定位格架对超临界水冷堆堆芯换热的影响

在压力为25 MPa、质量流速800kg/(m~2·s)、热流密度600kW/(m2·K)的条件下,对超临界水冷堆堆芯(燃料棒直径D=8.0mm,节径比P/D=1.2)类三角形子通道内超临界水的换热特性进行研究。用结构化的六面体网格以及计算流体动力学软件ANSYS CFX对堆芯的换热情况进行了模拟。以无定位格架子通道为基准,对比阻流片型定位格架对通道内流体换热的作用。结果表明,在超临界压力下阻流片型定位格架能够明显增强换热,降低最高包壳温度。另外,不同焓值区定位格架对换热的影响存在差异。     

半周加热内螺纹管中超临界水传热特性的数值模拟及机理分析

为了研究700℃高效超超临界锅炉和超超临界循环流化床锅炉水冷壁管中工质水的流动传热规律和机理,采用SST k-ω湍流模型模拟了大比热容区内半周加热条件下长度为2 m、水力直径为19 mm的垂直上升四头内螺纹管中超临界水的流动传热特性。结果表明:半周加热条件下内螺纹管内壁温度和热流密度呈现类似抛物线分布,在内壁热流密度变化不大的局部区域(圆周角φ=0°～90°),内壁温度在肋底与背风侧交点处达到最大值,在肋顶与迎风侧交点处达到最小值,内壁热流密度的变化趋势与之相反,这是由内螺纹肋的旋流作用造成的,内壁热流密度的周向分布不是影响超临界水传热特性的唯一因素;超临界水发生传热强化现象主要是由于其在边界层内的比热容份额较大导致的,而与湍动能的大小无关。     

七棒束内超临界流体流动传热的数值分析

在7棒束内超临界氟利昂流体传热试验数据的基础上,采用ω类湍流模型SST和RSO以及ε类湍流模型RNG和SSG对三组典型工况进行了数值计算.结果表明:近壁面区域采用低雷诺数模型处理的ω类模型能定性捕捉到壁面的局部传热弱化现象,而壁面函数法的处理方式不能预测到此类现象;受7棒束非对称热边界条件和强浮力的影响,棒束不同通道内流率分布存在强烈的不均匀性,中心棒和周边加热棒的传热特性存在显著差异.为了准确预测棒束内超临界流体的传热特性特别是典型传热弱化现象,应重点完善湍流模型的近壁面区域处理方式.     

超临界压力下航空煤油在竖直管内的传热研究

对竖直上升管内超临界压力下航空煤油的传热特性进行了实验研究。分析了不同质量流量、热流密度、压力和进口温度对超临界压力下航空煤油传热特性的影响。实验结果表明,提高质量流量或进口温度均使煤油传热效果变好。而热流密度对流体传热的影响主要在于改变了流体和壁面温度,热流密度越大,传热系数越高。压力对煤油传热影响不大,一般情况下,提高压力会恶化传热。超临界状态下,煤油物性变化很大,因此对煤油的传输和热力学性质的准确计算是研究超临界压力下传热现象的关键。利用拓展的对比态法来计算煤油的密度和传输特性,如黏度、热导率等。给出了煤油在超临界压力下的传热关联式,其计算值和实验值吻合良好。     

粗糙圆管内超临界航空煤油湍流换热特性分析

给出了国产航空煤油RP-3的三组分热物性替代燃料模型。采用k-ε湍流模型结合增强壁面处理的方法对超临界压力下航空煤油RP-3在圆形粗糙冷却通道中的流动与换热过程进行数值研究。分析了粗糙元形状、高度以及间高比等因素对其超临界流动和传热特性的影响规律,探究了人为粗糙度强化超临界航空煤油换热的机理。结果表明,人为设置粗糙元能使壁面附近产生局部回流区和旋涡结构,强化煤油与受热壁面间的对流换热。通过合理布置粗糙元结构,能大幅降低圆形冷却通道的壁面温度,有效抑制航空煤油的超临界传热恶化现象的发生。     

偏心方形管内超临界水传热特性数值研究

采用SSG湍流模型,对偏心方管内超临界水传热特性进行三维数值研究。研究了不同偏心距对通道内传热特性的影响及通道内的周向传热不均匀特性。研究结果表明:偏心方形管内存在着强烈的传热不均匀现象,在远离大比热区的低焓值区和高焓值区,偏心距离的增加对内壁温度和换热系数的影响不大。随着偏心距的增加,方形管壁面最高温度逐渐升高,边通道传热系数要强于角通道;超临界水的物性变化也会影响偏心方形管内周向不均匀性,从而影响传热特性。     

带定位格架的5×5棒束通道内过冷沸腾流动传热数值研究

基于流体体积函数(VOF)两相流模型,通过在控制方程中加入适当的质量源项和能量源项,建立了过冷沸腾模型,利用该模型对5×5定位格架棒束通道内的过冷沸腾现象进行数值模拟,研究了有无过冷沸腾现象2种工况下的传热特性以及入口速度、入口过冷度等因素对燃料组件内传热特性的影响。结果表明:有过冷沸腾现象时流体的传热效果优于无过冷沸腾现象,在有过冷沸腾现象时,总体上通道内传热系数与入口速度呈正相关,与入口过冷度呈负相关;在局部区域,相变传热占据主导因素,过冷沸腾程度越强,流体的传热效果越好。     

铅铋合金冷却快堆PBWFR子通道参数敏感性研究

摘要： 基于COBRA-IV开发出了适用于铅铋合金冷却组件和堆芯的子通道热工水力分析程序SUBAS,并利用其对铅铋合金冷却组件进行了详细的子通道分析,主要分析了不同燃料棒数目对组件内的温度场和速度场的影响;对湍流交混模型、横流压降系数、换热系数模型等做了相关的参数敏感性分析。研究结果表明：燃料棒数目的增加会导致组件内外质量、动量和能量的交换更加困难,各类通道的温度都有所升高;定位格架不仅增加了组件压降,而且降低了相邻通道之间的横向流动;湍流交混模型对组件的温度场和速度场影响较大,需要重点研究。     

轴向射流对旋流管状火焰传热影响的实验研究

为了探究轴向气体流动特征对旋流管状火焰传热性能的影响,在实验中引入轴向喷出的N₂作为稀释剂。研究了不同的轴向流量、喷嘴孔径及喷孔数量下的火焰结构及传热规律,重点分析了不同流动条件下管状燃烧室内火焰径向传热的温度分布特征。分析结果表明：旋流管状火焰能将一定量的气体工质快速升温至1 000℃以上;随着轴向稀释气体流量的增加,火焰根部被吹离切向入口,火焰锋面向燃烧室下游移动,导致火焰根部温度显著降低,而且最高温度区域也向下游移动,最高温度值也有所降低;随着喷嘴孔径的增大,火焰锋面位置无明显变化,而火焰根部向喷嘴侧移动,且相同轴向位置的径向温度更高,即喷嘴孔径的增加有助于对轴向气流的快速加热;在当前实验条件下由于受到空间限制,喷孔数量的变化对火焰位置与温度分布无明显影响。     

超临界R134a在ORC系统中的换热特性分析

采用有限元分析软件FLUENT对超临界有机朗肯循环系统中的换热器进行数值分析,进而对其进行设计和优化。超临界R134a在长为2 100 mm,直径为4 mm的光管换热器中分别进行向上流动和向下流动。换热边界条件是定热流密度,流体的进口压力P为4.5 MPa,进口温度Tin为349 K。分别探究热流密度q、质量流量G、热流质量比q/G和浮升力对传热特性的影响。采用无量纲数Bo来预判浮升力对传热的影响。虽然Morky的工作流体不是超临界R134a,但他们的经验公式也能用来更好地预测超临界R134a在光管中向上流动的传热特性。     

垂直内螺纹管内超临界水的二次流特性

运用计算流体力学方法,采用SSG雷诺应力模型对高温条件下垂直内螺纹管中超临界水的二次流特性进行了研究。研究发现:内螺纹的导流作用在垂直于主流的方向产生了很强的二次流场,主流焓的变化对二次流场的基本结构影响很小。内螺纹管周向和径向上的传热系数分布不均,超临界水在螺纹顶部区域的传热效果远好于螺纹底部。在螺纹顶部表面形成的单个旋涡产生一个压力相对较低的区域,传热增强。在大比热区,由于流体热物性急剧变化,传热的不均性更加显著。螺纹底部壁面是内螺纹管周向传热的薄弱位置,其内部结构的优化应重点集中在该区域。     

Steady-state thermal-hydraulic analysis of SCWR assembly

Among the six gen-IV reactor concepts recommended by the gen-IV international forum (GIF), supercritical water-cooled reactor (SCWR), the only reactor with water as coolant, achieves a high thermal efficiency and, subsequently, has economic advantages over the existing reactors due to its high outlet temperature. A thermal-hydraulic analysis of the SCWR assembly is performed in this paper using the modified COBRA-IV code. Two approaches to reduce the hot channel factor are investigated: decreasing the moderator mass flow and increasing the thermal resistance between moderator channel and its adjacent sub-channels. It is shown that heat transfer deterioration cannot be avoided in SCWR fuel assembly. It is, therefore, highly required to calculate the cladding temperature accurately and to preserve the fuel rod cladding integrity under heat transfer deterioration conditions.     

Numerical investigation on conjugate heat transfer to supercritical CO2 in membrane helical coiled tube heat exchangers

ABSTRACT

Conjugate heat transfer to supercritical CO₂ in membrane helical coiled tube heat exchangers has been numerically investigated in the present study. The purpose is to provide detailed information on the conjugate heat transfer behavior for a better understanding of the abnormal heat transfer mechanism of supercritical fluid. It could be concluded that the supercritical fluid mass flux and vertical/horizontal placement would significantly affect the abnormal heat transfer phenomenon in the tube side. The flow field of supercritical fluid is affected by both the buoyancy and centrifugal force in the conjugate heat transfer process. The local wall temperature and heat transfer coefficient in the tube side would rise and fall periodically for the horizontal heat exchanger, but this phenomenon will gradually disappear with the increase of the mass flow rate or fluid temperature in the tube side. The dual effects of buoyancy force and centrifugal force lead to the deflection of the second flow direction for the vertical placement, which further results in the heat transfer deterioration region on the top-generatrix wall for the downward flow being larger than that for the upward flow.     

带有横向微沟槽的矩形通道内超临界液化天然气换热强化特性模拟研究

提出了一种用于超临界液化天然气换热的微小通道换热器整体性能提高的被动式强化技术并进行了数值模拟验证和设计优化.在普通的矩形微小通道内利用3D激光打印技术在壁面加工横向圆弧形微沟槽以强化换热能力.首先对圆弧形微沟槽的槽深、槽宽和相邻两槽道中心距等几何尺寸进行了优化计算,然后讨论了在使用强化技术后工质温度在跨越临界温度的1...     

Numerical study of convective heat transfer to supercritical water in rectangular ducts

Numerical investigation has been performed to analyze forced convective heat transfer to supercritical water in horizontal rectangular ducts. Convective heat transfer near the critical region in the rectangular ducts is strongly influenced by large variations of thermodynamic and transport properties of supercritical fluid with gravity force, especially close to pseudocritical temperature. Fluid flow and heat transfer characteristics such as velocity, temperature, and local heat transfer coefficient with water properties distribution in the ducts are presented. Flow accelerates along the horizontal ducts because of decreased water density from heat transfer at the duct walls. Center of large flow recirculation in the duct section locates near the middle of vertical surface and additional secondary recirculation in clockwise direction appears with the increase of duct height. Local wall temperature severely varies along the inner surface of the duct section and its variation depends on aspect ratio of the duct. The heat transfer coefficient distributions along the ducts for various aspect ratios are compared with the proximity effect to the critical pressure.     

Evaluation of the Heat Transfer Correlations for Supercritical Pressure Water in Vertical Tubes

A large number of studies have been carried out on the flow and heat transfer of supercritical pressure fluids in the past decades. However, there are still some uncertainties and deficiencies in the accurate prediction for supercritical fluid heat transfer coefficient due to the large and fast variations of fluids properties in the so-called pseudo-critical region. In this paper, 15 correlations were selected from the literature and were compared with each other to verify their capability in predicting heat transfer coefficient of supercritical pressure water in vertical tubes. Based on the comparison between the calculation results of the existing heat transfer correlations and the experimental data obtained from the open literature, it was found that the Swenson et al. correlation and the Hu correlation can reasonably predict the heat transfer coefficient of supercritical water in the pseudo-critical region. After evaluating these correlations, the authors conducted polynomial fitting for the collected experimental data and got a new correlation for heat transfer coefficient of water at supercritical pressures. The new correlation can fit well with the experimental data even in the neighborhood of pseudo-critical temperature.     

Numerical investigation on heat transfer of supercritical water in a roughened tube

ABSTRACT

The turbulent mixed convection heat transfer of supercritical water flowing in a vertical tube roughened by V-shaped grooves has been numerically investigated in this paper. The turbulent supercritical water flow characteristics within different grooves are obtained using a validated low-Reynolds number κ-ε turbulence model. The effects of groove angle, groove depth, groove pitch-to-depth ratio, and thermophysical properties on turbulent flow and heat transfer of supercritical water are discussed. The results show that a groove angle γ = 120° presents the best heat transfer performance among the three groove angles. The lower groove depth and higher groove pitch-to-depth ratio suppress the enhancement of heat transfer. Heat transfer performance is significantly decreased due to the strong buoyancy force at T_b = 650.6 K, and heat transfer deterioration occurs in the roughened tube with γ = 120°, e = 0.5 mm, and p/e = 8 in the present simulation. The results also show that the rapid variation in the supercritical water property in the region near the pseudo-critical temperature results in a significant enhancement of heat transfer performance.     

Effects of choking on flow and heat transfer in micro-channels

The gas flows through micro-channels are encountered in many engineering applications such as the cooling devices of electronic chips, semiconductors, micro-electro-mechanical systems (MEMS), etc. Many works have been performed to investigate the flow and heat transfer characteristics generally occurring in the micro channels. According to these investigations, the majority of heat was transferred in the entrance region of the channel, due to high strain rate of the developing flow. These findings are valid only for unchoked micro channel flows. Once the gas flow is choked, the major flow features may be changed but no detailed works have been carried out to date. In these regards, the choked flow characteristics should be known to investigate the heat transfer phenomena in the micro channel flows. In the present study, numerical simulations have been used to provide detailed flow and heat transfer characteristics of micro-channel gas flows. The main objectives of the present effort are to understand the evolution of choking inside micro-channels with isothermally-heated-walls and to elucidate the regions of high heat transfer. The results obtained show that for choked flow conditions, high heat transfer is generated at both the entrance and the exit of the micro-channel. The exit effects like increased strain rate, high temperature gradient and the thinning of the boundary layer cause a rapid increase in heat transfer at the exit of the micro-channel. The location where the flow is choked is practically important in determining the heat transfer phenomena at the vicinity of the channel exit.