Drying of Porous Materials in the Presence of Solar Radiation

ABSTRACT

We present a combined heat and moisture transfer model for predicting the drying characteristics of porous building materials exposed to solar radiation. The model has been validated for convective drying using published data and for radiative drying using results of an experimental study carried out using a solar lamp to simulate solar radiation conditions. Actual and predicted moisture content profiles and the drying rates when compared give favourable results.     

恒热流工况下管内对流换热(火用)传递特性

从管内传热与流动的特点出发,基于热力学第一、第二定律和非平衡热力学理论,对壁面恒热流工况下,管内充分发展段对流换热的火用传递过程进行了研究,定义了对流换热传火用系数、火用流密度、传火用Nusselt数和对流换热火用传递方程,并导出相应的计算式;讨论了Reynolds数、量纲1热通量和不同截面位置等参数对对流换热火用传递过程的影响;最后将传火用和传热的结果进行了比较.     

Surface radiative transfer in gas‐to‐gas cocurrent microheat exchanger

The influence of surface radiative transfer in parallel flow microheat exchanger is numerically studied for its importance at high temperatures and for small flow dimensions. For these heat exchangers, the role of radiation is beneficial when the convective heat transfer to the annulus flow exceeds the convective heat transfer from the core flow. For this case, radiation improves the heat exchanger performance by decreasing the logarithmic mean temperature difference and by increasing the capacity, effectiveness, and volumetric heat transfer coefficient. Additional surface area is made available for convection to the annulus flow, thereby increasing the specific heat transfer surface for fixed geometry. Therefore, a high emissivity layer over the surfaces of microheat exchanger can improve the heat exchange performance. The active heat transfer area weighted by the convective heat flow rates is introduced as the true measure of heat exchanger compactness. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

基于低Reynolds数k-ε模型的超临界流体对流换热的快速计算模型

基于边界层理论,针对管内变物性湍流流动,建立了一种简单的数值模型,对超临界压力下水在直管内的湍流强制对流换热进行了数值研究。计算结果与实验数据比较表明,该数值模型不仅能正确地反映超临界工况下压力、质量流速和壁面热通量等参数对传热系数的影响,而且整个计算过程简洁而高效;计算中分别应用了两种均由Jones和Launder提出的低Reynolds数k-ε湍流模型(JL模型),发现JL1模型(c_1=1.55)适用于常物性管流,但在模拟超临界工况时计算值比实验值偏低很多;JL2模型(c_1=1.45)明显高估了常物性管流的传热系数,但在模拟超临界工况时在大部分参数范围内有着良好的预测精度。     

多尺度特征对纳米多孔材料辐射特性的影响

基于辐射等效介质理论,建立了纳米多孔材料的光学和辐射特性参数理论模型,计算了纳米多孔材料的有效热辐射特性参数。基于非灰体各向异性的Rosseland近似,建立了纳米多孔材料辐射换热模型,计算了纳米多孔材料的辐射热导率,分析了含不同遮光剂纳米多孔材料的辐射热导率与颗粒体积份额、颗粒平均直径的关系,揭示了纳米多孔材料内部辐射换热与多尺度结构特征和材料属性间的本构关系,为纳米多孔材料结构参数优化设计提供了依据。     

工业锅壳煤粉锅炉炉膛出口烟温计算方法研究

工业锅壳煤粉锅炉因其诸多优点,得到广泛应用。炉膛出口烟气温度可以为锅炉的设计提供重要参考。为了优化锅炉设计,分析了现有的烟温计算方法——73标准和《燃油燃气锅炉》简化算法。并根据能量衡算及辐射、对流传热方程建立了一维稳态传热数学模型,提出了具体的计算方法,通过10 t/h示范锅炉的验证,与已有的2种计算方法进行对比,表明该方法计算简便、准确。但在实际运行中需要考虑水侧热阻的影响。     

通气振动流化床与浸没水平管间局部传热系数

提出了测试振动流化床与浸没水平管局部传热系数的新方法 ,就通气振动流化床与浸没水平管间的局部传热特性进行了实验研究 .结果表明 ,局部传热系数随水平管圆周位置而变 ,这种变化受流化气速、振动频率和振幅的影响 ,通气条件下达到最佳传热效果的振动强度比不通气时低 ,振动条件下达到最佳传热效果的流化气速比不振动时小 ,通气条件下振幅和频率对局部传热系数具有大致相同的影响趋势 .实验还表明 ,局部传热系数随颗粒粒径的增加而减小 .     

3-D comparative behaviour of discrete transfer and imaginary planes methods for furnace modelling

The discrete transfer and imaginary planes methods are compared for accuracy and computation time in cartesian and cylindrical enclosures filled with grey gas. The reference solutions are given by the classical zone method. Comparisons are done for surface heat fluxes and gas radiative sources. A wide range of surface emissivities and gas absorption coefficients is analysed. In general, the discrete transfer method reveals superior for accuracy but consumes more computation time. The imaginary planes method loses accuracy when a high number of spatial divisions is used, however it is easy to apply.     

磁场调控磁性纳米流体流动和传热研究进展

磁性纳米流体在实现能量高效和可控传递领域极具发展潜力。本文综述了磁场作用下磁性纳米流体对流换热及沸腾换热的最新进展,主要包括强制对流换热、混合对流换热、自然对流换热、池沸腾换热及管内沸腾换热等方面的实验研究,分析了磁场类型、强度、梯度、频率、方向及磁铁位置等对磁性纳米流体流动和热传输特性的影响,指出可通过改变外加磁场来实现对磁性纳米流体流动和传热的控制,并探讨了磁性纳米流体流-磁耦合作用下的传热机理以及目前所面临的挑战。在此基础上,提出了未来磁场调控磁性纳米流体对流换热和沸腾换热的主要发展方向：制备稳定的磁性纳米流体,建立系统有效的流动和传热理论模型,并从微介观尺度诠释热-流-磁耦合协同换热机理。     

辐射换热球面微元对称等分离散坐标法

基于优化计算几何原理 ,提出了球面微元对称等分 (sphericalsurfacesymmetricalequaldividing ,SSD_N)离散坐标方法 .经双动量矩检验计算及有精确解的辐射换热计算 ,与传统的、精度较高的几种方法相比 ,SSD_N 的精度较高 ,某些N取值的SSD_N 方法精度甚至高于S_N 方法 .关于计算迭代次数的比较表明 ,SSD_N 方法更能节省计算时间     

纳米流体对流传热的研究进展

将纳米颗粒加入到传统换热介质中形成的纳米流体是一种新型的强化传热介质。它不仅具有较高的单相对流传热系数,而且分散稳定,不容易磨损和堵塞管道。文中综述了纳米流体在对流条件下强化传热的实验研究进展及强化传热模型,并对实验结果和强化传热的机理及模型进行了简要分析。发现:纳米流体强化对流传热的效果与颗粒和基液的属性等有关;强化传热主要是由于导热系数的增加和纳米颗粒的运动及重新分布引起的物性变化等影响。同时提出了纳米流体对流传热研究中存在及有待改进的一些问题。     

Mixed convection heat transfer in porous media in the non-darcy regime

In this study mixed convection heat transfer in a homogeneous porous duct of square cross section in a horizontal orientation is examined. Results from a generalized Forchheimer model are compared with that from the Darcy model. The heat transfer rate and the flow behavior depend on the following parameters: Grashof number, Gr = Q'gβKa/kv², an axial flow pressure drop parameter, ζ = (aK/vμ)dp'/dz', an inertial parameter ξ = mK/a, appearing in the Forccheimer model and the Prandtl number, Pr = C_pμ/k. In the Darcy limit, ξ → 0, the role of the axial flow parameter, λ is reduced to a mere scale factor and the flow behavior is determined by a single parameter, λ = Gr · Pr. Both the Darcy and the Forchheimer models exhibit dual solutions and a hysteresis behavior over a certain range of Gr. Such parametric dependence can be used as an additional tool along with carefully designed experiments to determine the importance of inertial and Prandtl number effects on convective heat transfer in porous media.     

纳米流体在芯片微通道中的流动与换热特性

对去离子水及体积分数分别为0.15%和0.26%的水基γ-Al₂O₃纳米流体在当量直径为194.5 μm的硅基梯形芯片微通道内的层流流动和换热特性进行了实验研究。考察了Reynolds数、Prandtl数以及体积分数对流动换热的影响。结果发现,使用纳米流体后,压降无明显增加,纳米流体的流动阻力特性与去离子水基本相同;对流换热Nusselt数较去离子水有明显提高,且随着体积分数的增加而增加;相同泵功下换热热阻显著下降。实验还发现纳米流体的强化传热效果在较高温度时更加明显。根据实验数据得到了梯形硅微通道内低浓度纳米流体的层流对流换热关联式。研究结果对于集成高效芯片散热系统设计具有重要意义。     

渐扩渐缩波纹通道内脉动流的传热强化

采用曲线坐标系下压力与速度耦合的SIMPLER算法, 数值研究了周期性渐扩渐缩波纹通道内脉动流动与换热情况,流动Re数的范围为10～1000,Pr数为0.7.计算考察了不同Re数、脉动频率以及振幅对通道内强化传热和压力损失的影响.研究结果表明,稳态下Re数大于40以后,流动出现分离,Re数大于600以后,流动显得复杂和混乱;受入口脉动流的影响,通道内的旋涡发生周期性的脱落、增长和迁移,从而增强了流体之间的扰动和掺混,强化了传热;传热的强化效果随着流动Re数、脉动频率以及振幅的增大而增强;另外,流动阻力随着无量纲时间呈正弦规律变化,但与脉动频率无关.     

三角形布管方式下2种换热器的场协同分析

基于采用周期性全截面计算模型得到的帘式折流片换热器和折流板换热器壳程流体流动和传热数值计算结果,应用场协同原理对二者传热性能进行了分析。分析了帘式折流片换热器在壳程不同位置处的速度和湍流度,以及场协同角和对流传热系数,并与折流板换热器相同位置处的情况进行了对比。由于折流板壳程流体为横向流动,而帘式折流片壳程总体上是纵向流动,故折流板换热器的平均流速和湍动度稍高于帘式折流片换热器,平均流速为帘式折流片换热器的1.15倍,其湍动程度为帘式折流片换热器的1.4倍;折流板换热器2条验证线上的场协同角的平均值均小于帘式折流片换热器。研究结果为管壳式换热器结构改进和性能提升提供了参考依据,同时帘式折流片换热器的这种结构特点对于节能降耗的研究也具有重要意义。     

A bifurcation study of convective heat transfer in a hele-shaw cell

Steady-state multiplicity characteristics of convective heat transfer within a Hele-Shaw cell are investigated. The Navier-Stokes equations and the energy equation are averaged across the narrow gap, d, of the cell. The resulting two-dimensional, stationary equations depend on the following parameters: (i) the length to height aspect ratio γ, (ii) the tilt anle ? (iii) the Prandtl number Pr, (iv) an inertia parameter ξ = d²/ 12a², and (v) the Grashof number. Gr = Q_gβga⁵/kv². Here a is the height of the cell and Q, is the heat generation rate per unit volume. The complete structure of symmetric and asymmetric stationary solutions are traced using recent algorithms from bifurcation theory. In the double limit of ξ → 0 and Gr → ∞ such that Ra = 4GrPrξ remains finite (where Ra is the Rayleigh number for the Darcy model) the Hele-Shaw model reduces to that of the Darcy model.     

Frontal transformation modes of structured energetic heterogeneous systems

The transformation of structured energetic heterogeneous systems is studied taking into account radiative heat transfer. It is shown that at the combustion front of porous heterogeneous media, radiative heat transfer has a significant effect on the structure of the front and the dynamics of its propagation. In the case of a nonlinear dependence of heat transfer on temperature, the reaction zone is concentrated in a narrow temperature range in the region of high temperatures. The dimensions of the reaction zone coincide with the width of the heating zone. In the case of dominant radiative heat transfer in multilayer systems, there may be quasihomogeneous or relay-race dynamics behavior of combustion-wave propagation, depending on the scale of heterogeneity. Oscillations in the displacement of the high-temperature zone are possible even for small scales of heterogeneity (microheterogeneous combustion mode).__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 2, pp. 51–61, March–April, 2005.     

介质辐射换热有限元法的扩散综合加速算法

发展了一个基于有限元法的 “扩散综合加速算法”,用于改善传统“源项迭代”技术在求解散射占优型介质辐射问题时计算效率较低的缺陷。该算法通过P₁扩散近似方程修正散射源项,提高迭代计算的准确度,从而实现了求解效率的提升。通过几个算例的分析,结果表明：当前的加速算法是准确高效的,能够明显提高介质辐射有限元法的计算效率,缩短计算时间。随着介质光学厚度和反照率的增大,算法的加速效果越趋明显,计算时间大幅缩短。     

竖直圆管内纳米流体自然对流沸腾的临界热通量研究

对大气压下纳米流体在竖直细小圆管内自然对流沸腾特性和临界热通量(CHF)进行了试验研究.工质使用了水和水-氧化铜的纳米流体.试验中加热管长L=200～500 mm,管径d =2.1～5.4 mm和纳米流体浓度=0.1～1.0 %(wt).试验主要研究了不同浓度的纳米流体对CHF的影响.试验结果表明:相对于纯水而言,随着纳米浓度的增加,纳米流体的沸腾特性有所劣化,这主要是因为纳米颗粒在传热面上形成吸附层降低了传热表面的粗糙度,减少了表面活性核化密度,增加了传热面热阻,因此降低了传热能力.随着纳米浓度的增加,纳米流体的临界热通量也随之增加.纳米流体的临界热通量不仅与管长与管径比有关,而且还与纳米浓度有关;氧化铜颗粒质量浓度为1 %(wt)的纳米流体的CHF比纯水的增加了30%以上.     

The imaginary planes method for the calculation of radiative heat transfer in industrial furnaces

Radiation is usually the dominant mode of heat transfer in furnace enclosures. The imaginary planes method is one of the new methods for calculating the radiative heat transfer in such systems. In this study, a general procedure was developed for its application in furnace modelling. Various case studies were carried out for a number of furnaces, and the results were compared with those of the zone method. It was found that the new method combines good accuracy with low computation time and appears to be a promising method for the determination of heat flux and temperature distributions within furnace enclosures.