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 wall.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 prediction of vortex flow and thermal separation in a subsonic vortex tube

This work was aimed at gaining understanding of the physical behaviours of the flow and temperature separation process in a vortex tube. To investigate the cold mass fraction’s effect on the temperature separation, the numerical calculation was carried out using an algebraic Reynolds stress model (ASM) and the standard k-ε model. The modelling of turbulence of com-pressible, complex flows used in the simulation is discussed. Emphasis is given to the derivation of the ASM for 2D axisymmet-rical flows, particularly to the model constants in the algebraic Reynolds stress equations. The TEFESS code, based on a staggered Finite Volume approach with the standard k-ε model and first-order numerical schemes, was used to carry out all the computations. The predicted results for strongly swirling turbulent compressible flow in a vortex tube suggested that the use of the ASM leads to better agreement between the numerical results and experimental data, while the k-ε model cannot capture the stabilizing effect of the swirl.

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(英文)

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.

带翅片式涡流发生器的圆管内的传热强化（英文）

The article presents the influence of winglet vortex generators(WVGs)placed in the core flow area on thermal performance enhancement of a tube heat exchanger.The experiment was carried out in a uniform wall heat-fluxed tube by varying turbulent airflow for Reynolds number ranging from 5300 to 24000.In the present 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 RPand RBvalues were evaluated and compared with those for smooth tube and tubes with twisted tape or wire coil.The measurement reveals that the WVGs enhance considerably the heat transfer and friction loss above the plain tube,wire coil and twisted tape.The Nusselt number and friction factor increase with the increment of RBand Re but with the decreasing RP.The average Nusselt numbers for the WVGs with various RBare in the range of 2.03–2.34 times above the plain 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 mechanisms in the winglet-inserted tube.

内带分散的V型翅片的热交换方管的强化传热（英文）

The article presents an experimental and numerical study on thermal performance enhancement in a constant heatfluxed square-duct inserted diagonally 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(R_B=e/H = 0.075,0.1,0.15 and 0.2),fin pitch to duct height ratio(R_P= 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 R_B but the decrease of R_P.The inserted square-duct at R_B = 0.2 and R_P = 0.5 provides the highest heat transfer and friction factor while the one with R_B= 0.1 and R_P= 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.

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 characteristics in a circular tube with 45°V-baffles with isothermal wall.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 wall of the inter-baffle cavity and lead a drastic increase in heat transfer 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.

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.

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 betterηthan 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 optimumηof 2.17 is seen for the one with a certain hole size.The PWs provideηat about 5%–8%above the solid winglets.

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.

Laminar Periodic Flow and Heat Transfer in a Rectangular Channel with Tri-angular Wavy Baffles

This paper presents a numerical analysis of laminar periodic flow and heat transfer in a rectangular constant temperature-surfaced channel with triangular wavy baffles (TWBs).The TWBs were mounted on the opposite walls of the rectangular channel with inline arrangements.The TWBs are placed on the upper and lower walls with attack angle 45?.The numerical is performed with three dif-ferent baffle height ratios (BR=b/H=0.05 0.3) at constant pitch ratio (PR) of 1.0 for the range 100 ≤ Re ≤ 1000.The computational results are shown in the topology of flow and heat transfer.It is found that the heat transfer in the channel with the TWB is more effective than that without baffle.The in-crease in the blockage ratio,BR leads to a considerable increase in the Nusselt number and friction factor.The results indicate that at low BR,a fluid flow is significantly disturbed resulting in inefficient heat transfer.As BR increases,both heat transfer rate in terms of Nusselt number and pressure drop in terms of friction factor increase.Over the range examined,the maximum Nu/Nu0 of 7.3 and f/f0 of 126 are both found with the use of the baffles with BR=0.30 at Re=1000.In addition,the flow structure and temperature field in the channel with TWBs are also reported.