不同状态方程对超临界二氧化碳强迫对流传热中流动加速因子的影响

明显的流动加速效应对超临界二氧化碳强迫对流传热有重要影响。流动加速因子是表征流动加速效应强度的重要无量纲数，在建立流动加速因子过程中需要用到二氧化碳状态方程。理论分析了选取理想气体状态方程和van der Waals方程对建立的流动加速因子合理性的影响，并结合实验数据进行了评估。结果表明，基于van der Waals 方程建立的流动加速因子可以较好地预测流动加速效应引起的传热恶化区域，而基于理想气体状态方程建立的流动加速因子与实验结果存在较大偏差。分析表明体积膨胀系数和体积压缩系数反映了流动加速现象的本质，以体积膨胀系数和体积压缩系数来建立流动加速因子更合理。      

竖直圆管内超临界二氧化碳强迫对流传热实验研究

为分析超临界二氧化碳强迫对流传热特性,开展竖直圆管内超临界二氧化碳强迫对流传热特性实验研究。实验结果表明:实验参数范围内存在明显的浮升力效应和流动加速效应;对于向上流动工况,随浮升力效应增强超临界二氧化碳从强迫对流传热过渡至混合对流传热,最后发展为自然对流传热,传热能力有弱化到逐渐恢复直至强化;对于向上流和向下流动工况,流动加速效应皆弱化传热。基于实验数据建立新的超临界流体传热关联式,在实验工况范围内95.03%的预测值与实验值偏差为±30%以内。     

竖直圆管内超临界流体混合对流传热特性理论分析

理论推导了变密度引起的浮升力效应和流动加速效应对超临界流体混合对流传热特性的影响。结果表明,浮升力效应和流动加速效应通过改变壁面边界层外缘的切应力影响湍流对传热传质的贡献,进而改变超临界流体混合对流传热特性。浮升力效应通常在加热区域入口及上游区域表现明显,而流动加速效应在主流区流体温度达到拟临界温度时更显著。与实验研究结果对比发现,新建立的浮升力因子和流动加速因子可较好地预测竖直圆管内超临界流体混合对流条件下拟临界区域的局部传热特性。     

浮升力效应和流动加速效应对超临界二氧化碳传热影响理论分析

从边界层基本特性出发，分析了拟临界区强变物性导致的浮升力效应和流动加速效应对近壁面区域超临界二氧化碳传热特性的影响机理，基于受力分析推导得到了2种效应作用下的超临界流体传热弱化起始点理论判据。研究结果表明，对于加热工况竖直向上流动，浮升力效应和流动加速效应均会导致近壁面区域切应力减弱，进而影响近壁面区湍流的生成与扩散，最终导致传热弱化；2种效应作用下传热弱化起始点判据分别为浮升力因子Bu=1.16×10-5和流动加速因子Ac=2.91×10-6，上述阈值与实验结果吻合良好。      

过冷流动沸腾汽泡浮升直径的理论研究

汽泡浮升直径模型已成为两相流领域理论分析与数值计算方法的重要子模型。为研究各力对汽泡浮升的影响规律,本文理论推导了过冷流动沸腾汽泡的受力方程,建立了预测汽泡浮升直径的无量纲模型,并与实验数据进行了对比验证,分析了汽泡浮升直径随各无量纲数的变化规律。结果表明,无量纲模型能准确预测水与R113工质中汽泡的浮升直径;浮升直径随Ja的增加而升高,随Re、Ca、Pr与Ar的增加而降低;结合无量纲数的定义,可认为生长力与表面张力抑制了汽泡的浮升效应,导致浮升直径升高;剪切升力与浮升力促进了汽泡的浮升效应,导致浮升直径降低。     

垂直圆管内超临界R134a对流传热实验研究

为研究超临界压力下的对流传热特性,对超临界压力氟利昂R134a在内径25 mm垂直圆管中的受热上升流动传热进行了实验研究,获得压力4.5 MPa和4.7 MPa、质量流速G=400~700 kg/(m2·s)、热流密度q=30~60 k W/m2的实验数据,对换热强化和传热恶化的规律和特性以及其影响因素进行了分析。结果发现,在拟临界区附近,超临界压力R134a出现明显的强化换热现象。在低质量流速或高热流密度下发生传热恶化,其恶化边界为q/G0.06 k J/kg。在特定的工况下观察到两次传热恶化:第一次发生在临近入口区域,在不同流体入口温度下均观测到恶化;第二次发生在远离入口区域,仅在一定流体焓值范围内存在。实验参数敏感性分析表明,传热强化随质量流速的增加、热流密度的减小、压力的降低而增加,而传热恶化则相反。     

管束效应对含空气蒸汽冷凝传热影响数值分析

采用数值模拟的方法分析管束效应对管外含空气蒸汽冷凝传热的影响。基于3×3管束，分析了管间距在1.5d~5d（d为管径）范围内的管束效应及管间距对局部和平均冷凝传热性能的影响。在管间距为1.5d条件下讨论了管束结构对冷凝传热性能的影响。结果表明，管束效应包括高浓度空气层的抑制传热作用和管束抽吸效应的强化传热作用。随着管间距的减小，第2类和第3类传热管主要受高浓度空气层的影响，第1类传热管主要受管束抽吸效应的影响。当管间距为1.5d时，第2类和第3类传热管的传热系数分别比单管恶化了6%和29%，而第1类传热管比单管增加了2.5%；在1.5d管间距条件下，管束抽吸效应随管列数的增加而明显增大，导致管束平均冷凝传热系数（hb）逐渐增大。当管列数达到20列时，hb高于单管。      

矩形窄缝通道近壁汽泡滑移和浮升可视化实验研究

采用高速摄像仪从宽面和窄面拍摄、观察矩形窄缝通道内近壁汽泡滑移、浮升运动,发现在加热面倾斜朝上和竖直向下流动沸腾时汽泡易于浮升。汽泡浮升后,其运动速度迅速增加。由于浮升汽泡处于过冷流体中,其界面上发生冷凝,导致汽泡界面形状变化较大。基于可视化实验结果,从受力角度分析不同流动方式和加热面放置方式对近壁汽泡滑移和浮升的影响。     

基于加速度效应的超临界水传热特性模型研究

超临界水冷堆（SCWR）运行在水的热力学临界点（22.1 MPa，374℃）之上，堆内冷却剂处于超临界状态，物性变化剧烈，与常规压水堆临界热流密度（CHF）导致包壳表面壁温飞升不同，超临界压力下的传热恶化是在变物性的影响下使得包壳表面温度相对缓慢上升，传统的热点判定方法和偏离泡核沸腾比（DNBR）限值等传热特性分析方法不再完全适用，因此，预测超临界水传热恶化时包壳壁温对SCWR的安全分析相当重要。本文基于边界层方程推导了超临界水传热关系式的加速度效应修正项，基于圆管实验数据，对加速度效应修正项的相关系数进行拟合获得超临界水传热特性半经验关系式，通过数据对比，该关系式在正常传热和传热恶化工况下均具有较好的适用性。本文获得的超临界水传热特性半经验关系式可为SCWR堆芯设计分析提供支持。      

超临界压力下竖直圆管内不同流体的传热特性

为了深入认识超临界压力下不同流体传热中的共性反映出的传热机理及物性导致的特性差异,以水和氟利昂R134a为工质分别在SWAMUP回路和SMOTH回路上开展了竖直圆管内上升流传热试验。在正常传热、传热强化、小质量流速时浮升力导致传热恶化和大质量流速时加速效应导致传热恶化的工况中,氟利昂和水的换热系数（HTC）随无量纲温度表现出一致的变化规律。浮升力无量纲数_πB增大,换热系数与经典关系式计算值之比减小;加速效应无量纲数_πA较小时,换热系数比随_πA的增大而增大,达到峰值后换热系数比随_πA的增大而减小。_πB对超临界水试验数据的相关性更佳,而_πA对超临界氟利昂试验数据的相关性更好。无量纲数表征的超临界压力下传热规律的高度相似性初步验证了以模化流体氟利昂R134a研究超临界水传热特性是合理可行的。     

Numerical analysis of thermal-hydraulic behavior of supercritical water in vertical upward/downward flow channels

Investigations on the thermal-hydraulic behavior in the SCWR fuel assembly have obtained a significant attention in the international SCWR community. However, there is still a lack of understanding of the heat transfer behavior of supercritical fluids. In this paper, the numerical analysis is carried out to study the thermal-hydraulic behaviour in vertical sub-channels cooled by supercritical water. Remarkable differences in characteristics of secondary flow are found, especially in square lattice, between the upward flow and downward flow. The turbulence mixing across sub-channel gap for downward flow is much stronger than that for upward flow in wide lattice when the bulk temperature is lower than pseudo-critical point temperature. For downward flow, heat transfer deterioration phenomenon is suppressed with respect to the case of upward flow at the same conditions.     

Heat transfer to a supercritical pressure fluid flowing in sub-bundle channels

Supercritical pressure water cooled reactor (SCWR) has been regarded as an innovative nuclear reactor. For the design and development of the SCWR, heat transfer performance under supercritical pressure is one of the most important indicators. In this paper, experimental data are presented on the heat transfer to a supercritical pressure fluid flowing vertically upward and downward in a small diameter heated tube and two sub-bundle channels with three heater rods and seven heater rods, using HCFC22 as the test fluid. Downstream of grid spacer for the sub-bundles, heat transfer enhancement was observed in the upward flow, but not in the downward flow. The enhancement was remarkable especially when the heat transfer deterioration occurs in the fully developed region unaffected by the spacer. The heat transfer correlation for the downstream region of the spacer was developed in the normal heat transfer of sub-bundles. In the fully developed region for the sub-bundle, occurrence of the heat transfer deterioration was suppressed or degree of the deterioration was moderated. At high mass velocity for downward flow in the seven rod sub-bundle, oscillation of heat transfer was observed in the region of the enthalpy over the pseudocritical point.     

超临界水四棒束传热数值分析

超临界水冷堆(SCWR)开发的关键是棒束内超临界水(SCW)的热工水力特性。本文针对超临界水四棒束流动传热实验进行CFD数值模拟,SSG湍流模型的计算结果与实验结果吻合良好。分析结果表明,流动方向对棒束截面内流量分布有显著影响。与下降流相比,尽管上升流时棒束间流动搅混较弱,但上升流时棒束截面流量及壁面周向温度分布更加均匀,加热棒壁面温度更低。可见,棒束横截面上的流量分布是影响加热棒壁面流动传热的主要因素。     

两流程2×2棒束超临界水传热实验研究

为支持欧盟2×2燃料组件入堆考核实验,开展了2×2棒束内超临界水传热实验研究。设计了两流程实验段,两流程之间通过矩形金属框隔离。2×2加热棒束通过定位格架安装在金属框中,超临界水通过第1流程向下流动然后进入第2流程冷却加热棒,第1流程通过金属框吸收第2流程流体的热量而温度升高。在2×2棒束横截面上存在明显的周向温度分布不均匀性。系统分析了实验参数包括质量流速、热流密度和实验压力对棒束平均换热系数的影响。结果表明,系统参数对棒束传热的影响规律与对单管和环管的一致。     

Numerical investigation of acceleration effect on heat transfer deterioration phenomenon in supercritical water

It is important to understand the heat transfer deterioration (HTD) phenomenon for specifying cladding temperature limits in the fuel assembly design of supercritical water-cooled reactor (SCWR). In this study, a numerical investigation of heat transfer in supercritical water flowing through vertical tube with high mass flux and high heat flux is performed by using six low-Reynolds number turbulence models. The capabilities of the addressed models in predicting the observed phenomena of experimental study are shortly analyzed. Mechanisms of the effect of flow structures and fluid properties on heat transfer deterioration phenomenon are also discussed. Numerical results have shown that the turbulence is significantly suppressed when the large-property-variation region spreads to the buffer layer near the wall region, resulting in heat transfer deterioration phenomenon. The property variations of dynamic viscosity and specific heat capacity in supercritical water can impair the deterioration in heat transfer, while the decrease of thermal conductivity contributes to the deterioration.     

Mixed convection heat transfer to carbon dioxide flowing upward and downward in a vertical tube and an annular channel

This paper addresses three main subjects in supercritical heat transfer: (1) difference in thermal characteristics between upward and downward flows; (2) effect of simulating flow channel shape; (3) evaluation of the existing supercritical heat transfer correlations. To achieve the objectives, a series of experiments was carried out with CO₂ flowing upward and downward in a circular tube with an inner diameter of 4.57 mm and an annular channel created between a tube with an inner diameter of 10 mm and a heater rod with an outer diameter of 8 mm. The working fluid, CO₂, has been regarded as an appropriate modeling fluid for water, primarily because of their similarity in property variations against reduced temperatures. The mass flux ranged from 400 to 1200 kg/m² s. The heat flux was varied between 30 and 140 kW/m² so that the pseudo-critical point was located in the middle of the heated section at a given mass flux. The measurements were made at a pressure of 8.12 MPa, which corresponds to 110% of the critical pressure of CO₂. The difference between the upward and downward flows was observed clearly. The heat transfer deterioration was observed in the downward flow through an annular subchannel over the region beyond the critical point. Several well-known correlations were evaluated against the experimental data, and new correlations were suggested for both a tube and an annular channel.     

Heat transfer in two-component internal mist cooling: Part II. Mechanistic modeling

The prediction of a mechanistic, three-dimensional, two-phase flow model is compared with experimental heat transfer data presented in the experimental part of this study for steady, internal, nozzle-generated, gas/liquid mist flow in vertical channels. The mechanistic model is based on the modification of the KIVA-3V computer code. The KIVA-3V code has been modified to solve the heat conduction equation in the surrounding structure with either steady or pulsed heat generation simultaneously with the fluid transport equations, and allow modeling of the various channel geometries and droplet injection methods. Among the numerically examined operating and design parameters are: the liquid atomization nozzle design, heat flux, carrier gas velocity and inlet temperature, liquid mass fraction at inlet, and flow direction. Comparison is made between the experimental data for wall and fluid bulk temperatures and heat transfer coefficients, and the predictions of the numerical model. Overall, reasonable agreement is obtained for downward mist flow, in particular at moderate heat fluxes; at high heat fluxes, the model slightly underpredicts the local heat transfer coefficients. For upward mist flow, the model underpredicts the local heat transfer coefficients typically by about 20%, and appears to predict dryout at the test section exit earlier than experiment. Some parametric and sensitivity calculation results are also presented and discussed.     

Numerical study on flow and heat transfer characteristics in the rod bundle channels under super critical pressure condition

In this study, the 3D flow and heat transfer characteristics in rod bundle channels of the super critical water-cooled reactor were numerically investigated using CFX codes. Different turbulent models were evaluated and the flow and heat transfer characteristics in different typical channels were obtained. The effect of pitch-to-diameter ratio (P/D) on the distributions of surface temperature and heat transfer coefficient (HTC) was analysed. For typical quadrilateral channel, it was found that HTC increases with P/D first and then decreases significantly when P/D is <1.4. There exists a “flat region” at the maximum value when P/D is 1.4. If P/D is larger than 1.4, heat transfer deterioration (HTD) occurs as main stream enthalpy is quite small. Furthermore, the HTD under low mass flow rate and the non-uniformity of circumferential temperature were also discussed.     

带绕丝2×2棒束超临界水传热实验研究

针对两流程带绕丝定位的2×2棒束实验本体,开展了超临界水的流动传热实验研究。实验结果表明,在带绕丝的2×2棒束中,同一横截面上存在显著的周向温度不均匀现象。在较小的G/q工况下出现传热恶化现象,且存在二次恶化现象。由于绕丝的搅混作用,流道下游的传热恶化得到抑制。实验表明,棒束传热规律与单管类似,对比结果指出Bishop等（1964）的计算公式能较好地预测实验结果。同时与不带格架的2×2棒束相比,绕丝具有强化传热的作用。