong for small numbers of these degrees of freedom but negligible when the number of these degrees exceeds 20. This allows us to restrict the analysis to the first 20 arbitrarily chosen degrees of freedom of n-dodecane molecules when considering the effects of inelastic collisions. The mass flux at this boundary decreases almost linearly with increasing vapour density at the same location for all above-mentioned models. For any given vapour density at the outer boundary of the kinetic region the values of the mass flux are smaller for the model, taking into account the contribution of internal degrees of freedom, than for the model ignoring these degrees of freedom. It is shown that the effects of inelastic collisions lead to stronger increase in the predicted droplet evaporation time in Diesel engine-like conditions relative to the hydrodynamic model, compared with the similar increase predicted by the kinetic model considering only elastic collisions. The effects of a non-unity evaporation coefficient are shown to be noticeable for gas temperatures of 1500 K. The application of the rigorous kinetic model, taking into account the effects of inelastic collisions and a non-unity evaporation coefficient, and the model taking into account the temperature gradient inside droplets, is recommended when accurate predictions of the values of droplet surface temperature and evaporation time in Diesel engine-like conditions are essential.