Abstract

Critical heat flux (CHF) is an important heat transfer deterioration phenomenon during boiling heat transfer. It has been extensively studied, especially in the field of nuclear energy. Previous research has found that downward-facing heat transfer is worse than upward-facing and should be paid more attention. In this paper, the boiling heat transfer process under different flow rates and inlet distances is investigated. Seven experimental cases were made including a pool boiling case. The experiment studied the effect of inlet distances under small flow rates, which is not covered by previous research. Analysis of the CHF mechanism included surface temperature curves, boiling curves, bubble behaviors, and heat transfer coefficient. The fluctuation of the surface temperature of forced convention cases was observed due to the bubbles sliding along the heating surface. The phenomenon of vapor film fragmentation could also be found. The results show that even at small flow rates, CHF occurring time is postponed and the CHF value increases. Reducing inlet distance or increasing flow rate can both promote boiling heat transfer, thereby enhancing CHF.

摘要

临界热通量(CHF)是沸腾传热过程中重要的传热退化现象。它已被广泛研究，特别是在核能领域。以往的研究发现，朝下的传热比朝上的传热差，应该引起更多的关注。本文研究了不同流速和入口距离下的沸腾传热过程。制作了七个实验案例，包括一个池沸腾案例。该实验研究了小流量下入口距离的影响，这是以前的研究未涵盖的。CHF机理分析包括表面温度曲线、沸腾曲线、气泡行为和传热系数。由于气泡沿受热面滑动，观察到强制对流情况下表面温度的波动。还可以发现汽膜破碎现象。结果表明，即使在小流量下，CHF 发生时间也会推迟，CHF 值会增加。减少入口距离或增加流速都可以促进沸腾传热，从而提高 CHF。