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).     

Study on thermal and fluid flow characteristics in turbulent channel flows with multiple twisted tape vortex generators

This research has been performed to study the influences of multiple twisted tape vortex generators (MT-VG) on the heat transfer and fluid friction characteristics in a rectangular channel. The experiments conducted using the twisted tapes with three different twist ratios (y/w = 2.5, 3.0 and 3.5) for generating different swirl and turbulent intensities in the channel. The twisted tapes are assembled to obtained the MT-VG with three different free-spacing ratios, s/w = 1.66 (5 tapes), s/w = 1.25 (7 tapes) and s/w = 1.0 (9 tapes). The results for the Reynolds number ranged from 2700 to 9000 at constant Prandtl number, Pr = 0.7, using air as the test fluid, are examined. In the studied range, the presence of channel with MT-VG leads to increase in heat transfer rate over the use of smooth channel around 10.3 to 169.5%. The channel with the smaller twist ratio (y/w) and free-spacing ratio (s/w) provides higher heat transfer rate and pressure loss than those with the larger of twist ratio and free-spacing ratio under similar operation conditions. In addition, correlations of Nusselt number (Nu) and friction factor (f) have been developed and the thermal enhancement index at constant pumping power is also 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.     

Performance assessment in a heat exchanger tube with alternate clockwise and counter-clockwise twisted-tape inserts

The article presents an experimental study of turbulent heat transfer and flow friction characteristics in a circular tube equipped with two types of twisted tapes: (1) typical twisted tapes and (2) alternate clockwise and counterclockwise twisted tapes (C–CC twisted tapes). Nine different C–CC twisted tapes are tested in the current work; they included the tapes with three twist ratios, y/w = 3.0, 4.0 and 5.0, each with three twist angles, θ = 30^o, 60^o and 90^o. The experiments have been performed over a Reynolds number range of 3000–27,000 under uniform heat flux conditions, using water as working fluid. The obtained results reveal that the C–CC twisted-tapes provide higher heat transfer rate, friction factor and heat transfer enhancement index than the typical twisted-tapes at similar operating conditions. The results also show that the heat transfer rate of the C–CC tapes increases with the decrease of twist ratio and the increase of twist angle values. Depending on Reynolds number, twist ratio and twist angle values, the mean Nusselt numbers in the tube fitted with the C–CC twisted tapes are higher than those with the typical ones and the plain tube around 12.8–41.9% and 27.3–90.5%, respectively. The maximum heat transfer enhancement indexes of the C–CC twisted tapes with θ = 90^o for y/w = 3.0, 4.0 and 5.0, are 1.4, 1.34 and 1.3, respectively. In addition, correlations of the Nusselt number and the friction factor for using the C–CC twisted tapes are also determined. Both predicted Nusselt number and friction factor are within ±15% and ±15% deviation compared to the experimental data.     

Numerical modeling and optimization of perforated twisted tape insert based on hybrid ANSYS-RSM approach

Heat exchangers have a wider scope in numerous applications, such as space heating, chemical industries, power stations, and so on. Due to heat loss and the thermal properties of the materials involved, the overall performance of a heat exchanger is questionable. Therefore, studies related to heat transfer techniques are appreciated in the research community. Thus, the present study numerically investigates the heat transfer performance of a horizontal heat pipe equipped with hexagonal perforated twisted tape inserts. Further, the numerical solutions of Nusselt number (Nu), friction factor (f), and thermal performance factor (TPF) are optimized with the help of response surface methodology (RSM). The parameters investigated in this study are Reynolds number (Re), which varies between 500 and 1500, input heat supply (Q) between 100 and 1000 W, and pitch ratio (3.2, 4, and 5.2). ANSYS fluid flow fluent was used to perform flow simulations for three different twisted tape inserts: horizontal, vertical, and alternate hexagon perforations. Optimum solutions are obtained at 1000 W heat supply, 1500 Re, and p/d_i = 4 from alternatively perforated twisted tape inserts. The optimum Nu, f, and TPF achieved are 119.545, 0.437, and 1.82, respectively. Also, the proposed RSM optimization method is evidenced with a maximum of 2.673% error during the confirmatory test.     

Convective heat transfer in a circular tube with short-length twisted tape insert

This work presents an experimental study on the mean Nusselt number, friction factor and enhancement efficiency characteristics in a round tube with short-length twisted tape insert under uniform wall heat flux boundary conditions. In the experiments, measured data are taken at Reynolds numbers in a turbulent region with air as the test fluid. The full-length twisted tape is inserted into the tested tube at a single twist ratio of y/w = 4.0 while the short-length tapes mounted at the entry test section are used at several tape length ratios (LR = l_s/l_f) of 0.29, 0.43, 0.57 and 1.0 (full-length tape). The short-length tape is introduced as a swirling flow device for generating a strong swirl flow at the tube entry before decaying along the tube. On the other hand, the full-length tape (LR = 1.0) is expected to produce a strongly swirling flow over the whole tube. The variation of heat transfer and pressure loss in the form of Nusselt number (Nu) and friction factor (f) respectively is determined and depicted graphically. The experimental result indicates that the short-length tapes of LR = 0.29, 0.43 and 0.57 perform lower heat transfer and friction factor values than the full-length tape around 14%, 9.5% and 6.7%; and 21%, 15.3% and 10.5%, respectively. In addition, it is apparent that the enhancement efficiency of the tube with the short-length tape insert is found to be lower than that with the full-length one. The mean deviation between measured and correlated values of the Nusselt number is in the order of ± 7% in the range of Reynolds numbers from 4000 to 20,000.     

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.     

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 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.     

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.     

Heat transfer enhancement by helically twisted tapes inducing co- and counter-swirl flows

An experimental investigation has been conducted for determining heat transfer enhancement by inserting helically twisted tapes, to induce co- and counter-swirl flows, (the tapes are symbolized as co-HTT and C-HTT, respectively). Tape pitch ratio (p/D) was varied between 1.0 and 2.0, while tape width ratio (w/D) and twist ratio (y/w) were fixed at 0.2 and 3.0, respectively. The experiments were performed for fully developed turbulent flow with Reynolds number range (Re) between 6000 and 20,000, under uniform wall heat flux condition. At similar conditions, the use of Co-HTT results in lower Nusselt number and friction factor but higher thermal performance factor than that of C-HTT. Nusselt number and friction factor increase with decreasing pitch ratio, while thermal performance factor possesses opposite trend. In addition, the empirical correlations for Nusselt number, friction factor and thermal performance factor as functions of the Reynolds number (Re), Prandtl number (Pr) and tape pitch (p/D), were developed through a multi-variant linear normal regression.     

Three-dimensional laminar slip-flow and heat transfer in a rectangular microchannel with constant wall temperature

Three-dimensional laminar slip-flow and heat transfer in rectangular microchannels having constant temperature walls are studied numerically using the finite-volume method for thermally and simultaneously developing flows. The Navier–Stokes and energy equations are solved with velocity slip and temperature jump at the wall. A modified convection–diffusion coefficient at the wall–fluid interface is defined to incorporate the temperature-jump boundary condition. Validity of the numerical simulation procedure is established and the effect of rarefaction on hydrodynamicaly developing flow field, pressure gradient and entrance length is analyzed. A correlation for the fully developed friction factor is presented as a function of Knudsen number (Kn) and aspect ratio (α). The influence of rarefaction on the Nusselt (Nu) number is investigated for thermally and simultaneously developing flows. The effect of velocity slip is found to increase the Nu number, while the temperature-jump tends to decrease it, and the combined effect could result in an increase or a decrease in the Nu number. In the fully developed region, there could be high as 15% increase or low as 50% decrease in Nu number is plausible for the range of parameters considered in this work.     

Forced convective heat transfer by swirling impinging jets issuing from nozzles equipped with twisted tapes

Forced convective heat transfer on the impinged plate associated with swirling impinging jets (SIJ) issuing from nozzles inserted by twisted tapes has been investigated. Swirling impingement jets with several swirl rates were generated by twisted tapes at different twist ratios (y/W = 3, 4, 5, and 6). The experiments were performed by locating nozzle at 4 different jet-to-plate spacings of L/D = 2, 4, 6 and 8. A jet Reynolds number varied between 4000 and 16000. Attributing to a high momentum transfer rate, an efficient heat transfer was obtained by using the jet with a small jet-to-plate spacing and the twist tape with a large twist ratio at high Reynolds number. At small jet-to-plate spacings (L/D = 2 and 4), swirling impinging jets gave higher heat transfer rate than conventional impinging jets while at large jet-to-plate spacings (L/D = 6 and 8), the opposed result was obtained. Over the range examined, only SIJ induced by the twisted tape at a twist ratio of 6 consistently provided higher average Nusselt numbers than CIJ.     

Heat transfer and pressure drop in rectangular channel with compound roughness of V-shaped ribs and deepened scales

A novel heat transfer enhancement (HTE) roughness with V-shaped ribs and deepened scales is devised. Performances of heat transfer and pressure drop in a rectangular channel fitted with such HTE surfaces are experimentally examined for both forward and backward flows in the Re range of 1000–30000. Relative to the smooth-walled pipe flow conditions, HTE ratios for the present test channel with forward and backward flows, respectively, reach 9.5–13.6 and 9–12.3 for laminar flows and 6.8–6.3 and 5.7–4.3 for turbulent flows. Comparisons of heat transfer data, pressure-drop measurements and thermal performance factors with previous results collected from varieties of HTE devices demonstrate the superiorities of this compound HTE device. The decrease of HTE ratio as Re increases for turbulent flows, which is a common setback for several HTE elements, is almost diminished in the channel fitted with present compound HTE surfaces. Experimental correlations of heat transfer and friction coefficient for the tested channel with forward and backward flows are derived for design applications.     

Parametric study on the heat transfer characteristics in a circular tube with helical tape insert under laminar flow conditions

In the present study, the effect of a helical coiled tape on the heat transfer characteristics in a tubular heat exchanger has been studied. Numerical analysis is carried out in a tubular heat exchanger with a helical coiled tape insert under laminar flow conditions. The heat transfer characteristics, like Nusselt number (Nu) and friction factor (f), have been determined. Numerical modeling is undertaken and validated using analytical results. To study the parametric variation, simulation is carried out for different width ratios (WR) and twist ratios (TRs). It is observed that Nu increases as the WR is increased and the TR is reduced. The thermohydraulic performance index (THPI) is measured and found to be higher for a combination of a higher WR and a higher TR. From the tested values, it is seen that for a combination of WR = 3 and TR = 1.25, the maximum THPI was observed. It is concluded that both the tape width and pitch will significantly influence the thermohydraulic characteristics in a tubular heat exchanger with a helical tape insert inserted.     

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%。     

Numerical investigation on the heat transfer characteristics in a circular pipe using multiple twisted tapes in laminar flow conditions

Introducing passive devices in the form of inserts helps to increase the heat transfer characteristics in a circular pipe. In the present work, the feasibility of using two twisted tapes has been numerically studied under laminar flow conditions. Two twisted tapes with configurations of co‐swirl and counter‐swirl conditions were simulated for Re ranging from 1000 to 2500. Heat transfer characteristics like Nusselt number, friction factor, and thermohydraulic performance index (THPI) were investigated. Even the effect of spacing of the tubes inside the pipe was numerically studied. Results indicated that the insertion of two tapes will improve the performance of the circular tube compared with a single tape in terms of a higher Nu and higher THPI. It is observed that for the co‐swirl condition for a Reynolds number of 1000, Nu and THPI is 38% and 29% higher than using a single tape, whereas, for the counter‐swirl condition, they are 43% and 34% higher than that of the single tape condition. Also, it is revealed that counter swirl positioning of the tape with an L/D ratio 0.56 outperformed all other configurations with the highest THPI.     

Heat transfer of rectangular narrow channel with two opposite scale-roughened walls

An experimental study of heat transfer and pressure drop in a rectangular channel roughened by scaled surfaces on two opposite walls with flows directed in the forward and downward directions for Reynolds numbers (Re) in the range of 1500 ⩽ Re ⩽ 15,000 was performed. Nusselt number ratios between the scale-roughened and smooth-walled ducted flows (Nu/Nu_∞) were in the range of 7.4–9.2 and 6.2–7.4 for laminar forward and downward flows respectively. The Nu/Nu_∞ values for turbulent developed flows in the scale-roughened channel with forward and downward flows were about 4.5 and 3 respectively. A comparison of present data with reported results using different types of surface roughness demonstrated the better thermal performances of present scale-roughened channel with forward flow at conditions of Re > 10,000. Experimental correlations of heat transfer and friction coefficient were derived for the present scale-roughened rectangular channel.     

3-D Numerical simulation of swirling flow and convective heat transfer in a circular tube induced by means of loose-fit twisted tapes

The article presents the application of a mathematical model for simulation of the swirling flow in a tube induced by loose-fit twisted tape insertion. Effects of the clearance ratio defined as ratio of clearance between the edge of tape and tube wall to tube diameter (CR = c/D = 0.0 (tight-fit), 0.1, 0.2 and 0.3) on heat transfer enhancement (Nu), friction factor (f) and thermal performance factor (η) are numerically investigated for twisted tapes at two different twist ratios (y/w = 2.5 and 5.0). The simulation is conducted in order to gain an understanding of physical behavior of the thermal and fluid flow in the tube fitted with loose-fit twisted tape under constant wall temperature conditions in the turbulent flow regime for the Reynolds number ranging from 3000 to 10,000. The Navier–Stokes equation in common with a energy equation is solved using the SIMPLE technique with the standard k–ε turbulence model, the Renormalized Group (RNG) k–ε turbulence model, the standard k–ω turbulence model, and Shear Stress Transport (SST) k–ω turbulence model. The numerical results show that the predictions of heat transfer (Nu) and friction factor (f) based on the SST k–ω turbulence models are in better agreement with Manglik and Bergles [R.M. Manglik, A.E. Bergles, Heat transfer and pressure drop correlations for twisted-tape inserts in isothermal tubes, part II: Transition and turbulent flows, Transaction ASME, Journal of Heat Transfer, 115 (1993) 890–896.] than other turbulence models. The mean flow patterns in a tube with loose-fit twisted tapes in terms of contour plots of velocity, pathline, pressure, temperature and turbulent kinetics energy (TKE) are presented and compared with those in a tube fitted with tight-fit twisted tapes. It is visible that the twisted tape inserts for y/w = 2.5 with CR = 0.0 (tight-fit), 0.1, 0.2 and 0.3 can enhance heat transfer rates up to 73.6%, 46.6%, 17.5% and 20%, respectively and increase friction factors up to 330%, 262%, 189%, and 160%, respectively, in comparison with those of the plain tube. The tube with loose-fit twisted tape inserts with CR = 0.1, 0.2 and 0.3 provide heat transfer enhancement around 15.6%, 33.3% and 31.6% lower than those with CR = 0.0 (the tight-fit twisted tape). The heat transfer augmentation is expected to involve the swirl flow formation between the tape and a tube wall. In addition, the simulation for thermal performance factor (η) of a tube with the loose-fit twisted tape and the tight-fit twisted tape under the same pumping power is also conducted, for comparison.