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。     

自然循环临界热流密度实验研究

本文介绍高压自然循环临界热流密度(CHF)实验研究。结果表明,对于稳定流动,自然循环与强迫循环CHF值几乎没有差別;不稳定流动将导致CHF降低。文中还给出了自然循环CHF工况预算模型。     

窄矩形通道自然循环流动停滞与临界热流密度研究

在华北电力大学自然循环实验室进行了自然循环条件下窄矩形通道内的临界热流密度(CHF)实验,对实验中出现的流动停滞及传热恶化现象进行了观察。提出自然循环饱和沸腾条件下,窄矩形通道内的流动停滞-传热恶化发生机理。即自然循环流量漂移发生后会产生流型变迁不稳定,继而造成流量的持续波动,并导致停滞现象,从而使出口附近的液膜层在一定的热流密度下被完全蒸发并引起CHF现象。而窄矩形通道内,由于受间隙尺寸的限制,蒸汽流对加热面上的液膜层产生挤压作用,加热面上液膜层厚度因此会变得较薄,在较小的加热量下便能发生传热恶化。基于机理分析,给出了相应的计算模型。引入了考虑窄通道间隙尺寸效应的无量纲约束数Nconf和反映自然循环流动特点的特征因子C,分别对模型进行了修正。根据实验结果,对计算模型进行了多元回归拟合,并对其准确性进行了验证。通过对实验结果与模型计算值的比较发现,随着通道入口流速和系统压力的增大,CHF均增大;而随着出口干度的增大,CHF会减小。     

自然循环临界热流密度实验现象及分析

对自然循环系统矩形通道内临界热流密度进行实验研究。研究发现:实验本体增加功率后,环状流液膜蒸干,壁温出现持续快速上升,实验本体出口发生沸腾临界。根据壁温的上升趋势和出口处流体的临界含汽率可以判断自然循环系统出现的临界热流密度(CHF)类型为干涸(Dryout)型。当自然循环系统沸腾临界出现时,自然循环流量出现明显的上升。根据理论分析可知:沸腾临界发生时导致自然循环流量上升的主要原因是环状流转变成弥散流,附在加热壁面的液膜消失,摩擦压降迅速减小。     

反应堆失流事故下堆芯自然循环冷却能力的模拟实验研究

为分析不同热工参数对强迫循环向自然循环过渡过程及稳定自然循环阶段传热及流动特性的影响,在内径为6 mm的竖直实验段内开展实验,研究不同入口过冷度、热流密度及初始流量对强迫循环向自然循环过渡过程以及稳定自然循环过程的影响。实验结果表明：入口过冷度及热流密度对自然循环向强迫循环过渡过程及稳定自然循环过程的换热能力有明显影响,而初始流量对该过渡过程及稳定自然循环的换热能力的影响并不显著。     

低压自然循环复合流量脉动实验研究

在低压流动沸腾不稳定性实验中,研究了自然循环流动在不同入口过冷度下的演化过程。对实验中的流动沸腾不稳定性入口流量信号进行快速傅里叶变换,基于振幅和频率特性区分了3种流动脉动模式:小幅流量脉动、复合流量脉动和逆流。分析了加热功率和入口过冷度对自然循环不稳定性的影响。根据加热段出口水温变化得到了出口的流型变化,当流量波动振幅较小时加热段出口流体始终是饱和状态,而当流量波动振幅较大时,加热段出口为单相液体和两相混合物交替通过。给出了这3种流量脉动的边界图,分析了热流密度和入口过冷度对流量脉动模式的影响。结果表明:出口含气率大于0时发生流动不稳定性,热流密度达到间歇干涸型临界热流密度时发生逆流。     

低压自然循环复合流量脉动实验研究

在低压流动沸腾不稳定性实验中，研究了自然循环流动在不同入口过冷度下的演化过程。对实验中的流动沸腾不稳定性入口流量信号进行快速傅里叶变换，基于振幅和频率特性区分了3种流动脉动模式：小幅流量脉动、复合流量脉动和逆流。分析了加热功率和入口过冷度对自然循环不稳定性的影响。根据加热段出口水温变化得到了出口的流型变化，当流量波动振幅较小时加热段出口流体始终是饱和状态，而当流量波动振幅较大时，加热段出口为单相液体和两相混合物交替通过。给出了这3种流量脉动的边界图，分析了热流密度和入口过冷度对流量脉动模式的影响。结果表明：出口含气率大于0时发生流动不稳定性，热流密度达到间歇干涸型临界热流密度时发生逆流。     

RELAP5对自然循环下临界热流密度的数值分析

临界热流密度(CHF)是导致沸腾传热变化而使发热元件表面发生传热恶化的现象。RELAP5等系统程序的CHF模型对于传热系统的安全分析有重要影响。基于RELAP5程序对单棒及三棒束自然循环CHF实验进行建模,并在CHF实验数据基础上对RELAP5中CHF预测值进行对比分析。实验装置是带有一个向上流动通道的自然循环回路。其中单棒束加热测试段由一根轴向非均匀加热的电加热棒及圆管外壁组成的环管状流道,三棒束流道由三根相同的轴向非均匀加热棒与三叶型的外管组成。实验条件为低压、低流量的自然循环流动:入口压力110～270 kPa、入口过冷度为10～70 K、自然循环流量0～400 kg/(m2·s)。依次以质量流量、入口压力和过冷度为基准参数对比分析实验值和RELAP5预测值。结果表明在低压、低流量及自然循环条件下,RELAP5中的CHF预测值随着质量流量的增大而增大,与入口压力及过冷度之间的依赖关系不明显。通过对实验值与模型计算值的比较得到,单棒束RELAP5模型预测的CHF值偏高于实验值,而三棒束RELAP5模型的预测值较实验值偏低。     

起伏条件下自然循环密度波不稳定性研究

以自编程序运动条件下自然循环分析程序PNCMC研究起伏条件下两相自然循环密度波不稳定性。研究表明:起伏条件下自然循环系统会发生密度波流动不稳定性现象,起伏引起的流量振荡动与密度波振荡相叠加,最终形成复杂的复合振荡;在长周期(13、23 s)起伏时流量的振荡周期与起伏周期相等,但一个起伏周期内存在多次振荡;在短周期(3 s)起伏时,复合振荡存在着2种状态,低功率时的复合振荡周期性差,表现出不可预测的在某个起伏周期内振荡的突然增强;高功率时,复合振荡表现出较好周期性;起伏条件下系统稳定边界是整体上移,而且周期越小,上移量越大。     

开式自然循环系统不稳定性实验研究

核电站非能动安全系统依靠自然循环将热量导出,然而系统中的驱动压头、流动阻力、流量等相互影响,容易引发流动不稳定性,导致流动沸腾临界热流密度大幅降低,带来安全问题。因此,流动稳定性是自然循环热力系统的重要研究对象。本文以开式竖直加热圆管为实验本体,通过实验和物理建模的方法,研究了自然循环条件下管内间歇沸腾现象和流动不稳定特性。研究结果表明：随着热流密度的升高,系统将经历初始状态、低频脉动、高频脉动和稳定的自然循环四个阶段。基于实验数据,建立了不稳定边界的预测模型,预测值和实验值符合良好,误差在±10%内。     

低压高过冷度下自然循环流动不稳定性实验研究

对具有长直上升段的自然循环系统，开展了流动不稳定性实验研究。同时，详细分析了低压、高入口过冷度条件下典型的流动不稳定现象。实验表明：自然循环系统的结构、流体的热边界条件会影响自然循环的运行特性及流动不稳定性类型。较高入口过冷度下，高热流密度导致系统脱离稳态后，很难重新回到稳定的两相自然循环流动状态。随着热流密度的提高，系统会经历间歇沸腾、复合动态流动不稳定性等状态。依据实验结果得到了高入口过冷度下的不稳定性边界图。在两相振荡期间，自然循环驱动压头和回路阻力的主要影响因素集中在长直上升段和加热段。加热段出口积聚的大量气泡对上、下游流体的强烈挤压作用是流量大幅振荡及逆流的主要原因。     

HRTL-5自然循环稳态特性

基于一维两相四方程漂移流模型,采用数值模拟的方法对5MW低温核供热堆热工水力模拟回路（HRTL-5）的自然循环稳态特性进行模拟,分析了HRTL-5自然循环流量特性及其参数效应。结果表明：1)漂移流模型比均相流模型更适用于HRTL-5;2)当系统压力为1.5MPa时,系统自然循环流量随加热热流密度的升高而增加;3)当系统压力为0.5MPa时,系统自然循环流量随加热热流密度的升高先增加后减小;4)自然循环流量随加热段入口欠热度的升高而减小;5)当加热热流密度较低时,〖JP3〗系统自然循环流量随压力的升高而减小,当加热热流密度较高时,系统自然循环流量随压力的变化呈现复杂状况。     

The effect of flow direction and magnitude on CHF for low pressure water in thin rectangular channels

Critical heat flux (CHF) at low flow condition can become important in an MTR-type research reactor under a number of accident conditions. Regardless of the initial stages of these accidents, a condition which is basically the decay heat removal by natural convention boiling can develop. Under such conditions, burnout may occur even at a very low heat flux. In view of this, the CHF at low-flow-rate and low-pressure conditions has been studied for water flowing in thin rectangular channels.Experiments were carried out with two types of rectangular test sections, namely, the one heated from one wide side and the other heated from two opposite sides. In order to observe the effects of gravity, CHF was measured both in upflow and downflow. The CHF at complete bottom blockage was also studied.The results indicate that burnout can occur at a much lower heat flux than pool-boiling CHF or than predicted by the conventional correlations. There was observed a minimum CHF at complete bottom blockage and at very low downflow. The low CHF at very low downflow appears to be due to the stagnation of the bubble in the heated section. This fact indicates that special care should be taken in analyzing the boiling phenomenon which occurs when the coolant flow is very low in a low pressure system.     

Investigation on the characteristic of CHF in various flow pattern regimes based on look-up table data

The critical heat flux (CHF) is one of the important phenomena limiting the maximum rate of heat transfer and hence power rating of nuclear reactors. The thermal hydraulic phenomena like pressure drop, heat transfer, stability, etc. depends upon the flow pattern in the system. The CHF phenomenon is also closely related to the two-phase flow patterns. It is important to investigate the dependence of CHF on the flow pattern regimes to understand the underlying mechanisms. The present investigation reveals that CHF generally increases with mass flux in the churn/slug region. However, in the annular region the CHF decreases with increase in mass flux. Considering the dependency of the CHF trend on the flow pattern regime, it will be useful to develop CHF models, which are specific to the flow pattern regime. The data of CHF look-up table has been considered in this investigation since this approach is one of the most reliable methods for the prediction of CHF and is being used in several best-estimate thermal-hydraulic system codes, such as RELAP5, CATHARE and CATHENA. The pressure, mass flux and quality have been considered as important thermal hydraulic parameters to characterize the flow pattern during CHF under various operating condition.     

自然循环与强迫循环条件下阻力及换热特性对比

为研究自然循环和强迫循环条件下流动及换热特性的区别,以单面加热窄矩形通道为研究对象,在压力0.2 MPa、实验段入口欠热度30~60℃的条件下,分别进行了强迫循环和自然循环条件下流动及换热实验。等热流密度条件下的阻力实验研究表明:在层流区,强迫循环和自然循环条件下的阻力特性几乎相同;在湍流区,修正后的Blasius关系式能同时适用于强迫循环和自然循环条件下的阻力预测;通过对比发现,强迫循环和自然循环条件下的转捩点雷诺数以及过渡态雷诺数区间几乎相同。换热实验研究表明:在湍流区,适用于强迫循环条件下的Gneilinski关系式能对自然循环换热能力较好预测;通过分析发现,在本实验研究范围内,自然循环与强迫循环条件下换热能力无明显区别。     

Critical heat flux under zero flow conditions in vertical annulus with uniformly and non-uniformly heated sections

The experimental study of water CHF (critical heat flux) under zero flow conditions has been carried out in an annulus flow channel with uniformly and non-uniformly heated sections over a pressure range of 0.52–14.96 MPa. In the present boiling system, the CHFs occur in the upper region of the heated section, in contrast to the results in the experiments for boiling tubes conducted by several investigators. The general trend of the CHF with pressure is that the CHF increases up to a medium pressure of about 6–8 MPa and decreases as the pressure is further increased. A comparison of the present data with the existing flooding CHF correlations shows that the correlations depend greatly on the effect of the heat flux distribution. When the correction terms with the density ratio and the effect of the heat flux distribution proposed in the present work are used with the CHF correlation based on the Wallis flooding correlation, it predicts the measured flooding CHF within an RMS error of 9.0%.     

Critical heat flux for finned and unfinned geometries under low flow and low pressure conditions

This paper presents the experiment and analysis for the critical heat flux (CHF) in a vertical annulus with finned and unfinned geometries under low flow and low pressure conditions. To consider the fin effect on CHF, the tests were performed on both finned heater and unfinned heater having same dimension as finned heater without fins. An analytical model was applied to estimate the heat flux and temperature distributions along the periphery of the finned geometry. The physical phenomena observed during the experiments are discussed and the parametric trends of the obtained data are examined to investigate the CHF characteristics for the finned geometry. A new correlation is proposed to predict the CHF for both finned and unfinned geometries at low flow and low pressure conditions. The developed correlation predicts the experimental data with an RMS error of 13.7%.