共查询到18条相似文献,搜索用时 184 毫秒
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对窄缝通道内过冷条件下单蒸汽泡运动特性进行了实验研究,分析了过冷度对单蒸汽泡运动特性的影响。实验结果表明,过冷条件下,单蒸汽泡在上升过程中,其尺寸不断减小,形状也不断改变;单蒸汽泡界面存在冷凝现象,过冷度越大,直径减小越快,同一直径蒸汽泡的纵横比在一个范围内波动;过冷条件下,单蒸汽泡的z向速度和x向速度都随着直径的增大先增加后减小,均在直径约10 mm时具有最大值;单蒸汽泡z向速度则随着过冷度的增大而增大,而x向速度在零上下波动,随过冷度增大略有增大。过冷度会影响窄缝通道内单蒸汽泡的行为特性,并进一步影响流型形成与演变。 相似文献
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竖直矩形窄缝通道内近壁汽泡生长和脱离研究 总被引:1,自引:1,他引:0
可视化研究窄缝通道内汽泡生长和脱离对于揭示窄缝通道内的沸腾传热机理具有重要意义。本文采用高速摄影仪从宽面和窄面可视化观察了常压条件下矩形窄缝通道内汽泡核化生长和脱离规律。研究结果表明,汽泡在核化点生长时,汽泡底部与加热面存在一小的接触面,总体而言,汽泡在生长过程中基本呈球状。在相同热工参数下,不同核化点处汽泡生长规律基本相同,但汽泡脱离直径相差较大。窄缝通道内汽泡生长速率小,脱离时间较长,可采用修正的Zuber公式预测窄缝通道内汽泡生长直径。在同一拍摄窗口内,统计分析了热工参数对汽泡平均脱离直径的影响规律。随热流密度的增加,汽泡平均脱离直径减小;随入口欠热度的增加,汽泡平均脱离直径减小;随主流速度的增加,汽泡平均脱离直径减小。 相似文献
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通过汽泡受力分析,构建窄缝通道内汽泡脱离直径预测模型。基于可视化实验获得的汽泡轴心倾斜角、前后接触角以及底部接触直径等数据,评价分析汽泡界面参数对脱离直径预测的影响,进而确定适用于本实验工况下窄缝通道内汽泡受力模型求解的界面输入参数,获得了窄缝通道汽泡脱离直径的预测值。利用竖直和倾斜条件下可视化实验获得的58个数据对汽泡脱离直径预测模型进行了验证,预测值和实验值符合较好。基于验证的汽泡脱离直径模型评估了各个力的地位和作用,应用分析了热工参数对汽泡脱离直径的影响。 相似文献
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板状燃料元件中的矩形窄缝通道具有宽高比大的几何特征,高度方向速度梯度大、分布陡峭,发生过冷沸腾时,近壁面汽泡运动行为将受其影响而改变,其中汽泡滑移现象对沸腾换热影响较大。本文针对矩形窄缝通道中的汽泡滑移行为,构建了包含滑移热流的壁面热流分配模型,并建立机理性的汽泡受力模型和滑移模型计算汽泡脱离直径、浮升直径和滑移距离等辅助参数,开发了一套适用于矩形窄缝通道内向上流动沸腾的壁面沸腾模型。选用Nuthel窄缝通道沸腾实验进行数值模拟验证,结果表明:本文模型可以较好地预测1~4 MPa中低压工况窄缝通道向上流动沸腾的壁面过热度,最大误差相比RPI模型由80%降低至17%;蒸发热流份额和近壁面空泡份额相比RPI模型更低。 相似文献
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板状燃料元件中的矩形窄缝通道具有宽高比大的几何特征,高度方向速度梯度大、分布陡峭,发生过冷沸腾时,近壁面汽泡运动行为将受其影响而改变,其中汽泡滑移现象对沸腾换热影响较大。本文针对矩形窄缝通道中的汽泡滑移行为,构建了包含滑移热流的壁面热流分配模型,并建立机理性的汽泡受力模型和滑移模型计算汽泡脱离直径、浮升直径和滑移距离等辅助参数,开发了一套适用于矩形窄缝通道内向上流动沸腾的壁面沸腾模型。选用Nuthel窄缝通道沸腾实验进行数值模拟验证,结果表明:本文模型可以较好地预测1~4 MPa中低压工况窄缝通道向上流动沸腾的壁面过热度,最大误差相比RPI模型由80%降低至17%;蒸发热流份额和近壁面空泡份额相比RPI模型更低。 相似文献
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Numerical analyses of a two-dimensional single bubble in a stagnant liquid and in a linear shear flow were conducted in the present study using the volume of fluid method, which is based on the local-instantaneous field equations. It was clarified that this method gives qualitatively appropriate predictions for the effects of the Morton number and the Eotvos number on fluctuating bubble motion in a stagnant liquid. Calculated velocity and pressure distributions indicated that the Karman vortex causes a sinuous movement of the bubble. As for the bubble motion in a linear shear flow, calculated bubbles migrated in a lateral direction. The direction of the lateral migration agreed to available experimental data. It was also confirmed that (i) the direction or the magnitude of the lateral migration is affected by the Eotvos and the Morton numbers, and (ii) the interaction among the internal flow of the bubble, the wake of the bubble and the external shear flow plays an essential role for the lateral migration. 相似文献
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弹状流的液弹部分受气弹尾部影响,其水力特性参数沿流动方向存在分区的不一致性。本文对竖直窄矩形通道中弹状流液弹内参数的分布特性进行了研究。结果表明:液弹内气泡在近壁面附近所受径向力较为平衡,气泡频率较大;随着远离气弹尾部,管道中间气泡频率逐渐增大。根据气泡频率波动变化将液弹分为3个区域,尾流区占液弹长度的40%~45%,过渡区占10%~15%,主流区占40%~50%。尾流区和主流区内,空泡份额呈“三峰型”分布;随着气相流速的增加,尾流区内近壁面处峰值逐渐增大,管道中间峰值逐渐下降,但主流区内情况相反。气泡直径随气相流速的增大而变大,且液弹内气泡聚合和破碎现象较少。 相似文献
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To explore the mechanism of boiling bubble dynamics in narrow channels, two types of channels are investigated which have I- and Z-shaped with width of 2 mm. Using VOF model and self-programming, the whole flow field is simulated with two different kinds of media, namely, water and ethanol. The influence of wall contact angle on the process of bubble generating and growth is studied, and the relationship between different channel shapes and the pressure drop is also investigated taking into account the effects of gravity, viscosity, surface tension and wall adhesion. The bubble generation, growth and departure processes are analyzed through numerical simulation and self-programming, and the influence of interface movements and changes on internal pressure difference and average surface heat transfer coefficient is investigated by using geometry reconstruction and interface tracking. It is found that wall contact angle has a great influence on the morphology of bubble. The smaller the wall contact angles are, the more round the bubbles are, and the less time the bubbles take to depart from the wall. The variation of contact angle also has effect upon the heat transfer coefficient. The greater the wall contact angle is, the larger the bubble-covered area is, thus the wall thermal resistance gets higher, and bubble nucleation is suppressed, and the heat transfer coefficient becomes lower. The role of surface tension in the process of boiling heat transfer is much more important than the gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance the heat transfer. The heat transfer coefficient of Z-shaped channels is larger than that of I-shaped ones, while the pressure drop of the former is obviously higher. In addition, surface tension and viscosity significantly impact the pressure drop of boiling system, and different specific heat and boiling point values result in different heat transfer coefficients. The simulation results in this paper match well with the experimental data revealed in other sources, both show that the heat transfer coefficient of water is higher than that of ethanol and Z-shaped channels have better heat transfer capability. 相似文献
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《Journal of Nuclear Science and Technology》2013,50(8):671-672
The authors have been investigating geysering, which may appear during start-up of a natural circulation boiling reactor. From the results, it became clear that a large slug bubble covering the entire channel and its condensation in the upper plenum were indispensable for geysering to occur. In this work, an image processing unit has been developed in order to measure the coalescence process of multiple slug bubbles. The proposed system was used to measure the coalescence process of multiple slug bubbles flow in vertical circular channel on air-water system. It was shown that a following slug bubble was influenced by the preceding slug bubble as the distance between the slug bubbles became 5 times inner channel diameter. The rise velocity of the following slug bubble increased due to the pressure drop induced by the wake behind the preceding slug bubble. The bubbles coalesced together in a fraction of a second. 相似文献
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X.Y Fu M Ishii 《Nuclear Engineering and Design》2003,219(2):143-168
The prediction of the dynamical evolution of interfacial area concentration is one of the most challenging tasks in two-fluid model application. This paper is focused on developing theoretical models for interfacial area source and sink terms for a two-group interfacial area transport equation. Mechanistic models of major fluid particle interaction phenomena involving two bubble groups are proposed, including the shearing-off of small bubbles from slug/cap bubbles, the wake entrainment of spherical/distorted bubble group into slug/cap bubble group, the wake acceleration and coalescence between slug/cap bubbles, and the breakup of slug/cap bubbles due to turbulent eddy impacts. The existing one-group interaction terms are extended in considering the generation of cap bubbles, as well as different parametric dependences when these terms are applied to the slug flow regime. The complete set of modeling equations is closed and continuously covers the bubbly flow, slug flow, and churn-turbulent flow regimes. Prediction of the interfacial area concentration evolution using a one-dimensional two-group transport equation and evaluation with experimental results are described in a companion paper. 相似文献
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气泡形状变化对两相流动和热质传递过程有着重要影响。本文利用高速摄像仪和粒子图像测速(PIV)技术对生长和浮升过程中气泡的形状振荡特性进行了实验研究,分析了不同注气流量下气泡形状及其周围流场的变化规律。实验结果表明,气流量对脱离后气泡的形状有着明显影响,在低流量下,脱离气泡逐渐由长椭球形转变为扁椭球形,而在高流量下,脱离气泡与生长气泡的聚合将导致气液界面大幅振荡。此外,振荡气泡对其尾流区内生长气泡形状的影响同样不容忽视,其会引起生长气泡所受惯性作用增强,进而导致气泡形状波动。 相似文献
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《Journal of Nuclear Science and Technology》2013,50(12):895-904
Several experimental results show that bubbles can easily be captured in the wake formed by leading bubbles when multiple bubbles are rising in a liquid. It is suggested from this experimental result that the effect of bubble wake should be included in the constitutive relationships representing the interfacial drag force. In the present study, steam-water bubbly flow experiments were performed to develop a new interfacial drag force model including the effect of bubble wake. Since the validity of the existing constitutive equations have been tested mainly for two-phase flow in small-diameter pipes, our study focused on two-phase flow in a large-diameter pipe. Using a one-dimensional two-fluid model, the applicability of the new interfacial drag force model to our experimental conditions was investigated. As a result, it was shown that the present model markedly improves the accuracy of the predicted results. It was therefore demonstrated that the present bubble wake model is effective at least for the conditions which were used for model development. Its applicability to different conditions will be discussed in a subsequent study. 相似文献