临界热流密度物理机理实验理论研究

Accurately predicting critical heat flux is paramount important to well design and safely operate a nuclear power plant. However, key governing principles of critical heat flux occurrence are still unclear and existing theoretical prediction models of critical heat flux are insufficient. The main difficulties consist in: under high heat flux conditions close to the critical heat flux, how to observe the boiling phenomena clearly; and how to obtain the nucleation site density on the heater surface and quantify the instability characteristics of thin liquid film adjacent the heated wall. In order to overcome the difficulties mentioned above, this study will develop an innovative experimental method "thin liquid film boiling" which will provide the rationale to observe the thin liquid film instability and bubbles' dynamcs by a high speed camera directly and measure the thin liquid film characteristic thickness by a non-invasive liquid film thicknsess dianostic technique of confocal optical sensor under high heat flux conditions while without losing the key physics of boiling and critical heat flux. Based on the experimental results, it is expected to discover the physical mechanisms of boiling heat transfer, especially the physical mechanisms of critical heat flux, so as to develop a newly theoretical model for the better prediction of critical heat flux. In addition,two methods for heat transfer enhancement and critical heat flux enhancement will be investigated to understand their basic physical mechanisms, one is decreasing the contact angle of heater surface and the other is fabricating an optimized artificial cavity pattern.

临界热流密度的准确预测对于核电的设计和安全运行都有着重要的意义。然而对于临界热流密度触发的物理机理还没有清晰的认识,理论预测模型还不完善。这一研究的主要难点在于：在高热流密度的工况下，如何观察近加热壁面的沸腾现象并准确测量汽化核密度和如何获得近壁面薄液层的动态特性。为了克服这些困难，本项研究将开发一种创新性的实验研究方法"液膜沸腾实验",从而采用高速摄像系统直接观测高热负荷下近加热壁面的气泡动态特性和液膜不稳定特性，同时采用共焦光学探针这一非接触式测量技术测量近加热壁面液膜厚度的动态特征。进而深入揭示沸腾传热的物理机理，特别是临界热流密度触发的物理机理，以此来建立更具有物理意义、更加准确实用的临界热流密度预测模型，为强化临界热流密度提供理论依据和技术建议，并研讨降低表面接触角和优化人工汽化核阵列对于强化沸腾传热和临界热流密度的影响作用。

临界热流密度的准确预测对于核电的设计和安全运行都有着重要的意义。然而对于临界热流密度触发的物理机理还没有清晰的认识,理论预测模型还不完善。这一研究的主要难点在于：在高热流密度的工况下，如何观察近加热壁面的沸腾现象并准确测量汽化核密度和如何获得近壁面薄液层的动态特性。..本项研究，开发了一种创新性的实验研究方法“液膜沸腾实验”， 通过液膜沸腾实验与池沸腾实验的临界热流密度的测量对比，验证了“尺度分离理论”，为采用液膜沸腾来研究临界热流密度奠定了理论基础。在液膜沸腾实验中，采用高速摄像系统直接观察了高热负荷下近加热壁面的气泡动态特性和液膜不稳定特性，阐述了气泡底下干斑的形成、润湿和扩张的动态特性与热流密度的关系，揭示了临界热流密度触发的物理机理；同时，采用共焦光学探针这一非接触式测量技术测量了近加热壁面液膜厚度的动态特征，获得了三种液层（micro-layer、macro-layer、bulk-layer）特征厚度随热流密度的变化关系，并建立了Macro-layer的经验关系式，为临界热流密度物理机理模型的建立奠定了重要的基础。..采用电化学方法改变金属表面和纳米流体的方法，探索了强化临界热流密度的方法，初步探讨了这些技术在核能系统中的应用前景。研究了石墨烯和石墨膜材料的优良传热特性，发现了该材料在沸腾过程中对于传热具有良好的适应性，当被使用作为沸腾表面时，通过自身形貌的改变能大大提高临界热流密度，将来有希望在热科学领域内得到广泛应用。