非均匀加热条件下内插扭带管强化传热模拟分析

以水为工作介质,采用欧拉多相流模型和非平衡沸腾模型,当流速在0.3~0.7m·s^-1范围内、工作压力为4.5MPa、热流密度为2MW·m^-2时,数值模拟了内插扭带管和光管管内流动过冷沸腾传热。对比了两种管道的换热系数、气泡份额、流动速度、流场流线、固体组件温度和压降,分析了内插扭带管的综合性能。结果表明,与光管相比较,内插扭带管的换热系数提高约6%~90%,压降增大约200%~250%,得到流速在0.4~0.6m·s^-1范围内时内插扭带管的综合性能评价因子η为1.1~1.3。     

Experimental investigation of heat transfer and friction coefficient of the water/graphene oxide nanofluid in a pipe containing twisted tape inserts under air cross-flow

The water/graphene oxide nanofluid effect in a pipe equipped by twisted tape inserts under air cross-flow is investigated and the optimal tape geometry is determined. The range of internal and external Reynolds numbers are: 3800<Re_o<21500 and 550<Re_i<2000. Heat transfer and pressure drop increase by increasing Re and inserts width and heat transfer performance coefficient increased up to 1.4, indicating enhanced heat transfer compared to undesirable pressure drop. On the other hand, the heat transfer coefficient is 26% higher when compared with water in a plain tube. According to the results, this method is a good alternative in heat exchangers.     

弓形折流板换热器中折流板对换热器性能的影响

本文采用Bell-Delaware换热器设计方法研究了传统弓形折流板换热器加热轻油时在不同管束排列角度下,改变换热器壳侧折流板间距以及改变折流板的窗口高度对管壳式换热器的壳体内径、换热管数目、壳侧换热系数及壳侧压降的影响.     

Condensation heat transfer and pressure drop characteristics of R-134a inside the flattened tubes at high mass flux and different saturation temperature

In this study, heat transfer coefficients and pressure drops of R-134a inside round and flat tubes are investigated experimentally with mass flux of 450, 550, and 650 kg m^?2 s^?1 at saturation temperatures of 35°, 40°, and 45°C. The effects of mass flux and saturation temperature on heat transfer coefficient and pressure drop are examined. The maximum enhancement factor and pressure drop penalty are obtained by flat tube (FT-2) up to 2.101 at 450 kg m^?2 s^?1 and 3.01 at 650 kg m^?2 s^?1, respectively. The correlation for flat tubes is proposed to predict the heat transfer coefficient within ±20% error.     

错排环布圆管翅片式换热板芯的实验研究

本文利用传热传质之间的比拟关系研究了错排环布圆管换热板芯的平均传热特性及阻力特性。实验中我们采用了三种翅片间距(Tp)、三种管排数(Nrow)以及三种管数(Ntube)组成的27种板芯结构,传质实验采用萘升华的方法来进行。然后通过三种限制条件对不同翅片间距、不同管排数和不同管数下的换热板芯的传热性能进行了比较。最后利用最小二乘法得出了具有工程指导意义的准则关联式。     

Heat Transfer and Pressure Drop Characteristics in Turbulent Flow Through a Tube

An experimental investigation has been carried out for turbulent flow through a tube with perforated strip inserts. Strips were of mild steels with circular holes of different diameters. Flow varies, with ranging Reynolds numbers from 15,000 to 47,000. Air velocity, tube wall temperatures, and pressure drops were measured for a plain and strip-inserted tube. The heat transfer coefficient and friction factor were found to be 2.80 times and 1.8 times, respectively, that of the plain tube. The heat transfer performance was evaluated and found to be 2.3 times that of the plain tube based on constant blower power.     

EXPERIMENTAL STUDY OF HEAT TRANSFER FROM A DISCRETE SOURCE TO A CIRCULAR LIQUID JET WITH ANNULAR COLLECTION OF THE SPENT FLUID

This study reports an experimental investigation of evaporative heat transfer and pressure drop of R-134a flowing downward inside vertical corrugated tubes with different corrugation pitches. The double tube test section is 0.5 m long with refrigerant flowing in the inner tube and hot water flowing in the annulus. The inner tubes are comprised of one smooth tube and three corrugated tubes with different corrugation pitches of 6.35, 8.46, and 12.7 mm. The test runs are performed at evaporating temperatures of 10°C, 15°C, and 20°C; heat fluxes of 20, 25, and 30 kW/m²; and mass fluxes of 200, 300, and 400 kg/m²s. The experimental data obtained from the smooth tube are plotted with flow pattern map for vertical flow. Comparisons between smooth and corrugated tubes on the heat transfer and pressure drop are also discussed. It is observed that the heat transfer coefficient and frictional pressure drop obtained from the corrugated tubes are higher than those from the smooth tube. Furthermore, the heat transfer coefficient and frictional pressure drop increase as the corrugation pitch decreases. The maximum heat transfer enhancement factor and penalty factor are up to 1.22 and 4.0, respectively.     

高效传热管内凝结换热性能及阻力性能的实验研究

本文以ＨＦＣ１３４ａ和ＨＣＦＣ２２为工质对光管及两种不同槽型的强化传热管（ＤＡＥ－２管与ＤＡＥＣ管）的水平管内凝结换热进行了对比实验研究、研究发现,ＤＡＥ－２管平均换热系数比光管提高了１４０％～１７０％,而单位长度阻力损失增加了５０％～１００％,ＤＡＥＣ管平均换热系数比光管提高了１６０％～２００％,同时单位长度阻力损失增加了７０％～１３０％。此外,本文给出了ＤＡＥ－２管和ＤＡＥＣ管平均换热系数及阻力损失的计算关联式,可用于冷凝器设计。     

Enhancing the thermal performance of AL2O3/DI water nanofluids in micro-fin tube equipped with straight and left-right twisted tapes in turbulent flow regime

This article communicates the thermal performance, heat transfer rate, and friction factor of Al₂O₃/DI water nanofluids at different concentrations in a micro-finned tube with tube helical inserts for different twist ratios. The thermal performance, heat transfer coefficient, and friction of the present study is also compared with a plain tube for validation. From the study, it is identified that the micro-finned tube with tube insert performance is higher as compared with a plain tube. Similarly, an empirical relation for Nusselt number (Nu) and friction factor (f) is estimated for straight twisted tube and left-right combination. The deviation between experimental and theoretical values for left-right twist and straight twist is found as 3 and 7% for Nusselt number and 7 and 9% for friction factor, respectively. Similarly, while analyzing the thermal performance, it was found that the maximum performance achieved was with a micro-fin tube with left-right twist with nanofluid concentration of 0.2%.     

Investigation on Heat Transfer and Pressure Drop of Copper–Water Nanofluid Flow in Plain and Perforated Channels

This article reports an experimental study on copper–water nanofluid flow inside plain and perforated channels. The effects of flow rate and nanoparticle concentration on the heat transfer and pressure drop are studied. It is found that the perforated channel has a remarkable heat transfer enhancement of 24.6%. Furthermore, by using the copper–water nanofluid instead of the base fluid, the heat transfer coefficient as well as pressure drop are increased for both plain and perforated channels. A noticeable thermal performance factor of 1.34 is obtained for the simultaneous utilization of both the heat transfer enhancement techniques considered in this article.     

Experimental analysis of conical baffles with rifts on heat transfer and pressure drop

In the present study, convective heat transfer to the air from a heating tube attached to conical baffles with rift was experimentally examined. The air entering the test section first contacts the large surface of the conical baffle. Therefore, the conical baffle both directs the air toward the heating surface and increases the heat transfer surface area. In the experiments, baffles with inclination angles of 45°, 60°, and 80° were used. The baffles were placed on the heating tube at the pitch of 15 mm. The temperature of the heating fluid (water) was kept fixed at 65°C. In addition to the riftless baffles, the experiments were carried out by using baffles with a rift spacing of 1.5 and 3.5 mm so that the boundary layer separation mechanism could be accelerated. Experimental results for eight different velocities of airflow (2–20 m/s) were presented. For the inclination angle of 60°, the increase in the heat transfer of the baffle with rift was 13% at a rift spacing of 1.5 mm and 4% at a rift spacing of 3.5 mm according to the riftless baffle. In addition, for the inclination angle of 60°, the pressure drop values of the riftless and the rift spacing of 1.5 and 3.5 mm were almost the same.     

内插弹簧转子的管内换热及转子旋转特性

管内插入物是强化管内换热的有效方法,内插可旋转结构在强化换热的同时还具有良好的除垢抑垢效果。实验研究了内插螺旋弹簧转子的管内换热以及转子的转动特性。结果表明,与光管相比,换热增强30%,但流动阻力增加5倍左右,转子的参数需要进一步优化以减小阻力。转子的转速与来流速度呈线性递增关系,与入口处转子相比,下游转子的转速逐渐降低,单螺旋转子转速降低的幅度大于双螺旋转子。双螺旋结构连续稳定运行半年以上,该转子强化换热及除垢抑垢性能稳定。     

Comparative Study on Heat Transfer Enhancement of Low Volume Concentration of Al2o3–Water and Carbon Nano-Tube–Water Nano-Fluids in Transition Regime Using Helical Screw Tape Inserts

This study reports the comparison of heat transfer and friction factor characteristics of helical screw inserts in Al₂O₃–water and carbon nano-tube–water nano-fluids through a straight pipe in transition regime with constant heat flux boundary condition. Experiments were carried out by using 0.15% volume concentration of Al₂O₃–water and carbon nano-tube–water nano-fluid with helical tape inserts of twist ratio, TR = 1.5, 2.5, and 3. The thermal performance of helical screw tape inserts with the carbon nano-tube–water nano-fluid is found -to be higher when compared to the Al₂O₃–water nano-fluid. In addition, the maximum enhancement in heat transfer was obtained for the carbon nano-tube–water nano-fluid with helical tape inserts of twist ratio 1.5. The increase in pressure drop of the Al₂O₃–water nano-fluid with helical screw tape inserts is found to be higher compared to the carbon nano-tube–water nano-fluid helical screw tape inserts at lower value of twist ratio.     

一种内螺纹强化管的冷凝实验研究

实验研究了环保替代制冷工质R410A、R22在水平强化管内冷凝换热特性,探索了热流密度、水流速度对换热特性、压降的影响。实验测试管为内螺纹强化管,长度为5.2 m,外径为9.52 mm。实验结果表明:制冷剂R410A、R22的传热系数和压降随热流密度的增大而增大,同时内螺纹管的换热系数还随管外冷却水流量的增加而升高,压降随冷凝温度的升高而降低,而R410A比R22有更好的换热效率和较小的压降。     

BOILING HEAT TRANSFER MECHANISMS IN A HORIZONTAL TUBE BUNDLE

The effect of thermal environment on boiling heat transfer performance in a section of a horizontal tube bundle was investigated using R-113 as the working fluid. The in-line tube bundle has five columns and 27 rows with a pitch-to-diameter ratio of 1.3. Heal transfer coefficients obtained from the instrumented tube in the tube bundle with only one tube heated while the other tubes remained unhealed and with all the lubes in the bundle heated are reported for a range of heat flux, pressure, mass flow rate, and quality. The results showed that heat transfer coefficient of a tube in a heated bundle is slightly higher than that in an unhealed bundle, with the variation of heat transfer coefficient decreasing as heat flux, mass flow rate, or pressure increased. It was also found that higher quality would tend to improve the heat transfer. However, the effect of quality disappeared as heat flux, mass flow rate, and pressure increased. Based on the experimental data, the mechanism of the heat transfer augment due to thermal environment was analyzed. It was proposed that fluid agitation and thin liquid film formation are two main factors for a heated bundle to have better transfer performance than an unhealed bundle,     

几何参数对低波纹通道流动与换热特性的影响

本文对几种不同几何模型的低波纹通道进行了传热及阻力性能数值研究,在一定的流速范围内得出了传热和阻力的特性曲线.分析了通道高度、波纹波峰高度、通道宽度对流动与换热的影响.结果表明,通道高度越小,换热越强,同时压降也增加;波纹波峰高度越大,换热加强,压降也相应增加;通道宽度越大,换热几乎不变,但压降随之降低.     

波纹板式空冷器阻力与传热特性实验研究

在可改变风量和热水流量的实验条件下,对波纹板式空冷器的阻力与传热特性进行实验研究。得到了空气侧的阻力降关联式以及两侧的对流换热系数关联式,其适用于热水雷诺数在2000-8000之间、空气雷诺数在2000-10000之间。在相同工况下,比较了波纹板式、光管式和翅片管式空冷器的性能指标,结果表明:迎面风速在2.45-4.1 m/s之间,波纹板式空冷器传热系数达到100-160 W/m2/℃;约比光管式提高70%,但只有以管束外表面为基准的翅片管式传热系数的六分之一;板式空冷器单位体积换热量约是翅片管式空冷器的1.5倍,是光管式的15倍;板式空冷器单位功耗换热量约是光管式空冷器的5.5倍,而翅片管式空冷器与光管式空冷器则相差不大。     

Heat Transfer Characteristics for Condensation of R134a in a Vertical Smooth Tube

In this study, condensation of pure refrigerant R134a vapor inside a smooth vertical tube was experimentally investigated. The test section was made of a copper tube with inside diameter of 7.52 mm and length of 1 m. Experimental tests were conducted for mass fluxes in the range of 20–175 kg/m²s with saturation pressure ranging between 5.8 and 7 bar. The effects of mass flux, saturation pressure, and temperature difference between the refrigerant and tube inner wall (ΔT) on the heat transfer performance were analyzed through experimental data. Obtained results showed that average condensation heat transfer coefficient decreases with increasing saturation pressure or temperature difference (ΔT). In addition, for the same temperature difference (ΔT), heat can be removed from the refrigerant at a higher rate at relatively low pressure values. Under the same operating conditions, it was shown that average condensation heat transfer coefficient increases as mass flux increases. Finally, the most widely used heat transfer coefficient correlations for condensation inside smooth tubes were analyzed through the experimental data. The best fit was obtained with Akers et al.'s (1959) correlation with an absolute mean deviation of 22.6%.     

HEAT TRANSFER AND PRESSURE DROP MEASUREMENTS FOR IN-TUBE CONDENSATION OF CFC-113 USING MICROFIN TUBES AND WIRE INSERTS

Accurate, repeatable heat transfer and pressure-drop measurements have been made for condensation of CFC-113 with downflow inside enhanced microfin tubes and tubes containing twisted-wire inserts. In the latter case measurements have also been made for CFC-113/air mixtures. The heat transfer rate was calculated from the coolant flow rate and temperature rise, the latter measured using a 10-junction thermopile with careful attention paid to adequate coolant mixing and isothermal immersion of the thermopile leads. The surface temperature was found from thermocouples embedded in the tube wall. One plain tube, nine microfin tubes (with different fin heights, helix angles, and number of fins), and four twisted-wire inserts (with different wire densities) were tested. Enhancement ratios (i.e., vapor-side heat transfer coefficient for the enhanced tube divided by that for a smooth tube at the same vapor-side temperature difference and vapor inlet velocity) between 1.6 and 5.6 for the microfin tubes and between 1.2 and 1.6 for the twisted-wire inserts were found, with values depending on vapor-side temperature difference, vapor inlet velocity, and air inlet mole fraction in the case of CFC-113/air mixtures. The microfin tubes showed moderate pressure-drop penalties of around 50% compared to the plain tube, while the twisted-wire inserts showed increasing pressure-drop penalty with increasing wire density.     

EXPERIMENTAL INVESTIGATION OF HEAT TRANSFER AND PRESSURE DROP CHARACTERISTICS OF W-TYPE SPIRALLY FLUTED TUBES

A new w-type spirally fluted tube with enhanced heat transfer characteristics was developed and manufactured. The heat transfer and pressure drop characteristics of tubes with various geometric parameters were measured over a range of Reynolds numbers from 8,000 to 30,000 for water flowing horizontally through the tube with steam condensing on the outside surface. Experimental correlations for the pressure drop and the heat transfer coefficient were developed from a large amount of data using multivariable linear regression analysis. Factors influencing the heat transfer and flow were analyzed. The experiments showed that heat transfer coefficient can be increased by 3–8% with the pressure drop reduced by 5–10% compared with an ordinary spirally fluted tube.