Heat transfer enhancement in a tube using delta-winglet twisted tape inserts

Heat transfer, flow friction and thermal performance factor characteristics in a tube fitted with delta-winglet twisted tape, using water as working fluid are investigated experimentally. Influences of the oblique delta-winglet twisted tape (O-DWT) and straight delta-winglet twisted tape (S-DWT) arrangements are also described. The experiments are conducted using the tapes with three twist ratios (y/w = 3, 4 and 5) and three depth of wing cut ratios (DR = d/w = 0.11, 0.21 and 0.32) over a Reynolds number range of 3000–27,000 in a uniform wall heat flux tube. The obtained results show that mean Nusselt number and mean friction factor in the tube with the delta-winglet twisted tape increase with decreasing twisted ratio (y/w) and increasing depth of wing cut ratio (DR). It is also observed that the O-DWT is more effective turbulator giving higher heat transfer coefficient than the S-DWT. Over the range considered, Nusselt number, friction factor and thermal performance factor in a tube with the O-DWT are, respectively, 1.04–1.64, 1.09–1.95, and 1.05–1.13 times of those in the tube with typical twisted tape (TT). Empirical correlations for predicting Nusselt number and friction factor have been employed. The predicted data are within ±10% for Nusselt number and ±10% for friction factor.     

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.     

Experimental investigations on heat transfer and frictional characteristics of a turbulator roughened solar air heater duct

An experimental investigation has been carried out to study the heat transfer coefficient and friction factor by using artificial roughness in the form of specially prepared inverted U-shaped turbulators on the absorber surface of an air heater duct. The roughened wall is uniformly heated while the remaining three walls are insulated. These boundary conditions correspond closely to those found in solar air heaters.The experiments encompassed the Reynolds number range from 3800 to 18000; ratio of turbulator height to duct hydraulic mean diameter is varied from, e/D_h = 0.0186 to 0.03986 (D_h = 37.63 mm and e = 0.7 to 1.5 mm) and turbulator pitch to height ratio is varied from, p/e = 6.67 to 57.14 (p = 10 to 40 mm). The angle of attack of flow on turbulators, α = 90° kept constant during the whole experimentation. The heat transfer and friction factor data obtained is compared with the data obtained from smooth duct under similar geometrical and flow conditions. As compared to the smooth duct, the turbulator roughened duct enhances the heat transfer and friction factor by 2.82 and 3.72 times, respectively. The correlations have been developed for area averaged Nusselt number and friction factor for turbulator roughened duct.     

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.     

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 study of the effect of winglet location on heat transfer enhancement and pressure drop in fin-tube heat exchangers

Local heat transfer coefficients were measured on fin-tube heat exchanger with winglets using a single heater of 2 inch diameter and five different positions of winglet type vortex generators. The measurements were made at Reynolds number about 2250. Flow losses were determined by measuring the static pressure drop in the system. Results showed a substantial increase in the heat transfer with winglet type vortex generators. It has been observed that average Nusselt number increases by about 46% while the local heat transfer coefficient improves by several times as compared to plain fin-tube heat exchanger. The maximum improvement is observed in the re-circulation zone. The best location of the winglets was with ΔX = 0.5D and ΔY = 0.5D. The increase in pressure drop for the existing situation was of the order of 18%.     

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.     

Development of correlations for Nusselt number and friction factor for solar air heater with roughened duct having multi v-shaped with gap rib as artificial roughness

This paper presents the results of an experimental investigation of heat transfer and friction in the flow of air in rectangular ducts having multi v-shaped rib with gap roughness on one broad wall. The investigation encompassed Reynolds number (Re) from 2000 to 20,000, relative gap distance (G_d/L_v) values of 0.24–0.80, relative gap width (g/e) values of 0.5–1.5, relative roughness height (e/D) values of 0.022–0.043, relative roughness pitch (P/e) values of 6–12, relative roughness width ratio (W/w) values of 1–10, angle of attack (α) range of 30°–75°. The optimum values of geometrical parameters of roughness have been obtained and discussed. For Nusselt number (Nu), the maximum enhancement of the order of 6.74 times of the corresponding value of the smooth duct has been obtained, however the friction factor (f) has also been seen to increase by 6.37 times of that of the smooth duct. The rib parameters corresponding to maximum increase in Nu and f were G_d/L_v = 0.69, g/e = 1.0, e/D = 0.043, P/e = 8, W/w = 6 and α = 60°. Based on the experimental data, correlations for Nu and f have been developed as function of roughness parameters of multi v-shaped with gap rib and flow Reynolds number.     

Heat transfer augmentation in a circular tube with perforated double counter twisted tape inserts

The present study explored the effects of perforated double counter twisted tapes on heat transfer and fluid friction characteristics in a heat exchanger tube. The twisted tapes with four different porosities of R_p = 1.2, 4.6, 10.4 and 18.6% were used as counter-swirl flow generators in the test section. The experiments were conducted in a circular tube in turbulent flow regime with Reynolds number ranging from 7200 to 50,000 using air as the working fluid under uniform wall heat flux boundary condition. The experimental results demonstrated that the Nusselt number, friction factor and thermal enhancement efficiency were increased with decreasing porosity except porosity of 1.2%. The results also revealed that the heat transfer rate of the tube fitted with tapes were significantly increased with corresponding increase in friction factor. In the range of the present investigation, heat transfer rate and friction factor were obtained to be around 80 to 290% and 111 to 335% higher than those of the plain tube values, respectively. Based on constant blower power, the highest thermal enhancement efficiency of 1.44 was achieved. In addition, the empirical correlations of Nusselt number, friction factor and thermal enhancement efficiency were developed based on the experimental data.     

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.     

Heat transfer enhancement by flow bifurcations in asymmetric wavy wall channels

The enhancement characteristics of heat transfer, through a transition scenario of flow bifurcations, in asymmetric wavy wall channels, are investigated by direct numerical simulations of the mass, momentum and energy equations, using the spectral element method. The heat transfer characteristics, flow bifurcation and transition scenarios are determined by increasing the Reynolds numbers for three geometrical aspect ratios r = 0.25, 0.375, and 0.5, and Prandtl numbers 1.0 and 9.4. The transition scenarios to transitional flow regimes depend on the aspect ratio. For the aspect ratios r = 0.25 and 0.5, the transition scenario is characterized by one Hopf flow bifurcation. For the aspect ratio r = 0.375, the transition scenario is characterized by a first Hopf flow bifurcation from a laminar to a periodic flow, and a second Hopf flow bifurcation from a periodic to quasi-periodic flow. The periodic and quasi-periodic flows are characterized by fundamental frequencies ω₁, and ω₁ and ω₂, respectively. For all the aspect ratios and Prandtl numbers, the time-average mean Nusselt number and heat transfer enhancement increases with the Reynolds number as the flow evolves from a laminar to a transitional regime. For both Prandtl numbers, the highest increase in the Nusselt number occurs for the aspect ratio r = 0.5; whereas, the lowest increases happen to r = 0.25. The increase of the Nusselt number occurs at the expense of a higher pumping power, which, for both Prandtl numbers, grows as the aspect ratio increases from r = 0.25 to r = 0.5 for reaching a specific Nusselt number. This enhancement is obtained without the necessity of high volumetric flow rates associated with turbulent flow regimes, which demand much higher pumping powers. Significant heat transfer enhancements are obtained when the asymmetric wavy channel is operated in the appropriate transitional Reynolds number range.     

Confined,milliscale unsteady laminar impinging slot jets and surface Nusselt numbers

Different modes of unsteadiness which develop within confined, laminar impinging slot jets of millimeter-scale are considered, including experimental measurements and numerical predictions of different flow characteristics, including spatially-resolved distributions of local Nusselt numbers measured on a constant heat flux surface. The effects of Reynolds number, and nozzle-to-plate distance on the local Nusselt number are investigated for a slot nozzle width B of 1.0 mm, Reynolds numbers Re from 120 to 200, nozzle-to-plate distances H/B from 0.75 to 12.5, and a nozzle aspect ratio y/B of 50. Observed are several different types of unsteady slot jet behavior, including: (i) a flow fluctuations/flapping motion mode of unsteadiness which is present for B = 1.0 mm, 4.75 ? H/B ? 5.5, and 120 ? Re ? 140, (ii) an intermittent flapping motion of the jet column which is present for B = 1.0 mm, 9 ? H/B ? 11.25, and 120 ? Re ? 200, and (iii) a continuous sinusoidal oscillation state, which is observed to be present for Re = 160 and H/B = 10. The flow fluctuations/flapping motion mode of unsteadiness, and the intermittent flapping motion of the jet column are both associated with local maxima in local, stagnation point Nusselt number distributions. The variations of these stagnation point Nusselt numbers associated with these two modes of unsteadiness are characterized by correlations which provide the dependence upon Reynolds number and normalized nozzle-to-plate distance ratio, H/B. Also described is the lateral variation of local Nusselt numbers for five nozzle-to-plate distances H/B of 2, 6, 8, 10, and 12, and Reynolds numbers from 120 to 200.     

Numerical study of turbulent slot jet impingement cooling on a semi-circular concave surface

An investigation of the flow field and heat transfer characteristics of a slot turbulent jet impinging on a semi-circular concave surface with uniform heat flux has been carried out numerically in this study. The turbulent governing equations are solved by a control-volume-based finite-difference method with a power-law scheme and the well-known k–ε turbulence model and its associate wall function to describe the turbulent structure. In addition, a body-fitted curvilinear coordinate system is employed to transform the physical domain into a computational domain.Numerical computations have been conducted with variations of jet exit Reynolds number Re_2B (5920 ? Re_2B ? 23,700), dimensionless jet-to-surface distance H/B (0.5 ? H/B ? 12), dimensionless jet width B/D (0.033 ? B/D ? 0.05) and the heat flux q″ (1663 W/m² ? q″ ? 5663 W/m²). The theoretical model developed is validated by comparing the numerical predictions with available experimental data in the literature. The variations of local Nusselt numbers along the semi-circular concave surface decrease monotonically from its maximum value at the stagnation point. The numerical results show that the local Nusselt numbers are reasonably predicted with a maximum discrepancy within 15%. As the Reynolds number fixes, the effect of the impingement distance (H/B) on the average Nusselt (Nu_avg) is not significant except at low H/B = 0.5. This study provides fundamental insight into turbulent slot jet impingement cooling on the semi-circular concave surface.     

Investigation of heat transfer enhancement by perforated helical twisted-tapes

Influence of perforated helical twisted-tapes (P-HTTs) on the heat transfer, friction loss and thermal performance characteristics under a uniform heat flux condition is reported. The P-HTTs were obtained by perforating typical helical twisted-tapes (HTTs) with a prospect to reduce the friction loss of fluid flow. The experiments were conducted using P-HTTs' three different diameter ratios (d/w) of 0.2, 0.4 and 0.6, and three different perforation pitch ratios (s/w) of 1, 1.5 and 2. The helical pitch ratio and twist ratio were fixed at P/D = 2 and y/w = 3. Tests were performed for Reynolds number between 6000 and 20,000. The experiments using the plain tube and the tubes with HTTs were also carried out for assessment. The experimental results reveal that the use of P-HTTs leads to the reduction of friction loss as compare to that of HTT. Heat transfer, friction loss and thermal performance factor increase as d/w decreases and s/w increases. For the present range, the maximum thermal performance factor of 1.28 is obtained by using the P-HTT with d/w = 0.2 and s/w = 2.0 at the Reynolds number of 6000. In addition, the empirical correlations for Nusselt number, friction factor and thermal performance factor give accurate predictions within ± 4%, ± 6% and ± 3%, respectively.     

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 and friction factor correlation for artificially roughened duct with metal grit ribs

This paper presents the results of an experimental investigation of heat transfer to the airflow in the rectangular duct of an aspect ratio 10:1. The top wall surface is made rough with metal ribs of circular cross section in staggered manner to form defined grid. The roughened wall is uniformly heated and the other walls are insulated. This geometry of duct closely corresponds to that used in solar air heaters. The effect of grit geometry [i.e., relative roughness height of grid (e/D_h), relative roughness pitch of grit (p/e), relative length of grit (l/s)] on the heat transfer coefficient and friction factor is investigated. The range of variation of system parameters and operating parameters is investigated within the limits, as e/D_h: 0.035 to 0.044, p/e: 12.5–36 and l/s: 1.72–1, against variation of Reynolds number: 4000–17,000. It is observed that the plate of roughness parameters l/s = 1.72, e/D_h = 0.044, p/e = 17.5 shows optimum performance. Correlations for Nusselt number and friction factor in terms of above parameters are developed which reasonably correlate the experimental data.     

Control of jet impingement heat transfer in crossflow by using a rib

Experimental studies are carried out to investigate the jet impingement heat transfer in crossflow by liquid crystal thermography (LCT). The aim is to assess the possibility of controlling heat transfer by using a rib. The crossflow Reynolds number spans from 80,000 to 160,000 and the velocity ratio ranges from 1.0 to 2.8. The results show that the presence of rib can significantly modify the heat transfer pattern of impinging jet. For all the tested cases, the presence of rib makes the Nusselt number profiles across the stagnation point change from a classical bell-shaped profile to a plateau-like pattern, indicating the enhanced heat transfer region expands more as the rib is present. In particular, the presence of rib has a more pronounced effect on the enhancement of heat transfer at lower velocity ratios (R = 1.0 and R = 1.4). However, in such cases, the local heat transfer in the rib corner region deteriorates. At higher velocity ratios (R = 2.0 and R = 2.8), the presence of rib makes the heat transfer rate more uniform, but meanwhile, it is found that the impinging jet effect tends to be weaker.     

An experimental study on the effects of a new swirl generator on thermal performance of a circular tube

This study experimentally focuses on the effects of a swirl generator on the thermal performance of a heat exchanging tube. The applied swirl generator is a helically twisted tube with a five-lobe cross section. As the main outcome, the thermal performance of the test tube equipped with the swirl generator are evaluated using the heat transfer rate in the form of Nusselt number and pressure drop in the form of friction factor. Water is used as the working fluid in the experiments performed for different Reynolds numbers from 6000 to 30,000. The different values of twist-angle (90 ≤ θ ≤ 360) and length (2 ≤ l ≤ 4) are investigated as the main geometrical parameters of the swirl generator. The results show that the swirl generator offers an enhancement up to 85% in the Nusselt number and an increase up to 52% in the friction factor. Therefore, the swirl generator presents a thermal performance up to 1.65. This study presents some correlations to predict the Nusselt number and the friction factor of the test tube equipped with the swirl generator.     

Milliscale confined impinging slot jets: Laminar heat transfer characteristics for an isothermal flat plate

Heat transfer and overall visualized flow characteristics of confined, laminar milli-scale slot jets are investigated, as they impinge upon an isothermal flat target plate, with a fully-developed profile at the nozzle exit. The effects of Reynolds number Re and normalized nozzle-to-plate distance ratio H/B are investigated for Re = 120–200, H/B = 2–10, and B = 1.0 mm, with a nozzle aspect ratio of y/B = 50. Instantaneous visualizations of overall slot jet flow structure show unsteady lateral distortions of jet columns at experimental conditions corresponding to the presence of continuous sinusoidal oscillations. Also apparent in flow visualization sequences are smoke signatures associated with instantaneous vortex structures which form as secondary flows develop in fluid which, initially, is just adjacent to the jet column. For each Reynolds number considered, local stagnation region Nusselt numbers Nu_o decrease dramatically as H/B increases to become greater than 7.2–13.2, as the Reynolds number is maintained constant at a value from 200 to 120, changes which occur just as continuous sinusoidal oscillation of the jet column begins to develop. The further development of continuous sinusoidal oscillating motion results in an approximate collapse of stagnation region Nusselt numbers measured at different Re and H/B values. When surface thermal boundary condition data are compared, the constant surface temperature data are generally higher than the constant surface heat flux data near the stagnation location, and lower at locations where x/B is greater than 1–2. The constant surface temperature data also show relatively low values with only very small changes with x/B, for x/B values which are greater than about 5.0. As such, the results illustrate the sensitivity of Nusselt numbers for laminar boundary layer and laminar slot jet flows to thermal boundary condition, as well as the restrictions on near-wall temperature gradients which result from a constant surface temperature thermal boundary condition.     

An experimental study of pressure loss and heat transfer in the pin fin-dimple channels with various dimple depths

An experimental study was conducted to investigate the effects of dimple depth on the pressure loss and heat transfer characteristics in a pin fin-dimple channel, where dimples are located on the endwall transversely between the pin fins. The pin fin-dimple channels considered consist of ten rows of pin fin-dimple combined structure. The pin fin transverse spacing-to-diameter ratio S/D = 2.5, the streamwise spacing-to-diameter ratio X/D = 2.5, the pin fin height-to-diameter ratio H/D = 1.0. The dimples have a print diameter the same with the pin fins, but have three different dimple depth-to-diameter ratios, i.e. δ/D = 0.1, 0.2 and 0.3. The experimental results, mainly the average Nusselt number and friction factor, for the pin fin-dimple channels with various dimple depths have been obtained and compared with each other for the Reynolds number range of 8200–50,500. The study showed that, compared to the baseline pin fin channel, the pin fin-dimple channels have further improved convective heat transfer performance by up to 19.0%, and the pin fin-dimple channel with deeper dimples shows relatively higher Nusselt number values. The study still showed dimple depth-dependent pressure loss behaviors for the pin fin-dimple channels compared to the pin fin channel, and the pin fin-dimple channel with shallower dimples shows relatively lower friction factors by up to 17.6% over the studied Reynolds number range. Furthermore, three-dimensional conjugate computations have been carried out for similar experimental conditions, and the computations showed the detailed characteristics in the distribution of the velocity and turbulence level in the flow, which revealed the underlying mechanisms for the associated dimple depth-dependent pressure loss and heat transfer characteristics in the pin fin-dimple channels.