Numerical investigation on flow and convective heat transfer of aviation kerosene at supercritical conditions

In this paper,characteristics of flow and convective heat transfer of China RP-3 kerosene in straight circular pipe were numerically studied.Navier-Stokes equations were solved using RNG k-turbulence model with low Reynolds number correction.The thermophysical and transport properties of the China RP-3 kerosene were calculated with a 10-species surrogate and the extended corresponding state method(ECS) combined with Benedict-Webb-Rubin equation.The independence of grids was first studied and the numerical results were then compared with experimental data for validation.Under flow conditions given in the paper,the results show that deterioration of convective heat transfer occurs when the wall temperature is slightly higher than the pseudo-critical temperature of kerosene for cases with wall heat flux of 1.2 and 0.8 MW/m 2.The degree of the heat transfer deterioration is weakened as the heat flux decreases.The deterioration,however,does not happen when the heat flux on the pipe wall is reduced to 0.5 MW/m 2.Based on the analysis of the near-wall turbulent properties,it is found that the heat transfer deterioration and then the enhancement are attributed partly to the change in the turbulent kinetic energy in the vicinity of pipe wall.The conventional heat transfer relations such as Sieder-Tate and Gnielinski formulas can be used for the estimation of kerosene heat convection under subcritical conditions,but they are not capable of predicting the phenomenon of heat transfer deterioration.The modified Bae-Kim formula can describe the heat transfer deterioration.In addition,the frictional drag would increase dramatically when the fuel transforms to the supercritical state.     

H型翅片管束空气流动及换热特性

为了对不同排列方式的H型翅片管进行数值分析,建立不同布置方式下H型翅片管束流动及换热数值模型,获得两种典型布置下的流动及传热特性变化规律。结果表明,H型翅片特殊沟槽结构使背风侧形成纵向冲刷流场,存在明显的三维流动特征,使得其具有自清理、不易积灰的功能;错排翅片基管传热系数大于等间距排列的基管,并且随速度增大,换热效果差异越明显;管束行距对翅片管传热系数的影响明显大于列距的影响,管束行距增大,引起流道内空气平均速度下降,传热系数降低,流道损失减小。     

层流脉动流动对流换热数值分析

为了改进换热器设计方法及提高设备换热效率,通过建立恒定热流密度加热的圆管内层流脉动流动数学模型,对流量脉动条件下的流动与传热特性进行了数值分析.研究结果表明,脉动流动时速度与压降发生同周期的变化,但两者之间存在相位差,随着脉动频率增加,相位差逐步趋近于π/2;脉动速度的径向分布在一定频率范围内出现环状效应,频率越大速度脉动幅值越小,但环状效应越明显;温度脉动幅度随脉动频率、普朗特数增加而减小,随速度脉动幅度增加而增加.     

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．     

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。     

水平光管和内螺纹强化管内换热性能的研究

本文通过建立制冷循环系统，主要研究光管和内螺纹铜管对制冷效果的影响，采用局部表面传热系数的分析方法，较准确地表现换热器管内制冷机单相和两相的换热性能，文中考虑了制冷剂的蒸发干度对制冷效果的影响，并把试验结果和kandikcar得出的经验公式计算结果进行了对比，得出如下结论：内螺纹铜管的换热效果要比光管高出2倍以上。     

Theoretical investigations on heat transfer to H_2O/CO_2 mixtures in supercritical region

The supercritical H_2O/CO_2 mixture is the working fluid to drive a turbine in a novel power generation system with coal gasified in supercritical water. This system is promising because of zero pollution emission in contrast to the conventional coal-fired power plant. Heat transfer coefficients of the supercritical H_2O/CO_2 mixtures are important to design heat transfer devices in this system, which is similar to the role of heat transfer to supercritical water in conventional systems. However, heat transfer to supercritical mixtures has received less attention. Here, we show that the supercritical mixtures with H_2O being one of the components, have similar convection heat transfer behavior to supercritical pure fluids for temperatures and pressures above the critical point of H_2O. This phenomenon was demonstrated from two aspects. Firstly, the forced convection heat transfer coefficients of supercritical mixtures were numerically calculated using the simulation model for supercritical pure fluids and using the thermophysical properties(density, heat capacity, thermal conductivity and viscosity) of supercritical mixtures as input.The calculated results agree well with experimental data in the supercritical region. Secondly, the calculated results also agree well with the correlations for supercritical pure fluids. The mechanisms were investigated by molecular dynamics simulations on the diffusion of supercritical mixtures. These results lay the foundation for predicting convection heat transfer coefficients of supercritical mixtures and for designing heat transfer devices with supercritical mixtures as heat transfer fluids.     

不同强化换热管内流动沸腾换热特性对比

采用实验方法对比制冷剂R410A在6根强化换热管和1根光滑管内的流动沸腾换热特性. 实验测试段饱和温度为6 ℃,进出口干度分别为0.2和0.9,质量流速变化范围为80～350 kg/（m²s）.实验结果表明：三维强化管相对光滑管流动沸腾换热系数的强化倍率可达1.14～1.53,因为强化表面上的凹痕阵列能够增强两相间湍动、提高汽化核心数目并打断液膜边界层制造分离流和二次流从而强化换热. 三维强化管中,管1EHT在低质量流速范围内具有较好的换热性能,而管2EHT在相对较高的质量流速时强化性能更优;齿形参数不同的3根内螺纹管间的换热系数差距较大,其中当内螺纹管螺旋角足够大时齿高与液膜厚度之比相近的内螺纹管具有较好的换热性能.     

具有凸出热源的空调室内的复杂对流换热研究

以空调房间为背景，建立了一侧竖壁有一个发热器、下壁面有两个凸出热源的二维空腔内对流换热的数学模型，并采用了数值计算的方法对不同的加热器温度、送回风方式、进风口空气的参数以及热源强度分别进行了模拟。最后根据计算结果，着重讨论了参数的改变对房间内温度场和流场分布的影响，并比较了不同空调方式条件下房间内的温度和气流分布。该研究对实际的工程应用有一定的指导作用。     

具有凸出热源的空调室内的复杂对流换热研究

以空调房间为背景,建立了一侧竖壁有一个发热器、下壁面有两个凸出热源的二维空腔内对流换热的数学模型,并采用了数值计算的方法对不同的加热器温度、送回风方式、进风口空气的参数以及热源强度分别进行了模拟.最后根据计算结果,着重讨论了参数的改变对房间内温度场和流场分布的影响,并比较了不同空调方式条件下房间内的温度和气流分布.该研究对实际的工程应用有一定的指导作用.     

Experimental and numerical investigations on convective heat transfer of dual piezoelectric fans

An investigation is performed to study the convective heat transfer performance under dual piezoelectric fans. Three main aspects are involved in the current study. Firstly, vibration tests for dual specific piezoelectric fans actuating at the first-mode resonant frequency are conducted to illustrate the influence roles of vibrating phase difference and fan-to-fan pitch on the piezoelectric fan vibration amplitude. Secondly, heat transfer measurements are made to compare the heat transfer among single fan, dual fans in-phase and dual fans out-of-phase. Thirdly, three-dimensional numerical simulations are conducted to reveal the influence mechanism of dual piezoelectric fans on heat transfer. The results show that, the vibrating phase difference of dual fans has nearly no influence on the displacement velocity and amplitude of piezoelectric fan related to single fan once the dimensionless pitch (P/W) is beyond 1.2. The dual piezoelectric fans produce nearly the same peak heat transfer coefficient as that of single fan case. Of particular is that the dual fans operating in-phase produce more favorable heat transfer than the dual fans operating out-of-phase, especially in the gap zone between dual fans. Due to the interaction between dual fans, the streaming flow induced by one vibrating fan suffers the action of sweeping flow of another vibrating fan when they operate out-of-phase. While for the dual fans operating in-phase, the streaming flows induced by vibrating fans merge together to form stronger wall jet flow in the region between two fans.     

套管式地下换热器传热模型及其特性分析

通过对套管式地下换热器传热过程的分析,在已有套管式地下换热器传热模型基础上,考虑管内流动和传热,提出了集管内流动与土壤导热相耦合的传热分析模型,并利用有限元数值计算方法进行了传热特性的分析。讨论了埋管管径组合和流体流速对流体出口温度及单位埋管换热量的影响。此外,还系统地研究了连续运行模式和可变负荷运行模式的地下传热特性,阐述了地下换热器高效运行的控制策略和影响。     

建筑外壁面换热系数对室内自然对流传热影响

为研究建筑外壁面换热系数与供暖室内自然对流换热的关联性,采用修正的湍流k-ε模型对外墙外壁面不同换热系数情况下的室内自然对流换热过程进行了数值分析并对比了热负荷值。结果表明:在满足室内供暖温度(18℃)要求条件下,考虑外界辐射和蒸发对外壁面换热过程的作用(外壁面换热系数取8.1 W·m-2·°C-1),所得散热器表面的换热能力要低于按照暖通设计规范取值(外壁面换热系数取23.3 W·m-2·°C-1)所得结果,两种取值方式对室内温度场和舒适性的影响很微弱,但所得室内热负荷之间的差异达到了6.2%。     

Heat transfer study on solid and porous convective fins with temperature-dependent heat generation using efficient analytical method

A simple and highly accurate semi-analytical method, called the differential transformation method（DTM）, was used for solving the nonlinear temperature distribution equation in solid and porous longitudinal fin with temperature dependent internal heat generation. The problem was solved for two main cases. In the first case, heat generation was assumed variable by fin temperature for a solid fin and in second heat generation varied with temperature for a porous fin. Results are presented for the temperature distribution for a range of values of parameters appearing in the mathematical formulation（e.g. N, εG, and G）. Results reveal that DTM is very effective and convenient. Also, it is found that this method can achieve more suitable results in comparison to numerical methods.     

印刷电路板换热器中S-CO2换热特性数值分析

为了探究超临界二氧化碳(S-CO2)在直通道印刷电路板换热器(PCHE)中换热特性,本文采用数值模拟方法对此问题进行了研究.计算分析了S-CO2在印刷电路板换热器中的换热特性,并对比了在单独改变一侧通道入口温度、工作压力及工质流量时对流换热系数和壁面努塞尔数的变化趋势.研究结果表明:由于工作压力不同,超临界二氧化碳工质...     

Role of viscous heating in entransy analyses of convective heat transfer

Science China Technological Sciences - The entransy theory is widely used and found to be effective in thermal analyses and optimizations. Some researchers considered the entransy variation due to...     

Numerical simulation of heat transfer of latent functionally thermal fluid in tubes with coaxially inserted cylindrical bars in laminar

The heat transfer of latent functionally thermal fluid in three kinds of tubes with coaxially inserted cylindrical bars is numerically researched using equivalent specific heat model, and the flow fields are analyzed with field synergy field. It is found that in the tubes with coaxially inserted cylindrical bars, the heat transfer effects of functionally thermal fluid become more and more pronounced with the Ste decreasing. This is similar to be case of functionally thermal fluid flowing in smooth straight tubes. Compared with the results receiving from smooth straight tubes, the heat transfer of functional thermal fluid in tubes with coaxially inserted cylindrical bars has been significantly enhanced. And this effect becomes more apparent as the diameter of coaxially inserted cylindrical bars increases meanwhile, however, energy consuming of the tubes shows the same trend. Supported by the Key Project of National Natural Science Foundation of China (Grant No. 50436020)     

多孔介质对封闭腔体内对流传热传质的影响

利用两区域法,其中在多孔介质区域利用Forheimer-Brinkman-Darcy方程,纯流体区域使用Navier-Stokes方程对部分填充多孔介质的二维封闭腔体内的自然对流传热传质进行数值研究。采用有限元法辅之以流体与多孔介质交界面上的连续性弱约束条件对两区域方程进行求解,分析了多孔介质厚度、渗透率及孔隙率的变化对封闭腔体内传热传质的影响。数值结果表明:当多孔介质厚度小于0.2时,其厚度的增加可明显削弱传热传质;大于0.2时,其影响明显减弱。渗透率从10^-3降低至10^-6时,腔体中流动减弱,导致平均传热传质速率降低。随孔隙率增加平均传热传质速率近似线性增加。     

装载机管片式散热器流动与传热特性数值分析

应用CFD软件对装载机管片式散热器空气侧流场、压力场和温度场进行了CFD研究。根据不同的空气质量流速,采用k-ε湍流模型对空气流动阻力和散热器换热系数进行数值模拟。与试验结果相比,CFD分析的空气流动阻力和散热器换热系数的平均误差小于10%,计算精度较高,该CFD模型可直接用于工程设计。