自然循环流动不稳定条件下的传热特性实验研究

Experimental Study on Heat Transfer Characteristic for Natural Circulation Flow Instability

为探究流动不稳定性机理,在低压自然循环系统中开展了一系列相关实验,分析了不同流量振荡模式下自然循环的沸腾传热机制及局部传热特性。实验表明:中、低热流密度下出现的较规则的周期性振荡由加热段内流动沸腾诱发,壁面过热度不会随流量振荡而大幅度变化;高热流密度下自然循环系统出现的周期性不规则振荡现象中,流动沸腾类型间的相互转变不是流量波动的唯一原因。大幅度的流量脉动可能在高热流密度下导致沸腾临界的发生,出口壁面出现间歇性干涸,局部传热系数下降的同时伴随壁温的短暂飞跃。随着热流密度的提高,自然循环系统可能出现持续性干涸。

A series of natural circulation experiments were carried out in order to explore the flow instability mechanism.The flow boiling mechanism and local heat transfer characteristics were analyzed for different natural circulation oscillation modes.The experimental results show that the periodic and regular oscillation is induced by flow boiling under low and/or medium heat flux.Meanwhile,the wall superheat varies within a small range when flow rate oscillates.Under high heat flux conditions,an irregular but periodic oscillation is observed in the experiment.The transition of different boiling regimes is not the unique reason for this oscillatory circulation mode.Large amplitude oscillation is likely to trigger transient critical boiling.Periodic dryout occurs near the outlet and gives rise to the decrease of local heat transfer coefficient and the fly-up of wall temperature.With the increase of heat flux,continuous dryout might take place in natural circulation system.