U形竖管内超临界甲烷传热异常行为机理研究

Mechanism Analysis of Abnormal Heat Transfer Behavior of Supercritical Methane in a U-Tube

关键词:  航空发动机  超临界流体  U形管  迪恩涡  空气预冷系统

Key words:  Aero engine  Supercritical fluid  U-tube  Dean vortex  Air precooler

摘要:

针对航空发动机的空气预冷系统U形竖管超临界流体传热异常问题，对超临界压力甲烷U形竖管内传热行为进行了数值研究。探究了热流密度、质量流量和运行压力对换热的影响，从超临界甲烷管内温度、速度、流动状态及无量纲数变化出发，阐述了超临界甲烷在U形管内异常传热现象形成的机理。结果表明，在较高热流密度（95kW·m^-2）下，浮升力导致的自然对流是上升直管段内传热恶化的主要原因，运行压力的升高抑制了物性变化，促进了传热恶化向正常传热方向恢复。弯管段内的二次流使混合对流转变为强制对流，二次流形成的迪恩涡改善了径向温度分布不均匀性，强化了弯管段及其后续直管段的传热，且在弯管顶部位置传热的强化作用最为显著。

Abstract:

Aiming at the abnormal heat transfer of supercritical fluid in U-shaped tube of air pre-cooling system of aircraft engine， the heat transfer behaviour in U-shaped tube of supercritical pressure methane was numerically studied. The effects of heat flow density， mass flow rate and operating pressure on heat transfer were discussed， based on the change of temperature， velocity， flow state and dimensionless number in supercritical methane tube， the mechanism of abnormal heat transfer of supercritical methane in U-shaped tube is expounded. The results show that the natural convection caused by buoyancy is the main reason for the deterioration of heat transfer in the rising straight pipe section at a high heat flux （95kW·m^-2）. The increase of operating pressure inhibits the change of physical properties and promotes the deterioration of heat transfer return to the normal heat transfer direction. The secondary flow in the elbow section transforms the mixed convection into forced convection， Dean vortex formed by secondary flow improves the non-uniformity of radial temperature distribution， the heat transfer in the elbow section and its subsequent straight pipe section is enhanced， and the heat transfer enhancement effect is most significant at the top of the elbow.