Experimental research on stabilities, thermophysical properties and heat transfer enhancement of nanofluids in heat exchanger systems

Stable TiO_2–water nanofluids are prepared by a two-step method, stabilities of nanofluids are investigated by precipitation method and transmittance method respectively, and thermal conductivities and viscosities are also measured. An experimental system for studying the heat transfer enhancement of nanofluids is established,and heat transfer and flow characteristics of TiO_2–water nanofluids in heat exchanger systems with a triangular tube and circular tube are experimentally studied. The effects of nanoparticle mass fractions(ω = 0.1 wt%–0.5 wt%) and Reynolds numbers(Re = 800–10000) on the heat transfer and flow performances of nanofluids are analyzed. Fitting formulas for Nusselt number and resistance coefficient of nanofluids in a triangular tube are put forward based on the experimental data. The comprehensive performances of nanofluids in a triangular tube are investigated. It is found that nanofluids in a triangular tube can significantly improve the heat transfer performance at the cost of a small increase in resistance coefficient compared with that in a circular tube, especially the resistance coefficients are almost the same between different nanoparticle mass fractions at turbulent flow. It is also found that the comprehensive evaluation index η decreases with Reynolds number at laminar flow but a critical maximum value appears at turbulent flow.

Forced Convection Heat Transfer Coefficient and Pressure Drop of Diamond-Shaped Fin-Array

Forced convection cooling of fins on a high-temperature wall has been used to cool high-power electronic devices. We numerically calculated and experimentally measured the forced convection heat transfer coefficient and pressure drop of a diamond-shaped fin-array with water flow in this study, which had been reported to have a self-induced flip-flop flow phenomenon. Although the flip-flop flow phenomenon occurred in calculations, it was not observed in experiments. The heat transfer and pressure drop of the diamond-shaped fin-array could be estimated with equations for turbulent flow in tubes.

Analysis of Heat Transfer of a Flat Plate Oscillating Heat Pipe, for Different Surfaces

对更大的力量密度和电子系统的更小的尺寸的最近、经常的要求刺激了象热管子那样的不同的被动的热转移方法的新图案的生长。特别地， OHP (震荡的热管子) 是相对新奇的设备，能够在长、短的距离上移开高热率与不温度落下。这研究专注于设计，造并且装配一个测试钻塔以便完全分析通过 5 拐弯的 deionised 水的流动模式板震荡在不同的热输入下面的热管子，它被二精力 M.Sc 在工程的学院和玛丽女王伦敦大学的材料科学里做。学生们。水的充满的比率是 40% 。而且， OHP 热表演上的试验性的研究被执行以便检验不同表面的效果湿条件：超级吸水的、吸水、清洗的黄铜。它被表明液体蛞蝓的形成和沿着隧道的水的蒸汽塞子。试验性的结果证明吸水的表面趋于是更多的精力有效。热转移表演超级吸水、吸水分别地比由 5-12% 和 15-20% 的黄铜高。

非牛顿流体在叶片式静态混合器中的传热强化特性

非牛顿流体在化学、食品和材料等领域具有关键作用,但高表观黏度使其通常具有低传热特性。叶片式静态混合器作为一种高效的传热强化设备,在化工过程强化中具有鲜明的技术特色。本文针对非牛顿流体在Kenics静态混合器(KSM)和Lightnin静态混合器(LSM)内的流动和传热特性进行了比较研究。重点分析了体积流量、元件长径比及流体浓度等参数对羧甲基纤维素(CMC)幂律流体的流动及传热影响。结果表明,在相同工况下,随着CMC溶液的体积流量增大,管道内的换热系数和压降均增大。静态混合元件的插入使得换热系数和压降显著提高且Lightnin元件产生的影响更明显。元件长径比和CMC溶液浓度影响流体在管道内的流动和换热。随着长径比的减小,传热性能得到提高,但其增大的阻力系数影响占据主导地位,综合换热性能系数(PEC)降低。溶液浓度的增大使得管道换热能力逐步削弱,并对管道内压降的提升有显著影响,综合换热性能降低。总结得出,在体积流量为4.5×10^(-4)m^(3)/s,质量分数为0.374%、长径比为3.0时,KSM的PEC最大为2.114。

各向异性Kelvin泡沫胞元高径比对其流动传热特性的影响

飞行器热管理系统中所用的高孔隙率泡沫材料具有质量轻、比表面积大等优点,孔隙尺度几何参数的优化设计对有效提升综合传热性能、降低结构质量至关重要。以各向异性Kelvin泡沫胞元为研究对象,采用混合热格子Boltzmann方法以及多GPU加速技术,实现了同时考虑泡沫骨架导热和多胞元孔隙尺度强迫对流换热的共轭传热数值模拟。在上述基础上,分别选取了Re=10、100、1000计算条件,开展了不同胞元高径比(H/D=0.5、0.75、1.0、1.5、2.0)对其流动传热特性的影响规律研究。结果表明,在不同Re条件下,随着各向异性Kelvin泡沫H/D的减小,其泡沫内部的流动阻力及努塞尔数均增大,且随着Re的增大,对流换热的作用增强,H/D的影响也更为明显。此外,相比于流动阻力,结构传热性能受H/D的影响变化速率更大,由此造成综合传热因子(j/f^(1/3))与H/D成反比关系,即减小Kelvin胞元H/D可有效提高泡沫结构的综合传热性能。

叠层交错凹穴微通道热沉内流动与传热特性

为改善日益呈现微型化和高功率化的微化工器件的散热性能,提出一种叠层交错凹穴微通道热沉。运用数值模拟方法研究该微通道热沉内流动和传热特性,考查扩缩比φ和高度比β对热性能的影响,并与直微通道热沉和单层三角形凹穴微通道热沉分别进行对比,评估其综合性能C_(PE)。结果表明:具有叠层交错凹穴的微通道热沉的努塞尔数Nu和摩擦因数f分别比单层三角形凹穴微通道高10.1%—34.4%和5.9%—28.7%。Nu、f、C_(PE)均随φ的增大而减小。当固定φ值且β=0.5时,Nu、f、C_(PE)在相应雷诺数Re下达到最大值。在相同Re条件下,β在0.25—0.75内的变化不会引起流动阻力的显著波动。叠层交错凹穴微通道热沉可以加强流体的混合,中断热边界层的再发展,具有强化传热作用。

R14和R23及其非共沸混合物在水平管内沸腾换热特性的模拟研究

本文采用数值模拟方法,应用VOF多相流模型及Lee相变模型,研究了R14和R23工质在蒸发温度为208.15K时的管内流动沸腾特性。同时,基于准静态理论提出了一种改进的传热传质模型,并利用该模型预测了非共沸工质R14/R23的流动沸腾特性。结果表明,基于液相组分浓度差的组分相间传质模型获得的模拟数据与实验结果具有较好的一致性。在相同蒸发温度下,纯工质的饱和气相密度对其在管内的流动沸腾换热具有显著影响。对于R14、R23及其非共沸混合物R14/R23,其流动沸腾传热系数随干度的增加均呈近线性增大趋势,且低沸点纯R14工质换热系数的增大趋势最显著,R23次之,R14/R23混合物最小。在相同工况下,混合工质R14/R23(0.5:0.5)的传热系数均小于其纯组分的对流传热系数,分别比R14、R23小30%和20%以上,其根本原因在于混合工质存在组分间传质阻力。

水滴形热管阵列预冷器综合换热特性数值研究

提出了一种应用于高超声速飞行器进气预冷的水滴形热管阵列预冷器构想,以实现进气预冷的高效换热和低流动损失。基于将热管处理为恒定温度导热体的简化模型假设,采用数值模拟方法研究了水滴形热管预冷器的流动换热特性以及水滴热管形状、纵向间距和来流马赫数的影响,并与具有相同截面积的圆形热管预冷器进行了对比分析。结果表明,水滴外形减少了外掠高温来流在热管尾缘脱落涡的产生,流动变得平滑。与圆形热管预冷器相比,水滴形热管预冷器极大地减小了高温来流流动损失,仅为圆形热管阵列的30%。虽然温降有所降低,但综合性能因子大于圆形热管预冷器。在本文的研究参数范围内,存在相对较优的水滴形状和热管纵向间距,可以获得更高的综合换热特性;随着来流马赫数增大,预冷器的综合换热特性有所提升。

新型鸟喙式涡流发生器强化传热数值模拟

将仿生鸟喙式结构引入涡流发生器中,结合Fluent软件在Re=6000~26800范围内对其换热能力j、流动阻力f与综合热性能C_(PEC)变化规律进行研究。结果表明:鸟喙式涡流发生器是一种高换热低流阻的高效换热元件,在Re=13000时C_(PEC)较传统矩形翼提高62.35%。此外其综合热性能随着迎流攻角α的增大先增大后减小,在α=30°时达到最大,较空通道提高26.83%。增大斜截角度β是降低流动阻力的一种有效方式,β=24°的结构较无截角结构流阻f降低47.03%。

变径孔射流冲击冷却的流动与传热特性数值研究

采用数值方法开展了变径孔射流冲击冷却的流动与传热性能研究。设计了3种变径孔射流冲击冷却结构,即收缩孔、直孔、扩张孔。对比了不同雷诺数情况下3种孔型射流冲击冷却的流动与传热特性。分析了雷诺数和孔型对射流冲击冷却流动性能、传热性能和综合热力性能的影响。拟合得到了不同孔型冲击冷却的平均努塞尔数、压力损失系数和综合热力系数有关于雷诺数的经验关联式。结果表明:与直孔冲击冷却相比,雷诺数为6000~30 000时,收缩孔冲击冷却的压力损失系数增加了约16.67倍~21.09倍,靶面平均努塞尔数提升了约0.96倍~1.85倍;扩张孔冲击冷却的压力损失系数降低了约22.55%~33.68%,靶面平均努塞尔数降低了约5.62%~10.85%。在3种孔型中,收缩孔冲击冷却的传热性能最优、流动性能最差,直孔冲击冷却的传热性能和流动性能都处于中等水平,而扩张孔冲击冷却的传热性能最差、流动性能最好。当雷诺数小于24 000时,扩张孔冲击冷却的综合热力性能最优,其次为直孔冲击冷却,而收缩孔冲击冷却的综合热力性能最差。当雷诺数大于等于24 000时,3种孔型冲击冷却的综合热力性能很接近。研究结果可以为未来先进燃气轮机内部变径孔冲击冷却的设计提供一定的参考。

Experiments of flow and heat transfer performances for air-cooling laminated turbine vane

Flow resistance and heat transfer coefficients of typical double wall laminated film cooling configuration within a turbine vane were experimentally studied.The specimen was in large scale,and made of transparent organic glass.Laminated configuration consisted of double wall laminates,pin-fins,staggered arrays of impingement and film holes.The number ratio of impingement holes,pin-fins and film holes was2∶1∶1.Five experiment vanes were installed in static cascade,and experiments were carried out under constant heat flux.Re of internal cooling air in the experiment was from 104 to 2×105,and Re of external fluid was from 105 to 3×105.The experiment results show that flow resistances of front channel and back channel of the vane are in the same level,and both of them decrease as Re of cooling air increases.Nuof front channel is slightly higher than that of back channel.Both of them increase as Re of cooling air increases.And experiment results were obtained from experiment vanes were compared with that obtained from laminated flat plates,and the tendency of the results agrees well.

加装螺旋导流板的双层铜管换热器传热性能分析

为提高螺旋铜管(SCT)换热器的综合传热性能,通过在换热器内加装螺旋形导流板,采用双向流固耦合计算方法,分析了不同入口流速和导流板安装位置下对换热器内部双层SCT的振动和传热性能的影响。数值结果表明:双层SCT换热器外层SCT的振动强度明显高于内层;无振动时内层SCT的传热系数高于外层,有振动时外层SCT的传热系数高于内层;随着入口流速增加,内外层SCT的振幅均增大,传热系数提高;导流板的安装位置会直接影响SCT的振动和传热特性,且在SCT呈现最大振动强度和最佳传热性能时的安装位置并不相同;对比振动强化传热参数Q PEC和综合传热性能因子R JF发现,双层SCT换热器的Q PEC和R JF均高于单层,两螺旋导流板分别安装在换热器的左、下位置时,Q PEC最大,安装在换热器的左、上位置时,R JF最大且比安装在左、下位置时最高增大了7.99%。该研究可为高性能SCT换热器的设计提供理论和数据支撑。

湿工况下带翼翅片管传热传质特性的数值研究

采用壁面反应原理建立冷凝传热模型,模拟研究平直翅片管、矩形翼翅片管以及弓形翼翅片管的通道内湿空气流动及传热传质特性。结果表明:当湿空气入口温度为328 K,水蒸气质量分数为11.2%时,矩形翼翅片管和弓形翼翅片管的传热因子相比于平直翅片管分别提高了24.9%—35.7%、26.7%—36.4%,传质因子分别提高了24.1%—35.4%、25.9%—38.3%,但矩形翼翅片管和弓形翼翅片管通道内工质的流动阻力系数分别增加了19.4%—25.3%、16.8%—22.9%,说明了矩形翼和弓形翼均可提高翅片管的传热传质特性,但也造成了阻力损失的增加。通过对综合传热性能评价因子的分析,矩形翼翅片管和弓形翼翅片管的综合传热性能评价因子值均>1,其综合传热性能均优于平直翅片管,且弓形翼翅片管综合传热性能最优。

锯齿型翅片内不同温区氦气流动与传热性能对比及节距对翅片性能影响的研究

为分析板翅式换热器在低温工况下的流动换热性能与常温工况下的差异和翅片节距对其性能影响,建立氦气在锯齿翅片通道内流动与传热的数值模型,开展数值模拟分析。结果表明:低温氦气在锯齿型翅片通道内的传热性能优于常温氦气。在氦气入口温度为20 K时,j因子比300 K时提升1.1%~55.9%,比77 K时提升0.41%~13.01%;在氦气入口温度为77 K时,j因子比300 K时提升0.53%~38.47%。低温氦气在锯齿型翅片通道内的流动性能比常温氦气差。在氦气入口温度为20 K时,f因子比300 K时高16.84%~28.87%,比77 K时高3.98%~23.19%。翅片节距大小在不同温区下对翅片流动与传热性能均有显著影响。在不同温区下,j因子随着翅片节距增大而减小,f因子随着翅片节距减小而增大。低雷诺数时,JF因子随着翅片节距减小而增大,而高雷诺数时则出现相反的趋势。

内置多叶扭带的碳化硅换热管强化传热特性研究

为了研究碳化硅换热管的流动和传热特性,达到增强综合换热能力的目的。采用数值模拟方法对内置4种不同结构扭带的换热管进行分析,确定四叶扭带模型作为后续研究对象,对比了在4种自定义的节距和宽度两种结构参数下,不同入口风速工况的换热管的热力性能。结果表明:扭带宽度对换热管的换热能力相较于节距具有更显著的强化作用,四叶扭带节距为360 mm、宽度为45 mm时换热管的综合换热能力最强,为1.59,此时换热管的努塞尔数相较光管提升了244.37%。

含相变材料的充填体内置采热管群热压性能研究

通过将采热管群与充填采矿技术相结合可提取矿井中的地热能,有效缓解高地热带来的热害问题。本文以采热管群为研究对象,分析了充填体放热过程中传热流体的温度、速度和压力分布,讨论了雷诺数、管径、管间距和管排方式对采热管群性能的影响。结果表明:当雷诺数从100增加到5000时,换热量从2.24×10^(3)kJ增加到16.78×10^(3)kJ,综合评价因子从0.087增加到1.31。当管径从4 mm增加到6 mm时,换热量从15.07×10^(3)kJ增加到17.11×10^(3)kJ,综合评价因子从1.157增加到1.388。当管间距从130 mm增加到170 mm时,换热量从14.24×10^(3)kJ增加到16.25×10^(3)kJ,综合评价因子从1.013增加到1.263。此外,四路并联布置可以保证较高的换热量和较低的压降,从而获得较高的综合评价因子。该研究为充填体中采热管群的设计和优化提供了理论参考。

翅片管换热器内烟气对流换热模拟研究

以螺旋翅片管式换热器为研究对象,选取烟气进出口压降表征换热器流动阻力的大小、努塞尔数(Nu)表征换热器对流换热的强弱,利用Fluent软件模拟分析不同烟气流速下翅片结构参数对换热器流动阻力和换热效果的影响,并引入性能因子综合评价翅片结构参数对换热器换热性能的影响。结果表明:烟气流速一定时,翅片高度增加、厚度加厚和间距缩短均会使换热器进出口压降和Nu增加,在增大换热器流动阻力的同时,换热效果得到增强;翅片结构参数和性能因子的变化规律与Nu相反,选取高度较低、厚度较薄、间距较稀疏的翅片参数时换热器的综合性能更好;此外,与结构参数不同,烟气流速加快时Nu和性能因子均增大,能提升翅片管换热器的综合换热性能。

管式炉中流体流动与传热性能的优化设计研究

本项目以石油化工等工业领域广泛使用的管式加热炉为对象,以计算流体动力学为主要工具,通过建立标准的k-ε湍流模型,利用PDF非绝热燃烧模型以及基于离散坐标的辐射传热模型,对加热炉内的流动、传热、传质过程进行数值模拟。本项目的研究成果将为实现炉膛中的燃烧过程的有效调控、优化模型参数和提高炉膛中的热效率等方面提供重要的科学依据。

Study on Thermal Insulation Performance of Cross-Laminated Bamboo Wall

In recent years,bamboo,as a green building material,has attracted more and more attention worldwide.Inspired by the investigation of cross-laminated timber in structural systems,a new engineered cross-laminated bamboo(CLB)consisting of the cross lamination of bamboo scrimber plates is proposed in this paper.To evaluate its potential in structural applications,the thermal insulation performances of the CLB walls and CLB walls with the EPS foam plate were studied and evaluated by the temperature-controlled box-heat flow meter method.Test results indicated that the thermal insulation performance improved with the increase of thickness,but different wall configurations had little effect on the thermal insulation performance under the same thickness of the CLB wall.The thermal insulation performance of EPS-CLB composite wall was much better than that of CLB wall.In addition,a relatively acceptable accuracy of the theoretical calculations was proved.Finally,the influence of different locations of the EPS foam plate on heat transfer coefficient can be neglected as it was studied based on the validated numerical models.

Friction factor and heat transfer of nanofluid in the turbulent flow through a 90°bend

Heat transfer and energy performance of Al_(2)O_(3)/water nanofluid in a 90°bend with circular cross-section are investigated in the range of Reynolds number(Re)from 5000 to 30000,particle volume concentration(Φ)from 0.005%to 4%,Schmidt number(Sc)from 9870 to 296100,Dean number(De)from 6636 to 14847.The momentum and energy equations of nanofluid together with the dynamic equation for nanoparticles are solved numerically with the particle convection,diffusion,coagulation and breakage taken into consideration.Some results are validated by comparing with the available experimental or numerical results.The effect of Re,Φ,Sc and De on the friction factor and heat transfer of Al_(2)O_(3)/water nanofluidis discussed.The results showed that the particle number decreases along the pipeline.Increasing De,Sc leads to a decrease and increase ofΦ,respectively.The mean particle diameter and particle polydispersity increase with increasing Debut with decreasing Sc.The friction factor increases with increasingΦ,Sc De and Pr but with decreasing Sc.The ratio of energy performance evaluation criterion(PEC)for the Al_(2)O_(3)/water nanofluid to the base fluid increases with increasing Re,Φand De,but with decreasing Sc.Finally,the expression of ratio of energy PEC for the nanofluid to the base fluid as a function of Re,Φ,Sc and De is derived.