强对流条件下使用p-VOF方法的低温圆管二维结霜模拟

Two-dimensional simulations of frost formation on a cold tube under strong convection by using the p-VOF method

为进行强对流条件下低温表面的结霜模拟,新发展了称为p-VOF方法的结霜模拟方法,该方法是一种用于有相变发生的多相流模拟的伪VOF方法:通过显式求解简化的质量守恒方程得到霜相的体积分数变化,替代CFD软件中原有的体积分数方程的求解。使用p-VOF方法对强对流条件下低温圆管的二维结霜进行了模拟,分析了强对流条件下低温圆管结霜的大致规律与特征。在常物性霜层条件下,来流湿度越大或管壁温度越低,霜层生长速率越快,霜层越厚;来流速度越大,在结霜初期的霜层生长速率越大,但霜层生长速率明显减慢的时间越早,最终的霜层也越薄。研究表明p-VOF方法能够正确反映圆管外形下流动、传热与霜层形貌变化的相互耦合关系,可用作强对流下的低温圆管二维动态结霜模拟的基础方法。

The pre-cooling heat exchanger is an important component of a new type of hypersonic engine,which cools the high-temperature intake air to-130℃ to increase the airflow rate and improve engine performance. However, the frosting due to the water vapor in the atmosphere is a key factor restricting the application of pre-coolers in the new type engines. The flow velocity(above 10 m/s) through the pre-cooler is much faster than that in most frosting researches. The condition that the incoming air velocity is greater than10 m/s can be considered as a strong convection condition. The frost layer is dense under strong convection,which is different from those with dendrite structures under natural convection or low-velocity conditions. A new frosting simulation method called p-VOF has been developed to simulate the frosting under strong convection conditions. This method is a pseudo VOF method for multiphase flow simulations with phase transition. It obtains the volume fraction change of the frost phase by explicitly solving the simplified mass conservation equation instead of the original volume fraction equation. The p-VOF method was used to simulate the twodimensional frosting simulation on a cold tube under strong convection to confirm its applicability to simulate the frosting under such conditions. The general mechanism and characteristics of the frosting were analyzed. And the effects of air velocity, air humidity, and tube surface temperature on the frosting and flow were studied. Results showed that the higher the air humidity or the lower the tube surface temperature, the larger the frost growth rate and the thicker the frost layer. And higher air velocity leads to larger frost growth rate at the beginning of frosting, but earlier slowing down of the frost growth, yielding a thinner frost layer.