Forced Convection Regime Research Articles

Flow film boiling on a sphere in the mixed and forced convection regimes

Saturated flow film boiling on a sphere has been numerically studied in this work for both vertical and horizontal flow configurations. The simulations were performed using a numerical methodology developed by the authors for boiling flows on three-dimensional unstructured meshes. For interface capturing, the coupled level set and volume of fluid method is used. The interface evolution, vapour wake dynamics and heat transfer have been thoroughly investigated by varying the saturated liquid flow velocity, sphere diameter and wall superheat. The relative importance of both the buoyancy and the inertial forces is described in terms of the Froude number $(Fr)$ . The vapour bubble evolves periodically at low $Fr$ values, while a stable vapour column develops at high $Fr$ values. The interface evolution pattern obtained in the present work is in good agreement with the results of experimental studies available in the literature. For all the values of $Fr$ , a stable vapour column develops for a large-diameter sphere and releases vapour bubbles of varying sizes. Furthermore, for a large-diameter sphere, surface capillary waves are observed at the interface, similar to the observations of some of the experimental studies available in the literature. The flow in the liquid and vapour wakes appears to be strongly coupled. The heat transfer in the present work is estimated using the spatially and temporally averaged Nusselt numbers. Finally, an fast Fourier transform analysis of the space-averaged Nusselt number reveals a strong interaction among the different forces.

Investigation of Laminar Forced and Mixed Convection in Horizontal Mini Tubes with Uniform Wall Heat Flux Boundary Condition

In this study, a numerical model is established to study the forced and mixed convection heat transfer in the laminar region for horizontal mini tube with different diameters (1.2 mm to 3.0 mm) under the uniform wall heat flux boundary condition. The Reynolds number is set from 100 to 2,000 and the uniform wall heat flux boundary condition is set at 10 and 20 kW/m2. The model is verified with well-established correlation to guarantee good accuracy. The existence of mixed convection is examined by (1) the ratio between the heat transfer coefficients at the top and the bottom of the tube, and (2) the temperature contour plot. Based on the simulation results, the existence of buoyancy superimposed on the main flow is strongly related to the tube diameter, the dimensionless location, the Reynolds number, and the heat flux applied. Under the same heating condition, for small diameter tube with insufficient length, buoyancy effect can never establish. The existing flow regime maps for macro tube were used to predict the forced and mixed convection regimes from the simulated mini tube results and discrepancy was found. A new flow regime map is hence developed for mini tubes, and it classified the simulation results with good accuracy.

Numerical analysis of a parallel triple-jet of liquid-sodium in a turbulent forced convection regime

In the present study, we have applied a combined wall-resolving dynamic Large-Eddy Simulation (LES) (for the velocity field) and Direct Numerical Simulation (DNS) (for the temperature field) approach for mixing of parallel triple-jets with different temperatures of liquid sodium in a turbulent forced convection regime. Because of the high thermal conductivity of sodium (a low-Prandtl fluid), we adopted the dynamic Smagorinsky subgrid closure for the unresolved velocity scales, while the thermal scales are fully resolved. Furthermore, the Time-dependent Reynolds-Averaged Navier-Stokes (T-RANS) approach with the high-Reynolds number variant (i.e. with the wall functions as boundary conditions along solid boundaries) of the four-equation eddy viscosity model (k−ε−kθ−εθ) was applied. The fine-mesh LES/DNS provided a close agreement with the experimental data for both velocity and temperature fields (for both first- and second-moments). In contrast, the coarse-mesh LES/DNS overestimated the turbulent kinetic energy profiles at different distances from the inlet plane. The T-RANS results confirmed a good agreement with the mean streamwise velocity and turbulent kinetic energy, as well as the mean temperature profiles. Finally, the analysis of power spectral density distributions of the temperature signal revealed that all simulation techniques captured a dominant flow frequency originating from the induced Kelvin-Helmholtz instabilities between the side and central jets. The presented combined dynamic LES/DNS approach is recommended for future simulations of the turbulent forced convection flows of low Prandtl fluids, especially if thermal fatigue effects need to be predicted correctly.

Convective heat transfer efficacy of an experimentally observed non-Newtonian MWCNT-Fe[formula omitted]O[formula omitted]-EG hybrid nanofluid in a driven enclosure with a heated cylinder

This study provides an in-depth analysis of the heat transfer (HT) characteristics of a hybrid nanofluid (HNF) composed of Multi-walled carbon nanotubes (MWCNT), Iron oxide (Fe3O4), and Ethylene glycol (EG), which is loaded in an enclosure with a heated cylinder. As per the exploratory report, this HNF exhibits Newtonian and non-Newtonian behavior depending on the percentage of nano-constituents suspended in the base fluid EG. The governing equations describing the convective HNF flow associated with the boundary conditions are numerically solved using the finite volume method. The study explores the HT performance of a loaded HNF by varying the volume fractions (ϕ), the driving condition of the vertical sides, and the aspect ratio (W∗) of the enclosure. The study investigates the variation of the Richardson number (Ri=0.01∼100) concerning the interactions between shear and buoyancy forces executed on the HNF. The study finds that the combination of the lower aspect ratio W∗=2.1 filled with non-Newtonian HNF with ϕ=1.8% in the forced convection regime (Ri=0.01) and the condition of both aiding and opposing forces applied to the side walls shows superior HT performance. The non-Newtonian HNF with ϕ=1.8% shows HT enhancement of 9.27% for aiding forces and 39.63% for opposing forces when Ri=0.01, revealing ultimate enhancement in the average Nusselt number (Nuavg). The study also investigates entropy production (Savg) from a thermodynamics perspective to assess the system’s thermal performance. The value of Savg is about 11.54 for ϕ=1.8% and Ri=0.01, and that value is comparable for both aiding and opposing force conditions. Results conclude that HT consistently enhances with nanoparticle addition in the forced convection regime. However, HT enhancement is only observed for non-Newtonian cases of HNF in the natural convection regime, suggesting the limitation of the HNF usage in the HT study. Additionally, the study discusses and compares different simulation results to comprehensively evaluate the HT characteristics and the system’s thermal performance, guiding the best benefit of the studied hybrid nanofluids in a thermal engineering system.

Phase diagram and permeability evolution for dissolving vertical fractures in a gravity field

Rock fractures always provide the main flow pathways in geological systems. When exposed to reactive flow, mineral dissolution causes different dissolution regimes and significantly impacts permeability change. Previous studies have examined dissolution regimes and permeability evolution in horizontal geometry, but dissolution dynamics and permeability changes of vertical fractures in a gravity field remain unexplored. Here, we conduct flow-dissolution experiments combined with a pore-scale modeling approach to investigate regime transition and permeability evolution in vertical fractures. We validate the modeling approach using experimental results and illustrate the importance of gravitational effects in dissolving vertical fractures. Our 3D numerical simulations reveal the regime transition, represented by a critical Richardson number Ri, from the forced convection regime to the buoyancy-driven convection regime as the importance of gravity increases. In the buoyancy-driven regime, gravitational effects promote the development of localized dissolution/flow channels. However, gravitational effects become negligible in the forced convection regime where wormholes and uniform patterns can be separated by the critical Pe number. Using the critical Ri and Pe numbers, we establish a phase diagram of dissolution regimes for vertical fractures in a gravity field, exhibiting good agreement with experiments. By correlating this phase diagram with permeability-aperture curves, we directly estimate the power law exponent of permeability evolution for each dissolution regime. This approach extends the classic phase diagram of dissolution regimes to consider gravitational effects on vertical fractures and provides a guide to directly estimate the parameters for permeability evolution, which is essential in reactive-transport modeling at the field scale.

Study on heat transfer characteristics of pulsating heat pipe heat exchanger with asymmetric structure

The Pulsating Heat Pipe (PHP), lauded for its efficacy in heat transfer, is distinguished by its uncomplicated architecture, economical production, diminutive form, and robust adaptability to environmental conditions. This study delineates the design of an asymmetric PHP heat transfer apparatus, achieved through the alteration of select conduit lengths within the comprehensive circulation system. The thermal transfer efficacy of this apparatus was empirically scrutinized under dual dissipation modalities: natural and forced convection. It was observed that the asymmetric PHP, when devoid of oscillatory activity, maintained a heat source temperature of 25∘C, whereas the temperature escalated to 26∘C upon the initiation of pipe vibration. Under the regime of forced convection, the asymmetric PHP demonstrated expedited activation, reduced initiation temperature, and heightened oscillatory behavior compared to its natural convection counterpart, thereby facilitating the phase change condensation within the condensation segment and ensuring efficient, sustained heat dissipation. Consequently, this bolstered the PHP’s heat transfer capabilities. The thermal resistance exhibited a declining trajectory under both dissipation strategies, with forced convection consistently yielding lower thermal resistance than natural convection. Nonetheless, the decrement in thermal resistance was gradual near the critical startup juncture and throughout the initiation phase. The PHP’s equivalent thermal conductivity displayed an upward trend in tandem with the escalation of the heat source’s temperature under both dissipation methods. Despite the superior heat transfer performance at elevated heat source temperatures, the efficiency of natural convection dissipation remained suboptimal, necessitating the application of forced convection to the condensation segment to further enhance the PHP’s thermal transfer proficiency and the overall device performance.

The appearance of oscillatory flows within a horizontal packed bed partly heated from below

We report on a numerical investigation of the unsteady mixed convective flow in a channel through a horizontal porous layer under localised isothermal heating from below to probe the oscillatory flow and thermal characteristics. The study was carried out for water flow within a plane channel containing stainless steel spherical particles. For this study, the channel height/particle diameter ratio was H/d=100 and the porosity was ɛ=0.35. The effects of the imposed pressure-driven flow characterised by the Péclet number (0.1≤Pe≤1000), and heating represented by the Rayleigh number (105≤Ra≤2×107), are investigated. The results reveal that for low Péclet numbers, steady free convective flows are dominant, while for high Péclet numbers, steady forced convective flows dominate. Importantly, no oscillatory flows are found to develop in the free and forced convection regimes. However, for moderate Péclet numbers, oscillatory mixed convective flows occur with different periodic, quasi-periodic, and chaotic flow behaviours. In addition, for low Rayleigh numbers, Ra≤106, the Nusselt number is not influenced by the Péclet number for Pe≤1. Nevertheless, for high Rayleigh numbers, Ra≥2×106, it is observed that there always exists a critical Péclet number for which the Nusselt number is a minimum. The effects of the Péclet and Rayleigh numbers on the local and average temperature disparity between the fluid and solid phases are then investigated.

Impacts of Permeability Heterogeneity and Background Flow on Supercritical CO2 Dissolution in the Deep Subsurface

AbstractMotivated by CO2 capture and sequestration (CCS) design considerations, we consider the coupled effects of permeability heterogeneity and background flow on the dissolution of a supercritical CO2 lens into an underlying deep, confined aquifer. We present the results of a large‐scale Monte Carlo simulation study examining the interaction of background flow rate and three parameters describing multi‐Gaussian log‐permeability fields: mean, variance, and correlation length. Hundreds of high‐resolution simulations were performed using the PFLOTRAN finite volume software to model CO2 dissolution in a kilometer‐scale aquifer over 1,000 years. Predictive dimensionless scaling relationships relating CO2 dissolution rate to heterogeneity statistics, Rayleigh (Ra) and Péclet (Pe) numbers were developed for both gravitationally dominated free convection to background flow‐dominated forced convection regimes. An empirical criterion, Pe = Ra3/4, was discovered for regime transition. All simulations converged quickly to a quasi‐steady, approximately linear dissolution rate. However, this rate displayed profound variability between permeability field realizations sharing the same heterogeneity statistics, even under mild permeability heterogeneity. In general, increased heterogeneity was associated with a lower mean and higher variance of dissolution rate, undesirable from a CCS design perspective. The relationship between dissolution rate and background flow was found to be complex and nonlinear. Dimensionless scaling relationships were uncovered for a number of special cases. Results call into question the validity of the Boussinesq approximation in the context of modest‐to‐high background flow rates and the general applicability of numerical simulations without background flow.

Analysis of free and forced convections in the flow of radiative viscous fluid with oxytactic microorganisms

The prime intend behind the current effort is to explicate flow attributes of magnetically influenced Newtonian fluids toward a stretchable sheet under the novel physical impact of oxytactic microorganisms in a comparative manner for free and forced convections. In addition, modified Fourier and Fick’s laws are implemented to examine the change in temperature and concentration distributions in a more realistic manner by accounting thermal and mass relaxation parameters in the flow. The obtained PDEs are reduced into the non-linear ODEs by employing similarity variables. Due to the complexity of parametrically based differential equations, a numerical scheme based on a finite-difference approach is implemented via the MATLAB built-in routine known as BVP4C. Flow-controlling parameter effects on associated distributions are evaluated through graphs and tables. Subsequently, the influence of flow-controlling parameters on associated distributions is revealed through pictures in a comparative manner for different convection regimes. Additionally, the quantities such as heat and mass fluxes along with the density of motile microorganisms are also illustrated. From the thorough analysis of the current investigation, it is inferred that velocity distribution enhances for free and forced convections, whereas the temperature of the fluid diminishes against the mentioned convective regimes. It is manifested that the Nusselt number is more in the situation of free convection instead of the forced convection situation. The magnitude of the skin friction factor is more in case of free convection as compared to the forced convection regime. It is also reported that by uplifting the magnitudes of concentration and thermal relaxation parameters, depreciation in associated heat and mass transfer rate arises. In addition, it is also reported that with the increment in the magnetic field, buoyancy ratio, bioconvection parameters, and Rayleigh number skin friction accelerate, while the behavior is quite opposite in case of stretching the ratio parameter.

Multi-objective hydrothermal optimization of nanoparticles shape in nanofluid flow through an annular pipe having metal foam fins

The objective of this contribution is twofold. First, a multi-objective hydrothermal optimization is performed to determine the optimum nanoparticles (NPs) shape in nanofluid flow through an annular pipe having metal foam fins, in the forced convection regime. Second, the suitability of the performance evaluation criterion (PEC), as a common method to analyze the hydrothermal performance of thermal systems, is examined by comparing the outcomes of the multi-objective optimization method with those obtained by maximizing PEC. It is found that, depending on the value of the Reynolds number, the cylindrical-shaped NPs or the blade-shaped NPs exhibit the highest heat transfer performance. The lowest value of the required pumping power belongs to the spherical-shaped NPs. The results indicate that PEC attains its maximum value in the spherical-shaped NPs, the NPs fraction of 0.01, the nondimensional fin length of 0.4, the Darcy number of 0.01, and the medium porosity of 0.75. However, the multi-objective algorithm provides several optimum circumstances, each for a certain criterion indicating the suitability of this strategy for hydrothermal performance optimization of the system.

Analysis of the laminar flow and enhanced heat transfer rate through a triangular array of cylinders embedded in a fluid-saturated porous media with mixed convection

An investigation of the laminar flow and enhanced heat transfer rate through a triangular array of cylinders embedded in a fluid-saturated porous media is considered. Mixed convection covering forced convection cases has been studied using a finite-volume approach considering the local thermal equilibrium model. Darcy–Brinkman–Forchheimer momentum equations are used to characterize the porous media in question. The study is constrained to the low and intermediate Peclet number (Re = 5 to 40, Pr = 50) and high range of Darcy number (Da = 10−3 to 10−1) for different cylinder spacings (0.7 ≤ φ ≤ 0.99). It is anticipated that porous media would enhance heat transfer, but it derives a multiple order in pressure drop indubitably not desirable in many heat applications. Investigation reveals that both mean drag coefficient (CD ) and Nusselt number (Nu) are strong functions of Da and φ; however, the influence of the buoyancy parameter is mainly witnessed in higher permeability levels and high Peclet numbers. The study also details the effect of governing parameters on mean gap velocities and pressure coefficients. Surprisingly, recirculation wakes exist for the lowest cylinder spacing (φ = 0.7) in high fluid momentum. Low φ and high permeability are desirable in a forced convection regime, whereas both φ and Da should be high in the case of mixed convection. This article also quantifies the combined effect of the Darcy number and buoyancy parameter on the heat-transfer enhancement ratio. A maximum of 112% increase in Nu for φ = 0.7 and 452% for φ = 0.99 are reported at Da = 10−1, but at the cost of higher pressure drop in the latter case.

Experimental adjustment of the turbulent Schmidt number to model the evaporation rate of swimming pools in CFD programmes

Water evaporation rate is among the most significant parameters to design and select air conditioning systems in buildings with indoor swimming pools. Experimental correlations are today widely used to estimate water evaporation rate, although discrepancies of up to 80% among existing correlations have been shown. An alternative to these empirical methods is the calculation of evaporation rate through computer fluid dynamics techniques. One of the most significant parameters to solve the mass transfer at the air-water interface in these models is the value of the turbulent Schmidt number. Although this value depends on air and water conditions (i.e., temperatures, velocities, and vapour pressure, among others), commercial computer fluid dynamics programmes set a fixed value by default. This study presents a new value through an experimental adjustment. A total of 40 experimental tests have been performed in a wind tunnel under typical conditions in indoor swimming pools. Afterwards, the adjustment was validated with data from 145 experimental tests reported in the scientific literature. The mean relative error in the evaporation rate using the turbulent Schmidt number was 7%, as against 25% using the value by default. The maximum error was reduced from 35% to 15% in forced convection regime.

Effects of non-Darcy mixed convection over a horizontal cone with different convective boundary conditions incorporating gyrotactic microorganisms on dispersion

This paper investigates the influence of dispersion impact on mixed convection flow over a horizontal cone within a non-Darcy porous medium. Multiple convective boundary conditions are applied to address the heat, mass and motile microorganism transfer phenomena. This paper incorporates the dispersion effect for gyrotactic microorganisms due to biological and environmental applications. By imposing appropriate similarity transformations, the nonlinear partial differential equations governing flow, temperature, concentration, and microbe fields are reduced to a system of ordinary differential equations & then solved using the MATLAB BVP4C function. The computation of grid independence test is analyzed for different flow profiles to show the precision of the points. In a few instances, our present numerical data is compared with previously published works, leading to excellent agreement. The non-Darcy effect, as well as mixed convection values from 0.1 to 0.9 and buoyancy parameters from 0.2 to 0.8, all significantly affects the velocity profile. The reduction in the microorganism profile is brought on by the increase in the bioconvection Lewis parameter and bio convection peclet number between 0.3 and 1. In the absence of dispersion, the variation of Biot numbers between 0.5 and 2, favor heat, mass, and motile microorganism transfer the most in the range of mixed convection parameter 0.5 to pure forced convection 1. Thermal, solutal and microorganism dispersion coefficients a, b, c that lie between frac{1}{7} and frac{1}{3} and higher values of modified peclet number ranges from 2 to 10 cause increased dispersion effects which lower flow transfer rates mostly in forced convection regime.

Effect of pulsation on flow and thermal characteristics of a wall jet

In the current investigation, a computational analysis of the thermal and dynamic characteristics of a pulsed incompressible laminar wall jet is performed under a forced convection regime. The considered wall is isothermal with a temperature dissimilar from that of the surrounding fluid. The nozzle outlet is marked by a periodic and parabolic profile of the longitudinal velocity as well as a uniform temperature. The dimensionless transport equations governing the heat and momentum transfer phenomenon are solved using the finite difference method with irregular mesh. Our results are validated against those which emanate from the non-pulsed jet in its different regions. The spatial and temporal evolution of the wall jet characteristics such as the velocities, the friction coefficient, the jet spreading, and the Nusselt number are presented and analyzed. It was revealed that the inclusion of pulsation leads to the spreading of the jet and the curtailment of the potential core length relative to a steady jet. Moreover, the pulsation strengthens the entrainment and mixing of the ambient fluid and therefore improves the heat transfer in the potential cone area.

Effects of flow fluctuations and buoyancy on heat transfer from a sphere in non-Newtonian nano fluids: Potential for intensification

This work numerically explores the combined effects of buoyancy-induced flow and pulsating velocity for nanofluids (power-law type fluids) over a sphere to ascertain the potential of this approach for process intensification. The pertinent field equations are solved numerically for a range of dimensionless variables as, Richardson number (0 ≤ Ri ≤ 5), amplitude of fluctuations (0 ≤ A ≤ 0.8), frequency of pulsation (π/4 ≤ ω* ≤ π), Reynolds number (5 ≤ Re ≤ 100), power-law index (0.3 ≤ n ≤ 1.5) and Prandtl number (1 ≤ Pr ≤ 100). The aiding-buoyancy flow suppresses the transient effects of the flow pulsations. The recirculation length (wake) is considerably shortened as the buoyancy-driven flow strengthens. Even a weak buoyancy-induced flow (for instance, Ri = 1) is sufficient to mar the influence of pulsatile velocity. The Richardson and Prandtl numbers do not influence the phase difference in the growth of the momentum boundary layer. Overall, the improvement in the Nusselt number is significantly lowered by the aiding-buoyancy mixed convection than that in the forced convection regime. While this may seem like a detrimental effect on the overall heat transfer, this offers a potential tool for modulating the rate of heat transfer for the processing of temperature-sensitive materials wherein the main consideration is to limit the maximum temperature in the system. The present results can be used to devise an operating envelope of conditions depending upon the envisaged application. Finally, the mean Nusselt number is correlated by using the modified velocity and viscosity scales accounting for the contribution of the mean flow and the buoyancy-induced flow.

Direct numerical simulation of turbulent heat transfer in liquid metals in buoyancy-affected vertical channel

This article investigates the impact of buoyancy force in a vertical channel with combined natural and forced convection liquid metal flow using direct numerical simulation (DNS). In particular, a liquid metal with Pr = 0.025 and Re = 4667 is considered in the combined natural and forced convection regime (Ri = 0, 0.25, 0.50 and 1.00). The liquid metal is driven upward while buoyancy aids the flow close to the hot wall and opposes the flow close to the cold wall. The influence of buoyancy on turbulent heat transfer and mean flow are analyzed. Turbulent transport is enhanced in aiding flow and weakened in opposing flow with increasing Richardson number for low-Prandtl-number fluids. The enhancement of normalized Reynolds stresses is more significant in the opposing flow compared to the aiding flow. Turbulent heat flux also increases and, consequently, enhances heat transfer. Besides, the budgets of streamwise and wall-normal turbulent heat flux, temperature variance, Reynolds shear stress and turbulent kinetic energy are investigated and related to the variations of the quantities in mean fields. The budgets of temperature variance and turbulent kinetic energy are nearly balanced by shear production and dissipation. As for the budget of Reynolds shear stress, streamwise and wall-normal turbulent heat flux, temperature pressure-gradient correlation contributes to the budget significantly in addition to production and dissipation. The DNS results can not only shed light on the heat transfer mechanism of turbulence in liquid metal flows, but also be used for validating and improving turbulent models for low-Prandtl-number fluids, which could be useful for developing Gen IV liquid-metal-cooled fast reactors.

Numerical simulation of a low Prandtl number flow over a backward facing step with an anisotropic four-equation turbulence model

In recent years the use of liquid metals has become more and more popular for heat transfer applications in many fields ranging from IV generation fast nuclear reactors to solar power plants. Due to their low Prandtl number values, the similarity between dynamical and thermal fields cannot be assumed and sophisticated heat turbulence models are required to take into account the anisotropy of the turbulent heat transfer involving liquid metals. In the present work, we solve an anisotropic four-equation turbulence model coupled with the Reynolds Averaged Navier Stokes system of equations to simulate a turbulent flow of liquid sodium over a vertical backward-facing step. We implement an explicit algebraic model for Reynolds stress tensor and turbulent heat flux that takes into account flow anisotropic behavior. We study forced and mixed convection regimes when a uniform heat flux is applied on the wall behind the step. Linear isotropic approximations for eddy viscosity and eddy thermal diffusivity underestimate the turbulent heat flux components while this anisotropic model shows a better agreement with DNS results.

Periodic Flow of Non-Newtonian Fluid Over a Uniformly Heated Block With Thermal Plates: A Hybrid Mesh-Based Study

In this work, we analyze the characteristics of periodic flows in non-isothermal viscous fluid over a heated block in the presence of thermal plates at Reynolds number (Re=100). The unsteady, incompressible Navier–Stokes (NS) equations with suitable initial and boundary data in 2D are executed by the finite element technique using a highly refined hybrid mesh. The temporal discretization is performed by an implicit stable backward differencing in time and a stable choice of finite elements from the finite element library for spatial discretization. The discrete nonlinear system arising from this discretization is linearized by Newton’s method and then solved through a direct linear solver PARDISO. For this forced convective study, the range of dimensionless parameters, namely, the Prandtl number (Pr) and power law index (n), are varied from 1 to 10 and 0.6 to 1.4 with a low Grashof number varying as (1≤Gr≤10) to produce a forced convection regime, respectively. For the authentication, we have compared our results with the literature at a similar configuration. After simulation, the results accomplished in the velocity profile, pressure, isotherm contours, drag and lift coefficients (trajectory motion), average Nusselt number (Nuavg), etc. are considered. For convergence of solution at low shear rate (n<1), crosswind stabilization (CWS) function has been incorporated. It is observed that Nuavg becomes oscillatory for the shear-thinning case (n<1), while for the shear-thickening cases (n>1), it converges to a single value. Furthermore, the drag (CD) and lift (CL) coefficients are more pronounced for shear-thinning cases (n<1) as compared with shear-thickening cases (n>1).

Combined Large-Eddy and Direct Numerical Simulations of a Planar Jet with Heated Co-Flow with Medium and Low Prandtl Fluids

In the present work, we have applied a combined dynamic large-eddy simulation (LES) and direct numerical simulation (DNS) approach for a three-dimensional planar jet in a turbulent forced convection regime (Re = 18000) with a heated co-flow. Results from LES are compared with Reynolds Averaged Navier-Stokes (RANS) simulations and experimental data. We have analyzed flow and heat transfer features for four values of the characteristic Prandtl numbers (Pr = 0.71, 0.2, 0.025, and 0.006), which are representatives of air, He-Xe gas mixture, Lead-Bismuth Eutectic (LBE), and sodium, respectively. The latter two low-Prandtl fluids have been considered because of their role as primary coolants in advanced fast pool-type reactor prototypes (such as the Multi-purpose Hybrid Research Reactor for High-tech Applications (MYRRHA) at SCK•CEN, Belgium). We have provided detailed insights into instantaneous and long-term time-averaged behavior of the velocity and temperature fields (the first- and second-order moments). Furthermore, we have analyzed profiles of characteristic velocity and temperature time scales and dissipation rates, as well as the power spectra of the streamwise velocity component and temperature at several characteristic locations. The mean temperature profiles demonstrated rather low sensitivity for various values of the Prandtl number. In contrast, profiles of the temperature standard deviation exhibited larger variations, decreasing in magnitude with lower Prandtl values. Here presented results of the high fidelity numerical simulations (dynamic LES/DNS) for the low-Prandtl working fluids can be used for further development, testing, and validation of the advanced RANS-type turbulence models.

Turbulent heat transfer in a liquid metal tube flow with azimuthally inhomogeneous heat flux

An experimental study of the convective heat transfer in a turbulent liquid metal tube flow with azimuthally inhomogeneous heat flux is presented. Prior to the liquid metal experiments, the validation of the test section was realized using water. These results showed a very good agreement with literature data. For the liquid metal experiments, an eutectic alloy of gallium, indium and tin (GaInSn) was used. The Péclet number was varied between 1400 and 3600, thus in a regime of forced convection. Experiments with homogeneous heating over the full circumference of the tube and inhomogeneous heating over half of the circumference with the other half being insulated, were performed. The azimuthally averaged Nusselt number and the temperature distribution in the tube wall were investigated. The results suggest that the azimuthally averaged Nusselt number for water and liquid metal tube flows with inhomogeneous heating over the circumference can be calculated sufficiently well with literature correlations for uniform heat flux. For an inhomogeneous heat flux the azimuthal temperature gradient in the tube wall increases for higher Reynolds number and is more pronounced for GaInSn than for water. Furthermore, impurities like oxide particles significantly decrease the liquid metal convective heat transfer coefficient.