含内热源的梯形腔自然对流换热数值模拟

引言 封闭腔体内的自然对流换热问题在实际工程中有着重要应用,因而受到越来越多的关注,工程实际中常用的腔体结构有方腔、圆形腔、三角形腔等.     

利用3ω法同时测量纳米流体热导率和热扩散系数

提出了应用基于谐波探测技术的3ω法进行液体导热性能测量的方法。设计了3ω测试系统，测试了不同浓度和不同温度下纳米流体的热导率和热扩散系数，与文献中的测试结果进行了对比。实验中测试的热波信号较好地满足频域内的导热方程，说明采用交流电流加热可使流体的微对流作用得到有效减弱。采用基于多颗粒布朗运动的微对流（MSBW）模型预测了纳米流体的热导率。浓度比较低时TiO₂+蒸馏水、Al₂ O₃+蒸馏水纳米颗粒流体的热导率随温度增加呈线性增大，并且与液体的Prandtl数有关，在测试温度为18～65℃范围内，水的热导率随温度升高以及纳米颗粒的布朗运动所引起周围基液的微对流作用是纳米流体强化传热的两个重要机理。     

Numerical simulation of natural convection in a square enclosure filled with nanofluid using the two-phase Lattice Boltzmann method

Considering interaction forces (gravity and buoyancy force, drag force, interaction potential force, and Brownian force) between nanoparticles and a base fluid, a two-phase Lattice Boltzmann model for natural convection of nanofluid is developed in this work. It is applied to investigate the natural convection in a square enclosure (the left wall is kept at a high constant temperature (T_H), and the top wall is kept at a low constant temperature (T_C)) filled with Al₂O₃/H₂O nanofluid. This model is validated by comparing numerical results with published results, and a satisfactory agreement is shown between them. The effects of different nanoparticle fractions and Rayleigh numbers on natural convection heat transfer of nanofluid are investigated. It is found that the average Nusselt number of the enclosure increases with increasing nanoparticle volume fraction and increases more rapidly at a high Rayleigh number. Also, the effects of forces on nanoparticle volume fraction distribution in the square enclosure are studied in this paper. It is found that the driving force of the temperature difference has the biggest effect on nanoparticle volume fraction distribution. In addition, the effects of interaction forces on flow and heat transfer are investigated. It is found that Brownian force, interaction potential force, and gravity-buoyancy force have positive effects on the enhancement of natural convective heat transfer, while drag force has a negative effect.     

绝热圆腔内4根冷热微管阵列振动强化传热实验

目前针对微细管管外同时有自然对流和周期横向振动存在情况下的混合对流问题鲜有研究。为了探究这种现象, 进而开发一种新型的换热器, 本文提出了一种由4根冷热微细管组成的微管阵列, 将其平行放置于有中间流体的圆形腔体内, 并对管束施加微小横向振动的混合对流传热机理进行研究。详细描述了实验系统及实验方法, 以及后期对实验数据的处理方法, 得到了传热Nu数随管间自然对流强度Ra数, 强迫振动Re_v数等量纲为1参数的变化曲线;揭示了该结构形式下传热特性随冷热管温差、振动频率、微管振幅等传热规律。结果表明, 横向振动在小温差情况下传热强化效果更为明显。将本文实验数据与Lemlich单管振动实验中的数据进行对比, 表明了振动对4根微管阵列的强化传热效果比单根管更明显。     

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。     

脉冲加热下微加热器在Al2O3纳米流体中的沸腾换热

以Al₂O₃-H₂O纳米流体中的微加热器为研究对象,通过实验方法对脉冲加热条件下微加热器的温度响应曲线和气泡动力学行为进行了详细的研究。比较了在纯水及浓度为0.1%和0.2%的Al₂O₃-H₂O纳米流体中微加热器的温度变化和气泡动力学行为。发现在脉冲加热条件下,微加热器在不同浓度的纳米流体中将出现不同的温度响应曲线,加热膜表面的气泡动力学行为也不相同。实验表明,在脉冲加热条件下,微加热器在Al₂O₃-H₂O纳米流体中的换热效果要明显高于纯水,纳米粒子的浓度对于加热膜表面的气泡动力学行为有明显影响,对微加热器换热的影响也很大。最后根据实验结果以及纳米粒子对气液固三相线的影响,对实验中Al₂O₃纳米流体的换热情况进行了合理的解释。     

TiO_2-H_2O纳米流体的黏度特性实验

在去离子水中添加亲水性分散剂,经过超声振动制备了粒径为20 nm的4种不同体积分数的TiO2-H2O纳米流体。对纳米流体和水进行黏度测试,结果表明纳米流体的黏度值均大于水,且黏度随TiO2粒子体积分数的增加呈加速上升的趋势,随温度呈反比变化。体积分数越高的纳米流体,在较低温度下的黏度增加幅度比高温时大。流变曲线表明,在所配制的体积分数内,TiO2-H2O纳米流体的黏度不随剪切速率的变化而变化,为典型的牛顿型流体。     

Natural Convection in Heat Generating Oval Porous Enclosures: A Non‐Darcian Model

This paper presents a series of numerical simulations dealing with the problem of natural convection flows and associated heat transfer in an enclosure filled with a fluid‐saturated porous medium. The analysis is based on the finite element technique and incorporates the Brinkman‐extended Darcy model for an oval enclosure. The numerical results obtained for a modified Rayleigh number, Ra, Darcy number, Da, offset, E, and eccentricity, e, are presented and discussed. The numerical predictions for a square enclosure compared well with published data. It is found that any increase in Da or Ra results in a higher fluid velocity that is responsible for shifting the core of the flow. Moreover, at higher ovality (E = 0.5), asymmetric flow is observed even at the lower range of Rayleigh number (Ra ? 20), which may be attributed to the effect of curved isothermal wall.     

Laminar heat transfer of non-Newtonian nanofluids in a circular tube

Forced convection heat transfer behavior of three different types of nanofluids flowing through a uniformly heated horizontal tube under laminar regime has been investigated experimentally. Nanofluids were made by dispersion of γ-Al₂O₃, CuO, and TiO₂ nanoparticles in an aqueous solution of carboxymethyl cellulose (CMC). All nanofluids as well as the base fluid exhibit shear-thinning behavior. Results of heat transfer experiments indicate that both average and the local heat transfer coefficients of nanofluids are larger than that of the base fluid. The enhancement of heat transfer coefficient increases by increasing nanoparticle loading. At a given Peclet number and nanoparticle concentration the local heat transfer coefficient decreases by axial distance from the test section inlet. It seems that the thermal entry length of nanofluids is greater than the base fluid and becomes longer as nanoparticle concentration increases.     

多孔介质方腔内置芯片热流耦合的LBM数值模拟

基于格子Boltzmann方法的渗流多孔介质耦合双分布模型，对表征体元（REV）尺度下含电子芯片的多孔介质自然对流进行数值模拟研究，主要研究不同物性参数对多孔介质自然对流的影响以及单电子芯片尺寸、多芯片布局等因素对电子芯片表面散热性能的影响。得出了如下研究结果：对于恒温单芯片的多孔介质自然对流，在达西数Da=10-2时存在临界芯片尺寸。在临界芯片尺寸条件下，流场扰动较更小的芯片尺寸更强，传热性能却几乎不变。不同瑞利数Ra条件下临界芯片尺寸不同，Ra越大，临界芯片尺寸越大，在Ra=103时临界芯片尺寸为0.203125倍方腔边长，Ra=104时临界芯片尺寸为0.25倍方腔边长，Ra=105时临界芯片尺寸为0.390625倍方腔边长。当多孔介质渗透率降低时，即Da=10-4时，不存在临界芯片尺寸，且芯片表面和冷壁处的平均Nusselt数均随Ra的增大而增大。对于恒温多芯片的多孔介质自然对流，在多孔介质渗透率较大(Da=10-2)的情况下芯片横排布置可取得最佳换热效果，在渗透率较小(Da=10-4)时芯片布局宜采用对角分布。     

Influence of heat flux and Reynolds number on the entropy generation for different types of nanofluids in a hexagon microchannel heat sink

Based on the Second Law of Thermodynamics, the entropy generation is studied for laminar forced convection flow of different nanoparticles(Al_2 O_3, CuO and SiO_2) mixed with water through a hexagon microchannel heat sink(HMCHS). The effects of different heat fluxes and Reynolds numbers on the entropy generation for different nanofluids, volume fractions and nanoparticles diameter are investigated. The heat flux is in the range of 125 to 500 kW·m~(-2) and the Reynolds numbers vary between 200 and 1500. The thermal, frictional and total entropy generations are calculated by integrating the volumetric rate components over the entire HMCHS. The results clearly show that the rise in the heat flux leads to an increase in the thermal entropy generation for nanofluids and pure water but they don't have any influence on the frictional entropy generation. Moreover, when the Reynolds number increases, the frictional entropy generation increases while the thermal entropy generation decreases. The results revealed that at low heat fluxes and high Reynolds numbers, pure water gives the lowest entropy generation, while at high heat flux the nanofluid has to be used in order to lower the overall irreversibility.     

A new model to predict the densities of nanofluids using statistical mechanics and artificial intelligent plus principal component analysis

In this work,some thermodynamic properties of nanofluids such as Sb2O5:SnO2/(EG + H2O),ZnO/(EG + H2O),Al2O3/(EG + H2O),ZnO/(PEG + H2O),ZnO/PEG,and TiO2/EG were estimated from the extended Tao-Mason equation of state,together with the Pak and Cho equation in various temperature,pressure,and volume fractions.The equations of state using minimum input data and density at rooom temperature as scaling constants,were developed to estimated densities of the above mentioned nanofluids.Furthermore,the artificial neural network plus principal component analysis (PCA) technique (with 20 neuron in hidden layer) was performed over the whole range of available conditions.The AADs of the calculated molar densities of all nanofluids using the EOS and ANN at various temperatures and volume fractions are 1.11％ and 0.48％,respectively.In addition,the heat capadty and isentropic compressibility of the above mentioned nanofluids are predicted using obtained densities of EOS with the Pak and Cho equation.     

ZrO_2-水纳米流体管内层流的传热特性

制备了粒径为50 nm的ZrO2-水纳米流体,并通过添加分散剂NH4PAA改善纳米流体的稳定性。测量了4种不同质量分数(0.2%,0.4%,0.8%,1.2%)的ZrO2-水纳米流体在层流状态下的对流换热系数。实验结果表明:在相同雷诺数下,纳米流体的换热系数要比纯水的有所提高,并随着ZrO2纳米颗粒质量分数的增加而增大。当纳米流体的质量分数为0.2%,0.4%,0.8%,1.2%时,其平均换热系数比纯水分别提高了1.9%,2.4%,5.2%和8.8%。实验管道内的不同位置也影响纳米流体换热系数的提高,入口段的换热系数要比充分发展段提高得更明显,其主要原因是纳米颗粒对流体边界层的干扰。     

冷热圆管在封闭方腔内不同垂直位置的自然对流数值研究

利用有限体积法对冷热圆管在封闭方腔内不同垂直位置的自然对流现象进行了数值研究。讨论了瑞利数Ra和冷热圆管间距δ对方腔内自然对流流动与换热的影响, 其中瑞利数的变化范围为10³～10⁶, 圆管间距变化范围为0.3～0.6。为了揭示冷热圆管间的相互作用和圆管与方腔间的相互作用对自然对流换热与流动的影响规律, 比较分析了热圆管在上、冷圆管在下和热圆管在下、冷圆管在上两种情形下冷热圆管、方腔的自然对流换热能力的差异。研究表明:瑞利数的改变, 对方腔内温度场分布和涡流结构有显著影响;热圆管在下、冷圆管在上这种情形更有利于自然对流换热的进行;增加圆管间距δ, 热圆管和方腔的换热能力增强, 但冷圆管的换热能力却有所减弱。研究结果为核电站安全壳非能动余热排出系统的性能研究提供了理论依据。     

二氧化铈/水基纳米流体核沸腾传热特性

对CeO₂纳米流体进行了池沸腾传热特性研究,考察了CeO₂/水基纳米流体的热导率,静态接触角以及沸腾后表面沉积情况对沸腾传热的影响。结果表明,CeO₂纳米流体可提高沸腾传热系数,且纳米流体最佳质量分数为0.05％,其沸腾传热系数较去离子水提高36%。热导率以及接触角随纳米流体质量分数的增加而增加,在本实验范围内,热导率最大增加1%;而纳米流体接触角从50.5°增加到92.9°;表面沉积随纳米流体的质量分数增加越来越明显,去离子水在沉积表面的接触角发生较大变化（51.4°~134.4°）。纳米流体的热导率影响可忽略不计;而接触角和沸腾表面颗粒沉积对纳米流体的强化传热作用影响较大。     

表面辐射对部分填充吸湿性多孔介质的封闭腔体内热湿耦合传递的影响

根据多孔介质热质传递原理, 基于有限元的方法数值分析了具有表面热辐射的部分填充吸湿性多孔介质的封闭腔体内部自然对流流动及热湿耦合传递过程, 探讨了表面发射率、Rayleigh数和Darcy数等参数对封闭腔体内部自然对流流动及热湿耦合传递过程的影响, 研究结果表明, 壁面热辐射的作用可以提高多孔介质内部的温度, 而且随着表面发射率的增大, 多孔介质内部的水分逐步向其右上角迁移和聚集。另外, Darcy数、多孔介质与空气的热导率比对方腔内部多孔介质的热量传递和水分迁移影响较小。     

Natural convection heat transfer enhancement of different nanofluids by adding dimple fins on a vertical channel wall

In this numerical study, natural flow and heat transfer of nanofluids with Al_2O_3, TiO_2, Cu and CNT nanoparticles in a vertical channel with dimpled fins at Rayleigh number(Ra) of Ra = 3.25 × 10~7 to Ra = 1 × 10~8 are investigated by using the finite volume method. The obtained results revealed that, using CNT in volume fractions of 2% and 4%leads to significant heat transfer and at φ = 6%, using TiO_2 nanoparticles has a great effect on Nu number enhancement. Also, using solid nanoparticles in base fluid causes more uniform heat transfer distribution,especially in areas close to heated surface and by adding more volume fraction in base fluid, temperature level reduces. In general, according to temperature contours, reduction of wall temperature depends on the increase of Ra and volume fraction and the type of solid nanoparticles.     

Turbulent convective heat transfer of nanofluids through a square channel

This paper reports the results of experimental investigation on the heat transfer performance of Al₂O₃/H₂O and TiO₂/H₂O nanofluids through square channel with constant wall temperature boundary condition. The flow regime through channel is turbulent. The nanofluids used in this research are Al₂O₃/H₂O and TiO₂/H₂O with different nanoparticle concentrations. Based on the results of the present investigation, for specific Peclet number, convective heat transfer coefficient and Nusselt number of nanofluids are higher than those of distilled water. The enhancement increases with increasing nanoparticle concentration. The results also reveal that the convective heat transfer coefficient for Al₂O₃/H₂O nanofluid is relatively the same as that of TiO₂/H₂O nanofluid.     

Lattice Boltzmann simulation of alumina-water nanofluid in a square cavity

A lattice Boltzmann model is developed by coupling the density (D2Q9) and the temperature distribution functions with 9-speed to simulate the convection heat transfer utilizing Al₂O₃-water nanofluids in a square cavity. This model is validated by comparing numerical simulation and experimental results over a wide range of Rayleigh numbers. Numerical results show a satisfactory agreement between them. The effects of Rayleigh number and nanoparticle volume fraction on natural convection heat transfer of nanofluid are investigated in this study. Numerical results indicate that the flow and heat transfer characteristics of Al₂O₃-water nanofluid in the square cavity are more sensitive to viscosity than to thermal conductivity.     

永磁梯度磁场布置方式对空气自然对流换热的影响

梯度磁场可用于控制非导电弱磁性介质的自然对流换热过程。利用钕-铁-硼永磁系统的不同空间布置，构建了具有不同磁场强度分布的梯度磁场，通过数值模拟得到了不同永磁梯度磁场的磁场强度和磁加速度。对不同梯度磁场作用下的二维封闭腔内的空气自然对流换热过程进行了数值研究，获得了空气自然对流的流场和温度场，以及壁面局部Nusselt数和平均Nusselt数并进行了比较。结果表明：空气自然对流换热可以通过施加具有不同磁加速度的梯度磁场得到强化或控制。