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Microscales of Natural Convection 自然对流的微观尺度
Pub Date : 1997-11-16 DOI: 10.1115/imece1997-0888
V. Arpaci
A universal dimensionless number, ΠN∼NN1+Pr-1 ,Pr being the usual Prandtl number and NN the limit of ΠN for Pr → ∞, is introduced for all natural convection processes. For NN=Ra ,Ra being the usual Rayleigh number, ΠN describes buoyancy-driven natural convection. For NN=Ma ,Ma being the usual Marangoni number, ΠN describes thermocapillary-driven natural convection. For NN=TaPr ,Ta being the usual Taylor number, ΠN describes centrifugally-driven natural convection. In terms of ΠN, a thermal Kolmogorov scale relative to an integral scale, ηθℓ∼ΠN-1/3 is introduced for natural convection including buoyancy, thermocapillary and centrifugally-driven flows. Heat transfer associated with these flows is modeled byNu∼ΠN1/3 ,Nu being the usual Nusselt number. A variety of turbulent natural convection phenomena are shown correlating the model.
一个通用的无量纲数ΠN ~ NN1+Pr-1,Pr是通常的普朗特数,NN是Pr→∞的极限ΠN,被引入到所有自然对流过程中。对于NN=Ra,Ra为通常的瑞利数,ΠN描述浮力驱动的自然对流。对于NN=Ma,Ma是通常的Marangoni数,ΠN描述了热毛细管驱动的自然对流。对于NN=TaPr,Ta为通常的泰勒数,ΠN描述离心驱动的自然对流。根据ΠN,相对于积分尺度的热Kolmogorov尺度,ηθ r ~ ΠN-1/3引入了自然对流,包括浮力、热毛细和离心驱动的流动。与这些流动相关的传热用Nu ~ ΠN1/3来模拟,Nu是通常的努塞尔数。与该模型相关的各种湍流自然对流现象也被展示出来。
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引用次数: 0
Turbulent Heat Transfer in a Forced and Unforced Two Dimensional Air Jet Impinging on an Isoflux Surface 碰撞在等流面上的受迫和非受迫二维气流的紊流传热
Pub Date : 1997-11-16 DOI: 10.1115/imece1997-0894
Edmund Singer, A. Ortega
Mean heat transfer and flow characteristics of a two-dimensional air jet impinging on a uniform heat flux surface were investigated. Measurements were made in a steady unforced jet, and in a jet with externally introduced forcing. The jet issued from a 24:1 aspect ratio rectangular nozzle. The heat transfer characteristics are parameterized by jet Reynolds number based on the hydraulic diameter and on the nozzle-to-plate spacing. Flow conditions (mean, turbulent fluctuations and power spectra) exiting the jet were measured and used to interpret their influences on the stagnation point heat transfer. A comparison of an unforced impinging jet to a forced impinging jet showed enhancements in the heat transfer rate of up to 57% with forcing.
研究了二维空气射流撞击均匀热流面的传热特性和流动特性。测量是在一个稳定的无强迫射流中进行的,在一个有外部引入力的射流中进行的。喷射从一个长径比为24:1的矩形喷嘴喷出。传热特性由基于水力直径和喷嘴与板间距的射流雷诺数参数化。测量出射流的流动条件(平均、湍流波动和功率谱),并用来解释它们对驻点传热的影响。非强制撞击射流与强制撞击射流的比较表明,强制撞击射流的传热率提高了57%。
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引用次数: 1
Measurements of Fully-Developed Augmented Convection in a Symmetrically Grooved Channel 对称沟槽通道中充分发展的增强对流的测量
Pub Date : 1997-11-16 DOI: 10.1115/imece1997-0891
R. Wirtz, Feng Huang, M. Greiner
Measurements of fully-developed augmented convection and pressure drop of air flow in an isothermal, symmetrically grooved channel are reported for channel Reynolds numbers ranging from 800 to 5,000. Grooves, oriented transverse to the flow, are of triangular shape with dimensions that are comparable to the hydraulic diameter of the channel. The grooved section is designed to excite instabilities in the flow leading to increased mixing at sub-transitional Reynolds numbers. Local heat transfer measurements are made using a holographic interferometer. Interferograms, representative of the cross-span-average temperature of the air in the channel, are analyzed to produce data records of the air temperature distribution and the local heat flux along the grooved walls. Heat flux distributions are spatially averaged to produce a correlation of fully-developed Colburn j-factor for this surface configuration. A performance evaluation of the grooved surface applied to a simple heat exchanger shows that it provides thermal performance which is comparable to other surfaces commonly employed in compact heat exchangers.
在等温对称沟槽通道中,对完全发展的增强对流和空气流动的压降进行了测量,通道雷诺数从800到5000不等。沟槽,横向导向的流动,是三角形的形状,其尺寸与通道的水力直径相当。沟槽部分的设计是为了激发流动中的不稳定性,从而在亚过渡雷诺数下增加混合。局部传热测量采用全息干涉仪。通过分析通道内空气跨跨平均温度的干涉图,得到沿槽壁面的空气温度分布和局部热流密度的数据记录。热通量分布在空间上平均,以产生充分发展的科尔伯恩j因子对这种表面构型的相关性。对应用于简单热交换器的沟槽表面的性能评估表明,它提供的热性能可与紧凑型热交换器中常用的其他表面相媲美。
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引用次数: 1
Parallel Computing Strategies for a Disperse Phase Flow and Combustion Model 分散相流与燃烧模型的并行计算策略
Pub Date : 1997-11-16 DOI: 10.1115/imece1997-0927
W. Fiveland, K. L. Parker, R. Gansman
Flow and combustion models are being used to evaluate new designs and retrofit options for various industrial combustion systems. Combustion models being used today are often very modular and, since they apply serial algorithms, require long run times to produce results. It is common for solutions to take several days, and the use of finite rate chemistry and Lagrangian based particle models can lengthen run times to a week or more. The modularity of these methods makes them candidates for parallel computing. This paper presents results for a distributed computing algorithm using the PVM software, which is applied to the finite rate chemistry and particle transport modules. It is based on a master-slave algorithm in which the master doles work to a number of independent processors. A load balancing scheme is used to account for the variability in the time the slaves complete their work. PVM was successfully used for parallel computations in the finite rate chemistry and particle modules. Significant speedups were found for both modules, but the work clearly indicates the need to control granularity and the need to optimize the algorithm specifically for the processors being used. Future work is planned to improve the algorithms presented here as well as extending the work to other parts of the combustion model.
流动和燃烧模型被用于评估各种工业燃烧系统的新设计和改造方案。目前使用的燃烧模型通常是非常模块化的,由于它们采用串行算法,需要很长时间才能产生结果。解决方案通常需要几天的时间,使用有限速率化学和基于拉格朗日的粒子模型可以将运行时间延长到一周或更长时间。这些方法的模块化使它们成为并行计算的候选方法。本文给出了一种应用于有限速率化学和粒子输运模块的PVM分布式计算算法的结果。它基于一个主从算法,其中主模块工作于许多独立的处理器。负载平衡方案用于考虑从服务器完成其工作时间的可变性。PVM成功地应用于有限速率化学和粒子模块的并行计算。这两个模块都有显著的速度提升,但这项工作清楚地表明,需要控制粒度,需要针对所使用的处理器专门优化算法。未来的工作计划是改进这里提出的算法,并将工作扩展到燃烧模型的其他部分。
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引用次数: 0
Analytical Solutions for Inverse Radiative Transfer Optical Property Estimation 逆辐射传递光学特性估计的解析解
Pub Date : 1997-11-16 DOI: 10.1115/imece1997-0929
N. Mccormick
An overview is given of analytical algorithms for estimating the optical properties of a homogeneous slab or a homogeneous optically-thin, convex-shaped medium.
概述了用于估计均匀平板或均匀光学薄的凸形介质光学性质的解析算法。
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引用次数: 1
Measurements of the Convection Heat Transfer Coefficient for a Two-Dimensional Wall Jet: Uniform Temperature and Uniform Heat Flux Boundary Conditions 二维壁面射流对流换热系数的测量:均匀温度和均匀热流边界条件
Pub Date : 1997-11-16 DOI: 10.1115/imece1997-0899
R. Abdulnour, K. Willenborg, J. Mcgrath, J. Foss, B. Abdulnour
The local heat transfer coefficients for isothermal and uniform heat flux boundary conditions for a two-dimensional wall jet have been experimentally determined. Hot wire anemometry surveys were used to quantify the velocity field in the wall jet. A micro thermocouple was used to quantify the temperature field in the wall jet for the isothermal boundary condition. Infrared imaging was applied to measure the wall temperature for the uniform heat flux boundary condition. The difference in the local convection coefficients due to the different thermal boundary conditions was largest at the leading edge of the heated wall and decreased in the stream wise direction. The local convection coefficient was found to be insensitive to the thermal boundary condition in the more turbulent region of the wall jet.
实验确定了二维壁面射流等温和均匀热流边界条件下的局部换热系数。采用热线风速测量法对壁面射流中的速度场进行了量化。在等温边界条件下,利用微型热电偶定量分析了壁面射流的温度场。在均匀热流边界条件下,采用红外成像技术测量了壁面温度。不同热边界条件对局部对流系数的影响在受热壁面前缘最大,沿气流方向减小。在壁面射流湍流较大的区域,局部对流系数对热边界条件不敏感。
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引用次数: 0
A Numerical Study of Three-Dimensional Natural Convection in a Horizontally Heated Enclosure 水平加热罩内三维自然对流的数值研究
Pub Date : 1997-11-16 DOI: 10.1115/imece1997-0922
P. Oosthuizen
Flow in a rectangular enclosure with a square horizontal cross-section, whose lower horizontal surface is heated and whose upper horizontal surface is cooled and whose vertical side-walls are adiabatic, has been numerically studied. It has been assumed that the flow is laminar and that the fluid properties are constant except for the density change with temperature which gives rise to the buoyancy forces. The unsteady form of the governing equations, written in terms of the vector potential and vorticity vector functions and expressed in dimensionless form, have been solved using a finite-difference procedure based on the equations. The solution was started with no flow in the enclosure. The solution, in general, has the following parameters: The Rayleigh number Ra, the Prandtl number and the size A of the square cross-sectional shape compared to the height of the enclosure. Results have only been obtained for a Prandtl number of 0.7. Results for values of A between 1 and 4 for various relatively low values of Rayleigh number (up to 15000) have been obtained. The results have been used to determine the effect of A on the value of Ra below which there is no fluid motion in the enclosure and to examine the various flow patterns that arise as the value of Ra is increased. The effect of A on the variation of the mean Nusselt number with Ra has also been examined.
本文对下水平面受热、上水平面受冷、垂直侧壁绝热的方形水平截面矩形壁面内的流动进行了数值研究。假定流动是层流的,流体的性质是恒定的,除了密度随温度的变化而变化,这种变化引起浮力。控制方程的非定常形式(以矢量位势和涡量矢量函数表示,并以无因次形式表示)在此基础上用有限差分法求解。解决方案开始时在外壳中没有流动。一般情况下,该解具有以下参数:瑞利数Ra、普朗特数以及方形截面形状相对于外壳高度的尺寸A。结果只在普朗特数为0.7时得到。对于各种相对较低的瑞利数(高达15000),A值在1到4之间的结果已经得到。结果已用于确定A对Ra值的影响,在此值以下,外壳内没有流体运动,并检查随着Ra值的增加而出现的各种流动模式。还研究了A对平均努塞尔数随Ra变化的影响。
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引用次数: 0
Natural Convection From Vertical Helical Coils in a Cylindrical Enclosure 圆柱外壳中垂直螺旋线圈的自然对流
Pub Date : 1997-11-16 DOI: 10.1115/imece1997-0902
H. Taherian, Peter L. Allen
Experiments were conducted to investigate natural convection heat transfer from single enclosed helical coils in a vertical orientation. Coils having different diameter, pitch, and tube diameters were placed inside a cylindrical shell of fixed height and diameter, to form a natural convection shell-and-coil heat exchanger. Effects of tube diameter, coil surface area and coil pitch on heat transfer coefficient were studied. Nu-Ra correlations were presented based on different characteristic lengths. The data was correlated well by NuDhx = 0.002RaDhx0.630 using heat exchanger hydraulic diameter as the characteristic length.
在垂直方向上对单个封闭螺旋盘管的自然对流换热进行了研究。在固定高度和直径的圆柱壳内放置不同直径、节距和管径的盘管,形成自然对流的壳管换热器。研究了管径、盘管表面积和盘管间距对换热系数的影响。根据不同的特征长度给出了Nu-Ra的相关关系。以换热器水力直径为特征长度,用NuDhx = 0.002RaDhx0.630与数据进行了很好的关联。
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引用次数: 0
A Periodic Transient Method Using Liquid Crystals for the Measurement of Local Heat Transfer Coefficients 一种利用液晶测量局部传热系数的周期瞬态方法
Pub Date : 1997-11-16 DOI: 10.1115/imece1997-0896
J. Baughn, M. R. Anderson, J. E. Mayhew, Robert J. Butler
A new periodic transient method for the measurement of local heat transfer coefficients is described. In this method, the freestream temperature is periodically heated while the local surface temperature fluctuation is measured. The local heat transfer coefficient can be determined from the frequency, the ratio of the surface temperature fluctuation to the free stream temperature fluctuation and the wall thermal properties. Measurements on a Plexiglas flat plate with a laminar boundary layer are presented as a demonstration of this method. For this demonstration the free stream is electrically heated using a fine wire cloth. The surface temperature fluctuation is measured using liquid crystals with a hue analysis technique. The results show that this is a viable method for measuring local heat transfer coefficients. Its advantages are that it approximates a uniform temperature thermal boundary condition and it is not necessary to control or measure the initial wall temperature.
介绍了一种新的局部换热系数的周期瞬态测量方法。在这种方法中,在测量局部表面温度波动的同时,周期性地加热自由流温度。局部换热系数可由频率、表面温度波动与自由流温度波动之比以及壁面热性能确定。以层流边界层的有机玻璃平板为例,对该方法进行了实验验证。在这个演示中,自由流是用细钢丝布电热的。表面温度波动测量使用液晶与色相分析技术。结果表明,这是一种可行的局部换热系数测量方法。它的优点是近似于均匀温度的热边界条件,不需要控制或测量初始壁面温度。
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引用次数: 5
Mechanisms of Heat Transfer Enhancement in a Turbulent Boundary Layer Downstream of a Cylinder-Wall Junction 圆柱-壁面交界下游湍流边界层强化传热机理研究
Pub Date : 1997-11-16 DOI: 10.1115/imece1997-0890
D. Wroblewski, Q. Xie
The wall heat transfer enhancement behind a circular cylinder-wall junction was investigated experimentally for Reynolds numbers ranging from ReD = 21,000 to 54,000 for locations up to 12 diameters downstream of the cylinder leading edge. Surface heat transfer was studied using a fully-heated surface downstream of the cylinder to provide traditional heat transfer coefficients, and thin-film surface sensors flush-mounted on an unheated surface to provide adiabatic heat transfer coefficients. Flow field transport measurements were obtained with a triple-wire Reynolds heat flux probe. The enhancement could be attributed to two effects: (1) a local fluid dynamic effect attributed to increased eddy diffusivity and subsequent increased turbulent transport and (2) an upstream heating effect, caused by reduced wall temperatures in the region directly behind the obstacle, and their effect on the subsequent redevelopment of the boundary layer downstream. The adiabatic heat transfer coefficients obtained from the surface sensors provided misleading results, because of the breakdown in the analogy between heat and momentum transport.
在圆柱体前缘下游12倍直径范围内,实验研究了圆柱体-壁面交界处的壁面传热增强,雷诺数范围为ReD = 21,000至54,000。研究人员利用圆柱体下游完全加热的表面来提供传统的传热系数,而在未加热的表面上安装薄膜表面传感器来提供绝热传热系数。流场输运测量是用三丝雷诺热流密度探头进行的。这种增强可归因于两种效应:(1)涡旋扩散系数增加和随后湍流输运增加引起的局部流体动力效应;(2)上游加热效应,由障碍物后方区域壁面温度降低引起,并对随后下游边界层的再开发产生影响。从表面传感器获得的绝热传热系数提供了误导性的结果,因为在热量和动量输运之间的类比失败了。
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引用次数: 0
期刊
Heat Transfer: Volume 3
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