沸腾换热及临界热流的动态微液层模型

对于水平面上的池内饱和沸腾传热机理,本文提出了一种新的微液层理论模型。与以前的理论模型不同的是,微液层模型给出了传热面上气-液-固接触的动态构造。饱和沸腾换热的主要机理在于动态微液膜的蒸发并与单个气泡的动力学过程密切相关。该模型能很好地预测充分发展的核沸腾换热曲线。由沸腾曲线上的极大值而很自然地导出临界热负荷。理论计算结果和已有的实验的结果相当吻合。     

水射流冷却过程中表面热流密度的预测

建立了由内部一点测量温度值,选取多个时间步长计算未知的表面热流密度与换热系数的导热反问题算法。该算法采用有限容积法离散方程,附加源项法处理边界条件。将该计算模型应用于求解浸没水射流冷却过程中表面热流密度,将计算温度值与实验测温值进行比较验证了算法的正确性。在此基础上分析了水射流冲击过程中薄钢片表面热流密度随表面温度与时间的变化特性。     

高效换热器内冷凝换热热阻的实验研究

在水平强化管内R134a的冷凝换热实验中,对试验工况中管内流型进行计算预测,使用关联式对管内传热系数进行预测,以校核实验数据的可靠性,以质量流量、冷凝温度为变量,以总传热系数K、冷冻水传热系数h_w、制冷剂传热系数h_r为衡量指标,对强化管换热热阻进行研究。实验结果表明:总传热系数K、制冷剂传热系数h_r均随质量流量的增加而增大,且对应冷凝温度越低其值越大,而冷冻水传热系数h_w随质量流量的增加而稍有降低,冷凝温度对其值影响并不大。此外,随着质量流量的增加,冷冻水热阻占总热阻比值逐渐增加,但制冷剂热阻总是小于冷冻水热阻,且冷凝温度越低,两者的差值越大。换热管内外热阻比重IOR随着质量流量的增加而降低,随着冷凝温度的升高而增大。     

污垢热阻动态监测装置及其传热计算方法的研究

介绍了污垢热阻动态监测装置的构成及特点.根据传热学基本原理,对该套装置进行了光滑管对流换热性能测试,结果表明两侧流体热平衡最大偏差小于10%,实验结果与经验公式计算的Nu数相吻合,其最大偏差小于l0%.用该装置进行换热表面污垢的动态自动监测,可以得出准确可靠的实验结果.     

毛细微槽内的相变传热的实验研究

本文对矩形毛细微槽竖直板的相变传热特性进行了实验研究。结果表明毛细微槽对相变换热具有很大的促进作用。当壁面过热度较小时,相变换热形式主要是三相接触线附近的蒸发换热机制。而当过热度较大时,微槽内发生剧烈的沸腾。微槽内相变换热的临界热负荷有两种产生机理:其一是当微槽长度较大时微槽内由于流动阻力而产生的液体输运临界;另一机理是当微槽长度较小时的池内沸腾临界现象,亦即由动态微液层模型决定的临界机理。实验还得到了微槽强化传热的最佳优化尺寸。     

喷雾腔压力及喷嘴孔径对相变喷雾冷却性能的影响

为了满足大功率激光器件高热流密度及低表面温度的冷却需求,以R22为冷却工质,实验研究了在闭式系统中改变喷雾腔压力及喷嘴孔径对相变喷雾冷却中临界热流密度、冷却温度等冷却性能的影响,实验结果表明：在喷雾入口压力为0.8 MPa,喷雾高度为22 mm,入口温度为-3 ℃的实验条件下,当喷雾腔压力在0.2~0.4 MPa范围内变化时,随着喷雾腔压力的升高,临界热流密度值（CHF）先增大后减小,存在最优的临界热流密度,冷却壁面温度随着喷雾腔压力的升高而上升;当改变喷嘴孔径时,CHF存在最优值,过小及过大的孔径均会影响喷雾冷却性能;当喷嘴孔径为 0.4 mm,喷雾腔压力为0.34 MPa时, CHF值最高,为276.1 Wcm-2,其对应的被冷却表面温度为26.8 ℃,表面换热系数为 66 640 Wm-2K-1。     

微液层模型预测过冷沸腾的临界热流密度

本文利用微液层模型对过冷沸腾的临界热流密度（CHF）进行了理论预测。过冷沸腾的强化换热主要是通过单个气泡的形成和消失造成的对流换热强化而引起的。对等热流面,CHF在高过冷区趋近于常数;对等温面,CHF随过冷度的增加而增加。过冷度增加时,蒸发换热量减少,总热流密度主要由蒸发区外的导热引起。     

热阻对液氮冷屏性能的影响研究

超导元件需用液氦降温至极低工作温度才能实现超导性能,液氦温区仅靠真空多层绝热方式无法达到理想绝热效果,采用液氮冷屏隔断液氦和环境之间的传热,能够有效降低液氦系统蒸发损失和液氦用量。为研究热阻对液氮冷屏传热特性的影响,建立了液氮冷屏热阻模型,通过理论传热计算得到了不同热阻与冷屏板温度及传热量之间的关系,利用数值模拟软件对不同热导率材料和不同板厚下冷屏板的温度分布进行了分析。结果表明,最不利热阻为接触热阻和导热热阻,采用高导热系数材料及适度增加冷屏板厚度有助于提高冷屏板温度分布的均匀性,减小接触热阻和冷屏板表面发射率有助于提高冷屏隔热性能,为改善冷屏热屏蔽效果提供依据。     

阻燃型高真空多层绝热材料性能的实验研究

通过实验研究了用于低温储液罐的阻燃型高真空多层材料的绝热性能,得出了该材料的比热流与层数的关系和工程应用时的最佳层数。结合实验数据进行传热分析,表明:玻璃纤维纸和铝箔构成的高真空多层材料中固体导热和层间残余气体导热占主要部分,绝热性能还有改善空间;绝热层瞬态比热流随大气压力和环境温度的变化而成余弦曲线变化;绝热层有蓄热能力。     

升膜式板式换热器的换热性能研究

升膜蒸发是在换热器表面形成一层薄液膜,薄膜蒸发能够强化换热。文中研究采用光滑铜板的板式升膜蒸发器,以去离子水作为介质,在不同进水流量、不同加热量(热流密度)下,测定换热器某些点的局部换热系数,计算出总的换热系数,研究影响板式换热器升膜蒸发的因素和变化趋势。     

低温下固体接触热阻与接触电阻的实验研究

在低温工程中,两个接触固体之间存在着接触热阻与接触电阻,将对低温实验中热量及电流的传输产生显著的影响,是进行低温下物性研究的关键。自行研制了一套可同步实现固体接触热阻和接触电阻的测量装置,该系统具有较高的精度,可实现外界力、温度等对接触热阻的测量,同时具备接触电阻的实时测量功能。在此基础上,开展了外界压力、温度、电流对接触热阻和接触电阻的实验研究。实验结果显示:随着压力的增大,接触热阻与接触电阻随之减小;低温下,随着温度的增大接触热阻与接触电阻增大,接触电阻增大的速率要比接触热阻快。温度平衡时,20mA范围内的电流变化对接触电阻的影响显著,对接触热阻影响非常小。当界面温度达到室温后,首次观测到接触热阻和接触电阻会随着温度的增大而减小。     

The effect of defects on the low temperature thermal conductivity of yttrium iron garnet

The extremely low thermal conductivity at low temperatures of some yttrium iron garnet single crystals is attributed to sheets of defects revealed by chemical etching.     

低温下不锈钢导热系数的测量

在低温条件下利用稳态纵向热流法测量了制造低温阀门常用的316L不锈钢和17-4PH不锈钢的导热系数,通过实验表明两者的导热系数与316不锈钢的导热系数十分接近。     

The electrical resistance of platinum at repeated quenching

The reversible change in electrical resistance of platinum after quenching from high temperatures is studied by the repeated quenching method. The values measured for three different radii of the wire specimens are used to determine the quench temperature at which the concentration of quenched-in vacancies does not depend on the quenching rate or on the diameter of the specimen. The value (1070±50)°K, i.e. (0·53±0·02)T _m, was found for it, indicating a high stacking fault energy of platinum.     

The electrical resistance of platinum at repeated quenching

It is shown how the irreversible change in electrical resistance of platinum is related to the irreversible change in resistivity and dimensional changes of the samples. A preliminary interpretation is also given of the component connected with resistivity on the basis of the scattering of conduction electrons on defect formations produced mainly from the vacancies and stabilized by impurities, primarily oxygen.     

Anisotropic low temperature thermal resistance in polycrystalline magnetic thin films

The scattering of spin waves and the thermal resistance due to this scattering are calculated for polycrystalline magnetic thin films with a small overall uniaxial anisotropy. The scattering is caused by the random orientation of crystallites, and also by the magnetization ripple, which is likewise produced by this random orientation. The wavelength spectra of the ripple and of the spin waves at low energy are essentially influenced by the magnetostatic interaction, which is taken into account within Harte's linearized thin film approximation, and for comparison also within Hoffmann's approximation. Due to the anisotropy of the spectra, the contribution of the spin waves to the thermal resistance of permalloy films should be highly anisotropic at temperatures of 2 K and below.     

The thermal characteristics of a closed two-phase thermosyphon at low temperature difference

The applications of closed two-phase thermosyphons are increasing in heat recovery systems because of their high effective conductivity. However, their range of application is limited by the need for some minimum temperature difference between the evaporator and condenser sections to initiate nucleate boiling. In the project described, the steady state heat transfer characteristics for vertical two-phase closed thermosyphons at low temperature differences with R11, R22, and water as working fluid were studied experimentally. From these experimental results, the minimum temperature differences required to initiate and sustain boiling in the low temperature thermosyphons have been established for the above working fluids. A method for improving the performance at low temperature difference was devised on the basis of a thermal triggering system. Triggering could also be achieved by mechanical vibration.     

Theory of the thermal magnetocapacitance of multicomponent silicate glasses at low temperature

An explanation is put forward for the early findings of Zeller and Pohl (, Phys. Rev. B, 4, 2029) and, in particular, Stephens (, Phys. Rev. B, 13, 852), and then for the more comprehensive recent findings of Meißner and Strehlow () for the anomalous magnetic-field-dependent behaviour of the heat capacity of some multicomponent glasses at low temperature (0.1?<?T?<?4?K) and in weak-to-moderate fields (0?<?B?<?8T). An essential new ingredient of this theory, together with the standard two-level system approach for cold glasses, is a multi-minima local potential for the quantum motion of metallic ions between near-degenerate wells, separated by relatively shallow barriers. Taking the tunneling entities as independent and assuming a novel probability distribution for the parameters of the multi-welled potential, an expression for the heat capacity C(T,B) is obtained with features that match rather well the reported unusual experimental data for the glasses. The indication coming from this theory for the data available is that the entities involved in tunnelling are nanoclusters of coherent ions.     

The effect of frozen soil layer thickness on thermal emission at the wavelengths 3.6–11 cm

The results of experimental investigation of temporal dynamics of radiobrightness temperature at the wavelengths 3.6, 5, and 11 cm in the course of frost penetration and thawing are presented for the soils with different humus content. Using the method of simulating emission from a horizontally stratified medium, it is shown that radiobrightness temperature variation is related to changes in the frozen soil layer thickness. It is noted that that dielectric permittivity of the frozen layer at the temperatures around 0 °C is controlled by non-frozen bound water, whose content depends on the soil type. It is also underlined that in certain cases proper interpretation of the experimental data requires inclusion of the soil moisture motion towards the cold front into consideration. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 5–10, September, 2006.     

非晶合金中的低温电阻率极小行为研究

研究了非磁非晶合金Cu₆₀Zr₂₀Hf₁₀Ti₁₀和铁磁非晶合金Fe₆₁Co₇Zr₁₀Mo₅W₂B₁₅的低温电阻，用不同的模型对电阻温度曲线进行拟合.对两个样品中出现的极小值行为进行分析，探讨了无序结构和磁性状态对极小值行为的影响. 关键词： 非晶合金 电阻 极小值