环形管内沸腾传热与临界热流密度研究/Research of Flow Boiling Heat Transfer and Critical Heat Flux

2018-09-09 10:55:42

heat flow tube Annular CHF



环形管在核动力装置中有重要的应用背景,其沸腾传热特性的研究和临界热流密度的预测日益受到重视。本文对外管内径23mm、内管外径18.9mm、仅外侧加热的窄缝环形管进行了单相和沸腾传热特性实验研究。实验结果表明,环形管单相强化传热不显著,低于相同条件下的直管内紊流传热,而沸腾传热具有明显的强化效应。本文得到了传热系数的实验关联式。本文采用数值计算的方法对环形管CHF进行了研究。由于发生过冷沸腾临界热负荷(CHF)和环状流CHF时圆管和环形管的流型没有本质的区别,因此本文对圆管临界热流密度预测方法进行了改进,使之有效地应用于环形管内CHF的预测。按环形管内流场分布和温度场分布的特点,重新定义了环形管的水力直径和热周直径。采用重新定义后的当量直径对圆管过冷沸腾液相底层烧干机理模型和环状流液膜烧干模型进行修正。计算了水在竖直同心环形管内向上流动时的过冷沸腾和环状流CHF,加热方式有内侧加热、外侧加热、两侧同时加热等。数值预测结果与文献数据比较表明:过冷沸腾CHF模型在p=1.27~18.6MPa、G>500kgm-2s-1、xc=-0.5~0参数范围内预测良好;环状流CHF模型在p=0.57~15.012MPa、G=198~3790kgm-2s-1、xc=0.07~0.86参数范围内预测良好。

The annular channels have profound background in nuclear industry, therefore more and more attention is paid on the heat transfer character and the prediction of the critical heat flux. In present study, the single-phase forced convection and flow boiling heat transfer in an annular tube is experimentally investigated with outer tube wall heating condition. The ID of the outer tube is 23mm and the OD of the inner tube is 18.9mm. The test results show that the enhancement of heat transfer of single-phase flow in annular tube is not remarkable which is less than that in circular tube under the same condition, whereas the saturated boiling heat transfer is enhanced because of the small channel geometry. The correlations to calculate the single-phase heat transfer and boiling heat transfer are obtained. Furthermore, the prediction of critical heat flux is numerical studied as well. Since the flow regime at both subcooled boiling CHF and annular flow CHF in the annular tube has no distinction with that in circular tube, the well-established mechanistic models about critical heat flux commonly used in the circular tube flow can be modified and employed in the annular tube flow. According to the special characteristics of the flow velocity field and the fluid temperature field, a hydrodynamic diameter and a heat diameter are re-defined. With the re-defined diameter, the subcooled boiling CHF model is modified based on the liquid sublayer dryout model, and the annular flow CHF model is also modified based on liquid film dryout model. The two modified models is employed to calculate CHF with water vertical flowing in a concentric annular tube and with all types of heat conditions which includes inner tube heating, outer tube heating, bilaterally tube heating. Good agreement is obtained between the numerical predictions and experimental data with the following parameter ranges: for subcooled boiling CHF, p=1.27~18.6MPa, G>500kg/(m2s), xc=-0.5~0; for annular flow CHF, p=0.57~15.01MPa, G=198~3790kg/(m2s), xc=0.07~0.86.