Heat transfer augmentation in a helical-ribbed tube with double twisted tape inserts

Turbulent convective heat transfer characteristics in a helical-ribbed tube fitted with twin twisted tapes have been investigated experimentally. The experiment was carried out in a double tube heat exchanger using the helical-ribbed tube having a single rib-height to tube-diameter ratio, e/D_H = 0.06 and rib-pitch to diameter ratio, P/D_H = 0.27 as the tested section. The insertion of the double twisted tapes with twist ratio, Y, in the range of 2.17 to 9.39 is to create vortex flows inside the tube. The inserted ribbed tube is arranged in similar directions of the helical swirl of the twisted tape and the helical rib motion of the tube (called co-swirl). Effects of the co-swirl motion of the ribbed tube and the double twisted tapes with various twist ratios on heat transfer and friction characteristics are examined. The results obtained from the ribbed tube and the twin twisted tape insert are compared with those from the smooth tube and the ribbed tube acting alone. The experimental results reveal that the co-swirling inserted tube performs much better than the ribbed/smooth tube alone at a similar operating condition. The co-swirl tube at Y ≈ 8 yields the highest thermal performance at lower Reynolds number (Re). In addition, the correlations of Nusselt number and friction factor as functions of Re, Pr and Y are also proposed.     

Studies on heat transfer and friction factor characteristics of turbulent flow through a micro-finned tube fitted with left–right inserts

Experimental investigation on heat transfer and friction factor characteristics of micro-finned tube fitted with full-length twisted tape and left–right twisted tape inserts of twist ratio 7.44, 8.27 and 11.17 has been presented. The experimental data obtained were compared with those obtained from plain tube published data. The heat transfer coefficient enhancement for left–right twisted inserts is higher than that for full-length twisted tape inserts for a given twist ratio. The empirical correlation for Nusselt number, friction relating Reynolds number and twist ratio was formed for both twisted tape inserts and left–right inserts and found to fit the experimental data within ±3.6% and ±2.5% for Nusselt number and ±7% and ±8.5% friction factor, respectively. Performance evaluation analysis was made and the maximum performance ratio was obtained for left–right twisted tape inserts.     

Effect of full length twisted tape inserts on heat transfer and friction factor enhancement with Fe3O4 magnetic nanofluid inside a plain tube: An experimental study

Turbulent convective heat transfer and friction factor characteristics of magnetic Fe₃O₄ nanofluid flowing through a uniformly heated horizontal circular tube with and without twisted tape inserts are estimated experimentally. Experiments are conducted in the particle volume concentration range of 0 < φ < 0.6%,twisted tape inserts of twist ratio in the range of 0 < H/D < 15and Reynolds number range of 3000 < Re < 22000. Heat transfer and friction factor enhancement of 0.6% volume concentration of Fe₃O₄ nanofluid in a plain tube with twisted tape insert of twist ratio H/D = 5 is 51.88% and 1.231 times compared to water flowing in a plain tube under same Reynolds number. Generalized regression equation is presented for the estimation of Nusselt number and friction factor for both water and Fe₃O₄ nanofluid in a plain tube and with twisted tape inserts under turbulent flow condition.     

Estimation of heat transfer coefficient and friction factor in the transition flow with low volume concentration of Al2O3 nanofluid flowing in a circular tube and with twisted tape insert

Experiments to evaluate heat transfer coefficient and friction factor for flow in a tube and with twisted tape inserts in the transition range of flow with Al₂O₃ nanofluid are conducted. The results showed considerable enhancement of convective heat transfer with Al₂O₃ nanofluids compared to flow with water. It is observed that the equation of Gleninski applicable in transitional flow range for single-phase fluids showed considerable deviation when compared with values obtained with nanofluid. The heat transfer coefficient of nanofluid flowing in a tube with 0.1% volume concentration is 23.7% higher when compared with water at number of 9000. Heat transfer coefficient and pressure drop with nanofluid has been experimentally determined with tapes of different twist ratios and found to deviate with values obtained from equations developed for single-phase flow. A regression equation is developed to estimate the Nusselt number valid for both water and nanofluid flowing in the transition flow Reynolds number range in circular plain tube and with tape inserts. The maximum friction factor with twisted tape at 0.1% nanofluid volume concentration is 1.21 times that of water flowing in a plain tube.     

Thermal characteristics in a heat exchanger tube fitted with dual twisted tape elements in tandem

The paper presents a comparative investigation of enhanced heat transfer and pressure loss by insertion of single twisted tape, full-length dual and regularly-spaced dual twisted tapes as swirl generators, in a round tube under axially uniform wall heat flux (UHF) conditions. The investigation encompassed the Reynolds number based on the inlet tube diameter (D) ranging from 4000 to 19,000. The experiments are performed using single twisted tapes and full-length dual twisted tapes with three different twist ratios (y/w = 3.0, 4.0 and 5.0) and also regularly-spaced dual twisted tapes with three different space ratios (s/D = 0.75, 1.5 and 2.25). The effects of major parameters on heat transfer and friction factor are discussed and the results from both single and dual twisted tape inserts are compared with those from the plain tube. The result shows that the heat transfer of the tube with dual twisted tapes is higher than that of the plain tube with/without single twisted tape insert. For both single twisted tape and full-length dual twisted tapes, Nusselt number (Nu) and friction factor (f) tend to increase with decreasing twist ratio (y/w). The average Nusselt number and friction factor in the tube fitted with the full-length dual twisted tapes at y/w = 3.0, 4.0 and 5.0, are respectively 146%, 135% and 128%; and 2.56, 2.17 and 1.95 times of those in the plain tube. For the regularly-spaced dual twisted tapes, the heat transfer rate is decreased with increasing space ratio (s/D). The average Nusselt numbers in the tube fitted with the regularly-spaced dual twisted tapes (s/D) of 0.75, 1.5 and 2.25 are respectively, 140%, 137% and 133% of that in the plain tube. With the similar trend mentioned above, all dual twisted tapes with free spacing yield lower heat transfer enhancement in comparison with the full-length dual twisted tapes (s/D = 0.0).     

Turbulent heat transfer and pressure drop in tube fitted with serrated twisted tape

This paper presents an original experimental study on compound heat transfer enhancement in a tube fitted with serrated twisted tape. The serrations on two sides of the twisted tape with twist ratio of 1.56, 1.88, 2.81 or ∞ are the square-sectioned ribs with the identical rib-pitch and rib-height. The local Nusselt number and Fanning friction factor increase as the twist ratio decreases in the tube fitted with smooth or serrated twisted tape. In the Re range of 5000–25 000, heat transfer augmentation attributed to the serrated twisted tape falls in the range of 250–480% of the plain-tube level. That is about 1.25–1.67 times the heat transfer level in the tube fitted with smooth twisted tape. Fanning friction factors are respectively decreased and increased in the tubes fitted with smooth and serrated twisted tapes as Re increases. Based on the same pumping power consumption, the thermal performances of the tubes with smooth and serrated twisted tapes are compared. A set of empirical correlations that permits the evaluation of the Nusselt number and the Fanning friction factor in the developed flow region for the tubes fitted with smooth and serrated twisted tapes is generated for engineering applications.     

Heat transfer enhancement by using CuO/water nanofluid in corrugated tube equipped with twisted tape

Heat transfer enhancement by using CuO/water nanofluid in corrugated tube equipped with twisted tape is presented. The investigated ranges are (1) three different CuO concentrations: 0.3, 0.5 and 0.7% by volume (2) three different twist ratios of twisted tape: y/w = 2.7, 3.6 and 5.3 (3) two different arrangements of twisted direction of twisted tape relative to spiral direction of corrugated tube: parallel and counter arrangements, and (4) Reynolds number from 6200 to 24000. The results achieved from the use of the nanofluid and twisted tape, are compared with those obtained from the uses of nanofluid alone and twisted tape alone. The experimental results reveal that at similar operating conditions, heat transfer rate, friction factor as well as thermal performance factor associated with the simultaneous application of CuO/water nanofluid and twisted tape are higher than those associated with the individual techniques. Evidently, heat transfer rate increases with increasing CuO/water nanofluid concentration and decreasing twist ratio. In addition, the twisted tape coupled with corrugated tube in counter pattern offer higher heat transfer performances than the ones in parallel pattern. Over the range studied, the maximum thermal performance factor 1.57 is found with the use of CuO/water nanofluid at concentration of 0.7% by volume in corrugated tube together with twisted tape at twist ratio (y/w) of 2.7 (in counter arrangement), for Reynolds number of 6200 where heat transfer rate and friction factor increase to 2.67 times and 5.76 times of those in the plain corrugated tube.     

扭带强化传热性能的数值与实验研究

通过模拟和实验的方法研究在湍流工况下(3 000Re10 000)换热器管内插入不同扭带模型后的传热特性和阻力特性。区别于传统螺旋扭带,提出一种顺时针与逆时针交替扭转的正反扭带。对不同扭率的传统扭带以及扭率为3的无缺口和半圆缺口正反转扭带进行模拟计算,并将模拟结果与实验结果进行对比验证。结果表明:在湍流流态下,雷诺数越小,扭带的强化换热效果表现越好;对不同扭率的扭带,其努塞尔数、摩擦系数和综合性能指标随扭率的减小而增大;扭率为3时,两种正反扭带的强化换热效果均优于传统扭带,无缺口正反扭带的换热效果最好;模拟计算的结果数据与实验结果数据比较,最大误差不超过8%。     

Experimental Study on Laminar Forced Convection of Al2O3/Water and Multiwall Carbon Nanotubes/Water Nanofluid of Varied Particle Concentration with Helical Twisted Tape Inserts in Pipe Flow

In this study an experimental investigation has been carried out to analyze the laminar forced convection of Al₂O₃/water and multiwall carbon nanotubes (MWCNT)/water nanofluids through uniformly heated horizontal circular pipe with helical twisted tape inserts. Tests were conducted for varied range of nanoparticle volume concentration (0.15%, 0.45%, 0.60%, and 1%) and helical tape inserts of twist ratios of 1.5, 2.5, and 3. The heat transfer enhancement and the increase of friction factor of nanofluids with helical inserts are compared with that of pure water results with plain tube without inserts. The Nusselt number is found to increase with the increase in Peclet number and nanofluid concentration. The MWCNT/water nanofluids with helical screw tape inserts exhibits higher thermal performance compared to Al₂O₃/water nanofluid. The maximum thermal performance factor was found to be 1.79 and 1.99 for Al₂O₃/water and MWCNT/water nanofluids with helical twisted tape inserts, respectively. The pressure drop for Al₂O₃ nanofluid is found to be higher compared to the MWCNT nanofluid for all the twist ratio of helical screw tape inserts.     

Flow and heat transfer behaviors of phase change material slurries in a horizontal circular tube

The flow and convective heat transfer behaviors of microencapsulated phase change material (MPCM) slurries in a horizontal circular tube have been experimentally investigated. The slurry consisted of microencapsulated 1-bromohexadecane (C₁₆H₃₃Br) and water, with the mass fractions of MPCM varying from 5% to 27.6%. The pressure drop and local heat transfer coefficients were measured, and the influences of capsule fractions, heating rates, and flow structures on heat transfer performance were also studied. Heat transfer coefficients measured for MPCM slurry are significantly higher than for those for single-phase fluid flow in laminar flow conditions, but exhibit more complicated phenomena at low turbulent conditions. Moreover, a new simple heat transfer correlation equation was proposed that accurately predicts the local heat transfer coefficients of laminar MPCM slurry flow in a horizontal circular tube.     

Experimental investigation of twisted tape inserts performance on condensation heat transfer enhancement and pressure drop

A comprehensive experimental investigation is conducted on the augmentation of heat transfer coefficients and pressure drop during condensation of HFC-134a in a horizontal tube at the presence of different twisted tape inserts. The test section is a 1.04 m long double-tube counter-flow heat exchanger. The refrigerant flows in the inner copper and the cooling water flows in annulus. The experiments are performed for a plain tube and four tubes with twisted tapes inserts of 6, 9, 12 and 15 twist ratios. The pressure drop is directly measured by a differential pressure transducer. It is found that the twisted tape with twist ratio of 6 gives the highest enhancement in the heat transfer coefficient and the maximum pressure drop compared to the plain tube on a nominal area basis. For this case the enhancement in heat transfer and the pressure drop are increased by 40 and 240% in comparison with to the plain tube. It is observed that the twisted tape with the twist ratio of 9 has the best performance enhancing the heat transfer with the minimum pressure drop. Also empirical correlations are developed to predict smooth tube and swirl flow pressure drop. Predicted results are compared to experimental data and it is found that these correlations are reliable for pressure drop estimation.     

Experimental and numerical study of convective heat transfer and fluid flow in twisted oval tubes

Twisted oval tube heat exchanger is a type of heat exchanger aims at decreasing the pressure drop of the shell side. In the present study, heat transfer and pressure drop performances of twisted oval tube have been studied experimentally and numerically. The experimental study of the twisted oval tube shows that heat transfer process can be enhanced but also with an increasing of pressure drop when compared with the smooth round tube. The effects of geometrical parameters on the performance of the twisted oval tube have been analyzed numerically. The result reveals that the heat transfer coefficient and friction factor both increase with the increasing of axis ratio a/b, while both decrease with the increasing of twist pitch length P. The influence of a/b and P on the overall performance of the twisted oval tubes are also studied. Aiming at obtaining the heat transfer enhancement mechanism of the twisted oval tube, secondary flow, total velocity and temperature distributions of flow section are given. From the analysis it can be concluded that the emergence of twist in the twisted oval tube results in secondary flow. It exists in the form of spiral flow when a/b is big, but in the form of up and down when a/b is small. It is this secondary flow that changes the total velocity and temperature distributions of the twisted oval tube when compared with a smooth oval tube with the same sectional geometric parameters. Then the synergy angle between velocity vector and temperature gradient is reduced and the heat transfer process is enhanced.     

Investigation on heat transfer and pressure drop during swirl flow boiling of R-134a in a horizontal tube

An experimental investigation has been carried out to study the effect of twisted tape inserts on heat transfer enhancement and pressure drop in a horizontal tube during swirl flow boiling of R-134a. The test-evaporator was an electrically heated horizontal copper tube and twisted tapes with different twist ratios of 6, 9, 12 and 15 were inserted one by one. The data were acquired at the refrigerant mass velocities of 54, 86, 114 and 136 kg/s m². The twisted tape inserts increases the boiling heat transfer coefficients and the pressure drop across the test-evaporator. An empirical correlation has also been developed to predict the swirl flow pressure drop in the test-evaporator.     

Studies on heat transfer and friction factor characteristics of thermosyphon solar water heating system with helical twisted tapes

Experimental investigation of heat transfer, friction factor and thermal performance of thermosyphon solar water heater system fitted with helical twisted tape of various twist ratios has been performed and presented. The helical twisted tape induces swirl flow inside the riser tubes, which increases the heat transfer and pressure drop. The empirical correlations developed for Nusselt number and friction factor with various twist ratios (Y = 3, 4, 5, 6) are fitted with the experimental data with a discrepancy of less than ±4.54% and ±6.13% respectively. The results are compared with a plain tube collector at the same operating conditions. Conclusions made from the results show that heat transfer enhancement in twisted tape collector is higher than the plain tube collector with minimum twist ratio and gradually decreases with increase in twist ratio. The overall thermal performance of twisted tape collector is found to increase with increase in solar intensity.     

Heat transfer and pressure drop in a spirally grooved tube with twisted tape insert

In this paper, experimentally determined pressure drop and heat transfer characteristics of flow of water in a 75-start spirally grooved tube with twisted tape insert are presented. Laminar to fully turbulent ranges of Reynolds numbers have been considered. The grooves are clockwise with respect to the direction of flow. Compared to smooth tube, the heat transfer enhancement due to spiral grooves is further augmented by inserting twisted tapes having twist ratios Y ? 10.15, 7.95 and 3.4. It is found that the direction of twist (clockwise and anticlockwise) influences the thermo-hydraulic characteristics. Constant pumping power comparisons with smooth tube characteristics show that in spirally grooved tube with and without twisted tape, heat transfer increases considerably in laminar and moderately in turbulent range of Reynolds numbers. However, for the bare spiral tube and for spiral tube with anticlockwise twisted tape (Y = 10.15), reduction in heat transfer is noticed over a transition range of Reynolds numbers.     

Experimental studies on heat transfer and friction factor characteristics of forced circulation solar water heater system fitted with helical twisted tapes

Experimental investigation of heat transfer, friction factor and thermal performance of twisted tape solar water heater with various twist ratios has been conducted and the results are compared with plain tube collector for the same operating conditions with Reynolds number varied from 3000 to 23,000. Experimental data from plain tube collector is validated with the fundamental equations and found that the discrepancy is less than ±5.35% and ±8.80% for Nusselt number and friction factor, respectively. Correlations have been developed for Nusselt number and friction factor with various twist ratios (Y = 3, 4, 5, 6) and are compared with the experimental values. Results conclude that, heat transfer and pressure drop are higher in twisted tape collector compared to the plain one. Among the various twist ratios, the minimum twist ratio 3 is found to enhance the heat transfer and pressure drop due to swirl generation. As the twist ratio increases, the swirl generation decreases and minimizes the heat transfer and friction factor.     

CFD Simulation of Heat Transfer Augmentation in a Circular Tube Fitted with Alternative Axis Twisted Tape in Laminar Flow under a Constant Heat Flux

CFD simulation of heat transfer and friction factor characteristics in a circular tube fitted with twisted tape inserts in laminar flow and constant heat flux is conducted using FLUENT version 6.3.26. Plain and alternate axis twisted tape inserts with twist ratios (y = 2.93, 3.91, 4.89) and alternative angles (β = 30^°, 60^°, 90^°) have been used for the simulation. The simulated results are matched with the literature correlations for a plain tube with a discrepancy of less than ±8% for Nusselt number and ±6.25% for friction factor. The results also revealed that the heat transfer in terms of the Nusselt number increased with increases of Reynolds number, alternative angles, and decreases of twist ratio. Among the various twist ratios and alternative angles, the twist ratio of y = 2.93 and alternative angle β = 90^° offered a maximum heat transfer enhancement. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(4): 384–396, 2014; Published online 3 October 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21089     

Thermal characteristics in round tube fitted with serrated twisted tape

The article presents an investigation of the effect of twisted tape with serrated-edge insert on heat transfer and pressure loss behaviors in a constant heat-fluxed tube. In the experiments, the serrated twisted tape (STT) was inserted into the entire test tube with a constant twist ratio in order to generate a continuous swirling airflow. Two geometry parameters of the STT to be considered in the present work are the serration width ratio and the serration depth ratio. The measurements have been conducted for the airflow rate based on Reynolds numbers in the turbulent regime from 4000 to 20,000. The experimental results of the STT inserted tube are compared with those of the plain tube fitted with typical twisted tape (TT). The results show that the heat transfer rate in terms of Nusselt number, Nu increases with the rise in the depth ratio but decreases with raising the width ratio. The heat transfer rate is up to 72.2% and 27% relative to the plain tube and the TT inserted tube, respectively. The use of the STT leads to higher heat transfer rate and friction factor than that of the TT for all cases. The thermal performance factor of the STT tube under constant pumping power is evaluated and found to be above unity indicating that using the STT tube is advantageous over the TT tube or the plain tube. In addition, the empirical correlations developed by relating the serration width ratio, serration depth ratio and Reynolds number are determined.     

Compound heat transfer enhancement of a dimpled tube with a twisted tape swirl generator

Friction and compound heat transfer behaviors in a dimpled tube fitted with a twisted tape swirl generator are investigated experimentally using air as working fluid. The effects of the pitch and twist ratio on the average heat transfer coefficient and the pressure loss are determined in a circular tube with the fully developed flow for the Reynolds number in the range of 12,000 to 44,000. The experiments are performed using two dimpled tubes with different pitch ratios of dimpled surfaces (PR = 0.7 and 1.0) and three twisted tapes with three different twist ratios (y/w = 3, 5, and 7). Experiments using plain tube and dimpled tube acting alone are also carried out for comparison. The experimental results reveal that both heat transfer coefficient and friction factor in the dimpled tube fitted with the twisted tape, are higher than those in the dimple tube acting alone and plain tube. It is also found that the heat transfer coefficient and friction factor in the combined devices increase as the pitch ratio (PR) and twist ratio (y/w) decrease. In addition, an empirical correlation based on the experimental results of the present study is sufficiently accurate for prediction the heat transfer (Nu) and friction factor (f) behaviors.     

Investigation of twisted tape inserted solar water heaters—heat transfer, friction factor and thermal performance results

Heat transfer in a solar water heater could be enhanced by means of twisted tapes, inserted inside the fluid flow tubes, which induce swirl flow and act as turbulence promoters. Experimental investigations for a solar water heater with twisted tape inserts having twist pitch to tube diameter ratio ranging from 3–12 have been carried out for varying mass flow rates. The results on heat transfer and friction data have been found to compare well with available results. Within the range of investigated parameters, the heat transfer in the twisted tape insert collectors has been found to increase by 18–70%, whereas the pressure drop increased by 87–132%, as compared to plane collectors. An expression correlating the Nusselt numbers in twisted tape and plane collectors, the twist pitch ratio has been developed in the form of Nu_s/Nu=1.3+2.88/y, which predicts the heat transfer within the range of the present investigation. Results conclude that such collectors would be preferable for higher grade energy collection as it is also at higher rate.Solar water heaters having twisted tape inserts inside the flow tubes perform better than the plane ones. It has been observed that heat losses are reduced (due to the lower value of the plate temperature) consequently increasing the thermal performance by about 30% over the plane solar water heaters under the same operating conditions. The effect of twisted-tape geometry, flow Reynolds number and intensity of solar radiation on the thermal performance of the solar water heater has been presented. It has been found that the twisted-tape collectors perform remarkably better in the lower range of flow Reynolds number (Re≈12,000), beyond which the increase in thermal performance is monotonous. It has also been found that such collectors might perform even better at higher values of intensity of solar radiation.