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Order of Magunitude Analysis of stratified natural convection and mixed convection penetration flow of cold liquid into a vertical channel with hot,forced flow

燃料集合体入口チャンネル高温強制対流への低温流体の混合対流潜り込み流れの支配パラメータ次元解析

Tokuhiro, Akira; 小林 順 

Tokuhiro, Akira; Kobayashi, Jun

高速炉を模擬した上部プレナムから燃料集合体への低温流体の混合対流潜り込み流れの実験研究が行われた。この潜り込み現象は、DRACS (Direct Reactor Auxiliary Cooling System)を使用した自然循環による崩壊熱除去時のある条件下において発生し、炉心流量を決定する自然循環ヘッドと炉心の冷却性に影響を与える可能性がある。本試験では上部プレナムと燃料集合体入口チャンネルを単純化した試験体を用いて、垂直チャンネル内の高温上昇流へ低温流体が潜り込む現象に対して瞬間的な温度分布と流速分布の計測を行った。その結果、瞬間的な温度分布および流速分布の時間変化はほぼ同一であることが確認された。潜り込み現象を定量的に把握するため、単位時間当たりの現象の発生頻度によって判断した。得られた実験データにより潜り込み初生条件とチャンネルへの潜り込み深さを、与えられたPr数に対してRe数とGr数の組み合わせて表すことに成功した。本報告では、得られた結果が他の同様な研究と若干の違いがあるが、実験データを充分に取り入れた次元解析に基づき、無次元数の組み合わせを提案する。

In the thermal-hydraulic analysis of nuclear reactors and other energy-related systems, the proper scaling of simulation experiments and separate effect studies, is first determined by the identification of the relevant dimensionless group(s) and then by the reproduction in magnitude of the range. While most experiments contain some idealizations in geometry so that well-known dimensionless groups can be easily identified, the non-ideal nature of any given experiment may appear for instance, in the presentation of data using the selected dimensionless numbers. If this is the case, a reconsideration of the Scale Analysis or Order of Magnitude Analysis (OMA) may reveal slight modifications to classic results that originate from ideal cases. The present work summarizes OMA by way of example; that is, a reconsideration of the stratified natural convection problem from which a stratified Nusselt number is introduced. Two data sets are presented in support of the described modification. This is then followed by an experimental investigation on penetration of cold liquid into a modeled subassembly channel under mixed convection, from the hot plenum of a protypicaI FBR design. The penetration phenomenon occurs under certain natural circulation conditions during the operation of the DRACS (direct reactor auxilialy cooling system) for decay heat removal and can influence the natural circulation head that determines the core flow rate and consequently the core cooling rate. In the present experiment, a simplified test section simulating the upper plenum and a subassembly channel was constructed wherein both temperature and velocity measurements were made to investigate the penetration of cold water into a vertical channel with upward flowing hot water. Upon comparing temperature and velocily data we found an overall similarity in their respective spatio-temporal distributions as expected. The data suggested correlations describing the onset of flow penetration and ...

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