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

The present study numerically explores the flow and heat transfer in “U-type” curved pipes of either partial or full curvature, which are often used as integral parts of heat exchangers and heat sinks in industrial applications such as solar thermal systems, among many others. Specifically, the case of a high-Prandtl-number Newtonian fluid (thermal oil) in laminar forced convection inside partially and fully curved “U-bends” is investigated, in order to illustrate the effect of the generated secondary flows on the various local and global scales. The 3D steady-state simulations are performed using the free and open source computational fluid dynamics software “OpenFOAM”. The analysis of the computational results is initially based on data at selected characteristic locations of interest; they are properly established and further explored by extracting internal field variables, in the form of 2D contours of quantities of interest, such as temperature and vorticity. A parametric study on the flow rate and curvature impact, namely in terms of increasing Re and De numbers ( Re  = 100, 1000, 2000), on the heat transfer process inside the U-bends is conducted comparatively for both geometries. The presence of secondary flows is confirmed and the differences between the geometries under investigation are visually illustrated. For moderate and high Re numbers, the presence of multiple counter-rotating secondary cells is generally observed, while the partially curved U-bend (“composite”) displays a more complex flow topology. This behavior is projected on heat transfer through the examination of temperature distribution on inner and outer arcs of both geometries. It is shown that the composite bend causes an abrupt decrease and oscillations on temperature distributions, which are found to be related with phenomena like flow impingement, separation and re-attachment. The Pr number effect is illustrated through a comparison with a lower Pr fluid and its impact is demonstrated both inside and downstream of the U-bends. The averaged Nusselt distribution along the length of both curved ducts further highlights the impact of these phenomena on local heat transfer and provides initial evidence in favor of the composite bend. The overall performance of the investigated curved geometries is compared based on a “performance factor” criterion and a superior performance of the composite U-bend is further established, attaining a value of almost 45% for higher flow rates. Finally, the effect of each U-bend on the straight pipe downstream of their respective exit is quantified. High divergence from the regular “combined hydrodynamic and thermal entrance flow” is found for the forced convection inside the downstream pipe. A characteristic overshoot of the downstream-to-upstream ratio Nusselt number is found for moderate and higher flow rates at small lengths from entrance (z*). The partially curved U-bend downstream effect seems to be greater for the investigated cases of Re  = 100, 2000, while the fully curved outperforms the composite for the moderate Re case ( Re  = 1000). The source of this discrepancy cannot be clearly identified through this work and further investigation is needed on this matter. All in all, the simulations performed here show that partially curved U-bends can potentially be advantageous, in terms of performance, compared to the standard fully-curved pipes for laminar forced convection applications. These results can provide a good starting point for the optimal design of such ducts for several heat transfer operations. [ABSTRACT FROM AUTHOR]

The geometry considered in the present work (serpentine pipe) is a sequence of U-bends of alternate curvature. It is characterized by pipe diameter, d = 2 a and bend diameter, D = 2 c . The repeated curvature inversion forces the secondary flow pattern, typical of all flows in curved ducts, to switch between two mirror-like configurations. This causes (i) pressure drop and heat or mass transfer characteristics much different from those occurring either in a straight pipe or in a constant-curvature pipe, and (ii) an early loss of stability of the base steady-state flow. In the present work, four values of the curvature δ = a / c (0.2, 0.3, 0.4 and 0.5) were considered. For each value of δ , the friction velocity Reynolds number Re τ = u τ a / ν was made to vary in steps between 10 and 50. Fully developed flow was simulated using a three-dimensional, time-dependent finite volume method and computational grids with a number of nodes ranging from ∼1.8 to ∼4.6 × 10 6 , according to the curvature. The computational domain included two consecutive and opposite bends and thus coincided with the minimum spatially repetitive unit. Heat transfer was also simulated for uniform wall heat flux conditions and a Prandtl number of 1. A complex scenario of transitions was predicted, leading from the base steady-state, top-down symmetric flow to turbulence through intermediate regimes which included steady-state asymmetric and time-periodic flows. For all curvatures, at the highest value of Re τ investigated (50) the flow was turbulent and exhibited top-down symmetric time averages. [ABSTRACT FROM AUTHOR]

3D numerical simulations are performed in order to investigate the thermofluids characteristics of the oblique fin heat sink under natural convection. Detailed parametric study is carried out on the effects of geometrical parameters, including main channel width, oblique channel width, fin length, fin width, offset ratio, and fin rotational angle. Through detailed sensitivity analysis, it is found that fin oblique angle and fin rotational angle are the most critical parameters affecting the performance of oblique fins; followed by the main channel width, fin length, and offset ratio. Yet, the effects of fin width and oblique channel width are very minor. Based on sensitivity analysis, an optimum oblique fin heat sink configuration is obtained and a novel symmetrical oblique fin is proposed and compared with the continuous fin heat sink. Subsequently, an experimental verification is made to validate the results of the optimum oblique fin heat sink. The simulation is in line with the experimental data, and the proposed oblique fin heat sink offers more than 3 °C reduction in the maximum heat sink temperature when compared to the continuous fin heat sink. • Performance of oblique fins under natural convection is investigated. • Detailed parametric study is performed on geometrical parameters. • Fin oblique angle and fin rotational angle play pivotal role on performance. • Optimum oblique fin design can reduce maximum heat sink temperature more than 3 °C. • Optimum oblique fin heat sink is validated with experiments. [ABSTRACT FROM AUTHOR]

The steady laminar fluid flow and heat transfer characteristics in the channel formed by the flat tube and the dimpled fin are investigated numerically. The results show that the dimple radius, the dimple pitch and the fin spacing affect the average Nusselt number and friction factor, while the effect of the dimple radius and the fin spacing is larger than that of the dimple pitch. The average Nusselt number, the friction factor and the intensity of secondary flow in the dimpled fin case are larger than those in the plain fin case at the same Re , respectively. The correlations of Nusselt number, the friction factor with Re and other parameters are reported. The significant result is that whenever the same intensity of secondary flow is produced by increasing the radius of the dimples or other structural parameter such as increasing the fin spacing and the dimple pitch, the same heat transfer intensity is obtained. This implies that the intensity of secondary flow determines the heat transfer characteristics on the fin surfaces. [ABSTRACT FROM AUTHOR]

A serpentine pipe is a sequence of parallel straight pipe segments connected by U-bends. Its geometry is fully characterized by pipe radius, a , bend curvature radius, c and length of the straight segments, l . The repeated curvature inversion forces the recirculation (secondary flow) pattern to switch between two specular configurations, which may enhance mixing and heat or mass transfer with respect to a constant-curvature pipe at the cost of an increase in pressure drop. In the present work, fully developed laminar flow and heat transfer in serpentine pipes were investigated by numerical simulation. The curvature δ = a / c was made to vary between 0.1 and 0.5 while the parameter γ = l / c was made to vary between 0 and 8; for each geometry, the friction velocity Reynolds number Re τ = u τ a / ν was made to vary between a very low value (5), yielding almost creeping flow, and the highest value Re τ ∗ still yielding steady laminar flow (∼35–40 in most cases). For Re τ ≤ Re τ ∗ results were obtained for values of the Prandtl number between 1 and 100; predicted values of the friction coefficient and of the Nusselt number were compared with experimental results and correlations proposed in the literature. For Re τ > Re τ ∗ convergence to steady flow was not achieved and an oscillatory behaviour of the solution was observed, indicating a transition to unsteady regimes which deserves a more focused study. [ABSTRACT FROM AUTHOR]

A Galerkin spectral method has been applied to solve numerically the problem of fully developed laminar mixed convection in horizontal concentric annuli with axially uniform heat flow per unit length, uniform temperature along the periphery of the heated wall, thermal insulation on the remaining surface. Results have been obtained for PeRa values up to 10 7 , for Pr = 0.7, 5 and ∞, for inner to outer diameter ratio η = 0.2 in case of inner wall heated and for η = 0.2, 0.4 and 0.6 in case of outer wall heated. Computations allow understanding the onset of secondary flows and their influence on both fluid dynamics and heat transfer. In particular, the distortion of the axial shear stress effects the reduction of the mass flow rate at a given axial pressure gradient. On the other hand, boundary layer thinning determines an increase in the Nusselt number up to a factor 2.5 on average in the investigated range as summarized by suitable correlations. Eventually, results have been compared with other numerical computations and experimental data only available for restricted ranges of the governing parameters. [ABSTRACT FROM AUTHOR]

Abstract: This paper investigates the behaviour of two non-mixing fluids flowing in a heated curved channel. In simulating this immiscible fluid flow, a multi-phase model based on VOF approach is developed for steady, turbulent, incompressible fluid flow through curved channel. The analysis examines the influence of centrifugal forces arising from channel curvature on the phase distribution and flow patterns, and their impact on the thermal characteristics. Unique flow features are identified through the interactions between the fluid phases and vortex structures of the immiscible flow in curved channels. The convective heat transfer in curved channel is appraised in relation to the behaviour of fluid components and curved channel flow parameters. Finally, the study presents a Second-Law irreversibility analysis whereby a thermal optimisation technique is developed for immiscible fluid flow through heated curved channels. [Copyright &y& Elsevier]

A novel heat transfer device for narrow space was proposed to meet high power battery heat dissipation. Based on the battery heat generation data, the heat transfer enhancement mechanism of the secondary flow pattern is studied. Then establish the general heat transfer correlation for this liquid-cooled structure with a specific aspect ratio. The results show that the channel with rib angle of 90° has higher Nu as well as ƒ and can achieve the same heat transfer performance with smaller flow rate. This is due to its transition flow pattern of twist-tape vortex and stagnant vortex, which can enhance the contact heat transfer performance of mainstream cold fluid while inhibiting the growth of boundary layer. The increase in rib number gradually decreases the gain of Nu with a minimum increment of less than 1%; while ƒ almost gradually increases with a common difference of 0.05. From the perspective of heat transfer and energy saving, increasing rib number does not essentially improve the heat transfer performance. Because the flow field at different rib number is just a mapping of the complete flow field at the corresponding length. Based on the above data, for the flat channel with aspect ratio of 10:3 in narrow space, the second-order general heat transfer correlation for different hydraulic diameter is established considering the Re , dynamic viscosity, rib angle and rib number. Finally, the coefficient of determination (R2) of the correlation is 0.9973, which can provide guidance for battery thermal management engineering applications. [ABSTRACT FROM AUTHOR]

In this study, microchannel heat sinks with symmetric and parallel wavy microchannels are studied in a wide range of Reynolds numbers from 50 to 700 via a three-dimensional fluid-solid conjugate model. The results show that the symmetric configuration yields higher Nusselt numbers than the parallel one, and it is especially true at higher Reynolds numbers and larger amplitude-to-wavelength ratios. The heat transfer enhancement can be attributed to the fact that the symmetric configuration induces four Dean vortices in the cross sections perpendicular to the flow path, whereas there are only two Dean vortices in the parallel configuration, leading to stronger coolant mixing and hence the higher Nusselt numbers in the symmetric configuration. However, because of the presence of channel throats, the symmetric configuration also achieves a significantly high pressure penalty. As a result, the overall performance of the symmetric configuration is slightly lower than that of the parallel configuration. To enhance the performance of wavy microchannel heat sinks, modified symmetric and parallel wavy configurations are proposed, in which several transverse gaps are added into ribs to connect adjacent microchannels. The results demonstrate that such a secondary branch design significantly enhances Nusselt numbers for both the parallel wavy configurations owing to the enhanced fluid mixing between adjacent microchannels. Moreover, it is found that, as compared with its original wavy configurations, secondary branches slightly increase the pressure drop across the modified parallel configuration. Thus, the overall performances are markedly enhanced for the modified parallel configurations. As expected, secondary branches greatly reduce the pressure drop across the modified symmetric configuration. Beyond expectation, the suction effect of channel throats due to secondary branches can weaken the Dean vortices in the valley vs valley regions and worsen the heat transfer performance, especially for the heat sink with larger Re and amplitude-to-wavelength ratios. As a result, the modified symmetric wavy configurations are suggested to employ with a wider gap and small amplitude-to-wavelength ratio. Generally speaking, the modified parallel wavy configuration is more dominant than the symmetric one. • Symmetric and parallel wavy microchannel heat sinks are studied numerically. • Parallel configuration shows higher Nu but larger pressure drop penalty. • Modified wavy configurations are proposed with secondary branch design. • Modified parallel configurations yield better overall performance. • Modified symmetric configuration achieves better overall performance partly. [ABSTRACT FROM AUTHOR]

Numerical simulation is performed to evaluate the privilege of inserting stationary or rotating twisted tape inside a tube as a heat transfer enhancement technique. The main emphasis of this work is placed on understanding the effect of pitch distance on the hydrothermal performance of such a system. The obtained results are validated with published experimental findings. The results of numerical study shows that the heat transfer, pressure drop and total energy consumption are increased by inserting a twisted tape inside a tube. Under the stationary condition, decreasing the pitch distance results in higher Nusselt number together with more friction factor. As the twisted tape starts to rotate (RTT1 case), both Nusselt number and friction factor are further increased. However, decreasing the pitch distance from L/2 to L/6 exhibits a marginal impact on the Nusselt number and friction factor. Further increase in the angular velocity (RTT3 case), although improved the average Nusselt number, comes at the expense of much more friction factor as well as energy consumption. Considering performance evaluation criterion (PEC) as a metric to assess the interaction between heat transfer and energy consumption, it is found that rotating twisted tape is beneficial at lower Reynolds number. Among the design parameters analysed in this research, the highest PEC number of 1.5 corresponds to the case of stationary twisted tape at Reynolds number of 1000 and twisted tape pitch of L/6. [ABSTRACT FROM AUTHOR]

In this paper, we consider the flow and heat transfer behaviour of turbulent upward and downward flows of supercritical n-decane, in order to reveal the features of heat transfer deterioration (HTD) that would be expected in relevant active regenerative cooling systems for scramjet engines. Specific focus is placed on key velocity-field features that appear in these flows. Following the validation of six turbulence models, the SST k - ω and RNG k - ϵ models are found to be suitable for simulating the upward and downward flow cases, respectively. "M" type velocity profiles (a non-monotonicity of the velocity along the radial direction) are observed, which arise due to a spatially-varying interplay between the inertial and viscous forces in the flow domain, while larger velocity gradients in the buffer layer are also observed that contribute to the phenomenon of HTD. Furthermore, it is found that the secondary flows as well as the different mass fluxes that arise due to the velocity increase from the wall to the flow core zone (i.e., the influencing range and intensity of cross-sectional kinetic energy), respectively, are observed in the HTD development region, as well as the HTD peak area and degradation regions. A zone of higher thermal diffusion appears in the near-wall region, which acts as a thermal barrier and contributes to HTD. • The inertial force and the viscous force play a major role in the upward and downward flow, respectively. • A large velocity gradient in the buffer layer and the non-monotonicity of velocity contribute to the HTD. • A thermal barrier is generated under the large heat diffusion near the heated wall and thus causes the HTD. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a numerical investigation for examining the secondary vortex motion and associated heat transfer process in fluid flow through curved passages. Overcoming current modelling limitations, the study formulates an improved simulation model based on 3-dimensional vortex structures for describing secondary flow and its thermal characteristics. For developing laminar fluid flow through curved rectangular ducts, the analysis performs a detailed parametric study involving the contours of helicity and outer duct wall pressure gradient for a range of flow rates, duct aspect ratios, duct flow curvatures and external wall heat fluxes. The flow conditions leading to hydrodynamic instability and Dean vortex generation in curved passages are carefully analysed, identifying the flow and geometrical parametric influences. Active interaction of buoyancy force on fluid motion arising from wall heating is considered where aspects of boundary layer separation is used in recognising thermal enhancement due to secondary flow. The simulated results are validated with the available experimental data. Surpassing precision and reliability of current practise, the study develops two numerical approaches offering improved capabilities for capturing the onset of hydrodynamic instability and resulting Dean vortices. These techniques are compared and assessed for their merits of application in determining the onset of secondary flow hydrodynamic instability with and without external wall heating. [Copyright &y& Elsevier]

Abstract: A numerical study is performed to examine the characteristics of fluid flow and convective heat transfer in a helical square duct rotating at a constant angular velocity about the center of curvature. Due to the combined effects of rotation (the Coriolis force), torsion and curvature (the centrifugal force), the flow behaviors become very complicated. A wide range of parameters is covered in this work. The variations of flow structure and temperature distribution with the force ratio F (the ratio of the Coriolis force to the centrifugal force) and the torsion are examined in details. The effects of rotation and torsion on the friction factor and Nusselt number are also studied at length. And also multiple solutions for a rotating toroidal square duct have been obtained. Certain hitherto unknown flow patterns are found. The present work shows both the nature of flow behaviors and the characteristics of heat transfer in a rotating helical square duct. [Copyright &y& Elsevier]

Abstract: Non-isothermal flow with convective heat transfer through a curved rectangular duct of aspect ratio 2 is numerically studied by use of the spectral method with a temperature difference between the vertical outer (heated) and inner (cooled) sidewalls. Numerical calculations are carried out for the Grashof numbers over the Dean number . In the present paper, two cases of the Grashof numbers and are discussed in detail. After a comprehensive survey over the parametric ranges, five branches of steady solutions are found using the Newton–Raphson iteration method for both the cases. Linear stability characteristics of each branch are then studied. It is found that among multiple steady solutions obtained, only one steady solution is linearly stable for a single range of the Dean number for , for , on the other hand, linear stability region exists in three different intervals of the Dean number on the same branch. Nusselt numbers are calculated as an index of the horizontal heat transfer for differentially heated vertical sidewalls. It is found that the convection due to the secondary flow, enhanced by the centrifugal force, increases heat transfer significantly from the heated wall to the fluid, and whence the flow becomes periodic and then chaotic, as the Dean number increases, the rate of heat transfer increases remarkably with respect to a straight channel. [Copyright &y& Elsevier]

When a temperature-induced variation of fluid density occurs in a rotating curved pipe, the Coriolis force, the centrifugal-type buoyancy force and the centrifugal force will contribute to the generation of the secondary flow. The interaction of these body forces complicates the characteristics of flow and heat transfer. A numerical study is performed to examine the combined effects of rotation (Coriolis force), curvature (centrifugal force) and heating/cooling (centrifugal-type buoyancy force), and solutions are obtained for a wide range of parameters. The combined effects of these comparable order of magnitude forces on the flow structure, temperature distribution, friction factor and Nusselt number are examined in detail. Present works show the natures of the flow and heat transfer in rotating curved pipes. [Copyright &y& Elsevier]

A numerical simulation is presented to describe the secondary flow characteristics in the flow through curved rectangular ducts that are heated on the outer (concave) wall. The buoyancy body forces due to external heating are incorporated in the formulation. The governing equations of momentum, mass continuity and energy of the flow are non-dimensionalised and transformed into vorticity-streamfunction. The equations are discretised using a finite volume method and the dynamical variables are defined on a staggered grid. The resulting penta-diagonal system of linear algebraic equations is solved using the Strongly-Implicit-Procedure. Numerical computations are performed for the flow through rectangular ducts of aspect ratios 1 to 8 and for the Dean number lying in the range 20 to 500. The external wall heat fluxes of 0 to 200 W·m−2 are considered. The computation accurately captures the influence of external heating on the secondary vortex motion and, predicts the occurrence of Dean vortices in non-isothermal flow through the curved ducts. With wall heating, the flow pattern rapidly deviate from the symmetrical vortex motion observed under isothermal conditions. The formation of Dean vortices still takes place in the flow although it is postponed to higher flow rates where the flow symmetry is gradually re-established. The number of Dean vortices formed in the flow is strongly influenced by the aspect ratio of ducts. Convective heat transfer is significantly enhanced by the secondary flow particularly when the Dean vortices emerge at the outer wall. The predicted characteristics of secondary flow and heat transfer conform to and agree well with the available experimental data. [Copyright &y& Elsevier]

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

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

Soot particulate matter deposition on the tubes of exhaust gas recirculation (EGR) coolers would form an insulating deposit layer, which deteriorates the thermal efficiency of the cooler and increases the pressure drop substantially. The objective of this research is to examine the influence of using helical tubes instead of straight horizontal tubes on the development of deposit layer. An experimental setup has been designed and constructed to examine the thermal performance of different types of EGR coolers under both clean and fouling conditions. Two sets of experiments have been performed, i.e. with and without soot particles, and in each set three types of EGR coolers have been examined, namely straight tube cooler, and two helical tube coolers of 5 and 10 windings. The effectiveness of the helical tube cooler is highest in case of 10 windings and lowest in case of a straight tube cooler which is due to the increase in the heat transfer surface area. The thermal resistance of the fouling layer in case of a straight tube cooler and 10 windings cooler has stabilized, however the thermal resistance in case of 5 windings is not stable and it is increasing with time. It has been concluded that there is a cleaning action during the fouling experiment in case of the helical tube coolers, and that is due to the secondary flow, and the efficiency of cleaning is a function of the intensity of the secondary flow. The thermal performance of the 5 windings cooler is not stable due to the development of the fouling layers, while in case of the 10 windings is stable, and that means there is a critical number of windings for the helical tube EGR coolers above which fouling diminishes. This critical number of windings is a function of the gas speed, particles size, tube diameter, radius of curvature and the winding pitch. The secondary flow in helical tubes agitates the boundary layer and decreases the pressure drop, such that it is advised to use helical tube coolers with more windings from a thermal and a fluid mechanics point of view. • Secondary flow in helical tubes brushes off deposited particles in the tubes. • Intensity of cleaning in helical tubes is a function of secondary flow. • There is a critical number of windings above which fouling diminishes. • Secondary flow in helical tubes decreases the pressure drop across the tubes. [ABSTRACT FROM AUTHOR]

In this paper, the thermal-hydraulic performance in the downstream of the spacer grid in a 7 × 7 rod bundle is numerically studied. The guide tubes effects are revealed by the comparisons of the flow and heat transfer behavior in two types of rod bundles, one without guide tubes and the other with guide tubes. It is found that the cross flow diagonals are fully developed at Z = 2 D h and unable to sustain at Z = 8 D h downstream of the spacer grid. The guide tubes discontinue the cross flow diagonals and would decrease the secondary flow intensity. The downstream temperature distribution is influenced by the evolution of the secondary flow as well as the guide tubes. The non-uniformity of the temperature distribution has the lowest value at Z = 2 D h and the guide tubes would make the temperature distribution more uneven. Furthermore, the normalized Nusselt number in both fuel rod bundles reaches the peak at Z = 2 D h and then decreases consistently. However, the heat transfer can be enhanced by 15% at least because the secondary flow still exists. The normalized Nusselt numbers downstream of the spacer grid in the rod bundle with guide tubes are smaller than those without guide tubes. Overall, the guide tubes would inhibit the secondary flow and weaken the heat transfer capability downstream of the spacer grid. [ABSTRACT FROM AUTHOR]