流道内涡街诱导弹性体振荡强化传热分析

以具有固定扰流圆柱的冲孔矩形涡流发生器通道内换热强化为研究对象,采用任意拉格朗日-欧拉(ALE)法模拟了流道内涡街诱导弹性体振荡强化换热现象。通过对比分析流道内的流动损失E、净换热量Q和热效率因子η,研究了弹性体抗弯刚度及安装角度对通道内流体流动和换热的影响。结果表明：流体经刚性圆柱产生涡街,诱导弹性体发生振荡,尾涡在弹性体达到最大形变时从后缘脱落进入下游中,其形成的二次流不断扫掠热边界层,使得壁面附近热流体与来流冷流体相互掺混,从而有效强化换热;抗弯刚度越低,弹性体振荡位移越大,在低流动损失前提下产生的净换热量越高,从而显著提高换热效率;弹性体安装倾角为π/4、抗弯刚度为0.25时净换热量提升最显著,较常规通道提升88.5%,较刚性体通道提升82.7%;热效率因子在采用弹性体的通道内均高于刚性体情况,弹性体安装角度β为π/2时热效率因子最高,且其在弹性体任何抗弯刚度下较常规通道均有提升。     

深层埋管式能源桩换热性能影响因素分析

提出一种新型的能源桩换热管型式,即深层埋管式能源桩。利用Comsol Multiphysics建立三维方法模拟桩体-土体传热,一维方法模拟管内水动态传热传质的数值模型,考虑了土体温度随深度的变化,模拟出口水温随时间的变化规律并计算换热量,比较深层埋管式与传统的1-U型、1-W型能源桩的换热量,分析了桩径、桩体导热系数、桩体密度、桩体比热容等不同参数对新型深层埋管式能源桩换热量的影响。模拟结果表明：以运行50 h为例,深层埋管式的总体换热量比1-U型、1-W型分别高122%、54%;而对于单位管长换热量,深层埋管式比1-U型、1-W型分别高9%、50%,桩径从0.5 m增加到1 m,换热量增加14.3%;桩体导热系数从1.2 W/(m∙K) 增大至2.5 W/(m∙K),换热量增加9.6%;桩体密度从1 800 kg/m³增大到2 600 kg/m³,换热量增大0.8%;桩体比热容从637 J/(kg∙K) 增大到1 037 J/(kg∙K),换热量增大1.1%。因此深层埋管式的热性能优于传统1-U型和1-W型,在满足能源桩力学性能的前提下,为了提高深层埋管式能源桩换热性能,可以适当增大桩径。对于桩体材料的选择,应该选择导热系数较高的材料。密度和比热容对换热量的提升影响不大。     

几种带冷却结构的双层管换热及流动对比

对相同质量流量下的光管、双层光管、带冷却结构（肋、扰流柱、凹坑、螺旋通道）的双层管等不同结构的管流动进行了流固耦合三维数值模拟,获取了固体壁温的分布特征;对各结构下,外层壁冷热侧温差、冷气温升、流动特性及综合换热效率进行了研究分析。研究结果表明：相同质量流量下,带螺旋通道双层管的外层壁冷热侧温差最小、综合换热效率最高;凹坑结构双层管与双层光管的流动及换热特性相似,流阻较小但换热效果也较差;扰流柱和肋结构双层管的流动换热特性相近,其温度分布均匀性、换热量介于双层光管和螺旋通道双层管之间,其流阻大且综合换热效率低。     

通道中簧片流固耦合振荡的强化传热

采用任意拉格朗日-欧拉(Arbitrary Lagrangian-Eulerian,ALE)方法,研究低雷诺数下内置倒转簧片通道的流动与传热,模拟不同抗弯刚性系数下倒转簧片的振动现象、流场特性及其对通道传热的影响。结果表明:随抗弯刚性系数的增大,倒转簧片在均匀来流中存在偏转模式、大幅度拍打模式、小幅度拍打模式和稳定模式4种运动模式;与普通通道相比,大幅度拍打模式下,通道壁温降低,平均努塞尔数显著提高;小幅度拍打模式下,通道壁温有较小程度的降低,平均努塞尔数有一定提高;稳定模式下,簧片对流场扰动极小,通道壁温与普通通道几乎相同,平均努塞尔数基本不变;偏转模式下,由于簧片始终偏向通道下侧,阻塞了通道下侧的流动,通道壁温显著升高,平均努塞尔数显著降低;在实际工程应用中,应避免使倒转簧片处于偏转模式和稳定模式下,尽量使其处于大幅度拍打模式下,才能有效强化通道内传热。     

换热表面结垢对U型管蒸汽发生器传热性能的影响分析

U型管蒸汽发生器的壳侧沉积了来自二回路系统中的腐蚀产物,结垢导致热量聚积在金属换热管上,容易造成垢下热点腐蚀,危害设备安全。为了明确结垢对蒸汽发生器传热性能的影响,本研究基于仿真平台APROS建立了U型管蒸汽发生器的分布式模型,并根据已公开论文中的数据进行了模型准确性验证;推导了污垢热阻与表面换热系数之间的关系式,分析了不同结垢厚度、位置对U型管蒸汽发生器换热区域的传热管壁面温度、流体温度、传热系数、热流密度等的影响程度。研究结果表明：随着结垢程度的加剧,蒸汽发生器的换热效率不断降低,出口蒸汽品质不断下降;结垢对沸腾段换热效率的影响比对过冷段换热效率的影响更大。     

扰流柱对间断交叉肋流动换热特性的影响研究

为了探究扰流柱对间断交叉肋通道流动与换热特性的影响,针对不同扰流柱数量和排布位置建立了不同的交叉肋模型,并通过数值模拟的方法,计算了各模型的阻力系数比、强化换热系数以及综合热效率3个性能指标的变化情况。研究结果表明：随着扰流柱数量的增大,阻力系数比和强化换热系数逐渐增大,而综合热效率不断下降;在进口雷诺数为20 000时,14柱模型与32柱模型相比,阻力系数比升高了15.4%,强化换热系数升高了32%,综合热效率提高了2.6%;将相同数量的扰流柱排布在通道内的不同位置对综合热效率的影响并不明显。     

自除污型干式污水源热泵系统的实验研究

提出一种具有快速除污功能的干式壳管式污水源热泵机组,并对其在桑拿洗浴中心的运行特性进行现场测试。实时监测生产热水过程中机组的运行参数,并验证污水蒸发器的除污特性。研究结果显示:热水加热时间为23min/次,供应热水量约120L/次,温度为51℃,可充分满足用水温度需要;机组COP最高达3.67,最低为2.51,平均值为2.91;30d内污水蒸发器换热量由8300W降低到5600W,约为干净状态换热量的67.5%,除污后污水换热量提升到8100W,基本恢复初始换热量,除污型污水换热器除污和换热效率较高;除污前后,蒸发温度由3.9℃提高到6.1℃,表明该除污功能可大大改善换热系数。     

土壤间接热脱附强化换热性能研究

针对土壤间接热脱附换热效率低的问题进行了强化中心管换热的研究,采用数值模拟的方法建立了不同肋片及不同排布方式的土壤热脱附中心管模型,通过改变进口烟气的流速,探究最佳肋片结构.模拟结果表明:在热脱附中心管中添加肋片能显著的提升其换热效率,其中螺旋凸管的传热效果最好,且螺距越小其换热效果越好,在相同进口流速下其换热系数提升...     

通道内柔性体流固耦合对流动和换热的影响

以新型冲孔矩形涡流发生器流道内的流动与换热为背景,模拟了流道内插柔性体的流固耦合对流换热过程,研究柔性体涡流发生器的振荡对通道内涡量场及其换热性能的影响,探究通道内柔性体流固耦合强化换热的机理。结果表明:柔性体在变形过程中会使流场中产生尾涡,且当其在一个拍打周期内达到最大弯曲程度时,尾涡会在后缘脱落到尾流中,使得冷、热流体剧烈掺混,从而起到强化换热的作用;随着Re的增大,换热强度提高得更快,在Re=3 000时通道中安装单个柔性体涡流发生器可以使通道对流换热强度提高39.4%。     

具有防融霜装置的蒸发器结霜特性及性能仿真研究

以具有防融霜装置的蒸发器为研究对象,建立基于理想最小防融霜补热量和结霜量无量纲关联式的空气源热泵动态模型,并依据实验数据验证其精确性,通过数值模拟分析不同环境参数对具有防融霜装置的蒸发器性能的影响。结果表明：后置式与跨越式系统蒸发器换热系数要高于前置式与传统电辅热系统,霜层厚度更小,分布更均匀;当相对湿度一定时,蒸发器换热系数随温度升高而增大;当空气温度不变时,换热系数随相对湿度的下降而减小,霜层更均匀;温度对蒸发器换热系数的影响比相对湿度更大。     

Effect of aspect ratio on saturated boiling flow in microchannels with nonuniform heat flux

Microchannel two‐phase flow is an effective cooling method used in microelectronics, in which the heat flux density is unevenly distributed usually. The paper is focused on numerical study the effect of aspect ratio on the flow boiling of microchannels with nonuniform heat flux. The heat source is a three‐dimensional (3D) integrated circuit. 3D microchannel model and volume of fluid method are coupled in numerical simulation. The results show that the aspect ratio has no relationship with the two‐phase pressure drop of the microchannel. It has a certain influence on the distribution of bubble shape. In terms of the heat transfer coefficient, the aspect ratio has a certain influence on a section of the inlet. Due to the nucleate boiling, the convective heat transfer in the remaining areas is the dominant factor and the average heat transfer coefficient is mainly determined by the heat flux at the bottom of the channel.     

A Possible Strategy for the Performance Enhancement of Turbine Blade Internal Cooling With Inclined Ribs

Ribbing the internal passages of turbine blades with 45 deg inclined ribs is a common practice to achieve a good compromise between high heat transfer coefficients and not too large pressure drop penalties. Literature studies demonstrated that, for channels having a large aspect ratio, the effect of the secondary vortices induced by angled ribs is reduced and the heat transfer performance is degraded. In order to enhance the performance, a possible strategy consists in introducing one or more longitudinal ribs (intersecting ribs) aligned to the main direction of flow. The intersecting ribs cut the ribbed channel into separate sub-channels and markedly affect the secondary flows with consequent increases in heat transfer performance. Experiments were performed for a rectangular channel with a large aspect ratio (equal to five) and 45 deg inclined ribs, regularly spaced on one of the principal walls of the channel. The effect of one and two intersecting ribs on friction and heat transfer characteristics has been investigated. The ribbed surface of the channel has been electrically heated to provide a uniform heat flux condition over each inter-rib region. The convective fluid was air. Heat transfer experiments have been conducted by using the liquid crystal thermography. Results obtained for the ribbed channel without intersecting rib and with one/two intersecting ribs are compared in terms of dimensionless groups.     

Boiling Heat Transfer for Freon R21 in Rectangular Minichannel

In this paper, we study the boiling heat transfer of upward flow of R21 in a vertical mini-channel with a size of 1.6 × 6.3 mm. The heat transfer coefficient was measured as a function of heat flux for a wide range of vapor quality and for two levels of mass flow rate, G = 215 kg/m²s and G = 50 kg/m²s. The standard deviation of wall superheat over channel perimeter and in time was determined from the measurement of the wall temperature along the channel perimeter. Different heat transfer mechanisms were revealed depending on flow patterns. The main heat transfer mode for large mass flux is convective boiling. We also figure out the mode when the evaporation of thin liquid films makes the essential contribution to heat transfer. The modified models of Liu & Winterton and Balasubramanian & Kandlikar describe the experimental data well for regime when the convective boiling makes the main contribution to the heat transfer.     

Instability,heat transfer and flow regime in a two-phase flow thermosyphon loop at different diameter evaporator channel

In this study, the influence of different channel geometries on heat transfer, flow regime and instability of a two-phase thermosyphon loop, is investigated. Instabilities in flow regime and heat transfer, at low and high heat fluxes, are observed. Bubbly flow with nucleate boiling heat transfer mechanism, confined bubbly/slug flow with backflow for small channel height (H) and finally slug/churn flow at high heat fluxes are observed. This study shows that flow and thermal instability increases as channel height (H) decreases and also heat transfer coefficient increases with increasing channel height and heat flux. Bubbly flow characterizes the flow regime at high heat transfer coefficients while confined bubbles, backflow and intermittent boiling are more significant for low channel heights with lower heat transfer coefficient and critical heat flux.     

Heat Transfer and Flow Pattern Characteristics for HFE-7100 Within Microchannel Heat Sinks

This study investigates the heat transfer characteristics and flow pattern for the dielectric fluid HFE-7100 within multiport microchannel heat sinks with hydraulic diameters of 480 μm and 790 μm. The test results indicate that the heat transfer coefficient for the smaller channel is generally higher than that of the larger channel. It is found that the heat transfer coefficients are roughly independent of heat flux and vapor quality for a modest mass flux ranging from 200 to 400 kg m^?2 s^?1 at a channel size of 480 μm and there is a noticeable increase of heat transfer coefficient with heat flux for hydraulic diameters of 790 μm. The difference arises from flow pattern. However, for a smaller mass flux of 100 kg m^?2 s^?1, the presence of flow reversal at an elevated heat flux for hydraulic diameters of 480 μm led to an appreciable drop of heat transfer coefficient. For a larger channel size of 790 μm, though the flow reversal is not observed at a larger heat flux, some local early partial dryout still occurs to offset the heat flux contribution and results in an unconceivable influence of heat flux. The measured heat transfer coefficients for hydraulic diameters of 790 μm are well predicted by the Cooper correlation. However, the Cooper correlation considerably underpredicts the test data by 35–85% for hydraulic diameters of 480 μm. The influence of mass flux on the heat transfer coefficient is quite small for both channels.     

Optimal mixed convection for maximal energy recovery with vertical porous channel (solar wall)

A vertical open-ended channel filled with a porous medium, with an imposed heat flux and a heat loss coefficient on one of its walls, is studied numerically. A fan can enhance the self-driven flow, and therefore a mixed convection regime is considered. The objective is to maximize the overall energy recovery (heat transfer to the fluid minus fan power). Correlations are developed for optimal pressure drop to be imposed by the fan and maximal energy recovery, as a function of the Rayleigh number, the channel aspect ratio, and the heat loss coefficient. The optimal allocation of the total energy losses (i.e., sum of the heat loss and fan power) is shown. Potential applications include solar wall and solar chimney used for ventilation and preheating of makeup air in buildings.     

Two-Phase Heat Sinks with Microporous Coating

To minimize flow boiling instabilities in two-phase heat sinks, two different types of microporous coatings were developed and applied on mini- and small-channel heat sinks and tested using degassed R245fa refrigerant. The first coating was epoxy based and was sprayed on heat sink channels, while the second coating was formed by sintering copper particles on heat sink channels. Minichannel heat sinks had overall dimensions 25.4 mm × 25.4 mm × 6.4 mm and 12 rectangular channels with a hydraulic diameter 1.7 mm and a channel aspect ratio of 2.7. Small-channel heat sinks had the same overall dimensions, but only three rectangular channels with hydraulic diameter 4.1 mm and channel aspect ratio 0.6. The microporous coatings were found to minimize parallel channel instabilities for minichannel heat sinks and to reduce the amplitude of heat sink base temperature oscillations from ～6°C to slightly more than 1°C. No increase in pressure drop or pumping power due to the microporous coating was measured. The minichannel heat sinks with porous coating had on average 1.5 times higher heat transfer coefficient than uncoated heat sinks. Also, the small-channel heat sinks with the “best” porous coating had on average 2.5 times higher heat transfer coefficient and the critical heat flux was 1.5 to 2 times higher compared with the uncoated heat sinks.     

Effects of heat flux,vapour quality,channel hydraulic diameter on flow boiling heat transfer in variable aspect ratio micro-channels using transparent heating

Experiments on flow boiling heat transfer in high aspect ratio micro-channels with FC-72 were carried out. Three channels with different hydraulic diameters (571, 762 and 1454 μm) and aspect ratios (20, 20 and 10) were selected. The tested mass fluxes were 11.2, 22.4 and 44.8 kg m^?2 s^?1 and heat fluxes ranging from 0–18.6 kW m^?2. In the present study, boiling curves with obvious temperature overshoots are presented. Average heat transfer coefficient and local heat transfer coefficient along stream-wise direction are measured as a function of heat flux and vapour quality respectively. Slug-annular flow and annular flow are the main flow regimes. Convective boiling is found to be the dominant heat transfer mechanism. Local heat transfer coefficient increases with decreasing hydraulic diameter. Moreover, the effect of hydraulic diameter is more significant when mass flux is higher. The unique channel geometry is considered as the decisive reason of the flow regimes as well as heat transfer mechanisms.     

Numerical study of laminar heat transfer and pressure drop characteristics in a water-cooled minichannel heat sink

With the rapid development of the information technology (IT) industry, the heat flux in integrated circuit (IC) chips cooled by air has almost reached its limit about 100 W/cm². Some applications in high technologies require heat fluxes well beyond such a limitation. Therefore the search of a more efficient cooling technology becomes one of the bottleneck problems of the further development of IT industry. The microchannel flow geometry offers large surface area of heat transfer and a high convective heat transfer coefficient. However, it has been hard to implement because of its very high pressure head required to pump the coolant fluid though the channels. A normal channel could not give high heat flux although the pressure drop is very small. A minichannel can be used in heat sink with a quite high heat flux and a mild pressure loss. A minichannel heat sink with bottom size of 20 mm × 20 mm is analyzed numerically for the single-phase laminar flow of water as coolant through small hydraulic diameters and a constant heat flux boundary condition is assumed. The effects of channel dimensions, channel wall thickness, bottom thickness and inlet velocity on the pressure drop, thermal resistance and the maximum allowable heat flux are presented. The results indicate that a narrow and deep channel with thin bottom thickness and relatively thin channel wall thickness results in improved heat transfer performance with a relatively high but acceptable pressure drop. A nearly-optimized configuration of heat sink is found which can cool a chip with heat flux of 256 W/cm² at the pumping power of 0.205 W. The nearly-optimized configuration is verified by an orthogonal design. The simulated thermal resistance agrees quite well with the result of conventional correlations method with the maximum difference of 12%.     

Investigation of heat transfer characteristics of hydrogen combustion process in cylindrical combustors from microscale to mesoscale

In the field of micro and mesoscale combustion, the feature of flame-wall thermal coupling is of great significance because of its small scale nature. Thus, this work provides a comprehensive heat transfer analysis in cylindrical combustors from the perspective of numerical simulation. The combustor has a fixed length-to-diameter aspect ratio of 10, and the channel diameter is scaling up from 1 mm to 11 mm to explore the influence of chamber dimension on heat transfer and flame structure. The distribution of convective and radiative heat flux on inner surface, contribution of thermal radiation are given. Moreover, the role of radiation in flame structure is analyzed, and the convective and radiative heat losses are quantitatively analyzed. We find that radiative heat flux is smaller compared to convective heat flux, and the proportion of radiative heat flux becomes larger with an increasing diameter. Thermal radiation does not change the flame structure when the diameter is less than 3 mm. When the diameter is greater than 5 mm, thermal radiation changes the location of flame front. The heat loss becomes larger at a smaller diameter, and heat loss ratio can reach approximately 73.6% in the combustor with diameter of 1 mm.