Flow boiling heat transfer tests were conducted to evaluate the Critical Heat Flux (CHF) and Heat Transfer Coefficient (HTC) of conventional and accident tolerant fuel (ATF) cladding materials, i.e., bare Zircaloy-4 (Zr4), and Zircaloy-4 coated with Chromium using physical vapor deposition (PVD) (Zr4-Cr-PVD) and cold spray process (Zr4-Cr-CS), respectively. The tests were performed on a single heater rod with a uniform heat flux profile, at the atmospheric pressure, inlet temperature (24 °C) and mass flow rate (750 kg/m²s). HTC’s were increased by 5.2% and 3.3% on Zr4-Cr-CS compared to bare Zr4 and Zr4-Cr-PVD. Improved HTCwas attributed to the increased void fraction and high roughness (Ra = 532 nm) of the Zr4-Cr-CS. In detail, the micro-cavities, which act as bubble seeds, were entirely distributed on the Zr4-Cr-CS, and these micro-cavities generated smaller and faster bubbles, thus the void fraction increased by 10.9% compared to bare Zr4. HTC is improved by these smaller and faster bubbles which can remove heat from the heater rod surface more efficiently. To identify the exact location of CHF, the surface temperature profile was measured using advanced fiber-optic sensors which have high temporal/spatial resolution (distance between point to point: ~2.5 mm, frequency ~100 Hz). CHF occurred at 80–95% along the heated length and showed an 11.6% reduction on Zr4-Cr-CS compared to bare Zr4. We hypothesize the CHF reduction was caused by the lower wettability of the Zr4-Cr-CS which reduces the liquid supply to the surface, and vigorous bubble accumulation owing to high void fraction near the outlet, which results in early formation of the vapor film. During the post-CHF quenching phase, the rougher Zr4-Cr-CS showed larger cooling rates compared to the bare Zr4 and Zr4-Cr-PVD, preventing of oxidation by chromium layer. In conclusion, we demonstrate that the high roughness on the Zr4-Cr-CS increased the number of micro-cavities on the surface, resulting in 5.2% increase in HTC and improvement in the quenching heat transfer performance whereas CHF was 11.6% reduced compared to bare Zr4. Overall statistically there is little effect in the coating with regards to CHF and slight improvement in HTC.

进行流沸腾传热测试以评估常规和耐事故性燃料（ATF）包层材料（即裸Zircaloy-4（Zr4）和Zircaloy-4涂层）的临界热通量（CHF）和传热系数（HTC）分别使用物理气相沉积（PVD）（Zr4-Cr-PVD）和冷喷涂工艺（Zr4-Cr-CS）镀铬。测试是在大气压，入口温度（24°C）和质量流量（750 kg / m ²）下，在具有均匀热通量曲线的单个加热棒上进行的s）。与裸Zr4和Zr4-Cr-PVD相比，Zr4-Cr-CS上的HTC分别提高了5.2％和3.3％。改进的HTC归因于Zr4-Cr-CS的增加的空隙率和高粗糙度（Ra = 532 nm）。详细地讲，充当气泡种子的微腔完全分布在Zr4-Cr-CS上，这些微腔产生的气泡更小，更快，因此与裸Zr4相比，空隙率提高了10.9％。这些更小，更快的气泡可以改善HTC，这些气泡可以更有效地从加热棒表面清除热量。为了确定CHF的确切位置，使用具有高时空分辨率（点对点之间的距离：〜2.5 mm，频率〜100 Hz）的高级光纤传感器测量了表面温度曲线。CHF沿加热长度的发生率为80–95％，显示为11。与裸Zr4相比，Zr4-Cr-CS减少了6％。我们推测，CHF降低是由于Zr4-Cr-CS的较低润湿性而引起的，该润湿性降低了向表面的液体供应，并且由于出口附近的高空隙率而导致了剧烈的气泡积聚，这导致了早期的蒸汽膜形成。在CHF后淬火阶段，与裸露的Zr4和Zr4-Cr-PVD相比，较粗糙的Zr4-Cr-CS显示出更大的冷却速率，从而防止了铬层的氧化。总之，我们证明Zr4-Cr-CS上的高粗糙度增加了表面上的微腔数量，导致HTC增大了5.2％，淬火传热性能提高，而CHF相比降低了11.6％。裸Zr4。