THERMO-FLUID DYNAMIC RESEARCH ON THE CHARACTERISTICS OF HEAT REMOVAL FROM THE NUCLEAR FUEL RODS NEAR THE SPACER

通过使用两种不同类型的隔离器，研究了支撑核棒的隔离器的几何形状对隔离器附近热流体现象的影响。一种是环形垫片，另一种是网格式垫片。特别注意的是：(a)引起干斑形成的机制，(b)干斑形成的频率和(c)间隔器下侧附近的温度分布。结果总结如下：(1)即使间隔片与加热管表面之间存在间隙，由于表面张力的作用，冷却水也会在间隔片之间停滞。因此，在本实验中，使用带间隙的间隔器的情况下，主要是排水导致在间隔器下方形成干斑。特别是在网格型垫片的情况下，间隙不是周向均匀的，表面张力也在周向上起作用。与环形垫片相比，水更紧密地保持在最小的间隙区域内，就像固体一样。因此，旱地的形成被限制在最窄间隔间隙区域的正下方区域，并且在网格间隔情况下比环形间隔情况下更频繁。虽然实验已经证明栅格式隔震器的临界热流密度比环形隔震器小，但其原因一直存在争议。本研究的结果是目前为止最可能的机制。(2)温度分布表明：(a)由于热边界层受到隔板内气泡增长的干扰，在有隔板的情况下，靠近底部的温度比没有隔板的情况下低。气泡可以自由移动的间隙空间在网格型间隔器的情况下被限制为更宽的间隙区域，但在环型间隔器的情况下不受限制。环形隔板对加热棒冷却的混合效果强于栅格隔板。这一实验事实也为环形垫片优于栅格垫片提供了依据。(3)目前的实验结果表明，即使在环空流动的情况下，液膜的排水是引起加热棒热临界状态即烧烬的更合理的控制因素，而不是传统认为的引起烧烬的夹带沉积理论。少

The effects of the geometry of the spacer, which supports the nuclear rods, on the thermo-fluid phenomena near the spacer were investigated by using two different types of the spacers. One is the ring type and the other is the grid type spacers. The special attention was focused on (a) the mechanism to cause the drypatch dormation, (b) the frequency of the drypatch formation and (c) the temperature distribution near the down side of the spacer. The results are summarized as follows.(1) Even if there is a gap between the spacer and the heating tube surface, the coolant water stagnates in between the spacer due to the surface tension force. Therefore it is mainly the drainage that causes the drypatche formation just underneath the spacer in the case of the present experiment where the spacer with the gap is used. Especially in the case of the grid type spacer the gap is not circumferentially uniform, and the surface tension acts in the circumferential direction as well. Then the water is … More kept more tightly within the smallest gap region than the ring type spacer and acts as if it is solid body. And accordingly the drypatch formation is limited in the region just beneath of the narrowest spacer gap region and more frequent in the grid spacer case than the ring type spacer.Although it has been made experimentally clear that the critical heat flux is smaller in the case of the grid type spacer than the ring type spacer, the reason of it is in the midst of a dispute. The result obtained in the present research is the most probable mechanism so far.(2) The temperature distribution shows that (a) the temperature near the bottom side of the spacer becomes lower in the case with the spacer than without the spacer because the thermal boundary layr is disturbed by the growing bubbles within the spacer. The gap space where the bubbles can move freely is limited to the wider gap region in the case of the grid type spacer but is not limited in the case of the ring type spacer. Then the mixing effect on the cooling of the heating rods is stronger in the case of thr ring type spacer than the grid type. This experimental fact also gives a ground That the ring type spacer is superior to the grid type.(3) The present experimental results show that the drainage of the liquid film even in the case of the annular flow is the more reasonable controlling factor to cause the thermally critical situation of the heating rods, i.e., the burnout, rather than the entrainment deposition theory, which is conventionally supposed to be the mechanism to cause the burnout. Less