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Abstract Numerical simulations are performed to examine the thermal and flow characteristics of curved vortex generators (VGs) on the fin surfaces of fin-and-tube heat exchangers. The performance of VGs with different curvatures are evaluated at various attack angles. In addition, the effects of parameters including position, size and inclination angle are also investigated. The heat transfer performance and pressure loss are analyzed using the dimensionless parameters Nu/Nu 0 , f / f 0 , and R =(Nu/Nu 0)/(f / f 0)1/3 with Re Dc ranging from 405 to 4050. The numerical result reveals that the value of R shows an upward trend with the increase of Re Dc , indicating a better thermal hydraulic performance is obtained at higher Re Dc for the present study. It is also found that the VGs with a curvature of 0.25 show best thermal hydraulic performance and the highest value of R = 1.06 is achieved at an angle of attack of β = 15°. Further parametrical study reveals that when the VGs are deployed at a non-dimensional horizontal distance of S 1 ∗ = 0.232 and vertical distance of S 2 ∗ = 0.697, better thermal hydraulic performance is achieved. The VGs with a smaller height and larger inclination angle show better thermal hydraulic performance at low Re Dc (Re Dc < 1620), while at high Re Dc the VGs with a larger height and smaller inclination angle present higher value of R. Highlights • The effect of curvature for curved vortex generators is evaluated. • The effect of attack angle for curved cortex generators is analyzed. • The vortex structure around vortex generators is studied. • The parameters of curved vortex generators are optimized. [ABSTRACT FROM AUTHOR]

Highlights • Three new types of arc-winglet vortex generator (VG) are proposed. • The two types of planar arc-winglet VGs improve the secondary flow behind the tubes. • The curved arc VG can effectively guide more fluid into the region behind the tubes. • The comprehensive performance for curved arc VG is better than that of planar arc VG. Abstract Vortex generator can effectively enhance the heat transfer performance on the fin surface by introducing the secondary flow and intensifying the mixing of fluid. In this study, four new types of arc-winglet vortex generator (VG) are proposed and the three-dimensional CFD numerical simulation was employed to investigate the fluid flow and heat transfer characteristics. The results indicate that compared with the conventional rectangular-winglet VG, the two types of plane arc VGs provide a significant improvement on the size of secondary flow behind the tubes and all the wake region size of the fins with the four types of arc-winglet VGs decreased to some extent, especially for the two types of curved arc VGs. The heat transfer enhancement of the fin with curved arc VGs is mainly completed by guiding more fluid to flow into the previous wake region and then scour the downstream tubes directly. In addition, compared with the rectangular-winglet VGs, the Nusselt number averagely increases by 11.5% for the equal-perimeter arc VGs fin, 19.9% for the curved equal-area arc VGs fin, and 35.9% for the curved equal-perimeter arc VGs fin. Although the equal-perimeter arc VG and the curved equal-perimeter arc VG have the same geometry size, the Nusselt number of the fin with the former one averagely increases by 22.2% while the pressure drop only increases by 13.5%. [ABSTRACT FROM AUTHOR]

Highlights • A new type of tear-drop delta vortex generators (VGs) is introduced. • The VGs with higher secondary flow intensity performer better in heat transfer. • The field synergy principle is used to clarify the mechanism of heat transfer. • The parameters of tear-drop delta VGs are optimized. Abstract Numerical simulations are performed to examine the thermal and flow characteristics of tear-drop delta vortex generators (VGs) on the fin surfaces of fin-and-oval-tube heat exchangers. The VGs deployed in a “common flow up” configuration behind tubes can induce longitudinal vortices and reduce the area of wake region, which results in significant heat transfer enhancement and negligible augmentation of pressure drop compared with plain fins without VGs. The heat transfer performance and pressure loss are analyzed using the dimensionless parameters j / j 0 , f / f 0 , and R = (j / j 0)/(f / f 0)1/3 with Re Dc ranging from 255 to 1533. The results indicate that the tear-drop delta VGs have a better thermal-hydraulic performance than plane delta VGs and the value of R reaches as high as 1.06–1.23 at the ratio of division for chord length (l 1 / l 2) being 2/3. Further parameters study reveals that when the height of VGs is 0.6 times of fin pitch, the lateral length is 0.3 times of fin pitch, and the chord length is 1.2–1.4 times of fin pitch, the optimal overall performance can be obtained. The mechanism of heat transfer enhancement is investigated by intensity of secondary flow and field synergy principle. It suggests that for the case with higher Nusselt number, the corresponding secondary flow intensity is higher and the synergy angle is smaller. In addition, for each case, where the Nusselt number is higher, the secondary flow intensity is higher and the synergy angle is smaller. [ABSTRACT FROM AUTHOR]

Highlights • A helical coil with reversed loops show unique pressure drop and heat transfer characteristics. • The thermal effectiveness of a common helical coil can be improved by adding reversed loops. • Adding reversed loops may enhance turbulence mixing in helical coil heat exchanger. • New friction factor and heat transfer correlations for a helical coil with reversed loops have been postulated. Abstract In this experimental study, flow and heat transfer characteristics of water through a newly designed helical coil heat transfer test section were investigated. A structural modification of the traditional helical coil has been implemented to improve its thermal effectiveness. Specifically, a 360° plastic reversed loop was added after each 180° of the main helical coil loop to redistribute the flow’s velocity profile, which should have a direct effect on the thermal performance of the helical coil heat transfer test section. Results reveal that reversed loops do enhance heat transfer rates by more than 20%, while pressure drop penalty also increases but not at the same rate. The curvature of the reversed loops with respect to the main coil is a critical factor in terms of thermal performance. Furthermore, as the size of the reversed loop decreases, the heat transfer enhancement and pressure drop penalty increment for the modified helical coil test section increase as well. Accordingly, it is suggested that there may exist an optimum helical coil structural modification for which the best tradeoff between pressure drop penalty and heat transfer enhancement can be found. [ABSTRACT FROM AUTHOR]

The Printed Circuit Heat Exchanger (PCHE) has attracted a lot of attentions recently due to its high efficiency and compactness, and the airfoil fin has excellent thermal–hydraulic performance compared with the zigzag channel in PCHE. Two novel fins were proposed in this study based on the configuration of NACA 0020 airfoil fin, in order to further improve the performance of airfoil fin PCHE. The thermal–hydraulic performance and the enhancement mechanism of the fins were numerically investigated using supercritical CO 2 as the working fluid. The results showed that one of the novel fins has better comprehensive performance and less entransy dissipation than the NACA 0020 airfoil fin used in PCHE, since the j factor of this novel fin is 2.97–6.15% larger, and the pressure drop is 0–4.07% less than the NACA 0020 airfoil fin under the selected conditions. And the staggered arrangement and the proper shape of fins could reduce the effect of boundary layer and improve the thermal–hydraulic performance effectively. In addition, the enhancement mechanism of the novel fin PCHE could be explained with field synergy principle very well. [ABSTRACT FROM AUTHOR]

Three new coiled flow inverter (CFI) heat exchangers with heat transfer area ( A t ) of 0.88 m 2 (4-banks), 0.44 m 2 (2-banks) and 0.22 m 2 (1-bank) were designed, fabricated and commissioned to develop a more compact heat exchanger. More than 800 experiments were carried-out in counter-flow configuration. The hot and cold fluids were used in the tube and shell side of CFI heat exchanger, respectively. Tube side flow rates were varied from 50–300 kg/h for water and 30–100 kg/h for compressed air, respectively. The shell side flow rates were varied from 100–2000 kg/h for water and 400–750 kg/h for air, respectively. The overall heat transfer coefficients of present CFI heat exchangers were found in good agreement with the results reported for the CFI heat exchangers of A t = 1.76 m 2 . The proposed CFI heat exchanger with A t = 0.22 m 2 provides 2.2–4.5 and 2–3.6 times higher overall heat transfer coefficient as compared to SHE and PHE of A t = 1.76 m 2 , respectively. The present study offers a small footprint CFI heat exchanger to meet the same heat transfer duty as compared to previously reported study. [ABSTRACT FROM AUTHOR]

The experimental investigation on thermal analysis of conical coil heat exchanger is presented in this paper. Fifteen conical coils of different cone angles (0° (helical), 45°, 90°, 135°, 180° (spiral)), with three different tube sizes (ID × OD, 8 × 10, 10 × 12 and 12 × 15) are fabricated and analysed with, same average coil diameter ( D m = 200 mm) and tube length ( L = 3 m). The experimentation is carried out with hot and cold water of flow rate 10–100 lph ( Re – 500 to 5000) and 30–90 lph respectively. The observations recorded were leads to analysis the heat exchanger for heat transfer and pressure drop characteristics at different flow rate of cold and hot fluid. The various parameters (heat transfer coefficient ( h i ), Nusselt number ( Nu ) effectiveness ( ε ) and friction factor ( f )) are estimated for the heat exchanger. The analysis indicates that, Nu and f are the functions of flow rates, tube diameter, curvature ratio and cone angle. Nu increase with increase in tube side flow rate whereas it reduces with increase in shell side flow rate, increase in cone angle and increase in tube diameter. The effects of cone angle, tube diameter and fluid flow rates on heat transfer and pressure drop characteristics are detailed in this paper. The empirical correlations are proposed to bring out the physics of the thermal aspects of the conical coil heat exchangers. [ABSTRACT FROM AUTHOR]

Low return temperatures are a prevailing issue in district cooling systems negatively affecting operating costs and energy efficiency. In this study, three aspects of district cooling substation design and control were investigated with the aim to increase the return temperatures: 1) secondary supply temperature setpoint, 2) primary flow rate and 3) the flow rate relation between the primary and secondary flows. Two different control strategies limiting the secondary setpoint and the primary flow were tested in four buildings supplied by district cooling. Also, the secondary flow was measured along with an NTU analysis and predictions with a heat balance and a support vector regression model. The results showed the control strategies successfully increased the primary return temperature with 0.6–1.6 °C and eliminated flow in the saturation zone. The primary and secondary flows were shown to be unbalanced in fourteen of sixteen substations causing a low heat exchanger temperature effectiveness. The preferred method for predicting the secondary flow was support vector regression. The novelties of this paper are the conducted field tests and measurements with associated analyses, contributing with knowledge about the actual operation of district cooling substations and outcomes when implementing improvement measures to increase the primary return temperature. • Field test of two district cooling control strategies: setpoint and flow limitation. • Setpoint limitation increased primary return temperature with 0.6–1.6 °C. • Flow limitation increased primary return temperature with 1.2 °C. • Field measurements of secondary flow revealed unbalanced primary/secondary flows. • Predictions of secondary flow in heat balance and support vector regression model. [ABSTRACT FROM AUTHOR]

Numerical simulation of flow and heat transfer characteristics in four trisection helical baffle heat exchangers with equilateral triangle tube layouts was conducted. The heat exchangers have the same helical pitch but four different baffle shapes or connections, i.e., a circumferential overlap (CO) scheme, an end-to-end (EE) scheme, a blocked V-notches (BV) scheme, and a middle axial overlap (MO) scheme. The single vortex secondary flow in each helical cycle and the leakage flow patterns in the V-notches of the adjacent baffles of these schemes are clearly depicted on the meridian slices and unfolded hexagon slices. The results for nine tubes in a 60° sector and four tube layers reveal that the local heat transfer coefficient of the center tube is much higher than that of the other tubes for all the schemes. The results show that the CO scheme has the highest shell-side heat transfer coefficient and comprehensive indexes, the BV scheme has the highest pressure drop and the worst comprehensive indexes, and the MO scheme has the lowest values of both shell-side heat transfer coefficient and pressure drop and the second highest comprehensive indexes. The average values of comprehensive index h o /Δ p o of CO scheme in the calculated range is respectively 16.5%, 27.3% and 13.5% higher than that of the EE, BV and MO schemes. [ABSTRACT FROM AUTHOR]

Abstract: Secondary flow is the flow on the cross section normal to the main flow. It can greatly enhance the convective heat transfer. Mounting or punching vortex generators (VGs) on heat transfer surface can produce secondary flow. The heat transfer of circle tube bank fin heat exchanger with VGs or without VGs on the fin surface having ten different configurations (different fin spacing, height and attack angle of VGs) was numerically studied. It is found that if a non-dimensional parameter based on the absolute vorticity flux is introduced to specify the intensity of secondary flow, the volume average non-dimensional intensity of secondary flow has one-to-one correlation with Nusselt number for all the cases studied, but there is no such correlation with the friction factor. Considering the pressure drop produced by the shape of VGs, the ratio of cross section area reduction after introducing VGs is introduced to modify the friction factor. At the same fin spacing, both Reynolds number and the volume average non-dimensional intensity of secondary flow have one-to-one correlation with the modified friction factor. These results reveal that secondary flow intensity determines the heat transfer characteristics of the fin surfaces of circle tube bank fin heat exchanger only. [Copyright &y& Elsevier]

Abstract: To improve understanding of the heat transfer enhancement for shell side of a double-pipe heat exchanger with helical fins and pin fins, the three-dimensional velocity components for the shell side with and without pin fins were measured experimentally by using laser Doppler anemometer (LDA) under cylindrical coordinate system and the fluid flow characteristics, i.e., the distributions of the axial velocity, secondary flow, stream function as well as vorticity were analyzed by employing orthogonal helical coordinate system. The results showed that, for the shell side only with helical fins at large pitch, there was a pair of vortex near the upper and lower edge of the rectangular cross section; the weakest secondary flow occurred at the center. By pin fins being installed, the three-dimensional velocity components in the helical channel were strongly changed. The extra two vortices emerged at the center of the cross sections. The fluctuation of the velocity at the center of the cross sections before and behind a pin was increased sharply by the pin, especially for the situation behind a pin. [Copyright &y& Elsevier]

Abstract: Steady heat transfer enhancement has been studied in helically coiled-tube heat exchangers. The outer side of the wall of the heat exchanger contains a helical corrugation which makes a helical rib on the inner side of the tube wall to induce additional swirling motion of fluid particles. Numerical calculations have been carried out to examine different geometrical parameters and the impact of flow and thermal boundary conditions for the heat transfer rate in laminar and transitional flow regimes. Calculated results have been compared to existing empirical formulas and experimental tests to investigate the validity of the numerical results in case of common helical tube heat exchanger and additionally results of the numerical computation of corrugated straight tubes for laminar and transition flow have been validated with experimental tests available in the literature. Comparison of the flow and temperature fields in case of common helical tube and the coil with spirally corrugated wall configuration are discussed. Heat exchanger coils with helically corrugated wall configuration show 80–100% increase for the inner side heat transfer rate due to the additionally developed swirling motion while the relative pressure drop is 10–600% larger compared to the common helically coiled heat exchangers. New empirical correlation has been proposed for the fully developed inner side heat transfer prediction in case of helically corrugated wall configuration. [Copyright &y& Elsevier]

A novel twisted annulus between two counter-twisted oval tubes is proposed for a double-tube heat exchanger. The oval tubes of the annuli have same twist pitch, but twist with opposite twisting directions. The thermohydraulic performance of the annuli with different aspect ratios and twist ratios is numerically presented. The results indicate that strong secondary flow is generated in the twisted annulus, which can significantly promote the heat transfer. The maximum Nu and f of the twisted annuli are separately 157% and 118% larger than those of the corresponding straight annuli. The largest value of the thermal performance factor is 1.98 in the studied range of geometrical parameters. Correlations of Nu and f as a function of Re , twist ratio and aspect ratio are proposed with the deviations within ±10% and ±8%, respectively. [ABSTRACT FROM AUTHOR]

Longitudinal vortices generated by winglet type vortex generators mounted on fin surface improve fluid mixing with disruption of boundary layer growth, which results in convective heat transfer enhancement. The present study reports the three-dimensional numerical investigation of airflow through heated fin-tube heat exchangers in presence of common flow down configured rectangular winglet pairs (RWPs) for staggered arrangements of circular tubes. It is well known that the thermal performance of a fin-tube heat exchanger is majorly influenced by various locations of winglets corresponding to tube centre. Henceforth, fluid flow and heat transfer characteristics of different possible RWP locations concerning each tube are examined. Further, for higher performance, a search for an effective angle of attack ranging from 15° to 60° is also performed for optimized locations. The effect of different inlet flow conditions is also investigated for Reynolds number ranges from 2000 to 10,000. Thermohydraulic performance is studied by considering Nusselt number, friction factor, secondary flow intensity and thermal performance factor. Irreversibilities due to RWPs are analysed by calculating the entropy generation rate due to viscous and heat transfer effects. Obtained results illustrate that the selection of promising RWP locations plays a vital role in improving thermal performance. The relationship between heat transfer, secondary flow and irreversibility parameters are also reported. • Performance of staggered tube arrangement in presence of winglets is analysed. • Promising and counter-productive locations are examined by using JF. • Longitudinal and horseshoe vortices promote heat transfer enhancement. • Irreversibilities due to thermal and viscous effects are analysed. • Upstream located RWPs show higher thermal-hydraulic performance. [ABSTRACT FROM AUTHOR]