太阳能空气加热器管道中填充三角形小翅片扰流器的湍流传热的实验研究简

The paper presents an experimental study on the heat transfer and flow friction characteristics in a solar air heater channel fitted with delta-winglet type vortex generators （DWs）. The experiments are conducted by varying the airflow rate for Reynolds number in the range of 5000 to 24000 in the test section with a uniform heat-flux applied on the upper channel wall. Firstly, the DW pairs are mounted only at the entrance of the lower wall of the test channel （called DW-E） to create multiple vortex flows at the entry. The effect of two transverse pitches （Re= Pt/H= 1 and 2） at three attack angles （a= 30°, 45° and 60°） of the DW-E with its relative height, b/H= 0.5 （half height of channel） is examined. Secondly, the 30° DWs with three different relative heights （blH = 0.3, 0.4 and 0.5） are placed on the upper wall only （absorber plate, called DW-A） of the test channel. The experimental result reveals that in the first case, the 60° DW-E at Rp = 1 provides the highest heat transfer and friction factor while the 30° DW-E at Rp = 1 performs overall better than the others. In the second case, the 30° DW-A at b/H= 0.5 yields the highest heat transfer and friction factor but the best thermal performance is found at b/H = 0.4.     

交叉间断肋片与凹槽强化传热的实验与数值研究(英文)

Experimental and numerical investigations have been conducted to study turbulent flow of water and heat transfer characteristics in a rectangular channel with discontinuous crossed ribs and grooves.The tests investigated the overall heat transfer performance and friction factor in ribbed and ribbed-grooved channels with rib angle of 30°.The experimental results show that the overall thermo-hydraulic performance for ribbed-grooved channel is increased by 10%-13.6% when compared to ribbed channel.The investigation on the effects of different rib angles and rib pitches on heat transfer characteristics and friction factor in ribbed-grooved channel was carried out using Fluent with SST(shear-stress transport) k-ω turbulence model.The numerical results indicate that the case for rib angle of 45° shows the best overall thermo-hydraulic performance,about 18%-36% higher than the case for rib angle of 0°.In addition,the flow patterns and local heat transfer characteristics for ribbed and ribbed-grooved channels based on the numerical simulation were also analyzed to reveal the mechanism of heat transfer enhancement.     

正方形管内顺序放置扰流线圈后的传热行为(英文)

Effects of insertion of tandem wire coil elements used as turbulator on heat transfer and turbulent flow friction characteristics in a uniform heat-flux square duct are experimentally investigated in this work. The experiment is conducted for turbulent flow with the Reynolds number from 4000 to 25000. The wire coil element is inserted into the duct with a view to generating a swirl flow that assists to wash up the flow trapped in the duct corners and then increase the heat transfer rate of the test duct. Apart from the full-length coil, 1D and 2D length coil elements placed in tandem inside the duct with various free-space lengths are introduced to reduce the friction loss. The results obtained from these wire coil element inserts are also compared with those from the smooth duct. The experimental results reveal that the use of wire coil inserts for the full-length coil, 1D and 2D coil elements with a short free-space length leads to a considerable increase in heat transfer and friction loss over the smooth duct with no insert. The full-length wire coil provides higher heat transfer and friction factor than the tandem wire coil elements under the same operating conditions. Also, performance evaluation criteria to assess the real benefits in using the wire coil insert into the square duct are determined.     

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.     

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.     

太阳能空气加热器管道中填充三角形小翅片扰流器的湍流传热的实验研究(英文)

The paper presents an experimental study on the heat transfer and flow friction characteristics in a solar air heater channel fitted with delta-winglet type vortex generators(DWs).The experiments are conducted by varying the airflow rate for Reynolds number in the range of 5000 to 24000 in the test section with a uniform heat-flux applied on the upper channel wall.Firstly,the DW pairs are mounted only at the entrance of the lower wall of the test channel(called DW-E)to create multiple vortex flows at the entry.The effect of two transverse pitches(RP Pt/H 1 and 2)at three attack angles(α30°,45°and 60°)of the DW-E with its relative height,b/H 0.5(half height of channel)is examined.Secondly,the 30°DWs with three different relative heights(b/H 0.3,0.4 and0.5)are placed on the upper wall only(absorber plate,called DW-A)of the test channel.The experimental result reveals that in the first case,the 60°DW-E at RP1 provides the highest heat transfer and friction factor while the30°DW-E at RP1 performs overall better than the others.In the second case,the 30°DW-A at b/H 0.5 yields the highest heat transfer and friction factor but the best thermal performance is found at b/H 0.4.     

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.     

带V型折流板圆管中流动结构与传热的三维数值研究(英文)

A 3D numerical investigation has been carried out to examine periodic laminar flow and heat transfer character-istics in a circular tube with 45° V-baffles with isothermal wal . The computations are based on the finite volume method (FVM), and the SIMPLE algorithm has been implemented. The fluid flow and heat transfer characteristics are presented for Reynolds numbers ranging from 100 to 2000. To generate main longitudinal vortex flows through the tested section, V-baffles with an attack angle of 45° are mounted in tandem and in-line arrangement on the opposite positions of the circular tube. Effects of tube blockage ratio, flow direction on heat transfer and pressure drop in the tube are studied. It is apparent that a pair of longitudinal twisted vortices (P-vortex) created by a V-baffle can induce impingement on a wal of the inter-baffle cavity and lead a drastic increase in heat trans-fer rate at tube wall. In addition, the larger blockage ratio results in the higher Nusselt number and friction factor values. The computational results show that the optimum thermal enhancement factor is around 3.20 at baffle height of B=0.20 and B=0.25 times of the tube diameter for the V-upstream and V-downstream, respectively. ? 2014 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. Al rights reserved.     

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.     

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.     

Enhanced heat transfer in rectangular duct with punched winglets

Thermal performance of a heat exchanger duct with punched winglets(PWs) mounted on the upper duct wall has been examined for Reynolds number(Re) ranging from 4100 to 25,500. In the present experiment, two types of PWs: punched delta-and elliptical-winglets(P-DW and P-EW) with four punched-hole sizes were tested at a fixed attack angle, optimal relative pitch and height. Also, data of solid delta-and elliptical-winglets(DW and EW) were included for comparison. The investigation has shown that the P-DW yields higher thermal-performance enhancement factor(η) than the P-EW. Although the solid DW and EW with no punch have the highest heat transfer and friction loss, the PWs yield better η than the solid ones. For PWs, the P-DW with smaller hole size has the peak heat transfer and friction loss around 5.7 and 40 times over the smooth duct, respectively but the optimum η of 2.17 is seen for the one with a certain hole size. The PWs provide η at about 5%–8% above the solid winglets.     

xperimental Study of Heat Transfer Enhancement and Friction Loss Induced by Inserted Rotor-assembled Strand （I） Water

The single-phase pressure drop and heat transfer in a rotor-assembled strand inserted tube were measured using water as the working fluid. Experiment using a smooth tube was carried out to calibrate the experimental system and the data reduction method. In the experiment, fixed mounts were used to eliminate the entrance effect. The experimental results of smooth tube show that employment of fixed mounts leads to a visible bias of friction factor at relative low Reynolds numbers, although it does not significantly affect the Nusselt numbers. The measured data of inserted tube reveal that rotor-assembled strand can significantly improve heat transfer with the Nusselt number increased by 101.6%-106.6% and the overall heat transfer coefficient increased by 58.1%-67.4% within the Reynolds number range of 20000 to 36000. Meanwhile, friction factor increases by 52.2%-84.2% within the same Reynolds number range. The correlations of Nusselt number and friction factor as function of the Reynolds number and Prandtl number were determined through multivariant linear normal regression.     

Convective mass transfer enhancement in a membrane channel by delta winglets and their comparison with rectangular winglets☆

Numerical calculations were conducted to simulate the flow and mass transfer in narrow membrane channels equipped with delta winglets, which are often used as longitudinal vortex generators to enhance heat transfer in heat exchanger applications. The channel consists of an impermeable solid wall and a membrane. The delta winglets are attached to the solid wal surface to enhance the mass transfer near the membrane surface and sup-press the concentration polarization. The winglet performance was evaluated in terms of concentration polariza-tion factor versus consumed pumping power. Calculations were implemented for NaCl solution flow in a membrane channel having a height of 2.0 mm for Reynolds numbers ranging from 400 to 1000. The delta wing-lets were optimized under equal pumping power condition, and the results of optimization suggest winglet height of 5/6 of the channel height, aspect ratio of 2.0, attack angle of 30°, and a winglet interval equal to the chan-nel height. The optimal delta winglets were compared with the optimal rectangular winglets we found previous-ly, and it is shown that the rectangular winglets yield a somewhat better performance than the delta winglets. ? 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. Al rights reserved.     

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.     

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.     

An empirical study on vortex-generator insert fitted in tubular heat exchangers with dilute Cu–water nanofluid flow

The heat transfer enhancement(HTE) in tubular heat exchangers fitted with vortex-generator(VG) inserts is experimentally investigated. The studied four parameters and ranges are: winglets-pitch ratio(1.33, 2.67, and 4),winglets-length ratio(0.33, 0.67, and 1), winglets-width ratio(0.2, 0.4, and 0.6), and Reynolds number(5200to 12200). The testing fluids are the water and Cu–water nanofluid at the volumetric fraction of 0.2%. The results obtained on HTE, pressure drop, and performance evaluation criterion(PEC) are compared with those for water in a smooth tube. It is found that the VG inserts with lower winglets-pitch ratio and higher winglets-length/width ratios present higher values of HTE and pressure drop. Over the range studied, the maximum PEC of 1.83 is detected with the Cu–water nanofluid inside the tube equipped with a VG insert at the winglets-width ratio of0.6 for the maximum Reynolds number, when the heat transfer rate and pressure drop are 1.24 times and 2.03 times of those in the smooth tube. Generalized regression equations of the Nusselt number, friction factor, and PEC are presented for the tubular heat exchangers with the VG inserts for both water and Cu–water nanofluid.It is concluded that the main advantage of the VG inserts is their simple fabrication and considerable performance, particularly at higher Reynolds number.     

缩放管内插双扭带复合强化传热的数值模拟(英文)

The paper presents a 3D numerical simulation of turbulent heat transfer and flow characteristics in converging-diverging tubes(CDs) and converging-diverging tubes equipped with twin counter-swirling twisted tapes(CDTs).The effects of Reynolds number(Re 10000-20000),pitch length(P 11.25,22.5 mm),rib height(e 0.5,0.8,1.1 mm),pitch ratio(δ1:8,5:4,8︰1),gap distance between twin twisted tapes(b 0.5,4.5,8.5 mm) and tape number(n 2,3,4,5,6) on Nusselt number(Nu),friction factor(f) and thermal enhancement factor(η) are investigated under uniform heat flux conditions,using water as working fluid.In order to illustrate the heat transfer and fluid flow mechanisms,flow structures in CDs and CDTs are presented.The obtained results reveal that all geometric parameters have important effects on the thermal performance of CD and CDT,and both CD and CDT show better thermal performance than plain tube at the constant pumping power.It is also found that the increases in the Nusselt number and friction factor for CDT are,respectively,up to 6.3%-35.7% and 1.75-5.3 times of the corresponding bare CD.All CDTs have good thermal performance with η greater than 1 which indicates that the compound heat transfer technique of CDT is commendable for the maximum enhanced heat transfer rate.     

Review on the effect of heat exchanger tubes on flow behavior and heat/mass transfer of the bubble/slurry reactors

Bubble/Slurry bubble column reactors(BCR/SBCR) are intensively used as multiphase reactors for a wide range of application in the chemical, biochemical and petrochemical industries. Most of these applications involve complicate gas–liquid/gas–liquid–solid flow behavior and exothermic process, thus it is necessary to equip the BCR/SBCR with heat exchanger tubes to remove the heat and govern the performance of the reactor. Amounts of experimental and numerical studies have been carried out to describe the phenomena taking place in BCR/SBCRs with heat exchanger tubes. Unfortunately, little effort has been put on reviewing the experiments and simulations for examining the effect of internals on the performance and hydrodynamics of BCR/SBCR. The objective of this work is to give a state-of-the-art review of the literature on the effects of heat exchanger tubes with different types and configurations on flow behavior and heat/mass transfer, then provide adequate information and scientific basis for the design and the development of heat exchanger tubes in BCR/SBCR, ultimately provide reasonable suggestions for better comprehend the performance of different heat exchanger tubes on hydrodynamics.     

不同折流板结构螺旋折流板换热器传热性能的比较(英文)

Numerical simulations were performed on flow and heat transfer performances of heat exchangers having six helical baffles of different baffle shapes and assembly configurations, i.e., two trisection baffle schemes, two quadrant baffle schemes, and two continuous helical baffle schemes. The temperature contour or the pressure contour and velocity contour plots with superimposed velocity vectors on meridian, transverse and unfolded concentric hexagonal slices are presented to obtain a full angular view. For the six helix baffled heat exchangers, the different patterns of the single vortex secondary flow and the shortcut leakage flow were depicted as wel as the heat transfer properties were compared. The results show that the optimum scheme among the six configurations is a circumferential overlap trisection helix baffled heat exchanger with a baffle incline angle of 20° (20°TCO) scheme with an anti-shortcut baffle structure, which exhibits the second highest pressure dropΔpo, the highest overal heat transfer coefficient K, shel-side heat transfer coefficient ho and shel-side average comprehensive index ho/Δpo.