密度成層内自然対流の生成条件と熱伝達率の精密測定

精确测量密度分层内自然对流的发生条件和传热系数

• 批准号: 08650251
• 负责人: 姫野 修廣
• 金额: --
• 依托单位: Shinshu University
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (C)
• 财政年份: 1996
• 资助国家: 日本
• 起止时间: 1996 至 无数据
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-08650251/
• 关键词: 自然対流; 安定密度成層; 二重拡散; 熱伝達

本研究では、主として矩形容器内にサッカロース水溶液の密度成層を形成し、これを加熱熱流束一定、冷却面温度一定の条件で両側面より加熱・冷却を行い、局所および平均熱伝達率の測定を行った。実験は加熱熱流束、初期密度勾配、初期温度、冷却面温度をパラメータに行い、以下の知見を得た。温度差による浮力と濃度差による浮力の比を表すパラメータに従い形成される分離対流層の厚さが決定され、それにより熱伝達率は大きく変化する。一般に層厚さが薄いほど対流が弱いので熱伝達率は小さくなるが、層厚さがある程度以上になると、境界層の先端効果により逆に層厚さの薄い方が熱伝達率は向上する。このことは分離対流層が形成された準定常状態と、溶質の物質伝達により層が消失して全域にわたって濃度が一様となった定常状態との熱伝達率を比較すると、層厚さが薄い場合には分離対流層が存在する場合の方が定常状態に比べて熱伝達率は小さいが、層がある程度以上厚くなると逆に定常状態の熱伝達率よりも大きくなることから確認された。また初期温度と冷却面温度の関係については、初期温度が冷却面温度に近い場合には対流層は加熱面側から形成されるが、初期温度と冷却面温度との温度差が大きくなるにつれ冷却面側からも形成されるようになり、初期対流層に大きな影響を及ぼす。しかし本実験のように熱流束一定の条件下では、層が薄く対流が弱くて熱伝達が悪い層では、加熱・冷却面間の温度差が大きくなり対流の駆動力が増大して層を厚くするように作用するので、対流層の不均一を押さえる方向に働き、初期温度の影響によってそれほど熱伝達率は大きな影響を受けなかった。しかし、壁温一定の条件下では対流層の厚さが不均一となって熱伝達率に大きな影響を及ぼす可能性があり、この点については今後さらに検討する予定である。

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