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Al2O3–water nanofluids along with stationary and rotating twisted tape inserts are used to increase the rate of heat transfer in a plain tube. The simulations are conducted through varying the desi...

An experimental study on optimum spacing between grooved tubes is reported in this paper. Two grooved tubes having pitch of 10 mm and 15 mm and a plain tube were considered for the heat transfer analysis. The spacing between two tubes with same pitch was varied from 10 mm to 35 mm with a step size of 5 mm. Velocity of air flowing over the tube surfaces was changed from 0.4 m/s to 1 m/s using a blower fan. Based on Nusselt number (Nu) the optimum spacing between the tubes was decided. The optimum spacing between grooved tubes of pitch 10 mm and 15 mm was compared with that of plain tubes. From the experimental analysis it was noticed that with increase in air velocity (increase in Reynolds number) the tube surface temperature reduced irrespective of any tube considered. Nu increased with increase in air velocity for all the tubes. The important conclusion drawn from the present study was that, there exists a limiting spacing (optimum) between the tubes above which no change in Nu was observed. Spacing of 30 mm was found to be the optimum spacing between the tubes irrespective of its surface geometry modifications.

One of the techniques used for heat transfer enhancement is inserting conical rings inside the tube to act as turbulators. In the present work, the performance of these conical inserts is investigated numerically. Air is used as the working fluid with Reynolds number (Re) range of 6000–25000. Conical ring inserts were applied through three configurations of; convergent conical rings (CR), convergent–divergent conical rings (CDR) and divergent conical rings (DR). Each arrangement with diameter ratios of (d/D = 0.3, 0.4, 0.5, 0.6, and 0.7) and pitch ratios of (PR = 2, 3, and 4) were employed. Both Nu and f increased with the decrease in the conical ring d/D and PR. The average Nu obtained from using (CR), (CDR) and (DR) arrays were found to be 330%, 419% and 765% more than Nu of the plain tube, respectively. The best enhancement tube efficiency was found to be 1.291 for the divergent ring with d/D = 0.4 and PR = 2 for Re of 6000. The entropy generation increased with the increase in Re for all the conical ring cases. Also, entropy generation increased with the decrease in the d/D. A new correlation was proposed for Nu, f and the enhancement tube efficiency with variable parameters and compared with published experimental data from the literature.

The effect of perforated twisted tapes with alternate axis (PATT), perforated twisted tapes (PTT) and regular twisted tapes (TT) with twist ratios (TR) of 3,4 and 5 are compared by experimental investigation in a circular tube under constant heat flux condition. Using water as the testing fluid, experiments are conducted within the Reynolds number range of 3000–16000. The heat transfer rates in the tube fitted with PATT, PTT and TT are respectively improved up to 48.12%, 44.3% and 33% of that in the plain tube. Similarly, the increase in friction factor for PATT, PTT and TT is 19.1%, 17.6% and 15.85% respectively, compared to that of the plain tube. For all inserts, the empirical correlations were established for Nusselt number and friction factor along with plain tube and the obtained correlations are fitted with the experimental data. At constant pumping power, the maximum thermal performance factor obtained is 1.433, 1.396 and 1.24 respectively for PATT, PTT and TT. Hence, the passive element which gives better energy efficiency along with lower operating cost is suggested for practical heat transfer applications.

In this paper, a two-dimensional model of a latent heat storage is presented. The model describes the time-dependent solidification of a phase-change material (PCM) around a plain tube heat exchanger. Heat transfer and energy balance equations are combined to predict the heat flow rate from the solidifying PCM to the heat transfer fluid. The model utilizes the temperature-based approach combined with a quasi-stationary approximation. In contrast to most solidification modelling approaches, the model is characterized by low computational complexity which enables the simulation of the entire storage volume instead of only a small section. The model is compared with experimental data obtained from a latent heat storage test-rig with a vertical plain tube heat exchanger. The predicted power output matched the experimental results very well, demonstrating the high accuracy of the simulations. This is the first report of a quasi-stationary model of a latent heat storage being validated with a PCM other than water and thus demonstrates the applicability of this approach in a wider range of PCM. The presented model can be used as a fast and accurate tool to design latent heat storage units as well as to analyse the performance of existing storage tanks.

Thermal performance improvements for tubular flows using spirally coiled spring (SCS) inserts to disturb boundary layers by tripping near-wall separated vortices are proposed. Combined with the segmental solid cords to induce axial swirls, a new convenient passive heat transfer enhancement (HTE) method is formulated. The interval of segmental solid cords is fixed at one tube diameter (d); while the pitch ratios (P/d) of the SCSs are 0, 1, 1.5, 2, 2.5 and ∞. The axial Nusselt numbers (Nu), mean Fanning friction coefficients (f) and thermal performance factors (TPF) at Reynolds numbers (Re) between 750 and 70,000 are measured for each test tube. Acting by the interactive near-wall vortices and the axial swirls in the tubes with the SCSs and segmental solid cords, both HTE effectiveness and f augmentation are boosted from those developed in the tubes with SCSs alone. Relative to the plain tube heat transfer references over the developing ( Nu ‾ 0 DE ) and developed ( Nu ‾ 0 FD ) flow regions, the ratios of Nu ‾ DE / Nu ‾ 0 DE and Nu ‾ FD / Nu ‾ 0 FD are elevated to 1.81–9.04 and 1.01–8.21 at 750 ≤ Re ≤ 2000; and 1.11–2.41 and 1.02–2.33 at 10,000 ≤ Re ≤ 70,000 by the SCSs of 0 ≤ P/d ≤ ∞ without cord. For present SCSs of 0 ≤ P/d ≤ ∞ with segmental cords, the ratios of Nu ‾ DE / Nu ‾ 0 DE and Nu ‾ FD / Nu ‾ 0 FD are raised to 1.81–10.74 and 1.01–9.07 at 750 ≤ Re ≤ 2000, and 1.11–3.15 and 1.02–2.66 at 10,000 ≤ Re ≤ 70,000, respectively. With present geometrical and flow conditions tested, the TPF values for the tubes enhanced by the SCSs without and with segmental solid cords are in the respective ranges of 5.07–0.61 and 3.75–0.64. For each type of enhanced tubular flows, the Nu and f correlations are devised to assist the relevant applications.

In this paper, an experimental research was carried out to study the heat transfer and friction factor characteristics in a tube with a new kind of twisted tape insert. Four twisted tapes with different lengths (2400, 1800, 1200 and 600 mm) were used in the experimental study. The experimental data revealed that Nusselt number caused by these four twisted tapes were 1.45–1.90, 1.23–1.55, 1.18–1.45 and 1.15–1.37 times of that for the plain tube, while the values for friction factor were 3.69–5.75, 2.30–4.48, 2.15–2.75 and 1.21–1.31. The performance evaluation criteria (PEC) for length ratio of 1.0, 0.75, 0.50 and 0.25 is 0.94–1.06, 0.82–0.94, 0.89–1.04 and 0.89–1.06, respectively. What's more, correlations for Nusselt number and friction factor were established in the paper. The predicted values calculated by correlations were in good agreement with experimental data, within 7% for Nusselt number and within 10% for friction factor.

Thermal performance improvements for ribbed channels are attempted by installing auxiliary fillets on ribbed endwalls. Detailed heat transfer distributions over two opposite endwalls enhanced by 45° ribs and a row of tilted three-dimensional fillets of 1/2 rib heights with streamlined shape are measured at five flow configurations using steady-state infrared thermography method. Local, area-averaged and channel-averaged Nusselt numbers (Nu, N ‾ u and Nu ‾ ‾ ), Fanning friction factors (f) and thermal performance factors (TPF) for each test channel are examined with Reynolds number (Re) in the range of 5000–15,000. The HTE properties and associated f augmentations for the compound HTE measures are assessed by comparing the N ‾ u and f data detected from present test channels with the Nusselt numbers (Nu∞) and f factors (f∞) of smooth plain tube. Empirical correlations evaluating N ‾ u and f factors for present ribbed channels with/without in-lined/staggered fillets are generated. With present in-lined (staggered) fillets on two opposite ribbed endwalls, the HTE ratios in terms of Nu ‾ ‾ /Nu∞ are raised to 4.43–4.21 (3.85–4.05) with forward flows and 4.31–4.02 (4.14–4.06) with backward flows at 5000 ⩽ Re ⩽ 15,000; whereas the corresponding f/f∞ ratios and TPF values fall in the respective ranges of 6.5–3.35 and 7.3–3.6 (7.5–5.6 and 9.06–7.39) with forward flows and 2.37–2.81 and 2.22–2.62 (1.97–2.29 and 1.99–2.09) with backward flows. As the Nu ‾ ‾ /Nu∞, f/f∞ and TPF values for present ribbed channels with in-lined/staggered fillets are generally raised from the ribbed channel counterparts of 3.93–3.61 ( Nu ‾ ‾ /Nu∞), 1.63–1.53 (f/f∞) and 2.41–2.35 (TPF), the improved thermal performances by fitting the tilted fillets to enhance the rib-induced vortical flows are demonstrated.

The use of heat transfer enhancement techniques, can improve the thermal performance of the tubes. In this study, the convective heat transfer from nanoparticles TiO2–SiO2 was dispersed to W/EG in the plain tube, under constant wall heat flux studied numerical and experimental. The type of nanofluid used is the TiO2–SiO2 base fluid EG/water mixture. The volume concentrations used were 1.0, 2.0 and 3.0%. The Reynolds number (Re) used ranges from 2900 to 11,200. The effect of nanofluids on heat transfer coefficients and friction factors is presented in this work. The results show that heat transfer increases with Reynolds number for numerical and experimental in plain tube. Hybrid nanofluids at volume concentration of 3.0% had the highest amount of Nusselt and the highest friction factor was followed by 2.0% and then 1.0%. Experimental and numerical results are compared in terms of Nusselt number average deviation found was 8.8, 8.9 and 7.9% for the volume concentration of 1.0, 2.0, and 3.0% in this study. The friction factor average deviation is 4.1, 3.8 and 3.5% for the volume concentration of 1.0, 2.0, and 3.0%, respectively.

The analysis of heat transfer design methods in condensing inside plain tubes under existing influence of shear stress is presented in this paper. The existing discrepancy in more than 30-50% between rating heat transfer coefficients and experimental data has been noted. The analysis of existing theoretical and semi-empirical methods of heat transfer prediction is given. The influence of a precise definition concerning boundaries of phase flow (it is especially important in condensing inside horizontal tubes), shear stress (friction coefficient) and heat flux on design of heat transfer is shown. The substantiation of boundary conditions of the values of parameters, influencing accuracy of rated relationships, is given. More correct relationships for heat transfer prediction, which showed good convergence with experiments made by different authors, are substantiated in this work., {"references":["W. 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With isopentane as working fluid, the heat transfer performances for corrugated, nodal and horizontal grain tubes are simulated. The structural parameters of the three kinds of tubes are compared with those of the plain tube. The numerical results using computational fluid dynamics are validated with theoretical values. For the corrugated, nodal and horizontal grain tubes, the heat transfer enhancements(HTEs) are 2.31–2.53, 1.18–1.86 and 1.02–1.31 times of those of the plain tube, respectively. However, the improved HTEs are at the expense of pressure losses. The drag coefficients are 6.10–7.09, 2.06–11.03 and 0.53–1.83 higher, respectively. From the viewpoint of comprehensive heat transfer factor, the corrugated tube is recommended for engineering applications, followed by the horizontal grain tube.

Simultaneous heat transfer and friction factor experiments for plain and micro-fin tubes were conducted in our previous study. The results showed that the heat transfer characteristics of the micro-fin tubes were the same as for the plain tube. However, the friction factor characteristics of the micro-fin tubes in the transition region were different compared to the plain tube. This type of friction factor data cannot be easily correlated by the traditional regression method. Therefore, a logistic dose-response curve-fitting method is proposed in this study. This particular method has recently been used by other researchers in correlating the friction factor data in plain tubes. In this study, three sets of micro-fin tubes friction factor data with different inlets from our previous study were correlated by the logistic dose-response curve-fitting method. All the fully developed friction factor data for the entire flow regime were predicted by a composite logistic dose-response function within ±11% deviat...

The thermohydraulic characteristics of the circular tube equipped with Left–Right helical blade rotors were reported; meanwhile, the influence of the assembled mode of Left–Right helical blade rotors on heat transfer, friction factor and thermal performance factor characteristics of the enhanced tube was also investigated. The experimental results revealed that the Nusselt number and friction factor of the tube with helical blade rotors was 77.9–182.2% and 22.1–71.1% higher than that of plain tube, respectively. The Nusselt number of the tube with Left–Right helical blade rotors increased with the decreasing numbers of Right helical blade rotors; the enhanced tube with the ratio of Left–Right helical blade rotors number as 1/3 generated the highest Nusselt number than others. The friction factor of the enhanced tube increased with the decreasing numbers of Right helical blade rotors and the decreasing number ratios. The thermal performance factor of Left–Right helical blade rotors increased with the decreasing number of the Right helical blade rotor; meanwhile, the enhanced tube with the ratio of Left–Right helical blade rotors number as 1/3 generated the highest thermal performance factor.

This paper presents experimental results for the nucleate pool boiling of R134-a on plain and micro-finned tubes at pressures between 6.1 bar and 12.2 bar. Local measurements of the temperature enabled us to identify the physical mechanism responsible for the enhancement of the heat transfer due to the micro-fins. The experimental heat transfer coefficients were then compared with various correlations obtained for a plain tube. The validity of the correlation furnished by Gorenflo and Kenning (2010) was tested for the micro-finned tube.

Experimental investigation of heat transfer and friction factor characteristics of a circular tube fitted with plain twisted tapes (PTT) and trapezoidal-cut twisted tapes (TTT) with twist ratios y = 2.0, 4.4, and 6.0 was carried out. The experimental data obtained from plain tube and PTT were verified with the standard correlation to ensure the validation of experimental results. The experimental results revealed that heat transfer rate, friction factor, and thermal enhancement factor in the tube equipped with TTT were significantly higher than in the tube fitted with PTT and the plain tube. The additional disturbance and secondary flow in the vicinity of the tube wall generated by the TTT compared to that induced by the PTT was determined as the reason for enhancement. Subsequently, an empirical correlation was also formulated to match with experimental results with ±6% and ±5% variation respectively for Nusselt number and the friction factor.

An experimental investigation has been carried out to find the heat transfer coefficient during condensation of R-134a vapor inside a horizontal tube. Experiments were conducted for the condensation of R-134a inside a plain tube and tubes with different twisted tape inserts. Twisted tapes with different twisted ratios of 6, 9, 12 and 15 were inserted in the refrigerant side, one by one, in the full length of test-condenser. For each inserted tube and the plain tube, test runs were carried out for the mass velocities of 92, 110, 128 and 147 kg/s-m2. An empirical correlation has also been developed to predict the enhanced heat transfer coefficient.

New experimental data are reported for forced-convection condensation of ethylene glycol on a set of nine single, copper, integral-fin tubes. The first set of five tubes had fin height and thickness of 1.6 and 0.25mm, respectively, with fin spacings of 0.25, 0.5, 1.0, 1.5, and 2.0mm. The second set of four tubes had fin spacing and thickness of 1.0 and 0.5mm, respectively, and fin heights of 0.5, 0.9, 1.3, and 1.6mm. The fins were rectangular in cross section. All tubes had a fin root diameter of 12.7mm. A plain tube of outside diameter 12.7mm was also tested. The tests, which were performed at a near constant pressure of ∼15kPa, covered vapor velocities between 10 and 22m∕s and a wide range of heat fluxes. The best performing tube was that with fin spacing, height, and thickness of 0.5, 1.6, and 0.25mm, respectively, which had an enhancement ratio (compared to the plain tube at the same vapor-side temperature difference and vapor velocity) of 2.5 at the lowest vapor velocity tested, increasing to 2.7 at the highest. For all but two of the tubes, the effect of vapor velocity on the heat-transfer coefficient of the finned tubes was less than on the plain tube, leading to a decrease in enhancement ratio with increasing vapor velocity. For two of the tubes, however, the enhancement ratio increased with increasing vapor velocity, which is the opposite trend to that found in most earlier experimental studies. This effect was thought to be due to the slight reduction in condensate flooding between the fins of these two tubes because of vapor shear.

Experimental data of the local heat transfer coeffcient of flow boiling ammonia in dependence of vapor fraction, mass flux and local heat flux is presented. Two horizontal test sections of 450 mm length and an inner diameter of 10 mm have been used, one being a plain tube, one being a spirally low finned tube. A constant wall temperature boundary has been aimed for the test section by heating with a fluid condensing on the tube outside. Local heat transfer coeffcients and pressure drops have been measured in the range −40

An experimental investigation has been carried out to find the condensing side heat transfer coefficient, ho, during condensation of steam over a plain tube, a circular integral-fin tube(CIFT) and a spine integral-fin tube(SIFT). The CIFT and SIFT have enhanced the ho by a factor of 2.5 and 3.2 respectively. The experimental values of ho for CIFT were compared with those predicted from different models. The Honda and Nozu model underpredicted the values of ho in a range of 10 to 20 percent.

This paper reports numerical investigations of heat transfer and friction factor characteristics in swirling flow conditions using CFD simulation. A commercial CFD package, FLUENT 6.3.26, was used in this study. 3D models for circular tube fitted with classical and quadrant-cut twisted tape (QCT) inserts with three twist ratios (y= 2.93, 3.91, and 4.89) and different cut depths (w= 0.5, 1.0, and 1.5 cm) were generated for the simulation. The data obtained from the CFD simulation were verified with the literature correlations of plain tube with the discrepancy of less than ±8% for Nusselt number and ±10% for friction factor. The results show that there was a significant increase in heat transfer coefficient and friction factor in the tube fitted with quadrant-cut twisted tape (QCT) with decreasing of twist ratio (y) and cut depth (w). Furthermore, the configuration of QCT insert with a twist ratio ofy= 2.93 and a cut depth ofw= 0.5 cm offered higher heat transfer rate and friction factor than other twist ratios.

In present study, the characteristics of particulate fouling, crystallization fouling and their composite fouling for the convergent-divergent tube were investigated. The comparative fouling experiments were carried out between the convergent-divergent tube and its corresponding plain tube. The experimental results show that the convergent-divergent tube has an advantage both in heat transfer performance and in antifouling performance over the plain tube.

New experimental data are reported for condensation of ethylene glycol at near atmospheric pressure and low velocity on six three-dimensional pin-fin tubes. Enhancements of the vapour-side, heat-transfer coefficients were found between 3 to 5.5 when compared to a plain tube at the same vapour-side temperature difference. Heat-transfer enhancement was found to be strongly dependent on the active surface area i.e. on the proportion of the tube and pin surface not covered by condensate retained by surface tension. For all the tubes, vapour-side, heat-transfer enhancements were found to be approximately 3 times the corresponding active-area enhancements. The best performing pin-fin tube gave a heat-transfer enhancement of up to 5.5; 17% higher than those obtained from ‘optimised’ two-dimensional fin-tubes reported in the literature and about 24% higher than the ‘equivalent’ two-dimensional integral-fin tube (i.e. with same fin root diameter, longitudinal fin spacing and thickness and fin height).Copyright © 2010 by ASME

Heat transfer and pressure drop data are presented for equilateral-triangular pitch tube bundles containing low-finned tubes with fin counts between 0.4 and 1.06 fins per mm. Two types of finned tubes were tested. The first was a 19-mm O.D. integral finned tube with a 1.65-mm fin height and with 0.75 and 1.06 fins per mm. The second was a 31.75-mm O.D. plain tube wound with 3.18-mm soldered fin stock and containing 0.4, 0.71, and 0.98 fins per mm. A new low-fin heat transfer correlation was developed which predicted these new data and additional data from five other sources within an error range from minus 20 percent to plus 31 percent. A necessary dimensionless group required to obtain this accuracy was the fin diameter/fin spacing ratio. Existing friction factor correlations were not successful in predicting these new pressure drop results. A new low-fin friction factor correlation was developed which predicted these data and additional published data within an error range from minus 19 percent to plus ...