环形通道内钠流动起始沸腾壁面过热度实验研究

对环形通道内金属钠起始沸腾壁面过热度进行实验研究。实验段长800 mm,环形通道外径10 mm,内径6 mm。电加热元件最高热流密度为846 kW/m²,进口过冷度为63.1～287.8 ℃,质量流量为7.2～122.0 kg/h,系统压力为0.85～28.79 kPa。实验结果表明,起始沸腾壁面过热度随热流密度和进口过冷度的增加而升高,随质量流量和系统压力的增加而降低。拟合得到了关于起始沸腾壁面过热度的半经验关系式,关系式计算结果与实验数据符合良好。     

Experimental research on the incipient boiling wall superheat of sodium

Incipient boiling wall superheat of sodium flowing in annulus was experimentally investigated. The annulus was 800 mm in length, 6 mm as inside diameter and 10 mm as outside diameter. The heat flux in the experiment was from 128 to 846 kW/m², with inlet subcooling from 63.1 to 287.8 °C, mass flow rate from 7.2 to 122.0 kg/h and system pressure from 0.85 to 28.79 kPa. The experimental results indicated that the incipient boiling wall superheat increased with the increasing heat flux and inlet subcooling. And lower liquid velocity and system pressure could result in a higher incipient boiling wall superheat. Furthermore, a semi-empirical correlation was obtained from the experimental results. It was also found that the predicting results agreed well with the experimental data.     

Sodium boiling experiments in a 19-pin bundle under loss-of-flow conditions

An experimental study was conducted on transient sodium boiling in a 19-pin electrically heated LMFBR fuel subassembly mockup under loss-of-flow conditions. In each run the inlet flow was reduced or stopped at constant heater power. There was no strong effect of temperature ramp rate on incipient-boiling (IB) wall superheat. The observed coolant voiding was initially limited to the center subchannel because of steep temperature gradient in the bundle. The bulk pressure rise registered upon initial vaporization was markedly lower than the vapor pressure corresponding to the IB wall superheat. The pressure pulse generated at vapor bubble collapse correlated reasonably well with the re-entrant liquid velocity, but the measured value was very much smaller than the calculation by sodium hammer analysis.     

Experimental study on convective boiling heat transfer in narrow-gap annulus tubes

Since convective boiling or highly subcooled single-phase forced convection in micro-channels is an effective cooling mechanism with a wide range of applications, more experimental and theoretical studies are required to explain and verify the forced convection heat transfer phenomenon in narrow channels. In this experimental study, we model the convective boiling behavior of water with low latent heat substance Freon 113 (R-113), with the purpose of saving power consumption and visualizing experiments. Both heat transfer and pressure drop characteristics were measured in subcooled and saturated concentric narrow gap forced convection boiling. Data were obtained to qualitatively identify the effects of gap size, pressure, flow rate and wall superheat on boiling regimes and the transition between various regimes. Some significant differences from unconfined forced convection boiling were found, and also, the flow patterns in narrow vertical annulus tubes have been studied quantitatively.     

Pool boiling heat transfer enhancement on porous surface tube

The passive residual heat removal exchanger (PRHR HX),which is a key equipment of the passive residual heat removal system,is installed in an elevated pool.Its heat transfer performance affects security and economics of the reactor,and boiling heat transfer in the liquid surrounding the exchanger occurs when the liquid saturation temperature exceeded.The smooth tubes,which are widely used as heat transfer tubes in PRHR HX,can be replaced by some enhanced tubes to improve the boiling heat transfer capability.In this paper,the pool boiling heat transfer characteristics of smooth tube and a machined porous surface tube are investigated by using high-pressure steam condensing inside tube as heating source.Compared with smooth tube,the porous surface tube considerably enhances the boiling heat transfer,and shortens the time significantly before reaching the liquid saturation temperature.Its boiling heat transfer coefficient increases from 68% to 75%,and the wall superheat decreases by 1.5oC.Combining effect of condensation inside tube with boiling outside tube,the axial wall temperatures of heat transfer tube are neither uniform nor linear distribution.Based on these investigations,enhance mechanism of the porous surface tube is analyzed.     

Heat transfer from a totally blocked fuel subassembly of a liquid metal fast breeder reactor: Part I. Experimental investigation

The safety issues of liquid metal fast breeder reactors (LMFBR) are crucial due to the fact that a highly reactive and hazardous fluid like liquid sodium is used as coolant. One of the extreme cases, which can occur in a fuel subassembly of an LMFBR, is a total blockage of liquid inside the subassembly, which may lead to boiling of sodium. The present study addresses this problem by conducting experiments on a 19-rod bundle assembly enclosed inside a tall hexagonal enclosure. Liquid sodium is used as the heat transfer fluid. The natural convection mode of heat transfer is the main focus of investigation with a co-flowing air through an annular packed bed to simulate the neighbouring subassemblies. The maximum temperature achieved under different rates of power generations and air flow conditions are observed. Also the radial temperature distributions at different planes under different operating conditions of power and air flow rates have been observed. The results are of significant importance for validating analysis for the purpose of prediction of boiling incipience in an LMFBR subassembly under totally blocked condition.     

低压低流速条件下的过冷沸腾换热特性

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。     

An Attempt of High Gamma Dose Measurement

Transient sodium boiling experiments have been conducted in an electrically heated single-pin annular channel under loss-of-flow conditions. Each run was made by reducing or stopping the flow at constant heater power. Records were obtained of the changes of temperatures, flow rates, pressures and voids. The experimental results were compared with analytical calculations.

There was no strong effect of temperature ramp rate on the incipient-boiling (IB) wall superheat in the range of the present experimental conditions.

The pressure rise at the initial vaporization compared well with the vapor pressure corresponding to the IB wall superheat. The pressure pulse occurred at the vapor collapse correlated reasonably well with the collision velocity between the re-entrant liquid columns, but the measured value was less than the theoretical value predicted from the sodium hammer analysis.

The observed voiding pattern agreed fairly well with the calculation by the NAIS-P2 code (single-bubble slug expulsion model). The initial expulsion acceleration of liquid agreed well with the theoretical prediction from the one-dimensional liquid column expulsion analysis. The thickness (0.05 and 0.29 mm) of the residual liquid film became thinner with higher IB wall superheat.     

Loss-of-flow experiment in a 37-pin bundle LMFBR fuel assembly simulator

An experimental study was conducted on transient sodium boiling in an LMFBR fuel subassembly mockup under loss-of-flow conditions. In the test section, an electrically heated 37-pin bundle was centered in a hexagonal tube. The measured maximum IB wall superheat was 36°C, and the effects of heat flux, temperature rise rate, and system pressure were unclear. Boiling was initiated at the end of the heated section, the bubble expanded mainly to the upstream central subchannels and to the downstream unheated section according to the expansion of the saturated temperature region. When the voided zone covered the whole flow cross-section, the void pattern changed to the one-dimensional slug ejection-type and the inlet flow decreased rapidly. Dryout occurred after the inception of flow reversal in the wide region of the bundle.     

环形通道内液态金属钠流动换热特性实验研究

对液态金属钠在环形通道内的单相流动换热特性进行了实验研究。结合实验数据,将液态金属钠单相流动分为层流区(Re≤2 000)、过渡区(2 000Re≤4 000)及湍流区(Re4 000),分别拟合得到不同流态下摩擦系数的计算关系式,并拟合得到液态金属钠环形通道内换热特性的相应关系式。结果表明:液态金属钠单相流动特性与常规流体(如水)类似,其层流区摩擦系数略大于水,湍流区与水的很接近。液态金属钠对流换热过程中,导热项占较大份额,同时Nu随Pe的增大而略有增大。     

Transient Boiling of Sodium in a Seven-Pin Bundle under Loss-of-Flow Conditions

Transient sodium boiling experiments have been conducted in an electrically heated 7-pin bundle under loss-of-flow conditions. Each run was made by reducing or stopping the inlet flow at constant heater power.

There was no strong effect of temperature ramp rate on the incipient-boiling (IB) wall superheat and the data were scattered with values as high as ~190°C in the range of the present experimental conditions.

The observed coolant voidings, except in some high superheat runs, were initially limited in the central subchannel because of the steep temperature gradient in the bundle, and then spread slowly. In order to describe these voiding processes, a two-dimensional voiding model was required. The initial expulsion acceleration of liquid was higher with higher IB wall superheat. The measured values were lower than the theoretical prediction from the one-dimensional liquid column expulsion analysis.

The bulk pressure rises at the initial vaporization were less than the vapor pressure corresponding to the IB wall superheat. The pressure pulse occurred at the vapor collapse correlated reasonably well with the re-entrant liquid velocity, but the measured value was less than the prediction from sodium hammer analysis.     

惰性气体对液态金属起始沸腾过热度的影响

惰性气体对降低液态金属起始沸腾过热度有重要作用。通过考虑壁面孔穴内惰性气体的体积变化及扩散作用,分析了池式沸腾核化过程中局部参数的变化,并研究了惰性气体作用下不同变量对液态金属起始沸腾过热度的影响趋势。结果表明,惰性气体可引起复杂的次级效应,为进行准确的热流密度、压力等变量的影响分析,应充分考虑核化过程中惰性气体的影响。     

Incipient Boiling of Sodium in an Annular Channel Heated by a Single-Pin with Blockage

Abstract

In simulation of partial failure affecting a fast breeder core, experiments on sodium boiling were performed in a single vertical channel of annular cross section (15 mm I.D.), indirectly heated by high flux heater pin (6.5 mm O.D., 60 cm heating length) and provided with a blockage disk (11mm O.D., 1mm thick, obstructing 42% of the channel area). The experimental conditions were; Pressure of cover gas: 1.0 kg/cm² abs., Heat flux: 0–80 W/cm², Flow rate of sodium: 1–4/l min.

The results revealed that the degree of incipient boiling superheat is reduced to a fraction of the value obtained previously for direct joule heating under similar conditions, and that, consequently, the intensity of pressure pulses is correspondingly reduced. On the other hand, the incipient boiling superheat increases, together with the extent of its scattering, with rising intensity of the heat flux.

Local boiling was observed to precede the onset of bulk boiling, causing small pressure spikes to be detected by all three pressure sensors installed along the test channel. The pressures evaluated from the measured velocities of the liquid sodium column based on single-bubble model were compared with the measured pressure signals, and a fairly good agreement was obtained.     

液钠沸腾两相流流型与临界热流密度(CHF)机理实验研究

通过大量的液态金属钠临界热流密度 (CHF)的实验研究 ,结合液钠两相传热流动特性及液钠的物性特点 ,分析了起始沸腾流型 ,泡状流 ,块状流 ,环状流和双向环状流的热工水力特性 ;并从实验结果出发 ,深入分析了液钠发生临界热流密度时的气泡爆炸和液膜撕裂或局部蒸干的两种传热恶化机理     

Fundamentals of liquid metal boiling thermohydraulics

Developments in space and in nuclear-energy have demonstrated the heat-transfer utility of liquid metals in both low-pressure and high-temperature applications.Compared with water or organic liquids, liquid metals are advantageous in the design of thermal features, as for heat transport at high-temperatures at normal pressures.Nowadays, sodium is used in fast breeder reactors, because of its low neutron absorption. Furthermore, liquid metals have also been used in cooling valves, in heat pipes, and for driving Stirling engines.In fast reactors, boiling represents a disturbing occurrence which must be avoided, or controlled as much as possible.The main factors determining the course of the disturbance are the inception superheat, the flow patterns and the boiling characteristics. These factors are connected in a rather complex way. This paper deals with basic research in the liquid-metals boiling field, including the inherent tendency of liquid metals to superheat before the initial boiling point, the flow pattern during boiling, and the two-phase flow boiling characteristics.     

自然循环流动不稳定条件下的传热特性实验研究

为探究流动不稳定性机理,在低压自然循环系统中开展了一系列相关实验,分析了不同流量振荡模式下自然循环的沸腾传热机制及局部传热特性。实验表明：中、低热流密度下出现的较规则的周期性振荡由加热段内流动沸腾诱发,壁面过热度不会随流量振荡而大幅度变化;高热流密度下自然循环系统出现的周期性不规则振荡现象中,流动沸腾类型间的相互转变不是流量波动的唯一原因。大幅度的流量脉动可能在高热流密度下导致沸腾临界的发生,出口壁面出现间歇性干涸,局部传热系数下降的同时伴随壁温的短暂飞跃。随着热流密度的提高,自然循环系统可能出现持续性干涸。     

Boiling of Potassium in Vertical Tube under Transverse Magnetic Field

The experimental study was conducted to examine the applicability of boiling potassium two-phase flow to the blanket cooling of fusion power reactors. A high flux heater pin of 44mm heating length and 6.5mm O. D. with eight thermocouples of 0.5mm O. D. was inserted from the bottom of a vertical channel which was made of a 4m long, 14.9mm I.D. and 17.5mm O. D. stainless steel tube and placed in a D.C. magnet of 50cm long poles. The experimental conditions were the heat flux: 0~67W/cm², the magnetic field strength: 0~1.8T, the Ar cover gas pressure: 1.0bar, the potassium level above heater: 1.8m, and the temperature of upper unheated section: 400°C.

In the absence of magnetic field, boiling occurred intermittently, repeating the cycles between superheating with moderate temperature fluctuation and desuperheating with condensing shock pulses. When a weak magnetic field was applied, the temperature fluctuation was enhanced by natural convection, the incipient boiling superheat was reduced, and the boiling pattern became continuous. With increasing the magnetic field strength, the fluctuation was suppressed and the incipient boiling superheat increased but tended to level off around 1.5 T. Violent incipient boiling caused by a large superheat inherent in liquid metals was mitigated by magnetohydrodynamic interaction under a transverse magnetic field of 1.0T or larger, and subsequently followed by continuous saturation boiling with small fluctuation. No burn-out of the heater pin occurred in spite of symptom of dryout within the experimental range: q=67W/cm² and B=1.5T.     

基于CFD方法的矩形窄缝通道内过冷流动沸腾模型研究

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。     

Experimental investigation of turbulent heat transfer to liquid sodium in the thermal entrance region of an annulus

An experimental investigation has been carried out for turbulent heat transfer to liquid sodium in the thermal entrance region as well as in the thermal fully developed region. The experiments were made with a step-function heat flux distribution in the inner wall of an annulus with the Reynolds number varying from 6000 to 60 000. the inner diameter of the channel was 17.2 mm and the outer diameter 31.8 mm. The velocity profile was fully developed at the start of heating. The experimental results were found to be in good agreement with the theoretical prediction. Finally, an empirical relation was obtained for the heat transfer in the fully-developed region.     

Incipient Boiling Pressure Pulse of Sodium under Forced Convection by Direct Heating

An account is given of experiments on incipient boiling pressure pulses imparted by liquid sodium, performed in a forced convection loop. The sodium is heated by direct current applied to the liquid metal itself in a vertical tube of 14.9 mm inner diameter through which the medium pumped. The experiments were carried out with the following ranges:

Input power: 6–12kW,Pressure at condenser: 0.6–1.0 kg/cm², Inlet temperature: 780^°–850^°C, Flow rate: 1.0–3. 5 l/min

Typical pressure pulses registered during incipient boiling presented the form of damped oscillation. The initial pressure pulses were dependent on the location of boiling inception along the test section, the incipient boiling pattern and the degree of superheat. The initial pressure pulses observed were in the range of 0–3. 5 kg/cm². The pressure pulses increased with the degree of superheat, which ranged 0^°–150°C.

The magnitudes of pressure pulses exceeded the saturation vapor pressure corresponding to the temperature of superheated liquid. Consideration of a simplified model indicated that, during incipient boiling, the pressure in the vapor phase oscillates while approaching asymptotically the saturation pressure.