Heat Transfer In Turbulent Flow Research Articles

Direct numerical simulation of compressible turbulent channel flows over porous boundaries

The impact of porous boundaries on skin friction and heat transfer in compressible turbulent channel flows is explored through direct numerical simulations. Utilizing the volume-averaged Navier–Stokes equations for the porous media, we examine flows at Mach numbers Mb=0.3 and Mb=1.5, with friction Reynolds numbers ranging from 176 to 220, across three permeability levels: 0 (impermeable), 1.6×10−5, and 1.0×10−4. Our findings reveal a dual impact of porous boundaries on skin friction: a direct reduction due to averaged slip velocity at the interface and an indirect increase from amplified Reynolds shear stress due to enhanced wall-normal permeability. This permeability weakens the wall-blocking effect, leading to intensified wall-normal velocity fluctuations and consequently higher Reynolds stress. Additionally, the Mach number significantly influences flow structures, particularly at higher permeabilities, where it disrupts near-wall streaks at Mb=0.3 but has a diminished effect at Mb=1.5. Thermal analysis confirms the validity of the generalized Reynolds analogy for the mean temperature in these flows, with the turbulent Prandtl number approximating 1, in line with the strong Reynolds analogy. Moreover, a strong spatial correlation between heat flux and wall shear stress fluctuations underscores the intertwined dynamics of momentum and thermal transport at the porous–fluid interface.

Temperature statistics of settling particles in homogeneous isotropic turbulence

Heat transfer of settling particles in homogeneous isotropic turbulence is investigated in one-way coupled Eulerian–Lagrangian direct numerical simulations. In the absence of gravity, our observations highlight that there was a correlation between particles concentration and temperature fronts, which are characterized by high-temperature gradients. The particle clustering is mainly influenced by the preferential concentration and the non-local effect, the latter being the memory of their interaction with the flow along their trajectories. Nevertheless, gravity significantly affects the clustering of particles, which further influence the heat transfer of particles with the fluid. We find that the heat transfer of particles is intricately related to particle dynamic inertia and thermal inertia, which can be quantified by the particle Stokes number St and particle thermal Stokes number Stθ, respectively. In this study, we observe that the variance of particle temperature rate of change is independent of Stθ at small thermal inertia but inversely proportional to Stθ2 for large thermal inertia. In the presence of gravity, the variance converges to Stθ−1 for particles with negligible thermal inertia. Our findings reveal that the second-order structure function of the particle temperature, indicating the enhancement of Lagrangian scalar intermittency, adheres to a power-law behavior with limited thermal inertial particles in the dissipation region, denoted as r2. However, as Stθ increases, the power law of the structure function of the particle temperature is disrupted leading to ‘thermal caustics’. The present findings of the heat transfer behavior of settling particles advance the understanding of gravity effect, and are valuable for the modeling of heat transfer in particle-laden turbulent flows.

Performance evaluation and enhancement of turbulent flow and convective heat transfer characteristics for turbine blade internal cooling

For turbine blade internal cooling, the performance of turbulent flow and convective heat transfer in the cooling channel is numerically investigated via the computational fluid dynamics method, where the cooling channel adopts the combination of transverse rib arrangements and round-edged rib configurations or the combination of oblique rib arrangements and round-edged rib configurations. Under these two combination conditions, the simulation result shows that in the wide Reynolds number range of Re = 20 000–80 000, the heat transfer enhancement and the flow loss reduction, such as the larger normalized Nusselt number of Nu/Nu0 and smaller normalized friction factor of f/f0, are simultaneously realized by the oblique rather than transverse rib arrangement. In the oblique rib channel, the relationship between turbulent flow and convective heat transfer is for the first time revealed by the relationship between secondary vortices and turbulent kinetic energies. Based on the relation between secondary vortices and turbulent kinetic energies, the overall performance of turbulent flow and convective heat transfer for the oblique rib channel is first optimized by various normalized rib height, e/Dh, and rib spacing, p/e, and then evaluated by both overall performance factors of OPT1 and OPT2. Both OPT1 and OPT2 are larger at e/Dh = 0.062 and p/e = 15.00 in the wide range of e/Dh = 0.047–0.101 and p/e = 10.00–15.00. As a result of the comprehensive evaluation of OPT1 and OPT2, the combination of e/Dh = 0.062 and p/e = 15.00 is eventually employed by the oblique rib channel.

Experimental, numerical and deep learning modeling study of heat transfer in turbulent pulsating pipe flow

Pulsating heat transfer is important in many engineering applications but is not fully understood or easily modeled. This work helps understand the phenomenon and demonstrates the efficacy and utility of deep learning modeling for pulsating heat transfer. The convective heat transfer coefficient (HTC) of pulsating flow through a copper tube was experimentally investigated at seventy-seven different operating conditions (500,000 time-series data points were collected at each operating condition) corresponding to different pulsating waveforms, frequencies, mass flow rates and pressures. The intensity of pulsation ranged from mild to strong enough to cause reverse flow. The literature on pulsating HTC is mixed, with several researchers reporting increased heat transfer and several other reporting decreased heat transfer relative to steady flow. The experimental data showed that both could be true depending on the ratio of the amplitude to mean flow velocity (amplitude ratio AR). The enhancement ratio (ER) of pulsating HTC relative to steady-flow HTC was measured to vary between 0.48 and 2.18 for AR ranging from 0.64 to14.42. A numerical study showed that this was only a (positive) correlation; fundamentally the overall pulsating HTC could be approximately explained by the variation of instantaneous Reynold’s number (Re) during the pulsating cycle. The distribution of instantaneous Re and hence the ER is primarily determined by the pulsating waveform, and this explains the disparate observations by different researchers. Long Short-Term Memory (LSTM) networks have been used to model pulsating HTC as a function of pulsating waveforms occuring across a measurement orifice. The ‘Toy Model’ concept where physics-based variables are used to increase generalization was implemented with the LSTMs for increased model robustness. These models could be used to maximize pulsating heat transfer by optimizing the waveform.

Comparison of Heat Transfer and Friction in Pipes With Various Internal Roughness

Abstract The heat transfer and friction factor of turbulent pipe flows with different internal roughness are experimentally investigated. Three types of roughness in forms of a mesh, hemispherical elements, and a coil are added to the interior of pipes with an inner diameter of 52.5 mm. The working fluid is air, and the Reynolds numbers vary from 20,000 to 90,000 in increments of 10,000. For investigating the heat transfer, the pipe wall is heated to 375 °C while the inlet air remains at the room temperature. The measurements show that the mesh-type roughness results in a maximum Nusselt number, Nu, increase of approximately 6%, the pipes with hemispherical roughness increased the Nu by a maximum amount of 30%, and the coil increased Nu by up to 60% compared with the smooth pipe. The maximum increase of friction factor is 40% for the pipes with mesh-type roughness, 30% for pipes with hemispherical roughness, and 67% for pipes with coil roughness. The experimental results indicate that adding hemispherical and coil roughness to the internal surface of the pipe can lead to a significant improvement in the rate of heat transfer while adding a mesh-type roughness can have marginal improvements and comes with a large frictional loss penalty. The analysis shows that the highest thermohydraulic performance is achieved using the hemispherical roughness elements.

The Role of Particle Inertia and Thermal Inertia in Heat Transfer in a Non-Isothermal Particle-Laden Turbulent Flow

We present an analysis of the effect of particle inertia and thermal inertia on the heat transfer in a turbulent shearless flow, where an inhomogeneous passive temperature field is advected along with inertial point particles by a homogeneous isotropic velocity field. Eulerian–Lagrangian direct numerical simulations are carried out in both one- and two-way coupling regimes and analyzed through single-point statistics. The role of particle inertia and thermal inertia is discussed by introducing a new decomposition of particle second-order moments in terms of correlations involving Lagrangian acceleration and time derivative of particles. We present how particle relaxation times mediate the level of particle velocity–temperature correlation, which gives particle contribution to the overall heat transfer. For each thermal Stokes number, a critical Stokes number is individuated. The effect of particle feedback on the attenuation or enhancement of fluid temperature variance is presented. We show that particle feedback enhances fluid temperature variance for Stokes numbers less than one and damps is for larger than one Stokes number, regardless of the thermal Stokes number, even if this effect is amplified by an increasing thermal inertia.

Effects of splitter plate and mass ratio on flow-induced vibration and heat transfer characteristics of a circular cylinder in turbulent flow: A numerical study

The two-degree-of-freedom (2DOF) flow-induced vibration (FIV) of a circular cylinder with heat transfer in turbulent flow is numerically investigated. The cylinder with an attached splitter plate is elastically supported and allowed to vibrate in both in-line and transverse directions. The unsteady RANS and energy equations along with the SST k-ω turbulent model are coupled with the equation of vibration to study the FIV of the cylinder. The effects of the attached plate length (L/D = 0.5, 1.0, and 1.5) and the mass ratio (mr=2 and 6.76) on the heat transfer rate, vortex shedding patterns, vibration trajectories, FFT, and cylinder displacements are studied in a wide range of Reynolds number (1156–9246) and reduced velocity (2–16). The results indicate that FIV can have either a positive or negative effect on heat transfer enhancement, depending on the splitter plate length and the type of cylinder vibration response. At high Reynolds numbers, the cylinder vibration response is observed in the galloping region, accompanied by a significant vibration amplitude. In this region, for L = 0.5D, vibration has a positive effect on heat transfer, while for other lengths, this effect is negative. For L = 1.5D and a reduced velocity of 5.5, contributions of vortex-induced vibration (VIV) and surface area in the heat transfer enhancement are 23.5 % and 44.3 %, respectively. By reducing the mass ratio from 6.76 to 2.6 for L = 0.5D, the heat transfer rate increases by approximately 40 % compared to a stationary cylinder.

Recent Advances in Computational Convective Heat Transfer Study in a Sub-Channel for Nuclear Power Reactor and Future Directions

A nuclear power reactor's primary use is to generate thermal energy, which in turn produces electricity. The primary heat source is a nuclear fission event occurring inside the fuel rod. The convection heat is transmitted through the coolant by the heat energy generated at the fuel rod wall boundary. Better heat transfer is produced in the flow area by turbulence and irregularity. As a result, turbulent flow heat transfer may present a significant challenge when predicting and assessing the thermal performance of nuclear power reactors. Computational techniques in convective heat transfer have become indispensable for solving challenging issues in the fields of science and engineering thanks to the development of current sophisticated numerical methods and high-performance computer hardware. The development of novel computational techniques and models for complicated transport and multi-physical phenomena is constantly in demand throughout applicable disciplines. This chapter's objective is to provide some recent developments in computational techniques for convective heat transfer, taking into account research interests in the community of mass and heat transfer, and to showcase relevant applications in nuclear power plant engineering domains including future directions. This study describes the most recent advancements in nuclear reactor convective heat transfer research utilizing the computational fluid dynamics (CFD) method, particularly at Ansys Fluent. This work examines the convective heat transfer and fluid dynamics fluid dynamics for turbulent flows across three rod bundle sub-channels that are typical of those employed in the PWR-based VVER type reactor. In this paper, CFD analysis is carried out using the software tool Ansys Fluent. Temperature distribution profile, velocity profile, pressure drop, and turbulence properties were investigated in this study. Boundary conditions i.e. temperature, velocity, pressure, heat flux, and heat generation rate were applied in the sub-channel domain. The main obstacles and bright spots for the CFD methods in nuclear reactor engineering are discussed, which helps to further its further uses. We intend to research a full-length fuel bundle model for VVER-1200 in the future to gather specific fluid characteristic data and use the findings to analyze safety and operate nuclear power facilities in Bangladesh. This paper presents a thorough analysis of the sub-channel thermal hydraulic codes used in nuclear reactor core analysis. This review discusses several facets of previous experimental, analytical, and computational work on rod bundles and identifies potential future directions based on those earlier studies.

Turbulent Flow Heat Transfer and Thermal Stress Improvement of Gas Turbine Blade Trailing Edge Cooling with Diamond-Type TPMS Structure

Additive manufacturing allows the fabrication of relatively complex cooling structures, such as triply periodic minimal surface (TPMS), which offers high heat transfer per unit volume. This study shows the turbulent flow heat transfer and thermal stress of the Diamond-TPMS topology in the gas turbine blade trailing edge channel. The thermal-fluid-solid analysis of the Diamond-TPMS structure, made of directionally solidified GTD111, at the nearly realistic gas turbine condition is executed, and the results are compared with the conventional pin fin array at the Reynolds number of 30,000. Compared to the baseline pin fin structure, the Diamond-TPMS model distributes flow characteristics more uniformly throughout the channel. The overall heat transfer enhancement, friction factor ratio, and thermal performance are increased by 145.3%, 200.9%, and 32.5%, respectively. The temperature, displacement, and thermal stress in the Diamond-TPMS model are also distributed more evenly. The average temperature on the external surface in the Diamond-TPMS model is lower than the baseline pin fin array by 19.9%. The Diamond-TPMS network in the wedge-shaped cooling channel helps reduce the volume displacement due to the material thermal expansion by 29.3%. Moreover, the volume-averaged von Mises stress in the Diamond-TPMS structure is decreased by 28.8%.

New insight into the heat transfer and pressure drop correlations for laminar and turbulent flow in a circular tube with a twisted-tape insert: A discriminated dimensional analysis

Discriminated dimensional analysis combined with the scale analysis has been implemented to obtain new correlations describing the dimensionless pressure drop as Hagen number and Nusselt number for laminar and turbulent swirl flow in a circular tube with a classical twisted-tape insert. The new insight into the phenomenon revealed that the swirl parameter Swh, as a criterion of similarity, governs the process in the entire region of swirl fluid flow in a circular tube with a classical twisted-tape insert. The validity of the analysis has been demonstrated by an excellent agreement between a large experimental data set for isothermal pressure drop and Nusselt number from many different sources with the predictions of the Hgh+−Swh and Nuh+−Swh correlations.

Heat Transfer and Entropy Generation of Turbulent Flow in Corrugated Channel using Nanofluid

This paper presents a numerical study of two-dimensional, turbulent flow with heat transfer in a corrugated channel using 3% of Al2O3 nanoparticles volume fraction dispersed in a basic fluid (water) with a particle diameter of 30nm. The equations governing the flow are: the energy equation to model heat transfer, the momentum equations and the continuity equation. To simulate different values ​​of the Reynolds number ranging from 1000 to 6000, the CFD code Ansys- Fluent based on the finite volume method is used. For turbulence modeling, the k-ε realizable model was used. The results are validated by other scientific work. Nu, skin friction coefficient, pressure drop and PEC factor are explored. We noted that the increase in the Reynolds number leads to an increase in the average Nusselt and a decrease in the coefficient of friction. Particular attention is paid to the generation of transfer entropy, friction and general entropy as well as the Bejan number. The transfer entropy generated in the fluid crossing the channel as well as the Bejan number decrease with the increase of the Reynolds while that of the friction increases. In addition, the use of nanofluid (Al2O3/water) gives better heat transfer than pure water.

Comparison study on thermo-hydraulic performance of circular and airfoil shaped wire-wrapped rod bundle

The turbulent flow behavior and heat transfer performance of wire-wrapped rod bundles have a significant impact on the thermal–hydraulic characteristics and safety of Generation IV nuclear reactors. In this study, the airfoil-shaped wire-wrapped bundle is proposed to improve the comprehensive performance of advanced reactor cores, and the thermal–hydraulic performance has been compared with the circular-shaped wire-wrapped bundle. Special attention has been paid to the temperature variation and friction pressure loss caused by the wire wraps. Specifically, the generation of the secondary flow is the main factor. The airfoil-shaped wire-wrapped bundle exhibits a half friction pressure loss, a more uniformed temperature distribution, and a reduction of the hot spot temperature compared to the circular-shaped wire-wrapped bundle. Moreover, the airfoil-shaped wire shows a 16.3% increase in Nusselt number and a 2.0% increase in friction factor compared to circular-shaped wire, resulting in an 8.7% increase in performance evaluation criteria (PEC). The airfoil-shaped wire-wrapped bundle demonstrates better comprehensive performance than the circular-shaped wire-wrapped bundle in terms of PEC.

Wall friction and heat transfer in turbulent pulsating flow in a square duct

For the first time, a DNS study of pulsating turbulent flows and heat transfer in a square duct is performed. Calculations were carried out at Re=2200 in the current-dominance mode for eight values of the oscillation period and three values of the Prandtl number. It was found that the time-mean friction coefficient in the studied pulsating flows has a lower value than in the stationary flow. The difference reaches −14.7%. The values of the Nusselt numbers are also decreasing, albeit by a smaller amount. The features of secondary flows of Prandtl’s 2nd kind at different phases of the flow oscillations are studied. The profiles of the wall friction stress and heat transfer coefficients and their evolution over time are determined. The time-oscillations of the wall friction are entirely determined by the oscillations of the main flow, being ahead of it in phase by π/4, which coincides with the behavior of laminar pulsating flows at high frequencies. The oscillations in heat transfer on the wall are determined by oscillations in the intensity of turbulent heat exchange, and the change in the latter occurs almost in phase with a change in the volume-average intensity of the transverse velocity fluctuations.

Comparative study of heat transfer and pressure drop in turbulent flow of a singular and hybrid nanofluids into a horizontal pipe

Comparative study of heat transfer and pressure drop in turbulent flow of a singular and hybrid nanofluids into a horizontal pipe

Modulation of Turbulence Flux Budgets by Varying Fluid Properties in Heated High Prandtl Number Flow

The accurate computational modelling of momentum and heat transfer in turbulent wall-bounded flow is still a challenging task, especially for fluids with high molecular Prandtl numbers. Due to the steep temperature gradients, the strong variation of fluid properties becomes an additional important issue. The present Direct Numerical Simulation study specially addresses this problem considering a representative working fluid in terms of a real heat transfer oil with temperature-dependent material properties. Near the heated wall, the increase in molecular viscosity significantly attenuates the turbulent convective fluxes of both momentum and heat, despite the markedly increased enthalpy fluctuations. As the thermal conductivity decreases markedly less with temperature than the viscosity, the dampened turbulent mixing significantly reduces the Nusselt number, whereas the skin friction coefficient only marginally differs from the constant fluid property case. The attenuation of the radial velocity fluctuation consistently reduces the production terms in the budgets of the turbulent momentum and heat fluxes, while the production of the enthalpy fluctuations is only shifted away from the wall without significant change in peak level. The also attenuated redistribution in the turbulent axial stress budget reduces the pressure-strain based steering of turbulence towards isotropy. Facing a consequently more anisotropic turbulent motion poses a further difficulty to standard turbulence modelling mostly developed for isotropic turbulence.

Data-driven estimation of entropy production by large scale motions in an intermediate turbine duct

In modern gas turbines, the physics of three-dimensional inflow for aggressive intermediate turbine ducts are strongly associated with large-scale coherent motions, which contribute to the production of irreversible aerodynamic and thermodynamic energy losses. To address this problem, an extended data-driven approach for estimating the irreversible losses through viscosity dissipation and heat transfer in turbulent flows is proposed. An aggressive intermediate turbine duct connected to a high-pressure turbine is numerically investigated using the detached-eddy simulation framework. The influence of tip leakage vortices and rotor wakes on the aerodynamics in the duct is assessed through quadruple proper orthogonal decomposition analysis. The entropy production rate corresponding to the coherent structures is then identified, and the irreversible losses in the duct subject to unsteady flow motions are quantitatively evaluated. The results indicate that the coherent structures in the intermediate turbine duct are strongly associated with the upstream conditions, including the tip leakage flow and rotor wakes. Direct viscous dissipation accounts for the majority of the irreversible losses, with coherent perturbed dissipation contributing to 7.25% and 5.54% of the total for tip gap sizes of 1.5% and 3.5% span, respectively. The streamwise development of coherent perturbations is restricted compared with that in the mean structure, and the stochastic part contributes little to the viscous dissipation. The proposed method provides a systematic procedure for estimating the irreversible energy losses in unsteady turbulent flow, and highlights the significance of unsteady aero-thermodynamics in the design of turbomachinery.

Analytical and numerical investigations of fluid flow and heat transfer in turbulent slip incompressible flow in a cylindrical channel

Based on two analytical approaches (Prandtl model and von Kármán model), turbulent flow in a cylindrical channel with flow slippage on the wall was considered. The results of the analytical solution are supplemented with numerical simulations based on the lattice Boltzmann model (LBM). Validations of the analytical model against numerical computations showed their good agreement. The analytical model enabled obtaining relations for the velocity profile, hydraulic resistance coefficient and Stanton number, which make it possible to evaluate the influence of the slip effect (Kn) and the flow rate (Re) on their variation.

The effect of the triangular rib usage in the plate fin heat sinks on the pressure drop, base plate temperature, and entropy generation

Improving the thermal efficiency in plate fin heat sinks (PFHS) results in a reduction in the weight, volume and cost of the heat sink. This ensures long life and reliable operation of the cooled equipment. In this study, the performances of triangular ribbed PFHSs, which are formed by placing a staggered array of triangular ribs between plate fins, were numerically investigated. Pressure drop, base plate temperature and entropy generation are used for performance comparisons. Numerical heat transfer and flow analysis were performed in three-dimensional models with the help of Ansys Fluent program, which uses the finite volume method with incompressible, turbulent flow and conjugate heat transfer assumptions. The design parameters were determined as inlet velocity between 3 m/s and 9 m/s, number of plate fins 9, 12 and 15, plate fin heights 10 mm and 30 mm, triangular rib heights 0.1 mm and 0.3 mm. As a result, the heat sink base temperature decreases with the increase of velocity, fin height, number of plate fins and triangular rib height, lower pressure drop values were obtained in the case of Hrib=0.1 mm compared to the unribbed case. If the design with the least entropy production is desired as the optimum design, there is a %10.64 increase in the base temperature and a significant decrease in the pressure drop (220.88 Pa to 7.512 Pa). In this case, since the fin length is H=30 mm, the volume and weight of the heat sink also increase.

A three-dimensional fictitious domain method for direct numerical simulations of particle-laden flows with heat transfer

In the present work, a three-dimensional fictitious domain method for particulate flows with heat transfer is proposed. For the case of fixed particle temperature, an iterative scheme for the temperature Lagrange multiplier is proposed, in order to determine its initial value and overcome the spurious oscillation of the explicit scheme at the initial time stage for different initial fluid and particle temperatures. Both explicit and implicit schemes are proposed for the solution of coupled fluid and solid temperature equations in the case of freely evolving particle temperature. The implicit scheme is suited to the case of large density ratios, specific heat ratios, or thermal conductivity ratios. Our method for the case of fixed particle temperature is verified via the test problems of a stationary hot sphere heating the surrounding quiescent fluid, a fixed sphere, and spheroid, respectively, in uniform flow, and sedimentation of a sphere and spheroid, respectively, in a vertical channel. We propose a new correlation of particle Nusselt number for an isolated sphere in a relatively small domain. Our code for the case of varying particle temperatures is verified via the effective thermal conductivity of a motionless sphere and the rising of a catalyst particle in an enclosure. Our method is applied to the sedimentation of a sphere at different Grashof numbers, specific heat ratios, and conductivity ratios. In addition, some preliminary results on heat transfer in turbulent channel flows laden with neutrally buoyant spherical and spheroidal particles, respectively, from fully resolved simulations with our method are reported.

Heat Transfer and Pressure Loss of Turbulent Flow in a Wedge-Shaped Cooling Channel With Different Types of Triply Periodic Minimal Surfaces

Abstract Additive manufacturing enables highly efficient cooling fabrications such as triply periodic minimal surface (TPMS), which provides excellent heat transfer per unit volume. In a wedge-shaped channel representing trailing edge turbine blade cooling, conventional pin fins are replaced with different TPMS structures due to their topological features to enhance the flow mixing and heat transfer, strengthen the structural integrity, and reduce the manufacturing material. The turbulent flow and heat transfer characteristics of solid- and sheet-based TPMS models, including gyroid, diamond, and Schoen-I-graph and wrapped package (IWP), are numerically investigated. The heat transfer, pressure loss, and thermal performance are compared at Reynolds numbers of 10,000–30,000. Notably, among the studied TPMS structures, the diamond-sheet structure is selected as the optimal model. Compared to the baseline pin fin structure at an equal Reynolds number, it remarkably increases the overall heat transfer by up to 163.2%, the pressure loss by 181.8%, and the thermal performance by up to 77.3%. The numerical results indicate that the gyroid- and diamond-sheet structures effectively organize and interact with the cooling fluid, reducing low-velocity recirculation flow in the tip region of the trailing edge. The flow in the diamond-sheet network is distributed more evenly from the root to the tip region, improving the temperature uniformity throughout the channel. Overall, the diamond-sheet TPMS structure could effectively improve the heat transfer performance, temperature uniformity, and structural integrity in the turbine blades' trailing edge, thereby potentially extending the durability of the turbine blades.