低熔点四元硝酸盐圆管内受迫对流换热特性CSCD

为了验证经典对流换热关联式对于新型的低熔点四元硝酸盐的适用性,该文研究了不同工况下低熔点四元混合硝酸盐在套管式换热器内的受迫对流换热特性。通过套管式换热器内管中的熔盐与壳侧的导热油的换热实验,测量得到熔盐与导热油在套管式换热器实验段的进出口温度,并获得了熔盐与导热油的总传热系数。通过威尔逊分离法从总传热系数中分离得到熔盐侧的对流换热系数,从而研究圆管中熔盐的对流换热特性。结果表明:该低熔点四元盐在研究的充分发展紊流区域内,其雷诺数在1×10~4~5×10~4之间,普朗特数在4.9~15.5之间,熔盐和导热油的总换热系数在670~1300 W/(m^2·K)之间,熔盐侧的对流换热系数在2900~7800 W/(m^2·K)之间变化。根据实验数据拟合得出了低熔点四元盐在圆管中紊流段的对流换热关联式。将实验数据与经典的对流关联式比较发现,经典关联式仍然适用于低熔点四元混合硝酸盐的管内对流换热。该研究为熔融盐在太阳能热发电中的实际应用提供了参考数据。     

Numerical simulation of forced convective heat transfer of molten salt in tubes

熔融盐作为太阳能热发电中的重要的传热蓄热介质,其强制对流传热特性对如何强化对流传热和设计以熔融盐作为工质的换热器具有很重要的指导意义。建立了熔融盐-油套管式换热器内不同种类的熔融盐在不同工况下的强制对流传热模型,数值模拟并理论研究了管内熔融盐强制对流传热特性,提出数值模拟的熔融盐对流传热关联式;并将数值模拟结果与实验结果进行了对比。在此基础上分析了流速对管内熔融盐对流传热系数和热流密度的影响规律。结果表明在Re处于10000～60000的范围内数值模拟和实验结果具有较好的一致性。     

高熔点熔盐管内传热性能实验分析

搭建试验台研究高熔点熔盐在水平圆管的紊流传热特性,并分析圆管内局部传热规律。该试验雷诺数范围在(1.0～3.6)×10~4之间,普朗克数在4.75～8.00之间,根据实验数据并参考经典实验关联式,采用多元线性拟合熔盐平均传热努塞尔数实验关联式,其最大拟合误差为±10%;并将实验值与经典圆管紊流传热关联式以及低熔点熔盐传热实验关联式的计算值比较。结果表明:在工程允许范围内,部分经典对流传热关联式可描述熔盐管内传热特性;而低熔点熔盐传热实验关联式不适用于高熔点熔盐传热特性。     

三维双侧水平强化管R245fa凝结换热性能试验

由于制冷剂R11和R123对臭氧层有破坏作用,为完成环保新工质R245fa对R11和R123的替代工作,对R245fa在内螺纹外斜翅片的三维双侧强化管外的凝结换热性能进行试验。数据处理过程中,采用Wilson图解法获得管内水侧对流换热系数及其计算关联式,再利用热阻分离法获得管外凝结换热系数。研究表明:试验中管内对流换热系数高于管外冷凝换热系数,所以管外侧的传热热阻是占据主导地位的传热热阻;相对于光管,R245fa在三维双侧强化管管内换热强化换热倍率为3.58,管外强化换热倍率为2.48;对实验数据进行拟合,得到管外换热系数的变化规律和凝结换热关联式。     

湿烟气在开孔翅片管外凝结换热数值模拟

为高效回收湿烟气全热并对烟气冷凝换热设备进行优化设计,以设计的紧凑式开孔翅片管换热器为对象,采用欧拉壁膜(EWF)模型与组分输运模型耦合研究低温烟气在翅片管换热器中的凝结换热规律。数值模拟得到的凝结速率及对流凝结换热系数与实验结果最大偏差分别为13.4%和10.9%。结果表明：对流凝结换热系数随入口水蒸气质量分数和烟气流速增大以及管壁温降低而增大,翅片开孔可以起到均压、破坏温度边界层、截断液膜、加快凝结液排出进而强化传热的作用,基于模拟拟合的关于改进雅各布数J的烟气对流凝结关联式与实验数据平均相对误差为13.1%,模拟关联式对于90%的实验结果预测误差在-20%～+20%以内。     

水平方管底面加热熔盐混合对流模拟研究

利用数值模拟方法研究熔融盐在底面加热水平方管内的混合对流传热过程,分析熔融盐混合对流的流动和传热特性等。得到了沿流动方向各横截面处的温度、速度分布图及流线图,并对传热特性以及Nu数的变化规律进行探讨。结果表明,在高热流密度情况下非均匀加热壁面时,浮升力效应使主流核心区的形状随流动距离的增加而发生变化;在通道横截面上形成对称的二次涡流;局部Nux数在相同热流密度下随Re数的增加而增大;在Re数相同数时Nux数会随着Ri数的增加而增加;数值模拟点与湍流混合对流传热关联式的吻合度较高,其偏差在±15%以内。     

椭圆钎焊与双金属轧制翅片管传热与阻力性能的试验对比

对椭圆钎焊翅片管和双金属轧制翅片管换热器的传热及阻力性能进行试验对比研究,试验得到了一系列工况下的传热数据与管外空气流动阻力数据,给出了相应的传热系数、流动阻力曲线。从总传热系数中分离出管外空气侧的对流换热系数,得到了具有一定应用价值的管外换热的计算关联式;拟合得到了管外阻力计算关联式。结果表明:椭圆钎焊翅片管比双金属轧制翅片管的传热系数约高9%,管外换热系数约高17%,且管外空气流动阻力约低11%。     

水平管内R1234yf流动冷凝换热和压降的实验研究

在1根光管、2根微肋管内对R1234yf两相流动冷凝换热进行实验,实验工况设定为冷凝温度(40±0.5)、(43±0.5)和(45±0.5)℃,质量流速为400～900 kg/(m~2·s),实验段进口制冷剂干度为0.80～0.85、出口制冷剂干度为0.15～0.20,进而从关联式拟合机理上分析各关联式对管内换热系数、压降的预测效果。结果表明:管内换热系数、压降均随流速的增加、冷凝温度的降低而增大,且微肋管内换热系数、压降均大于光管内换热系数及压降,其中,1号微肋管内换热系数最高,2号微肋管内压降最大;对于光管换热系数、压降,Thome关联式和Fridel关联式预测效果最佳,其预测平均误差均在3%以内,而Wang et al关联式和Chisholm et al关联式预测误差最大,其预测平均误差在25%以上;对于微肋管内换热系数、压降,Cavallini et al关联式和Haraguchi et al关联式分别表现出较好的预测效果,其平均预测误差分别-15.43%和-15.68%。     

烟气冷凝器换热特性实验研究

天然气燃烧产生的烟气中含有大量的水蒸气,回收烟气中的余热可以提高天然气的热利用率。为了研究烟气冷凝器在变工况下的对流冷凝换热特性,采用湿空气模拟天然气燃烧产生的烟气,实验研究了不同水蒸气含量、冷却水进口温度和流量、湿空气进口温度和流量对冷凝器凝结水率、平均对流换热系数以及沿程温度的影响,并根据实验数据拟合了对流冷凝传热的准则关联式。结果表明：烟气进口的相对湿度和冷却水的进口温度对烟气冷凝器对流冷凝换热特性的影响最大;拟合的传热关联式与实验的误差在20%以内,为天然气烟气冷凝器的设计提供了依据。     

倾斜平板热面朝上自然对流换热实验研究

对不同倾斜角度的平板在空气中的自然对流换热系数进行了实验测量.采用直接电加热方法对倾斜平板进行加热,在倾斜铜板背面嵌人式布置热电偶测量平板表面平均温度,同时测量电加热时的电压及电流.实验结果表明,随着倾斜表面与空气温差的增加,实验得到的平均努谢尔特数与经典的准则关联式得到的努谢尔特数呈现不同的变化趋势;随着倾斜角度的增加自然对流换热得到了强化,而且当倾斜平板趋于水平或者竖直时实验结果与经典准则关联式的计算结果偏差逐渐增大,最大偏差达到51%.     

熔盐与过冷沸腾水传热特性实验研究

为研究熔盐蒸汽发生器的传热特性,以Solar Salt二元硝酸盐为工质,对熔盐与过冷沸腾水在蒸汽发生器中的传热规律进行实验研究.实验结果表明:熔盐传热系数随着水侧与熔盐的质量流速的增大而增大,随着水侧压力的升高而减小;当水侧在过冷水状态时,熔盐传热系数随着熔盐入口温度的升高而增大;当水侧在过冷沸腾状态时,熔盐传热系数随...     

Turbulent convective heat transfer with molten salt in a circular pipe

In order to understand the heat transfer characteristics of molten salt and testify the validity of the well-known empirical convective heat transfer correlations, an experimental study on turbulent convective heat transfer with molten salt in a circular tube was conducted in this paper. Molten salt circulations were realized and operated in a specially designed system over 1000 h. The flow rates and temperatures of molten salt and mineral oil at the inlet and outlet in the test section were measured and the average forced convective heat transfer coefficients of molten salt were determined by least-squares method. Finally, heat transfer correlations of turbulent flow with molten salt in a circular tube were obtained. Good agreement was observed between the experimental data of molten salt and the existing well-known correlations. The experimental data of molten salt in the present work are consistent with experimental results reported by different references in a wide range of Prandtl numbers from 0.7 to 59.9.     

Forced—Flow Convection for Liquid Methanol Flowing through Microchannels

Experiments were conducted to investigate the single phase forced-flow convection of methanol flowing through microchannels with rectangular cross-section. The fully-developed turbulent convection regime was found to be initiated at aboutRe=1000 ∼ 1500. The fully developed turbulent heat transfer can be predicted by the well-known Dittus-Boelter correlation with mere modification of the original empirical constant coefficient 0.023 to 0.00805. The transition and laminar heat transfer behaviors in microchannels are highly peculiar and complicated, and heavily affected by liquid temperature, velocity and microchannel size.     

Heat Transfer Characteristics of Printed Circuit Heat Exchanger with Supercritical Carbon Dioxide and Molten Salt

Molten salt and supercritical carbon dioxide (S-CO_2) are important high temperature heat transfer media,but molten salt/S-CO_2 heat exchanger has been seldom reported.In present paper,heat transfer in printed circuit heat exchanger (PCHE) with molten salt and S-CO_2 is simulated and analyzed.Since S-CO_2 can be drove along passage wall by strong buoyancy force with large density difference,its heat transfer is enhanced by natural convection.In inlet region,natural convection weakens along flow direction with decreasing Richardson number,and the thermal boundary layer becomes thicker,so local heat transfer coefficient of S-CO_2 significantly decreases.In outlet region,turbulent kinetic energy gradually increases,and then heat transfer coefficient increases for turbulent heat transfer enhancement.Compared with transcritical CO_2 with lower inlet temperature,local heat transfer coefficient of S-CO_2 near inlet is lower for smaller Richardson number,while it will be higher for larger turbulent kinetic energy near outlet.Performance of PCHE is mainly determined by the pressure drop in molten salt passage and the heat transfer resistance in S-CO_2 passage.When molten salt passage width increases,molten salt pressure drop significantly decreases,and overall heat transfer coefficient slightly changes,so the comprehensive performance of PCHE is improved.As a result,PCHE unit with three semicircular passages and one semi-elliptic passage has better performance.     

Investigation on forced convective heat transfer of molten salts in circular tubes

In this paper, experiments are carried out to obtain convective heat transfer coefficients of turbulent flow and transition flow of molten Hitec salts in a circular tube. The present experimental data together with experimental data of four kinds of molten salts from the existing literature are correlated for transitional and turbulent convective flow respectively. In addition, the Prandtl number dependence of convective heat transfer with different working fluids is examined. It is shown that the present experimental data are in good agreement with existing correlations.     

常压下狭缝流动沸腾传热的研究

以蒸馏水为工质，在常压下，对间隙为1mm的环形狭缝通道中的流动传热进行了实验研究。分别将狭缝通道中的单相强制对流和过冷沸腾的实验数据与传统的Dittus-Boelter型关系式的计算结果进行了比较。通过分析狭缝通道中流动沸腾的传热特性认为，过冷沸腾传热比单相强制对流传热加强；质量流速对狭缝通道中的流动沸腾传热有较大影响。     

Experimental study on the heat transfer characteristics of fluoride salt in the new conceptual passive heat removal system of molten salt reactor

A new conceptual design of a passive residual heat removal system (PRHRS) has been proposed for molten salt reactor. High‐temperature heat pipes are used in this new design to improve the system inherent safety and make the PRHRS more compact. An experimental system using fluoride salt FLiNaK has been constructed to validate and support the future design of PRHRS of molten salt reactors. In this research, tests on the natural convection heat transfer of FLiNaK in the drain tank with an inclined heat pipe inserted at different heights were performed. The temperature distribution of fluoride salt in the tank was analyzed. The height of heat pipe and the bulk temperature of FLiNaK have little influence on the normalized salt temperature distribution. However, with the height of heat pipe increasing, the temperature difference of molten salt decreases and heat transfer coefficient of natural convection increases. In addition, the empirical correlations of natural convection heat transfer between liquid FLiNaK and inclined heat pipe are obtained within the range of Rayleigh numbers from 3.97 × 10⁶ to 1.16 × 10⁷. The comparisons show that a good agreement with less than 5% deviation is obtained between the proposed correlations and the test data.     

非均匀热流下水平管内混合对流大涡模拟研究

针对以槽式太阳能集热器为背景的高密度、高度非均匀热流下水平管内的混合对流换热问题,采用大涡模拟方法,研究了热流密度非均匀性对水平管内混合对流瞬态涡结构、脉动强度、湍流热通量及局部平均壁温的影响;揭示了非均匀热流下自然对流对管内湍流特性的影响规律;提出了适用于不同热边界条件下管内混合对流换热的强化措施。结果表明:均匀热流时,自然对流会抑制管顶部的湍流脉动,使流动层流化,造成传热能力局部恶化;非均匀热流时,随着自然对流的增强,近壁面速度脉动强度先减小后增大,二次流逐渐增强,换热能力逐渐提高,故管内换热能力受湍流脉动与二次流协同影响;在自然对流影响下,均匀加热时管顶部可采用针对层流的强化换热措施,非均匀加热时需着重提高管底部高热流区域的湍流脉动与涡强度。     

窄缝环形流道内流动传热特性的实验研究

对竖直及水平窄缝环形流道内单相水受迫对流换热进行了实验研究。详细介绍了实验装置和方法。实验结果表明:窄缝环形流道内流动换热的Nu数与普通圆管内公式计算值不同,不能采用传统的圆管内的对流换热模型来计算。窄缝环形流道进行对流换热时,从层流向紊流过渡较普通圆管明显提前。分析了测壁温与分离系数法之间的差别,应用修正的Dittus Boelter公式所计算的窄缝环形流道内的Nu数与实验数据吻合较好。图6参9     

波纹管内流动与传热规律的数值计算

采用三维层流及低雷诺数湍流模型对波纹管内流动与传热性能进行了数值模拟,模拟结果与试验结果吻合良好.通过数值计算拓宽了波纹管流动与传热关联式的参数范围,发现在较大雷诺数(RP)范围内波纹管阻力系数随Re的变化趋势表现为指数规律.考察了不同波纹高度、波纹间距对流动与传热的影响,并对模型参数进行了综合性能评价,结果表明:波纹高度对波纹管内流动与传热的影响较波纹间距更显著;波纹管结构的强化传热性能只有在高Re条件下才得以体现,Re越大,波纹管综合性能因子也越高.通过数值计算得到了波纹管流动与传热的最优结构参数及最佳传热雷诺数范围.