Search

• Numerical simulation of steam-liquid two-phase flow with swirling flow in different twisted tubes is carried out. • The volume fraction, velocity and temperature distribution are studied under the influence of swirling flow. • Un-uniform of the circumferential heat transfer coefficient is studied. • The absolute vorticity flux and secondary flow intensity of twisted tubes are quantitatively analyzed. • Comparison between two-fluid three flow field with the droplet model and EE+GENTOP concept is carried out. The Eulerian-Eulerian (EE) simulation approach + GENeralized TwO Phase (GENTOP) concept and two-fluid three-flow field with droplet were used to simulate the heat and mass transfer. Analysis encompassed the examination of steam and liquid phase distributions, as well as flow pattern transitions. Heat transfer enhancements were quantified using a comprehensive heat transfer factor. The disturbance intensity of the secondary flow was characterized by the absolute vortex flux. Results revealed a correlation between the intensity of the secondary flow and axial length, indicative of progressive fluid disturbance enhancement. Additionally, the maximal motion of the steam phase vortex was observed both near the wall and channel center. The EE+GENTOP concept exhibited superior capability in reproducing multi-scale flow including small-scale dispersed bubbles and large-scale continuous steam (coalescence and break-up). For investigations into the entrainment and deposition of liquid droplets in annular flow, the selection of a two-fluid three-flow field with the droplet model is recommended. [ABSTRACT FROM AUTHOR]

• Secondary flow was introduced to improve the performance of serpentine channel BTMS. • A grid + elliptical groove serpentine channel plate is proposed in this paper. • Pump power is reduced by 92.6 % and cooling efficiency coefficient is increased by 11.097 times. • Clear the coolant flow trajectory and eddies distribution. • Sensitivity analysis to further clarify the mechanism of cooling. To improve the thermal and economic performance of liquid cooling plate for lithium battery module in the distributed energy storage systems, on the basis of the traditional serpentine liquid cooling plate, the unidirectional secondary channels and grooves are added, combined to three kinds of serpentine cold plates for the battery module. By contrast, the cold plate with both elliptical groove and secondary channel has the best comprehensive performance. Though the thermal performance is slightly reduced, the consumption of pump power is reduced by 92.6 % and the cooling efficiency coefficient is improved by 12.32 times compared to the original cold plate. The cooling temperature targets of the battery pack are that T max less than 40℃ and Δ T avg less than 3 K. In order to achieving the best performance of the cold plate, the influence of the structural parameters and hydrodynamic parameters on the performance index is analyzed, and the sensitivity analysis is carried out which clear the operational mechanism and key design parameters of the cold plate. The results show that different design parameters would generate the change of eddy current which is influential to the cooling performance. Besides, v and D have a significant effect on the performance of the cold plate, followed by d and n, and a has almost no effect on the cooling performance. [ABSTRACT FROM AUTHOR]

• Curve channel with periodical wave structure is proposed to enhance heat transfer. • Secondary flow in curve-wave channel generates more vortices disturbing main flow stream. • Thermal boundary layer in curve-wave channel redevelops periodically along main flow direction. • Heat transfer enhanced by curve-wave channel outweighs its pressure penalty. • The optimum wave amplitude can be obtained by evaluating the dimensionless performance factor. Flow and heat transfer in curve channel have been studied extensively due to the presence of the secondary flow induced by the centrifugal force. However, for the curved channel with small curvature, weakened centrifugal force reduces the intensity of secondary flow and heat transfer enhancement is attenuated accordingly. For sake of maintaining high heat transfer efficiency in curve channel with a wide range of curvature, periodical wave structure was introduced into the smooth-curve channel by this study. A three-dimensional model of the curve-wave structure was established and numerical simulation was carried out to explore the flow characteristics and heat transfer performance in the proposed channel. In addition, the effects of the wave amplitude on the channel performance were analyzed. The results showed that the heat transfer of smooth-curve channel can be enhanced remarkably by applying wave structure on the channel wall, and the overall performance factor indicates that the heat transfer argumentation outweighs the pressure loss penalty. It was also found that the heat transfer in the curve-wave channel can be further enhanced by increasing wave amplitude. The most effective heat transfer enhancement can be achieved based on the evaluation of the overall performance factor. [ABSTRACT FROM AUTHOR]

Highlights • The best placement of the semicircular tube for horizontal flows is obtained. • The heat transfer behavior has a lot to do with the flow characteristics near the wall. • The entransy dissipation theory can explain the local heat transfer behavior very well. • A new buoyancy criterion concerning vorticity is introduced for horizontal flows. Abstract The buoyancy has great influence on the convective heat transfer of supercritical pressure fluids whose thermal properties change drastically near the pseudocritical point. Thus, numerical investigations on the coupled heat transfer characteristics of supercritical pressure CO 2 (S-CO 2) in horizontal semicircular channels are conducted in the present study. The effect of the secondary flow induced by buoyancy on the local thermal performance is analyzed in detail, and the dimensionless number Se , which represents the absolute vorticity flux in the main flow, is employed to describe the secondary flow and estimate the buoyancy effect. The numerical results show that the buoyancy effect gets smaller with the increase of the mass flow rate, and the heat transfer performance in the asymmetric flow is superior to that in symmetric flows at low mass flow rate. The buoyancy could significantly improve the thermal performance on the top wall but deteriorate that on the bottom wall in the hot side, which is exactly opposite in the cold side. The local wall heat transfer enhancement could be attributed to thinner thermal boundary layer due to the secondary flow, and the entransy dissipation theory could well explain the local heat transfer behavior. The criterion Se/Re gives better prediction for the buoyancy effect on both overall and local heat transfer than the existing ones. [ABSTRACT FROM AUTHOR]

Highlights • Fluid flow and heat transfer performances in jackets under the four types of DWP arrangement are compared. • The comprehensive evaluation factor of CFU-V DWP is between 1.005 and 1.082. • The best range of attack angle of the DWP is between 25° and 30°. • The comprehensive evaluation factor decrease with the reducing of curvature of the jacket. Abstract In this paper, multiple delta winglet pairs (DWP) vortex generators were periodically mounted on the heated wall of the half coiled jacket to reinforce heat transfer capability. The jacket is approximated as a helical duct with semicircular cross section. The height of DWP is half the jacket pipe and the circumferential spacing angle between the adjacent DWPs is fixed as θ 1 = 90°. Numerical investigations were performed to compare fluid flow and heat transfer performance in jackets (Re = 4000–18,000) under four types of DWP arrangement. Results according to the present study show that the common-flow-up (CFU) configuration of DWP has more advantages in heat transfer enhancement than the common-flow-down (CFD) one. The DWP with CFU-V arrangement provides best overall heat transfer performance when the attack angle β of DWP is between 25° and 30°. This is due to stronger vortices produced by it, slower dissipation of the coupled vortices and subsequently emerged additional vortices. Its comprehensive evaluation factors JF range is from 1.005 to 1.082. The comprehensive heat transfer enhancement effect of CFU-V DWP decreases with the reducing of the jacket curvature ε. [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 • Measurements of post-dryout heat transfer in helical coils are obtained. • Curvature significantly enhances post-dryout heat transfer in helical coils. • The post-dryout heat transfer coefficient of helical coils decreases in quality. • Heat transfer correlation for post-dryout regime of helical coils is developed. Abstract Post-dryout heat transfer for steam-water flow in helical coils is experimentally investigated. The inner diameters of the helical coils are 12.5 mm and 14.5 mm, and the coil diameters range from 180 mm to 380 mm. The experimental system pressure covers the range 2–8 MP. The results indicate that the centrifugal force induced secondary flow has a great influence on post-dryout heat transfer in helical coils. In the low quality post-dryout region, because of the high redeposition rate induced by secondary flow, the heat transfer coefficient of helical coils is significantly higher than that of straight tubes. As the total droplet mass decreases with increasing quality, the heat transfer performance of helical coils decreases and continuously approaches that of straight tubes. In high quality post-dryout region, the bulk fluid in helical coils is closer to thermodynamic equilibrium state due to the secondary flow, and the heat transfer coefficient may be exceeded by straight tubes. For the effects of flow parameters, the post-dryout heat transfer performance decreases in heat flux and increases in mass flux. For system pressure, due to the change of the dominant heat transfer mechanism, the post-dryout heat transfer decreases in the low quality region but increases in the high quality region with increasing system pressure. Based on a detailed analysis of the effects of various dimensionless parameters, a new correlation containing a quality correction is proposed and provides good prediction accuracy. [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]

In this study, a new shaped continuous mixer with simple mixing structure is proposed, aiming to suppress the short pass phenomenon to gain high mixing performance. Therefore, fluid motion of secondary flow by centrifugal force and swirling flow is focused. Furthermore, validity was examined by numerical analysis. Flow visualization was performed, and the mixing performance was verified using the electric conductivity method and transmission luminance measurement. As a result, a horizontal disc mixing tank with a fillet shape as the shape of the mixer was proposed. In the radial direction, it is predicted that forced vortex occurs near the center and Quasi-free vortices occurs at the opposite edge. Moreover, at opposite edge in the vertical direction, a pair of upper and lower vortices was observed. It was confirmed that a secondary flow was generated at the opposite edge. Concentration dispersion rapidly progresses in the vicinity of the side opposite edge of the mixing tank. It is confirmed that dispersion in the vicinity of the outlet part is small and uniformly mixed in the mixing tank regardless of the angle. It is regard as the consequence of the shear effect caused by the swirl flow generated near the outlet portion and the cross-sectional mixing due to the secondary flow generated at the opposite edge. It was found that it has mixing performance of about N = 9–14, comparing to the complete mixing tank train model. [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]

The thermal and hydraulic performances of an air-cooled, planar, oblique-finned heat sink (OF HS) were investigated for two oblique angles: 30° and 45°. The conjugate heat transfer between the heat sink and the air flow were computed numerically in ANSYS Fluent for a range of air flow rates for the smallest periodically repeating portion of the heat sink. The RNG k-ε turbulence model with enhanced wall treatment was used to solve the fluid flow and heat transfer. Followed by the experimental validation, the numerical results were scrutinized further to understand the effects of the flow field on the measured heat transfer performances. Apart from boundary layer disruption, vortices generated within the secondary channels due to flow separation particularly improved the advection component of the convective heat transfer, resulting in a heat transfer enhancement, exceeding the pressure drop penalty. The strong flow mixing, showing chaotic behavior, enabled a more uniform increase in the air temperature in the streamwise direction, utilizing the cooling potential of the air flow more effectively. 30° oblique-finned heat sink was observed to induce higher rates of secondary flow rates and improve the heat transfer performance more than its 45° counterpart. Due to the migration of the flow in the direction of the secondary channels, the heat transfer enhancement was compromised at high Reynolds numbers, the pressure drop penalty exceeded the heat transfer enhancement, causing a reduction in the thermal-hydraulic performance factor. The effects of flow migration on the flow and temperature fields were investigated with full domain numerical simulations. Experimental investigations showed significant improvements in the junction temperatures. [ABSTRACT FROM AUTHOR]

In the current study, we experimentally investigated the two-dimensional heat transfer with different thermal boundary conditions and secondary flow in a rotating smooth channel. The Reynolds number, based on the channel hydraulic diameter ( D = 80 mm ) and the bulk mean velocity ( U m = 1.82 m / s ) is 10000, and the rotation number ranges from 0 to 0.52. The mean density ratio ( d . r . = ( T w - T b ) / T w ) is about 0.1 in the current work using transparent heater glass (Indium Tin Oxide) to provide uniform heat flux. Three different thermal boundary conditions (Case A: four walls are heated, Case B: leading and trailing walls are heated, and Case C: only leading or trailing wall is heated) are taken into consideration to investigate the heat transfer distribution on the leading and trailing side. The heat transfer results show that thermal boundary condition has a significant effect on the details of the distribution of heat transfer on the leading and trailing side, especially on both end of the Z/D directions. The ratio Nu/Nu 0 of case B is about 10% larger than that of case A on leading side, and the difference between case B and case C is not obvious. To explain the phenomena, secondary flow in the rotating channel without wall heated was measured by PIV. The vortex-pair is found on the cross-section. Two small vortex-pair appear with the increase of rotation numbers, and then the intensity of the small vortex-pair is weakened near the leading side when the rotation number equals 0.52 in the current work. More details of the two-dimensional distribution of heat transfer and secondary flow in the rotating channel are presented in this paper. [ABSTRACT FROM AUTHOR]

• Increasing number of ribbed walls enhances channel heat transfer for angled ribs. • For angled ribs, four-sided ribbed channels provide the greatest performance. • Friction and heat transfer roughness functions are derived using four-sided data. • Designers can use correlations to predict friction and heat transfer in channels. Regionally averaged heat transfer distributions and friction characteristics are experimentally investigated for fully developed, turbulent flow in a rectangular channel with a width-to-height aspect ratio (AR=3). 45-deg angled rib turbulators are placed on one, two, and four sides of the channel in parallel and criss-cross configurations with a pitch-to-height ratio (P / e = 10) and a rib blockage ratio (e D h = 4.1 %). For a roughness Reynolds number range ( e + = 25 − 500) , results show that increasing the number of ribbed walls improves the overall channel heat transfer for the 45-deg parallel ribs. Results reveal that four-sided ribbed channels provided the greatest thermal performance followed by the two-sided and one-sided ribbed channels, respectively, for both angled rib designs. On the basis of the law of wall similarity, friction and heat transfer roughness functions are derived using the four-sided data reported in this work. Furthmore, friction factors and heat transfer coefficients of similar angled rib designs were obtained from open literature and similarly correlated to derive more robust friction and heat transfer correlations over the roughness Reynolds number range. Thermal designers can use these proposed correlations to predict friction and heat transfer in rectangular cooling channels (AR=1-5) with various numbers of ribbed walls. The measured friction factor and heat transfer coefficient of this study can be predicted using these semi-empirical correlations with maximum errors of 9% and 7%, respectively. [ABSTRACT FROM AUTHOR]

Mass and momentum transfer characteristics in and downstream of a 90° elbow are studied experimentally with the aid of the plaster dissolution method, surface flow visualization in the elbow and the cross-sectional velocity field measurement by stereo particle image velocimetry (SPIV) downstream of the elbow. The experiments are carried out for the elbow with the radius to diameter ratio 1.5 at moderate Reynolds number Re = 5 × 10 4 . The mass transfer measurements in the elbow indicate that the major change of mass transfer coefficient is observed along the inner wall of the elbow, where low mass transfer coefficient is found in the first half of the inner wall and it increases abruptly in the second half of the elbow, which is followed by a gradual recovery of the mass transfer in the downstream, while the mass transfer on other wall does not change so much with that of the straight pipe. These features are mainly due to the flow acceleration in the first half of the inner wall and the following flow separation in the second half of the inner wall, where the high turbulent energy production is expected from the SPIV measurement downstream of elbow. These results indicate that the mass transfer coefficient in and downstream of the elbow is highly modified by the flow separation and secondary flow in the elbow, which may generate the high turbulent energy production in the second half of the inner wall in the elbow. [ABSTRACT FROM AUTHOR]

In this study, the conjugate heat transfer performance of an enhanced planar heat sink design, comprising cross-connected alternating converging–diverging channels, was analyzed for forced air convection conditions. Numerical simulations were performed in ANSYS Fluent 15.0 using the RNG k–ε turbulence model accompanied by the enhanced wall treatment option to resolve the air flow and evaluate the heat transfer. Numerical results, which were validated experimentally, were utilized to investigate the flow and the temperature fields. The converging–diverging channel sections induced secondary flows through the cross connections, repeatedly disturbing the thermal and hydraulic boundary layers over the leading edges of the fin sections. The performance of the proposed heat sink design was benchmarked against the conventional straight channel heat sink of equivalent dimensions. Significant heat transfer enhancement was observed. However, the vortices, generated as a result of the separation of the secondary flows, were observed to prevent the heat transfer performance from being further improved and cause an excessive increase in the pressure drop penalty. [ABSTRACT FROM AUTHOR]

• The influence of secondary flow vortex on heat transfer and flow resistance was investigated. • The local flow heat transfer of supercritical LNG under the cross pseudophase transition are studied. • The effects of airfoil shape and structure on heat transfer and flow resistance are studied. As the core equipment of floating storage and regasification unit (FRSU), the function of liquefied natural gas(LNG)vaporizer is to heat the LNG from -162 °C to normal temperature under supercritical pressure, which is a pseudophase transition process. This paper numerically investigates the local flow and heat transfer of supercritical LNG during the pseudophase transition process of three airfoil channels with different structures and shapes under transcritical temperature condition. The secondary flow generated by the shape and structure of the airfoil has an important influence on the local flow heat transfer performance. Especially in the liquid-like region, the three airfoil fins not only generate mixed disturbance of the velocity vector in the y-direction, but also generate secondary vortices in the z-direction, which can more effectively interfere with the boundary layer and enhance the heat transfer performance. As the temperature increases, the density and the viscosity of the supercritical LNG in the pseudo-critical and gas-like regions decrease, the vortexes formed in the three airfoil fins appear above or below the tail of the airfoil. The research shows that under the selected conditions, Fin-2 has the best hydraulic performance, especially in the gas-like region, and its f -factor can be lower than 0.02. Because Fin-2 changes the curvature of the leading edge and minimizes the impact surface, it is conducive to the rapid discharge of local fluid, thus reducing the flow resistance and generating secondary flow at the tail of the airfoil. Fin-3 has the best heat transfer performance, and its j -factor is 5–10% higher than that of other airfoil fins. This is the tail shape of the airfoil is concave, and the secondary vortices are located in the concave area, which disturbance the boundary layer and improve the heat transfer process. [ABSTRACT FROM AUTHOR]

• The configurations of OFOW induce strong secondary flow at the leading edge, and the secondary flow remains considerably strong throughout the plate. The coupling of intensified secondary flow and leading-edge effects significantly enhances heat transfer, though suffers from increased pressure loss penalty. • Both V wave pointing directions in the second plate can only induce fragmentized vortices, which results in the weakened interaction of the fluids between the sidewalls and channel center. • Wave amplitude A is the most dominant parameter. When A = 0.3 at Re ≈ 400, j of OFOWs is 4.2 times enhanced than the plain channel and 1.8 times than the OSF, while f is 5.5 times larger than the plain channel and 2.3 times than the OSF. Area goodness factor j/f is only 20% smaller than the plain channel and 24% smaller than the OSF, while JF is enhanced by 155% than the plain channel and 34% than the OSF. • Wave pointing direction and phase angle in the successive offset plates have minor effects. At the same amplitude A, the four configurations of OFOW show comparable heat transfer and pressure loss characteristics. An offset fin with oblique waves (OFOW) which is a combination of offset fin (OSF) and V-shaped oblique wavy fin (OWF) has been proposed to further enhance heat transfer in single phase laminar flows. The heat transfer and pressure loss characteristics of different configurations of the OFOW are investigated numerically. The 3D unsteady periodic model with the H1 thermal boundary condition is adopted to explore the effects of V wave pointing direction, phase angle and wave amplitude. The local distributions of velocity, secondary flow, wall shear stress and heat flux as well as friction factor f and Colburn factor j are presented and compared with those of the plain channel, OSF and OWF with the same aspect ratio and hydraulic diameter. It is found that the OFOW can further enhance heat transfer than the OSF and OWFs, i.e., at Re ≈ 400 with wave amplitude of A = 0.3, the Colburn factor j is around 4.2 times of that of the plain channel. It is shown that opposite V wave pointing directions in the successive offset plates cannot induce long-range counter-rotating vortices, and V wave amplitude A is the most dominant parameter, where wave pointing direction and phase angle in the successive offset plates have minor effects. [ABSTRACT FROM AUTHOR]

The characteristic of flow and heat transfer in double-layered microchannels with cavities and ribs is investigated numerically. The intensity of secondary flow ( Se ) and field synergy principle are used to evaluate the heat transfer performance. The results show that the effect of arrangement of cavities and ribs on friction factor and Nusselt number is obviously. The opposite cavities and aligned ribs aside the microchannel wall (Channel C) exhibit the highest intensity of secondary flow, thus resulting in the highest Nusselt number and pressure drop simultaneously. From the view point of field synergy principle, Channel C has the smallest value of field synergy angle β and the highest value of field synergy number Fc , which indicates the best synergy relationship between velocity and temperature fields. Lastly, a thermal enhanced factor is used to evaluate the comprehensive effect of flow friction and heat transfer. The thermal enhanced factor of Channel C increases with Se firstly and then decreases. The offset cavities and ribs aside the microchannel wall (Channel B) shows the best comprehensive effect in the range of Reynolds number and Se studied here. Therefore, Reasonable geometric design of double-layered microchannels must be considered to enhance heat transfer and decrease pressure drop as well. [ABSTRACT FROM AUTHOR]

The secondary flow vortices make the flow field near the endwall highly three-dimensional. The jet-to-secondary flow interaction and the interaction between multiple film rows are important for the understanding of endwall film cooling characteristics and the improvement of prediction method. Single row film cooling, three combinations of two-row film cooling as well as a full coverage configuration with five rows of film cooling on a typical flat vane endwall with matched averaged blowing ratios and density ratio to the engine condition were investigated experimentally. Detailed measurements of adiabatic film cooling effectiveness distribution using PSP technique as well as the pressure distribution of the endwall were performed. The single row film cooling results were used to test the applicability of the superposition law on the prediction of endwall multi-row film cooling effectiveness compared to the multi-row experimental results. The results indicated that the superposition method could not give quantitatively reliable film cooling effectiveness of the endwall multi-row cases especially for the full coverage film cooling configuration. The upstream film injections could alter the endwall secondary flow and affect the coolant spreading of the downstream row. Moreover the interaction between the downstream and upstream injections increased the mixing loss and induced a higher turbulence level which made the coolant from the upstream row dissipate faster. [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]

The secondary flow vortices make the flow field near the endwall highly three-dimensional. The jet-to-secondary flow interaction may help to make an efficient film cooling arrangement to cool the endwall. Six rows of cylindrical hole injection placed at different locations on a typical flat vane endwall with different density ratios and matched averaged blowing ratios to the engine condition were investigated experimentally in this work. Detailed measurements of adiabatic film cooling effectiveness distribution using PSP technique as well as the pressure distribution of the endwall were performed. The results showed that the endwall film cooling was mainly influenced by the passage vortex and endwall cross flow which prevented the cooling jet from covering the regions close to the leading edge and pressure side. The theoretical analysis indicated that the local blowing ratio of endwall film cooling increased as the local static pressure increased. The cooling jets with higher local blowing ratios were more sensitive to the change of the coolant supply pressure. Film injections located in high static pressure zones with a high momentum flux ratio had the potential to reduce the intensity of secondary flow and provided coolant to the near leading edge and pressure side regions. [ABSTRACT FROM AUTHOR]

• Jet impingement on a heat sink with discontinuous staggered ring ribs is proposed. • The secondary flow enhances the heat transfer in the wall jet region. • The ring-ribbed structure greatly improves temperature uniformity of the heat sink. The temperature uniformity of the bottom surface of the heat sink is one of the hot research topics in the field of jet impingement heat transfer enhancement. In this study, jet impingement on a heat sink with novel discontinuous staggered ring ribs in the wall jet region is put forward to improve the local heat transfer by combining a single micro-jet impact with the secondary flow channel. The flow and heat transfer characteristics of the confined liquid jet impinging on the ribbed head sink under different structural parameters of ribs and jet Reynolds numbers are numerically studied. The V2F turbulence model is used for numerical calculation and the obtained results are compared with the experimental results. The results show that discontinuous staggered ring ribs in the wall jet region of the heat sink promote the formation of secondary flow and strengthen the fluid-solid conjugate heat transfer, that effectively improve the temperature uniformity of the entire heat sink. The temperature difference of the ring-ribbed heat sink is about 91.82% lower than that of the flat plate heat sink when the Reynolds number is 8000. When the outlet shrinkage ratio decreases, or when the position radius of the first layer of ribs or arc angle of a rib on the first layer increases, the temperature uniformity of the ring-ribbed heat sink is gradually improved. However, the lifting range of the outer edge of the heat sink decreases. Also, when the jet Reynolds number increases to 20,000, the maximum temperature difference of the heat sink decreases down to 1.6 K. [ABSTRACT FROM AUTHOR]

A combined experimental and numerical investigation is carried out to study the characteristics of laminar flow and forced convection heat transfer in a square cross-section wavy “serpentine” microchannel with the upper wall insulated and other side walls held at constant temperature. Experimental measurements of convective heat transfer and pressure drop are performed for 30/70% and 10/90% by weight mixtures of glycerine/water over a range of Dean number from 0.6 to 80. Complementary three-dimensional computational fluid dynamics numerical simulations are also conducted for the same conditions. The results show that the growth of secondary-flow vortices promotes fluid mixing in the serpentine microchannel and leads to an enhancement of the convective heat transfer. As a consequence the serpentine microchannel is able to enhance the performance of heat transfer relative to a straight microchannel over the entire range of Dean number. Meanwhile, at these values of Prandtl number the relative pressure-drop losses increase with increasing Dean number. These increased pressure-drop losses are rather modest over the whole range of Dean number compared with the significant enhancement in heat transfer. [ABSTRACT FROM AUTHOR]

In this work, the secondary flow and recirculation flow motions initiated at the bifurcations and bends in symmetric fractal-like microchannel networks with two branching levels were studied through three-dimensional simulations. The effects of Reynolds number and aspect ratio on the vortices generation were investigated. And the influences of these flow behaviors on the hydrodynamic and thermal characteristics of the fractal-like microchannel networks were demonstrated. It was found that for a fractal-like microchannel network, both the flow rate and the aspect ratio have large influences on the evolution of the vortices which therefore promoted the fluid mixing and enhanced the efficiency of the heat transfer greatly. As a result, the fractal-like microchannel network with a smaller aspect ratio of 0.333 was verified to have the lowest pressure drop and highest heat transfer performance compared with the other microchannel networks under investigation in our study. [ABSTRACT FROM AUTHOR]

The performance of a novel design of a three-section plate-fin heat sink employing channels of stepwise decreasing hydraulic diameter is numerically and experimentally evaluated. Prototype heat-sink and inlet–outlet manifold configurations have been manufactured and characterized, in terms of thermal resistance and induced pressure drop, in a closed fluid loop experimental rig and for flow rates in the range of 20–40 mL/s. The flow development and particularly the secondary flow pattern inside the heat sink are investigated by means of a numerical three-dimensional model. The findings of the study establish that the induced pressure drop, which does not exceed 1000 Pa for the considered flow rates, is primarily attributed to the flow friction in the third-heat sink section, which is of microscale dimensions and the effect of the inlet–outlet manifold system is very small (0.5% of the total). In terms of heat transfer, the effect of buoyancy in the first heat-sink section has a beneficial impact on thermal performance maintaining the thermal resistance constant, at a value approximately equal to 0.015 K/W, regardless of the flow rate of the cooling fluid. Heat transfer is also enhanced in the successive sections due to the effect of contraction-induced longitudinal vortices. Finally, it is proven that the heat sink wall exhibits a more uniform temperature distribution with the decrease of the Reynolds number. [ABSTRACT FROM AUTHOR]

The superior capability of bearing thermal and mechanical loads to other types of open cellular materials has led to advances in developing new periodic cellular materials. We introduce a lightweight X-type lattice fabricated via the metal sheet folding and present its thermo-fluidic characteristics in single-phase forced convection. For fixed porosity, thermal conductivity and Reynolds number, the X-type lattice provides overall heat removal capacity up to two times higher than reference periodic cellular materials. The unique morphology of the X-type lattice results in a large scale spiral primary flow, which interacts with several secondary flows. These fluid flow behaviors and the induced complex flow mixing substantially enhance heat transfer on both the substrate and ligaments. However, the X-type lattice causes roughly three times higher pressure drop than reference periodic cellular materials for a given Reynolds number. Overall, superior heat transfer is achieved by the X-type lattice for a fixed pumping power. [ABSTRACT FROM AUTHOR]

This paper is aimed towards presenting the CFD study of vortex tube carried out to gain an understanding about influence of thermo-physical properties such as thermal diffusivity, Prandtl number, specific gas constant and thermal conductivity of different gases and turbulence models on its performance. The energy separation has been observed for eight different gases as working fluid. For the first time in CFD analysis of vortex tube, H 2 has been used as working fluid and its effect investigated. To understand the complex nature of highly compressible, turbulent and swirling flow within the vortex tube, different turbulent models, namely, one equation Spalart–Allmaras, two equations Standard k – ε and Standard k – ω model are used. Additionally, every turbulence model has been tested by using Second Order Upwind (SOU) and Quadratic Upstream Interpolation for Convective Kinetics (QUICK) scheme. Results indicated that magnitude of energy separation increases with increase in thermal diffusivity and thermal conductivity of the gas, with H 2 being an exception. H 2 also defies the criteria of decreasing molecular weight for improved energy separation. Magnitude of energy separation decreases with value of specific gas constant. Prandtl number of gas does not show any influence on energy separation magnitude. The energy separation magnitude predicted by using Spalart–Allmaras model in the cold region is better than that predicted by Standard k – ε and Standard k – ω model. Standard k – ε model combined with QUICK scheme presents itself as suitable model to predict the flow physics appropriately. A recirculating secondary flow has also been identified by all the models. The overall prediction of energy separation effect and flow physics parameters has been in good agreement with experimental results. [ABSTRACT FROM AUTHOR]

Secondary flow is employed in micro channel heat sink (MCHS) by introducing slanted passage in the channel wall between the adjacent channels in alternating orientation. The intervallic passage causes disruption in the hydrodynamic boundary layer and redevelopment at the leading edge of the following wall. This phenomenon decreases the average thermal boundary layer thickness, thus enhances the heat transfer performance with minor pressure drop due to combined effect of thermal boundary layer re-development and flow mixing. The data presented in comparison of simple MCHS. The results showed that the overall performance of MCHS with alternating slanted passage (MASP) increased by 146% and thermal resistance reduced to 76.8% when compared with simple MCHS. It has to be highlighted that contradicting from the usual phenomenon where improvement in heat transfer accompanied with increment in pressure loss, thermal enhancement in present study comes together with reduction in pressure drop up to 6% compared to the simple one. [ABSTRACT FROM AUTHOR]

• Heat transfer correlations of RP-3 are established in the helical coiled tube. • Correlations include the parameters of density, specific heat and grashof number. • Re, Nu , and centrifugal richardson number increase exponentially along the tube. • Experimental data for various fuels and structures are used to verify the correlation. The laminar heat transfer characteristics of RP-3 in the liquid phase and supercritical phase were experimentally studied at 5 MPa in the helical coiled tubes. The influences of secondary flow and buoyancy on heat transfer were analyzed under different heat fluxes, mass flow rates and helical diameters. The Nu increased with the increase of curvature. When the heat flux was 150 kW/m2, the mass flow rate was 1 g/s, and the wall temperature was higher than the critical temperature, the Re, Nu , and centrifugal Richardson number increased exponentially along the tube length. By introducing the combined parameters of density, specific heat and Grashof number, the correlations of RP-3 were established in the helical coiled tube under supercritical laminar mixed convection. In addition, the experimental data under different fuels, helical diameters and tube diameters were used to verify the above correlation. [ABSTRACT FROM AUTHOR]

Abstract: The present paper deals with the longitudinal heat transportation by an oscillatory flow in a curved tube heat transportation pipe. The velocity and temperature fields during oscillatory water flow in a curved tube with inner diameter of 10mm were numerically simulated using the commercial software FLUENT. To clarify the effect of the curvature on heat transportation, the ratio of the tube radius to the curvature radius was varied from 0.0125 to 0.075, and oscillatory flow frequency was varied from 0.1 to 2.0Hz. The effective thermal diffusivity exhibited a peak at a curvature ratio around 0.05–0.07 and reached a value 12 times higher than that of a straight tube. It was also found that the dispersion of fluid particles due to secondary flow caused the enhancement of heat transportation. [Copyright &y& Elsevier]

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: A computational fluid dynamics methodology is developed to study steady laminar flow and heat transfer in periodic zigzag channels with square cross-sections, considering the effect of chaotic advection. The computational domain consists of up to 14 repeating zigzag units with smoothly joined inlet and outlet sections. The results show fully-developed periodic flow fields can be achieved for Re <200; however, a spatially periodic flow field is no longer seen for higher Reynolds number due to the enhanced effect of chaotic advection with increasing Reynolds number; for Re >400 the flow field becomes transient. The flow field shows that the secondary flow pattern varies significantly at different axial locations. Highly enhanced heat transfer appears in the regions where strong Dean vortices occur, as indicated by more pronounced twisting of flow streaklines. For Re >200, the unit-averaged heat transfer enhancement and pressure-drop penalty (relative to straight passages) are no longer uniform from unit to unit but mean values of both of these performance metrics increase with increasing Reynolds number over the whole range of 50< Re <400. [Copyright &y& Elsevier]

Abstract: Pulsatile flow across two circular cylinders with different geometric arrangements is studied experimentally using the particle image velocimetry method and numerically using the finite element method. This investigation is motivated the need to optimize gas transfer and fluid mechanical impedance for a total artificial lung, in which the right heart pumps blood across a bundle of hollow microfibers. Vortex formation was found to occur at lower Reynolds numbers in pulsatile flow than in steady flow, and the vortex structure depends strongly on the geometric arrangement of the cylinders and on the Reynolds and Stokes numbers. [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]

Abstract: This paper provides an analysis on cooling tower fouling data taken in seven 15.54mm I.D. helically ribbed, copper tubes at R e =16,000. The geometric parameters include number of rib starts, helix angle, and height. A series of semi-theoretical fouling correlations as a function of the product of area indexes and efficiency indexes are developed. The mass transfer coefficient is calculated by the Von-Karman analogy. The wall shear stress is obtained from the pressure drop. The correlations and the analyses can be used directly to assess the fouling potential of helical-rib in actual cooling water systems. [Copyright &y& Elsevier]

In the present study, the modified formulations of the partially-averaged Navier-Stokes (PANS) model were suggested for accurate prediction of the secondary flow by resolving the anisotropy of turbulence. The main characteristics of the modified PANS (mPANS) models were to resolve the wide range of turbulent length scale by decreasing f k value in the region where the anisotropic behavior of turbulence dominates. The information on the region, where there is highly anisotropic turbulence, was procured by using the budget analysis and the anisotropy invariant map. The level of how much f k should be reduced was determined by model factors, which were set to be able to resolve the secondary flow in a square duct and around a prolate spheroid. To identify the reliability of the model factors, the mPANS models were applied to the flow around the KRISO very large crude oil carrier at Froude number of 0.142. The mPANS models showed improved predictions of the hook-shape pattern of the streamwise component of velocity and secondary vortex on the propeller plane compared to the original PANS (oPANS) results by decreasing the modeled turbulent kinetic energy. • The modified formulations of the PANS (mPANS) model were suggested for accurate prediction of the secondary flow by resolving the anisotropy of turbulence. • To procure the information on the region with highly anisotropic turbulence, the budget analysis and the anisotropy invariant map were adopted. • The level of how much fk should be reduced in the highly anisotropic region was determined by the model factors, which were set to resolve the secondary flow in the square duct and around the prolate spheroid. • To identify the reliability of the model factors, the mPANS models were applied to the flow around KRISO very large crude oil carrier. • The mPANS models showed improved predictions of the secondary flow and richer content of the resolved flow structures compared to the original PANS model. [ABSTRACT FROM AUTHOR]

Abstract: The Reiner–Rivlin viscoelastic model was used to investigate numerically on the pattern and strength of the secondary flows in rounded corners square ducts. The influence of rheological properties on flow domain was studied. The governing equations for steady state, laminar fully developed flow were solved by finite difference method. It is shown the secondary flow of a Reiner–Rivlin fluid has a negligible effect on the axial flow and f.Re. Viscous and elastic behaviors as well as the rounding of the corners have important effects on the secondary flows created by viscoelastic fluid. [Copyright &y& Elsevier]

Angled ribs were employed to induce a secondary flow as well as to cause the flow separation and reattachment. They were mounted on the inner surface of a tube with an angle between the plane of each rib and the mean flow direction of 75°, 60° and 45° for the angled ribs and 90° for the transverse ribs. Heat transfer experiment, flow visualization experiment and numerical simulation were conducted in order to clarify the flow structure and its effects on the circumferential heat transfer distribution. The present study explicitly demonstrates the heat transfer superiority of angled ribs over conventional transverse ribs due to a development of the rib-induced secondary flow in a form of a pair of vortices, which convey the colder and higher momentum fluid from the core region towards the heated wall and augment heat transfer especially at the bottom region of the tube. [Copyright &y& Elsevier]

• Numerical simulation of steam-liquid two-phase flow in rifled tube with dryout is carried out. • The inhibitory effect of rifled tube on dryout is studied. • Un-uniform of the circumferential wall temperature caused by the distribution of steam and liquid relative content and velocity is studied. • The absolute vorticity flux and secondary flow intensity of rifled tube are quantitatively analyzed. The two-fluid three-flow field mathematical model is used to carry out the numerical simulation of heat transfer characteristics of steam-liquid two-phase flow in dryout and post-dryout region of the rifled tube. The absolute vorticity flux and intensity of the secondary flow are introduced to quantitatively characterize the heat transfer enhancement. The results show that, compared with smooth tube, the rifled tube can greatly reduce the rise of wall temperature and have a significant effect on dryout inhibition. The cross-section wall temperature of rifled tube presents a periodic distribution. The wall temperature at the intersection of the root and rib side is higher due to the fluid retention. After the dryout occurs, the wall temperature is higher than other locations due to the continuous liquid film on the crest being torn. The secondary flow strength and heat transfer enhancement degree of fluid in rifled tube increase with the increase in axial height. The absolute vortex flux, intensity of the secondary flow and absolute value of the vorticity in main flow direction can reflect the degree of fluid disturbance caused by internal thread. [ABSTRACT FROM AUTHOR]

• Numerical simulation of steam-liquid two-phase flow in rifled tube with dryout is carried out. • The inhibitory effect of rifled tube on dryout is studied. • Un-uniform of the circumferential wall temperature caused by the distribution of steam and liquid relative content and velocity is studied. • The absolute vorticity flux and secondary flow intensity of rifled tube are quantitatively analyzed. The two-fluid three-flow field mathematical model is used to carry out the numerical simulation of heat transfer characteristics of steam-liquid two-phase flow in dryout and post-dryout region of the rifled tube. The absolute vorticity flux and intensity of the secondary flow are introduced to quantitatively characterize the heat transfer enhancement. The results show that, compared with smooth tube, the rifled tube can greatly reduce the rise of wall temperature and have a significant effect on dryout inhibition. The cross-section wall temperature of rifled tube presents a periodic distribution. The wall temperature at the intersection of the root and rib side is higher due to the fluid retention. After the dryout occurs, the wall temperature is higher than other locations due to the continuous liquid film on the crest being torn. The secondary flow strength and heat transfer enhancement degree of fluid in rifled tube increase with the increase in axial height. The absolute vortex flux, intensity of the secondary flow and absolute value of the vorticity in main flow direction can reflect the degree of fluid disturbance caused by internal thread. [ABSTRACT FROM AUTHOR]

• Modified single-particle tracking method based on aerosol mass flow rate was employed. • The highest LDEF on a sharp-bent tube were observed right after the bending region for all different conditions. • At low Re, most of particles were deposited at the outer wall. • At high Re, particles were uniformly deposited along the azimuthal angles due to its strong secondary flow. The characteristics of fluid flow on a sharp-bent tube under various conditions were analyzed. Numerical simulations for analyzing the particle deposition locations and patterns on a sharp-bent tube were conducted by using the modified single-particle tracking analysis based on aerosol mass flow rate. Through the numerical calculation, we showed that after the bending point in a sharp-bent tube, the faster axial velocity occurred near the outer wall, and the boundary layer at high Reynolds number became thinner. Furthermore, the faster radial velocity near the tube wall was observed at less developed-flow region at high Reynolds number owing to the stronger secondary flow. The nanoparticle deposition locations and patterns were systematically examined in various viewpoints including cumulative number of deposited particles, local deposition enhancement factor, and particle deposition pattern according to azimuthal angles. We found that most of the nanoparticles were deposited on the outer wall right after the bending point owing to outward-sloping flow. Moreover, the difference in relative deposition efficiency along the azimuthal angles at each section in the sharp-bent tube was reduced as Reynolds number increased. This is because the nanoparticles near the wall were well mixed due to the strong secondary flow at high Reynolds number. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Nanoparticle penetration through a sharp-bent tube was examined. • Effects of particle sizes and flow rates on penetration were investigated. • Diffusion loss was dominant in low Schmidt number. • Secondary flow was enhanced by high Reynolds number. • Empirical correlation curve as a function of Peclet number was obtained. In the present study, nanoparticle transport through a sharp-bent tube, i.e., elbow connection, was systematically examined by using a particle size ranging from 3 to 50 nm. In the experiments, particle size and flow rate significantly affected the penetration efficiency. To be specific, the smaller particles which had higher diffusion coefficient were more likely deposited on the sharp-bent tube and the higher flow rate reduced the flow-directional nanoparticle residence time resulting in increased penetration efficiency. To explain the experimental penetration efficiency on the sharp-bent tube, characteristics of fluid flow on the sharp-bent tube were studied numerically. The flow field calculations showed that the recirculation pattern occurred at the corner of the sharp-bent tube, and the flow separation and reattachment were observed at the inner wall right after the bending point. Additionally, when compared to higher Reynolds number, the intensity of the secondary flow was weaker at lower Reynolds number as well as its center point was located farther from the tube wall. Therefore, the nanoparticle residence time on the sharp-bent tube became longer and a smaller number of particles penetrated the tube at lower Reynolds number. Based on the experimental data, the penetration efficiency on the sharp-bent tube was predicted by the correlation fitting curve. The relative penetration efficiency on the sharp-bent tube was also obtained by comparing it to the penetration efficiency on the straight tube. The strong diffusion transport rate and weak advection transport rate induced more particle losses due to secondary flow after bending point, resulting in the decreased relative particle efficiency. [ABSTRACT FROM AUTHOR]

• Quantitative experimental study on secondary flow field in the cross-section of tube. • Three areas with different secondary flow characteristics are formed by three factors. • Two areas with opposite secondary flow direction are divided by a horizontal line. • Horizontal line position and secondary flow vortices dominate flow field structure. • Heat flux and mass flow velocity significantly affect secondary flow characteristics. In recent years, the characteristics of mixed convective heat transfer and heat transfer deterioration, which are seriously affected by secondary flow in the horizontal tube, have received increasing attention in the field of new heat exchange system development, such as the supercritical water heat exchangers and nuclear reactor cooling system. To gain insight into the mechanisms of secondary flow in horizontal heated tubes, experiment-based visualization of the secondary flow field in the cross-section is performed in thermally developing, laminar and transitional mixed convection. Deionized water is used as the working fluid, and the test section is a circular quartz glass tube with an internal diameter of 25.8 mm and length of 300 mm. Transparent conductive oxide heating films, which are used to uniformly heat the working fluid, are deposited on the outer surface of the quartz glass tube by the ion deposition method. The operating heat flux is varied from 1.15 kW/m2 to 9.24 kW/m2, the mass flow velocity is varied from 15 kg/(m2∙s) to 35 kg/(m2∙s), and the pressure is varied from 0.1 to 1.8 MPa. The experimental results indicate that fluid rises up along the tube wall under the effect of buoyancy in the vicinity of the tube wall. In the central region of the cross-section, an approximately horizontal flow boundary line is formed, which divides the center region into two parts that have significantly different secondary flow characteristics. Above the flow boundary line, the fluid slowly moves upward under the effect of thermal acceleration; below the flow boundary line, the fluid accelerates to descend under the effect of gravity. With increasing heat flux, both the flow stratification phenomena and buoyancy effect are enhanced, which leads to higher secondary flow velocity in the vicinity of the tube wall and downward motion of the flow boundary line and secondary flow vortices. A higher mass flow velocity weakens the flow stratification phenomena; thus, the flow boundary line and secondary flow vortices move upward, and the secondary flow velocity increases in the upper half cross-section. As the pressure is less than 1.8 MPa, it has little influence on the secondary flow velocity field. The results of this study confirm that secondary flow shares a close relationship with the mechanism of heat transfer deterioration. [ABSTRACT FROM AUTHOR]

• Several buoyancy criteria are used to evaluate the mixed convection. • The redistributed flow and temperature field with buoyancy force are investigated. • The induced secondary flow due to buoyancy are observed and analyzed. • The mechanisms of three typical heat transfer states with laminar flow is discussed. To ameliorate the thermal management of a cooling system in an advanced engine, the heat transfer characteristics affected by the buoyancy-driven supercritical hydrocarbon fuel flow in a rectangular channel are numerically explored in detail. Several common buoyancy criteria (Gr / Re 2, Gr / Re 2.7 and Gr q / Gr th) are established and a three-dimensional numerical model is simulated with a verified SST k-ω turbulence model at real working conditions. The test results demonstrate that the buoyancy force, essentially induced by the relatively large density gradient under the severe thermal stratification environment, dramatically redistributes the flow structure and temperature distribution. The buoyancy-aided flow accelerates the formation of secondary flow, which exerts the positive role in promoting the transitional trend towards laminar flow from the initial developed turbulent state. Besides, it decreases the wall shear stress and thermal diffusion in near-wall fluid as the thickness of thermal boundary layer increases. In particular, the laminar-like flow is considered to be responsible for the occurrence of the typical 1st HTD (Heat transfer deterioration) and the 2nd HTD. In addition, a local EHT (Enhanced Heat Transfer) behavior can be regarded as a consequence of a very slight recovery of Gr q / Gr th and Gr / Re 2.7 with a sharply increase in the wall shear stress and Nusselt number. [ABSTRACT FROM AUTHOR]