Negative Thermal Transport in Conduction and Advection

Negative refractive index has drawn a great deal of attention due to its unique properties and practical applications in wave systems. To promote the related physics in thermotics, here we manage to coin a complex thermal conductivity whose imaginary part corresponds to the real part of complex refractive index. Therefore, the thermal counterpart of negative refractive index is just negative imaginary thermal conductivity, which is featured by the opposite directions of energy flow and wave vector in thermal conduction and advection, thus called negative thermal transport herein. To avoid violating causality, we design an open system with energy exchange and explore three different cases to reveal negative thermal transport. We further provide experimental suggestions with a solid ring structure. All finite-element simulations agree with theoretical analyses, indicating that negative thermal transport is physically feasible. These results have potential applications such as designing the inverse Doppler effect in thermal conduction and advection.     

有机发光二极管的热分析与热设计

采用基于计算流体动力学的热学模拟仿真与瞬态热学测试技术分析了OLED的热学特性,研究并讨论了输入功率、面板取向、风速等实际应用变量对OLED面板结温的影响。研究结果表明,OLED的结温与衬底及封装盖表面存在明显的温度梯度,且此温度梯度随输入电流增加大幅增大。OLED的热学特性与面板取向、气流速度密切相关。     

降低热不稳定性的激光二极管泵浦固体激光器

通过谐振腔腔结构理论研究了在端面泵浦固体激光器中,等效热反射镜曲率半径变化时,振荡光半径的变化速率理论和实验都表明,通过调整谐振腔的参量,使得该变化速率接近零,可以有效地抑制热效应导致的激光功率不稳定性。     

多量子阱超辐射发光二极管(SLD)热分布计算

对由8个量子阱所组成的条形超辐射发光二极管(Super luminescent diode,SLD)进行了热分析,计算了不同器件结构下的热阻和温度分布。计算结果表明,热阻变化受芯片宽度和长度的影响较大,可以达到两个数量级;当注入功率达到1W时,有源区的温度将接近50K。该分析对有效地设计芯片的结构,减少温度升高对SLD稳定性的影响具有指导意义。     

激光二极管端面泵浦多段复合板条激光器热效应

针对激光二极管端面泵浦的多段渐变浓度复合板条激光增益介质,提出了两种选取掺杂浓度的方法,并分别计算了多段渐变浓度复合板条增益介质的热量、温度及应力分布.结果表明,与单一掺杂浓度板条增益介质相比,采用多段渐变浓度复合增益介质可显著降低增益介质内部的温度梯度及最大热应力,从而提高了激光器整体的损伤极限泵浦功率,有利于激光器的功率升级.     

激光二极管阵列泵浦固体激光器热设计研究

对激光二极管阵列（LaserDiodeArray,LDA）泵浦固体激光器热设计进行了研究。通过采用计算热阻的方法,根据LDA的发热功率选择热电制冷器（ThermaolElectronicConrtoller,TEC）和相应的散热器,采用强迫风冷的方式对重复频率为25Hz,输出单脉冲能量为80mJ的LDA泵浦固体激光器进行了精确的温度控制。试验结果表明,在-40℃～55℃环境温度范围内,激光器性能稳定,满足技术指标要求。     

热容型大功率半导体激光器瞬态热特性

为研究热容型大功率半导体激光器在低环境温度、高瞬时功率、长工作间歇时间条件下的应用,建立三维瞬态热传导模型,通过有限元法计算得出热沉三维尺寸对半导体激光器瞬态热特性的影响。选取尺寸为26.6 mm×11.5 mm×4 mm的热沉进行热容型半导体激光器的封装测试,获得其在-20℃和-30℃环境温度下连续工作3.5 s过程中有源区温度随时间的变化曲线,并与数值计算的结果进行对比。结果表明,两者在误差范围内能够很好地吻合。     

高功率密度激光二极管叠层散热结构的热分析

高功率窄间距列阵叠层是提高激光二极管泵浦源光功率密度的有效途径,而封装散热热沉的结构设计在其热管理上占据至关重要的作用。本文利用ANSYS有限元分析方法,对叠层间距、绝缘陶瓷厚度以及陶瓷底面与散热恒温面距离等几个影响高功率密度激光二极管叠层封装散热的重要因素进行了分析,得到器件的最高温度随结构参数变化的规律,并对上述参数进行优化。最后,利用优化结果设计出一种适用于高功率密度激光二极管叠层泵浦源的高效有源散热热沉结构,大幅提高了器件的散热能力,并降低了所需冷却水的水泵功耗需求。     

激光二极管端面泵浦多段复合板条激光器热效应

针对激光二极管端面泵浦的多段渐变浓度复合板条激光增益介质,提出了两种选取掺杂浓度的方法,并分别计算了多段渐变浓度复合板条增益介质的热量、温度及应力分布.结果表明,与单一掺杂浓度板条增益介质相比,采用多段渐变浓度复合增益介质可显著降低增益介质内部的温度梯度及最大热应力,从而提高了激光器整体的损伤极限泵浦功率,有利于激光器的功率升级.     

LD抽运钕玻璃薄片放大器热管理结构

基于传热学基本理论,借助有限元分析方法,对LD端面抽运钕玻璃薄片激光放大器激光介质的瞬态热效应进行数值模拟.通过对不同边界条件下钕玻璃薄片内部温度场及应力场的分析,研究了LD端面抽运钕玻璃薄片的热效应特性,并对比了不同热管理方案下钕玻璃薄片的温度分布.结果表明:采用白宝石窗口结合外围热沉TEC制冷的冷却方案最佳.此种冷却方案下钕玻璃薄片内最高温度在非抽运面中心处,约120℃.虽然比直接对抽运面水冷方式的冷却效果稍差,但是可以避免抽运面直接水冷所引起的一些问题.相关研究结果也表明,由于钕玻璃热阻较大,即使加强径向散热,抽运区的热量仍然难以从径向疏散,对热管理方案进行优化时应该考虑从尺寸较小的轴向进行散热.     

激光二极管端面泵浦Yb∶YAG薄片激光器的热效应

通过激光二极管端面泵浦Yb∶YAG薄片工作特点分析,建立了符合实际情况的热模型.热模型考虑了介质薄片具有端面绝热、周边恒温冷却、后端面镀膜达到充分利用泵浦光能量等特点.分析并建构了Poisson方程新的求解方法,得出了Yb∶YAG薄片内部温度场以及泵浦端面热形变场的一般解析表达式.理论研究结果表明:若激光二极管泵浦功率为20 W、耦合到薄片泵浦面的泵浦光高斯半径为250 μm时,长度3 mm、半径1.5 mm的5.0at.%Yd:YAG薄片泵浦端面的最高温升为44.5℃,最大热形变量为0.83 μm.     

聚合物发光二极管面光源的光热特性

运用传输矩阵法及MATLAB软件模拟计算了以MEH-PPV为发光材料的PLED面光源出光率,分析了发光层、PEDOT层、ITO层对器件出光率的影响.模拟分析了玻璃表面半球形、圆台形、圆锥形3种微形貌对器件出光率的影响.仿真结果表明,玻璃表面为圆锥形微散射层对提高器件出光率最有利,且出光均匀性最好.最后分析了PLED面光...     

Advection and dispersion in time and space

Previous work showed how moving particles that rest along their trajectory lead to time-nonlocal advection–dispersion equations. If the waiting times have infinite mean, the model equation contains a fractional time derivative of order between 0 and 1. In this article, we develop a new advection–dispersion equation with an additional fractional time derivative of order between 1 and 2. Solutions to the equation are obtained by subordination. The form of the time derivative is related to the probability distribution of particle waiting times and the subordinator is given as the first passage time density of the waiting time process which is computed explicitly.     

Slow Modes in Passive Advection

The anomalous scaling in the Kraichnan model of advection of the passive scalar by a random velocity field with nonsmooth spatial behavior is traced to the presence of slow resonance-type collective modes of the stochastic evolution of fluid trajectories. We show that the slow modes are organized into infinite multiplets of descendants of the primary conserved modes. Their presence is linked to the nondeterministic behavior of the Lagrangian trajectories at high Reynolds numbers caused by the sensitive dependence on initial conditions within the viscous range where the velocity fields are more regular. Revisiting the Kraichnan model with smooth velocities, we describe the explicit solution for the stationary state of the scalar. The properties of the probability distribution function of the smeared scalar in this state are related to a quantum mechanical problem involving the Calogero–Sutherland Hamiltonian with a potential.Partially supported by NSF Grant DMS-9205296     

半导体激光泵浦复合晶体固体激光器的热效应

为了验证复合晶体使用到半导体泵浦的固体激光器中与非复合晶体的区别,提高半导体泵浦的固体激光器的工作效率,开展了半导体激光泵浦YAP/Tm∶YAP复合晶体固体激光器的热效应的验证实验。采用有限元分析法,模拟了晶体温度及热应力的分布,并分析了热透镜长度的变化情况。结果发现,与非复合晶体相比,复合晶体的温度和热应力均有不同程度的下降,复合晶体工作时的最高温度降至其80%,热应力降至其70%。同时也验证了热透镜焦距不随非掺杂晶体长度的增大而改变,这也意味着复合晶体不能有效提高复合激光的光束质量,但是可以确保输出激光光束质量的稳定性。因此可以证实,使用复合晶体能够有效改善激光器的温度和力学特性,但不能优化固体激光器的光束质量。     

大功率、宽光谱超辐射发光管的温度稳定性研究

超辐射发光管（SLD）的温度稳定性是通过芯片制作、耦合封装和应用控制三个环节实现控制. 在一定的驱动电流下,通过提高耦合效率、改善温控器件的几何参数以及器件整体的热分布参数,实现SLD大功率、宽光谱的光功率输出,出纤光功率达到750μW／100mA,光谱宽度大于40nm. 工作温度范围为－40～＋70℃,功率波动小于2 ％.     

Thermal Performance of Laser Diode Array under Constant Convective Heat Transfer Boundary Condition

Three-dimensional heat transfer model of laser diode array under constant convective heat transfer coefficient boundary condition is established and analytical temperature profiles within its heat sink are obtained by separation of variables. The influences on thermal resistance and maximum temperature variation among emitters from heat sink structure parameters and convective heat transfer coefficient are brought forward. The derived formula enables the thermal optimization of laser diode array.     

端面抽运方形复合YAG温度场及热透镜研究

采用数值方法研究了方形截面复合Nd∶YAG激光晶体棒在端面抽运情况下的温度分布及热透镜效应,得到复合激光晶体内温度场的数值解和径向温度梯度引起的轴向传播光相位延迟分布ΔΦT(r),并与同条件下普通晶体的结果进行了比较.结果表明,吸收系数越大,两种晶体中最高温度相差也越大.采用复合晶体可以大幅度降低激光晶体内的温升,同时减小端面形变造成的透镜效应.复合晶体的ΔΦT(r)与普通晶体相似,且不满足对半径r的二次方关系.在rwp时,ΔΦT(r)相对于参考球面透镜的相位畸变Δφ随r的增大而急剧增大.     

Completely Conservative and Oscillationless Semi-Lagrangian Schemes for Advection Transportation

In this paper, we present a new type of semi-Lagrangian scheme for advection transportation equation. The interpolation function is based on a cubic polynomial and is constructed under the constraints of conservation of cell-integrated average and the slope modification. The cell-integrated average is defined via the spatial integration of the interpolation function over a single grid cell and is advanced using a flux form. Nonoscillatory interpolation is constructed by choosing proper approximation to the cell-center values of the first derivative of the interpolation function, which appears to be a free parameter in the present formulation. The resulting scheme is exactly conservative regarding the cell average of the advected quantity and does not produce any spurious oscillation. Oscillationless solutions to linear transportation problems were obtained. Incorporated with an entropy-enforcing numerical flux, the presented schemes can accurately compute shocks and sonic rarefaction waves when applied to nonlinear problems.     

环形激光二极管抽运棒状激光器中瞬态温度和热应力分析

直接从激光二极管发光强度的角分布出发,采用光线追迹方法获得激光棒内的热沉积分布,在此基础上采用热传导模型和热力模型,比较了不同抽运功率、不同棒半径下达到稳态温度分布的时间,并且对稳态和瞬态热应力进行了详细模拟计算。结果表明,采用环形激光二极管阵列侧面抽运的棒状激光器中的热效应问题十分严重,不同的抽运结构参量下,温度分布不同;达到稳态所需时间随棒半径增大而增加,而不受抽运功率的影响;抽运功率越大,棒内温差增大,热应力也越大;热破坏主要集中于激光棒中心区域和表面区域。