Zr-4表面特性及冷却剂过冷度 对骤冷沸腾传热的影响

Effect of Surface Characteristics of Zr-4 and Subcooling Degree of Coolant on Heat Transfer of Quenching Boiling

核燃料棒再淹没骤冷沸腾是堆芯失水事故后避免堆芯熔化采取的重要事故缓释措施。本文采用不同粒度砂纸打磨得到不同粗糙度的Zr-4表面,研究了Zr-4表面粗糙度和冷却剂过冷度对骤冷沸腾过程冷却速率及沸腾换热的影响。对所制备的不同表面粗糙度实验段进行了骤冷沸腾可视化实验,同时测量实验段内部温度,通过导热反问题反演得到骤冷过程表面温度及热流密度。结果表明:表面粗糙度对膜态沸腾换热的影响较小,但粗糙度较大的表面更早地触发了表面-液体接触,强化了骤冷沸腾;而粗糙度较小时,粗糙度对骤冷沸腾的影响较小;当粗糙度进一步减小时,由于表面接触角的增大,骤冷沸腾持续的时间增长。此外,随着冷却剂过冷度的增大,膜态沸腾气膜厚度减薄,维持稳定气膜的最小膜态沸腾温度增大,骤冷速率增强。本文为揭示压水堆破口事故燃料棒再淹没过程的流动沸腾换热机理提供一定的理论基础。

During a postulated accident in water-cooled nuclear reactors,such as loss-of-coolant accident,the reactor core can be partially or completely uncovered from coolant and consequently becomes overheated.The cold liquid is reinjected into the nuclear reactor core to effectively cool the superheated fuel rods.In this process,the quenching boiling will be encountered in the fuel rod surface.The reflooding quenching boiling of nuclear fuel rod is an important accident mitigation measure to avoid the core melting after the loss-of-coolant accident of the core of pressurized water reactor.In this work,quenching experiments of Zr-4 alloy with different roughness were performed in subcooled distilled water pool at atmospheric pressure.The surface of Zr-4 was polished by abrasive paper with different mesh numbers.The effects of Zr-4 surface roughness and coolant subcooling degree on cooling rate and boiling heat transfer during the quenching boiling were studied.The visualization experiments of quenching boiling were carried out on different surface roughness test sections,and the internal temperatures of test sections were measured at the same time.The image processing technique was used to obtain the evolution of vapor film thickness during quenching boiling of fuel rod and the inverse heat conduction problem was employed to obtain the surface temperature and heat flux during the quenching.The experimental results show that the surface roughness has little effect on the film boiling heat transfer.However,the surface with higher roughness triggers the surface-liquid contact earlier and strengthens the quenching boiling.When the roughness is small,the effect of roughness on quenching boiling is weak.When the roughness is further reduced,the duration of quenching boiling increases due to the increase of the surface contact angle.In addition,with the increase of the liquid subcooling,the vapor thickness of film boiling decreases,the minimum film boiling temperature to maintain stable vapor film increases,and the quenching rate increases.Based on the experimental data,the relationship between the minimum film boiling temperature(T min)and the coolant subcooling degree(T sub)is established with the average error of 2.4%.This study provides a theoretical basis for the boiling heat transfer mechanism during the reflooding process of nuclear reactor core at a loss-of-coolant accident.