水平花瓣螺纹管内超临界二氧化碳换热数值研究

Numerical investigation on heat transfer of supercritical carbon dioxide in horizontal petal-ribbed tubes

中文关键词: 花瓣螺纹管  超临界  二氧化碳  增强换热  浮升力  旋流  二次流

英文关键词: petal-ribbed tube  supercritical  carbon dioxide  enhanced heat transfer  buoyancy  swirling flow  secondary flow

中文摘要:

      针对布雷顿循环换热器的增强换热问题，提出了花瓣螺纹管的方案，开展了水平管内超临界二氧化碳换热数值研究．探究了不同运行压力和花瓣数目下的换热机制．通过温度、速度、湍动能分布讨论了浮升力和花瓣旋流耦合下的非均匀换热问题．定量描述了旋流引起二次流的强弱．基于综合换热系数(PEC)表征了花瓣螺纹管相比光滑管的增强换热程度．结果表明：PEC高于1.2，浮升力导致畸变的温度场和速度场．热边界层和速度边界层厚度周向不均匀，通道右上侧湍流增强和换热增强最显著．随着运行压力提高，浮升力和旋流均减弱，两者的耦合作用减弱，花瓣层周向不均匀换热减弱．花瓣数目越多，浮升力越弱，旋流越强，耦合作用越弱，周向换热越均匀．

英文摘要:

      The petal-ribbed tube is proposed to enhance the heat transfer of Brayton cycle heat exchangers, and investigation on heat transfer of supercritical carbon dioxide in horizontal tubes is numerically conducted. The heat transfer mechanisms under different operating pressures and petal numbers are studied. The non-uniform heat transfer under the coupling of buoyancy and petal swirling flow is analyzed through the temperature, velocity and turbulent kinetic energy distributions. The secondary flow strength caused by swirling flow is quantitatively described. The enhanced heat transfer degree of petal-ribbed tube compared to smooth tube is evaluated based on the performance evaluation coefficient (PEC). Results indicate that the PEC is higher than 1.2, and the buoyancy leads to the distorted temperature field and velocity field. The thickness of thermal boundary layer and velocity boundary layer are circumferential non-uniform, and the turbulence enhancement and heat transfer enhancement on the upper-right side of channel are most significant. With the increasing of operating pressure, both buoyancy and swirling flow weaken, and the coupling effect between buoyancy and swirling flow weakens, resulting in a weakening of circumferential non-uniform heat transfer in petal layers. The more petals there are, the weaker the buoyancy, the stronger the swirling flow, the weaker the coupling effect, and thus the more uniform the circumferential heat transfer.