装填有矩形切口螺旋带的管中的湍动传热和压降(英文)

Heat transfer, friction factor and thermal enhancement factor characteristics of a double pipe heat exchanger fitted with square-cut twisted tapes (STT) and plain twisted tapes (PTT) are investigated experimentally using the water as working fluid. The tapes (STT and PTT) have three twist ratios ( , 4.4 and 6.0) and the Reynolds number ranges from 2000 to 12000. The experimental results reveal that heat transfer rate, friction factor and thermal enhancement factor in the tube equipped with STT are significantly higher than those fitted with PTT. The additional disturbance and secondary flow in the vicinity of the tube wall generated by STT are higher compared to that induced by the PTT is referred as the reason for better performance. Over the range considered, the Nusselt number, friction factor and thermal enhancement factor in a tube with STT are respectively, 1.03 to 1.14, 1.05 to 1.25 and 1.02 to 1.06 times of those in tube with PTT. An empirical correlation is also formulated to match with experimental data of Nusselt number and friction factor for STT and PTT.     

圆形管中填充带圆钉的螺旋状金属带时的传热和流动阻力研究(英文)

This paper deals with the experimental investigation on Nusselt number, friction factor and thermal enhancement factor of a double pipe heat exchanger equipped with twisted tape consisting wire nails (WN-TT) and plain twisted tapes (P-TT) with three different twist ratios of y 2.0, 4.4 and 6.0. Test runs are conducted using the water as the working fluid with Reynolds number range between 2000 and 12000 for WN-TT and P-TT. It is found that Nusselt number, friction factor and thermal enhancement factor in the tube equipped with WN-TT appreciably higher than those in the tube fitted with P-TT and plain tube. Over the range considered Nusselt number, friction factor and thermal enhancement factor in a tube with WN-TT are respectively, 1.08 to 1.31, 1.1 to 1.75 and 1.05 to 1.13 times of those in tube with P-TT. The better performance of WN-TT is due to combined effects of the following factors: (1) common swirling flow generated by P-TT, (2) additional turbulence offered by the wire nails. Empirical correlations for Nusselt number, friction factor and thermal enhancement factor are also formulated from the experimental results of WN-TT and P-TT.     

插入双面有三角形翅片的金属带对热交换管中流动和传热特性的影响

The convective heat transfer and friction behaviors of turbulent tube flow through a straight tape with double-sided delta wings(T-W)have been studied experimentally.In the current work,the T-W formed on the tape was used as vortex generators for enhancing the heat transfer coefficient by breakdown of thermal boundary layer and by mixing of fluid flow in tubes.The T-W characteristics are(1)T-W with forward/backward-wing arrangement,(2)T-W with alternate axis(T-WA),(3)three wing-width ratios and(4)wing-pitch ratios.The experimental result reveals that for using the T-W,the increases in the mean Nusselt number(Nu)and friction factor are,respectively,up to 165%and 14.8 times of the plain tube and the maximum thermal performance factor is 1.19.It is also obvious that the T-W with forward-wing gives higher heat transfer rate than one with backward-wing around 7%. The present investigation also shows that the heat transfer rate and friction factor obtained from the T-WA is higher than that from the T-W.In addition,the flow pattern and temperature fields in the T-W tube with both backward and forward wings were also examined numerically.     

Enhanced heat transfer in a heat exchanger square-duct with discrete V-finned tape inserts☆

The article presents an experimental and numerical study on thermal performance enhancement in a constant heat-fluxed square-duct inserted diagonal y with 45° discrete V-finned tapes (DFT). The experiments were carried out by varying the airflow rate through the tested square duct with DFT inserts for Reynolds number from 4000 to 25000. The effect of the DFT with V-tip pointing upstream at various relative fin heights and pitches on heat transfer and pressure drop characteristics was experimentally investigated. Both the heat transfer and pressure drop were presented in terms of Nusselt number and friction factor respectively. Several V-finned tape characteristics were introduced such as fin-to duct-height ratio or blockage ratio (RB=e/H=0.075, 0.1, 0.15 and 0.2), fin pitch to duct height ratio (RP=P/H=0.5, 1.0, 1.5 and 2.0) and fin attack angle,α=45°. The experimental results reveal that the heat transfer and friction factor values with DFT inserts increase with the increment of RB but the decrease of RP. The inserted square-duct at RB=0.2 and RP=0.5 provides the highest heat transfer and friction factor while the one with RB=0.1 and RP=1.5 yields the highest thermal performance. Also, a numerical simulation was conducted to investigate the flow structure and heat transfer mechanism inside the tested duct with DFT inserts.     

Heat transfer augmentation in a circular tube with winglet vortex generators☆

The article presents the influence of winglet vortex generators (WVGs) placed in the core flow area on ther-mal performance enhancement of a tube heat exchanger. The experiment was carried out in a uniform wall heat-fluxed tube by varying turbulent alrflow for Reynolds number ranging from 5300 to 24000. In the pres-ent work, the WVGs with an attack angle of 30° were inserted into the test tube at four different winglet pitch ratios (RP=P/D) and three winglet-width or blockage ratios (RB=e/D). The experimental results at various RP and RB values were evaluated and compared with those for smooth tube and tubes with twist-ed tape or wire coil. The measurement reveals that the WVGs enhance considerably the heat transfer and friction loss above the plaln tube, wire coil and twisted tape. The Nusselt number and friction factor increase with the increment of RB and Re but with the decreasing RP. The average Nusselt numbers for the WVGs with various RB are in the range of 2.03–2.34 times above the plaln tube. The thermal performance for the WVGs is found to be much higher than that for the wire coil and twisted tape and is in a range of 1.35–1.59. Also, a numerical investigation is conducted to study the flow structure and heat transfer enhancement mecha-nisms in the winglet-inserted tube.     

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.     

Thermal performance of tubular heat exchanger with multiple twisted-tape inserts

The paper presents an experimental investigation on enhanced heat transfer and pressure loss characteristics by using single, double, triple, and quadruple twisted-tape inserts in a round tube having a uniform heat-fluxed wal . The investigation has been conducted in the heat exchanger tube inserted with various twisted-tape numbers for co-and counter-twist arrangements for the turbulent air flow, Reynolds number (Re) from 5300 to 24000. The typical single twisted-tape inserts at two twist ratios, y/w=4 and 5, are used as the base case, while the other multiple twisted-tape inserts are at y/w=4 only. The experimental results of heat transfer and pressure drop in terms of Nusselt number (Nu) and friction factor (f), respectively, reveal that Nu increases with the increment of Re and of twisted-tape number. The values of Nu for the inserted tube are in a range of 1.15–2.12 times that for the plain tube while f is 1.9–4.1 times. The thermal enhancement factor of the inserted tube under similar pumping power is evaluated and found to be above unity except for the single and the double co-twisted tapes. The quadruple counter-twisted tape insert provides the maximum thermal performance.     

Numerical investigation on flow and heat transfer characteristics of corrugated tubes with non-uniform corrugation in turbulent flow

Based on finite volume method, the pressure drop and heat transfer characteristics of one smooth tube and ten different axisymmetric corrugated tubes, including two with uniform corrugation and eight with non-uniform corrugation, have been studied. A physical model of the corrugated tube was built, then the numerical simulation of the model was carried out and the numerical simulation results were compared with the empirical formula.The results show that: the friction factor decreases with the increase of Reynolds number ranging from 6000 to 57000, the value of which in the corrugated tubes with non-uniform corrugation(tube 03–10) are smaller than those with uniform corrugation(tube 01–02). The geometry parameters of tube(01) have advantages on the heat transfer enhancement in low Reynolds number flow region(from 6000 to 13000) and tube(07–08)have advantages on the heat transfer enhancement in high Reynolds number flow region(from 13000 to 57000). The vortex, existed in each area between two adjacent corrugations called second flow region, is the root of the enhancement on heat transfer in the corrugated tubes. The effectiveness factor decreases with the increasing of Reynolds number and the performances of the corrugated tubes with pitch of 12.5 mm have advantages than these of 10 mm under the same corrugation geometric parameter.     

转子组合式装置强化传热与阻力实验研究(II)22号润滑油(英文)

The paper described experimental investigation of heat transfer and single-phase pressure drop through tubes with different rotor-assembled strands inserted in the Reynolds number range of 800-9000 with lubricant as working fluid. In the experiment, fixed mounts were employed to eliminate the entrance effect. The experimental results showed that the employment of fixed mounts led to a visible bias of friction factor in the laminar regime while it could not affect the Nusselt numbers significantly. Experiment for the tube inserted with rotors-assembled strand showed remarkable improvement for heat transfer with the Nusselt number increased by 200%-225% in the laminar regime and 125%-160% in the transitional regime. Meanwhile, the friction factor increased inevitably by 200%-300% within the same range of Reynolds number. The comparison of different rotor-assembled strands in-serted tubes and plain tube showed that the heat transfer benefited from the increase of the diameter of rotor-assembled strand with the same lead and the decrease of the lead of rotor-assembled strand, so does the friction factor. Based on experimental data and thorough multivariant linear normal regression method, the correlations of average Nusselt number and friction factor are established.     

Turbulent forced convection in a heat exchanger square channel with wavy-ribs vortex generator☆

Turbulent forced convective heat transfer and flow configurations in a square channel with wavy-ribs inserted diagonally are examined numerical y. The influences of the 30° and 45° flow attack angles for wavy-ribs, blockage ratio, RB=b/H=0.05–0.25 with single pitch ratio, RP=P/H=1 are investigated for the Reynolds number based on the hydraulic diameter of the square channel, Re=3000–20000. The use of the wavy-ribs, which inserted diagonal in the square channel, is aimed to help to improve the thermal performance in heat exchange systems. The finite volume method and SIMPLE algorithm are applied to the present numerical simulation. The results are presented on the periodic flow and heat transfer profiles, flow configurations, heat transfer characteristics and the performance evaluations. The mathematical results reveal that the use of wavy-ribs leads to a higher heat transfer rate and friction loss over the smooth channel. The heat transfer enhancements are around 1.97–5.14 and 2.04–5.27 times over the smooth channel for 30° and 45° attack angles, respectively. However, the corre-sponding friction loss values for 30° and 45° are around 4.26–86.55 and 5.03–97.98 times higher than the smooth square channel, respectively. The optimum thermal enhancement factor on both cases is found at RB=0.10 and the lowest Reynolds number, Re=3000, to be about 1.47 and 1.52, respectively, for 30° and 45° wavy-ribs.     

扭曲管强化传热性能实验研究

在设计的蒸汽-水换热实验台上测试了5种不同导程的椭圆扭曲管和圆管的传热和流动阻力性能.分别对5种不同导程的扭曲管和圆管管内流体努赛尔数,摩擦系数和管内综合评价因子随雷诺数变化情况进行了对比.实验结果表明:扭曲管的强化传热性能明显,且导程越小,强化传热效果越好.在实验测试的雷诺数范围内,扭曲管的努塞尔数为光滑圆管的1.0...     

换热管内置螺旋形扭带阻力与传热特性的实验研究

为了研究螺旋形扭带阻力与传热特性,选取了不同宽度（6、7和8 mm）的3种扭率（2.0、3.0、4.0）、3种螺距比（1.5、2.0、2.5）的参数组合下共27根螺旋形扭带插入换热管内进行实验.实验结果表明,插入螺旋形扭带后换热管内流动阻力和传热效果都有明显提高.通过对实验数据的多元线性回归分析,建立了相应的阻力系数和努赛尔数关联式.并且由强化传热综合性能评价分析,在实验雷诺数范围内得出强化传热综合性能评价因子φ=1.063～1.587,证明了实验研究的扭带具有强化传热的应用价值.     

INFLUENCE OF NONUNIFORM TWISTED TAPE ON HEAT TRANSFER ENHANCEMENT CHARACTERISTICS

An experimental study of heat transfer and friction factor characteristics in a circular tube fitted by twisted tapes with nonuniform twist ratios is reported. The twisted tapes are used as swirl generators playing roles as heat transfer enhancers. The nonuniform twisted tapes examined in the present work have (1) sequentially increasing twist ratios (SL), (2) sequentially decreasing twist ratios (LS), (3) repeatedly increasing-decreasing twist ratios (SL/SL), (4) repeatedly decreasing-increasing twist ratios (LS/LS), (5) intermittently increasing-decreasing twist ratios (SL→LS), and (6) intermittently decreasing-increasing twist ratios (LS→SL). In addition, three different typical twisted tapes with twist ratios of 3.0, 4.0, and 5.0 (TT) were also evaluated. Apparently, LS yields monotonically increasing swirling intensity and SL decreasing swirling intensity, resulting in lower heat transfer rate and friction factor than the other four nonuniform twisted tapes, which possess extra fluid fluctuation. However, among the tapes tested, the SL/SL offers the maximum thermal performance factor of around 1.03, which corresponds to Nusselt number of around 36% and friction factor of 3.57 times over those of the plain tube.     

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.     

Heat transfer enhancement by twisted tapes with alternate-axes and triangular, rectangular and trapezoidal wings

The effects of twisted tapes with alternate-axes and wings on heat transfer, flow friction and thermal performance characteristics in a round tube are investigated. The influence of wing shape including triangle, rectangle and trapezoid is also studied. The experiments are performed using twisted tape with three different wing-chord ratios (d/W) of 0.1, 0.2 and 0.3 and constant twist ratio (y/W) of 4.0. The alternate-axis is made by arranging each plane of twisted tape to 60 degree difference relative to the adjacent plane. The results show that both heat transfer rate and friction factor associated by all twisted tape are consistently higher than those without twisted tape. Under the similar operating conditions, Nusselt number, friction factor as well as thermal performance factor given by the tape with alternate-axes and trapezoidal wings are higher than those given by the others. It is also found that that Nusselt number, friction factor and thermal performance increase with increasing wing-chord ratio. The maximum thermal performance factor of 1.42 is found with the use of twisted tape with alternate-axes and trapezoidal wings at d/W = 0.3, where the heat transfer rate and friction factor, respectively increase to 2.84 and 8.02 times of those in the plain tube.     

Enhancement of Turbulent Flow Heat Transfer in a Tube with Modified Twisted Tapes

Numerical simulations were performed to study the fluid flow and heat transfer in a tube with staggered twisted tapes with central holes. In the range of Reynolds numbers between 6000 and 28 000, the modified twisted tapes increased the Nusselt number by 76.2 ～ 149.7 % and the friction factor by 380.2 ～ 443.8 % compared to the smooth tube. Compared to the typical twisted tapes, the modified twisted tapes produced an acceleration flow through the triangle regions leading to the enhancement of heat transfer, and the holes in the modified tapes reduced the severe pressure loss. It was found that the modified twisted tapes decreased the friction factor by 8.0 ～ 16.1 % and enhanced the heat transfer by 34.1 ～ 46.8 % in comparison with the typical tapes. These results indicated that the performance ratio values of the tube with modified twisted tapes were higher than 1.0 in the range of Reynolds numbers studied. The computed performance ratios of the tube with modified twisted tapes were much higher than those of the tube with typical twisted tapes. This means that the integrated performance of the tube with staggered twisted tapes with central holes is superior to that of the tube with typical twisted tapes.     

管内层流强化传热的数值模拟

通过数值模拟,研究了85%甘油在含旋流片缩放管管内的传热与流阻特性,并与光滑管、缩放管的传热与流阻特性进行了对比。研究结果表明,层流时含旋流片缩放管的综合因子大于缩放管,且缩放管内插入6个小角度旋流片具有更好的传热性能。在有旋流片段可以通过减薄边界层厚度与增加有效传热温差的双重途径提高传热速率,而在旋流片下游段则可以利用有效传热温差的缓变特性继续维持较大的传热速率,从而能避免过多的流体置换次数,极大幅度的降低流体的输送功耗。     

旋流片支撑缩放管管束传热与流动的数值模拟

通过数值模拟,以水为工质,研究了湍流时旋流片支撑缩放管管束管间的传热综合性能,并与无支撑光滑管管束、无支撑缩放管管束及旋流片支撑光滑管管束的传热性能进行了对比,分析了不同结构参数对其传热综合性能的影响;揭示了旋流片支撑缩放管管间流体速度场与温度场之间的协同性。结果表明:旋流片支撑急扩慢缩型缩放管管束相对急缩慢扩型具有更好的传热综合性能,且都比旋流片支撑光滑管管束的传热综合性能好,但二者都没有对应的无支撑缩放管管束综合性能好;具有小角度跟大扭率结构的旋流片更有利于旋流片支撑管束传热综合性能的提高;相对旋流片支撑光滑管管束与无支撑缩放管管束,旋流片支撑缩放管管束强化传热的原因在于传热场协同的增强作用更为明显。考虑管间支撑物支撑管束与抗振的必要性,旋流片支撑缩放管管束是一种高效的壳程强化传热措施,文中条件下通过优化,最大传热综合性能能达到1.057。