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This communication concerns some doubtful results included in the above paper. [ABSTRACT FROM AUTHOR]

The present paper, for the first time, examines the influences of utilizing oil-ZnO nanofluid with different volume fractions φ = 0.5 %, 1 %, and 2 % in alternating flattened tubes (AFTs) with different alternating pitch angles of 30°, 45°, 60°, and 90° on the performance of a dual-tube heat exchanger (DTHE). This work is conducted experimentally and numerically for the Reynolds number (Re) range of 300 < Re < 1900 for oil-ZnO nanofluid and Re = 2000 for water. Based on the experimental results, the optimal case is selected for the numerical simulations of AFTs. The performance evaluation criterion (PEC) is defined for the simultaneous evaluation of pressure drop (Δp) and heat transfer coefficient (HTC). The results demonstrate that the overall heat transfer coefficient (U) and Δp are augmented with the inlet flow rate and the alternating angle between the pitches. Therefore, the maximum heat transfer (HT) and Δp correspond to the AFTs with the angle of 90° (AF4) at Re = 1900. The PEC amount of AF4 shows a 56 % enhancement compared to the circular tube. It is also observed that using copper oxide nanoparticles inside the oil improves the HT rate and Δp in the heat exchanger. Besides, an increment in φ increases U and Δp; however, the values of PEC show that the positive effects of the nanofluid are larger than their negative impacts in such a way that the PEC is improved by 64 % when the nanofluid with φ = 2 % is utilized in AFTs compared to the circular tube. [ABSTRACT FROM AUTHOR]

Abstract Winglet pairs are promising longitudinal vortex generators which can be used to produce streamwise vortices that do not decay until further downstream and consequently increase heat transfer rate with comparatively lower pressure penalty. This paper deals with the effect of delta winglet vortex generator (DWVG) pairs on thermal and flow behaviors in a circular tube for Reynolds numbers (Re) range of 5000–25000. The DWVG pairs involved are the pitch ratio (PR = 9.6), four attack angles (α = 10°, 20°, 30° and 40°), three winglet height (h = 5 mm, 7.5 mm and 10 mm) and three spacing between leading edges (s = 10 mm, 15 mm and 20 mm). The experimental results indicate that the Nusselt number (Nu) increases with Re while friction factor (f) decreases with Re. Nusselt number and friction factors both are increasing with attack angle and winglet height, while the middle spacing yields the highest Nu and f. Maximum Nusselt number increment (Nu / Nu 0) with the DWVG pairs was observed as being 73% larger than that of smooth tube, while the maximum friction factor increment (f / f 0) was 2.5 times larger. Thermal enhancement factor (TEF) decreases with Re. The largest TEF obtained, 1.44, is with the combination of α30°s15 h 7.5 at Re = 5000. Compared with other types of VGs in published experimental research papers, the current DWVG pairs show better thermal performance than many of them. Vortices downstream of the DWVG are visualized with smoke flow for better understanding of the flow behavior. [ABSTRACT FROM AUTHOR]

Abstract This paper presents analyses of complex flows and heat transfer induced by twin oblique turbulent slot-jets of different directions (divergent, convergent or parallel) impinging a heated wall. A comparison of the heat transfer characteristics between perpendicular and three cases of twin oblique jets (parallel, convergent and divergent). The twin slot jets are located on a confining adiabatic wall at a distance of 8 slot jet width. Convective heat is investigated numerically examining the effect of Reynolds number (Re) and jet inclination angle (α). This problem is relevant to a wide range of practical applications including nuclear engineering devices, manufacturing, material processing, electronic cooling, drying paper or textile, tempering of glass, etc. All computations are performed using two dimensional large eddy simulations (LES) approach with Smagorinsky sub-grid scale (SGS) models. For all directions and inclinations of the jets, the location of the stagnation points is changed and hence, the location and magnitude of the maximum Nusselt number on the heated wall vary. When Reynolds number increases, Nusselt number is improved for all types of inclination. The averaged Nusselt number shows that the perpendicular impingement gives better heat transfer than that of the oblique jets. The poor heat transfer is obtained for the parallel oblique jets. For the same angle, divergent jets give smallest heat transfer than the convergent jets. Graphical abstract Twin-jets flow configuration. (a) Perpendicular (b) Divergent (c) Parallel (d) Convergent. Image 1 Highlights • Four types of flow patterns of impinging twin jet are studied by Large Dissipation Simulation. • Nusselt number is enhanced for all types of inclination for increasing Reynolds number. • Averaged Nusselt number of the perpendicular impingement gives better heat transfer than that of the oblique jets. • The poor heat transfer is obtained for the parallel oblique jets. • For the same angle, divergent jets give smallest heat transfer than the convergent jets. [ABSTRACT FROM AUTHOR]

The interaction of longitudinal vortices decreases the intensity of longitudinal vortices and inevitably affects the heat transfer performance of heat exchangers. In this paper, the effects of transverse distance of vortex generators (VGs) on the interaction of longitudinal vortices and the heat transfer performance are quantitatively studied. The increments of longitudinal vortex intensity, Nusselt number and friction factor resulted from the application of VGs are discussed in detail. The results show that the transverse distance of VGs obviously affects the interaction of longitudinal vortices, the heat transfer enhancement and the pressure loss characteristics of the heat exchanger. The interaction of co-rotating longitudinal vortices generated by VGs around the same tube is less affected by the transverse distance of VGs. While the interaction of counter-rotating longitudinal vortices generated by VGs around different tubes is closely related to the transverse distance of VGs. The interaction between counter-rotating longitudinal vortices plays a dominant role in the interaction process of longitudinal vortices. Optimal transverse distance of VGs exists for best heat transfer performance of the heat exchanger. Meanwhile, the transverse distance which leads to the worst heat transfer performance is also reported. For the largest Reynolds number studied in this paper, the maximum differences in the increments of intensity of longitudinal vortices, Nusselt number and friction factor for different transverse distance of VGs are 34.0%, 33.9% and 18.5%, respectively. [ABSTRACT FROM AUTHOR]

This paper analyses the effect of spirally corrugation in a simple tube on the heat transferred and friction factor using numerical simulations. The simulations have been validated with experimental data available in the literature. The study compares the behaviour of both smooth and spirally corrugated tubes considering turbulent flow at four Reynolds numbers (15- 40 × 10 3 ) and two Prandtl numbers (2.9 and 4.3). The main novelty of this paper is to perform a 3-D simulation because some previous studies using similar geometry were restricted to a 2-D analysis. For the smooth and corrugated tubes, stainless steel tubes with an inner diameter of 18 mm, a length of 6 m and a wall thickness of 1 mm were used. The corrugated tube has a corrugation depth of 0.43 mm and a helical pitch of 15.86 mm. The meshing process was performed using ANSYS Workbench (v.17.0) with an unstructured grid with a refined mesh near the wall to ensure that the laminar viscous sub-layer was captured. Hence, a k-epsilon ( k − ε ) turbulence model with a near-wall treatment was used in the proposed simulations. Two grids were used to perform a grid sensitivity analysis. The results for the corrugated tube indicate that the numerical model predicts an average Nusselt number within a maximum relative error of 17% compared with the experimental data, and the differences in the Fanning factor are lower than 9%. [ABSTRACT FROM AUTHOR]

Rising temperature of gas turbine inlet brings great challenge for blade temperature tolerance, thus high effective cooling structure for turbine blade is vital. In this paper, aerothermodynamic features including heat transfer and pressure drop of a Coriolis-applied rotating smooth U channel are studied in detail. Both experiment and numerical methods are utilized in the study. A rotating cooling structure experiment rig system has been established and validated. The experiments were conducted in the rotating cooling structure experiment rig. Besides, RANS was chosen in the simulation. In the experiment and numerical studies, Reynolds number is constant at 16000 and Rotation number ranges from 0 to 0.024. The results indicate that Coriolis force plays a dominant and positive role on heat transfer ability on trailing wall, and that bend outlet impingement effect is the primary factor compared to Coriolis force for the Nusselt number of smooth leading surface. Besides, Coriolis force at bend region probably suppresses the formation of K–H vortices, thus boosts the flow stability, reduce pressure loss of channel and weakens second heat transfer peak of impingement at the bend outlet. What's more, the total performance of the Coriolis-applied smooth U channel at Ro of 0.006 and 0.024 are 2.18 % and 8.51 % higher than the channel at Ro of 0, which means that rotation is good for the Coriolis-applied smooth U channel and that the channel has superior performance and is encouraging in future rotating blade cooling application. • A Coriolis-applied smooth U channel is proposed and investigated. • Detail heat transfer experiments on the channel were implemented. • Simulations were conducted to unveil the heat transfer and pressure drop mechanisms. • Rotation enhances overall heat transfer of the Coriolis-applied smooth U channel. • Rotation brings pressure loss of the Coriolis-applied smooth U channel decreased. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive investigation of heat transfer enhancement and pressure drop in a Coiled Flow Reverser (CFR) with a twisted tape insert. Both laminar (10 < Re < 2000) and turbulent (12,000 < Re < 18,000) regimes have been studied using both experimental and numerical methods. The effects of twist ratio of twisted tape, inlet Reynolds number, and perforating the twisted tapes have been investigated. The study found that the best performance in the laminar regime is achieved with a twisted tape with a twist ratio of 15.73 and no holes on it at a Reynolds number of 2000. In the turbulent regime, the same twisted tape performs the best at a Reynolds number of 17,000. The highest Thermal Performance Ratio (TPR) in laminar regime is 1.55 and in turbulent regime is 0.99. In comparison to the straight pipe, all types of CFRs show better performance in heat transfer enhancement. Also, as the Reynolds number increases, the friction coefficient decreases in all configurations. The study reveals that the twisted tape insert enhances heat transfer performance while increasing pressure drop. This study provides valuable insights into the design and optimization of heat exchangers with CFR. The findings can be used to improve the efficiency and performance of various industrial processes, such as refrigeration systems, air conditioning, and power plants. • Fluid flow and heat transfer in a coiled flow reverser (CFR) were studied. • Effects of tube insert on the heat transfer enhancement of CFR were investigated. • Effects of perforation of tube insert on heat transfer enhancement of CFR were examined. • The effects of tube insert at laminar flow are more than turbulent flow. • Perforation has no significant effects on the thermal performance ratio. [ABSTRACT FROM AUTHOR]

With the development of science and technology, the heat dissipation problem of high-density electronic products has gradually emerged in engineering practice. In this paper, the enhanced heat dissipation process of fluid induced oscillation through flexible membrane is proposed, which belongs to the multi-field synergistic bidirectional coupling effect of thermal-fluid-solid coupling. Through the comparative numerical analysis of the flow induced oscillation of the flexible membrane and the fixed rigid fin in the flow channel, the mechanism of the fluid induced oscillation of the flexible polyethylene membrane is revealed. Based on the two-way fluid-solid coupling method, a numerical model for the heat transfer process of polyethylene membrane with thickness of 0.02 mm in the fluid channel was established. The temperature field, Nu (Nusselt number) and PEC (Performance comparison index) number of the rigid fin and polyethylene membrane installed in the channel under different Reynolds numbers were calculated respectively. The ratio of the length of the membrane to the height of the channel was adjusted for multiple groups of calculations to verify the positive effect of the polyethylene membrane installed in the channel on the heat exchange effect. The results show that in the fully developed area of heat transfer, the channel with polyethylene membrane is better than that with rigid fins in terms of turbulence and vortex shedding; at Re = 4400, the comprehensive enhanced heat transfer factor of heat exchange is the largest, up to 1.27; as the ratio of the height of the new flexible membrane to the height of the channel is 1, the heat transfer enhancement effect is best. In the next step, the correlation theory between oscillation characterization parameters and heat transfer enhancement effect under pulsating fluid excitation will be investigated. [ABSTRACT FROM AUTHOR]

In this paper, the fluid flow behavior and cooling characteristics of a sweeping impingement jet were investigated. Also, the difference between free and confined impingement schemes was investigated. In addition, the conventional approach for the purpose of significantly reducing computational time by using a confined domain with a slip upper wall instead of an unconfined domain was evaluated. A 2D unsteady Reynolds averaged Navier-Stokes (URANS) simulation accompanied with the k-ω SST turbulence model is used in this study. The study has been conducted for a target wall with a constant heat flux of 3000 W/m2, jet-to-wall distance of 4, and a jet Reynolds number of 2500. The results show that the overall average cooling performance of the sweeping jet is better in the confined impingement scheme compared to that of the steady jet, while the steady jet is slightly better in the unconfined sweeping impingement scheme. Using a confined scheme with a slip upper wall does not reveal the complete thermal and flow behaviors, and the wall heat transfer distribution is very different from the unconfined domain. [ABSTRACT FROM AUTHOR]

This paper studied the buoyance-aided mixed convective heat transfer of air-cooling flow in a 5 m long and 1.2mX0.3 m cross-section vertical channel. One of the channel walls was uniformly heated and the rest three walls were unheated and insulated. Heat was transferred from the heated wall by convection and radiation, and heat was also transferred from unheated walls to the air flow by convection originated from their net radiation gain from the heated wall. The air flow rate, the inlet and outlet air flow temperatures, and local surface heat fluxes and temperatures on the heated wall were measured. The Reynolds number of air flow was from 2900 to 221,000 and Richardson number was changed from 0.006 to 90.6. The results showed that for walls with larger emissivity the total convection heat transfer was not deteriorated in mixed convection zone. Therefor a modified heat transfer correlation was developed for the aiding mixed convection in the asymmetrically heated channel considering both mixed convection and radiation. It predicted the experiment results with a deviation less than 18%. [ABSTRACT FROM AUTHOR]

Abstract Sidewall proximity, characterized by the gap distance (G) between border pin-fin column and sidewall in a pin-finned microchannel with in-line arrangement, plays a significant role on pressure drop and heat transfer characteristics. To better understand the thermal performance and explore the underlying mechanisms, a comprehensive comparison is numerically developed among three representative microchannels with gap-to-diameter ratios (G / D) of 0.6, 1.0 and 1.4, respectively. The Reynolds number investigated in this paper varies from 13 to 202. It is found that the gap distance severely influences flow distribution, streamline structure, velocity field and temperature distributions in a pin-finned microchannel. At a fixed Reynolds number, pressure drop of microchannel is continuously decreased while heat transfer is first enhanced and then reduced with the increase of gap distance. Among the three models, the microchannel with G / D = 1.0 possesses a comparatively superior heat transfer performance. In addition, extremely low local Nusselt numbers on both the base surface and the pin-fin surface near sidewall seriously deteriorate the overall heat transfer performance of the microchannel with G / D = 0.6. Furthermore, unremarkable heat transfer performance is also observed from the microchannel with G / D = 1.4 for its obvious decline of local Nusselt number on inner regions in spite of a rise on border region. Taking heat transfer and pressure drop into account simultaneously, the results show that a very small gap distance (i.e., G / D = 0.6) should be avoided for design of a pin-finned microchannel. Microchannels with middle gap distances (i.e., G / D = 0.9, 1.0, 1.1) have a relatively better overall thermal performance, which separately provide a superiority of 10.6–13.6% (G / D = 0.9), 10.0–13.5% (G / D = 1.0), 8.2–14.4% (G / D = 1.1) compared to the microchannel with G / D = 0.6. Finally, new correlations of friction factor and Nusselt number are developed by considering the effects of sidewall proximity. Highlights • Sidewall proximity influences on a pin-finned microchannel are observed. • Details of flow characteristics and heat transfer are discussed. • Suitable sidewall proximity is of importance to improve thermal performance. • New correlations of friction factor and Nusselt number are developed. [ABSTRACT FROM AUTHOR]

Abstract To overcome the deficiencies of parallel-flow shell and tube heat exchanger with round rod baffles (RRB-STHX), STHX with a new type of anti-vibration baffle (hexagon clamping baffle) and equilateral triangle cross sectioned wire coils (HCBetsw-STHX) is proposed in this paper. Response surface methodology (RSM) is adopted to investigate its thermal-hydraulic performance. Baffle distance, coil pitch, coil diameter and Reynolds number are considered as four design parameters. Analysis is conducted based on CFD to acquire the objective functions (Nu H , Nu H / Nu R , f H , f H / f R and PEC) for different combination of design parameters. The RSM with Central Composite design is used to identify the relationships between the objective functions and the design parameters. Variances of the linear term, quadratic term and interactive term for design parameters in the response variables are analyzed. The mechanism of heat transfer enhancement of HCBetsw-STHX is illustrated from the view of field synergy theory. The results indicate that the heat transfer enhancement of HCBetsw-STHX is quite better than that of RRB-STHX. In the given design space, the Nu H / Nu R is in the range of 1.335–1.720; the f H / f R is in the range of 5.462–12.936. The most significant factor on Nu H is Reynolds number, while the most significant effect factor on Nu H / Nu R , f H , f H / f R and PEC is coil pitch. The sensitivity of three objective functions (Nu H , f H and PEC) to design parameters is explored. In addition, the design parameters optimization for the maximum value of Nu H and PEC and that for the minimum value of f H are also carried out and corresponding flow structures are shown and illustrated. Highlights • A new parallel-flow heat exchanger with hexagon clamping baffles and wire coils is proposed. • Parameter sensitivity of factors on thermal-hydraulic performance is explored. • Design parameters optimization is explored to maximize the Nu and PEC and to minimize the f based on the generated response surface. [ABSTRACT FROM AUTHOR]

Abstract In the process of drilling, the tubing-casing annulus are easily to be eccentric. Heat transport phenomenon occurring in such tubes and annulus can be complex, and this paper investigates forced convection in a vertical eccentric annulus with different radius ratios and eccentricity (normalized by the radius difference). Physical experiments and numerical simulation have been conducted with constant inner tube inlet temperature and flow rate of hot water. Results show that with the increase of radius ratios, the convection heat transport coefficient increases. For a radius ratio of 1.875, the heat transport coefficient increases as the eccentricity increases when the Re > 2000 and it decreases when the Re < 2000. For a radius ratio of 2.25 and Re < 1500, the influence of eccentricity is trivial, but for Re > 1500, the increase of eccentricity strengthens the heat convection. For a radius ratio of 3.06, the heat transport coefficient increases as the eccentricity increases. An empirical correlation for Nusselt number is derived as a function of Reynolds number, Prandtl number and eccentricity. Highlights • For D o / D i = 1.875, the effect of eccentricity is identified into two zones. • For D o / D i = 3.06, the increase of eccentricity enhances the convective heat transport. • The increase of radius ratio strengthens the heat transport of annular channels. • With low eccentricity and low Reynolds number, the increase of radius ratio can hardly improve the heat exchange efficiency. • Three kind of empirical correlations are regressed through parametric analysis. [ABSTRACT FROM AUTHOR]

Abstract The advancements in the materials science and the ability of manufacturing new materials, make it possible for engineers to use the porous medium as a solid body that provide maximum surface contact with the fluid. In this paper, to increase the contact surface area and decrease the flow resistance, the solid tapered fins replace with porous one and its effect on convective heat transfer is investigated for different Reynolds number. To do this, first the heat transfer of the rigid tapered fins is studied and validated, then, the solid body is changed to the porous one and the influence of the Reynolds number was investigated on the heat transfer and pressure drop. The results show that in laminar flow the Nusselt number of the flow with porous medium is 33% higher and the pressure drop is 9.35% lower than the rigid one with the same conditions. Therefore the application of the porous medium at low velocities is much recommended but with increasing the velocity, the porous media is not preferable. Also, for the problem of flow around a bundle of porous tapered fins, an equation for Nusselt number is presented based on the Reynolds number. [ABSTRACT FROM AUTHOR]

Abstract This paper discusses on the heat and fluid flow around two co-rotating cylinders in the tandem arrangement. The non-dimensional rotating speed (R.S) varies in the range of 0 ≤ R.S ≤ 4 and different non-dimensional gap spaces such as G/D = 1.5, 2.0, and 3.0 are considered between the cylinders. Computations are performed at the Reynolds number of 200 with constant Prandtl number of 7.0. It is demonstrated that rotating the cylinders deforms the recirculating regions of both upstream and downstream cylinders in which the rate of this deformation changes as a function of the R.S and G/D. On the other hand, co-rotating the cylinders shows some additional events such as the azimuthal displacement of the front stagnation points and development of the negative lift coefficient for both cylinders. It is found that the instabilities of the shear layer for both upstream and downstream cylinders are maximum at R.S = 1 and with increasing the R.S, the vortex shedding suppresses around the cylinders due to dominating the fluid rotating zone. Finally, it is revealed that at higher R.S values, a uniform Nusselt number distribution can be observed on both cylinders regardless of the gap space between the upstream and downstream cylinders. Highlights • Heat and fluid flow of both cylinders strongly depends on the RS and G/D. • Vortex strength of both cylinders against the RS is stronger for small G/D values. • At high RS values, a uniform Nusselt number distribution develops on the cylinders. • The RS = 1 is found as the critical RS at Re = 200. • Oscillations of the lift and drag coefficients become maximum at RS = 1. [ABSTRACT FROM AUTHOR]

In a double-wall cooling system with multiple jet impingement the arrangement and size of effusion holes may change flow field and thereby change heat transfer characteristics. This paper stands in the view of internal cooling of turbine blades, and studied multiple-jet impingement heat transfer in double-wall cooling structures with narrow channels with pin fins and different-size effusion holes on the target wall. Five target plates were investigated including flat plate, pin fin plate and three pin fin plates with different-size effusion holes (effusion-to-jet diameter ratio of D e / D j = 0.5 , 1.0 a n d 1.5 ). Transient liquid crystal thermography experiments were conducted to explore the heat transfer characteristics on these target plates. The ratio of jet-to-plate spacing was fixed to be 1.5 and Reynolds numbers based on the jet diameter range from 15,000 to 30,000. The experimental results showed that the pin fins and effusion holes reduce the crossflow strength in downstream region, improve and uniform heat transfer on the whole target plate obviously. Compared with the flat plate, pin fin plate with effusion holes of D e / D j = 1.5 has highest averaged Nusselt number on the endwall. Numerical computations were carried out based on the experimental model, which revealed that the total heat transfer quantity on the pin fin plate with effusion holes of D e / D j = 1.5 can be increased by up to 51% comparing to that of the flat plate. Detailed interactional flow information between the wall jet flow, pin fins and effusion holes is expounded for the heat transfer improvement in the impingement-effusion structures. Moreover, conjugate heat transfer analyses were done to further investigate the overall cooling performance of the impingement-effusion structures. [ABSTRACT FROM AUTHOR]

In this paper, thermohydraulic characteristics and enhanced heat transfer mechanism in water–cooling oil heat exchanger with various shapes and attack angles of vortex generators (VGs) and spacing of double VGs are numerically investigated at different Reynolds number (Re) and oil–water mediums. A CFD-aided method, conjugated heat transfer model, is performed to elaborate the correlation between heat transfer and flow characteristics. The quantitative and qualitative results manifest that the longitudinal and secondary vortices, induced by VGs, generate synergistically the tapered and elliptical enhanced heat transfer regions on the heat exchange fins, but the inhibiting effect still exists between both, particularly in high attack angle or oil medium. Elliptical winglet VG with 30° in oil medium with Re = 349 and 15° in water medium with Re = 2726 gains the maximum thermal performance factor, reaching 1.69 and 1.28, respectively, compared without VGs. Furthermore, the appropriate configuration of two core heat transfer regions is a cost-optimal method to obtain in-depth synergistic heat transfer. The spacing of 4 mm shows the best synergistic heat transfer, and the maximum heat flux of its was elevated to 2.2% and 8.6% more than that of single VG in oil and water medium, respectively. [ABSTRACT FROM AUTHOR]

The droplet impacting on a substrate is widely encountered in daily life and industrial applications. In this paper, the impact dynamics and heat transfer are numerically simulated for both single droplet and binary droplets on a hot superhydrophobic substrate. The dimensionless numerical model is built up through the transient 2D axisymmetric model with a volume of fluid (VOF) model. The effect of the Weber number, Reynolds number, size ratio, contact angle on the spreading dynamics and heat transfer is investigated in details. It is found that the spreading factor and contact time increase with the increasing Weber number. Besides, the maximum spreading factor and contact time of binary droplets are larger than those of a single droplet under the same Reynolds and Weber numbers. The transient heat transfer rate of a single droplet is larger than that of binary droplets impingement due to larger spreading surface area-to-volume. The total input heat of a single droplet is generally larger than that of binary droplets except at large Weber numbers, and the equal-sized binary droplets have the larger total input heat than un-equal-sized binary droplets. For binary droplets impact on a hot superhydrophobic surface, the hot substrate can promote the spreading and retard the receding due to thermo-capillary effect, and thus will enhance heat transfer between the droplet and the hot substrate. These findings may be helpful in gaining insights into the dynamics and heat transfer of binary droplets impact on the hot substrate. [Display omitted] • Spreading factor and contact time of binary droplets are higher than single droplet. • Heat transfer of single droplet is higher than binary droplets. • Heat transfer is the best for equal-sized droplet. • Thermal-capillary effect promotes the spreading and retards the receding. [ABSTRACT FROM AUTHOR]

Augmentation of heat transfer without increasing the pressure drop has been a challenge in turbine blade internal cooling. Ribs are commonly used to achieve this goal and it is imperative to keep the pressure loss as low as possible. In this paper, the thermohydraulic performances of a number of perforated ribs are examined and compared. Three geometrical features of the ribs include hole inclination angle (0°, 30°, and 45°), relative hole inlet height (0.2, 0.4, 0.6, and 0.8), and the variation of the cross-sectional area of the circular holes (convergent, straight, and divergent). The ribs are mounted in a rectangular channel (A R = 2:1) and flows with a range of Reynolds numbers (10,000 to 25,000) are examined. A validated SST Gamma-Theta model is employed to evaluate the turbulent heat transfer and pressure loss characteristics. It is found that the permeable rib labeled as Case #212, improves the performance between 4.35 and 6.39% for flows with Reynolds numbers between 10000 and 25000. Among the investigated configuration parameters, the hole inclination has the highest effect on the thermal performance enhancement. • Study on heat transfer and pressure drop characteristics of perforated ribs. • Role of each geometric parameter of the perforated rib is determined. • Flow structure is investigated to obtain fundamental reasons for performances. • Best performance by perforated ribs for Reynolds number from 10,000 to 25,000. [ABSTRACT FROM AUTHOR]

In order to improve the cooling process in impingement synthetic jets, it is necessary to evaluate the influence of dimensionless parameters and the geometry of the flow field and heat transfer. In this paper, the effects of geometry (confined and unconfined impingement synthetic jets), and jet-to-surface spacings, Reynolds number and dimensionless stroke length on a three-dimensional unsteady impingement synthetic jet are studied. For this purpose, two types of turbulence models, namely the v 2 − f and S S T / k − ω , have been employed. The simulation results have been indicated that the v 2 − f model comparing to the S S T / k − ω model has a close agreement with the available experimental data. The results show that the flow field and the corresponding heat transfer distribution of the impingement synthetic jet are affected by geometry such that an unconfined impingement synthetic jet is more efficient in a cooling process relative to the confined case. Also, increasing jet-to-surface spacing affects the vortex structure and consequently the heat transfer. The stagnation heat transfer rate reaches to the maximum value at an optimum impingement distance as a result of the appropriate ventilation and the coherence vortex structures. The stagnation heat transfer rate experiences several extrema as a result of the increasing stroke length and under the higher stroke length, the impingement synthetic jet acts similar to the impingement continuous jet. [ABSTRACT FROM AUTHOR]

In this paper, the effect of vortex induced vibration (VIV) on convective heat transfer from an elastically mounted rigid circular cylinder in cross-flow is investigated numerically. The motion of cylinder is modeled by using a mass-spring-damping system. The effect of reduced velocity and damping ratio on the vortex formation, vortex shedding process, cylinder displacement amplitude, Nusselt number and the position of maximum local Nusselt number is studied. In this study the Reynolds Number and the mass ratio are 150 and 2, respectively. Furthermore, different values of reduced velocity and damping ratio are investigated ( U r = 3, 4, 5, 6, 7, 8 and   ζ = 0. 0.01, 0.05, 0.1). The numerical results demonstrate that the reduced velocity and damping ratio can affect the heat transfer considerably. The beating phenomenon is occurs at U r = 4 and ζ = 0.05 which leads to changes in the displacement amplitude and Nusselt number widely with respect to the time. In the beating phenomenon the total Nusselt number decreases in comparison with a stationary cylinder. The maximum heat transfer enhancement is obtained at U r = 4 , ζ = 0 . [ABSTRACT FROM AUTHOR]

Perforated plates interspaced with spacers are effectively used in recuperative heat exchangers. In this paper, we present generalized correlations for Colburn factor and friction factor of perforated plates as functions of all geometrical variables and Reynolds number. Heat transfer and flow friction data are generated through CFD modelling on a stack of perforated plates. The model is validated with the published experimental data. Correlations are developed through regression analysis over a large number of computed data generated under varying parametric conditions. Effects of various geometrical parameters on the thermos-hydraulic characteristics of perforated plates are discussed in terms of Colburn factor, friction factor and modified area goodness factor. [ABSTRACT FROM AUTHOR]

In this paper, a two-dimensional numerical simulation is carried out to understand the effect of confinement (blockage ratio β ) on fluid flow and forced convective heat transfer characteristics past a blunt headed cylinder. Utilizing air as an operating fluid, flow simulations are carried out for wide ranges of blockage ratios ( 1 10 ≤ β ≤ 1 3 ) and Reynolds numbers ( 60 ≤ Re ≤ 200 ) . The flow characteristics and heat transport are analysed critically for different β . The functional dependence of C D (Drag Coefficient) and C L (Lift Coefficient) on blockage ratio is examined. It has been found that C D reduces with increasing Re, while the Strouhal number and the average Nusselt number show an increasing trend when the blockage ratio is increased. The average Nusselt number also increases with increasing Re . [ABSTRACT FROM AUTHOR]

The present paper presents fluid flow and mixed convective heat transfer characteristics past a semi-circular cylinder at incidence with a tandem square cylinder in cross flow. Using air ( Pr = 0.71 ) as an operating medium, numerical simulations are performed for the range of Reynolds numbers, 10 ≤ Re ≤ 45 and incidence angles, 0 0 ≤ α ≤ 180 0 . The effect of thermal buoyancy is brought about by varying the Richardson number in the range 0 ≤ R i ≤ 2 . The mathematical model is firstly validated with the experimental and numerical results from the literature and found to be in good agreement. The steady separated flow is observed to become unsteady periodic under the superimposed thermal buoyancy. Furthermore, apart from thermal buoyancy effect, angle of incidence is found to play a pivotal role in bringing hydrodynamic instabilities and thereby vortex shedding for such steady mixed convective flows. Functional dependence of drag ( C D ) , lift ( C L ) , and moment ( C M ) coefficients on the combined influence of α and R i , is explored and analysed in detail. Additionally, other global quantities, such as local and average Nusselt number distribution, Strouhal number ( S t ) are determined with respect to the various ranges of parameters considered in the present investigation. [ABSTRACT FROM AUTHOR]

In this paper, 2-D numerical investigation and sensitivity analysis are performed on heat transfer rate and heat exchanger effectiveness of a double pipe heat exchanger filled with porous medium. The Darcy–Brinkman–Forchheimer model is applied to model the flow field in the porous zone. The sensitivity analysis is performed utilizing the Response Surface Methodology. The studied parameters are: Reynolds number (50 ≤ Re ≤ 250), Darcy number (10 −5 ≤ Da ≤ 10 −3 ), temperature difference between hot and cold fluids (30 ≤ ΔT ≤ 70) and the porous substrate thickness (1/3 ≤ δ ≤ 1). The obtained results showed that enhancement of the Nusselt number due to the increase in Reynolds and Darcy numbers is in the vicinity of the 77.84% for the case with δ = 2/3 and Da = 10 −5 to 10 −3 , and 203.25% for the case with δ = 1 and Re = 50 to 250. Furthermore, increasing porous substrate thickness reduces the mean Nusselt number until δ = 2/3 and then increases it. In addition, it is found that the heat exchanger effectiveness increases with the Re number and reduces with enhancement of the Da number. The sensitivity analysis showed that the sensitivity of the mean Nusselt number to the Re and Da numbers and the porous substrate thickness is positive, while the sensitivity of the heat exchanger effectiveness to the Re number is positive but to the Da number is negative. [ABSTRACT FROM AUTHOR]

In this paper, a numerical model is proposed to study the transition to three-dimensionality and the evolution of vortices in the wake of a blunt-headed cylinder considering air as the working fluid in the Reynolds number (Re) range 130–2000. An experimental setup is developed to validate the numerical model on the basis of heat transfer characteristics. Based on symmetry and time dependence characteristics of flow, it is categorized as dynamically steady symmetric flow (50 ≤ Re ≤ 200), dynamically steady unsymmetric flow (220 ≤ Re ≤ 270) and unsteady flow (Re ≥ 280). The vortex dislocation phenomenon showing intermittent low frequency pulsations during the transition to three-dimensionality is studied. Aerodynamic characteristics are studied by computing force coefficients and Strouhal number (St). Brief overview of heat transfer characteristics is given. Dynamic mode decomposition (DMD) analysis is carried out to determine quantitatively the critical Re at which secondary instability occurs and hence clear demarcation of the Re regime is obtained for the validity of two-dimensional analysis of blunt headed cylinder. [ABSTRACT FROM AUTHOR]

The present paper compares measured and calculated heat transfer parameters for a small-size slot supersonic nozzle. Heat transfer coefficient and temperature recovery factor were examined under conditions of partial laminarization of boundary layer caused by moderate flow acceleration and low Reynolds numbers. The transient heat transfer method was implemented for estimation of these values for the nozzle subsonic and supersonic parts. Maximum values of flow acceleration parameter K lied in the range 1.3 ... 2.75∙10−6. Experimentally measured value was normalized to the developed turbulent boundary layer heat transfer coefficient and reached St/St Rex = 0.5–0.7 and indicated partial laminarization phenomenon. It was shown that the temperature recovery factor hasn't been affected by the flow acceleration. The results are compared to numerical heat transfer predictions using sst, γ-sst and γ-Re θ -sst turbulence models. Numerical calculations and experimental data were found to be in reasonably good agreement for low acceleration level with K less than 0.75∙10−6. Nevertheless, γ-sst turbulence model made slightly conservative estimation whereas in contrast γ-Re θ -sst model overestimates quantities for the moderate flow acceleration conditions. • Heat transfer in a small-size slot supersonic nozzle has been investigated. • Effect of flow acceleration on partial laminarization was assessed. • Temperature recovery factor does not depend on the flow acceleration. • Heat transfer coefficient reduces up to two times. [ABSTRACT FROM AUTHOR]

This paper presents the forced convective heat transfer performances of novel airfoil heat sinks produced by Selective Laser Melting (SLM). Heat sinks with staggered arrays of NACA 0024 and NACA 4424 airfoil shaped fins were investigated experimentally and the results were compared with conventional heat sinks with circular and rounded rectangular fins. In addition, NACA 0024 heat sinks with angles of attack (α) ranging from 0° to 20° were also fabricated and the effects of the angle of attack (α) on the heat sink thermal performances were examined. Experiments were conducted in a rectangular air flow channel with tip ( CL t ) and lateral ( CL h ) clearance ratios of 2.0 and 1.55 and with Reynolds numbers (Re) ranging from 3400 to 24,000. Numerical studies were first performed to validate the experimental results of the circular finned heat sink and reasonably good agreement between the experimental data and numerical results were observed. Comparison of the experimental results showed that the heat transfer performances of the airfoil and rounded rectangular heat sinks exceeded those of the circular heat sink. The experimental Nusselt numbers were computed based on the heat sink base area (Nu b ) and the total heat transfer area (Nu t ). In comparison with the circular heat sink, highest enhancements in Nu b and Nu t of the NACA 0024 heat sink at α = 0° were 29% and 34.8%, respectively. In addition, the overall heat transfer performances of the NACA 0024 heat sinks were also seen to increase with increasing α. The results suggest that the streamline geometry of the airfoil heat sink has low air flow resistance, which resulted in insignificant bypass effect and thereby improving the heat sink thermal performance. In addition, the increase in α further improves the heat transfer performance of the NACA 0024 heat sinks through the formation of vortices which enhanced fluid mixing. Finally, based on the above mechanisms proposed, a semi-analytical model was developed to characterize the heat transfer performances of the NACA 0024 heat sinks for the range of α and Re tested. In comparison with the experimental data, reasonably accurate predictions were achieved with the model where the deviations in Nu b were less than 7% for Re ≥ 6800. [ABSTRACT FROM AUTHOR]

This paper investigates a Taylor-Couette-Poiseuille flow in an annular channel of a slotted rotating inner cylinder, corresponding to a salient pole hydrogenerator. The purpose of this study is to improve the understanding of flow and thermal phenomena in electrical machines using a simplified scale model. The validation of the numerical model for a specific configuration is first shown by comparing the results with the experimental data. A parametric study is also performed to investigate all main flow regimes and to derive correlations in terms of the Nusselt number distribution on the rotor pole face and sides. The results show that the Nusselt number is proportional to the tangential Reynolds number to the power 1/7 in the pole and inductive faces trailing side. This relationship is similar to the one encountered in classical Taylor-Couette-Poiseuille flows between two concentric and smooth cylinders. [ABSTRACT FROM AUTHOR]

Jet impingement cooling is usually used in zones with extremely high thermal loads. The major factor that limits impingement heat transfer uniformity is the crossflow. The impingement cooling with return hole is applied to reduce the negative effect of crossflow on the target surface heat transfer in present paper. The influences of jet-to-plate spacing (H / D), impingement target surface curvature (α), temperature ratio (TR) and Reynolds number on flow field and heat transfer performance are investigated in detail. The results indicate that the correlation between the overall area average Nusselt number of target surface and H / D , the overall area average Nusselt number decreases by approximately 5.76% with H / D increases by one unit. Increasing the impingement target surface curvature is beneficial to expand the spanwise heat transfer. The overall area average Nusselt number increases by 295.5% for Reynolds number varying from 10,000 to 50,000 at TR = 0.910, and the overall area average Nusselt number increases by 8.68% for the TR varying from 0.668 to 0.910 at Re = 30,000. The results indicate that impingement cooling with return hole can achieve high heat transfer coefficient and uniform heat transfer distribution. [ABSTRACT FROM AUTHOR]

This paper compares the thermohydraulic performance of varying curvature wavy microchannels, namely elliptical-wavy and sinusoidal-wavy microchannels. The amplitude of microchannels is fixed at 0.6 mm, and wavelengths are varied from 3 mm to 8 mm. Simulations are performed for the laminar flow of water (160 ≤ Re ≤ 900) considering three-dimensional conjugate heat transfer using ANSYS Fluent version 2020R2. The computational model has been validated by performing experiments on a serpentine-wavy microchannel, a special case of the elliptical-wavy microchannel, using micro-particle image velocimetry (μ-PIV) test facility. Both Nusselt number and friction factor increase with the increase in waviness. Elliptical-wavy microchannels have superior thermohydraulic performance than sinusoidal-wavy microchannels except for the channel of highest waviness at Re = 160. A microchannel with lower entropy generation has a higher thermohydraulic performance. Further, we observe centrifugal instabilities in both sinusoidal-wavy and elliptical-wavy microchannels at higher Reynolds numbers. Centrifugal instabilities in the elliptical-wavy differ from those in the sinusoidal-wavy microchannel by the asymmetric formation of additional vortices about the horizontal midplane in the channel cross-section. [ABSTRACT FROM AUTHOR]

This paper explores the effect of curved shape fin heights on open microchannel heat sink performance. Different geometries of rectangular, plano-convex, and plano-concave fins have been investigated by employing three-dimensional simulation. A variety of parameters including fin height, curve range coefficient, Reynolds number, and heat flux have been examined. At the same heat flux, Reynolds number, and fin height, the Nusselt number of the rectangular fin is lower than the two shapes of plano-convex and plano-concave. In Comparison of the two curved shapes, due to their geometries and fluid flow behavior, in the lower fin heights, the Nusselt number of the plano-concave is greater than the plano-convex. In the higher fin heights, the Nusselt number of the plano-convex is greater than the plano-concave. Even though at higher fins, where the Nusselt number of the plano-convex fins is greater than the rectangular and plano-concave shape, the plano-convex pressure drop is less than these two shapes. In a heat flux of 450 k W / m 2 , Reynolds number of 600, and a fins equivalent height of 0.6 mm, that the volume and mass of the fins are the same, there is an 18% difference between the maximum and minimum Nusselt number of plano-convex and plano-concave, by different curve coefficients. The obtained results are beneficial in designing novel shape fins in enhanced open microchannel heat sinks. [ABSTRACT FROM AUTHOR]

Mini-channel heat exchangers are extensively used to achieve heat dissipation with high heat flux, while the problem of flow maldistribution seriously impairs the thermal efficiency and operational reliability of heat exchangers. In this paper, the flow distribution characteristics of mini-channel heat exchangers with the conventional manifold model are numerically studied, and the influence of flow maldistribution on heat transfer is analyzed. To suppress the problem of flow maldistribution, an innovative approach for improving fluid uniformity by the built-in spiral baffle in the inlet manifold is proposed, and the effects of spiral baffles with different pitches and radii on flow distribution are discussed. Besides, the heat transfer characteristics and pressure drop under different manifold models are also compared. Finally, the variable-pitch spiral baffle is introduced to realize the stepped regulation of flow distribution. The results show that the conventional manifold model has serious flow maldistribution and causes the deterioration of heat transfer in front-end channels, especially at high Reynolds numbers. The improved manifold model greatly improves the uniformity of the fluid and temperature distribution inside mini-channel heat exchangers on the premise of sacrificing a small amount of pressure drop. The optimal pitch and radius of the spiral baffle are 20 mm and 9 mm respectively, and the flow maldistribution in almost all channels is controlled within the range of ±15% at this case. In addition, the variable-pitch spiral baffle further improves the flow uniformity of the mini-channel heat exchanger, and shows better overall performance than the equal-pitch spiral baffle. • The influence of flow maldistribution of the conventional manifold is analyzed. • An innovative method for improving the fluid uniformity is proposed. • The performances of spiral baffles with different parameters are compared. • Variable-pitch spiral baffle shows better comprehensive performance. [ABSTRACT FROM AUTHOR]

Swirling flows have applications in several areas which includes enhanced heat transfer and combustion. In these applications, swirling jets typically impinge on a target surface either orthogonally or at an angle. This paper presents an experimental and numerical investigation on the effects of jet-to-target spacing (z/d – 0.5 to 4) on heat transfer characteristics of orthogonally impinging swirling jets in comparison to geometrically similar cylindrical jet. Further, obliquely impinging jets (at z/d = 4) with anti-clockwise swirl (when viewing target surface from the jet) have been studied. Transient liquid crystal thermography experiments were carried out for above configurations at jet Reynolds number of 18,000 for orthogonal impingement and for Re ranging from 9000 to 18,000 for obliquely impinging swirling jet. The experimentally obtained local heat transfer when studied in conjunction with the flow-field predictions reveal that swirling jets can result in high heat transfer compared to conventional cylindrical jets at low z/d, where the area coverage of swirling jet is smaller than cylindrical jet. With increasing z/d, swirl jet impingement heat transfer showed significant reduction (while increasing coverage) while cylindrical jet impingement primary heat transfer zone was not very sensitive towards it. This is attributed to increased expansion of swirling jet exiting the nozzle at higher z/d, while cylindrical jet retaining its potential core for the z/d range investigated here. Obliquely impinging swirling jet had reduced stagnation zones due asymmetric arrangement of swirl passages with respect to target surface. [ABSTRACT FROM AUTHOR]

The main aim of this paper is to investigate the effect of nozzle geometry on the flow and heat transfer from a pulsed jet into a concave surface. Experiments have been carried out for pulsed circular jet and numerical simulations were done for circular, elliptical, rectangular and square geometries. Numerical solution was performed for the frequency ranges of 25 H z to 100 H z , the jet Reynolds number of 7000 , the dimensionless nozzle distance to concave surfaces of 2 and 5. Numerical results show a consistent agreement with experimental results and previous works. Accordingly, the geometry of nozzle directly affects the air entrainment ratio. In addition, with the increase of aspect ratio of the nozzle, average Nusselt number (Nu ave) decreases. By pulsating the inlet jet with pulse frequency of 100 H z , the average Nu number for circular/square and elliptical/rectangular jets increases 22 % and 15 % , respectively. Generally, the pulsation causes a reduction and an increase of the Nu ave at low and high frequencies, respectively, compared to that of the steady jet. At low frequencies, the Nu ave of circular jet decreases significantly in comparison to that of the square and elliptical jets. However, at high frequencies, circular jet shows higher Nu ave compared to other ones. As the distance between the jet and the concave surface increases, the effect of the nozzle shape on Nu ave decreases. [ABSTRACT FROM AUTHOR]

This paper proposed a swirl chamber with circumferential ribs to avoid the impacting of cross-flow to jets, thus enhancing the penetration ability of jet and swirl cooling heat transfer intensity. To reveal the mutual effects between jets and cross-flow, the heat transfer features and flow structures under different Reynolds numbers of smooth swirl chamber and novel rib-roughened swirl chambers were studied and compared using 3D steady numerical analysis. The influence of rib height ratio on flow and heat transfer features was investigated further. The investigation revealed that for smooth swirl chamber, the cross-flow seriously weakened the heat transfer intensity. In contrast, the novel rib-roughened swirl chamber had an excellent suppression effect on cross-flow, thus greatly enhancing the penetration ability of jet and strengthening the swirl cooling heat transfer intensity. Under the Reynolds number of 10,500, for smooth swirl chamber, the peak number of circumferentially-averaged Nusselt number decreased from 80 to 58 from the first pitch to the last pitch. While for the case that rib height ratio is equal to 0.5, it increased from 80 to 120. In addition, the suppression effects of circumferential ribs on cross-flow increased when the rib height ratio increased, likewise the heat transfer enhancement effect. When the rib height ratio is less than 0.5, the Nusselt number ratio was larger than the friction factor ratio, while when the rib height ratio is larger than 0.5, the Nusselt number ratio was less than the friction factor ratio. The best thermal performance can be achieved when the rib height ratio was 0.5. Besides, the heat transfer enhancement effect of circumferential ribs was more significant under a higher Reynolds number in the studying range. [ABSTRACT FROM AUTHOR]

The feasibility of achieving microscale heat transfer effects in macro geometries using conventional fabrication methods has recently been demonstrated. This paper looks at passive techniques, using nature-inspired Inverted Fish Scale geometrical design, to improve the heat transfer performance of the newly proposed system. In this study, an annular microchannel, with gap size of 300 μm, is formed by securing a cylindrical insert of mean diameter 19.4 mm within a cylindrical pipe of internal diameter 20 mm. The Inverted Fish Scale profile is introduced on the insert surface, so as to improve heat transfer through increasing the convective heat transfer coefficient of the flow, for a constant heat transfer area. Both experimental and numerical investigations are carried out to study the effect of the Inverted Fish Scale enhancement profile on the heat transfer and flow characteristics of the microscale flow. Single-phase liquid flow using distilled water is examined, with Reynolds number ranging from 1300 to 4600. The microchannel is considered hydraulically smooth, with length of 30 mm and hydraulic diameter of 600 μm. Results show that the Inverted Fish Scale (IFS) profile indeed has positive effect in enhancing heat transfer. The maximum convective heat transfer coefficient achieved in the whole study is 52.8 kW/m 2 ·K, using IFS insert with scale height of 0.21 mm and pitch length of 2.1 mm, at Reynolds number of 4300. This is more than twice the value using Plain insert at the same flow condition. The possible enhancement mechanisms include re-initialization of velocity and thermal boundary layers, flow recirculation and higher turbulence intensity. In addition, the thermo-hydraulic performance factor, which incorporates the undesirable increment in friction factor, is examined. The thermo-hydraulic enhancement of the IFS profile is generally found to be more effective for 1300≲Re≲3250. In particular, the IFS insert with scale height of 0.21 mm and pitch length of 2.1 mm performs 43% better than the Plain insert, at Reynolds number of 1700. New correlations for the average Nusselt number and friction factor are proposed for the IFS microchannel, to be used in the design of compact heat exchangers. Based on calculations, the present system is able to remove heat flux of up to 375 W/cm 2 . The pressure drop values of the system are all less than 3.3 bars, which may be overcome by a commercially available pump. The present study reiterates the feasibility of achieving microscale heat transfer effects in macro geometry systems, and demonstrates the effectiveness of the Inverted Fish Scale profile in enhancing heat transfer performance. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer enhancement in a parabolic trough receiver using wall-detached twisted tape inserts was numerically investigated. The resulting heat transfer, fluid friction and thermodynamic performance were determined and presented. The flow was considered fully developed turbulent, with Reynolds numbers in range 10 260 ≤ Re p ≤ 1 353 000 depending on the fluid temperature. The twisted tape's twist ratio and width ratio vary in the range 0.50–2.00 and 0.53–0.91, respectively. The numerical investigations are based on a finite volume method, with the realisable k – ɛ model for turbulence closure. The study shows considerable increase in heat transfer performance of about 169%, reduction in absorber tube's circumferential temperature difference up to 68% and increase in thermal efficiency up to 10% over a receiver with a plain absorber tube. An entropy generation analysis shows the existence of a Reynolds number for which there is minimum entropy generation for each twist ratio and width ratio. The optimal Reynolds number increases with increasing twist ratio and reducing width ratios. The maximum reduction in the entropy generation rate was about 58%. Correlations for heat transfer and fluid friction performance for the range of parameters considered were also derived and presented. [ABSTRACT FROM AUTHOR]

A novel type of pipe insert was developed to improve convection heat transfer in the present work. Heat transfer performance and pressure drop was numerically studied. Pipe inserts with different dimensionless spacer length (S/D = 3.33, S/D = 5, S/D = 6.67, S/D = 8.33, S/D = 10, S/D = 11.67) were investigated at the Reynolds number range of 100–1750. Liquid water was used as the working fluid. The results showed that the maximal Nusselt number was enhanced by 3.4–10.3 times as that of the smooth tube. The friction factor resulted in an increase of 5.6–13.5 times. Performance Evaluation Criterion (PEC) values were approximately 1.91–4.33. The Nusselt number increased with the decrease in dimensionless spacer length. A small spacer length resulted in a high heat transfer coefficient. However, it also brought a high flow resistance and eventually deteriorated the heat transfer performance. Therefore, a suitable dimensionless spacer length S/D = 6.67 was recommended in this paper. The effect of pipe shape (R/D = 3.33, R/D = 5, R/D = 6.67) on the thermal characteristic was also investigated in this study. It indicated that the pipe shape had a little impact on the heat transfer performance in the laminar regime. Compared with other inserts, pipe inserts can transfer more heat for the same pumping power for their structure, because their special structure make it possible for the fluid to flow from the central region to the wall region, which reconstructs the velocity profile and temperature profile in the tube and results in a high heat transfer performance. [ABSTRACT FROM AUTHOR]

The present paper explores the possibility of using wire-loop structures on the active plate of a parallel plate channel for efficient heat transfer augmentation. For this purpose, numerical simulations were carried out for periodically fully-developed turbulent flow in typical repeating modules using the realizable k – ε model. Experiments were also conducted in order to validate some of the results obtained from numerical simulations. For both studies, the Reynolds number was varied from 2000 to 20,000. The effect of three different loop-densities on fluid flow and heat transfer characteristics were investigated when wire-loops were placed perpendicular to the main flow direction, whereas the effect of loop-orientations on these parameters were studied for a fixed loop density of 2270 loops/m 2 . While for all investigated cases, substantial heat transfer augmentation was observed with wire-loop structures as compared to the empty parallel plate channel under the condition of identical pressure gradient, the thermal-hydraulic performance improved significantly with the increase in loop-density. The maximum attainable loop-density, however, was found to strongly depend on the loop-orientation owing mainly to the geometric as well as manufacturing constraints. It was also observed that loops, oriented diagonally to the main flow direction offer the best performance, although it is less sensitive to loop-orientation as compared to the loop-density. [ABSTRACT FROM AUTHOR]

A jet can be synthesized by ejecting and injecting fluid from and to an orifice or channel. When actuated, a synthetic jet delivers positive momentum but no net mass flow per cycle. Small, compact synthetic jet actuators can be fabricated to operate in the subaudible acoustic range and can be packaged in orientations that allow them to deliver cooling air flow to electronic devices. For cooling, the most promising orientation is one that delivers the jet flow in a direction normal to the heated surface such that it impinges on the surface as a periodic jet. In previous studies, numerical simulations were performed by the authors, utilizing an idealized canonical geometry, with the goal of eliminating actuator artifacts from the fundamental physics that drive the problem. The present paper reports on laboratory experiments that were undertaken in order to nearly replicate the idealized synthetic jet geometry and thus allow validation of the previous numerical investigations. The experiment was designed at a geometrically large scale, with jet slot widths of 4.2 and 6 mm. The amplitude and frequency at which the jet was actuated determined the Reynolds (Re) and Womersley (Ω) numbers which are the dominant non-dimensional groups. By maintaining Re and Ω in the laboratory experiments to match those of the small scale actuators, the laboratory experiments were geometrically scaled up to allow highly resolved measurements of the unsteady velocity field and the local time-dependent Nusselt number on the target heated surface. Experiments were performed at variable jet Re, Ω, and height from the target surface. The dependence of the surface averaged Nu to jet parameters agrees with the computational results when the convective heat transfer is significantly larger than the experimental heat losses, and when the fluid remains coherent with respect to the forcing frequency. Discrepancies found between numerical and empirical local data at large spacings, high Ω and low Re, suggest flow transition to turbulence and possible emergence of three dimensional effects, which were not accounted for in the computational analyses. [ABSTRACT FROM AUTHOR]

Integrated a fully developing three-dimensional heat transfer and flow model, a multi-objective optimization aims to fulfill the geometric design for double-tube heat exchangers with inner corrugated tube is investigated in this work with RSM. Dimensionless corrugation pitch ( p / D ), dimensionless corrugation height ( H / D ), dimensionless corrugation radius ( r / D ) and Reynolds number ( Re ) are considered as four design parameters. Considering the process parameters, the characteristic numbers involving heat transfer characteristic, resistance characteristic and overall heat transfer performance calculated by CFD, and are served as objective functions to the RSM ( Nu c , f c , Nu c / Nu s , f c /f s and η in this paper). The results of optimal designs are a set of multiple optimum solutions, called ‘Pareto optimal solutions’. It reveals the identical tendency of Nu c / Nu s and f c / f s reflecting the conflict between them that means augmenting the heat transfer performance with various design parameters in the optimal situation inevitably sacrificed the increase of flow resistance. According to the Pareto optimal curves, the optimum designing parameters of double pipe heat exchanger with inner corrugated tube under the constrains of Nu c / Nu s ≥ 1.2 are found to be P / D = 0.82, H / D = 0.22, r / D = 0.23, Re = 26,263, corresponding to the maximum value of η = 1.12. [ABSTRACT FROM AUTHOR]

In this paper the flow of an incompressible Newtonian fluid past a stationary unconfined spheroid under the mixed convective heat transfer regime is studied numerically. The spheroid is assumed to be maintained at a constant wall temperature. The effect of Reynolds number (Re), Prandtl number (Pr), aspect ratio ( E ) and Richardson number (Ri) on the average drag coefficient and Nusselt number is investigated. The range of parameters considered in this study is: 10 ≤ Re ≤ 100; 0 < Ri < 2 and 0.5 ≤ E ≤ 1.5 and for two Prandtl numbers of 1 and 5. The drag coefficient increases monotonically with the increase of surface temperature for all Reynolds number. Prolate spheroids have higher drag coefficient and Nusselt numbers compared to oblate spheroids. The variation of Nusselt number with Ri at a fixed Re is non monotonic at large Re and this effect is stronger for prolate spheroids when compared to oblates. For aspect ratios greater than unity and for Re > 60, Nusselt numbers for small Ri (∼0.5) are lower compared to Nusselt numbers for Ri = 0. This difference in Nusselt numbers becomes appreciable for larger Prandtl numbers. A correlation for average Nusselt number was developed using least squares regression analysis. [ABSTRACT FROM AUTHOR]

As an effective technology to reduce flow resistance, drilling holes on the surface of vortex generators had been studied extensively. In this paper, thermal performance of punched rectangular winglet vortex generators (PRWVGs) in turbulent regime (Reynolds number was in the range of 9,090 ∼ 21,210) was studied experimentally and numerically. Air was applied as the working fluid. The PRWVGs were inserted into the tube with three attack angles (α = 30 ° , 45 ° , 60 °). The punched holes were set in three different positions (g = 3 m m , 5 m m , 7 m m) in the vertical direction and at three different hole heights (d = 1 m m , 3 m m , 6 m m). The performance of PRWVGs was quantified by the heat transfer coefficient ratio (h / h 0), the friction factor ratio (f / f 0), and a combined thermal enhancement factor (T E F) of the two. The results revealed that h / h 0 as well as f / f 0 values of PRWVGs compared with those of planar VGs (without holes) were reduced. The flow and temperature fields of different cases were obtained by numerical simulations, and the experimental results were verified by analyzing the fluid flow behaviors. The results showed that the jet flow (defined as a jet emanating from the punched hole of a VG), would enhance the heat transfer efficiency in the recirculation area. And the T E F value of VGs was improved by punching holes on the surface of VGs. However, the shape of the jet flow depended on the position of the hole. When the position of the hole was lower in the vertical direction, the jet flow was effective in increasing its T E F value. Additionally, the size of the hole had a large effect on the T E F value. Besides, entropy generation analysis showed that PRWVGs were thermodynamically advantageous. When d = 1 m m , w = 5 m m , the Bejan number reached the minimum. Under this condition, the proposed PRWVGs performed best in practice. When the height of the hole was d = 1 m m , T E F was the maximum, which was 1.25 at α = 45 ° and R e = 9,090. [ABSTRACT FROM AUTHOR]

Impingement cooling is an important turbine blades cooling technique, but it is often not as efficient as required due to the deflection of downstream jets caused by crossflow. In this paper, a novel corrugated target surface is proposed to convert downstream oblique impingement into orthogonal impingement for higher heat transfer. To prove this design, the flow and heat transfer characteristics of impingement cooling with conventional flat target surface (Baseline case), full-corrugated (FC) target surface, and semi-corrugated (SC) target surface are compared over the jet Reynolds number ranging from 15,000 to 45,000. Results show that the Nusselt number of full-corrugated target surface is almost the same as that of flat target surface at the region corresponding to the first five jets, but its peak value and uniformity are significantly improved at the region corresponding to the last four jets, especially under high jet Reynolds numbers. In addition, with the increase of corrugation depth (H), the total heat transfer capacity and area-averaged Nusselt number increase, but this comes at the cost of increased friction loss. Comprehensively evaluating heat transfer and friction loss, only the FC with H = 0.8 D case always has better thermal performance than Baseline case under all computed conditions, and the maximum improvement can reach 5.8%. Last but not least, the SC with H = 0.8 D case is proposed based on the FC with H = 0.8 D case to further reduce the friction loss, and results show that its thermal performance is improved significantly due to the decreased friction loss and almost unchanged heat transfer. [ABSTRACT FROM AUTHOR]

The heat transfer performances of pipes with twisted tape inserts having periodically spherical dimple-protrusion patterned walls are numerically investigated by Ansys Fluent under a constant heat flux of 5 W/cm2. The inserts having three different densities (N = 0, 30, and 45) and three different kinds of arrangement (composed of only dimples, only protrusions, and a combination of dimples and protrusions) with a constant twist ratio of 3 were tested for Reynolds numbers (Re) within the range from 3000 to 27000. Comparing the turbulence models proposed the validated realizable k-ϵ model coupled with enhanced wall function to employ on the numerical simulations. While the highest Nusselt number (Nu) and friction factor (f) values for the presented study were found for the dense protruded twisted tape (D-PT) arrangement at Re values lower than 18000 and for dense dimpled-protruded twisted tape (D-DPT) arrangement for the Re greater than 18000, dimpled-protruded twisted tape (DPT) and dimpled twisted tape (DT) sequences reached the lowest f values of 0.365 and 0.322 at Re = 3000, respectively. D-DPT and D-PT sequences reached the highest thermal performance factor (TPF) values of 1.508 and 1.478 at Re = 3000, respectively. All patterned twisted tapes performed better Nu than conventional twisted tapes in the analyzing range and performed better TPF than typically twisted tapes in the Re region of less than 6000. [Display omitted] • The effect of DT, PT, DPT, D-DT, D-PT, and D-DPT on heat transfer enhancement performance was numerically investigated. • All patterned twisted tapes performed better TFT than typically twisted tapes in the Re region of less than 6000. • D-DPT and D-PT sequences reached the highest TPF values of 1.508 and 1.478 at Re = 3000, respectively. [ABSTRACT FROM AUTHOR]

A numerical method is utilized in this paper to study the effects of varying divider inclination angle on the flow structure and heat transfer characteristics in a ribbed two-pass channel for Reynolds number varying from 10,000 to 50,000. The studied divider inclination angles are − 3 ° , − 2 ° , − 1 ° , 0 ° , + 1 ° , + 2 ° and + 3 °. The ribs have an orientating angle of 45 °. The results show that the heat transfer in the bend region and in the passage upstream the bend is enhanced significantly and the heat transfer in the passage downstream the bend is reduced compared to a channel with a straight divider wall as the divider wall inclination angle varies from − 3 ° to + 3 °. Channels with inclined divider wall produce higher pressure drop and friction factors than a channel with straight divider wall. According to the results, a channel with − 1 ° inclined divider shows the best thermal performance among all the channels investigated. The flow structure and heat transfer characteristics for all the channels are depicted in detail and discussed in this study. [ABSTRACT FROM AUTHOR]

This paper aims to provide a novel enhanced heat transfer method for the internal tip surface of a U bend channel of relevance for a gas turbine rotor blade. The DWVGs (delta-winglet vortex generators) pair is arranged at multiple locations on the tip surface. Two types of vortex generators are studied, including common-flow-up and common-flow-down configurations. The inlet channel Reynolds number varies from 10,000 to 50,000. The topological analysis method is used to determine the formation and evolution of the vortices and to better understand the mechanism of the heat transfer enhancement. Results of skin-friction lines, topological portrait, Nusselt number, friction factor, thermal performance are included. The results show that due to the interaction among the vortices surrounding the vortex generators, the DWVGs pair in common-flow-up configuration has a slight heat transfer improvement, and is not sensitive to the tip location. However, the DWVGs pair in common-flow-down configuration placed at the downstream of the tip surface improved the heat transfer significantly as the induced vortices between the turbulator pairs effectively reduce the thickness of the boundary layer. Compared with the smooth U bend channel, the optimal design shows that the heat transfer and overall thermal performance can be increased by up to 7.4% and 6.8%, respectively. This study elaborates the flow and heat transfer processes from the perspective of topology, which is helpful in the design of cooling procedures of turbine blades. [ABSTRACT FROM AUTHOR]

Finned tube heat exchangers are among the most common types of heat exchangers that are used in engineering applications. Most of the tubes used in finned-tube heat exchangers are of circular type, but the oval tubes have more desirable features as they impose lower pressure drops with approximately the same amount of heat transfer. In this paper, a finned oval tube is studied experimentally and numerically to obtain temperature fields around the tube at different locations. Moreover, several 3D numerical simulations are performed to study the effects of fin length, fin spacing and diameter ratio of the tube cross section. Based on the obtained results, fin spacing of 1 mm is the optimum at all tube aspect ratios and fin lengths when Reynolds number is low while at high Reynolds numbers, fin spacing of 3 or 4 mm is the better choice. As to the fin length considerations, for a short fin, a spacing of 3 or 4 mm is the best choice at high Reynolds numbers. For larger fin lengths, sensitivity to fin spacing is less but a fin spacing of 3 or 4 is preferable at high Reynolds numbers. • The excellence of oval tubes with rectangular fins to circular tubes with rectangular fins in certain cases is observed. • Fixing major tube diameter and increasing tube aspect ratio culminates in increased pressure loss and decreased heat transfer. • Smaller fin spacing is desirable at low Reynolds numbers for all cases. • Larger fin spacing is desirable for high Reynolds numbers when tube aspect ratio and fin length are smaller. [ABSTRACT FROM AUTHOR]