1.Beijing Key Laboratory of Multiphase Flow and Heat Transfer for Low Grade Energy, North China Electric Power University, Beijing 102206, China 2.Key Laboratory of Power Station Energy Transfer Conversion and System, Ministry of Education,North China Electric Power University, Beijing 102206, China
Fund Project:Project supported by the National Key R&D Program of China (Grant No. 2017YFB0601801) and the Fundamental Research Funds for the Central Universities, China (Grant No. 2019QN032)
Received Date:17 September 2020
Accepted Date:08 October 2020
Available Online:06 February 2021
Published Online:20 February 2021
Abstract:Supercritical fluids are widely used in engineering technology, and the flow and heat transfer characteristics are very important for engineering design. However, due to the fact that the physical micro- and macroscopic behaviors of supercritical fluids are still open, neither the heat transfer mechanism nor the flow mechanism of supercritical fluids has been well revealed. It is widely believed that liquid-like (LL) and gas-like (GL) supercritical fluid are two phases distinguishable on a molecular scale. Only recently, has it become clear that the macroscopic transition from LL to GL supercritical state, when crossing the Widomline, is successfully detected in experiment, and explained based on the pseudo-boiling concept. In this paper, the abnormal flow and heat transfer behavior of supercritical CO2 are studied based on the pseudo-boiling theory. On the assumption that the transition from LL to GL is heterogeneous, an analysis method for pseudo-boiling heat transfer is developed from classical dimensional analysis and subcritical subcooled boiling theory of models. To analyze the pseudo-boiling resulting in heat transfer deterioration process of supercritical fluid, two dimensionless numbers which are π = (qw·ρl)/(G·Δi·ρg) and π13 = (qw·βpc·di)/λg are proposed to explain the anomalous heat transfer characteristics in vertical upward heating flow. The former π reflects the rate of conversion between gas-like and liquid-like fluid. The larger gas-like conversion rate promotes the rapid production of more high-temperature fluid in the near-wall region, and the latter π13 characterizes the temperature gradient of gas-like film near the wall: the larger temperature gradient causes the gas film to cover the wall surface. The heat transfer deterioration may occur when the cooler liquid-like fluid of the core region cannot rewet the hot wall adequately. The new dimensionless numbers can successfully explain the heat transfer deterioration of supercritical fluid flow induced by pseudo-boiling. Our work paves the way to understanding the heat transfer and flow for supercritical fluids which establishes a relation among the internal flow, heat transfer field characteristics, boundary conditions and physical properties based on the pseudo-boiling theory preliminarily. The results of dimensional analysis can be applied to the similarity theory analysis of different fluids, which is of significance for promoting the theoretical research of supercritical fluid heat transfer on the basis of pseudo-boiling concept. Keywords:supercritical carbon dioxide/ pseudo-boiling/ heat transfer deterioration/ dimensional analysis
其中, 类气膜密度ρg的定性温度为(Tb+Twi)/2, 类液密度ρl的定性温度为Tb, 如果类气膜的厚度很小, 类气膜的密度定性温度也可采用Tw估计, βpc为拟临界温度下的膨胀系数. 在图7中, 红色代表正常传热, 黑色代表恶化传热, 当热流密度为239.1 kW/m2时, 传热发生明显恶化时(见图7(a)), 对应的类气膜内的温度梯度也同样存在1个先增大后减小的趋势(见图7(b)), 这表明在恶化时, 近壁区物性剧烈变化, 较大的温度梯度使类气膜覆盖在壁面上, 热量集聚在近壁区. 但是, 对于低热流密度的正常传热, 类气膜内的温度梯度相对较小, 而是随着焓值缓慢地增大, 传热没有明显的恶化现象. 图 7 正常和恶化传热下类气膜内的温度梯度和内壁温随焓值分布 Figure7. Distribution of temperature gradients and inner wall temperature with enthalpy in gas-like film under normal heat transfer(NHT) and heat transfer deterioration(HTD).
这个无量纲数表征了类气膜的径向生长速度qw/(ρg·Δi)和主流流体的平均速度G/ρl之比. 如图8(a)所示, 给出了正常传热和恶化传热下的新无量纲数π、内壁温和换热系数随焓值分布, 对于正常传热, 类气膜生长速度相对于主流流体的速度较小, 热量被及时带走, 因此, 传热没有明显的恶化现象. 但是, 当类气膜生长速度相对较大时, 主流流体没有及时通过对流带走近壁区的热量, 热量集聚在近壁面, 传热恶化发生, 如图8(b)所示, 在传热恶化时, 这个无量纲数同样出现1个峰值. 图 8 正常传热和恶化传热下的新无量纲数π、内壁温Twi和换热系数h随焓值分布 Figure8. Distribution of the new dimensionless numberπ, inner wall temperature Twi and heat transfer coefficient h with enthalpy under normal heat transfer (NHT) and deteriorated heat transfer (HTD).
5.新无量纲数描述超临界流体拟沸腾作用机制采用新无量纲数描述拟沸腾诱导SCF传热恶化过程, 如图9(a)所示, 当传热发生明显的恶化时, 管内类气膜膨胀变厚, 在近壁区形成1个凸起. 如图9(b)所示, 当传热恶化时, 较厚的热类气膜覆盖在壁面, 管中心为相对较冷的类液流体, 较冷的类液流体不能充分润湿壁面导致传热恶化, 当类气膜变薄时, 传热又发生恢复, 类气膜越厚, 传热恶化越严重. 图9(c)给出了新无量纲数π和π13在管内轴向距离的局部分布, 较大的π表明较大的类气膜生长速度, 热量没有被及时带走, 而较大的类气膜温度梯度使类气膜覆盖在近壁区, 在峰值附近出现了1个峰值, 这个值越大, 流动越接近亚临界过冷沸腾过程, 故传热恶化, 这两个无量纲数一个表征类气膜生长速度与主流平均速度大小, 一个表征类气膜内的温度梯度大小. 图 9 新无量纲数作用超临界流体拟沸腾传热恶化机制 Figure9. New dimensionless action on number deterioration of pseudo-boiling heat transfer mechanism.