Turbulent Heat Transfer Research Articles

Numerical and experimental analysis of LBE flow and heat transfer characteristics in hexagonal rod bundles

Lead-bismuth eutectic (LBE) is a liquid with low Prandtl number and high thermal conductivity, generally used as the coolant in lead-cooled fast reactors of Generation-IV. The traditional Reynolds analogy is not appropriate for LBE, hence the theoretical analysis and experimental measurements are studied to interpret the flow and heat transfer of LBE in this article. In this way, a single-rod experiment on flow and heat transfer for LBE is performed and a new empirical correlation is fitted. To analyze the behavior of flow and heat transfer for low Prandtl number fluids, and also to validate the turbulence model and new correlation in the hexagonal rod bundles. The CFD simulations are performed for the XJTU 7-rod bundle, the KIT 19-rod bundle, and the ENEA 37-rod bundle based on ANSYS Fluent. The convective heat transfer coefficient of LBE, the temperature of heating wall at different sub-channels, and the temperature non-uniformity are investigated in this paper, to reveal the flow and heat transfer characteristics of LBE. The results show that the deviations of the section averaged Nusselt number are in 25%, and the deviations of the local temperature of LBE and heating wall are in 10℃. Grid spacers will result in a 30% rise in the section averaged Nusselt number, which will improve the turbulent intensity and heat transfer. Temperature non-uniformity is reduced as the Peclet number is increased. The validations demonstrate that the new empirical correlation provides accurate convective heat transfer coefficient predictions for LBE flow and heat transfer in hexagonal rod bundles with a relative derivation in 20%, which can be used as a reference for additional LBE simulations and experiments.

Polarized-depolarized Rayleigh scattering for simultaneous composition and temperature measurements in non-isothermal gaseous mixtures.

This paper demonstrates the application of polarized-depolarized Rayleigh scattering (PDRS) as a simultaneous mixture fraction and temperature diagnostic for non-reacting gaseous mixtures. Previous implementations of this technique have been beneficial when used for combustion and reacting flow applications. This work sought to extend its applicability to non-isothermal mixing of different gases. The use of PDRS shows promise in a range of applications outside combustion, such as in aerodynamic cooling technologies and turbulent heat transfer studies. The general procedure and requirements for applying this diagnostic are elaborated using a proof-of-concept experiment involving gas jet mixing. A numerical sensitivity analysis is then presented, providing insight into the applicability of this technique using different gas combinations and the likely measurement uncertainty. This work demonstrates that appreciable signal-to-noise ratios can be obtained from this diagnostic in gaseous mixtures, yielding simultaneous temperature and mixture fraction visualization, even for an optically non-optimal selection of mixing species.

Turbulent heat transfer in pipes with increased roughness through shavings of helical ribs

Large-eddy simulations of mircrostructured pipes are carried out in this study. The pipes are helically ribbed with several starts from which shavings are cut out along the rib. A Reynolds number of 16,000 and a Prandtl number of 9 are applied. Cyclic boundary conditions are used to simulate an infinitely long pipe and a LES turbulence model is applied. The results show that both the number of starts and the distance of the shavings along the rib, have an impact on turbulence and heat transfer. An optimum was found for the number of starts regarding heat transfer. The heat transfer area increases for a higher number of starts, but the flow-related heat transfer decreases. Thus, an optimal arrangement of starts was identified for this pipe configuration. The variation of the arrangement showed that the pressure loss can be reduced by an inline configuration, whereby the heat transfer is only slightly decreased. It can be shown that the simulations can contribute to optimising complex structures in the future and thus accelerate the development process of new geometries. It can be shown that the development of new heat exchanger pipes is also possible with complex structures, this can be demonstrated here for the first time for structures that can actually be fabricated.

Impact of human body shape on forced convection heat transfer

Predicting human thermal comfort and safety requires quantitative knowledge of the convective heat transfer between the body and its surrounding. So far, convective heat transfer coefficient correlations have been based only upon measurements or simulations of the average body shape of an adult. To address this knowledge gap, here we quantify the impact of adult human body shape on forced convection. To do this, we generated fifty three-dimensional human body meshes covering 1st to 99th percentile variation in height and body mass index (BMI) of the USA adult population. We developed a coupled turbulent flow and convective heat transfer simulation and benchmarked it in the 0.5 to 2.5m·s-1 air speed range against prior literature. We computed the overall heat transfer coefficients, hoverall, for the manikins for representative airflow with 2m·s-1 uniform speed and 5% turbulence intensity. We found that hoverall varied only between 19.9 and 23.2 W·m-2K-1. Within this small range, the height of the manikins had negligible impact while an increase in the BMI led to a nearly linear decrease of the hoverall. Evaluation of the local coefficients revealed that those also nearly linearly decreased with BMI, which correlated to an inversely proportional local area (i.e., cross-sectional dimension) increase. Since even the most considerable difference that exists between 1st and 99th percentile BMI manikins is less than 15% of hoverall of the average manikin, it can be concluded that the impact of the human body shape on the convective heat transfer is minor.

Analysis of macrosegregation during slab continuous casting using 3D-longitudinal 2D hybrid model

ABSTRACT To improve the computational efficiency of simulating the macrosegregation process that occurs during continuous casting of steel slabs, a new hybrid method combining three-dimensional and longitudinal two-dimensional models was developed to consider the turbulent flow, heat transfer, solidification, and solute transport phenomena. The reasonability of the performed computational domain division was determined, and the model accuracy was verified experimentally. Using this approach, the solute distribution characteristics and central segregation during continuous casting were examined in detail. The obtained results revealed that recirculation flows in the turbulent flow region play a critical role in the distribution of solute elements near the slab surface. Owing to the redistribution of solute elements at the solid–liquid interface and the movement of the solid–liquid interface towards the slab centre during continuous casting, the solute elements in the slab centre are continuously enriched, ultimately resulting in central segregation.

Computational fluid dynamics and multi-objective response surface methodology optimization of perforated-finned heat sinks

BackgroundThis study analyses turbulent flow and heat transfer characteristics of a three-dimensional perforated finned heat sink using computational fluid dynamics (CFD) and response surface methodology (RSM) methods. MethodsThe effects of perforation geometry, including size (1.85 ≤ σ ≤ 2) and cross-sectional shape (square, circular, hexagonal, and triangular), as well as the Reynolds number (25, 000 ≤ Re ≤ 40, 000) as design variables, on parameters such as friction drag force, pressure drag force, total drag force, and Nusselt number were investigated. In addition, using the well-known RSM technique, three accurate models were proposed for the percentage of heat transfer enhancement (PHTE), percentage of drag reduction (PDR), and percentage of weight reduction (PWR) as the most significant design objectives for any heat sink. RSM models served as the foundation for two- and three-objective optimizations. Significant FindingsResults indicate that fins with square perforations in high σ and fins with circular perforations in low σ could achieve exceptional thermal performance (PHTE > 70%). Compared to solid fins, fins with square perforations demonstrated improved PDR performance with a 20–40% reduction in total drag force. In addition, using fins with circular perforations can reduce the heat sink's weight by 45–65% compared to solid fins. Moreover, the optimal condition for the perforated finned heat sink can be attained by considering the values 40,000 and 1,946 for Reynolds number and σ, respectively, and by selecting a circular shape for the perforations. PHTE, PDR, and PWR increase in the optimal case by 68.98%, 35.87%, and 58.86%, respectively, compared to the base case (solid fin).

Thermal and hydraulic optimization of supercritical CO2 pillow plate heat exchanger with ellipse weld spots in CSP system

Pillow plate heat exchangers (PPHE) with compactness and high thermal performance are applying widely. In present article, PPHE with ellipse weld spots is first proposed, and turbulent flow and heat transfer of supercritical carbon dioxide (S-CO2) in PPHE channel is analyzed and optimized. The channel is made of many periodically overlapped metal plates, and the plate is a flat plate with many staggered convex-concave spots. Compared with channel section with concave spot, channel section with convex spot has higher flow velocity, lower flow fraction and higher heat transfer coefficient, so it dominates heat transfer process. Along flow direction, heat transfer coefficient in channel section with convex spot gradually increases to maximum in flat region, and then it drops to minimum in downstream region of convex spot, while friction factor has opposite tendency. A meaningful phenomenon is found that higher heat transfer coefficient (h) and lower friction factor (f) appear simultaneously in the same region. The flow and heat transfer behavior are significantly affected by weld spot geometries, and elliptic weld spot with larger ratio of spot length and width Ψ results in weaker vortices and fluid impingement, which remarkably reduces friction factor. The elliptic weld spot with Ψ = 1.4 is optimum geometry for maximum index of Nu/f1/3. In addition, correlations of f and Nusselt number (Nu) for Ψ = 0.6–1.4 and Re = 4132–6650 are developed.

New Insights in Turbulent Heat Transfer With Oil and Hybrid Nano-Oils, Subject to Discrete Heating, for Parabolic Trough Absorbers

Abstract The concentrated solar thermal systems, operating in the medium temperature range of 373–573 K, will be extremely useful for several industrial processes. However, the need for an in-depth understanding of the turbulent heat transfer in parabolic trough absorbers with pure and hybrid nano-oils, including the effect of buoyancy or gravity, is realized. This paper presents the Reynolds-averaged Navier–Stokes (RANS)-based turbulent heat transfer analyses in a 3D, long, straight for Reynolds number from 5000 to 20,000 and discrete heating conditions with different heat flux ratios such as 1, 5, 10, 20, 40, and 50 for pure oil and hybrid nano-oils having 1, 4, and 6% volume concentration of the nanoparticles. The major findings are, (a) gravity-induced anisotropy leads to high and low-speed fluid flows near the lower and upper walls, and temperature redistribution at a plane, which is beneficial, (b) the statistical axial-velocity deviates from the standard logarithmic law at a Reynolds number of 5000, and (c) the ratio of surface-area-averaged Nusselt number between the lower half and upper half of the tube is 4–12. Some important recommendations are (a) the effect of gravity must be included, (b) the local Richardson number may be used for improving the standard logarithmic law for the axial velocity, and (c) Nusselt number correlations are deduced for the upper half surface and lower half surfaces. The findings, albeit for limited parameters, will be useful for improving the heat transfer aspects in the parabolic trough absorber.

Turbulent Heat Transfer Augmentation in a Square Channel by Augmenting the Flow Pattern with Novel Arc-Shaped Ribs

Solar water heaters (SWHs) are widely used in HVAC industries as well as in households for different heating purposes. The present numerical simulation focuses on the investigation of the thermo-hydraulic performance of novel semi-arc-rib SWHs. Semi-arc-shaped ribs in the square channel of the absorber plates with different pitch and height ratios are investigated in this study. The present novel modification disturbs the boundary layers by generating vortices, and thus, enhanced fluid mixing takes place. Water with a Reynolds number (Re) ranging from 4000 to 25,000 is used as a working fluid, and a 1.0 kW/m2 heat flux is imposed on the tube wall. The results demonstrate a significant increase in the Nusselt number (Nu) as the fluid layers localize behind each rib near the absorber plates, and at the same time, the number of swirls generated inside the tube and the frictional losses both increased noticeably. To ensure the effectivity of the present novel SWH geometry, the thermo-hydraulic performance (η) for each case was calculated, and it was found that in all the cases, it was greater than unity, which signifies that the present semi-arc-rib SWH is promising and can be used in HVAC industrial and household applications.

On the mechanism of turbulent heat transfer in composite porous-fluid systems with finite length porous blocks: Effect of porosity and Reynolds number

The majority of literature studies on composite porous-fluid systems involve fully-developed porous channel flows where the porous media covers the whole length of the channel. These studies utilized periodic boundary conditions at the inlet and outlet. In these systems, the stagnation at the frontal face of the porous block, turbulent separation bubble over the porous-fluid interface, and flow leakage from the porous to non-porous regions do not exist. The existence of these flow features in the case of a finite porous block immersed in a channel flow modifies turbulent interactions across the porous-fluid interface. In contrast to the previous studies, this paper investigates the flow and thermal characteristics of turbulent channel flow containing a porous block with a finite length. To this end, pore-scale large eddy simulations are performed in composite porous-fluid systems with two porosities (53% and 91%) at three Reynolds numbers of 3600, 7200 and 14400. Flow visualization shows that two distinct regions are formed over the interface in low-porosity cases: Region#1 near the leading edge with organised hairpin structures and high flow leakage; Region#2 away from the leading edge with unorganised hairpin structures and lower flow leakage. In region#1, maximum turbulent fluctuations occur far away from the interface while they approach the interface in region#2. The results showed that by increasing either the Reynolds number or porous length, the location of maximum turbulence statistics approaches the interface. This observation supports earlier findings for fully-developed porous channel flows which are only valid in region#2. Whereas, with a low Reynolds number or a short porous length, the turbulent statistics peak far from the interface, consistent with the observations in region#1. Besides, it was found that increasing the porosity and Reynolds number reduces the flow leakage (from the porous region to the non-porous region) up to 50% and 10%, respectively, which in turn disrupts the patterns of contour-rotating vortex pairs and hairpin structures over the interface. It is further found that for a fixed Reynolds number, the overall Nusselt number for the high-porosity case is 2.6 times higher than that of the low-porosity case. The pressure drop for the low-porosity cases is 1.8 times more than that for the high-porosity cases.

Experimental study on turbulent convection heat transfer of supercritical CO2 in cooled inclined miniature tubes

This study explores experimentally turbulent convection heat transfer characteristics of supercritical carbon dioxide (CO2) in cooled miniature tubes (Dh = 1 mm) with various inclined angles (θ = 0, 30, 60 and 90°). The influence of tube inclined angles, CO2 mass flow rates and wall heat fluxes on wall temperature and heat transfer coefficient were analyzed. Results related to buoyancy effect and thereby affected heat transfer were discussed. It was found that the wall temperature and the heat transfer coefficient markedly increase with the increased inclined angle, indicating the gradual weakness of mixed convection flow with the increased inclined angle. This effect was strengthened as mass flow rate decreased. In addition, the liquid-like region witnesses more significant influence of buoyancy effect on the heat transfer coefficient. Increasing mass flow rate could promote the heat transfer intensification, accompanied by the flow shifting from mixed convention flow to forced convection flow. The heat transfer coefficient is independent of the wall heat flux in liquid-like region, and slightly increased with the augment of wall heat flux in gas-like region. Given that the previously existing heat transfer correlations showed large deviations in predicting the present experimental data, a set of new correlations were proposed for cooling heat transfer prediction of supercritical CO2 in inclined miniature tubes and yield excellent predictive capability, evidenced by overall mean obsolete error of 11.13%, 7.72%, 12.30% and 14.24% for the inclined angle θ = 0°, 30°, 60° and 90° respectively.

Heat transfer analysis of supercritical CO2 in a High-Speed turbine rotor shaft cooling passage

This study investigated the turbulent flow heat transfer in a high-speed turbine rotor shaft cooling passage using CFD. Supercritical CO2 is used as the coolant. The coolant temperature, heat transfer coefficient, convection heat transfer, and frictional heat transfer are investigated. The effect of clearance, mass flow rate, shaft rotational speed, and shaft temperature is also investigated. The result showed that the coolant temperature and heat transfer coefficient are independent of the radial clearance. The significant effect of mass flow rate on the coolant temperature is predicted. The convection heat transfer is substantially higher than the frictional heat transfer. This implies that the convection heat transfer is highly dominant over the frictional heat transfer by which the significant cooling provision could be achieved by the supercritical CO2 fluid through the annulus passage. A little agreement is found when the quantity of heat transfer is predicted using correlation and compared with the result obtained by CFD.

A Study on Accuracy Range of Multiphase Mixture Model for Turbulent Convective Heat Transfer Enhancement Simulation of Water-Al2O3 Nanofluid

Because of complications and cost of experimental studies, simulating heat transfer of nanofluids using the methods of computational fluid dynamics (CFD) has become a reliable approach to work with them. As Mixture model remains as one of the most heavily used CFD models to examine the heat transfer enhancement of nanofluids according to literature, finding out the range of nanofluid configurations for which the Mixture model is able to provide satisfactory results is a necessity. In this study, turbulent flow of water-Al2O3 inside a circular pipe under uniform wall temperature has been simulated in order to find out the conditions for Mixture model to yield reliable results in terms of predicting heat transfer enhancement of nanofluids. Along with depicting significant increase in heat transfer with particle concentration hike, the results suggested that Mixture model predicts heat transfer enhancement the most accurately for nanoparticle concentration of around 3% with an average discrepancy of less than 1% from experimental data, though the results for particle concentration range of 2.5% to 3.5% are quite satisfactory yielding average error lower than 8%.

An entropy viscosity method for large eddy simulation of turbulent thermal flow in a rotor–stator cavity

Turbulent flow and heat transfer in a rotor–stator cavity have fundamental importance in both academia of turbulence research and the industry of rotating turbomachinery. The main characteristic of the flow is that there is the centrifugal Ekman layer on the rotor and the centripetal Bödewadt layer on the stator, which are separated by a central rotating core. In this paper, an entropy viscosity subgrid model based on the large eddy simulation (LES) method is proposed to solve the complex flow with heat transfer in a rotating frame at high Reynolds numbers. The method is fully validated by the simulation of turbulent thermal flow in a closed stator–rotor cavity up to Re=106. By performing 12 simulations, the sensitivity of the simulation results to mesh resolution and the free parameters of entropy viscosity are systematically studied, and the proper range for the parameters is determined. In particular, it is found that the prediction on the mean flow and fluctuation from the simple turbulent diffusivity model, which scales linearly with the eddy viscosity, is as accurate as that from the alternative model that is a more computationally complex model. Despite the comparable accuracy, the entropy viscosity-based LES uses the mesh resolution two-order lower than that of direct numerical simulation; therefore, it is feasible to apply the LES to the flow at the practical Reynolds number in an aircraft engine, that is, Re≥107.

ヘリウムガスを用いた水平細管内強制対流定常熱伝達に関する数値解析

The numerical simulation of turbulent heat transfer for helium gas flowing in a horizontal small-diameter tube was performed. The numerical simulation conditions were 0.8 mm and 1.8 mm for inner diameters, and 102-128 m/s and 87-241 m/s for velocities at the inlet of the tube, respectively. The numerical solution was carried out by applying ANSYS FLUENT software. The standard k-ε model and a user-defined wall function were used for the analysis. The thickness of the conductive sublayer, which is the region where heat transfer by conduction is dominant, was estimated. As a result of the numerical simulation, it was found that the thickness of the conductive sublayer becomes thinner as the flow velocity increases and the inner diameter decreases. Moreover, the heat transfer coefficients of the simulation data were compared with the previous experimental data, and they were in good agreement with the experimental data.

Numerical study of location and depth of rectangular grooves on the turbulent heat transfer performance and characteristics of CuO-water nanofluid flow

This current work expresses numerical simulation of forced turbulent flow convection in a grooved cylinder. Rectangular grooves with a spacing of A = 1, A = 1.1, and A = 1.3, and groove depth to cylinder diameter of e/D = 0.1 and 0.2 were considered. This research concentrates on the effect of groove depth, location of the grooves and CuO nanoparticles on the heat transfer for Reynolds numbers 10000, 12,500, 15,000 and 17,500 in volume fractions of 0, 1, 2, 3 and 4% of nanoparticles. Results show that grooves improve heat transfer. This behavior at a lower A ratio results in a significant Nu number increase so that the highest Nu number occurs for A ratio of 1, 1.1 and 1.3. Increasing e/D ratio, due to increasing the channel section in this area, results in loss of velocity and dissipation of flow momentum, resulting in lower convective heat transfer and lower Nu number. Changing the pitch for e/D = 0.1 results in a 1.1 to 1.6 times increase of Nu number compared with the smooth channel, and for e/D = 0.2 this value is 1.1–1.5 times the smooth channel for similar Re, φ and geometry. Changing groove pitch at e/D = 0.1 results in a 2.1–2.9 times increase in friction factor compared with the smooth channel in similar conditions. For e/D = 0.2, this increase is 1.8–2.8 times the smooth channel. In low Re, the thermal performance is higher than in higher velocities. This is because the grooved channel acts as a smooth channel at high Re, and the average Nu does not have significant growth.

Dissimilar turbulent momentum and heat transfer associated with viscoelastic vibration in channel flow

This experimental study on the momentum and heat transfer in drag-reducing turbulent channel flow of surfactant solution, i.e., viscoelastic fluid, has revealed a viscoelastic vibration (VEV) as a novel type of ordered turbulent motion in the wall vicinity where the elastic energy is transformed into turbulent kinetic energy. The VEV is a wave-like motion of low-frequency modes in the streamwise direction and causes zero average Reynolds shear stress. However, thermal diffusion occurs continuously due to the movement of a fluid lump owing to the VEV via the additional action of pressure.

Numerical prediction of turbulent flow and heat transfer in buoyancy-affected liquid metal flows

The present paper investigates the capabilities of some selected Reynolds-Averaged Navier Stokes (RANS) based turbulence models in reproducing liquid metals thermal hydraulics Direct Numerical Simulation (DNS) data. For this purpose, forced and mixed convection conditions, both addressing buoyancy-aided and buoyancy-opposed flow, are considered. The paper mainly focuses on velocity and temperature fields estimation, providing a comparison between the RANS and DNS computations. The capabilities of the turbulence models are discussed with the aim to highlight which ones provide the best predictions.In particular, attention is paid to the approach adopted for the calculation of the turbulent heat flux contributions. Together with models assuming the commonly adopted Simple Gradient Diffusion Hypothesis (SGDH) approach and the Reynolds analogy, a model including the Algebraic Heat Flux Model (AHFM) approach is considered. While being a practical and robust approach to deal with turbulent heat fluxes, the SGDH approach shows intrinsic limitation in dealing with liquid metal thermal hydraulics, mainly because of their low-Prandtl number. The adoption of a more advanced AHFM method may instead relevantly improve the quality of the obtained predictions.The obtained results show that the selected model adopting the AHFM method provides definitively better predictions of the addressed phenomena with respect to the ones considering the SGDH approach. While some discrepancies are still observed for the velocity fields, the temperature fields are captured very well, suggesting a clear superiority of the AHFM model. The present paper thus provides further validation and supports the use of AHFM as a valuable tool to predict turbulent heat fluxes.

Numerical study on heat transfer characteris tics of supercritical water in straight and helical tubes

The subcritical steam generators in high-temperature gas-cooled reactor pebble-bed modules can be replaced by supercritical steam generators to improve thermal efficiency. The axial and circumferential nonuniform flow and heat transfer characteristics of supercritical water in straight and helical tubes with varying inclinations (0–90°) are simulated to investigate the combined effects of tube structure, flow direction, thermophysical properties, property variations, buoyancy, and centrifugal forces. With heat fluxes of 233–930 kW/m2 and inlet mass fluxes of 225 or 1260 kg/(m2 s) under a working pressure of 24.5 MPa, the friction coefficients, turbulent kinetic energy, secondary flow, temperature, and heat transfer coefficients are studied in the cross section of variable structure tubes. The wall temperature and heat transfer coefficients of four regions are compared in straight and helical tubes; the maximum heat transfer coefficient appeared before the pseudo-critical point. The heat transfer coefficients are the greatest at the bottom of the straight tubes, where fluidity was high, and at the outer of the helical tubes, where the fluid velocity was high under centrifugal force. The secondary flow in the helical tubes causes a nonuniformity in the wall temperature, causing the circumferential maximum wall temperature to move.

ANN model with feature selection to predict turbulent heat transfer characteristics of supercritical fluids: Take CO2 and H2O as examples

Heat transfer deterioration of supercritical fluids is difficult to predict accurately due to the numerous influencing factors. Traditional correlations failed to fit the relationships between many parameters over a wide range of experimental conditions. To this end, we developed a general artificial neural network model for simultaneously predicting the Tw and Nu of supercritical H2O and CO2, which is the first model currently available for multi-fluid prediction. To maintain the compactness of the model feature space, redundant features are eliminated by feature selection. This model was trained with circle tube data but still maintained high accuracy on the non-circle tube data, demonstrating its strong generalization and independence of channel shape. High accuracy, applicability, and convenience over a wide range of applications are embraced in the ANN model, which lays a solid foundation for engineering application of the ANN model. The generalizability of the ANN model could be further improved by incorporating other fluid data.