MODELING OF HEAT TRANSFER OF UPWARD ANNULAR FLOW IN VERTICAL TUBES

This article presents the modeling of heat transfer of upward annular flow in a smooth tube and a spirally internally ribbed tube. First, analytical models of two-phase flow dynamics and heat transfer of annular flow in flow boiling were derived from the liquid film momentum and energy equations for smooth tubes. Combined with empirical correlations for liquid droplet entrainment and deposition rates in annular flow, modeling of heat transfer of upward annular flow in the smooth tube was conducted. The predicted heat transfer coefficients of annular flow agree with the experimental results very well for the smooth tube. Based on the heat transfer model for smooth tubes, a simplified annular flow heat transfer model for the spirally internally ribbed tube was proposed by modifying the interfacial friction factor. The predicted heat transfer coefficients by the modified heat transfer model for the spirally internally ribbed tube agree with the experimental results to some extent. It is suggested that the heat transfer model for the spirally ribbed tube be further improved by modifying the correlations for liquid droplet entrainment and deposition rates in annular flow, which should describe the feature of annular flow in the spirally internally ribbed tube. Extensive experimental data are needed for this purpose.     

Enhanced Heat Transfer Characteristics of Upward Flow Boiling of Kerosene in a Vertical Spirally Internally Ribbed Tube

Experiments of upward flow boiling of kerosene in a vertical spirally internally ribbed tube and a vertical smooth tube were conducted, respectively, in the present study. The spirally internally ribbed tube has an inner diameter of 11 mm (an equivalent inner diameter of 11.6 mm) and an outer diameter of 22 mm. The smooth tube has an inner diameter of 15 mm and an outer diameter of 19 mm. The test tubes were uniformly heated by passing an electrical current along the tubes with an available heated length of 2500 mm. At the outlet of the test section, the experimental pressure was 3 bars. The experimental heat flux ranged from 28.5 to 93.75 kW/m². The experimental mass flux was 410, 610, and 810 kg/m²s, respectively. Both local and average flow boiling heat transfer coefficients were measured in the test tubes. The enhanced heat transfer characteristics of the flow boiling of kerosene in the spirally internally ribbed tube are presented by comparing the experimental heat transfer coefficients with those obtained in the smooth tube. It shows that the flow boiling heat transfer coefficients in the spirally internally ribbed tube are 1.6 to 2 times greater than those in the smooth tube. The physical mechanisms of the enhanced heat transfer characteristics of flow boiling in the spirally internally ribbed tube are analyzed. According to the experimental data, an expression for the flow boiling heat transfer coefficient of kerosene was found in terms of the Martinelli number for the spirally internally ribbed tube. The correlation is applicable to the design of heat exchange equipment, using the spirally internally ribbed tube as a heat transfer element under these test conditions.     

Study of vapor liquid two-phase frictional pressure drop in a vertical heated spirally internally ribbed tube

Experiments of vapor liquid two-phase frictional pressure drop of upward flow boiling in a smooth tube and in a spirally internally ribbed tube were conducted, respectively. The spirally internally ribbed tube has an outside diameter of 22 mm and an inside diameter of 11 mm (an equivalent inside diameter of 11.6 mm) and the smooth tube has an outside diameter of 19 mm and an inside diameter of 15 mm. The test tubes were vertically installed and uniformly heated by electricity to achieve flow boiling test conditions. The available heated length of both test tubes is 2500 mm. The working fluids are water and kerosene, respectively. The experimental pressure is 6 bar for flow boiling of water and 3 bar for flow boiling of kerosene. The exit vapor quality of the test sections is about 0.3. The two-phase Reynolds number ranges from 8000 to 28,000. The experimental two-phase frictional pressure drops in the smooth tube are compared with the predicted results by the two-phase flow homogeneous model and the Friedel formula (Friedel, 1979. Improved friction pressure drop correlation for horizontal and vertical two-phase pipe flow. European Two-phase Flow Group Meeting, Ispra, Italy), respectively. It shows that the experimental results agree with the Friedel formula better than the two-phase flow homogenous model. By comparison, the two-phase frictional pressure drops in the spirally internally ribbed tube are 1.6-2.7 times greater than that in the smooth tube. A physical explanation of the increase of the two-phase frictional pressure drop in the spirally internally ribbed tube is given. According to the two-phase flow homogeneous model, a correlation of two-phase friction factor is proposed for the spirally internally ribbed tube and it is applicable to pressures up to 6 bar.     

煤油在内螺纹管中流动沸腾强化传热特性

针对石油化工过程中沸腾传热设备技术改造的需要 ,在 3× 1 0 5Pa (绝对压力 )的条件下 ,对煤油在垂直内螺纹管中的上升流动沸腾传热进行了实验研究 ,将实验结果同煤油在光管中的实验结果进行了比较 .实验结果表明 ,内螺纹管中的沸腾换热系数是光管的 1 6～ 2倍 ,并且可在小温差条件下实现流动沸腾传热 .提出了煤油在内螺纹管中的流动沸腾换热系数的关联式 ,并对煤油在内螺纹管中的流动沸腾特性和强化传热机理进行了分析 .     

高效复合强化换热器的管程性能

通过搭建内螺纹扭曲椭圆管复合强化管换热器传热与压降性能测试平台,对内螺纹扭曲椭圆管复合强化管换热器的管程传热和流阻性能进行了实验研究,以实验数据为基础拟合得到其对流传热系数和流动摩擦阻力系数与管内流体Reynolds数的计算准则关系式,将内螺纹扭曲椭圆管与光滑圆管、内螺纹圆管、光滑扭曲管的管程传热、流阻以及综合性能进行对比分析,结果显示内螺纹扭曲管流动阻力略高于其他类型的换热管,传热效果和综合性能明显优于其他类型换热管。内螺纹扭曲管作为一种内螺纹和扭曲扁管强化传热技术的叠加技术,强化传热效果明显,具有重要的工程应用价值。     

Study of Single Phase Flow Heat Transfer and Friction Pressure Drop in a Spiral Internally Ribbed Tube

The experimental results of single‐phase flow heat transfer and pressure drop experiments in the turbulent flow regime in a spirally ribbed tube and a smooth tube are presented in this paper. The ribbed tube has an outside diameter of 22 mm and an inside diameter of 11 mm (an equivalent inside diameter of 11.6 mm) and the smooth tube has an outside diameter of 19 mm and an inside diameter of 15 mm. Both tubes were uniformly heated by passing an electrical current along the tubes with a heated length of 2500 mm. The working fluids are water and kerosene, respectively. The experimental Reynolds number is in the range of 10⁴–5 · 10⁴ for water and is in the range of 10⁴–2.2 · 10⁴ for kerosene. The experimental results of the ribbed tube are compared with those of the smooth tube. The heat transfer coefficients of the ribbed tube are 1.2–1.6 fold greater than those in the smooth tube and the pressure drop in the ribbed tube is also increased by a factor of 1.4–1.7 as compared with those in the smooth tube for water. The corresponding values for kerosene are 2–2.7 and 1.5–2, respectively. The heat‐transfer enhancement characteristics of the ribbed tube are assessed. This tube is especially suitable for augmenting single‐phase flow heat transfer of kerosene. Correlations for the heat transfer and the pressure drop for the spirally ribbed tube are proposed, according to the experimental data.     

垂直上升内螺纹管内流动沸腾传热特性

在压力9～22 MPa,质量流速450～2000 kg·m^-2·s^-1,内壁热负荷200～700 kW·m^-2的参数范围内,试验研究了用于1000 MW超超临界锅炉φ28.6 mm×5.8 mm垂直上升内螺纹水冷壁管内汽水流动沸腾传热。研究表明：内螺纹管内壁螺纹的漩流作用可抑制偏离核态沸腾（DNB）传热恶化,内螺纹管在高干度区发生蒸干型（DO）传热恶化。增大质量流速可推迟壁温飞升,壁温飞升幅度随质量流速增大而降低。热负荷越大管壁温越高,随热负荷增大管壁壁温飞升提前,且传热恶化后壁温飞升值增大。随着压力增加,壁温飞升发生干度值减小。内螺纹管汽水流动沸腾传热系数呈π形分布,传热系数峰值出现在汽水沸腾区。文中还给出了亚临界压力区内螺纹管单相区和汽水沸腾区的传热系数试验关联式。     

环状流起始点初始携带份额的分析计算

目前大多数研究采用液膜干涸模型预测气液两相环状流的临界热通量,其中确定环状流起始点、起始点和平衡状态的携带份额、携带现象的发生条件、液滴沉积率以及携带率对模型的计算精度都起着决定性的作用。通过大量研究目前已经得到了适用于不同条件下的经验关联式来预测此模型中的大多数参数,然而环状流起始点的初始携带份额至今仍需通过假设来确定。本文对蒸气-水为工质,压力为0.18～0.27 MPa条件下得到的实验数据进行了分析,通过计算值与实验值的对比提出了初始携带份额的经验关联式。比较结果表明,计算值与实验值的偏差在20%以内。     

基于查询表的环状流初始携带份额分析计算

气液两相流临界热通量对工业设备的安全性有重要影响,液膜干涸模型是预测气液相环状流临界热通量的有效方法。确定环状流起始点、初始和平衡状态的携带份额、携带现象的发生条件、液滴沉积率以及携带率对模型的计算精度都起着决定性的作用。通过大量研究目前已经得到了适用于不同条件下的经验关联式来预测此模型中的大多数参数,然而环状流起始点的初始携带份额至今仍需通过假设来确定。以广泛应用于核工业的临界热通量查询表为依据,针对界限含气率区间建立环状流液膜干涸模型,对使模型计算值满足查询表工况的初始携带份额进行分析,拟合得到其与Weber数及液相Reynolds数的关联式,预测值偏差大多在±30%内。     

内壁填充环状金属泡沫的管内流动凝结换热

通过采用在圆管内壁填充环状金属泡沫的方法强化管内对流凝结换热,实验研究了制冷剂R134a在内壁填充环状金属泡沫管内的流动凝结的压降和换热,克服了完全填充金属泡沫管流动阻力大的缺点。用于计算传热系数的管壁温度通过热电偶测量得到。综合分析了质量流速和两相流体干度对流动凝结压降及传热系数的影响。研究结果表明内壁填充环状金属泡沫管压降远大于光管,压降随质量流速和干度的增加而迅速增大且呈非线性。通过壁面温度分布和温度波动对内壁填充环状金属泡沫管内的两相流型进行判别,发现影响该类强化管凝结换热的两种主要流型：分层流和环状流。内壁填充环状金属泡沫管的凝结传热系数大于光管,且随着质量流速和干度的增加传热系数增大,该类强化管流动凝结传热系数是光管的2倍左右。     

水平微翅管内环状流两相强制对流蒸发换热的计算模型

对水平微翅管内的环状流流动和换热特性进行了分析.考虑微翅管内环状流液膜中的扰动和二次流的作用,借用粗糙管速度分布和摩擦相似函数建立了水平微翅管内环状流两相强制对流蒸发换热系数的预测模型.理论计算值与实验数据相比较,结果较令人满意.     

Scaling of enhanced heat exchanger tubes

The performance of inner-fin and spirally indented heat exchanger tubes is compared with plain smooth tubes under severe scaling conditions. Artificially hardened water was recirculated through a heat exchanger which contained an enhanced tube and a plain tube in parallel and the decrease in heat transfer coefficient due to scale deposition followed with time. Inner-fin and spirally indented tubes maintained advantages in heat transfer coefficient over plain tubes of 10 to 100% after scaling. Asymptotic fouling resistances were larger for the inner fin tubes than for plain tubes, and smaller for the spirally indented tubes at velocities above 3 ft/sec.     

三维内微肋管内凝结流型分区及分层波状流换热关联式

引　言水平管内的受迫对流膜状凝结广泛用于化学工业及制冷与空调的冷凝设备中 .对于冷凝器的设计 ,确定管内膜状凝结换热系数是至关重要的 .众所周知 ,水平管内受迫对流凝结在流动方向汽液两相流动会出现不同的流型 ,对于不同的流型 ,对应的换热机理和换热系数也各不相同 .因此 ,首先需要区分管内受迫对流换热的流型 ,在此基础上再针对不同的流型对其凝结换热过程进行研究 .对于整个凝结换热过程 ,通常是根据汽液两相流在不同流型时的流动及换热特征将凝结过程划分为不同的流型区域 ,并得到相应的流型转变的判据 ,然后通过实验对不同的流型…     

Heat transfer and friction factor of Therminol liquid phase heat transfer fluid in a ribbed tube

Experiments and simulations on flow and heat transfer behavior of Therminol-55 liquid phase heat transfer fluid have been conducted in a ribbed tube with the outer diameter and inner diameter 25.0 and 20.0 mm,pitch and rib height of 4.5 and 1.0 mm.respectively.Experimental results show that the heat transfer and thermal performance of Therminol-55 liquid phase heat transfer fluid in the ribbed tube are considerably improved compared to those of the smooth tube.The Nusselt number increase with the increase of Reynolds number.The increase in heat transfer rate of the ribbed tube has a mean value of 2.24 times.Also,the pressure drop results reveal that the average friction factor of the ribbed tube is in a range of 2.4 and 2.8 times over the smooth tube.Numerical simulations of three-dimensional flow behavior of Therminol-55 liquid phase heat transfer fluid are carried out using three different turbulence models in the ribbed tube.The numerical results show that the heat transfer of ribbed tube is improved because vortices are generated behind ribs,which produce some disruptions to fluid flow and enhance heat transfer compared with smooth tube.The numerical results prove that the ribbed tube can improve heat transfer and fluid flow performances of Therminol liquid phase heat transfer fluid.     

螺旋槽管中单相水与油湍流摩擦阻力与传热特性

对单相水与油分别在4根不同几何参数的螺旋槽管中湍流摩擦阻力与传热特性进行了试验研究。以动量传递与热量传递的模拟理论为基础,由试验结果的关联得到了动量传递粗糙度函数（R函数）与热量传递粗糙度函数（G函数）的表达式,并与前人工作进行了比较。针对油的Pr随温度变化较大的特性,提出了改进的强化传热性能评价方法,并据此对1号与2号试验管进行了评价。研究结果可用于换热器设计及优化螺旋槽管几何参数。     

3种管内强化管沸腾换热性能对比

对3种不同螺纹结构的强化管的管内沸腾换热性能进行对比实验研究, 采用R22为实验工质。3种强化管的内径和外径相同, 分别为6.9 mm和7.92 mm;管外均为光滑表面, 管内的强化结构参数则不同:螺纹的螺旋角变化范围为14°~18°, 螺纹高为0.15~0.22 mm, 螺纹槽宽度为0.1~0.2 mm。在给定进口和出口制冷剂的状态下, 通过改变管内工质的质量流速, 测试3根强化管的换热特性与质量流速的关系, 并进行性能对比。实验结果表明:3种螺纹管管内沸腾传热系数分别比光管高出60%~80%, 80%~120%和80%。分析认为, 当流动处于层状流或者层状流与环状流的过渡区时, 较大的螺旋角有利于换热;当流体处于环状流时, 较多的螺纹头数有利于换热。     

螺旋槽管槽深对定子水冷器传热与阻力特性的影响

螺旋槽管与折流栅组合的高效水冷器的传热和阻力特性与其管、壳程的结构因素密切相关,本文就螺旋槽管槽深对水冷器管、壳程传热与阻力特性的影响进行了实验研究.结果表明：壳程流速变化对水冷器传热系数的影响更加明显,即壳侧热阻相对较大,强化传热应以强化壳侧换热为主要目标;螺旋槽管槽深对水冷器传热系数的影响很大,应在实际工程设计中确保实际槽深符合设计要求;槽深越深,管壳程阻力也相应增加,即传热的强化是以阻力增加为代价.根据实验结果还得到了不同结构水冷器的管、壳程换热与阻力计算关联式.     

进口轴向叶片旋流器与螺旋槽管的管内复合强化传热

对进口轴向叶片旋流器与左旋螺旋槽管、右旋螺旋槽管以及对称交叉螺旋槽管组成的管内复合强化传热进行了试验研究,给出了管内阻力和传热试验结果,得到了考虑复合攻角（旋向）因素在内的管内阻力和传热准则关联式,并对传热和流阻的综合热力性能进行了定量评价．试验表明：相同泵功和换热面积条件下,旋流器与螺旋槽管反向复合后的换热量可提高至光管换热量的1．9倍．     

流体在粗糙管管内及管隙间湍流流动阻力与传热的数据关联

本文分析了流体在规整型粗糙强化管束的管内和管隙间的流动和传热特性,讨论了湍流流体在规整型粗糙壁面上和在光滑壁面上动量与热量传递的差别,推导出光滑管和粗糙管管内及管隙间湍流流动的流体摩擦阻力系数和传热准数方程的关联式,并能较好地关联螺旋槽管、螺旋低翅片管和缩放管管内与管隙间的流体阻力和传热的实验数据.     

Condensation with non-condensing gases inside vertical tubes with turbulent gas and film flow

Mass transfer coefficients in the turbulent gas flow of a vertical condenser tube are determined experimentally in a short test segment. When the film model is used to evaluate the data, the gas phase resistance to mass transfer is well described by a correlation obtained from single-phase pipe flow. No significant interaction between gas and film flow is observed, neither for laminar nor for turbulent film flow. A design method for long condenser tubes is developed considering superheating of the gas and subcooling of the falling film. When the film model and a single-phase pipe flow correlation are incorporated, the method matches with sufficient accuracy experimental data taken from the literature. To estimate the onset of entrainment in falling film exchangers a correlation for entrainment in annular two-phase flow is checked. The calculations are in good agreement with experimental data obtained in a long adiabatic tube.