自转清洗扭带管对流传热强化机理的实验研究

具有传热强化功能的自转螺旋扭带清洗防垢技术发展较快。应用激光测速仪LDV(Laser Doppler Velocimeter)实验研究自转清洗扭带管内流体的湍流特性。结果表明：在自转扭带竹带动下,管内流体的流动结构发生了反常态的变化。在近管壁环形区域内流体的轴向分速度明显比管中心区域的高,轴向湍流度比无白转扭带时大;切向分速度随半径的增大而增大,并且存在很大的径向湍流度。这些结果初步说明了自转螺旋扭带管对流传热强化的机理是：管内由扭带带动形成的强制旋流和轴向平行流叠加而形成的螺旋流动,以及近管壁环形区域内流速的增大,不仅加强了边界层流体的扰动以及边界层流体与主流流体的混合,并且使边界层厚度减簿,从而才使管内的对流传热得以强化。本文试验研究的结果为自转螺旋扭带管内对流传热强化机理的深入理论研究提供实验基础。     

循环流态化自动清洗式换热器的传热强化

液-固流态化传热强化的机理尚未完全清楚,已有文献对流态化粒子的粒度与传热系数的影响关系有较大分歧。从传热强化角度对水-沙流态化自动清洗的运行参数进行优化试验研究,得到的结果是:沙子粒度不是愈细愈好,而是D p 2mm(8目)左右为好;粒子体积分数以2.4%比较好;流速在0.2~0.28 m/s(R e为6 000~9 000)较好。虽然粒子体积分数低,但是在优化条件下的流态化传热强化幅度几乎可以达到一倍左右,并且阻力又很低。     

矩形通道内过冷流动沸腾传热特性试验研究

以高强化发动机缸盖材料蠕铁作为加热块材料,在矩形通道内开展了接近发动机冷却系统的不同流动工况下过冷沸腾传热特性的试验研究。流动工况取发动机常用范围:压力为0.10~0.25MPa,主流温度为60~95℃,流速为0.347~6m/s。研究结果表明:提高冷却液流速可以强化壁面对流换热强度,但是存在沸腾换热的低流速工况同样能够达到高流速工况下换热效果,系统压力和主流温度都会影响冷却液过冷度,进而影响沸腾传热效果。可视化结果表明:气泡直径增大、生长频率升高及气泡之间相互作用都会使气泡对边界层流体扰动增强,从而提高传热效率。在压力为0.2MPa、主流温度为95℃、流速为1m/s工况下,在壁温达到170℃以上时沸腾开始出现,在壁温达到210℃时,沸腾传热效率比单相对流换热提高了40%以上。     

内螺纹管在低温烟气中的传热特性研究

为了研究内螺纹管对低温烟气传热强化的效果,通过对内螺纹管换热器和光管换热器在低温烟气中的传热试验,比较分析内螺纹管和光管两种换热器在不同工况下的传热系数,根据试验数据计算拟合出试验传热关联式。结果表明:烟气流速对总的换热系数有较大的影响,工质水流速不变,烟气流速从2.0 m/s增加到3.0 m/s时,内螺纹管换热器的换热系数增长率为17.1%;管内工质水无相变时,工质水流速对总的换热系数影响不大;低温换热的热阻主要集中在烟气侧;内螺纹管可以强化低温烟气的换热,但强化效果不明显;内螺纹管工质水侧的传热关联式Nu=0.009Re~(0.985)Pr~(0.4)(1.1×10~4Re2.3×10~4)。     

内置螺旋纽带换热器强化传热性能实验分析

换热设备的应用遍布生产、生活的各个领域,随着能源成本的升高和碳排放量的增大,采用简单、高效的方式提高换热设备的传热效率对于节能、增效意义重大。螺旋纽带技术作为一种结构简单,实用性强的强化传热技术,强化传热的同时兼具防垢的性能,非常适用于现有设备的技术改造。采用实验的方法对光管换热器和装有自转螺旋纽带换热器的传热系数k及压降ΔP进行测定,通过实验数据的对比分析表明,在管道流速为1.5 m/s,加装螺旋纽带装置后换热器传热系数和管侧流体压降数值比光管时提高了近1倍。     

太阳辐射填料床储热特性研究

针对给定太阳日辐射曲线,研究集成蓄热单元的太阳光热系统的整体能量的动态转化特性及关键参数影响规律。结果表明：填料床总储热量与传热流体进口流速呈非线性变化,当传热流体进口流速 u_f =0.006 m/s时,填料床总储热量最大;在给定填料总容量和u_f =0.006 m/s的条件下,填料床高径比为5的填料床具有更高的储热能力;在该计算条件下,u_f =0.006 m/s、填料床高径比为5及填料量相对值为1时,太阳光热能实现最大程度上的转化和储存。     

高效宽负荷率超超临界锅炉垂直管圈水冷壁在低质量流速下的传热特性

在压力p=21~29.8 MPa、质量流速G=600~1 100kg/(m~2·s)、热负荷q=330~793kW/m~2工况范围内,对低质量流速优化内螺纹管的传热特性进行了实验研究,并根据实验数据得到了近临界压力区和超临界压力区的传热实验关联式.结果表明:在近临界压力区,亚临界部分的传热特性好于超临界部分的传热特性,质量流速增大能推迟传热恶化,热负荷增大则使传热恶化提前发生,内螺纹管抑制膜态沸腾(DNB)的能力有所减弱;在超临界压力区,压力越低,大比热容区内强化传热作用越显著,在其他条件一定时,超临界水的热物性变化对管内传热的作用由质量流速和热负荷共同决定;质量流速不变,继续增大热负荷,大比热容区内的传热将由强化转变为恶化.     

流速对非等温凝结表面Marangoni凝结换热特性的影响

在具有温度梯度的凝结表面上进行了水-酒精混合蒸气的Marangoni凝结的实验。研究了3种流速(V=2、4和6 m/s)对凝结表面上不同位置热流密度的影响。研究发现:流速对纯水蒸气和混合蒸气凝结时的表面温差和热流密度的影响是不同的。对纯水蒸气而言,流速增大后,表面温差和热流密度是增加的,并加剧了热流密度的分布不均(热流密度的相对差值在压力为31.2 kPa,流速2 m/s时为0.538,流速4 m/s时为0.6,流速6 m/s时为0.625)。对于混合蒸气,表面温差随流速的增加而减小,而热流密度增大很少(压力31.2 kPa,流速2 m/s时为0.186,4 m/s时为0.182,6m/s时为0.098)。     

电厂汽轮机冷凝器污垢纽带清洗技术研究

本文以调查实例强调了保持电厂汽轮机冷凝器传热面清洁的极端重要性 ,通过冷却水水垢热阻对其传热系数影响的计算 ,论证了即使只有 0 .2 5mm厚的水垢就可以使其下降30 % ,介绍了兼有自动清洗防垢和传热强化双重功能的自转纽带技术的原理 ,论述了试验研究结果 ,可以满足汽轮机冷凝器污垢清洗的要求 ,传热强化的幅度在 1 0 %以上 ,采用试验数据计算 2 5MW机组应用该技术可以获得每年百万元的增产效益     

油气冷凝回收系统中板式蒸发器的优化北大核心CSCD

对汽油蒸气冷凝回收系统中板式蒸发器进行了概述。对冷凝回收工艺提出了优化的三段式冷凝方式,并且基于Aspen HTFS板式蒸发器的设计方案,对冷凝回收系统中的三段板式蒸发器进行了系统的优化设计。结果表明:在温度为20℃、压力为101.325kPa的条件下,当进入冷凝回收系统的油气-空气混合气的体积流量为400m3/h时,该冷凝系统的预冷段、中冷段、深冷段的蒸发器传热面积宜分别设计为12.4、53.2、4.5m2,总传热板片数分别为33、135、19。     

Air-side heat transfer and friction characteristics of biofouled evaporator under wet conditions

The effects of biofouling on air-side heat transfer and friction characteristics under wet conditions of three biofouled finned tube heat exchangers and one clean finned tube heat exchanger were investigated experimentally. Experimental results indicate that the biofouled fin efficiency of the evaporator decreases by 15.5% compared with the clean evaporator under the condition of the biofouled area ratio of 60% at the inlet air velocity of 2.0 m/s; The ranges of friction fouling factor and heat transfer fouling factor are 19.8%–43.1% and −15.6%−13.1%, respectively; a small quantity of biofouled particles can enhance heat transfer at low Reynolds number, and the enhancement effect decreases with the increase of Reynolds number.     

Air-side heat transfer and friction characteristics of biofouled evaporator under wet conditions

The effects of biofouling on air-side heat transfer and friction characteristics under wet conditions of three biofouled finned tube heat exchangers and one clean finned tube heat exchanger were investigated experimentally. Experimental results indicate that the biofouled fin efficiency of the evaporator decreases by 15.5% compared with the clean evaporator under the condition of the biofouled area ratio of 60% at the inlet air velocity of 2.0m/s; The ranges of friction fouling factor and heat transfer fouling factor are 19.8%―43.1% and ―15.6%―13.1%, respectively; a small quantity of biofouled particles can enhance heat transfer at low Reynolds number, and the enhancement effect decreases with the increase of Reynolds number.     

Experimental Study of Fouling Resistance in Twisted Tube Heat Exchanger

This paper discusses fouling of a twisted tube heat exchanger under different conditions of fluid velocity and heat input. The fluid velocity was varied from 0.5 to 2.0 m/s, whereas the heat input to the heat exchanger was varied from 200 to 800 W. The experimental results show that for low fluid velocity of 0.5 m/s, the fouling resistance showed noticeable variation with respect to heat input, whereas for high velocity ranges, that is, 1.0–2 m/s, the variation in fouling resistance is less. The fouling in twisted tube steadily increases with time for different values of heat input from 1000 min onward for fluid velocity in the range from 1.0 to 2.0 m/s. It is also observed that fouling resistance curves overlap for various values of heat input. During the initial 1000 min of the test duration, the maximum fouling in a twisted tube heat exchanger decreases with increase in fluid velocity from 1.0 to 2.0 m/s. This behavior of the fouling rate can be attributed to the fact that at higher fluid velocity, flow becomes turbulent, and this in turn flushes the fouling particles. The time-series correlations for the fouling resistance are found to be logarithmic in nature.     

Forced Convection Film Boiling Heat Transfer Over a Vertical Cylinder

The knowledge of subcooled film boiling heat transfer is important as the basis of understanding the reflooding phenomenon during emergency cooling in a nuclear reactor under a loss-of-coolant accident. In this study, forced convection film boiling heat transfer from a vertical cylinder in Freon-113 flowing upward along the cylinder was measured for the flow velocities ranging from 0 to 1.3 m/s, and liquid subcoolings ranging from 0 to 20 K at pressures near atmospheric. A platinum heater with a diameter of 3 mm was heated by electric current. The heat transfer coefficients obtained are almost independent of vertical positions on the cylinder. The heat transfer coefficients are almost independent of velocity for the velocities lower than about 1 m/s and become higher for the velocities higher than 1 m/s. The heat transfer coefficients at each velocity are higher for higher liquid subcoolings. Improvement of film boiling heat transfer from the vertical cylinder with the increase in flow velocity is much less than that of horizontal cylinder in cross flow previously reported by the authors. This is mainly due to the difference of heat transfer enhancement mechanism; the former is the drag force on vapor flow acted by a liquid flow, and the latter is the pressure gradient near the front stagnation point caused by external potential flow.     

Heat transfer characteristics in a condenser of closed two‐phase thermosyphon: Effect of entrainment on heat transfer deterioration

Condensation heat transfer in a closed two‐phase thermosyphon is experimentally examined using two different types of test section. Test Section 1 is a straight‐pipe‐type thermosyphon, whereas Test Section 2 has a large‐diameter evaporator compared with a condenser to minimize entrainment at the evaporator. Condensation heat transfer in Test Section 1 shows much lower heat transfer coefficients than those estimated by a Nusselt theory. This low condensation heat transfer occurs due to a working fluid entrainment. It is confirmed from a result of Test Section 2 that the condensation heat transfer is similar to the values predicted by the Nusselt theory as far as the effect of the working fluid entrainment is negligible and flooding does not occur. A new correlation for the heat transfer coefficient considering the effect of entrainment is proposed. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(3): 212–225, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10030     

Heat transfer in the evaporator section of moderate-speed rotating heat pipes

Experiments were performed to investigate the heat transfer mechanism in the evaporator section of non-stepped rotating heat pipes at moderate rotational speeds of 2000–4000 rpm or accelerations of 40g–180g, and evaporator heat fluxes up to 100 kW/m². The thermal resistance of the evaporator section as well as that of the condenser section was examined by measuring the axial temperature distributions of the flow in the core region of the heat pipe and along the wall of the heat pipe. The experimental results indicated that natural convection heat transfer occurred in the liquid layer of the evaporator section under these conditions. The heat transfer measurements were in reasonable agreement with the predictions from an existing rotating heat pipe model that took into account the effect of natural convection in the evaporator section.     

海水淡化系统水平管降膜蒸发器传热系数研究

针对海水淡化系统水平管降膜蒸发器,总结和分析管内冷凝侧与管外蒸发侧的换热系数关联式,比较管内径、入口蒸汽流速、蒸汽冷凝温度、出口蒸汽干度对管内蒸汽冷凝侧换热系数的影响;研究传热温差以及喷淋密度对管外蒸发侧换热系数的影响。结合不同的污垢系数,进行了总传热系数的影响因素分析,为海水淡化系统的工程设计提供依据。     

Heat extraction from salinity-gradient solar ponds using heat pipe heat exchangers

This paper presents the results of experimental and theoretical analysis on the heat extraction process from solar pond by using the heat pipe heat exchanger. In order to conduct research work, a small scale experimental solar pond with an area of 7.0 m² and a depth of 1.5 m was built at Khon Kaen in North-Eastern Thailand (16°27′N102°E). Heat was successfully extracted from the lower convective zone (LCZ) of the solar pond by using a heat pipe heat exchanger made from 60 copper tubes with 21 mm inside diameter and 22 mm outside diameter. The length of the evaporator and condenser section was 800 mm and 200 mm respectively. R134a was used as the heat transfer fluid in the experiment. The theoretical model was formulated for the solar pond heat extraction on the basis of the energy conservation equations and by using the solar radiation data for the above location. Numerical methods were used to solve the modeling equations. In the analysis, the performance of heat exchanger is investigated by varying the velocity of inlet air used to extract heat from the condenser end of the heat pipe heat exchanger (HPHE). Air velocity was found to have a significant influence on the effectiveness of heat pipe heat exchanger. In the present investigation, there was an increase in effectiveness by 43% as the air velocity was decreased from 5 m/s to 1 m/s. The results obtained from the theoretical model showed good agreement with the experimental data.     

闭式深层海水供冷系统换热特性及优化设计研究

针对低纬度岛屿地区全年温度高、湿度大,空调全年运行时间长而常规能源运输成本高,传统高能耗空调系统难以适应的问题,提出一种利用深层海水供冷的闭式空调系统。根据换热特性的差异,将换热管道分为沿程垂直换热管道和海底换热盘管两部分,通过数值模拟研究管径、流速等参数对垂直管段和海底换热盘管段传热性能的影响。研究结果表明：垂直管段管径在0.6 m以内,流速在1~2 m/s范围可保证较高的换热性能;对于海底换热盘管段,最佳管径为0.025~0.050 m,最佳流速为0.4~0.8 m/s。在此基础上建立适用于垂直管段优化设计的费用年值数学模型,计算其比摩阻、流速,形成设计用水力计算表,并给出适用于海底换热盘管段工程设计的设计线算图及其修正公式。