含不凝性气体的蒸汽冷凝传热实验研究

对含不凝性气体的蒸汽在竖直圆管外表面冷凝传热进行实验研究,分析过冷度、压力、不凝性气体质量分数以及氦气占比对蒸汽冷凝换热的影响,给出冷凝传热过程中的经验关联式并同经典公式进行对比。结果表明:在压力不变的条件下,壁面过冷度同冷凝传热系数的变化趋势相反;实验范围内,未发生氦气分层现象;所得到的经验关联式具有更广的适用范围,且其与实验值的误差在±20%以内。     

含不凝性气体的蒸汽冷凝传热实验研究

对含不凝性气体的蒸汽在竖直圆管外表面冷凝传热进行实验研究,分析过冷度、压力、不凝性气体质量分数以及氦气占比对蒸汽冷凝换热的影响,给出冷凝传热过程中的经验关联式并同经典公式进行对比。结果表明:在压力不变的条件下,壁面过冷度同冷凝传热系数的变化趋势相反;实验范围内,未发生氦气分层现象;所得到的经验关联式具有更广的适用范围,且其与实验值的误差在±20%以内。     

竖直管束外含空气蒸汽冷凝的实验研究

本文对竖直管束及单管的管外冷凝换热进行了实验研究,分析了管壁面过冷度、混合气体压力和不凝性气体含量对管束外冷凝传热性能的影响,对比了管束与单管的传热特性,给出了管束外冷凝传热系数的计算关联式。研究结果表明,管束的平均冷凝传热系数随过冷度的增大而减小,随混合气体压力的增大而增大,随不凝性气体质量分数的增加而减小。在混合气体高压力、低不凝性气体含量时管束的传热效果明显优于单管。关联式计算值与实验值误差范围小于±10%。     

流速对含不凝气体的蒸汽冷凝的影响

蒸汽在安全壳内壁面上的冷凝是事故发生后安全壳内的气体向壳壁传热的主要方式,是影响钢制非能动安全壳压力响应的重要因素。针对事故后核电厂安全壳内的事故工况条件,在较宽的参数范围内开展了蒸汽冷凝传热过程的试验研究。试验压力为0.11~0.5 MPa(d),主流空气质量分数为29%~78%、壁面过冷度为26~60℃,混合气体平均流速0.4~1.9 m/s。试验结果表明:在0.9 m/s以下的低流速范围内,试验数据与经验关系式的计算结果符合较好;流速高于0.9 m/s时,流速成为影响含有不凝性气体的蒸汽凝结传热的主要因素之一;主流空气质量分数较低时,流速对含有不凝性气体蒸汽冷凝的传热系数的影响更加显著;对于伴有蒸汽冷凝的对流换热过程,由自然对流向混合对流转变的判据与单相对流换热过程不同。     

壁面过冷度对水平管内含空气蒸汽冷凝换热影响

为研究壁面过冷度对含不凝性气体蒸汽流动冷凝换热能力的影响,分别进行水冷却与空气冷却条件下空气-蒸汽混合气在水平管内的流动冷凝实验,分析空气入口质量分数,混合气入口流速以及管外冷却剂体积流量变化时,管内局部换热系数随壁面过冷度的变化规律。结果表明:在环状流及波状流范围内,局部换热系数随壁面过冷度的增加而增大;在分层流范围则随壁面过冷度的增加而减小;空气含量与混合气流速的增加,会使得环状流和波状流下壁面过冷度增大对冷凝换热能力的促进作用更为显著。     

含不凝性气体蒸汽浸没射流冷凝压力振荡实验研究

实验研究了不凝性气体（空气）含量、水温和蒸汽质量流速对蒸汽浸没射流冷凝压力振荡特性的影响,实验工况横跨冷凝振荡（CO）区和稳定冷凝（SC）区。结果表明：对于纯蒸汽射流,压力振荡主频随水温的升高而降低,振荡强度随水温的升高而升高;在CO区,振荡主频和振荡强度均随蒸汽质量流速的升高而升高;在SC区,振荡主频随蒸汽质量流速的升高而降低,振荡强度基本上不随蒸汽质量流速的变化而发生改变;对于含空气射流,随空气质量分数的增加,振荡主频总体呈下降趋势,振荡强度先迅速下降后小幅上升,在空气质量分数为0.05～0.1区域内振荡主频和振荡强度均存在极小值。     

水平管内蒸汽冷凝局部换热特性实验研究

以水蒸气为工质,实验研究了水平管内纯蒸汽冷凝的局部换热特性。实验选取换热管内径为25 mm、换热管进口压力为0.15~0.4 MPa、局部蒸汽的Re=5756~92289,分析了蒸汽压力及流速、壁面过冷度对冷凝传热系数的影响,并将采用现有关系式计算的冷凝传热系数与实验结果进行了对比。结果表明：冷凝传热系数随壁面过冷度的增大而减小,随压力的升高和流速的增大而增大;采用现有关系式计算的冷凝传热系数与实验值的偏差较大,关系式有待进一步改进;在实验范围内,由拟合换热关系式计算所得冷凝传热系数与实验结果的相对偏差在15%左右。     

不同管径与倾角下单管和管束外含空气蒸汽冷凝对比分析

为评估不同传热管结构参数下单管与管束外含空气蒸汽冷凝传热规律的差异,基于外径12～19 mm、倾角0°～90°的单管和3×3管束在压力0.2～1.6 MPa、空气质量份额12%～87%的参数范围内开展了试验研究。结果表明：不同压力范围内,管径和倾角对单管和管束的影响呈现不同的规律。在压力小于0.8MPa时,管束冷凝传热受管径和倾角影响的规律与单管总体一致,两者的冷凝传热系数均随管径和倾角的减小而增大。在0.8～1.6 MPa时,管束冷凝传热受管径和倾角的影响与单管存在明显差异。结合不凝性气体影响蒸汽冷凝传热的机制对所呈现的一致性和差异性规律进行了分析。     

几何参数对含空气蒸汽冷凝影响的数值分析

含空气蒸汽冷凝是反应堆失水事故时安全壳内重要的热工水力现象。已有研究多关注气体压力、温度等热工参数对传热特性的影响,而对几何参数的影响及其作用原理分析较少。采用三维CFD数值模拟方法,基于扩散边界层冷凝机理模型研究了管径(4~60 mm)、管长(0.1~7 m)及倾角(0°~90°)对含空气蒸汽冷凝传热特性的影响。结果表明,管径、管长及倾角均对含空气蒸汽冷凝传热特性有显著影响。平均冷凝传热系数随管径的增大而减小;随管长的增长先减小后增大,3 m左右达到最小值;随倾角的增大而增大。局部冷凝传热系数沿管长方向先迅速减小后缓慢增大。倾斜布置时,迎流面产生明显传热强化,向两侧逐渐减弱,背流面存在一定的传热抑制。     

低过冷流场中蒸汽水下喷注直接接触冷凝声学特性研究

低过冷流场中蒸汽水下喷注直接接触冷凝（DCC）是核动力舰船冷凝器热井鼓泡除氧的热力基础,该过程涉及强湍动、相变、多相流等复杂行为,可诱发强烈的流场压力脉动与辐射噪声,是影响舰船声隐身性能的重要因素。针对该问题,本文通过实验对流场过冷度2.8~14.7 ℃范围内蒸汽水下喷注DCC过程的汽羽流型演化和声学特性进行研究。结果表明,蒸汽质量流速和流场过冷度对汽羽流型演化具有不同的作用机制和影响规律。低过冷流场中,蒸汽水下喷注冷凝噪声具有宽频特征;流场过冷度极低时,低频噪声占辐射噪声的主导地位,随流场过冷度的增大,高频噪声的贡献逐渐提升。     

稳定蒸汽浸没射流冷凝传热系数评价

为分析稳定蒸汽浸没射流的传热特性,对3类典型冷凝传热系数开展评价。结果表明:平均传热系数实验值精度主要受界面面积计算模型影响,由冷凝驱动势和蒸汽质量流速表征的传统半经验关系式在不同孔径下的预测偏差较大,新增排放孔径为独立拟合变量的纯经验关系式适用范围更广且误差在±30%以内;界面传热系数的预测精度主要受汽羽微观参数取值的影响;由压力振荡主频表征的无量纲传热系数在低池水过冷度下与实验值偏差较大,关系式中纳入汽羽贯穿长度后,预测趋势与实验值类似。      

Experimental and empirical study of steam condensation heat transfer with a noncondensable gas in a small-diameter vertical tube

An experimental study was performed to investigate local condensation heat transfer coefficients in the presence of a noncondensable gas inside a vertical tube. The data obtained from pure steam and steam/nitrogen mixture condensation experiments were compared to study the effects of noncondensable nitrogen gas on the annular film condensation phenomena. The condenser tube had a relatively small inner diameter of 13 mm (about 1/2-in.). The experimental results demonstrated that the local heat transfer coefficients increased as the inlet steam flow rate increased and the inlet nitrogen gas mass fraction decreased. The results obtained using pure steam and a steam/nitrogen mixture with a low inlet nitrogen gas mass fraction were similar. Therefore, the effects of noncondensable gas on steam condensation were weak in small-diameter condenser tubes.A new correlation was developed to evaluate the condensation heat transfer coefficient inside a vertical tube with noncondensable gas, irrespective of the condenser tube diameter. The new correlation proposed herein is capable of predicting heat transfer rates for tube diameters between 1/2- and 2-in. because of the unique approach of accounting for the heat transfer enhancement via an interfacial shear stress factor.     

Forced convection condensation in the presence of noncondensable gas in a horizontal tube; experimental and theoretical study

To have a better understanding on forced convection condensation with noncondensable gas inside a horizontal tube, an experimental research and theoretical investigation were conducted under annular and wavy flow. The effects of noncondensable gas mass concentration, mixture gases velocity, pressure and inner wall sub-cooling on the condensation heat transfer have been analyzed. The results indicate that the local heat transfer coefficient increases with the increase of the mixture inlet velocity and pressure while decreases with the increase of the noncondensable mass fraction and wall sub-cooling. Based on the above conclusions, an empirical correlation for predicting the local heat transfer coefficient was proposed which showed a good agreement with the experimental data with an error of ±20%. Furthermore, a theoretical model using the heat and mass transfer (HMT) analogy method was developed including the suction effect. The heat transfer capacity for the film, gaseous boundary and convective heat transfer of the bulk gases were compared along the tube. Besides, the axial distribution of the bulk gases and liquid–gas interface temperatures inside the tube were analyzed. The present theoretical model fits better with the experimental data compared with Lee's and Caruso's models for stratified flow.     

Evaporation and condensation heat transfer with a noncondensable gas present

To evaluate the system pressure of an external water wall type containment vessel, which is one of the passive systems for containment cooling, the evaporation and condensation behavior under a noncondensable gas presence has been experimentally examined. In the system, steam evaporated from the suppression pool surface into the wetwell, filled with noncondensable gas, and condensed on the containment vessel wall. The system pressure was the sum of the noncondensable gas pressure and saturated steam pressure in the wetwell. The wetwell temperature was, however, lower than the supression pool temperature and depended on the thermal resistance on the suppression pool surface. The evaporation and condensation heat transfer coefficients in the presence of air as noncondensable gas were measured and expressed by functions of steam/air mass ratio. The evaporation heat transfer coefficients were one order higher than the condensation heat transfer coefficients because the local noncondensable gas pressure was much lower on the evaporating pool surface than on the condensing liquid surface. Using logal properties of the heat transfer surfaces, there was a similar trend between evaporation and condensation even with a noncondensable gas present.     

近临界压力区垂直内螺纹管临界热流密度实验研究

在近临界压力区,对垂直上升内螺纹管流动沸腾的偏离泡核沸腾(DNB)型临界热流密度(CHF)现象进行了实验研究。试验段采用ф35 mm×5.67 mm六头内螺纹管。实验参数范围为:压力18~21 MPa,质量流速500~1 000kg/(m~2·s),进口过冷度3~5℃,内壁热负荷40~960kW/m~2。实验得到了不同工况下的内壁温度和传热系数分布特性,分析了流动参数对内螺纹管中DNB型CHF的影响,并根据实验数据拟合出两相区的传热关联式与临界热流密度(qCHF)预测关联式。内螺纹管的qCHF实验数据被用于与光管的qCHF预测值进行对比,发现内螺纹管具有一定的CHF强化作用,但当压力越靠近临界压力时这种作用会被抑制甚至消失。实验结果表明:在近临界压力下,内螺纹管会在低干度区甚至过冷区发生DNB现象,压力的增大和质量流速的减小均会使DNB提前发生。qCHF随压力的减小和质量流速的增大而增大。在特定工况下,试验段不同截面会分别发生偏离泡核沸腾与蒸干。     

An experimental investigation of a passive cooling unit for nuclear plant containment

A set of condensation experiments in the presence of noncondensables (e.g. air, helium) was conducted to evaluate the heat removal capacity of a passive cooling unit in a post-accident containment. Condensation heat transfer coefficients on a vertically mounted smooth tube have been obtained for total pressure ranging from 2.48×10⁵ Pa(abs) to 4.55×10⁵ Pa(abs) and air mass fraction ranging from 0.30 to 0.65. An empirical correlation for heat transfer coefficient (h), has been developed in terms of a parameter group made up of steam mole fraction (Xs), total pressure (P_t), temperature difference between bulk gas and wall surface (dT). This correlation covers all data points within 20%. All data points are also in good agreement with the prediction of the diffusion layer model (DLM) with suction and are approximately 2.2 times the Uchida heat transfer correlation. Experiments with an axial shroud around the test tube to model the restriction on radial flow experienced within a tube bundle demonstrated a reduction of the heat transfer coefficient by a factor of about 0.6. The effect of helium (simulating hydrogen) on the heat transfer coefficient was investigated for helium mole fraction in noncondensable gases (X_He/X_nc) at 15, 30 and 60%. It was found that the condensation heat transfer coefficients are generally lower when introducing helium into noncondensable gas. The difference is within 20% of air-only cases when X_He/X_nc is less than 30% and total pressure is less than 4.55×10⁵ Pa(abs). A gas stratification phenomenon was clearly observed for helium mole fraction in excess of 60%.     

Condensation heat transfer characteristic in the presence of noncondensable gas on natural convection at high pressure

The steam-gas pressurizer in integrated small reactors experiences very complicated thermal-hydraulic phenomena. Especially, the condensation heat transfer with noncondensable gas under natural convection is an important factor to evaluate the pressurizer behavior. However, few studies have investigated the condensation in the presence of noncondensable gas at high pressure. In this study, therefore, a theoretical model is proposed to estimate the condensation heat transfer at high pressure using the heat and mass transfer analogy. For the high pressure effect, the steam and nitrogen gas tables are used directly to determine the density of the gas mixture and the heat and mass transfer analogy based on mass approach is applied instead of that based on the ideal gas law. A comparison of the results from the proposed model with experimental data obtained from Seoul National University indicates that the condensation heat transfer coefficients increase with increasing system pressure and with decreasing mass fraction of the nitrogen gas. The proposed model is also compared with other conventional correlations proposed in the literature. The proposed model demonstrates the capability to predict the condensation heat transfer coefficients at high pressure better than any other correlation. Finally, the condensate rate is compared to verify the application of the heat and mass transfer analogy at high pressure. The comparison results confirm that the heat and mass transfer analogy can be applied to evaluate the condensation heat and mass transfer at high pressure.