Natural convection heat transfer increase at the laminar-turbulent transition in the presence of instationary longitudinal vortices

Heat transfer of a liquid to a solid wall in natural convection is studied in the regime of laminar to turbulent transition in the presence of longitudinal vortices. These vortices lead to an increase in heat transfer, especially in their later stage before breakdown. This last increase is significant and even higher than one would expect for a turbulent flow. In this work heat transfer was studied numerically and in experiments using thermochromic liquid crystals.     

Numerical study of thermal instability in mixed convection flow over horizontal and inclined surfaces

This paper presents numerical study of thermal instability in mixed convection flow over horizontal and inclined plates. The criterion on the position marking on the onset of longitudinal vortices is defined in the present paper. The results show that the onset position characterized by the Grashof number depends on the Prandtl number, wave number, and the inclined angle φ from the horizontal. The flow is found to become more stable to the vortex mode of instability as the value of inclined angle increases, owing to a decrease in buoyancy force in the normal direction. However, the Prandtl number has a destabilizing effect on the flow. The results of the present numerical prediction show reasonable agreement with the experimental data in the literature.     

汽轮机排汽缸模型的数值模拟与结果分析

采用NEMECA商用软件包之Fine／TURBO求解三维定常的Navier—Stokes方程组，对蒸汽透平低压排汽缸内部的复杂三维流动进行了数值模拟，应用多区域结构化网格并将计算域出口向下游延伸。与已有的实验结果进行对比，揭示出排汽缸内复杂的旋涡运动，分析了影响排汽缸气动性能的总压损失系数和静压恢复系数等因素，指出了排汽缸优化设计的方向。     

尾随涡对风力机叶片气动性能影响机理研究

以Phase Ⅵ风力机叶片为研究对象,以r/R=30%、63%和95%处叶素为参考,建立与7、9、15 m/s试验风速下该风力机叶片附着涡环量沿展向分布相同的叶片模型,分析尾随涡对风力机当地翼型气动性能的影响机理。采用带转捩效应的SST k-ω湍流模型,对所建立的叶片模型和二维S809翼型的气动特性进行研究和对比分析。结果表明：旋转叶片尾随涡对分离现象产生抑制作用且随攻角的增大减弱;尾随涡的影响表现出多重效应,除了减小当地翼型的攻角,还降低其吸力面负压系数和压力面正压系数。     

Numerical study on particle distribution characteristics of slush hydrogen in a cryogenic tank

A numerical model considering phase change and heat transfer was established by the Euler-Euler two-fluid method to investigate the storage characteristics and two-phase flow field of slush hydrogen. Numerous numerical simulations were performed to discuss the effect of particle diameter (d_p = 0.02–0.5 mm), content of solid hydrogen (α_s = 10%–50%), and heat leakage (q = 50–200W·m⁻²) on the flow field. It was found that particle deposition could occur during the storage process, and there exist moving vortices with contrary directions under specific conditions. The sedimentation characteristics and vortex size are influenced by many factors including particle size, solid hydrogen content, and heat leakage. An increase in particle size could lead to the strengthening of precipitation and the expansion of the counterclockwise vortex region on the right side of the tank. And the increase in solid hydrogen content could result in more deposition and more collisions and friction between particles. Moreover, the increase in heat leakage could increase the area of the counterclockwise vortex. Numerical results of the deposition and flow field characteristics in the storage tank could clearly show the physical law of the slush hydrogen so that the uniform distribution of slush hydrogen could be promoted for efficient storage and application.     

平行平板通道内置螺旋线圈流动传热特性

通过实验与三维数值模拟相结合的方法,对内置螺旋线圈平行平板通道的流动及传热特性进行了研究,发现相对于无扰流填充物的平板通道,螺旋线圈的应用能够显著地强化传热,相同Re数下Nu数增幅为29%~141%,相应地阻力系数增幅为26%~175%。数值模拟的结果显示,流体受螺旋线圈的诱导可产生多纵向涡流动,增强了速度在垂直于壁面方向的分量,同时导致温度场发生明显改变,使得速度场与温度梯度场的协同性能得到提升,从而强化了传热。在700相似文献   

Core‐annular two‐phase flow in a gently curved circular channel

The velocity field of a core‐annular two‐phase flow through a curved channel is investigated. The case of fully developed flow with a concentric, circular fluid–fluid interface is considered. In the limit of a gentle axial curvature of the curved channel, an analytical solution is obtained in the form of a regular asymptotic expansion to first order. The condition on the physical parameters for the core to be concentric and circular is derived by accounting for the normal stresses at the interface. Two key features of the flow, the secondary circulations and the redistribution of the axial velocity, are described in detail. The viscous coupling of the two fluids at the interface leads to a variety of circulation patterns and axial velocity profiles, depending on the system parameters. The parameter space is divided into different regions using analytical conditions at the transition between flow regimes. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4871–4886, 2013     

High throughput particle separation with a mirrored deterministic ratchet design

Deterministic ratchets, also called lateral displacement arrays, consist of periodic arrays of obstacles, spaced such that suspended particles having a minimum size are displaced. They have potential for suspension separation and fractionation. We report on the development of designs with obstacle arrays mirrored along the center plane providing more efficient displacement compared to non-mirrored designs. Depending on the application (e.g. concentration or cleaning) the design can be oriented to the center or to the sides. Highest recovery of incoming particles was found 88% in the center orientation, while the strongest depletions were achieved with the side orientation. Halving the length of the array did not always yield similar separation to the conventional design, especially at higher concentrations. This implies that particle displacement was not pure deterministic, but influenced by stochastic particle–particle interactions. Fractionation experiments demonstrated high recoveries of large particles, which were not much influenced by presence of small particles.     

自由旋转卷弧翼弹箭的气动特性数值分析

为深入研究自由旋转卷弧尾翼弹箭的气动特性，利用计算流体力学方法及滑移网格技术对卷弧翼弹箭开展了时间精确非定常数值模拟。通过与文献对比的方式验证了模拟方法的有效性；分别利用网格无关性和时间步数无关性检验，确定出合适的网格数量和时间步数；在不同马赫数下对比、分析了自由旋转卷弧翼弹箭和固定卷弧翼弹箭的气动特性差异；考虑不同的翼、体差动滚转条件，研究了差动滚转角速度对自由旋转卷弧翼弹箭气动力矩特性的影响。结果表明：由于翼、体连接处黏性涡的强度不同，自由旋转卷弧翼弹箭的滚转力矩系数和马格努斯力矩系数明显小于固定卷弧翼；在马赫数0.8和1.1条件下，自由旋转卷弧翼弹箭的马格努斯力矩系数接近于0,其滚转力矩系数与差动滚转角速度基本呈线性关系，而马格努斯力矩系数与差动滚转角速度呈明显的非线性关系；受翼、体连接处黏性涡的影响，自由旋转卷弧翼弹箭的滚转力矩系数随马赫数增大而急剧减小，马格努斯力矩系数随马赫数增大而急剧增大；上述因素对卷弧翼弹箭的气动特性影响较大，在气动、弹道设计时须予以着重考虑。     

US Photography in the Philippines Before 1898: The Menage Expedition of 1890–1893

Between 1890 and 1893, two young Americans, Frank Swift Bourns and Dean Conant Worcester, travelled through the central and southern Philippines on a zoological expedition. In addition to collecting animal specimens, the two men took more than one hundred and fifty photographs. These photographs have not been given much attention by historians but they are an important set of images that help expand the understanding of photography in the Philippines in the late Spanish colonial era. This article discusses the circumstances surrounding the making of these images and provides a framework for interpreting their significance by applying the concept of the ‘photographic obsessions’ of Western explorers as defined and described by Willem van Schendel and his colleagues in the book The Chittagong Hill Tracts: Living in a Borderland.