摘要
亚临界沸腾包括界面蒸发和气泡动力学诱导的传热,但超临界传热是否存在类界面蒸发和类气泡传热以及两者间的转换缺少直接的实验证据.本文进行了超临界CO_(2)液池传热的实验研究,压力和液池温度分别为8—10 MPa和15℃.作为加热元件和感温元件,22 mm长和70μm直径的镍铬丝水平放置在液池中,光纤探针垂直放置,其顶端高于镍铬丝200μm.发现随热流密度或壁面过热度的持续增大,依次发生自然对流、类界面蒸发、类蒸发-沸腾转换、类沸腾4种传热模式.本文重点关注类界面蒸发和类沸腾传热以及两者间的转换.在类界面蒸发模式下,传热系数随壁面过热度增大略有下降,光纤输出小幅/高频信号,不存在主频,多尺度熵大,表征随机信号波动.在类蒸发-沸腾转换模式下,光纤输出大幅/低频周期信号,存在明显主频,多尺度熵小,代表有序的周期性脉动传热.在以类气泡为特征的类沸腾模式下,光纤信号波动幅度介于类蒸发和转换模式之间,主频不明显,多尺度熵也介于类蒸发和转换模式之间.研究获得了超临界类沸腾直接的实验证据,加深了对超临界传热机理的理解,为后续理论研究和工程应用提供了基础.
Subcritical boiling includes interfacial evaporation and heat transfer induced by bubble dynamics.However,for supercritical heat transfer,direct experimental evidence of the existence of pseudo-evaporation and pseudoboiling heat transfer,as well as the conversion between them is lacking.In this work,the experimental study of supercritical carbon dioxide pool heat transfer is conducted.The pressure and temperature of the cell are 8-10 MPa and 15 ℃,respectively.As heating element and temperature sensing element,a nickel-chromium alloy wire with a length of 22 mm and diameter of 70 μm is placed horizontally in the high-pressure cell.The fiber optic probe is placed vertically,with its tip 200 μm above the wire.Four heat transfer modes,i.e.natural convection,pseudo-evaporation,transition of evaporation and boiling,and pseudo-boiling,are found to occur sequentially with the increase of heat flux density or wall superheat.Natural convection occurs when the wall temperature is below the pseudo-critical temperature.This work focuses on pseudo-evaporation and pseudoboiling heat transfer and the transition between them.In the pseudo-evaporation mode,the heat transfer coefficient decreases slightly with wall superheat increasing.The fiber outputs a high frequency signal with small amplitude,and there is no dominant frequency.The multiscale entropy is large,characterizing random signal fluctuations.In the transition of evaporation and boiling mode,the fiber outputs a large-amplitude/lowfrequency periodic signal with a significant dominant frequency and small multiscale entropy,representing an ordered periodic pulsating heat transfer.In the pseudo-boiling mode characterized by bubble-like structure,the fiber signal fluctuation amplitude and multiscale entropy are between the counterparts of the first two modes i.e.natural convection mode and pseudo-evaporation mode.The dominant frequency is not obvious.The multiscale entropies in the specific case are calculated under different key parameters,such as dimensionality,time scale factor,and length of origin data.Optimal parameters are selected based on the best separation of heat transfer modes.Finally,pseudo-boiling can be distinguished from pseudo-evaporation by multiscale entropy of 0.9,and from transition of evaporation and boiling by multiscale entropy of 0.5.In this work,direct experimental evidence of supercritical-like boiling is obtained,which deepens the understanding of the supercritical heat transfer mechanism and provides a basis for theoretical studies and engineering applications in future.
作者
何孝天
徐进良
程怡玮
He Xiao-Tian;Xu Jin-Liang;Cheng Yi-Wei(Beijing Key Laboratory of Multiphase Flow and Heat Transfer for Low Grade Energy Utilization,North China Electric Power University,Beijing 102206,China;Key Laboratory of Power Station Energy Transfer Conversion and System,Ministry of Education,North China Electric Power University,Beijing 102206,China)
出处
《物理学报》
SCIE
EI
CAS
CSCD
北大核心
2023年第5期405-415,共11页
Acta Physica Sinica
基金
国家自然科学基金重点项目(批准号:52130608)
国家自然科学基金创新研究群体科学基金(批准号:51821004)资助的课题。
关键词
超临界流体
类沸腾
光纤探针
多尺度熵
supercritical fluid
pseudo boiling
fiber optic
multiscale entropy
