YD系列导热油的最高允许膜温

提出了ＹＤ系列导热油的稳定性机理，简要介绍了ＹＤ系列导热油在加热炉管内的最高允许膜温的计算方法。给出了它在最高允许膜温以下工作的适宜流速值及不同管径的膜温修正系数Φ。     

基于Fluent模拟的超声波流量计安装位置研究

以实验室现有泵性能测试装置为基础,开展了基于Fluent的超声波流量计安装位置研究,研究了90°弯管、阀门、管径和流速对管道内流体流场的影响。研究发现,对于充分发展的紊流或湍流,距离90°弯管较近处,流体的流速较为不均,在弯管的内侧会出现流速较低的现象。但在较小管径和较大流速或雷诺数时,超声波流量计的安装位置可以较10D适当提前。当流动状态处于层流时,与管壁接触处的流速明显低于管壁中间。然而随着流速的增加,流体逐渐转变为充分发展的湍流,管道中的流速逐渐变得均匀。     

槽式太阳能热发电站镜场内小荷载基础取消二次灌浆的可行性研究

对槽式太阳能热发电站中镜场内小管径导热油传输管道支撑钢柱的柱脚锚固技术进行了分析,对基础二次灌浆作用进行了分析讨论;对比中、美两国的行业规范,分析柱脚锚栓的受力性能,并对锚栓进行建模计算,探讨小管径导热油传输管道基础取消二次灌浆的可行性。     

有机热载体在螺旋盘管内流动换热特性影响因素分析

在考虑导热油物性变化和管壁热量输入的情况下,利用ANSYS CFX模拟软件数值模拟了螺旋直径D、管壁粗糙度K、导热油入口流速u、盘管布置方式及炉管直径d对导热油在盘管式有机热载体炉内层盘管的前10圈螺旋炉管内流动换热特性的影响。分析结果表明:导热油在有机热载体炉辐射受热面流速宜控制在3~4 m/s范围内。同时对其流动阻力系数和换热准则关联式进行拟合,为有机热载体炉优化设计提供借鉴。     

半月形折流挡板的参数对沿程摩阻系数的影响

采用数值模拟方法研究了雷诺数、折流挡板间距、数目和管径等参数对沿程摩阻系数的影响,分析了带有折流挡板的管流沿程摩阻系数的放大效应,并给出了高雷诺数下沿程摩阻系数的经验关联式。结果表明:当雷诺数大于20 000时,带有折流挡板的管流的沿程摩阻系数与雷诺数几乎没有关系;沿程摩阻系数随着管径的增大而减小,随着挡板间距的增大而减小,随着挡板数目的增大先增大后趋于定值;当带有折流挡板的管流等比例放大时,沿程摩阻系数会发生改变,可根据所提出的关联式重新计算沿程摩阻系数。     

水平光管内R410A与R134a流动冷凝换热特性的对比研究

为研究R410A与R134a在水平光管内的冷凝换热特性,在管内冷凝换热试验台上进行冷凝试验,分析质量流量、冷凝温度、测试水雷诺数Re、管径和制冷剂物性对换热系数和压降的影响。研究表明:换热系数、压降均随着质量流量的增加而变大,随冷凝温度的升高而减小,换热系数随测试水雷诺数Re的增加而减小,而测试水雷诺数Re对压降的影响相对较小;尽管R410A的换热系数随管径的减小而增大,而管径对R134a换热系数的影响并不显著,R134a与R410A的压降均随管径的减小而增大;单位压降换热系数随质量流量的增加而减小; Cavallini et al.关联式可较好预测R410A与R134a在光管内换热系数,而Shah关联式只能用于预测R134a的换热系数。     

不同雷诺数下90°城门洞形弯道内二次流及动能修正系数的研究

利用FLUENT软件并结合Realizable κ-ε湍流模型,对不同雷诺数下水平面上的90°有压城门洞形弯道进行三维数值模拟,研究了雷诺数为2×104~2×107的湍流区弯道内的二次流、动能修正系数随雷诺数改变的变化规律,并给出了二次流发展情况、流速分布均匀性的判断依据。结果表明,虽然雷诺数为二次流强度的主要影响因子,但位于该区内的雷诺数对二次流并无明显影响;雷诺数越大,水流混掺加剧致使水流越紊乱,而动能修正系数反而越小,断面流速分布越均匀;二次流能显著地削减动能修正系数值,且雷诺数越小,这种削减作用越大;压差系数可作为判断二次流的发展状况的依据;流速峰值与流速平均值的比值可作为判断城门洞形断面流速分布的均匀性的依据,且雷诺数越大,适用性越好。     

R410A和R134a在水平强化管内的冷凝换热性能

为研究强化管的冷凝换热性能和强化换热机理,采用实验的方法对R410A在外径6.35和8 mm的光管及内螺纹管(螺旋角为18°和28°)中的冷凝换热性能进行了研究,并与R134a进行对比,实验工况:冷凝温度30和35℃,质量流速400～1 100 kg/(m~2·s)。结果表明:螺纹管冷凝传热系数强化倍率均显著大于内表面扩展倍率;R134a强化因子大于R410A,强化管对粘度、表面张力较大的制冷剂强化效果更显著;8 mm管强化因子大于6 mm,管径较大时,换热提升效果更好;水侧雷诺数为14 000时,8 mm、28°螺纹管在质量流速为500 kg/(m~2·s)时,管内外侧热阻接近,强化效果较好。     

脉动流动强化传热的研究进展

介绍了脉动流动强化传热的机理、方法,分析了脉动频率、脉动振幅、雷诺数、管径、脉动型式、流体物理性质对传热特性的影响,概括了脉动流动强化传热的国内外研究现状,并提出了进一步研究的建议.     

预防导热油变质劣化的智能监测系统研究

讨论了导热油变质劣化的主要原因,分析了过热超温、氧化变质、化学污染等因素对导热油的品质影响及危害程度,研究了导热油样品分析和品质变化的智能监测,探讨了导热油炉及循环供热系统运行的智能监控,同时按照专家诊断远程网络系统给出的建议或方法,可以迅速解决在运行使用过程中对导热油品质变化产生影响的各种不利因素和存在的实际问题。     

Theoretical study of superheated vapor effect on liquid film condensation in a saturated porous medium using Forchheimer's model

In this work, the superheated vapor effect on liquid film condensation in a saturated porous medium using Forchheimer's model has been investigated analytically and numerically. The applied governing equations, the continuity equation, the Forchheimer equation, and the energy equation were transformed using the similarity transformation technique into a dimensionless form using a set of suitable variables and then solved numerically using the Runge–Kutta method. Results obtained were graphically drawn to illustrate the effects of superheated vapor and subcooled liquid on the liquid film condensation, temperature, and heat transfer rate through the porous medium. It was found that the film thickness is a function of dimensionless parameters related to the degree of subcooling and Grashof number without a superheating effect. Consequently, the Nusselt number depends on the square root of the Rayleigh number, the Grashof number, and the dimensionless film thickness. It was also found that if superheating exists, the liquid film thickness then depends on four dimensionless parameters related to the Grashof number, the degree of subcooling of the liquid, the extent of the superheating of the surrounding vapor, and a property ratio of the liquid and the vapor phase.     

新型多孔铜微通道热沉散热性能实验研究

新型多孔铜微通道散热技术采用多孔铜微通道结构,增加热沉与冷却工质的接触面积,提高热沉的散热性能。利用单室金属-气体共晶定向凝固工艺,通过控制冷却速度、过热度、气压等工艺参数,从而制备优质的多孔铜材料。根据多孔铜微通道热沉散热原理,搭建散热性能测试平台,研究冷却工质流量、多孔铜材料的孔径和孔隙率、入口截面斜率角对多孔铜微通道热沉散热性能的影响规律。结果表明:增加冷却工质流量有利于提高多孔铜微通道热沉的散热性能;在恒定体积流量下,减小孔径有利于提高多孔铜微通道热沉的散热性能;当多孔铜孔隙率为30.8%时,多孔铜微通道热沉散热性能最佳;入口截面斜率角对多孔铜微通道热沉散热性能的影响较小。     

Influence of viscosity variations on the forced convection flow through two types of heterogeneous porous media with isoflux boundary condition

A theoretical analysis of fully developed forced convection flow through a heterogeneous porous medium with isoflux boundary condition is carried out by invoking a varying viscosity model to determine its effect on the resulting heat and fluid flow characteristics. Two different types of permeability and thermal conductivity variations, in the transverse direction of the channel, are accounted for in the analysis, viz., continuous weak and step-wise variations. Analytical solutions are obtained and graphical illustrations are presented to reveal the influence of thermal conductivity, permeability and viscosity variations. The results indicate that the viscosity variations have significant effect on the resulting heat transfer characteristics interpreted in terms of the Nusselt number. Moreover, the results reveal that the heat transfer rate decreases with the increase of permeability ratio for strong viscosity variations, which cannot be captured if a constant viscosity model is assumed.     

燃气机热泵改变制冷剂流量的增益调度控制

燃气机热泵可以通过改变燃气机转速调节系统容量,系统容量的调节和压缩机转速的变化,需要电子膨胀阀调节制冷剂流量与之相匹配。采用实验方法建立蒸发器过热度模型,通过理论分析和实验测试,研究了燃气机热泵系统变转速调节和当过热度设定值改变时蒸发器过热度的控制策略。提出采用增益调度控制策略实现蒸发器过热度的控制,实验结果表明:改变燃气机转速时,过热度控制比较精确,波动范围在±0.5℃以内;过热度设定值改变时,最大超调量小于2℃,过热度响应速度快,具有很好的动态响应特性,达到稳态的时间不超过200 s。     

Analysis of influence of physical parameters on vapor-liquid flow behavior up to dryout in a heat-generating porous medium

In the present work the influence of various physical characteristics on the two-phase flow behavior in a self-heated porous medium has been studied using a numerical model, that is, the effects of heat generation rate, of porosity, of particle size, and of system pressure on the dryout process. To analyze the effect of these characteristics, the variation of both liquid volumetric fraction and liquid axial velocity is evaluated at the steady state or at the onset of a first boiled-out region. The analysis of computational results indicate that a qualitative tendency exists between the characteristics such as heat generation rate, porosity, effective particle diameter and the temporal development of the liquid volumetric fraction field up to dryout. In addition to these characteristics, a variation of fluid properties such as phase density, phase viscosity due to a change of system pressure can be used for gaining insight into the nature of two-phase flow behavior up to dryout.     

Thermally dependent viscosity and non-Newtonian flow in a double-pipe helical heat exchanger

A double-pipe helical heat exchanger was numerically studied to determine the effects of thermally dependent viscosity and non-Newtonian flows on heat transfer and pressure drop for laminar flow. Thermally dependent viscosities were found to have very little effect on the Nusselt number correlations for Newtonian fluids; however significant effects on the pressure drop in the heat exchanger were predicted. Changing the flow rate in the annulus can significantly affect the pressure drop in the inner tube, since the average viscosity of the fluid in the inner tube would change due to the change in the average temperature.The effects of non-Newtonian power law fluids on the heat transfer and the pressure drop were determined for laminar flow in the inner tube and in the annulus. The Nusselt number was correlated with the Péclet number for heat transfer in the inner tube. For the annulus, the Nusselt number was found to correlate best with the Péclet number and the curvature ratio. Pressure drop data were compared by using ratios of the pressure drop of the non-Newtonian fluid to a Newtonian fluid at identical mass flow rates and consistency indices.     

Viscous potential flow analysis of capillary instability with heat and mass transfer through porous media

The capillary instability of the cylindrical interface separating two viscous and incompressible fluids through porous medium is studied using viscous potential flow theory, when there is heat and mass transfer across the interface. The fluids are considered to be viscous and incompressible with different kinematic viscosities. A dispersion relation that accounts for the growth of axisymmetric waves is derived and stability is discussed theoretically as well as numerically. Stability criterion is given in terms of critical value of wave number. Various graphs have been drawn to show the effect of various physical parameters such as heat transfer capillary number, viscosity ratio, permeability and porosity on the stability of the system. It has been observed that porous media and heat transfer both have stabilizing effect while porosity has destabilizing effect on the stability of the system.     

Experimental Study on the Air-Side Performance of a Multipass Parallel and Counter Cross-Flow L-Footed Spiral Fin-and-Tube Heat Exchanger

This study conducted experiments on the air-side performance of novel L-footed spiral fin-and-tube heat exchangers that were faced with airflow at high Reynolds numbers (3500–13,000). The examined heat exchangers have a multipass parallel-and-counter cross-flow type of water flow arrangement. This flow arrangement is a combination of the parallel cross-flow and the counter cross-flow. This type of water flow arrangement may be the best fit for the reverse-flow system, because it can provide constant heat-exchange effectiveness for every flow reversal direction at the same airflow rate. Ambient air was used as a working fluid on the air side and hot water for the tube side. This way the effect of the number of tube rows on the heat transfer and friction characteristics of L-footed spiral fin-and-tube heat exchangers was clearly observed. The effect of the fin's outside diameter on the pressure drop was also studied. The results show that the number of tube rows has no significant effect on the air-side heat transfer or on friction characteristics at high Reynolds numbers. However, the fin's outside diameter shows a significant effect on the pressure drop. The pressure drop increases as the fin's outside diameter increases for the same number of tube rows.     

Numerical Investigation of the Effect of Baffle Orientation on Heat Transfer and Pressure Drop in a Shell and Tube Heat Exchanger With Leakage Flows

The commercial CFD code FLUENT is used to investigate the effect of baffle orientation and of viscosity of the working fluid on the heat transfer and pressure drop in a shell-and-tube heat exchanger in the domain of turbulent flow. The shell-and-tube heat exchanger considered follows the TEMA standards and consists of 76 plane tubes with fixed outside diameter, which are arranged in a triangular pitch. Two baffle orientations as well as leakage flows are considered. In order to determine the effect of viscosity on heat transfer and pressure drop, simulations are performed for the working fluids air, water, and engine oil with Prandtl numbers in the range of 0.7 to 206. For each baffle orientation and working fluid, simulations are performed using different flow velocities at the inlet nozzle. Heat transfer and pressure drop are reported in order to describe the performance of vertically and horizontally baffled shell-and-tube heat exchangers. The heat transfer coefficient is described as modified shell-side Nusselt number, which is defined similar to the VDI method.     

Laminar convective heat transfer in chaotic configuration

The convective heat transfer in chaotic configuration of circular cross-section under laminar flow regime at different values of Dean number and Prandtl number is investigated numerically. The chaotic configuration is the combination of 90° bends and coils. The insertion of equidistant 90° bends between the two consecutive coil produces the phenomenon of flow inversion. The hydrodynamics and heat transfer under laminar flow conditions in the chaotic configuration with constant wall flux as a boundary condition is studied. The control-volume finite difference method with second-order accuracy is used. The chaotic configuration shows a 25–36% enhancement in the heat transfer due to chaotic mixing while relative pressure drop is 5–6%. The effect of Prandtl number on fully developed heat transfer coefficient is also reported. It is observed that heat transfer increases with increase in Prandtl number. The stretching and folding phenomenon in chaotic configuration is observed and discussed for heat transfer coefficient and pressure drop in the chaotic configuration. The cyclic oscillation behavior in the heat transfer coefficient with downstream distance in the chaotic configuration and coiled tube is also observed and discussed. It appeared that heat transfer is strongly influenced by flow inversion. The effect of boundary conditions on heat transfer performance in the chaotic configuration as well as in the coiled tube is also carried out. The study is further extended to predict hydrodynamics and heat transfer with temperature-dependent viscosity in the chaotic configuration. A comparative study for heat transfer and friction factor is also carried out for constant and temperature-dependent viscosity in coiled tube and chaotic configuration. It was observed that the heat transfer under heating condition with temperature-dependent viscosity is higher as compared to the constant viscosity result while friction factor shows the reverse phenomenon in the chaotic configuration.