Improvement of Nozzle Performance for Flashing Expansion of Initially Subcooled Hot Water

研究表明，喷嘴喉道附近的热平衡程度会影响喷嘴的性能特性，如临界流量、喷嘴轴向压力分布、推力和推力系数。随着热非平衡程度的增加，闪起延迟时间变长，喷嘴内液滴加速不充分，气液比滑移增大。因此，减小喷管喉部附近的热不平衡程度是提高喷管性能的必要措施。首先，通过实验研究了传统会聚-发散型喷管的性能特点及效率下降的原因。其次，将压力欠冲（作为热不平衡程度之一）随减压速率的降低而减小的关系应用于喷嘴设计。结果表明，采用这种方法可以提高喷管的推力系数。第三，已知干扰刺激了闪烁的开始。为此，设计了在发散通道内安装细导线的喷嘴来制造扰动，并进行了试验。细丝喷嘴的推力系数较高。提出了一种确定喷嘴出口横截面积的方法。最后，从临界流动模型的角度测量轴向空隙率分布是很重要的。本文尝试用中子射线照相法测量喷嘴内轴向空隙率分布。

It was made clear that the degree of thermal nonequilibrium near a nozzle throat affects the performance characteristics of nozzles, such as the critial flowrate, the pressure profile along the axis of the nozzle, the thrust, and the thrust coefficient. As the degree of the thermalnonequilibrium increases, the delay time of flashing inception becomes longer, and droplets in the nozzle are not accelerated fully and the slip between gas and liquid ratio becomes larger. Thus, it is necessary to decrease the magnitude of the thermal nonequilibrium near the nozzle throat for improving the nozzle performance.First of all, the performance characteristics and the reason why the nozzle efficiency de-creases were studied experimentally using conventional convergent-divergent nozzles. Second, the relationship that a pressure undershoot (as one of the degree of thermal nonequilibrium) decreases with decreasing a decompressive rate was applied to the nozzle design. It was found that the thrust coefficient of the nozzle is improved in this manner. Third, it is known that disturbance stimulates the flashing inception. Thus, the nozzle installed thin wires in the divergent passage was designed to make disturbance and then tested. The nozzles with the thin wires showed higher thrust coefficient. A methodof determining the cross-secectional area at the nozzle exit was also proposed. Last, it is important to measure axial void fraction profiles from the viewpoint of a critical flow modeling. Measurement of the axial void fraction profiles in the nozzle was tried by using neutron radio-graphy.