浮升力对竖直上升圆管内超临界航空煤油换热的影响及评价准则

为深入理解航空发动机再生冷却热防护中的换热机理，对竖直上升圆管内超临界压力RP-3航空煤油的传热恶化开展了实验研究。探究了两类传热恶化的换热特征和形成机理，考察了热流密度、质量流量和进口压力对传热恶化的影响机制。获得了两类传热恶化的起始条件判别准则。以Nu/Nu0=0.75作为依据，建立了浮升力影响评价准则。通过格拉晓夫数修正实现了换热关联式预测。结果表明：两类传热恶化出现在不同条件下，低温区传热恶化源于边界层转捩和浮升力的综合作用，当进口雷诺数高于5730时其不再出现；高温区传热恶化的临界热流密度可以表述为质量流量的函数关系。当浮升力参数Grb/Re2.7和Gr/Re2分别高于10-7、10-3时，浮升力作用不可忽略。改进的换热关联式具有合理的预测精度，满足热防护系统设计的工程运用。

Influence and Evaluation Criterion of Buoyancy on Heat Transfer of Supercritical Aviation Kerosene in a Vertical Upward Circular Tube

To deeply understand the heat transfer mechanism in regenerative cooling thermal protection of aero engine, an experimental investigation on heat transfer deterioration of RP-3 aviation kerosene in a vertical upward circular tube under supercritical pressures was conducted. The heat transfer characteristics and formation mechanisms of two types of heat transfer deterioration were studied, and the influence mechanisms of heat flux, mass flux and inlet pressure on heat transfer deterioration were investigated. Two criteria for determining the initial conditions of heat transfer deterioration were obtained. Based on Nu/Nu0=0.75, an evaluation criterion of buoyancy effect was established. The correlation prediction of heat transfer was achieved by the modification of the Grashof number. Results indicate that these two types of heat transfer deterioration occur at different conditions. The heat transfer deterioration in low temperature regions originate from the combined effect of the boundary layer transition and the buoyancy, and it no longer appears when the inlet Reynolds number is higher than 5730. The critical heat flux for heat transfer deterioration in high temperature regions can be expressed as a function of mass flux. When the buoyancy parameters Grb/Re2.7 and Gr/Re2 are higher than 10-7 and 10-3, respectively, the buoyancy effect cannot be ignored. The modified heat transfer correlation has reasonable prediction accuracy and meets the engineering application of thermal protection system design.