超临界压力下二氧化碳分层水平流动的稳定性
The stability of stratified horizontal flows of carbon dioxide at supercritical pressures
AI总结：
本文通过阴影成像与壁温测量研究88.5 bar下二氧化碳水平槽道在顶/底加热时的分层稳定性，揭示稳定与不稳定分层对传热的相反作用。
摘要：
超临界压力下的流体在拟临界线附近表现出密度的大幅变化，因此浮力在其流体动力学中起着关键作用。本文通过实验研究了在88.5 bar和32.6°C条件下、水动力充分发展的二氧化碳水平槽道流动中的传热与湍流特性；实验分别从顶面或底面加热，以诱发强烈的垂直密度梯度。为了可视化流动并评估其传热性能，研究同时采用阴影成像技术与表面温度测量。结果表明，在适度加热条件下，两种加热构型中的流动均出现强烈分层，整体Richardson数Ri最高达到100。当二氧化碳从底部向上加热时，所形成的不稳定分层流动由日益显著的热羽流二次运动主导，增强了垂直混合，并使传热相对于中性浮力情形逐步改善。相反，由顶部加热诱发的稳定分层会抑制垂直运动，导致传热恶化，并且这种恶化变得与Reynolds数无关。光学结果为近拟临界区域内依赖方向的复杂传热动力学提供了新的认识。这些认识有助于可靠设计使用高物性变化流体的换热器，而这类换热器对于电力和工业供热脱碳至关重要。然而，结果也强调，需要进一步发展实验技术，以便为更广泛的非理想超临界条件生成可靠的参考数据。
英文摘要：
Fluids at supercritical pressures exhibit large variations in density near the pseudo critical line, such that buoyancy plays a crucial role in their fluid dynamics. Here, we experimentally investigate heat transfer and turbulence in horizontal hydrodynamically developed channel flows of carbon dioxide at 88.5 bar and 32.6°C, heated at either the top or bottom surface to induce a strong vertical density gradient. In order to visualise the flow and evaluate its heat transfer, shadowgraphy is used concurrently with surface temperature measurements. With moderate heating, the flow is found to strongly stratify for both heating configurations, with bulk Richardson numbers Ri reaching up to 100. When the carbon dioxide is heated from the bottom upwards, the resulting unstably stratified flow is found to be dominated by the increasingly prevalent secondary motion of thermal plumes, enhancing vertical mixing and progressively improving heat transfer compared to a neutrally buoyant setting. Conversely, stable stratification, induced by heating from the top, suppresses the vertical motion leading to deteriorated heat transfer that becomes invariant to the Reynolds number. The optical results provide novel insights into the complex dynamics of the directionally dependent heat transfer in the near-pseudo-critical region. These insights contribute to the reliable design of heat exchangers with highly property-variant fluids, which are critical for the decarbonisation of power and industrial heat. However, the results also highlight the need for further progress in the development of experimental techniques to generate reliable reference data for a broader range of non-ideal supercritical conditions.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
