脱湿过程中红黏土热导率的变化试验研究

自然状态下岩土体中的三相组成处于动态变化之中,对土体热传导性能的准确测试造成困难,通过热导率脱湿曲线(TCDC)可间接获取土体的传热性能。为此,以桂林红黏土为研究对象,采用压力板仪和KD2 Pro土壤热特性分析仪,研究了红黏土在脱湿过程中热导率的变化规律。试验结果表明:在进气值之前,土体饱和度不变,3种干密度的土体热导率都有小幅度增加,此阶段主要与吸力变化引起的土体密实度变化有关;当吸力超过进气值后,热导率随饱和度减小逐渐减小,此阶段含水率为影响热导率的主要因素。基于热导率试验结果,采用Lu模型和William模型对红黏土脱湿过程中热导率变化曲线进行拟合,整体上2模型对试验结果拟合较好,可以用于红黏土脱湿过程中热导率变化的模拟。     

R134a水平微细管内流动沸腾换热的实验研究

本文对R134a在水平微细管内的流动沸腾进行了实验研究。实验测试段选用了内径为1 mm、2 mm、3 mm共3种不同的水平光滑不锈钢管,实验的饱和温度为5~30℃,热流密度为2~70 k W/m2,流量范围为200~1500 kg/(m2·s)。实验结果表明:相同条件下,干涸前2 mm管较3 mm管换热系数平均增幅为11.6%,1 mm管较2 mm管换热增幅为26.3%,1 mm管径换热系数比3 mm管径平均增大40.8%。随着管径的减小,换热系数在更低的干度开始减小,质量流速和强制对流蒸发作用对换热系数的影响变小,热流密度的影响依然显著;塞状流和弹状流区域减小,泡状流和环状流区域增大。     

直流蒸汽发生器全范围流动与传热数值模拟

以BW公司实际运行的直流蒸汽发生器为原型,采用两流体三流场数学模型数值模拟直流蒸汽发生器二次侧单相液对流换热区、核态沸腾区、蒸干后区域、过热区(即全范围)的流动沸腾换热现象。结果表明:本文采用的模型可较好地预测直流蒸汽发生器全范围的流动与传热特性,数值模拟结果与运行数据符合较好;蒸干后质量含汽率低于热平衡质量含汽率,结合壁温变化趋势表明直流蒸汽发生器运行过程中蒸干后区域存在一定程度的热力非平衡;随着流动换热的发展,表面传热系数迅速上升,蒸干的发生导致传热性能急剧下降,在蒸干后区域及过热蒸汽区表面传热系数缓慢上升。     

Investigation of Pre- and Post-Dryout Heat Transfer in Upward Steam-Water Two-Phase Flow at Low Flow Rate

A heat transfer experiment was performed on steam-water two-phase flow in an annular flow path with a uniformly heated rod under the conditions of the mass flow rates from 0.2xlO⁶ to l.Ox 10⁶ kg/m²-h, inlet qualities from 0.5 to 1.0, heat fluxes below 4.7x 10⁵ W/m² and pressure of 31 bar. Dryout of the heater rod surface was observed resulting in the sharp rise of the heater rod surface temperature. Measured heat transfer coefficients were compared with the several empirical and semi-empirical correlations with the emphasis on the applicability of the correlations to the present test conditions being important in the analysis of the thermal hydraulic behavior during a LOCA of a nuclear reactor. The measured heat transfer coefficient in the pre-dryout region is lower than the existing correlations. The cooling of the heat transfer surface by the liquid phase in the post-dryout region is significant, which is neglected in the existing correlations. The heat transfer coefficients calculated for the post-dryout region by the Groeneveld correlation show good agreement with the presently measured results within the accuracy of 0~27%.     

Dryout Heat Flux for Core Debris Bed, (I)

For gaining basic data on decay heat coolability of debris bed in the post-accident heat removal, measurement of dryout heat flux was made, with stagnant water as coolant, in a 50mm I. D. pyrex glass cylinder vessel. The fuel debris bed subjected to decay heat was simulated by steel ball particles which were inductively heated with a power supply of 20 kHz and 30 kW. The bed was made of homogeneous size particles. An emphasis was placed on the influence of system pressure and particle size. The experiment covered the ranges over the steel ball diameters of 0.3–4.0 mm and the system pressure of 0.02–0.5 MPa.

The experimental results, as a whole, agreed fairly well with the prediction based on Lipinski's 0-D model with respect to the dependence of dryout heat flux both on pressure and on particle size. In detail, however, the dryout heat flux deviates toward a lower value at a higher pressure while to a higher value for a smaller size particle bed. Comparison of the results between the free and fixed beds suggests that the deviation to the higher side will be attributed to the channeling and/or levitation.     

Dryout Heat Flux for Core Debris Bed, (III)

Mixture of cylindrical steel pellets and Al₂O₃ balls, which simulated the intact fuel pellets and fragmented claddings, respectively, was inductively heated in a 50 mm I.D. pyrex glass cylinder filled with water, to investigate the coolability of TMI-2 type degraded core debris bed. The size of steel pellets was 11 mm dia. × 11 mm for BWR, 8 mm dia. × 12 mm for PWR and 5.5 mm dia. × 9 mm for FBR and Al₂O₃ balls were about 2 mm in diameter. The height of the debris bed was 25 cm or lower.

The dryout heat flux does not level off up to a bed height of 25 cm or over for the TMI-2 type bed while 8 cm or so in the bed of only steel balls. The dependence of dryout heat flux on the system pressure agrees with the Lipinski's 0-D model by adopting a proper equivalent diameter. When a simple number-weighted average is used as the equivalent diameter, the prediction gives a fairly good agreement with the experiment for FBR type bed but underestimations for the PWR and BWR type beds. It should be noted, that the small balls of less fraction, not the large pellets, substantially govern the dryout. When the coolant flow is allowed from the bottom, however, the dryout heat flux is enhanced up to the level for the complete vaporization of coolant, and small amount of mass flux or circulation head can greatly improve the coolability.     

Development of a Transient Boiling Transition Analysis Method Based on a Film Flow Model

A new single-channel, transient boiling transition (BT) prediction method based on a film flow model has been developed for a core thermal-hydraulic code. This method could predict onset and location of dryout and rewetting under transient conditions mechanically based on the dryout criterion and with consideration of the spacer effect. The developed method was applied to analysis of steady-state and transient BT experiments using BWR fuel bundle mockups for verification. Comparisons between calculated results and experimental data showed that the developed method tended to predict occurrence of rewetting earlier, however, onset time of BT and maximum rod surface temperature were well predicted within 0.6 s and 20°C, respectively. Moreover, it was confirmed that consideration of the spacer effect on liquid film flow rate on the rod surface was required to predict dryout phenomena accurately under transient conditions.     

CONVECTIVE BOILING IN PARALLEL MICROCHANNELS

Experiments were performed with clear water and with surfactant flowing in parallel triangular microchannels. The study is based on systematic measurements of temperature and flow pattern by infrared radiometry and high-speed digital video imaging. Different flow patterns were observed simultaneously in various microchannels at a fixed value of water or surfactant flow rates. Depending on flow and heat flux, pressure and temperature instabilities in the heated microchannels were studied. This work develops a practical modeling approach for two-phase microchannel heat sinks and also considers effect of surfactant on convective boiling in microchannels.     

富液式吸附隔板的研制

叙述了富液式吸附隔板的设计思想、技术途径、原料特点、产品性能及结构。     

直管式直流蒸汽发生器不同运行参数下两相流压降预测

将B&W公司的直流蒸汽发生器进行简化,采用常热流边界条件进行不同运行参数下直流蒸汽发生器二次侧流动与换热过程数值模拟,并与经典摩擦压降经验关联式进行对比。结果表明：Martinelli-Nelson关联式更适用于预测蒸干发生时两相流的摩擦压降;摩擦压降随质量含汽率增加整体呈现上升趋势,蒸干发生时摩擦压降的变化率明显增大;管内气液两相流摩擦压降随质量流量和热通量增加而增大,随运行压力增大而减小。其中质量流量、运行压力对摩擦压降的影响较明显,热通量对其影响较小。