Heat transfer enhancement of nanofluids in a double pipe heat exchanger with louvered strip inserts

The effect of using louvered strip inserts placed in a circular double pipe heat exchanger on the thermal and flow fields utilizing various types of nanofluids is studied numerically. The continuity, momentum and energy equations are solved by means of a finite volume method (FVM). The top and the bottom walls of the pipe are heated with a uniform heat flux boundary condition. Two different louvered strip insert arrangements (forward and backward) are used in this study with a Reynolds number range of 10,000 to 50,000. The effects of various louvered strip slant angles and pitches are also investigated. Four different types of nanoparticles, Al₂O₃, CuO, SiO₂, and ZnO with different volume fractions in the range of 1% to 4% and different nanoparticle diameters in the range of 20 nm to 50 nm, dispersed in a base fluid (water) are used. The numerical results indicate that the forward louvered strip arrangement can promote the heat transfer by approximately 367% to 411% at the highest slant angle of α = 30° and lowest pitch of S = 30 mm. The maximal skin friction coefficient of the enhanced tube is around 10 times than that of the smooth tube and the value of performance evaluation criterion (PEC) lies in the range of 1.28–1.56. It is found that SiO₂ nanofluid has the highest Nusselt number value, followed by Al₂O₃, ZnO, and CuO while pure water has the lowest Nusselt number. The results show that the Nusselt number increases with decreasing the nanoparticle diameter and it increases slightly with increasing the volume fraction of nanoparticles. The results reveal that there is a slight change in the skin friction coefficient when nanoparticle diameters of SiO₂ nanofluid are varied.     

3D numerical and experimental analysis for thermal–hydraulic characteristics of air flow inside a circular tube with different tube inserts

Numerical and experimental analyses were carried out to study thermal–hydraulic characteristics of air flow inside a circular tube with different tube inserts. Three kinds of tube inserts, including longitudinal strip inserts (both with and without holes) and twisted-tape inserts with three different twisted angles (α = 15.3°, 24.4° and 34.3°) have been investigated for different inlet frontal velocity ranging from 3 to 18 m/s. Numerical simulation was performed by a 3D turbulence analysis of the heat transfer and fluid flow. Conjugate convective heat transfer in the flow field and heat conduction in the tube inserts are considered also. The experiments were conducted in a shell and tube exchanger with overall counterflow arrangement. The working fluid in the tube side was cold air, while the hot Dowtherm fluid was on the shell side. To obtain the heat transfer characteristics of the test section from the experimental data, the ε-NTU (effectiveness-number of transfer unit) method is applied to determine the overall conductance (UA product) in the analysis.It was found that the heat transfer coefficient and the pressure drop in the tubes with the longitudinal strip inserts (without hole) were 7–16% and 100–170% greater than those of plain tubes without inserts. When the longitudinal strip inserts with holes were used, the heat transfer coefficient and the pressure drop were 13–28% and 140–220%, respectively, higher than those of plain tubes. The heat transfer coefficient and the pressure drop of the tubes with twisted-tape inserts were 13–61% and 150–370%, respectively, higher than those of plain tubes. Furthermore, it was found that the reduction ratio in the heat transfer area of the tube of approximately 18–28% may be obtained if the twisted-tape tube inserts are used.     

Experimental study of heat transfer enhancement with wire coil inserts in laminar-transition-turbulent regimes at different Prandtl numbers

Helical-wire-coils fitted inside a round tube have been experimentally studied in order to characterize their thermohydraulic behaviour in laminar, transition and turbulent flow. By using water and water–propylene glycol mixtures at different temperatures, a wide range of flow conditions have been covered: Reynolds numbers from 80 to 90,000 and Prandtl numbers from 2.8 to 150. Six wire coils were tested within a geometrical range of helical pitch 1.17 < p/d < 2.68 and wire diameter 0.07 < e/d < 0.10. Experimental correlations of Fanning friction factor and Nusselt number as functions of flow and dimensionless geometric parameters have been proposed. Results have shown that in turbulent flow wire coils increase pressure drop up to nine times and heat transfer up to four times compared to the empty smooth tube. At low Reynolds numbers, wire coils behave as a smooth tube but accelerate transition to critical Reynolds numbers down to 700. Within the transition region, if wire coils are fitted inside a smooth tube heat exchanger, heat transfer rate can be increased up to 200% keeping pumping power constant. Wire coil inserts offer their best performance within the transition region where they show a considerable advantage over other enhancement techniques.     

Heat transfer and friction factor characteristics in turbulent flow through a tube fitted with perforated twisted tape inserts

This work deals with the experimental investigation on Nusselt number, friction factor and thermal performance factor in a circular tube equipped with perforated twisted tape inserts with four different porosities of R_p = 1.6, 4.5, 8.9 and 14.7%. The experiments were conducted in a turbulent flow regime with Reynolds number ranging from 7200 to 49,800 using air as the working fluid under uniform wall heat flux boundary condition. The experimental results revealed that both heat transfer rate and friction factor of the tube fitted with perforated twisted tapes were significantly higher than those of the plain tube. Over the range investigated, Nusselt number, friction factor and thermal performance factor in the tube with perforated twisted tape inserts was found to be 110 –340, 110 –360 and 28–59% higher than those of the plain tube values, respectively. In addition, the empirical correlations of Nusselt number, friction factor and thermal performance factor were formulated from the experimental results of tape inserts.     

Experimental studies on heat transfer and friction factor characteristics of turbulent flow through a circular tube fitted with regularly spaced helical screw-tape inserts

Experimental investigation of heat transfer and friction factor characteristics of circular tube fitted with full-length helical screw element of different twist ratio, and helical screw inserts with spacer length 100, 200, 300, and 400 mm have been studied with uniform heat flux under turbulent flow condition. The experimental data obtained are verified with those obtained from plain tube published data. The effect of spacer length on heat transfer augmentation and friction factor, and the effect of twist ratio on heat transfer augmentation and friction factor have been presented separately. The decrease in Nusselt number for the helical twist with spacer length is within 10% for each subsequent 100 mm increase in spacer length. The friction factor for helical twist insert with spacer length 100 mm is very much close to value of friction factor for full-length helical twist for all Reynolds number and decreases by 5% for each 100 mm increment space length indicating that there is no much reduction in pumping power. Hence the helical screw inserts with spacer can only be used for heat augmentation only in turbulent flow with less reduction in pumping power. Empirical correlation were formed for explaining data and found to fit experimental data within ±10%, and ±20%, respectively, for Nusselt number and friction factor.     

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.     

Turbulent flow heat transfer and pressure loss in a double pipe heat exchanger with louvered strip inserts

In the present work, heat transfer and friction characteristics were experimentally investigated, employing louvered strips inserted in a concentric tube heat exchanger. The louvered strip was inserted into the tube to generate turbulent flow which helped to increase the heat transfer rate of the tube. The flow rate of the tube was in a range of Reynolds number between 6000 and 42,000. The turbulent flow devices were consisted of (1) the louvered strips with forward or backward arrangements, and (2) the louvered strip with various inclined angles (θ = 15°, 25° and 30°), inserted in the inner tube of the heat exchanger. In the experiment, hot water was flowed through the inner tube whereas cold water was flowed in the annulus. The experimental data obtained were compared with those from plain tubes of published data. Experimental results confirmed that the use of louvered strips leads to a higher heat transfer rate over the plain tube. The increases in average Nusselt number and friction loss for the inclined forward louvered strip were 284% and 413% while those for the backward louvered strip were 263% and 233% over the plain tube, respectively. In addition, the use of the louvered strip with backward arrangement leads to better overall enhancement ratio than that with forward arrangement around 9% to 24%.     

Heat transfer augmentation through the use of wire-rod bundles under constant wall heat flux condition

This article reports an experimental investigation on heat transfer, friction factor and thermal performance characteristics of turbulent flow (6000 ≤ Re ≤ 20,000) in heat exchanger tubes with wire-rod bundles as flow turbulators. The experiments were carried out at three different pitch ratios (P/D) of 1.0, 1.5 and 2.0 and three wire-rod number per bundle (N) of 4, 6 and 8. The experimental results show that Nusselt number increases with increasing Reynolds number and wire-rod number per bundle, and decreasing pitch ratio (P/D) of the turbulators. As compared to the results of the tube without wire-rod (the plain tube), heat transfer rate and friction factor are respectively increased in ranges of 3.5 to 68.8% and 156 to 353%, depending on the operating conditions. At the same pumping power, the use of wire-rod turbulators results in thermal performance factor up to 1.02 times of those of the plain tube. In addition, the correlations that developed from the present experimental data for Nusselt number, friction factor and thermal performance factor are also presented.     

Experimental investigation of heat transfer in a triple tube heat exchanger with coiled spring inserts

Experiments have been performed to investigate the effect of coiled spring inserts on heat transfer, pressure drop, and performance parameters of a triple tube heat exchanger (TTHX). Three different spring inserts having a pitch of 5, 10, and 15 mm are used and the diameter of the spring wire is taken as 1 mm. The experiments were carried out under a turbulent flow regime, with water as a working medium in parallel and counter flow configurations. The variation in different performance characteristics like heat transfer coefficient, Nusselt number, and effectiveness have been compared at various Reynolds numbers ranging between 4000 and 16,000 in the considered flow patterns. The Nusselt number of TTHX with the lowest pitch spring is found to be higher than that of the plain TTHX by 57.27% at Re = 4000 for the counter flow configuration. Both the thermal performance factor and effectiveness increased as the pitch of the spring insert was decreased. The effectiveness of TTHX with the lowest pitch spring insert is found higher than that of the plain TTHX by 43.84% in the counter flow pattern.     

Turbulent heat transfer and friction factor of Al2O3 Nanofluid in circular tube with twisted tape inserts

The thermophysical properties like thermal conductivity and viscosity of Al₂O₃ nanofluid is determined through experiments at different volume concentrations and temperatures and validated. Convective heat transfer coefficient and friction factor data at various volume concentrations for flow in a plain tube and with twisted tape insert is determined experimentally for Al₂O₃ nanofluid. Experiments are conducted in the Reynolds number range of 10,000–22,000 with tapes of different twist ratios in the range of 0 < H/D < 83. The heat transfer coefficient and friction factor of 0.5% volume concentration of Al₂O₃ nanofluid with twist ratio of five is 33.51% and 1.096 times respectively higher compared to flow of water in a tube. A generalized regression equation is developed for the estimation of Nusselt number and friction factor valid for both water and nanofluid in plain tube and with inserts under turbulent flow conditions.     

Experimental studies on heat transfer and friction factor characteristics of turbulent flow through a circular tube with small pipe inserts

Heat transfer performance and pressure drop tests were performed on a circular tube with small pipe inserts. These inserts with different spacer lengths (S = 100, 142.9 and 200 mm) and arc radii (R = 5, 10 and 15 mm) were tested at Reynolds numbers between 4000 and 18,000. Tap water was used as working fluid. The use of pipe inserts allowed for a high heat transfer coefficient with relatively low flow resistance. The Nusselt number and friction factor increase with the decrease in spacer length. Optimal results were obtained for S = 100 mm (R = 10 mm). Heat transfer rates and friction factors were enhanced by 2.09–2.67 and 1.59–1.85 times, respectively, to those in the plain tube. Performance evaluation criterion (PEC) values were approximately 1.79–2.17. The Nusselt number and friction factor increase with the decrease in arc radius. Small pipe inserts with R = 5 mm and S = 100 mm show maximal heat transfer rates of 2.61–3.33 and friction factors of 1.6–1.8 times those of the empty tube. The PEC values were 2.23–2.7. Compared with other inserts, pipe inserts can transfer more heat for the same pumping power for their unique structure.     

Pressure drop and heat transfer augmentation due to coiled wire inserts during laminar flow of oil inside a horizontal tube

An experimental investigation has been carried out to study the enhancement in heat transfer coefficient by coiled wire inserts during heating of engine oil inside a horizontal tube. The test-section was a double-pipe counter-flow heat exchanger. The engine oil flowed inside the internal copper tube, while saturated steam, used for heating the oil, flowed in the annulus. First of all, the data were acquired for the heating of engine oil while flowing in the plain tube. Later, seven coiled wires having pitches of 12–69 mm and wire diameters of 2.0 mm and 3.5 mm were put one by one in the oil-side of test-section. The effects of Reynolds number and coiled wire geometry on the heat transfer augmentation and fanning friction factor were studied. Finally, two empirical correlations have been developed for predicting the heat transfer enhancement of these coiled wire inserts. These correlations predict the experimental Nusselt number in an error band of ±20 percent.     

Experimental investigation on heat transfer enhancement of a tube with coiled-wire inserts installed with a separation from the tube wall

An experimental investigation was conducted for the thermo-hydraulic performance of a circular tube with coiled-wire inserts which were installed with a small separation from the inner wall of the tube. The wire inserts had an equilateral triangular cross-sections with a constant side length of e = 6 mm and they were coiled with three different pitch-to-diameter ratios: P/D = 1, P/D = 2, P/D = 3. A specific method was employed to coil the wires so that an edge of the triangle was oriented to face the incoming air flow. The coiled-wire inserts were installed with 1 and 2 mm separation from the inner tube wall so that the heat transfer enhancement due to the laminar boundary layer disturbance could be investigated. Experiments were performed for Reynolds numbers from 3429 to 26,663. The coiled-wire inserts led to a significant increase in both the heat transfer rate and friction factor over the smooth tube based on coil pitches and clearance. The maximum thermal performance was observed around 1.82 for the P/D = 1, s = 1 type at a Reynolds number of 3429. In conclusion, the laminar boundary layer disturbance can be effectively enhanced by using these types of coiled-wire inserts.     

Effect of flow geometry parameters on transient heat transfer for turbulent flow in a circular tube with baffle inserts

The effect of the flow geometry parameters on transient forced convection heat transfer for turbulent flow in a circular tube with baffle inserts has been investigated. The characteristic parameters of the tubes are pitch to tube inlet diameter ratio H/D = 1, 2 and 3, baffle orientation angle β = 45°, 90° and 180°. Air, Prandtl number of which is 0.71, was used as working fluid, while stainless steel was considered as pipe and baffle material. During the experiments, different geometrical parameters such as the baffle spacing H and the baffle orientation angle β were varied. Totally, nine types of baffle inserted tube were used. The general empirical equations of time averaged Nusselt number and time averaged pressure drop were derived as a function of Reynolds number corresponding to the baffle geometry parameters of pitch to diameter ratio H/D, baffle orientation angle β, ratio of smooth to baffled cross-section area S_o/S_a and ratio of tube length to baffle spacing L/H were derived for transient flow conditions. The proposed empirical correlations were considered to be applicable within the range of Reynolds number 3000 ⩽ Re ⩽ 20,000 for the case of constant heat flux.     

Numerical investigation on heat transfer and friction factor characteristics of laminar and turbulent flow in an elliptic annulus utilizing nanofluid

In this paper, a numerical investigation on heat transfer performance and flow fields of different nanofluids flows through elliptic annulus in a laminar and turbulent flow regimes. The three-dimensional continuity, Navier–Stokes and energy equations are solved by using finite volume method (FVM) and the SIMPLE algorithm scheme is applied to examine the effects of laminar and turbulent flow on heat transfer characteristics. This study evaluates the effects of four different types of nanoparticles, Al₂O₃, CuO, SiO₂ and ZnO, with different volume fractions (0.5–4%) and diameters (25–80 nm) under constant heat flux boundary condition using water as a base fluid were used. The Reynolds number of laminar flow was in the range of 200 ≤ Re ≤ 1500, while for turbulent flow it was in the range of 4000 ≤ Re ≤ 10,000. The results have shown that SiO₂–water nanofluid has the highest Nusselt number, followed by ZnO–water, CuO–water, Al₂O₃–water, and lastly pure water. The Nusselt number for all cases increases with the volume fraction but it decreases with the rise in the diameter of nanoparticles. In all configurations, the Nusselt number increases with Reynolds number. It is found that the glycerine–SiO₂ shows the best heat transfer enhancement compared with other tested base fluids.     

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.     

Heat transfer enhancements of low volume concentration Al2O3 nanofluid and with longitudinal strip inserts in a circular tube

The turbulent convective heat transfer and friction factor behavior of Al₂O₃ nanofluid in a circular tube with different aspect ratios of longitudinal strip inserts are studied experimentally. Experiments are conducted with water and nanofluid in the range of 3000 < Re < 22,000, particle volume concentration 0 < φ < 0.5% and longitudinal strip aspect ratios of 0 < AR < 18. The agreement between the values of Nusselt number obtained with water is satisfactory when compared with the data of Heish and Huang. Results indicate that heat transfer coefficients increase with nanofluid volume concentration and decrease with aspect ratio.     

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.     

Turbulent flow and heat transfer in discrete double inclined ribs tube

The turbulent heat transfer and flow resistance in an enhanced heat transfer tube, the DDIR tube, were studied experimentally and numerically. Water was used as the working fluid with Reynolds numbers between 15,000 and 60,000. The numerical simulations solved the three dimensional Reynolds-averaged Navier–Stokes equations with the standard k-ε model in the commercial CFD code, Fluent. The numerical results agree well with the experimental data, with the largest discrepancy of 10% for the Nusselt numbers and 15% for the friction factors. The heat transfer in the DDIR tube is enhanced 100 ～ 120% compared with a plain tube and the pressure drop is increased 170 ～ 250%. The heat transfer rate for the same pumping power is enhanced 30 ～ 50%. Visualization of the flow field shows that in addition to the front and rear vortices around the ribs, main vortices and induced vortices are also generated by the ribs in the DDIR tube. The rear vortex and the main vortex contribute much to the heat transfer enhancement in the DDIR tubes. Optimum DDIR tube parameters are proposed for heat transfer enhancement at the same pumping power.     

Numerical investigation of fluid flow and heat transfer over louvered fins in compact heat exchanger

Numerical investigation of fluid flow and heat transfer characteristics over louvered fins and flat tube in compact heat exchangers is presented in this study. Three-dimensional simulations of single and double row tubes with louvered fins have been conducted. Simulations are performed for different geometries with varying louver pitch, louver angle, fin pitch and tube pitch and for different Reynolds number. Conjugate heat transfer and conduction through the fins are considered. The air-side performance of heat exchanger is evaluated by calculating Stanton number and friction factor. The results are compared with experiment and a good agreement is observed. The local Nusselt number variation along the top surface of the louver is calculated and effects of geometrical parameters on the average heat transfer coefficient is computed. Design curves are obtained which can used to predict the heat transfer and the pressure drop for a given louver geometry.