Experiment on Boiling Heat Transfer of Refrigerant R134a in Mini-channels

The flow boiling heat transfer characteristics of refrigerant R134 a flowing inside two different kinds of minichannels are investigated. One channel is multi-port extruded with the hydraulic diameter of 0.63 mm,and the other one is rectangular with offset fins and a hydraulic diameter of 1.28 mm. The experiments are performed with a mass flow rate between 68 and 630 kg/(m^2·s),a heat flux between 9 and 64 kW/m^2,and a saturation pressure between 0.24 and 0.63 MPa,under the constant heat flux heating mode. It is found that the effect of mass flow rate on boiling heat transfer is related to heat flux,and that with the increase of heat flux,the effect can only be efficient in higher vapor quality region. The effects of heat flux and saturation pressure on boiling heat transfer are related to a threshold vapor quality,and the value will gradually decrease with the increase of heat flux or saturation pressure. Based on these analyses,a new correlation is proposed to predict the boiling heat transfer coefficient of refrigerant R134 a in the mini-channels under the experimental conditions.

Experimental study on convective boiling heat transfer in narrow-gap annulus tubes

Since convective boiling or highly subcooled single-phase forced convection in micro-channels is an effective cooling mechanism with a wide range of applications, more experimental and theoretical studies are re- quired to explain and verify the forced convection heat transfer phenomenon in narrow channels. In this experimental study, we model the convective boiling behavior of water with low latent heat substance Freon 113 (R-113), with the purpose of saving power consumption and visualizing experiments. Both heat transfer and pressure drop characteris- tics were measured in subcooled and saturated concentric narrow gap forced convection boiling. Data were obtained to qualitatively identify the effects of gap size, pressure, flow rate and wall superheat on boiling regimes and the tran- sition between various regimes. Some significant differences from unconfined forced convection boiling were found, and also, the flow patterns in narrow vertical annulus tubes have been studied quantitatively.

Boiling Heat Transfer in Circulating Fluidized Beds

A model is proposed to predict boiling heat transfer coefficient in a three-phase circulating fluidized bed (CFB), which is a new type of evaporation boiling means for enhancing heat transfer and preventing fouling. To verify the model, experiments are conducted in a stainless steel column with 39mm ID and 2.0m height, in which the heat transfer coefficient is measured for different superficial velocities, steam pressures, particle concentrations and materials of particle. As the steam pressure and particle concentrations increase, the heat transfer coefficient in the bed increases. The heat transfer coefficient increases with the liquid velocity but it exhibits a local minimum. The heat transfer coefficient is correlated with cluster renewed model and two-mechanism method. The prediction of the model is in good agreement with experimental data.

Forced Convection Boiling Heat Transfer and Dryout Characteristics in Helical Coiled Tubes with Various Axial Angles

对水和水蒸汽汽液两相流体在螺旋轴呈各种倾角放置的螺旋管内强制对流沸腾传热与烧毁特性进行了系统地试验研究，试验中系统及结构参数范围如下：压力：Ｐ＝０．４～３．０ＭＰａ质量流速：Ｇ＝１００～２４００ｋｇ／ｍ２·ｓ进口水温：Ｔ＝３０～８０℃出口干度：ｘ＝－０．０５～１．２管内壁面热负荷：ｑ＝０～５４０ｋＥ／ｍ２试验段结构参数：总长Ｌ＝６４４８ｍｍ，管内径ｄ＝１１ｍｍ，螺旋直径Ｄ＝２５６ｍｍ，螺旋升角β＝４．２７°螺旋管轴向放置倾角：水平位置（０°）、向上倾斜４５°（＋４５°）、垂直向上（＋９０°）、向下倾斜４５°（－４５°）．共进行了１０５０个工况的试验．试验结果表明，螺旋管内汽液两相流强制对流沸腾传热可以划分为核态沸腾区、两相流强制对流区、烧毁及烧毁后传热区等３种区域．通过数据处理和分析总结，给出了３区域间转变的边界方程，和３个区域内两相流传热系数的计算公式．根据试验观察和数据结果，对烧毁现象及烧毁点或区域发生的条件及机理进行了深入的分析研究，发现一系列有关现象的规律和特点，指出了其主要影响因素，并给出了烧毁点临界质量干度的预报公式．

Pressure Drop of Liquid–Solid Two-Phase Flow in the Vertical Tube Bundle of a Cold-Model Circulating Fluidized Bed Evaporator

A cold-model vertical multi-tube circulating fluidized bed evaporator was designed and built to conduct a visualization study on the pressure drop of a liquid–solid two-phase flow and the corresponding particle distribution.Water and polyformaldehyde particle(POM)were used as the liquid and solid phases,respectively.The effects of operating parameters such as the amount of added particles,circulating flow rate,and particle size were systematically investigated.The results showed that the addition of the particles increased the pressure drop in the vertical tube bundle.The maximum pressure drop ratios were 18.65%,21.15%,18.00%,and 21.15%within the experimental range of the amount of added particles for POM1,POM2,POM3,and POM4,respectively.The pressure drop ratio basically decreased with the increase in the circulating flow rate but fluctuated with the increase in the amount of added particles and particle size.The difference in pressure drop ratio decreased with the increase in the circulating flow rate.As the amount of added particles increased,the difference in pressure drop ratio fluctuated at low circulating flow rate but basically decreased at high circulating flow rate.The pressure drop in the vertical tube bundle accounted for about 70%of the overall pressure drop in the up-flow heating chamber and was the main component of the overall pressure within the experimental range.Three-dimensional phase diagrams were established to display the variation ranges of the pressure drop and pressure drop ratio in the vertical tube bundle corresponding to the operating parameters.The research results can provide some reference for the application of the fluidized bed heat transfer technology in the industry.

Surface-particle-emulsion heat transfer model between fluidized bed and horizontal immersed tube

A mathematical model, surface-particle-emulsion heat transfer model, ispresented by considering voidage variance in emulsion in the vicinity of an immersed surface. Heattransfer near the surface is treated by dispersed particles touching the surface and through theemulsion when the distance from the surface is greater than the diameter of a particle. A film withan adjustable thickness which separates particles from the surface is not introduced in this model.The coverage ratio of particles on the surface is calculated by a stochastic model of particlepacking density on a surface. By comparison of theoretical solutions with experimental data fromsome references, the mathematical model shows better qualitative and quantitative prediction forlocal heat transfer coefficients around a horizontal immersed tube in a fluidized bed.

Heat Transfer Enhancement by Fluidized Solid Particles in Gas Carrying Evaporation

Heat transfer characteristics are studied for gas carrying evaporation with fluidized solid particles in a vertical rectangular conduit. Experimental results show that heat transfer of gas carrying evaporation is enhanced and the superheat of liquid in contact with heating surface lowers remarkably by introducing solid particles. Nucleate boiling on the heating surface is suppressed to a considerable degree. The mechanism of heat transfer enhancement by fluidized solid particles is analyzed with the consideration of collisions of solid particles with the boiling vapor bubbles.

蛇形微通道内绝热和沸腾流动气液两相压降的实验研究

蛇形微通道因结构紧凑和换热能力强的特点在高新技术领域中具有较大的研究价值,目前对微细尺度蛇形通道内全流型演变过程中的压降研究分析相对较少。通过实验的方法研究了当量直径为0.65 mm和弯管段曲率半径为1.2、1.8、2.4 mm的蛇形微通道内绝热流动和沸腾流动气液两相压降变化规律。根据实验计算了质量流速为600 kg/(m^(2)·s)时压降的占比情况,分析了质量流速为600、1200、1800、2400 kg/(m^(2)·s)时压降的变化情况,并与文献中压降经验关联式进行对比,发现文献中大部分已有关联式并不适用于蛇形微通道内气液两相流型的预测。

Thermographic Observation of High-Frequency Ethanol Droplet Train Impingement on Heated Aluminum and Glass Surfaces

The present study considers the impingement of a train of ethanol droplets on heated aluminum and glass surfaces.The surface temperature is allowed to vary in the interval 140℃–240℃.Impingement is considered with an inclination of 63 degrees.The droplet diameter is 0.2 mm in both aluminum and glass surface experiments.Thermal gradients are observed with a thermographic camera.It is found that in comparison to glass,the aluminum surface displays very small liquid accumulations and better evaporation performance due to its higher thermal conductivity.The relatively low thermal conductivity of glass results in higher thermal gradients on the surface.The droplet impact area on the aluminum surface is smaller than the corresponding area for the glass surface.Interestingly,the liquid accumulation area is not symmetrical.Moreover,the extension of the droplet train impact region decreases on increasing the surface temperature because higher temperature values allow greater surface energy levels that enhance significantly the evaporation rate.

Hydrodynamic Pattern Investigation of Ethanol Droplet Train Impingement on Heated Aluminum Surface

Steady-state hydrodynamic patterns of ethanol droplet train impingement on the heated aluminum surface is investigated in the surface temperature range of 80°C–260°C using two different Weber numbers(We)of 618 and 792.Instead of a vertical train impingement,the droplet train is sent to the aluminum surface with an incline of 63 degrees.Changes in the spreading length are observed at different surface temperatures for two different We values,which are obtained by using two different pinholes with 100 and 150μm diameters.The greatest spreading length is seen at the lowest surface temperature(80°C)and it continuously decreases until the surface temperature of 200°C.Above 200°C,the spreading length remains stable which is most probably because of the Leidenfrost effect.The spreading lengths of the experiments with 100μm are 46.4%smaller than the experiments with 150μm.Also,splashing angles are observed for both We values.The ranges of splashing angle observations are 140°C–200°C and 170°C–185°C for We values of 792 and 618,respectively.

一种新的固液共轭沸腾传热LB模型

为了避免沸腾传热格子玻尔兹曼(LB)模型中因固液能量传递速率一致而导致的模拟误差,尝试性地将温度弛豫时间τT引入到固体和液体的物性参数λ、cp中,成功获得了不同的能量传递速率,得到了一种新的固液共轭沸腾传热格子玻尔兹曼伪势模型。在验证了模型的准确性、稳定性和合理性后,采用固液共轭传热模型和原始伪势模型对不同润湿性表面的沸腾传热过程进行了数值模拟。结果表明:由于原始伪势模型忽视了固、液区域不同的传热能力,导致计算获得的气泡根部周围存在范围较大的低密度相变区,该区域的存在对气泡产生了额外相间作用力,大大改变了气泡的壁面接触角。而共轭传热模型通过引入温度弛豫时间函数实现了对固体和液体区域不同的热物理性质的表征,获得的低密度相变区范围很小,程度更低,壁面接触角更接近于设置的气泡接触角。在除超亲水表面外的其他润湿性表面,共轭传热模型获得的实际接触角的最大相对误差为8.6%,较原始伪势模型降低了9.8%,更能准确地描述实际的沸腾换热微观过程。

中国大型LNG/FLNG绕管换热器研发进展与工业化应用

大型LNG/FLNG绕管换热器是FLNG和大型LNG生产建设的核心装备,技术门槛高,长期由美国APCI和德国Linde公司垄断。本文全面回顾了中国在大型LNG/FLNG绕管换热器研发和工业化应用方面的最新进展,主要包括:①构建了适用于烷烃工质冷凝与沸腾流动的高精度数值模拟方法,提升了数值计算的准确性和稳定性,数值模拟结果与实验数据相对误差在±20%以内;揭示了晃动及结构参数对烷烃工质复杂相变流动传热的影响机制,建立了烷烃工质复杂相变流型转换准则与分流型的流动换热关联式;开发了具有自主知识产权的大型LNG/FLNG绕管换热器仿真设计软件,其仿真结果与现场运行数据平均相对误差小于5%。②研制了300 m以上超长无缝换热铝合金管及3种适用于LNG/FLNG绕管换热器的新型均布器,形成了基于铝换热管的大型LNG/FLNG绕管换热器结构优化设计技术,开发出了中国首套全铝制30万m3/d LNG/FLNG绕管换热器,通过了CCS认证,并制定了相关企业标准。③搭建了全球首个LNG/FLNG绕管换热器工业化测试平台,完成了首套全铝制30万m3/d LNG/FLNG绕管换热器静止和晃荡工业化应用测试,结果表明国产LNG/FLNG绕管换热器在换热等方面性能良好。相关研究成果为未来自主设计和建造大型LNG/FLNG绕管换热器奠定了基础。

纳米流体强化传热进展综述

伴随电子设备的高速发展,制约其微型化、集成化的热交换设备高传热负荷成为需要解决的首要问题。作为高效、高传热性能的新型能量输运工质,纳米流体可以有效提高工质的导热性能,并改善散热系统的换热性能。因此,对于纳米流体强化传热技术的研究,不仅可以深入探究纳米流体在实际应用中的发展潜力,也有助于热交换设备传热性能的提高,具有广阔的市场应用前景和巨大的潜在经济价值。尽管现今已有大量针对纳米流体强化机理的科学研究,但研究结果的一致性较低,仍存在较大争议。因此,本文将从纳米流体在单相对流传热、池沸腾传热、流动沸腾传热三个方面进行全面的调研分析,针对纳米流体强化的传热机理进行总结和讨论。

R134a在菱形离散肋微小通道内的流动沸腾换热实验研究

研究了制冷剂R134a在角度分别为30°、60°和90°的菱形离散肋微小通道内的流动沸腾换热特性。微小通道内菱形离散肋分布区域长300 mm、宽20 mm,进口处饱和压力为(700±5)kPa,其他工况范围为:干度0~1,质量流率200~500 kg(/m2·s),热通量10~30 kW/m2。实验结果表明:离散肋中的流动沸腾换热受到核态沸腾和对流沸腾的共同作用,传热系数随质量流率和热通量的增加而增加,但随着干度的升高,热通量的作用减弱并趋于消失。此外,离散肋结构对流动沸腾换热有显著影响,相同工况下,90°菱形离散肋的传热系数高于30°和60°,且在高干度更显著。最后,基于实验数据和分析结论,提出了一个适用于预测不同结构离散肋微小通道中流动沸腾传热系数的计算关联式。

水平管道过冷沸腾换热的非稳态数值计算

为研究循环式冷却系中涉及的过冷沸腾流动传热特性,基于VOF多相流模型和Lee相变模型对一水平管道中的过冷流动沸腾过程进行非稳态数值计算。考虑到沸腾起始点的影响,在Lee模型的基础上引入Bergles提出的沸腾起始点关联式以对其进行修正。从热边界层发展和沸腾阶段的发展两方面分析水平管道过冷沸腾换热过程中的流动换热特性及其波动规律,总结了不同热流密度工况下相关参数的分布关系以及对流换热系数沿流动方向的分布规律。结果表明:热边界层的发展和沸腾不断加剧使得流场的不稳定性增加,加热区域后部对流换热系数的波动幅值是入口附近的2倍;热流密度的增加使得流动和换热参数沿流动方向的变化速度加快,热流密度为250 kW/m^(2)工况下,热边界层发展所影响区域约为150 kW/m^(2)工况下的60%;热边界层的发展使得加热段前部的对流换热系数呈现前高后低的特点。当受热区域热流密度较大时,换热设备可以通过减小换热通道长度的方式,在提升换热效率同时减小沸腾带来的换热系数波动的影响。

竖直矩形窄通道内流动沸腾研究进展

综述了几何与工况参数变化对竖直矩形窄通道内换热效果影响的传热实验研究,以及窄通道内气泡成核与移动、流型及其转换的研究,从传热模型、核态沸腾起始点、有效空泡起始点、流动不稳定起始点、临界热流密度的研究等方面分析和总结了矩形窄通道内流动沸腾的传热机理研究。结合目前实验与理论研究,总结了现存问题,为矩形窄通道内流动沸腾的进一步研究提供参考。

3D打印槽道结构槽宽对池沸腾传热特性的影响

表面微结构化是强化沸腾传热的重要手段,研究微结构尺寸对沸腾传热特性的影响规律意义重大。本文采用选择性激光熔化技术(SLM)制备了不同宽度的槽道结构试样,并对其进行了常压下池沸腾传热特性实验研究。结果表明,相比于光滑铜面,槽长2.3mm,槽宽0.5~2.3mm槽道结构的传热系数(HTC)与临界热通量(CHF)均有显著提升。槽道结构的CHF随着槽道宽度的增大先增加后减小,HTC随着槽道宽度的增大而减小。槽道宽度为0.9mm时CHF达到最大值331.5W/cm^(2),为光滑铜面的3倍,同时HTC为光滑表面的1.7倍。较小的槽道宽度增加了试样的传热面积,限制了气泡脱离直径进而增加气泡的脱离频率,是HTC提升的关键因素;而槽道内气液流动阻力限制与水动力不稳定性,是槽道结构CHF提升的关键控制因素。

微圆柱阵微通道内沸腾换热及气泡运动特性研究

基于流体体积(VOF)模型对恒定热流密度下的微圆柱阵通道内的流动沸腾换热特性展开数值研究,并利用几何重构法捕捉气液两相界面迁移变化,研究了流道内流速的分布和微圆柱高度对气泡分布、沸腾换热性能及沸腾不稳定性的影响。结果表明:微圆柱的存在增加了气泡核化点密度;气泡运动增加了工质与加热壁面接触的可能性;微圆柱高度较低时,易发生气塞现象,加热面散热不均导致局部异常过热,换热性能下降;微圆柱阵通过阻碍气泡反向流动来降低沸腾不稳定性,但微圆柱高度过高会造成换热系统振荡,影响传热可靠性。

EXPERIMENTAL RESEARCH OF CYLINDER TO BED HEAT TRANSFER IN AGITATED FLUIDIZED BEDS

Cylinder-to-bed heat transfer in agitated fluidized beds was studied experimentally. In the experiments, the aluminum particles were used as bed material, the diameter of those ranged from 0.5 mm to 2 mm. The effects of gas velocity, particles size, and agitator rotary speed on heat transfer were studied. From the experimental results, we have come to the following conclusion: (1) there are optimal ranges for airflow velocity and rotary speed to get optimal heat transfer coefficient; (2) the cylinder-to-bed heat transfer is greatly affected by gas velocity, rotary speed and particles sizes, and the effect of rotary speed on heat transfer is similar to that of gas velocity; (3) higher heat transfer coefficient is obtained with smaller particles.

三相循环流化床中沸腾传热特性

提出了一种新型蒸发沸腾传热方式,即汽液固三相循环流化床沸腾传热。实验表明:该传热方式具有强化传热和防、除垢效果;其传热系数比汽液两相流沸腾传热膜系数高1.5～2.0倍,并能长期保持传热壁面的洁净。实验中研究了不同种类的固体粒子(玻璃球、陶瓷球、钛粒和钢球)、粒子体积分率、液相流速以及加热蒸汽压力等因素对循环流化床沸腾传热的影响。