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.     

Experimental Studies on Heat Transfer by Natural Convection and Pool Boiling of Sodium in a Strong Magnetic Field

This report deals with an experiment on the heat transfer of liquid sodium, with particular reference to the effects brought by the application of a magnetic field on pool boiling. The test section, a heater pin of 6.5 mm diameter, was inserted vertically into the center of a sodium tank. The heating surface of the pin was parallel to the magnetic field as well as to the direction of gravity.

Under conditions of natural convection in a magnetic field, a sharp rise of the heating surface temperature was always seen to occur at some point when the heat flux was gradually increased, accompanied by the onset of sharp temperature oscillations.

The surface superheat required for the initiation of boiling decreased with increasing intensity of the applied magnetic field, reached a minimum, then increased again.

The surface temperature fluctuations in nucleate boiling was higher under magnetic field than when free of such influence. The critical heat flux for burn-out was not appreciably affected by magnetic field.     

Natural Convection Heat Transfer of Liquid Lithium under Transverse and Parallel Magnetic Fields

Stagnant liquid lithium contained in a vertically 1.3 m long and 46 mm I.D. 316-SS cylindrical vessel up to a 0.3 m level was heated by a concentrically inserted heater pin of 12.5 mm O.D. and 54 mm active length. The parallel magnetic field to the vessel was imposed by a superconducting magnet. The experiment covered the ranges of the lithium temperature: 320～510°C, the heat flux: 10～40 W/cm², the transverse B: 0～1.2 T (Ha = 0～2,730) and the parallel B= 0～3T (Ha = O～6,860).

The temperature fluctuation is enhanced by imposing a weak magnetic field of B = 0.1 ～ 0.3 T for both parallel and transverse fields and almost completely suppressed with increasing to B = 1 T in the case of the transverse field but its low frequency component still remains large, becoming oscillatory, up to B = 3 T in the case of parallel field. The heat transfer shows a similar trend to the temperature fluctuation. It increases singularly by a weak B especially in the perpendicular sector to the transverse magnetic field and decreases with increasing B. In the case of parallel magnetic field, the heat transfer increases in a weak field of B = 0.1 ～ 0.5 T, the same as in the transverse magnetic field, but it does not decrease so much in a strong field of B = 1～3 T, presenting a rather higher value than in B = 0 T.     

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.     

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.     

Influence of Magnetic Field on Boiling of Stagnant Potassium

Abstract

The influence of a magnetic field on the boiling of stagnant potassium was experimentally studied in the range of field intensity from 0 to 0.7 Tesla under low pressure cover gas. The results indicated that incipient boiling superheat and the location of boiling inception were not influenced by the magnetic field. On the other hand, the magnetic field very appreciably impeded the motion of the liquid column supported above the vapor bubble after boiling was initiated. Another effect of the magnetic field proved to be inhibition of vapor condensation in the void space during the period of descend of the liquid column. As a result the vapor in the void space remained uncondensed, to serve as permanent cushion for the liquid column, which maintained an oscillatory motion, to which corresponding oscillations were registered in the pressure and temperature of the vapor bubble, which changed along the vapor saturation line.     

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。     

Incipient Boiling Phenomena of Sodium under Forced Convection by Direct Heating

The results of an experimental study on incipient sodium boiling with upward forced convection loop are reported. Forced convection sodium in a vertical tube of 14.9mm I.D. was directly heated by direct current (–4V, –5,000A). The experiments were carried out under the following conditions:

Input power: 6–12kW, Pressure at the condenser: 0.9–1.1 atm

Flow rate: 1.0–3.0l/min, Inlet temperature: 7,500°–830°C

The incipient boiling phenomena observed were either violent single bubble ejection or else slow vapor formation depending on the conditions.

It was indicated from observations of the position of boiling inception that the boiling did not always occur at the position of maximum temperature.     

Heat Transfer and Temperature Fluctuation of Lithium Flowing under Transverse Magnetic Field

For providing background information on the liquid metal cooling of fusion reactors, the MHD effects on heat transfer of flowing lithium was experimentally studied with an emphasis on temperature fluctuations. The test section was constituted of a 15.8 mm I.D., 1.65 mm thick, 316-SS tubing and a 7.6 mm O.D. heater pin with nine 0.5 mm thermocouples. The experiment covered the range of B=0–1.0 T, U=0.2–4.0m/s, T=320–390°C and q=0–68 W/cm². The results are summarized as follows:

(1) With increasing magnetic flux density B, Nusselt number Nu decreases to a value of flat flow velocity model: 6.38 but rises in B=0.1–0.4T. (2) The RMS of temperature fluctuation has strong dependence on B and presents a peak near B=0.25T. (3) The singular rise of Nu is inferred to result from enhanced turbulence due to steepening of radial gradient of velocity near the wall as a result of MHD effect. (4) The transit velocity estimated from temperature cross-correlation in the parallel sector coincides with the mean velocity when B=0 but deviates toward a lower value with applying B. (5) Local flow velocity is decelerated in the magnetically-parallel sectors and accelerated in the perpendicular.     

Comments on “Dispersion Law in Anisotropic Scattering Media,by T. Hino & Y. Ozawa”

In the liquid metal boiling with the incipient boiling superheat, the pressure transient is large enough to be measured. In this paper the time constant of pressure transient τ in the growing stage of the vapor bubble is measured and its dependence on the channel geometry, the dynamic constraint and the liquid temperature is made clear. The evaporation coefficient C is also obtained by measuring the vapor pressure and the liquid column velocity. For potassium C is constant in our experiment and its value is (3.8±0.6)X10^?3. Using the measured C, the τs are computed with the momentum equation, the state equation and the Hertz-knudsen equation in the kinetic theory.     

Study on Pb-Bi-water direct contact boiling two-phase flow and heat transfer

For the development of 45w%Pb-55w%Bi cooled direct contact boiling water small fast reactor (PBWFR), Pb-Bi-water direct contact boiling two-phase flow loop has been fabricated and operated. The loop consists of a Pb-Bi flow loop (four heater pin bundle, a chimney, an upper plenum, a level meter tank, an air-water cooler, and an electromagnetic flow meter) and a water-steam flow loop (a pump, a preheated, an injection nozzle, the chimney, the upper plenum with mist separators and dryers, a condenser, a buffer tank, and an air-water cooler). At the rated operating condition system pressure is 7 MPa. The sub-cooled water was injected into a Pb-Bi flow in the chimney. A power of the heater pin bundle was controlled to obtain the inlet and outlet temperatures of the heater bundle. The Pb-Bi and steam flows were simulated analytically using one-dimensional models of frictional and form losses and a drag force. The Pb-Bi-steam two-phase frictional pressure loss was calculated by means of the two-phase flow multiplication factor of Lockhart-Martinelli model. It was found that Pb-Bi temperature decreased quickly in the chimney due to high heat transfer rate of Pb-Bi-water direct contact boiling. The volumetric overall heat transfer coefficient was 60–310 kW/m³K, and decreased with the superheat.     

Ejection Velocity and Residual Film during Initial Expulsion of Sodium in Direct Heated Channel

An experimental study of transient boiling of sodium by direct heating is presented. Liquid sodium circulating by forced convection in a vertical tube of 14.9 mm inner diameter is heated by direct current (～4 V, ～5,000 A) applied through the sodium.

The velocity of ejection of the vapor bubble and the variations in the thickness of the film remaining on the channel wall were measured during a single bubble ejection. Furthermore, a simplified model formulated around the inertia of the liquid column was used to derive estimates for comparison with measured data. The experiments were performed within the following ranges:

Input power: 6～12 kW

Pressure at the condenser.: 0.6～1.1 kg/cm²

Flow rate: 1.0～3.5 l/min

Inlet temperature.: 750°～850°C

The ejection velocity measured was between 2.0 and 16m/sec. The velocity increased with the degree of superheat, i.e., the velocity was governed by the degree of superheat in the region where the vapor phase expanded.

The film thickness ranged from near zero to 1.0 mm. The film thickness increased with ejection velocity.     

Experimental research on heat transfer to liquid sodium and its incipient boiling wall superheat in an annulus

Liquid sodium is mainly used as a cooling fluid in the liquid metal fast breeder reactor （LMFBR）, whose heat transfer, whether convective heat transfer or boiling heat transfer, is different from that of water. So it is important for both normal and accidental operations of LMFBR to perform experimental research on heat transfer to liquid sodium and its boiling heat transfer. This study deals with heat transfer with high temperature （300-700℃） and low Pe number （20-70） and heat transfer with low temperature （250-270℃） and high Pe number （125-860）, and its incipient boiling wall superheat in an annulus. Research on heat transfer involves theoretical research and experiments on heat transfer to liquid sodium. It also focuses on the theoretical analysis and experimental research on its incipient boiling wall superheat at positive pressure in an annulus. Semiempirical correlations were obtained and they were well coincident with the experimental data.     

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。     

Applications of Artificial Neural Network for the Prediction of Flow Boiling Curves

An artificial neural network (ANN) was applied successfully to predict flow boiling curves. The databases used in the analysis are from the 1960's, including 1,305 data points which cover these parameter ranges: pressure P=100–1,000 kPa, mass flow rate G=40–500 kg/m²-s, inlet subcooling ΔT_sub =0–35°C, wall superheat ΔT_w = 10–300°C and heat flux Q=20–8,000kW/m². The proposed methodology allows us to achieve accurate results, thus it is suitable for the processing of the boiling curve data. The effects of the main parameters on flow boiling curves were analyzed using the ANN. The heat flux increases with increasing inlet subcooling for all heat transfer modes. Mass flow rate has no significant effects on nucleate boiling curves. The transition boiling and film boiling heat fluxes will increase with an increase in the mass flow rate. Pressure plays a predominant role and improves heat transfer in all boiling regions except the film boiling region. There are slight differences between the steady and the transient boiling curves in all boiling regions except the nucleate region. The transient boiling curve lies below the corresponding steady boiling curve.     

水基磁性流体竖直加热棒下的池沸腾传热实验研究

研究了磁性微粒浓度、外加磁场对水基磁性流体在竖直加热棒加热情况下的沸腾传热影响;实验结果显示,磁性微粒浓度和外加磁场对磁性流体的沸腾换热有很大影响：对于中低浓度的磁性流体,存在一个最优的磁粒浓度,在该浓度下沸腾传热的强化效果最显著,施加磁场时,该结论仍然成立。施加磁场能强化磁性流体的沸腾传热。     

氧化层对液态金属起始沸腾过热度的影响模型

液态金属具有良好的浸润性,因此常具有很高的起始沸腾过热度。但在浸润性差的氧化层的影响下,起始沸腾过热度可大幅降低。为对氧化层作用进行分析,建立了氧化层的影响模型,并利用实验数据进行了验证。结果表明,氧化层作用下的核化过程可分为直接核化和界面移动后的核化两类,氧化层作用下的核化半径介于氧化层位置及界面最深处位置之间,其最佳估算值为两者的几何平均值。     

Direct Measurement of Neutron Flux

An experimental study was carried out on steady-state boiling of sodium flowing in the annular channel formed around an electrically heated simulation of a fuel pin. In the present experiment, the inlet temperature and flow rate were held constant, and the heat flux was gradually increased up to the inception of boiling. Thereafter, the heat flux was further increased step by step until the surface temperature of the heater pin marked a sharp rise, indicating the occurrence of dry-out. Records were obtained of the changes brought by the increasing heat flux to boiling phenomena, with particular reference to the behavior of the two-phase flow pattern and to the characteristics of boiling noise, as well as of the frequency of bubble formation.

It was made clear that there exists a region in which steady-state boiling will be established, and under these conditions the two-phase flow pattern changes sequentially from bubbly flow to slug flow and then to annular flow. This behavior of sodium boiling in a narrow channel is quite similar to the case of water.

With rising heat flux, the level of noise intensity associated with boiling first increased sharply to attain a maximum point, then decreased somewhat and remained constant thereafter until dry-out.

The frequency of bubble formation depended on the size of the bubble. The product of bubble frequency and equivalent diameter was found to be constant.     

Incipient Sodium Boiling Superheats in Bundle Geometry under Loss-of-Flow Conditions

Transient sodium boiling experiments were conducted using electrically heated pins arranged in 7-, 19- and 37-pin bundles to reproduce loss-of-flow conditions. The average degrees of superheat attained before inception of sodium boiling amounted to 47, 45 and 16°C respectively with the 7-, 19- and 37-pin bundles.

The experimental results indicated that the degree of superheat decreased with increasing number of heating pins in the bundle. This tendency can be explained from the fact that a larger bundle has a radially wider region of sodium saturation temperature at boiling inception comparatively—with the smaller cooling effect exerted by the peripheral subchannels—and consequently a larger number of active nuclei to trigger boiling and terminate the superheating phenomenon. No meaningful correlation was discerned between the degree of superheat and other factors like sodium velocity, rate of sodium temperature rise and intensity of applied heatflux.     

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.