圆管层流入口段耦合换热的数值模拟研究

采用控制容积法与蒙特卡罗法对圆管层流入口段参与性介质的辐射一对流耦合换热进行了数值模拟研究．在对流一扩散项的离散中,分别采用了一阶差分的指数格式与二阶差分的QUICK格式．通过模拟计算,比较分析了两种差分格式的结果差别,考察了入口段耦合换热的温度场、径向导热热流场以及对流换热壁面热流与努谢尔数的分布特征．研究表明,入口段速度边界层发展对势流区的排挤效应对耦合换热有明显影响,采用QUICK格式能够模拟这一现象,而一阶差分格式不能,后者过高地估计了对流换热强度;辐射换热在抑制对流换热作用的同时,使两种差分格式的模拟结果呈现偏差的区域由入口附近向出口方向扩展．     

Condensation heat transfer characteristics of vapor flow in vertical small-diameter tube with variable wall temperature

To explore the condensation characteristics of vapor flow inside vertical small-diameter tubes, the classical Nusselt theory is revised and an analytical model with variable tube wall temperature is established by considering the effect of surface tension exerted by condensate film bending as well as the effect of shear stress on vapor-liquid interface. The effects of various factors including tube wall temperature and gravity on flow condensation in small-diameter tubes are analyzed theoretically to show the heat transfer characteristics. Comparison with the experimental data indicates that the proposed analytical model is fit to reveal the fundamental characteristics of flow condensation heat transfer in vertical small-diameter tube.     

翼片诱导纵向涡强化层流对流传热数值模拟

利用三维数值模拟,分析了圆管内添加翼片后流体的流动结构和对流传热特性。模拟中,翼片与壁面呈45°倾斜放置,选取包含1个翼片的1／6通道进行研究。结果表明,翼片可在下游诱导产生2个旋转方向相反的纵向涡,形成对称的涡偶,涡偶外侧为背壁流,内侧为向壁流。纵向涡结构提高了流体在径向上的速度波动,在翼片下游靠近管壁处,最大速度可达到主流平均速度的80％,增强了对速度边界层的扰动。流场的改善使通道内的温度场分布更加均匀,与光滑通道相比,壁面附近的温度梯度可提高接近1个数量级。流体对壁面的冲刷作用使对流传热得到强化,相对于光滑通道,壁面局部Nu数可提高近50倍。纵向涡对通道内流体的强化传热作用随Re的增加而显著提高。     

Convective heat transfer enhancement of laminar flow of latent functionally thermal fluid in a circular tube with constant heat flux: internal heat source model and its application

Latent functionally thermal fluid is a kind of new two-phase fluid, in which phase change material is microencapsulated and suspended in a conventional single-phase heat transfer fluid (fig. 1). Such a slurry is of much larger apparent specific heat than conventional single-phase fluids during the phase change period, which enhances the heat transfer rate between the fluid and the tube wall. The flow rate and pump power required can be reduced evidently by using the slurries …     

变化的表面传热系数对磨削温度场的影响

详细分析了磨削弧内磨削液流的状态 ,引用温度边界层的概念 ,论证了磨削弧内的磨削液膜的状态 ,并由此得出磨削弧内的表面传热系数是随变化的壁温和位置而变化的 ,在求解温度场的分布时 ,变化的表面传热系数更符合实际的情况     

切应力作用下的层流饱和蒸发降膜流动和传热模型

处于切应力作用下的层流饱和蒸发降膜流动,具有广泛的工业应用价值和潜在的应用前景。基于力平衡和能量守恒原理,建立了在同向或反向切应力作用下层流饱和蒸发液膜传热特性的物理模型,推导出无量纲液膜厚度和表面传热系数与流动长度、界面切应力、界面对流换热强度、雷诺数间的非线性关系式,为从理论上揭示层流饱和蒸发液膜的传热机理提供了一种可供分析的数学模型。     

Simulation of rarefied gas flow and heat transfer in microchannels

Analysis and simulation of rarefied nitrogen gas flow and heat transfer were performed with the Knusden number ranging from 0. 05 to 1.0, using the direct simulation of Monte Carlo (DSMC) method. The influences of the Kn number and the aspect ratio on the gas temperature and wall heat flux in the microchannels were studied parametrically. The total and local heat fluxes of the microchannel walls varying with the channel inlet velocities were also investigated in detail. It was found that the Kn number and the aspect ratio greatly influence the heat transfer performance of microchannels, and both the channel inlet and outlet have higher heat fluxes while the heat flux in the middle part of channels is very low. It is also found that the inlet free stream flow velocity has small affect on the wall total heat flux while it changes the distribution of local heat flux.     

Convective heat transfer and flow characteristics of Cu-water nanofluid

An experimental system is built to investigate convective heat transfer and flow characteristics of the nanofluid in a tube. Both the convective heat transfer coefficient and friction factor of Cu-water nanofluid for the laminar and turbulent flow are measured. The effects of such factors as the volume fraction of suspended nanoparticles and the Reynolds number on the heat transfer and flow characteristics are discussed in detail. The experimental results show that the suspended nanoparticles remarkably increase the convective heat transfer coefficient of the base fluid and show that the friction factor of the sample nanofluid with the low volume fraction of nanoparticles is almost not changed. Compared with the base fluid, for example, the convective heat transfer coefficient is increased about 60% for the nanofluid with 2.0 vol% Cu nanoparticles at the same Reynolds number. Considering the factors affecting the convective heat transfer coefficient of the nanofluid, a new convective heat transfer correl     

Mechanism and numerical analysis of heat transfer enhancement in the core flow along a tube

The present study introduces the principles of enhanced heat transfer in the core flow to form an equivalent thermal boundary layer in the fully developed laminar tube flow, which consequently enlarges the temperature gradient of the fluid near the tube wall, and thereby enhances the heat transfer between the fluid and the tube wall. At the same time, the increase of flow resistance in the tube is not so obvious. Mechanism analysis and numerical calculation based on air and water have been carried out to verify the principle and method presented in this paper, which may bring positive effects to the design of heat exchanger with high heat transfer efficiency and low flow resistance. Supported by the National Key Basic Research Development Program of China (Grant No. 2007CB206903)     

变加热功率下不同形状微肋阵热沉内的对流换热

为探索变加热功率下微肋阵热沉内的对流换热规律,采用精密机械加工获得圆形、菱形和三角形微肋阵热沉,建立一体式加热试验系统,测试了微肋阵热沉的压力降、流动阻力系数、热阻等对流换热参数,研究Re为0~1 000时微肋阵内阻力及对流换热受加热功率的影响规律。研究结果表明,微肋阵内阻力系数先随加热功率增加而增大,圆形和菱形截面微肋阵中该现象在Re>400时消失,而三角形微肋阵在Re>250时消失。加热功率的增加强化了圆形和菱形截面微肋阵内的对流换热,三角形微肋阵的Nu在Re<250时随加热功率的增加而增大,当Re>250后则有所降低;加热功率对于圆形和菱形微肋阵热沉热阻的影响在Re<600时较为明显,而对于三角形微肋阵当Re>250后加热功率对于热阻的影响基本可以忽略。     

Enhancement of flow boiling heat transfer with surfactant

The surfactant additive octadecylamine (ODA) was used to enhance the flow boiling heat transfer of water in vertical copper tube, and the effects of the aqueous solution properties, mass fraction of ODA, mass flux and heat flux etc. on flow boiling heat transfer were investigated. In order to analyze the mechanism of enhancement on boiling heat transfer with ODA, the copper surface was detected by XPS, and the diagram of binding energy was obtained. The results show that ODA can be adsorbed on the surface of the copper wall, and affects the properties of the heating surfaces and enhances the flow boiling heat transfer of water. Only in low heat flux and in a suitable range of concentration, can ODA aqueous solution enhance flow boiling heat transfer, and the suitable mass fraction of ODA is in the range of 1 × 10⁻⁵ −5 × 10⁻⁵. In addition, compared with water, ODA aqueous solution does not increase the flow drag under the same experimental conditions. Foundation item: The Natural Science Foundation of Tianjin(No. 94220) Biography of the first author: QIU Yun-ren, doctoral student, born in Dec. 1966, majoring in heat and mass transfer in chemical engineering.     

螺旋肋片管与气固多相流传热特性研究

本文在试验的基础上对气固多相流横向冲刷螺旋肋片管的传热特性进行了研究。试验所用物料为平均粒径ｄｐ＾－＝４４μｍ电厂灰和ｄｐ＾－＝１２０μｍ细石英砂。颗粒重量浓度Ｍｓ在０￣４．５ｋｇ／ｋｇ范围内。结果表明,螺旋肋片管与气固多相流传热与气流Ｒｅ数,颗粒浓度以及颗粒尺寸均有关,在较细颗粒和较低Ｒｅ数下,气固多相流强化传热程度随颗粒浓度的增加而增加。     

换热系数准则式在燃气涡轮耦合计算中的应用

为节省涡轮叶栅气热耦合数值模拟时间及提高计算结果准确度,将应用较为广泛的迪图斯-贝尔特和格尼林斯基管内对流换热系数准则式应用于NASA-MarkⅡ高压燃气气冷涡轮叶栅气热耦合换热计算中,换热系数准则式引入温度差和入口效应等考虑涡轮内冷腔几何及工况的修正因子.采用商用软件CFX11.0的不同湍流模型进行数值模拟计算,计算...     

An experimental comparison of water based alumina and silica nanofluids heat transfer in laminar flow regime

Experimental investigations were carried out to determine the Al2O3/water and SiO2/water nanofluids flowing through a circular tube. convective heat transfer performance and pressure drop of Measurements show that the addition of small amounts of nano-sized Al2O3 particles to the base fluid increases heat transfer coefficients considerably, while the result for the silica nanofluids contradicts with the alumina nanofluids and this leads to some interesting results. In the case of alumina nanofluids, an average increase of 16% in convective heat transfer coefficient is observed with an average penalty of 28% in pressure drop. Moreover, flow resistance increases significantly compared to the base fluid even at very low concentrations of nanofluids. Finally, measured heat transfer coefficients are compared with predicted ones from the correlation of Shah under the same conditions.     

Heat transfer and flow characteristics in a channel with one corrugated wall

The present study numerically investigates the characteristics of three-dimensional turbulent flow and heat transfer in the channel with one corrugated wall heated with constant temperature by means of large eddy simulation.The corrugated wall is sinusoidal in the streamwise and spanwise directions.The Reynolds number in terms of bulk velocity and channel half-height is fixed at 2800 and the wave amplitude to wavelength ratio is varied in the rangeα/λ=0.01,0.02,0.04 in the streamwise direction andα/λ=0.01,0.02,0.04 in the spanwise direction.The results show that flow separation bubbles appear and near-wall streamwise vortices are generated with larger population in the upslope region of the bottom wall as wave amplitude increases.Compared with flat wall,the corrugated geometry increases the pressure coefficient and decreases the friction coefficient on the corrugated wall,and consequently increases the total drag coefficient owing to the increase of pressure coefficient,as expected,the heat transfer is higher.The waves in the spanwise direction converge the vortices into the trough along the streamwise direction and push them away from the bottom wall.Finally,thermal performance factor is defined and the effects of wave amplitude on the thermal performance are scrutinized.     

凝固换热器中地表水紊流冻结与换热特性

为分析采集凝固热热泵系统凝固换热器的特性,需要掌握地表水紊流工况的相变换热规律.基于准稳态近似方法与假想等壁温模型,分析凝固换热器中紊流水的管内冻结特性,并采用定义的当量平均表面换热系数比Kh和潜热显热比KSL讨论紊流水冻结换热特性.结果表明:紊流时,管半径和沿水流流动方向距离对冻结特性的影响不明显,雷诺数、管壁温和进口水温影响较为明显;随着无量纲凝固时间增长,换热性能急剧下降,入口水温与管壁温度越低、管径越小、管长越大、雷诺数越小,换热特性越好.所得结果将为凝固换热器优化设计及热泵系统性能改善提供重要参考.     

微型薄膜气敏元件芯片的传热系数和热功耗

热功耗是关系到气敏元件的应用、热特性分析和工艺设计的重要参数 ,至今无计算气敏薄膜元件热功耗的实用公式 ;在研究微型薄膜气敏元件引线的散热量 ,芯片传热系数随芯片尺寸和温度变化的规律的基础上 ,给出了热功耗的相关计算方法和公式 ,据此计算得出微型薄膜气敏元件的热功耗数值 ,与实验结果一致     

螺旋管内流动沸腾传热系数关联式拟合与误差分析

为了发展适于螺旋管内流动沸腾传热系数关联式,基于流动沸腾传热机理,引入参数D_n数来修正复杂管道对传热系数的影响,并经过回归计算确定D_n数的指数,从而发展了螺旋管内流动沸腾传热系数关联式。进一步分析了传热系数预测值与实验值的偏差随流量、干度的变化情况。利用以R134a为介质的螺旋管传热实验数据验证了该关联式的适用性,并采用平均相对误差(the mean relative error, MRE)和均方根误差(the root mean square error, RMSE)来衡量预测结果的准确性,计算出MRE在8.23%范围内,RMSE为0.532。该平均相对误差值和均方根误差值都比较小,表明回归计算结果符合要求。因此,复杂管道引入参数D_n数建立传热系数关联式是非常适用的,并值得推广应用。     

翅片换热器中天然气烟气的冷凝传热

对雷诺数在１００￣３００之间的单排冷凝翅片换热器进行了实验研究,并求出了传热因子、传质因了与雷诺数之间函数关系,为天然气冷凝热水器的冷凝换热器提供参考。