微通道中液氮的流动沸腾——换热特性分析

对微通道中液氮流动沸腾换热特性进行试验研究和分析。给出典型的沸腾曲线,分析壁温、干度和换热系数沿微通道管程的变化规律,考察热流密度、质量流量和压力对流动沸腾换热的影响。将126个试验数据点与四个换热关联式比较,并对微通道中流动沸腾换热机理进行分析。结果表明,在多数情况下干度和热流密度对沸腾换热系数的影响较小,换热系数主要决定于质量流量和压力,随两者增加而增加,换热以对流蒸发为主导机理。KLIMENKO关联式预测效果最好,TRAN微通道关联式次之,对常规管道得到广泛使用的CHEN关联式和SHAH关联式都远远高估了试验值。基于两相流压降和换热特性分析,推知微通道中的两相流流型不同于常规管道：在低干度情况下,流型以弥散泡状流为主;而在高干度情况下,流型以由雾状汽芯和不规则液膜组成的环状流为主。     

CO_2池沸腾换热关联式理论分析

总结了常见的池沸腾换热关联式。通过对池沸腾换热过程分析得出CO2在小热流密度和大热流密度范围下的一种分段的换热关联式。将新的拟合公式值和预测关联式值进行比较,得出CO2的拟合公式值与理论关联式及实验拟合关联式的预测值的偏差在±16%之内,具有一定的通用性。通过对CO2池沸腾换热过程的分析,得出池沸腾换热的影响因素及其变化规律,并总结了常用的强化池沸腾换热方法。     

混合制冷剂水平管内沸腾换热计算方法的回顾和评价

对混合制冷剂水平管内局部沸腾换热系数的计算方法及计算公式进行了分析比较，筛选出了与实验数据吻合较好的计算公式。结果表明，陈民公式对预测混合制冷剂管内局部沸腾换热系数准确度较好。     

电子元器件冷却中的对流换热

三、强制对流换热 1.矩形通道电子元件可以由单相强制对流来进行冷却,但其冷却效果,根据其与底板的固定方式(埋入或凸起)会有所不同,而且,根据其强化传热的方式,其结果也不一样,一般情况下,元件安装固定于矩形通道的壁面上,由于元件长度规格的多样化,使传热条件显得较复杂。 1)干装热源平装热源的强制对流换热的早期实验研究是由Baker完成的,他考虑到硅油和R—113平行流过小片状热源时,单相强制对流换热系数随着表面积的减小会明显增加,且会超过核沸腾以及基于二维边界层理论的计算结果,对流强化是由于小型加热元件引起的三维流动而产生的。     

再循环重力供液制冷系统换热性能可视化研究

建立了再循环重力供液制冷系统可视化试验台,在不同工况下对制冷系统进行实验研究,观察玻璃蒸发管内制冷剂的沸腾换热流动状态,研究分析重力供液蒸发器的传热特性。试验表明:再循环重力供液蒸发器内,制冷剂的沸腾换热出现了气泡流、气塞流、气弹流、分层流、波状分层流等流型。通过编程计算得到,经过修正的J.Chawla关联式和Kandlikar关系式分别在低温及高温工况下对沸腾换热系数有较好的预测,计算结果与试验值的偏差均在12.5%以内,采用两种关联式相结合的方法能较好地对重力供液蒸发器管内沸腾换热进行预测。     

用当地传热系数进行蒸发器传热性能的计算模拟

以Gungor和Winterton管内强制对流沸腾的通用关联式为基础,编制了计算程序,用当地传热系数对生产冷媒水的蒸发器的传热性能进行了模拟计算,理论计算值与试验结果吻合良好。     

机械密封对流传热系数数值研究

考虑动环、静环之间的摩擦生热,采用SSTk-ω湍流模型,对接触式机械密封腔内流体的对流换热进行数值研究,并采用基于SIMPLEC方法的有限体积法进行数值求解。结合流场分布特点,利用局部Nu数和平均Nu数分析动环、静环表面的对流换热性能。结果表明:静环表面对流换热系数的变化规律与动环转速和冲洗量之间的相对大小有关;动环表面的对流换热系数则主要与转速相关;采用不同模型得到的动环表面平均Nu数关联式在低Re数时差别不大,但高Re数下基于机械密封结构模型的关联式较基于圆筒模型的关联式适用性更好。     

平衡点跟踪型强迫对流换热实验装置的研制

依据空气横掠单管强迫对流换热原理,研制平衡点跟踪型强迫对流换热实验装置。装置利用单片机控制风速和加热功率等实验工况参数,对工况参数进行同步调整、实时采集分析,实现热平衡状态点跟踪。采用牛顿冷却公式和准则关系式自动计算努谢尔特数和雷诺数,自动拟合准则关系式、自动显示关联式曲线,实现了全自动数据分析。测试结果表明,在雷诺数40～4000范围内,实验结果与经验关联式吻合度较好,实现了热平衡点自动跟踪,达到了强迫对流换热实验实时自动测量要求。     

翅片管换热过程的数值模拟

以切削-挤压复合成型新工艺加工的新型翅片管为研究对象，给出大长径比的翅片管换热过程的数学模型及其有限元模型，利用有限元分析软件ANSYS的二次开发语言APDL编写程序，对翅片管换热过程中温度场的分布进行数值模拟，得到翅片管两侧的平均对流换热系数；对不同尺寸的翅片管进行换热过程模拟计算，并根据结果总结出翅片管几何参数对对流换热系数的影响规律；对有限元计算结果进行实验验证，两者结果比较吻合；并以计算结果为依据，对翅片管结构进行优化。     

管带式换热器空气侧传热传质与阻力性能的准则关联式

报道了管带式换热器空气侧换热及阻力性能的实验结果，给出了计算空气侧换热系数、析湿系数与流动阻力的准则关联式。这些公式的可信度较高，可在工程设计中应用。     

R717管内流动沸腾传热关系式评价分析

综述了R717管内沸腾传热试验研究;从7篇论文中搜集了1157组R717沸腾传热试验数据;利用试验数据评价了现有的36个管内流动沸腾换热关系式;研究了干度和管径对换热系数的影响;与CO2、N2和水3种自然制冷剂的传热特性进行了比较。文章获得了一些具有使用价值的结论,为R717管内沸腾传热计算公式的选用提供了指导,为R717流动沸腾传热的进一步研究提供了参考。     

Prediction of forced convective boiling heat transfer coefficient of pure refrigerants and binary refrigerant mixtures inside a horizontal tube

Forced convective boiling heat transfer coefficients were predicted for an annular flow inside a horizontal tube for pure refrigerants and nonazeotropic binary refrigerant mixtures. The heat transfer coefficients were calculated based on the turbulent temperature profile in liquid film and vapor core considering the composition difference in vapor and liquid phases, and the nonlinearity in mixing rules for the calculation of mixture properties. The heat transfer coefficients of pure refrigerants were estimated within a standard deviation of 14% compared with available experimental data. For nonazeotropic binary refrigerant mixtures, prediction of the heat transfer coefficients was made with a standard deviation of 18%. The heat transfer coefficients of refrigerant mixtures were lower than linearly interpolated values calculated from the heat transfer coefficients of pure refrigerants. This degradation was represented by several factors such as the difference between the liquid and the overall compositions, the conductivity ratio and the viscosity ratio of both components in refrigerant mixtures. The temperature change due to the concentration gradient was a major factor for the heat transfer degradation and the mass flux itself at the interface had a minor effect.     

Forced convective boiling in vertical tube for binary refrigerant mixtures of R11 and R113

An experimental study was carried out on convective boiling heat transfer for mixtures of R11 and R113 flowing in a uniformly heated vertical tube by measuring the wall and bulk temperatures, and the results were compared with an existing correlation. A reduction of the average heat transfer coefficient for mixtures was verified for flow boiling. It was observed that two kinds of boiling behavior existed depending on mass flux. It was also found that the Chen's correlation was particularly successful for the case of high mass rate flow in which convective boiling prevailed. However in the case of low mass rate flow where nucleate boiling was dominant, the Chen's correlation was found to be inappropriate. Mass transfer resistance in the liquid film played a vital role for determining the heat transfer coefficient of refrigerant mixtures. It has been also found that the equilibrium assumption was hardly applicable to the convective boiling phenomena.     

管内流动沸腾传热关系式对R410A的适应性研究

R410A作为一种替代制冷剂,已经大量用在工业生产中。R410A制冷系统的设计和研发需要进行R410A管内流动沸腾换热计算。目前有很多公式预测两相流流动沸腾换热系数,它们对R410A的适应性需要判断。本文从10篇论文中收集了1268组R410A流动沸腾传热实验数据,用这些数据对27个两相流流动沸腾换热关系式进行了评价,选出了较为精确的R410A管内流动沸腾换热关系式,为R410A管内流动沸腾换热计算的公式选择提供了依据,为提出精确度更高的R410A管内流动沸腾公式提供了参考。     

蒸发式冷凝器管外水膜与空气传热性能及机理的研究

建立了蒸发式冷凝器实验平台,测试了不同喷淋密度及迎面风速对管外水膜与空气传热与阻力性能的影响,结果表明管外水膜与空气传热受迎面风速的影响较大,而受喷淋密度影响较小。得到了管外水膜与空气传热系数计算式,最大相对标准偏差为12.2%,进而分析了蒸发式冷凝器中水膜与空气的传热机理。     

A study on the improvement of heat transfer performance in low temperature closed Thermosyphon

The study focuses on the heat transfer performance of two-phase closed thermosyphons with plain copper tube and tubes having 50, 60, 70, 80, 90 internal grooves. Three different working fluids (distilled water, methanol, ethanol) are used with various volumetric liquid fill charge ratio from 10 to 40%. Additional experimental parameters such as operating temperature and inclination angle of zero to 90 degrees are used for the comparison of heat transfer performance of the thermosyphon. Condensation and boiling heat transfer coefficients, heat flux are obtained using experimental data for each case of specific parameter. The experimental results are assessed and compared with existing correlations. The results show that working fluids, liquid fill charge ratio, number of grooves and inclination angle are very important factors for the operation of thermosyphons. The relatively high rate of heat transfer is achieved when the thermosyphon with internal grooves is used compared to that with plain tube. The optimum liquid fill charge ratio for the best heat transfer performance lies between 25% and 30%. The range of the optimum inclination angle for this study is 20°-30° from the horizontal position.     

Two-phase flow heat transfer of R-134a in microtubes

The characteristics of the two-phase flow heat transfer of R-134a in microtubes with inner diameters of 430 μm and 792 μm were experimentally investigated. The effect of the heat flux on the heat transfer coefficient for microtubes was significant before the transition quality. The boiling number expressed the interrelation between the heat flux and the mass about the heat transfer coefficients. The smaller microtube had greater heat transfer coefficients; the average heat transfer coefficient for the tube A (D _i = 430 μm) was 47.0% greater than that for the tube B (D _i = 792 μm) at G = 370 kg/m²·s and q″ = 20 kW·m². A new correlation for the evaporative heat transfer coefficients in microtubes was developed by considering the following factors: the laminar flow heat transfer coefficient of liquid-phase flow, the enhancement factor of the convective heat transfer, and the nucleate boiling correction factor. The correlation developed in present study predicted the experimental heat transfer coefficients within an absolute average deviation of 8.4%.