R600a喷雾冷却系统换热过程

以R600a压力式封闭系统喷雾冷却过程为研究对象,对其换热过程进行分析。对液滴撞击热面后的状态进行建模,分析了其运动状态。通过忽略液膜的对流换热,引入韦伯数来简化并修正雾滴与热源表面的对流换热系数关联式;借鉴二次成核理论,通过单位时间内,单位面积上覆盖的雾滴数量对核态沸腾换热系数关联式修正。通过上述分析,以对流换热和核态沸腾换热两种机理为中心,建立了新的换热系数关联式。通过与其他文献的关联式、实验测量值进行比较、不同工质进行比较、不同实验系统比较,发现该式预测值和实验测量值偏差在±20%以内,能够很好地预测压力式封闭系统喷雾冷却过程的换热系数。     

离子风强化平板对流传热的数值模拟

本文应用多物理场耦合计算软件COMSOL对离子风作用下强化平板对流换热的过程进行了数值模拟.研究了空气在高压电场下的对流换热现象,得到线-板电极模型相应的流场和温度场分布,并与文献中的实验结果进行了对比,验证了多物理场耦合数值方法的准确性。通过研究不同电压下传统风扇风冷、离子风和离子风与风扇共同作用下的对流换热系数变化规律,发现空气放电强化传热较传统方式有明显优势。还研究了不同极板距与板长比和不同板长情况下对流换热系数的变化规律.     

热复杂边界条件三维热应力场数值模拟研究

本文以制动盘为研究对象,基于三维对称有限元模型,运用顺序耦合数值模拟方法对制动盘制动过程传热与受力进行了探讨,分析了在热流密度、对流换热系数、辐射换热系数与时间相关的复杂的二、三类边界条件下,温度场与应力场的瞬时变化。研究结果表明,数值模拟结果与实验结果吻合程度高,证明了采用数值模拟方法对具有复杂边界条件的对象进行热应力研究与预测的可行性,同时为其他领域的传热与应力研究提供了理论依据。     

纳米流体对流换热的实验研究

建立了测量纳米流体对流换热系数的实验系统,测量了不同粒子体积份额的水-Cu纳米流体在层流与湍流状态下的管内对流换热系数,实验结果表明,在液体中添加纳米粒子增大了液体的管内对流换热系数,粒子的体积份额是影响纳米流体对流换热系数的因素之一。综合考虑影响纳米流体对流换热的多种因素,提出了计算纳米流体对流换热系数的关联式。     

通道内空气热磁对流的协同分析

为研究通道内空气热磁对流的流动规律,用数值模拟方法,研究了磁场作用下二维模型水平通道内的流动换热过程,获得了通道内的磁通密度分布和空气温度轴向分布,在此基础上,建立了一维通道内空气热磁对流的数学模型,就温度场和磁场的相对关系对水通道内热磁对流过程的影响进行了数值计算,获得了通道内不同温度场、磁场以及其不同相对位置下的通道空气流量,并讨论了磁极形状对空气流量及其变化的影响。     

空气与高温烧结水泥颗粒球间气固换热规律研究

本文对空气与高温烧结水泥颗粒的气固换热规律进行了非稳态实验研究和数值模拟。通过实验分析了空气速度、颗粒初始温度、颗粒大小对对流换热系数的影响,水泥颗粒直径为8.88 cm和5.94 cm,水泥颗粒初始温度约为500℃和700℃,空气速度范围1～4 m/s,得到了实验范围内空气绕流烧结水泥颗粒的对流换热经验关联式。结果表明:高温烧结水泥颗粒冷却过程中辐射换热热流密度与对流换热热流密度相当,不可忽略;实验所得准则数Nu_f与Re_d的指数关系为0.5。对直径为8.88 cm的水泥颗粒对应的试验工况进行了数值模拟,数值模拟结果与实验结果符合较好,误差在20%以内。     

不同磁致纵向涡形式对空气对流换热的影响

为揭示不同磁致纵向涡对通道内空气对流换热的影响规律,分别就两极和四极钕铁硼永磁体作用下的矩形通道内的对流换热进行了数值模拟。模拟以通道入口段的流动和换热为对象,得到了不同Re和不同壁温下的流场和温度场, 对流换热的Nu和阻力系数,以及场协同数Fc。结果表明,不同纵向涡形式下的流场和温度场的协同性不同,具有八纵向涡形式的对流换热的协同性优于四纵向涡形式,强化效果也优于四纵向涡。     

边界条件局部变化对二维发汗冷却的影响

本文通过对发汗冷却的多孔区域进行二维非热平衡数值模拟,研究了多孔介质区域进口处对流换热系数、冷却剂流量局部降低以及多孔介质受热表面热流密度局部增大对青铜、陶瓷两种不同多孔材料的温度场的影响。计算结果表明,冷却剂在进口处与多孔壁面对流换热系数的增大使多孔介质内部趋向于热平衡;热端壁面对流换热系数、冷却剂流量的局部变化对陶瓷多孔壁面在该局部区域的影响要大于青铜多孔壁面,但青铜多孔壁面受影响的区域更大,而冷却剂流量的局部降低对两种材料固体、流体间温差的影响程度基本一致。     

微细多孔介质中对流换热研究

本文对空气、氦气和二氧化碳流过微细多孔介质内部的对流换热进行了实验研究和数值模拟。实验段为烧结多孔介质,颗粒的平均直径为40～225μm,通过单吹法的实验方法进行瞬态实验,得到了多孔介质内部对流换热系数。数值模拟了气体流过微细多孔结构内部的对流换热的非稳态过程,得到多孔介质内部对流换热系数。数值结果与实验结果基本一致,并与已有结果基本吻合。     

冷却条件下管内超临界压力CO2对流换热研究

本文用数值计算和实验测量结合的实验方法对冷却条件下超临界压力二氧化碳在细圆管内(Din=2 mm)的局部对流换热进行了实验研究.结果表明,冷却条件下超临界二氧化碳局部对流换热系数在流体温度略高于准临界温度时达到峰值.本文还对该过程进行了数值模拟,比较了不同湍流模型的计算结果,根据数值模拟提供的信息分析了影响冷却条件下超临界压力二氧化碳换热的主要因素和物性变化对换热的影响.     

基于对流传热模型的LD端面泵浦圆形激光晶体的热效应研究

通过对半导体端面泵浦棒状Nd∶YAG晶体的热效应进行了理论分析,研究了端面抽运圆形截面激光晶体内部温度场,建立了符合条件的激光晶体热模型。考虑晶体侧面与冷却液之间的对流传热,以及晶体端面与外界非绝热边界条件,从而建立更为合理的边界条件,得出更符合实际的晶体的温度分布场。研究结果表明,考虑端面的对流传热后,计算的晶体中心温度降低,而相应的热焦距稍有增加;空气传热系数增加时,晶体中心温度明显降低,热焦距显著增加,减弱了晶体热效应。     

Effect of magnetic field gradients induced by microvasculature on NMR measurements of molecular self-diffusion in biological tissues

Presence of induced mesoscopic gradients of magnetic field in magnetically heterogeneous samples affects the measured value of apparent diffusion coefficient. This effect is investigated theoretically in the context of diffusion measurements in perfused biological tissues with blood as the paramagnetic compartment. It is shown that the apparent diffusion coefficient is sensitive to mutual correlations in vessel positions. Neglect of these correlations results in a failure of the commonly used model of microvasculature in which vessels are described as independently placed cylinders. The model is modified to account for intervessel correlations. The results indicate an underestimation of apparent diffusion coefficient in proportion to the magnetic susceptibility of intravascular compartment in agreement with published experimental data. The proportionality coefficient depends on the microvascular architecture. Comparison with experimental data yields a numerical value for a new model parameter that characterises the correlation in mutual positions of blood vessels.     

Time-dependent analytical model of thermal effects in continuous-wave end-pumped Tm:YAP lasers

A time-dependent analytical thermal model of temperature and stresses is developed to investigate the transient behavior of thermal effects in continuous-wave (CW) end-pumped laser rods. To validate the model, it is applied to a CW pumped Tm:YAP crystal and found to be in very good agreement with results of solving heat-transfer equation by finite-difference method and experimental results. The temperature contribution coefficient and stress contribution coefficient are derived from the analytical model. The temperature contribution coefficients obtained from analytical and numerical models are 1.29 K/W and 1.14 K/W, respectively. For the stress contribution coefficient of 0.34 MPa/W, the permissible maximum pump power of ∼471 W is obtained. The focal lengths of the Tm:YAP is measured in the range of 40∼90 mm, when the pump power is between 16 and 34 W.     

LD端面泵浦矩形Nd:YVO_4晶体的热效应研究

建立了矩形激光晶体的热传导模型。通过充分考虑晶体侧面与冷却液之间的对流传热,以及晶体端面与外界的热传导,建立了更为合理的边界条件。利用有限元差分法求解泊松方程,得到激光晶体内温度和温度场分布。研究结果表明,在考虑晶体的侧面对流和端面导热的情况下,得出了更符合实际的晶体温度场分布,计算的热焦距与实验测量结果更为吻合。     

Water molecules migration at oil–paper interface under the coupling fields of electric and temperature: a molecular dynamics study

Moisture is an important factor affecting the insulation properties of transformers. Due to the limitations of macroscopic experimental methods, the diffusion of water at oil–paper interface cannot be accurately measured. Therefore, molecular dynamics method was used in this work to establish oil–paper layer model of 10⁵ atoms. Through jointly analysing the aggregation degree, diffusion coefficient, free volume as well as radial distribution function of water molecules, the diffusion mechanism of water molecules at oil–paper interface was studied. The results show that when the initial water content in paper was high, water molecules would accumulate at oil–paper interface to form the local high-water region during heating. The polarisation of the electric field strengthened the hydrogen bonding interaction between water molecules and increased the probability of occurrence of the high-water region. Meanwhile, electric field reduced the free volume and diffusion coefficient of water molecules and rendered its diffusion coefficient anisotropic. What’s more, when the electric field was combined with the temperature field, the electric field played a leading role in the diffusion of water molecules while the temperature field was less affected. Diffusion coefficients of water molecules at different temperatures from molecular dynamics simulations were well consistent with experimental results, which verified the rationality of the model.     

Heat and mass fluxes in the presence of fast exothermic superficial reaction

In this paper heat and mass transfer phenomena are studied in a catalytic monolith with a fast exothermic reaction taking place at the walls at fully developing laminar flow for different values of the kinetic parameters. A two-dimensional model has been adopted to simulate the behaviour of the monolith reactor. The unsteady Navier–Stokes equations have been discretized by adopting the control volume approach and solved by means of the CFD-ACE+ package. The model surface reaction is parametrically varied to account for the effects of the perturbation generated by heat production associated with the reaction on flow field, temperature and concentration profiles and then on transport. Results show that Nu and Sh trends are not monotonic functions but that there exists a transfer enhancement due to the perturbation of the flow field. This increase is shown to be dependent on the kinetics parameters of the surface reaction. We show that the definition of the new driving force we previously proposed, which relates the transfer coefficients to the adiabatic temperature rise, is also able to describe the effect of the kinetic parameters if the pre-exponential factor and the activation energy are included in the correlation.     

不良导体导热系数的测量与探讨

根据利用物体散热速率求传热速率的实验思想,给出了橡胶盘导热系数的实验结果。并对散热速率做了分析,给出了一个修正方案。     

Spatial correlation of the high intensity zone in deep-water acoustic field

The spatial correlations of acoustic field have important implications for underwater target detection and other applications in deep water. In this paper, the spatial correlations of the high intensity zone in the deep-water acoustic field are investigated by using the experimental data obtained in the South China Sea. The experimental results show that the structures of the spatial correlation coefficient at different ranges and depths are similar to the transmission loss structure in deep water. The main reason for this phenomenon is analyzed by combining the normal mode theory with the ray theory. It is shown that the received signals in the high intensity zone mainly include one or two main pulses which are contributed by the interference of a group of waterborne modes with similar phases. The horizontal-longitudinal correlations at the same receiver depth but in different high intensity zones are analyzed. At some positions, more pulses are received in the arrival structure of the signal due to bottom reflection and the horizontal-longitudinal correlation coefficient decreases accordingly.The multi-path arrival structure of receiving signal becomes more complex with increasing receiver depth.     

多喷嘴氦冷偏滤器的优化与分析

利用ANSYS/CFX对聚变堆氦冷多喷嘴冷却偏滤器模块在相同高热负荷条件但不同冷却条件下进行了有限元数值分析,得到了偏滤器冷却单元的温度和换热系数分布图,将该温度场作为结构力学分析的加载条件,利用ANSYS/Mechanical对冷却模块进行了稳态条件下的结构应力分析,得到了具有不同结构参数的偏滤器模块的应力分布。通过分析各结构参数对偏滤器模型的热应力的影响,对结构参数进行筛选,得到内应力相对较低的结构设计,从而优化多喷嘴氦冷偏滤器结构。     

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利用ANSYS/CFX对聚变堆氦冷多喷嘴冷却偏滤器模块在相同高热负荷条件但不同冷却条件下进行了有限元数值分析,得到了偏滤器冷却单元的温度和换热系数分布图,将该温度场作为结构力学分析的加载条件,利用ANSYS/Mechanical对冷却模块进行了稳态条件下的结构应力分析,得到了具有不同结构参数的偏滤器模块的应力分布.通过分析各结构参数对偏滤器模型的热应力的影响,对结构参数进行筛选,得到内应力相对较低的结构设计,从而优化多喷嘴氦冷偏滤器结构.