A variant of the low-Reynolds-number two-equation turbulence model applied to variable property mixed convection flows

Previous work has demonstrated that the low-Reynolds-number model of Launder and Sharma (1974) offers significant advantages over other two-equation turbulence models in the computation of highly non-universal buoyancy-influenced (or “mixed convection”) pipe flows. It is known, however, that the Launder and Sharma model does not possess high quantitative accuracy in regard to simpler forced convection flows. A variant of the low-Reynolds-number scheme is developed here by reference to data for constant property forced convection flows. The re-optimized model and the Launder and Sharma formulation are then examined against experimental measurements for mixed convection flows, including cases in which variable property effects are significant.     

Synthesis of soluble polyimides for vertical alignment of liquid crystal via one-step method

A series of polyimides (PIs) were copolymerized from 4-dodecyloxy-biphenyl-3′,5′-diaminobenzoate (DBPDA), 3,3′-dimethyl-4,4′-methylene-dianiline (DMMDA) and 4,4′-oxydi(phthalic anhydride) (ODPA) via one-step method. The PIs possessed excellent solubility in polar aprotic solvents and easily formed thin flexible films by solution casting. The glass-transition temperatures (T_gs) of the PIs were in the range of 219-242 °C and thermal decomposition temperatures in nitrogen occurred above 350 °C. The resultant PI films exhibited high transparency at wavelengths greater than 400 nm and induced excellent uniform vertical alignment of liquid crystal (LC). Even after the rubbing process, the pretilt angles of LC were still above 89°. The PI seems to be a prospective material for alignment layers in flexible displays.     

Theoretical study of new acceptor and donor molecules based on polycyclic aromatic hydrocarbons

Functionalized polycyclic aromatic hydrocarbons (PAHs) are an interesting class of molecules in which the electronic state of the graphene-like hydrocarbon part is tuned by the functional group. Searching for new types of donor and acceptor molecules, a set of new PAHs has recently been investigated experimentally using ultraviolet photoelectron spectroscopy (UPS). In this work, the electronic structure of the PAHs is studied theoretically with the help of B3LYP hybrid density functionals. Using the ΔSCF method, electron binding energies have been determined which affirm, specify and complement the UPS data. Symmetry properties of molecular orbitals are analyzed for a categorization and an estimate of the related signal strength. While σ-like orbitals are difficult to detect in UPS spectra of condensed film, calculation provides a detailed insight into the hidden parts of the electronic structure of donor and acceptor molecules. In addition, a diffuse basis set (6-311++G**) was used to calculate electron affinity and LUMO eigenvalues. The calculated electron affinity (EA) provides a classification of the donor/acceptor properties of the studied molecules. Coronene-hexaone shows a high EA, comparable to TCNQ, which is a well-known classical acceptor. Calculated HOMO-LUMO gaps using the related eigenvalues have a good agreement with the experimental lowest excitation energies. TD-DFT also accurately predicts the measured optical gap.     

Rotational symmetry breaking in small-area circular vertical cavity surface emitting lasers

We investigate theoretically the dynamics of three low-order transverse modes in a small-area vertical cavity surface emitting laser. We demonstrate the spontaneous breaking of axial symmetry of the transverse field distribution in such a device. In particular, we show that if the linewidth enhancement factor is sufficiently large dynamical regimes with broken axial symmetry can exist up to very high diffusion coefficients ~ 10 μm²/ns.     

热管冰蓄冷结构设计

从技术角度研究和探讨了热管做为冰蓄冷装置的适用性;对各种热管用金属管材与工质的特性进行了科学比较;从而最终确定了适宜冰蓄冷工作的热管管材、热管工质及其基本尺寸;为热管冰蓄冷装置的深入研究及推广应用,提供了强有力的理论依据。     

用于高热流密度器件冷却的热管散热器实验研究

针对大型计算机服务器CPU的耗能量,探讨了一种新的热管排布方式的散热器,并对其散热性能进行了实验研究.研究结果表明,采用此种热管散热器,最高热流密度为74.3W/cm2,其冷却风速控制在4m/s即可满足芯片冷却要求.同时根据模拟计算得到的散热器底板温度分布,可有助于对热管排布方式的优化设计.     

振荡流热管可视化实验

本文通过振荡流热管可视化实验,研究热管传热性能与流型的变化。实验中可观察到,振荡热管内工质运动通常处于“间歇-振荡”交替运行的状态。随加热功率的增大,间歇时间所占比例逐步减小。随着加热功率的增大,热管内的流型也发生相应变化,实验中主要观察到了泡状流、塞状流、环状流、局部波状流等多种流型。汽泡的生成、长大、聚合、分离和湮...     

阿尔法磁谱仪配电系统的热控系统设计及实验研究

本文以阿尔法磁谱仪中的配电系统为研究对象,阐述了利用热管解决配电系统局部温度过高的热控问题。通过数值模拟,采用体点散热问题的仿生优化方法,获得了高导热性的热管在母板中的人工构造,构建了一种有效的热控系统。数值模拟和实验结果对比验证了数值模拟模型的可靠性和仿生优化方法的可行性,也说明了热管在均温和传热方面的优良作用。     

新型平板热管换热器热回收效率特性实验研究

本文针对普通住宅房间设计了一台新型平板式热管换热器,该换热器结构紧凑、体积小巧。为研究该换热器的使用条件,本文开展了不同工质(R113、R141b以及这两种工质的混合物)对该热管换热器换热效率影响的实验研究。整个实验在夏季工况下进行,热管真空度为1×10~(-3)Pa,充液量(灌入热管换热器内的工质体积与热管换热器体积之比)为1/3。实验结果表明:该热管换热器热回收效率较高。在整个风量范围内,R141b作为工质的热管换热器换热效果最好,最高效率达到了58.2%。     

高温及超高温热管的启动特性和传热极限

根据局部高温、高热流加热等特殊场合的需求,设计、加工了钠高温热管及锂超高温热管,并对热管的启动和极限特性进行了研究。高温及超高温热管能够顺利启动,但启动时应注意可能到来的声速传热极限。端部加热启动时,端部附近管体部分会形成一段等效的加热段,可以将工质从启动时的固体状态到全部熔化形成相变传热及流动循环的时间确定为启动时间。热管的传热极限特性表明,超高温热管应工作在更高的温度和更大的热流密度。热管的结构和工艺优化有利于启动和运行。