进风角度对椭圆管翅式换热器传热性能影响

对两排椭圆管翅式换热器实验元件在不同进风角度下（30°、45°、60°和90°）的换热性能进行了实验研究,结果表明,在测试的迎风速度范围内随着进风角度的减小换热器换热性能减弱,并给出了测试工况范围内的换热性能经验关联式;对不同进风角度下空气侧换热性能进行了数值计算,与实验结果进行对比,符合良好;最后对不同进风角度下换热器内不同通道内空气平均速度的分布进行了研究,解释了换热性能差异的原因,为相应的工程应用提供理论参考。     

Flow field in the freeboard of vortexing fluidized bed

A systematic investigation on the flow field in a vortexing fluidized bed cold model was reported. The gas velocity profiles within the freeboard with diameters of 0.19 m and 0.29 m were measured by using a five-hole pitot tube. A new parameter, called vortex number, V_or defined as the ratio of tangential velocity to axial velocity of the swirling gas stream, was proposed for representing the swirl intensity. V_or is found to be increased with secondary air velocity, and decreased with primary air velocity and diameter of secondary air nozzles. It is also found that the profile of swirl flow is significantly affected by the arrangement of the secondary air nozzles. The effects of inserted length of secondary air nozzles and geometric structure of expansion section on the swirl flow are also studied. Based on the experimental data, a correlation is presented to estimate the vortex number. Vortex number is found to be a function of the geometric structure of exhaust tube, diameter of secondary air nozzle and tangential air flow rate.     

Thermohydraulics of Turbulent Flow Through Heat Exchanger Tubes Fitted with Circular-rings and Twisted Tapes

The influences of circular-ring turbulators (CRT) and twisted tape (TT) swirl generators on the heat transfer enhancement, pressure drop and thermal performance factor characteristics in a round tube are reported. The circular-ring turbulators were individually employed and together with the twisted tape swirl generators in the heated section of the tube. Three different pitch ratios (l/D1.0, 1.5, and 2.0) of the CRT and three different twist ratios (y/W 3, 4, and 5) of the TT were introduced. The experiments were conducted using air as the working fluid under a uniform wall heat flux condition, for the Reynolds number between 6000 and 20000. The experimental results reveal that the heat transfer rate, friction factor and thermal performance factor of the combined CRT and TT are considerably higher than those of CRT alone. For the range examined, the increases of mean Nusselt number, friction factor and thermal performance, in the tube equipped with combined devices, respectively, are 25.8%, 82.8% and 6.3% over those in the tube with the CRT alone. The highest thermal performance factor of 1.42 is found for the combined device consisting of the CRT with l/D 1.0 and TT with y/W 3. The correlations of the Nusselt number, friction factor and thermal performance factor of the tubes with combined devices are also developed in terms of Reynolds number, Prandtl number, twist ratio and pitch ratio.     

内插不同形状涡产生器管内层流流动与传热特性的对比分析

内插扰流元件是一种可操作性强的管内强化传热方式,其强化传热机理主要是在管内诱导产生了二次流。在均匀壁温热边界条件下,对内插不同形状涡产生器管内层流流动与传热特性进行了数值分析。研究发现：在扭带基础上裁去部分面积相同的条件下,管内插等腰梯形涡产生器的换热能力最强,直角梯形涡产生器次之,矩形涡产生器的换热能力最差,管壁上的局部Nusselt数的峰值所在圆周位置及其大小与涡产生器形状有关,而不同形状的涡产生器对管内流动的阻力系数影响较小。插入涡产生器后,管内二次流强度参数Se和平均Nusselt数Nu均随Reynolds数Re的增大而增大,二者随Reynolds数Re的变化规律具有一致性。平均Nusselt数Nu与二次流强度参数Se呈幂函数相关,内插涡产生器管内的二次流强度决定了其对流换热强度。     

Turbulent heat transfer and pressure drop in plain tube bundles

The influence of turbulence intensity on heat transfer and pressure drop in tube bundles was measured in an open wind tunnel. The bundles consisted of one to three rows of plain tubes with in-line and staggered tube arrangements. The vertically arranged tubes were heated by saturated steam, condensing inside, and cooled outside with air in cross-flow. The turbulence intensity behind different grids varied in the air stream between 0.8% and 25%, and the air-side Reynolds numbers ranged from 1.5 × 10⁴ to 1.5 × 10⁵. The enhancement of the heat transfer coefficients for the bundles is due to an increase in the level of turbulence and also to a decreasing number of rows. The increase in the Nusselt number is about 42% for a single row and about 14% for a tube bundle with three rows.

From the pressure drop experiments it follows that over a wide range of Reynolds numbers the drag coefficient is almost independent of the inlet turbulence intensity.

In this paper the experimental results for single rows and for bundles with three rows are presented. The influence of the transverse and longitudinal pitch to diameter ratio on heat transfer and pressure drop is discussed.     

螺旋肋片形成非衰减性旋流的强化传热性能

对于螺旋肋片形成非衰减性旋流的传热和阻力特性进行了实验研究,并比较分析了非衰减性旋流与多翅管和衰减性旋流在传热和阻力特性方面的差异.研究结果表明,在传热系数上非衰减性旋流比多翅管和衰减性旋流分别提高60%～85%和49%,但非衰减性旋流的阻力损失也明显高于多翅管和衰减性旋流.综合热力性能的分析结果表明,在较低Reynolds数下非衰减性旋流的综合热力性能较好,当Reynolds数大于某一临界值 (本实验为18000)后衰减性旋流的综合热力性能较好.     

涡产生器翼高对管翅式换热器流动与传热的影响

采用数值模拟方法分析了三角形小翼式涡产生器翼高分别为1.5、1.75和2.0 mm时对圆管管翅式换热器空气侧传热及纵向涡强度的影响。结果表明：在相同Re下,随着翼高的增加,Nu、阻力系数 f 以及量纲1二次流强度Se_m都增大;所研究各模型的Se_m与Nu呈唯一对应关系,并且获得了Se_m与Nu的定量关系;以强化传热因子JF作为评价准则,得出翼高为1.75 mm时能够使换热器获得较优的综合强化传热效果。     

涡强化扁管管片散热器流动与传热的数值模拟

以空气为介质(Pr=0.698),通过数值模拟的方法,在Re=300～1800的范围内对涡强化扁管管片散热器初始段层流状态下的流动与传热进行了模拟分析,说明了涡产生器横向位置,即交错系数Sr改变时对局部Nu_local和横断面上的平均Nu_b的影响,并通过实验数据验证了数值模拟方法的正确性,证实了通过数值模拟的方法研究、开发换热板芯的可行性.     

矩形缩放管间插入旋流片的传热实验研究

为了研究新型换热器——矩形管束换热器的传热性能,采用矩形缩放管间插入一组旋流片来研究旋流片下游的传热与流阻性能,并与光滑矩形管的传热和流阻进行比较。实验结果表明:在实验范围内(19 000Re44 000),随着下游距离的增大,旋流片产生的自旋流逐渐衰减,局部Nu数和阻力系数减小,并且局部Nu数的下降速度要慢于阻力系数的;当下游距离在0—200 mm时,阻力系数下降的程度较大;旋流片下游在得到强化传热的同时,阻力增加也很大。     

Augmentation of heat transfer performance in coiled flow inverter vis-à-vis conventional heat exchanger

In the present work, primarily two studies were carried out to ascertain the performance of coiled flow inverter (CFI) as heat exchanger at pilot plant scale. In the first study, performance of CFI heat exchanger has been compared with conventional heat exchangers, i.e. shell and tube heat exchanger (SHE) and plate type heat exchanger (PHE) under identical heat transfer area and process conditions. Experiments were conducted with water flowing under laminar flow regime within the flow range of 30–300 kg/h in the tube side of SHE and PHE. Friction factor and Nusselt number calculated from present experimental study in SHE and PHE were compared with the experimental data previously reported for CFI heat exchanger (Kumar et al., 2007). The Number of Transfer units (NTU) calculated in the present study for CFI was nearly 3.7–7.5 times higher as compared to SHE and 2–2.5 times higher as compared to PHE. In the second part of the study, experiments were performed first time to investigate the pressure drop and heat transfer of compressed air flowing under turbulent flow condition in CFI heat exchanger at pilot plant scale. Hot air at elevated pressures (10–30 kg/cm²) in the tube side of CFI heat exchanger with flow range 3×10⁴<N_Re<1.4×10⁵ was cooled by either cooling water or ambient air. The friction factor and Nusselt number values for compressed air flowing in the CFI were also compared with the experimental data reported in the literature for coiled tube at ambient conditions. On the basis of experimental results, new correlations for friction factor and Nusselt number of compressed air flowing under turbulent flow condition in CFI heat exchanger have been developed.     

并流多通道进出口管壳式换热器壳程传热性能比较

对一种新型并流多通道进出口结构的壳程轴流管壳式换热器壳程局部传热性能进行实验研究,在有分布挡板与无分布挡板的情况下,分别对进口段局部努塞尔数Nu的分布、局部平均Nu的分布、以及换热器整体的传热和阻力性能进行了比较与分析。研究结果揭示了壳程进口段的局部表面Nu的分布规律,并给出了合理的机理分析。结果表明,分布挡板不但能够有效促进壳程流场和局部Nu的均匀分布,而且能够提高整体传热性能,且阻力增加较小。     

管内周期性自旋流强化传热的结构优化

在圆管内间隔插入多个旋流片的流道中,流体流动呈现周期性自旋流.用数值模拟方法对空气湍流情况的传热和流阻性能受相邻旋流片间距L_p、旋流角β和旋转角α3个因素的影响进行了分析,并得出旋流片的优化结构,而且实验验证了数值模拟预测的正确性.结果表明：α越大,传热性能越好,阻力越大;L_p越小,传热性能越好,阻力越大;β越大,传热性能越好,而阻力损失因受到摩擦阻力和形体阻力的综合影响而呈现非单调变化.旋流片的最佳结构参数为：旋流角β为20.3°,旋转角α为180°,相邻旋流片间距L_p为33d.与α相比,β对传热和流阻性能的影响更显著.     

Mass transfer ambiguities in swirling pipe flows

A mathematical model of mass transfer in the boundary layer of swirl flow is developed. The equations obtained for Sherwood number and mass transfer coefficient generalize the equations obtained by Levich for the flat plate to the case of mass transfer in axisymmetrical swirl flow. For the first time it is shown that, in swirl flows, the mass transfer is not completely controlled by integral flow parameters (Reynolds number and the swirl number) but depends essentially on the type of vortex symmetry. The left-handed helical vortices generate wake-like swirl flows and increase mass transfer in comparison with axial flows. The right-handed helical vortices generate jet-like swirl flows, which can decrease mass transfer.     

旋流动力稳定性对传热影响的研究

旋流动力稳定性是影响旋流传热和流动性能的重要因素, 今提出了基于旋流稳定性基本判据的瑞利判据和失稳指标b,分析旋流动力稳定性的影响因素及其对传热效果的影响作用。结果表明旋流强度、轴向分量速度和旋转流道的几何尺寸均会对旋流稳定性产生重要影响。随着旋流失稳指标b 的提高传热系数明显增加,对于Vz=2.14ms-1的旋流, b 由0增至500时传热系数提高100%以上。旋流动力稳定性的研究结果为旋流强化传热研究的深入提供重要的手段。     

EXPERIMENTAL INVESTIGATION OF HEAT TRANSFER IN AN AIR DUCT WITH AN INCLINED HEATING SURFACE

In this paper some experimental results showing the influence of inclination of a cylindrical heating surface immersed in an air duct on heat transfer coefficient are presented. The experiments were performed in a laboratory-scaled apparatus of square cross section with dimensions 120 ×120 mm and 1400 mm in height. Heat transfer surface was an electrical heater made of brass tube with outer diameter of 19 mm and 110 mm length. In each experiment the temperatures of the front and rear side of the heating surface, inclination angle, air velocity, inlet air temperature, and heater power were measured. It was concluded that heat transfer coefficient depends on flow conditions and angle of inclination. The maximum Nusselt number (Nu) was observed to occur about 45° inclination relative to the horizon, for the range of Reynolds numbers used in experiments. The values of heat transfercoefficients in the vertical position were very nearly the same as they were in the horizontal position. Based on the experimental results, a correlation was proposed for estimation of Nu at the desired flow velocity and inclination angle, relative to Nu at zero inclination.     

Experimental investigation of heat transfer in an air duct with an inclined heating surface

In this paper some experimental results showing the influence of inclination of a cylindrical heating surface immersed in an air duct on heat transfer coefficient are presented. The experiments were performed in a laboratory-scaled apparatus of square cross section with dimensions 120 ‐ 120 mm and 1400 mm in height. Heat transfer surface was an electrical heater made of brass tube with outer diameter of 19 mm and 110 mm length. In each experiment the temperatures of the front and rear side of the heating surface, inclination angle, air velocity, inlet air temperature, and heater power were measured. It was concluded that heat transfer coefficient depends on flow conditions and angle of inclination. The maximum Nusselt number (Nu) was observed to occur about 45° inclination relative to the horizon, for the range of Reynolds numbers used in experiments. The values of heat transfercoefficients in the vertical position were very nearly the same as they were in the horizontal position. Based on the experimental results, a correlation was proposed for estimation of Nu at the desired flow velocity and inclination angle, relative to Nu at zero inclination.     

纵向涡强化圆管内换热的数值模拟及性能分析

对布置有涡发生器小翼对的圆管内部湍流流动和强化传热特性进行了三维数值模拟。研究了涡发生器的形状和对数对流动传热特性的影响并采用综合性能指标进行优化。结果表明：涡发生器后横截面上产生的多纵向涡结构使管内流体混合更加充分,促进了壁面边界层与主流的动量和能量交换,提高了传热强度。每排4对矩形小翼时的换热性能最好,Nusselt数比光管平均提高了27.2%。相同形状下,每排4对涡发生器的综合性能均高于每排3对;相同对数下,梯形小翼的综合性能最好,三角形小翼次之,矩形小翼最差。每排4对梯形小翼时的整体综合性能最优,性能评价标准达到了0.97～1.07。     

强化缩放管内湍流对流换热

引　言缩放管是管内外凹凸肋面对称的异形管 ,对管内外两侧轴向流体均有良好的传热强化作用 ,而且凹凸曲面光滑 ,尺度较大 ,不容易产生积垢 .Мигай ,ВК[1] 研究了 6种不同结构的缩放管中的传热和阻力规律 ,提出了缩放管的优化结构 .近年来 ,国内外对缩放通道中流体换热进行数值模拟研究很多[2 ,3] ,强调如何准确描述这种典型有回流的椭圆形流动 ,但对其机理的研究尚不充分 .邓先和提出将缩放管与空心环支撑结构相配合 ,在壳管式换热器中得到了广泛的应用[4 ,5] .存在的不足之处是 :目前使用的缩放管强化传热效率不够高 ,经传热机理分…     

钛合金螺旋扭曲管内强迫对流实验分析

以去离子水为工质,研究了钛合金螺旋扭曲管内的强化传热与流阻性能,并与钛合金圆管比较;根据实验数据拟合螺旋扭曲管内努塞尔特数和阻力系数的实验关联式,并与经典对流传热关联式比较. 结果表明,Dittus?Boelter方程、Sieder?Tate方程、Михеев方程和Gnielinski方程适用于模拟螺旋扭曲管内对流换热特性.     

换热管内插扰流子的数值模拟研究

针对强化换热时较难避免的压降损失问题,以数值模拟为手段,对比分析了换热管中安装V字折流板（V-baf）、柱状扰流子（C-rod）,以及旋流子（S-gen）等扰流元件后,流体流动和换热情况。结果表明：扰流元件的加入,可以一定程度上强化传热;总结文献中已有的换热性能评价标准,综合考虑换热强化和压降损失,发现PEC（Performance Evaluation Criterion）是一种容易理解、易于计算的评价指标;经过对比3种扰流子的换热效果,其中旋流子诱导管内流体产生旋转流动,旋转流动可以使得流体在径向发生混合,增大核心区流体温度均匀程度,符合二次流强化与核心流强化原则;同时,旋转流动持续距离长,可以保持较长距离不衰减,换热综合效果较好。