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为探究低压低流速条件下的过冷沸腾换热特性,开展本实验研究。通过分析实验中采集的热工参数和可视化图像,探究了沸腾滞后现象、沸腾失稳现象以及沸腾换热特性。实验发现沸腾起始点壁面过热度较高,而沸腾的发生大幅提高了换热系数,因此出现了显著的沸腾滞后现象。实验中较为光滑的加热面可达到较高的过热度,而低压下快速产生的气泡尺寸较大,在较低的热流密度下气液界面发生剧烈变化,使气泡破裂为多个小气泡并成为核化点。在过冷沸腾换热系数的预测中,Dittus-Boelter对流换热关系式不再适用,采用Hallman关系式和Gnielinski关系式计算对流换热系数,并引入壁面过热度对池式沸腾换热系数进行修正,可使过冷沸腾换热系数的预测精度大幅提高。 相似文献
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H. Ohta 《Nuclear Engineering and Design》1997,175(1-2)
To clarify the relation between the liquid–vapor behavior and the heat transfer characteristics in the boiling phenomena, the structures of transparent heaters were developed for both flow boiling and pool boiling experiments and were applied to the microgravity environment realized by the parabolic flight of aircraft. In the flow boiling experiment, a transparent heated tube makes the heating, the observation of liquid–vapor behavior and the measurement of heat transfer data simultaneously possible. The heat transfer coefficient in the annular flow regime at moderate quality has distinct dependence on gravity provided that the mass velocity is not so high, while no noticeable gravity effect is seen at high quality and in the bubbly flow regime. The measured gravity effect was directly related to the behavior of annular liquid film observed through the transparent tube wall. In the pool boiling experiment, a structure of transparent heating surface realizes both the observation of the macrolayer or microlayer behavior from underneath and the measurements of local surface temperatures and the layer thickness. It was clarified in the microgravity experiments that no vapor stem exists but tiny bubbles are observed in the macrolayer underneath a large coalesced bubble at high heat flux. The heat flux evaluated by the heat conduction across the layer assumes less than 30% of the total to be transferred. The evaporation of the microlayers underneath primary bubbles just after the generation dominates the heat transfer in the microgravity, not only in the isolated bubble region but also in the coalesced bubble region. 相似文献
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This paper presents the results of visualization experiments that were carried out to investigate the dynamics of vapor bubbles generated in water pool boiling. In the experiments, vapor bubbles were generated on a vertical circular surface of a copper block containing nine cartridge heaters, and the contact angle of the heated surface was used as a main experimental parameter. The experiments were performed under subcooled as well as nearly saturated conditions. To enable clear observation of individual bubbles with a high speed camera, the heat flux was kept low enough to eliminate significant overlapping of bubbles. When the contact angle was small, the bubbles were lifted-off the vertical heated surface within a short period of time after the nucleation. On the other hand, when the contact angle was large, they slid up the vertical surface for a long distance. When bubbles were lifted-off the heated surface in subcooled liquid, bubble life-time was significantly shortened since bubbles collapsed rapidly due to condensation. It was shown that this distinct difference in bubble dynamics could be attributed to the effects of surface tension force. 相似文献
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在窄缝流道内发生沸腾换热现象时,由于沸腾产生的汽泡受窄缝流道的限制,受压变形而消除了汽泡表面张力对传热的影响。因此对此现象进行基础性理论研究具有很重要的意义。本文在常压下用蒸馏水对窄缝间隙为 0.75mm的垂直环形流道,进行了流动沸腾传热实验研究。实验段的有效加热长度为 900mm,其加热方式为内外侧双面加热,实验的流量变化范围在 1.67× 10- 5~ 5.83× 10- 5m3/s。通过实验得到了在不同质量流速和热流密度下双面加热的窄缝流道中内外侧沸腾换热系数随干度变化的分布和特点。研究结果表明,由于在窄缝流道中存在着大量的运动聚合受压变形汽泡,因此使内外侧沸腾换热系数都很高 (可达 105W· m- 2· K- 1以上 )。 相似文献
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Wen Peipei Chen Barclay G. Jones Ty A. Newell 《Nuclear Engineering and Design》2008,238(10):2693-2698
This study examines the bubble detachment phenomena under subcooled nucleate boiling conditions, in order to obtain a better understanding of the bubble dynamics on horizontal flat heat exchangers. Refrigerant R134a is chosen as a simulant fluid due to its merits of having smaller surface tension, reduced latent heat, and lower boiling temperature than water. Experiments are run with varying experimental parameters, e.g. pressure, inlet subcooled level, flow rate, etc. Digital images are obtained at frame rates up to 4000 frames/s, showing the characteristics of bubble movements. Bubble departure and bubble lift-off, which are described as bubbles detaching from the original nucleation sites and bubbles detaching from the horizontal heated surface respectively, are both considered and measured. Results are compared against the model proposed by Klausner et al. for the prediction of bubble detachment sizes. While good overall agreement is shown, it is suggested that finite rather than zero bubble contact area should be assumed, which improves the model prediction at the pressure range of 300–500 kPa while playing no significant role at a lower pressure of 150 kPa where the model was originally benchmarked. The impact of heated surface structure is studied whose results provide support to the above assumption. 相似文献
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Numerical simulation of bubble behaviors in subcooled flow boiling under swing motion 总被引:1,自引:0,他引:1
A numerical investigation of bubble behaviors in subcooled flow boiling of water under the effect of additional inertial forces has been performed considering energy and mass transfer during phase change based on the VOF (volume-of-fluid) method. The pressure ranges from 0.1 to 1.0 MPa, and heat flux from 200 to 500 kW/m2. The mass flow rate and inlet subcooling are specified at 320 kg/m2 s and 10 K, respectively. The liquid-vapor interface is captured using the piecewise linearity interpolation calculation (PLIC) geometry restructuring method. The simulations are carried out on upward water flow in a vertical, rectangular duct with single side heating surface. The pressure, velocity vector and temperature distribution around two isolated bubbles are studied firstly. The behaviors of bubble coalescence, sliding, detachment from the heated wall, and the bubble shape variation during lifetime are further examined. The bubble behaviors in the different pressure and heat flux are investigated. The simulated results of bubble growth rate and wall temperature are agreed well with the correlations in the literatures. The additional inertial forces caused by swing are negligible, but the fluctuation of mass flow rate caused by swing motion influences the forces acting on bubble significantly. Compared with the motionless condition, the pressure drop is increased and the fluctuation becomes acute as heat flux increases under the swing condition. 相似文献
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Two fundamental phenomena are significant when a shock pressure interacts with the large scale coarse mixing state. One is an intensive flow and the other is the surface area enhancement due to the disintegration of the hot drops. The effects of these phenomena on the transient heat transfer and behavior of vapor film under a shock pressure are investigated. Transient heat transfer of film boiling from an electrically heated platinum ribbon 2.5 mm wide and 0.15 mm thick was measured immediately after passage of a shock pressure from 0.1 to 0.7 MPa. The heater was set horizontally in a vertical shock tube which was filled with vapor liquid bubbly mixture and kept initially in the film boiling state. That is, the heater corresponds to a typical hot drop and the bubbles around it correspond to the coarse mixture around the drop. The liquid was Freon-113 with an initial void fraction in the range from 0 to 3%. When the shock wave arrives at the heater, intensive transient flow occurs due to collapse of bubbles around the heater. First, the effects of the initial void fraction, the intensity of the shock and the heated wall temperature on the transient heat fluxes and collapse of the vapor film were investigated experimentally and analytically under the shock pressure. Compared with a heated wall in the liquid alone, the transient heat flux at the heated wall increases and the collapse of the vapor film becomes easier in the bubbly mixture due to the transient flow. Effects of surface enhancement during the fragmentation process on the heat transfer rate and transient behavior of vapor film are investigated analytically by application of the newly proposed surface stretch model. It is made clear when the surface area is increasing, the vapor film is apt to collapse and the transient heat transfer is enhanced by the surface stretch. 相似文献
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V. V. Sorokin 《Atomic Energy》2009,106(1):17-25
Heat transfer from microfuel fill to a two-phase coolant by means of a phase transition can occur in two forms: evaporation
and bubble boiling. The evaporation regime proceeds on menisci along the lines of triple contact of phases. This regime is
stable and promising for technical applications. Bubble boiling occurs when the surface of the microfuel is overheated relative
to the saturation temperature. In the presence of boiling, the bubbles in the fill can be large (relative to the pore size)
and small. Evaporation and boiling with small bubbles occur simultaneously, the process is characterized by a high coefficient
of heat transfer, and the regime is stable and promising for applications. Several paths lead to a crisis of boiling. The
most dangerous process is one which gives rise to systemic instability of the uniform distribution of the phases in the fill.
Superheating of microfuel surfaces in this case is not an accident, but the mechanical action of the accompanying pressure
oscillations can be dangerous. Microfuel surfaces can become dry inside large volumes of steam, which arise in regions where
the fill porosity is elevated.
Translated from Atomnaya énergiya, Vol. 106, No. 1, pp. 16–22, January, 2009. 相似文献
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在中低压条件下,对矩形窄缝通道两相流动传热进行试验研究,分析两相流动传热的变化规律,拟合出饱和沸腾传热系数计算关系式,并采用简化的一维分析方法对两相压降进行分析计算。试验结果表明:在相同热平衡含汽率(x)情况下,两相流动压降随系统压力(p)的降低而增大,随系统流量的增大而增大的变化规律;p越低,两相流动压降随x的增加而增大越剧烈;流量越大,两相流动压降随x的增加而增大越剧烈。通过数据回归方法得到汽相湿周长比例因子F并拟合了计算关系式,其计算值与试验值符合得较好。矩形窄缝通道内饱和沸腾平均传热系数受p、质量流量及热流密度的影响较大。 相似文献
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沸腾换热具有传热温差低、换热系数高等优点,对提高设备的紧凑性、经济性和安全性具有重要意义.目前,核动力装置中主要以光管作为沸腾换热元件,沸腾换热强化的空间很大.本文针对核动力装置非能动余热排出换热器沸腾换热工况,以水为工质,对光管及3种强化管的管外沸腾换热特性进行实验研究,得出了4种管型的沸腾换热强化特性,并分析了各强化管的强化机理.整体针翅管表面大量螺旋排列的三维翅片增大了换热面积、延缓了汽泡在壁面附近聚合形成大汽膜,使沸腾换热得到强化;多孔管采用机械方法在壁面加工出大量微细小孔,汽化核心数量和汽泡脱离频率均大幅提高,因此,沸腾换热强化效果显著;绕丝针翅管是一种复合强化手段,兼有整体针翅管和多孔管的优点,沸腾换热强化效果也较好. 相似文献
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棒束燃料元件子通道间流体存在搅混与横向二次流,流动及阻力特性相较矩形通道、圆管等简单通道更为复杂。核动力舰船、船舶、小型浮动核电站等会受到海浪影响,经常处于倾斜、摇摆、垂荡等瞬变运动下。目前的相关研究多集中在低压工况的研究领域,高温高压自然循环运动条件下的研究较少。本文采用实验研究方法,对自然循环系统摇摆条件下棒束通道内流动传热特性进行了研究,获得了过冷沸腾和饱和沸腾两种条件下摇摆角度和摇摆周期对棒束壁面温度变化和传热系数的影响,并获得了摇摆周期内棒束通道内的传热系数计算关系式。结果表明,饱和沸腾传热系数变化比过冷沸腾的剧烈;在本文实验工况范围内,棒表面传热系数波动幅值随着摇摆幅度的增大而增大;摇摆条件下棒束通道过冷沸腾和饱和沸腾工况时均传热系数基本不变。 相似文献
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铬(Cr)涂层锆合金包壳是最有前途的耐事故燃料(ATF)的新型包覆材料之一,对其表面的气泡动力学进行研究有助于评估是否具有更好的传热性能。在常压下的Cr涂层锆合金包壳池式沸腾实验装置中对不同工艺方法下制备的Cr涂层锆合金包壳进行实验,研究了粗糙度等表面状态对气泡产生、长大以及脱离等气泡行为的影响。结果表明,气泡接触角与Cr涂层表面粗糙度有关,粗糙度越大,表面气泡接触角越小;不同涂层工艺下制备的4种Cr涂层锆合金包壳样件表面的气泡脱离直径范围为1.256~1.446 mm,气泡脱离频率范围为29.99~50.97 Hz;气泡脱离直径与粗糙度呈负相关,脱离频率与粗糙度呈正相关;气泡脱离直径预测模型与实验数据之间的偏差为±6%,脱离频率预测模型与实验数据之间的偏差为±3%。 相似文献
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Predictive study of the incipient point of net vapor generation in low-flow subcooled boiling 总被引:1,自引:0,他引:1
A predictive model of the initial point of net vapor generation, incipient point of net vapor generation (IPNVG) for low-flow subcooled boiling is developed in this paper. The IPNVG established in this model meets both the thermodynamic and hydrodynamic conditions. The thermodynamic condition is described by the heat balance at IPNVG. The amount of heat for steam generation is equal to that for bubble condensation at IPNVG. The force balance of the detached bubbles at IPNVG or the hydrodynamic condition is established to provide the diameter of the detached bubbles and the interfacial heat transfer coefficient for the calculation of heat transfer at IPNVG. This mechanism of the present model makes it applicable to low-flow subcooled boiling. Several coefficients involved in the proposed model are identified by Freon-12 experimental data. This model is compared with the experimental data obtained from different works. The data cover two working media of steam-water and Freon-12, two flow conditions of natural and forced circulations and relatively wide ranges of pressure, mass flux and heat flux. The predictions of this model agree with the data quite well. 相似文献
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《Nuclear Engineering and Design》2005,235(10-12):1251-1265
Population balance equations combined with a three-dimensional two-fluid model are employed to predict subcooled boiling flow at low pressure in a vertical annular channel. The MUltiple-SIze-Group (MUSIG) model implemented in CFX4.4 is extended to account for the wall nucleation and condensation in the subcooled boiling regime. A model considering the forces acting on departing bubbles at the heated surface is formulated. This model provides the capacity of complex analyses on the bubble growth and departure for a wide range of wall heat fluxes and flow conditions.Comparison of model predictions against local measurements is made for the void fraction, bubble Sauter mean diameter and gas and liquid velocities covering a range of different mass and heat fluxes and inlet subcoolings. Good agreement is achieved with the local radial void fraction, bubble Sauter mean diameter and liquid velocity profiles against measurements. However, significant weakness of the model is evidenced in the prediction of the vapour velocity. Work is in progress to circumvent the deficiency of the MUSIG boiling model by the consideration of additional momentum equations to better represent the momentum forces acting on the range of bubble sizes in the bulk subcooled liquid. 相似文献