Turbulent Mixed Convection Heat Transfer Research Articles

Numerical analysis of turbulent mixed convection heat transfer of molten salt in horizontal tubes with uniformly cooled heat flux

Molten salt is a widely used heat transfer medium in the field of energy systems. In the engineering design, mixed convection heat transfer often occurs in the high heat flux tube convection heat transfer process, which can significantly change the convective heat transfer capability. However, optimizing the heat exchange conditions can improve the heat transfer capacity of molten salt. In this study, turbulent mixed convection heat transfer characteristics of Hitec molten salt inside horizontal cooling tubes are investigated numerically using the Reynolds stress turbulence model. The results show that the secondary flow caused by buoyancy can lead to significant nonuniform in heat transfer and wall temperature on the wall surface of the horizontal tube cross section. In addition, the buoyancy parameter Bo is selected to characterize the buoyancy effect on Hitec mixed convection heat transfer inside a horizontally cooled circular tube, and within the Bo range from 2.82 × 10−8 to 1.61 × 10−5, as the Bo increases, the relative average Nusselt number for the upper half tube wall gradually increases, the relative average Nusselt number for the lower half tube wall decreases, and the relative average Nusselt number for the entire tube wall shows the combined results of the upper half wall and the lower half wall relative average Nusselt numbers, which slightly increases and then decreases. Furthermore, based on the numerical data, the critical buoyancy parameters Bocr,1 = 10−7 and Bocr,2 = 6.96 × 10−6 at which the buoyancy force starts to affect heat transfer and the average Nusselt number for the entire tube wall starts to decrease are proposed, and variations of the mixed convection heat transfer against the buoyancy parameter Bo are presented respectively for different regions of the horizontal cooling tubes. Based on this research, a reasonable and widely applicable buoyancy parameter selection criterion can be constructed to guide the future engineering designs.

Opposing Mixed Convection Heat Transfer for Turbulent Single-Phase Flows

Convection, wherein forced and natural convections are prominent, is known as mixed convection. Specifically, when a forced convection flow is downward, this flow is called opposing flow. The objectives of this study are to gain a comprehensive understanding of opposing flow mixed convection heat transfer and to establish the prediction methodology by evaluating existing correlations and models. Several heat transfer correlations have been reported related to single-phase opposing flow; however, these correlations are based on experiments conducted in various channel geometries, working fluids, and thermal flow parameter ranges. Because the definition of nondimensional parameters and their validated range confirmed by experiments differ for each correlation reported in previous studies, establishing a guideline for deciding which correlation should be selected based on its range of applicability and extrapolation performance is important. This study reviewed the existing heat transfer correlations for turbulent opposing flow mixed convection and the single-phase heat transfer correlations implemented in the thermal–hydraulic system codes. Furthermore, the authors evaluated the predictive performance of each correlation by comparing them with the experimental data obtained under various experimental conditions. The Jackson and Fewster, Churchill, and Swanson and Catton correlations can accurately predict all the experimental data. The effect of the difference in the thermal boundary conditions, i.e., uniform heat flux and uniform wall temperature, on the turbulent mixed convection heat transfer coefficient is not substantial. The authors confirmed that heat transfer correlations using the hydraulic-equivalent diameter as a characteristic length can be used for predictions regardless of channel-geometry differences. Furthermore, correlations described based on nondimensional dominant parameters can be used for predictions regardless of the differences in working fluids. The authors investigated the extrapolation performance of the mixed convection heat transfer correlations for a wide range of nondimensional parameters and observed that the Jackson and Fewster, Churchill, and Aicher and Martin correlations exhibit excellent extrapolation performance with respect to natural and forced convection flows, indicating that they can be applied beyond the parameter range validated experimentally.

Mixed convection heat transfer and transient flow of high-pressure gas in a large-scale cavity with rotating blade

Mixed convection heat transfer phenomenon, incorporating natural and forced convection process, occurs widely in natural and industrial fields. Nevertheless, few works have investigated numerically the turbulent mixed convection heat transfer within the large-scale structure under high-pressure condition. In this study, a three-dimensional numerical model of a large-scale closed cavity with two rotating blades and reactor has been built on account of the finite volume method, aiming to explore the mixed convection heat transfer and turbulent flow of internal high-pressure gas. Compressible gas with varying thermophysical properties was chosen as the heat transfer medium and the numerical model was solved by Unsteady Reynolds-Averaged Navier-Stokes (URANS). Turbulent flow and heat transfer, inside the enclosure and reactor, were compared through the variation of the Reynolds number (Re), gas pressure (P) and heating surface temperature (Th). The results signify that for given P and Th, varying Re has a consistent impact on the transient flow and heat transfer process inside the enclosure and reactor. The turbulent flow and thermal transport are enhanced as increasing the Re. When the Re = 900,000, an enhanced forced convection effect is found inside the reactor. Dissimilar impacts on the flow field and thermal transport inside the enclosure and reactor are found with changing gas pressure owing to the interaction of natural and forced convection. The flow field attenuates gradually as the increment in P, and thereby larger high-temperature zone is achieved at a smaller pressure inside the enclosure, whilst inside the reactor, faster heat conduction is attained at a bigger pressure. Changing heating surface temperature has a more significant influence on the flow field and heat transfer inside the reactor compared to the enclosure. Additionally, increasing the Re and Th has a smaller effect on the average Nusselt number (Nuavg) while a positive correlation between the Nuavg and P is obtained.

Numerical investigation of turbulent mixed convection flow and heat transfer characteristics of 2 × 2 subchannels with spacer grids

AbstractIn turbulent mixed convection conditions, flow and heat transfer can deteriorate under specific conditions, thus affecting the performance of the safety of the system, which could imperil the safety of the nuclear reactor. In this paper, by analyzing the velocity, temperature distributions, and other thermal‐hydraulic parameters, the steady‐state computational fluid dynamics approach with the SST k–ω turbulence model used in this study was verified. Then, flow and heat transfer characteristics in turbulent mixed convection inside 2 × 2 subchannels with and without spacer grids were investigated. The results showed that the velocity profiles were distorted and the heat transfer capacity was impaired due to the influence of buoyancy. The Nusselt number ratio (Nu/Nuf) between turbulent mixed convection and forced convection increased with the increase of Reynolds number (Re) until unity but decreased with the increases of Grashof number (Gr) and Buoyancy parameter (Bo*).

Computational model for turbulent heat transfer in buoyancy-influenced flows at supercritical pressures in circular tubes

Turbulent heat transfer at supercritical pressures in fluids flowing upward in heated circular tubes is very likely to be affected by buoyancy and, when this buoyancy is stronger than a certain value, mixed convection occurs. The turbulent mixed convection heat transfer is very complex, and none of the existing turbulence models can adequately simulates the experimental or DNS data including mixed convection. A hybrid turbulence model that blended two turbulence models, which used the Kolmogorov velocity scale and wall shear stress, respectively, has shown a good performance in predicting experiments or DNS. However, even the blended model has not been satisfactory for large buoyancy parameter (Bu). It is judged that the inadequacy is due to the treatment of the coefficient of the production term in the dissipation rate equation as a constant. The semi-local scaling concept has been found to be inadequate. As a remedy, the coefficient associated with the production term in the dissipation rate equation is treated as a function of turbulent boundary layer deformation and Bu, and an integrated density concept instead of local density is used in calculating friction velocity. The computational model improved from the previous blended turbulence model by introducing the two concepts satisfactorily predicts the DNS data.

Influence of system buoyancy and hydraulic diameter on turbulent mixed convection in a rectangular duct

We investigated buoyancy-aided, turbulent mixed-convection heat transfer in a vertical rectangular duct. In order to achieve highly buoyant conditions, mass transfer experiments instead of heat transfer ones were performed based on the analogy between heat and mass transfer. Reynolds numbers ranged from 2,700 to 25,450 and Grashof numbers were 3.30 × 105 and 5.15 × 106. The results of turbulent forced-convection experiments were consistent with previous studies. Notably, at the same axial position, local heat transfer was affected by downstream flow for different heated lengths, and heat transfer was affected by the hydraulic diameter. Five distinct flow regimes were observed based on buoyancy coefficients. Particle image velocimetry measurements revealed that variations in local heat transfer were closely related to the amount of turbulence produced, which was affected by differences in velocity between buoyant and forced flows. Turbulent mixed-convection heat-transfer correlations are proposed based on analytical models and parametric studies of the influence of system buoyancy and hydraulic diameter.

Numerical Study of Turbulent Mixed Convection Heat Transfer in a Semi-Circle Cavity Contains a Rotating Cylinder

The investigation of mixed convection inside the semi- circle cavity with a fixed cylinder and rotating has been performed numerically. The bottom wall was maintained to constant heat flux while the curved wall was subjected to ambient and an adiabatic rotating cylinder presence in center cavity . The computation fluid dynamics (CFD)modeling includes numerical solutions of the conservation equation for mass ,momentum and energy, numerical prediction by using ansys FLUENT version 15. The finite volume method were used for solving governing differential equations and the turbulence model was applied SST(K-ω) . The results were represented of streamlines, isotherm counters and distribution local Nusselt number at Rayleigh number 2.9×10 8 identical to heat flux q= 290w/m 2 and angular velocity of cylinder ω=0 ,62.8,83.73 and 89.5rad/s clockwise and inclination angle of cavity 0°,30°and 90°.The result shows that the effect of both rotating cylinder ,inclination angle led to enhancement heat transfer. Keywords: Mixed convection, turbulence model , inclination angle , semicircle cavity. DOI : 10.7176/JNSR/9-18-02 Publication date :September 30 th 2019

Hybrid LES/DNS of turbulent forced and aided mixed convection to a liquid metal flowing in a vertical concentric annulus

In the present study fully-developed turbulent forced and mixed convection heat transfer to a liquid metal flowing upwards in a concentric annulus is numerically investigated by means of Large Eddy Simulation (LES). The inner-to-outer radius ratio is 0.5. The Reynolds number based on bulk velocity and hydraulic diameter is 8900, while the Prandtl number is set to a value of 0.026. A uniform and equal heat flux is applied on both walls. Three different buoyancy strengths are considered, corresponding to onset of turbulence reduction, maximum impairment and recovery condition on the inner wall while recovery and enhancement develop on the outer wall. Due to the difference between thermal and hydrodynamic turbulent scales in liquid metals it is shown that with the same grid resolution a LES is performed for the flow field and at the same time a “thermal“ Direct Numerical Simulation (DNS) for the temperature field. From a detailed analysis of the two-point correlation functions of velocity and temperature fluctuations it emerges that a streamwise extent of 25δ and 40δ (being δ the half gap width) is necessary for forced and mixed convection, respectively, while a quarter of circumference is enough in azimuthal direction for this radius ratio, Reynolds and Prandtl number. Comparison of the forced convection flow field with available DNS simulations shows very good agreement. Nusselt numbers evaluated from the few available literature correlations for liquid metals flowing in an annulus give unsatisfactory results, mainly on the inner wall. The mixed convection results are thoroughly analyzed and discussed in terms of friction factor, Nusselt number, first and second order statistics, budgets of turbulent kinetic energy and budgets of temperature variance. The obtained data are also useful for validating Reynolds-Averaged turbulence models. Moreover, simulations with two coarser grids at the condition of maximum turbulence reduction are also compared with the reference results of the fine LES. It results that when turbulence is impaired the grid resolution in circumferential direction can be strongly coarsened.

Analysis of entropy generation behavior of supercritical water flow in a hexagon rod bundle

The numerical simulation was carried out to investigate the local entropy generation in turbulent mixed convection heat transfer of supercritical water within the inner sub-channel of SCWR. The SSG Reynolds stress model with enhanced wall treatment was used as a turbulence model. The circumferential heterogeneity of heat transfer was introduced into the entropy generation analysis. The mechanisms of entropy generation in the near wall region within the boundary layer and the region away from the wall were discussed in detail. The results show that the buffer layer has great influence on heat transfer enhancement, and both the viscous sub-layer and buffer layer act significantly in the entropy generation with respect to circumferential heterogeneity. Usually, the entropy generation in the narrow gap is relatively larger than that in the sub-channel center when comparing the contribution of entropy production caused by heat transfer to that caused by energy dissipation. The simulation findings may provide a theoretical direction for the conceptual design of SCWR.

Mixed convection heat transfer in a 5 × 5 rod bundles

In order to investigate the buoyancy effect on heat transfer in rod bundles, experiments were performed at low flow rate with Reynolds number ranging from 1×103 to 3×104 in a 5×5 squarely arrayed rod bundles. The diameter of the rods are 10mm with a pitch of 13.3mm, and the length of the test section is about 3m. The rods are heated uniformly by a DC power, and cooled by pressurized water. The experimental data indicates that Weisman’s correlation predicts well within a deviation less than 10% in forced turbulent convection, but overestimates the Nusselt number in mixed convection conditions. It’s been found that mixed convection heat transfer in rod bundles is similar to that in simple channels, with increasing of free convection component, the heat transfer tends to be impairs firstly, and then followed by enhancement. The experimental data also indicates that buoyancy effect on heat transfer should not be neglected even when Reynolds number is about 15,000 under some certain heating conditions. New empirical correlations have been proposed for the prediction of forced turbulent convection heat transfer and mixed convection heat transfer in rod bundles respectively.

Experimental study and numerical models assessment of turbulent mixed convection heat transfer in a vertical open cavity

Flow and heat transfer in a vertical open cavity with turbulent mixed convection is investigated experimentally and numerically. The internal size of the cavity is 1 m × 1 m × 1.2 m, with liquid nitrogen as the cold source and mica electronic heating plates as the heat source of the experiments. The velocity and temperature fields are obtained by hot-wire anemometry, particle image velocimetry and local temperature sensors for 4.6 × 104 ≤ Re ≤ 5.9 × 104 and Gr ≤ 1.8 × 1013. The secondary upward flow was found to occur near the heated walls. Three two-equation RANS turbulence models, RNG k-ε model, L-B Low Re k-ε model and SST k-ω model, are used in the numerical simulation with the same boundary conditions as experiments. Through comparison with experimental data, Low Re k-ε model performs overall the best in the accuracy in solving mixed convection problems in large-scale vertical cavities with strong buoyancy force.

Two phase simulation and sensitivity analysis of effective parameters on turbulent combined heat transfer and pressure drop in a solar heat exchanger filled with nanofluid by Response Surface Methodology

In this paper, a 2-D numerical investigation and a sensitivity analysis of combined turbulent mixed convection and radiation heat transfer have been done using two phase mixture model in a solar heat exchanger. Numerical simulations are carried out to study the effects of the four parameters, the Richardson number (10−2 ≤ Ri ≤ 102), volume fraction of nanoparticles (0.00 ≤ ϕ ≤ 0.04), nanoparticles diameter (40 nm ≤ dp ≤ 60 nm) and the wall surface emissivity (0 ≤ ε ≤ 1). The effective parameters analysis is processed utilizing the RSM (Response Surface Methodology). It is found that decreasing the Ri number and ϕ and also increasing in ε and dp increases the mean total Nusselt number. Due to a decrease in Ri number and increase in ϕ, ε and dp the pressure drop ratio enhances. The sensitivity of the mean total Nusselt number and pressure drop ratio to Ri number is negative. The sensitivity of the mean total Nusselt number to dp and ε is positive but to ϕ is negative. Moreover, the sensitivity of the mean Nusselt number to Ri number, ϕ, dp and ε is more than the sensitivity of the pressure drop ratio to these parameters.

Numerical investigation on heat transfer of supercritical water in a roughened tube

ABSTRACTThe turbulent mixed convection heat transfer of supercritical water flowing in a vertical tube roughened by V-shaped grooves has been numerically investigated in this paper. The turbulent supercritical water flow characteristics within different grooves are obtained using a validated low-Reynolds number κ-ε turbulence model. The effects of groove angle, groove depth, groove pitch-to-depth ratio, and thermophysical properties on turbulent flow and heat transfer of supercritical water are discussed. The results show that a groove angle γ = 120° presents the best heat transfer performance among the three groove angles. The lower groove depth and higher groove pitch-to-depth ratio suppress the enhancement of heat transfer. Heat transfer performance is significantly decreased due to the strong buoyancy force at Tb = 650.6 K, and heat transfer deterioration occurs in the roughened tube with γ = 120°, e = 0.5 mm, and p/e = 8 in the present simulation. The results also show that the rapid variation in the supercritical water property in the region near the pseudo-critical temperature results in a significant enhancement of heat transfer performance.

CFD Investigation of Turbulent Mixed Convection Heat Transfer in a Closed Lid-Driven Cavity

CFD Investigation of Turbulent Mixed Convection Heat Transfer in a Closed Lid-Driven Cavity

Entropy generation in turbulent mixed convection heat transfer to highly variable property pipe flow of supercritical fluids

In this study, a two dimensional CFD code has been developed to investigate entropy generation in turbulent mixed convection heat transfer flow of supercritical fluids. Since the fluid properties vary significantly under supercritical conditions, the changes of entropy generation are large. The contribution of each of the mechanisms of entropy production (heat transfer and energy dissipation) is compared in different regions of the flow. The results show that the mechanisms of entropy generation act differently in the near wall region within the viscous sub-layer and in the region away from the wall. The effects of the wall heat flux on the entropy generation are also investigated.

Heat transfer enhancement in parabolic trough collector tube using Al2O3/synthetic oil nanofluid

Abstract Three-dimensional fully developed turbulent mixed convection heat transfer of Al2O3/synthetic oil nanofluid in a trough collector tube with a non-uniform heat flux was numerically studied. The effect of AL2O3 particle concentration in the synthetic oil on the rate of heat transfer from the absorber tube was also investigated. Various nanoparticle concentrations (

Numerical study of entropy generation due to coupled laminar and turbulent mixed convection and thermal radiation in an enclosure filled with a semitransparent medium.

The effect of radiation on laminar and turbulent mixed convection heat transfer of a semitransparent medium in a square enclosure was studied numerically using the Finite Volume Method. A structured mesh and the SIMPLE algorithm were utilized to model the governing equations. Turbulence and radiation were modeled with the RNG k-ε model and Discrete Ordinates (DO) model, respectively. For Richardson numbers ranging from 0.1 to 10, simulations were performed for Rayleigh numbers in laminar flow (104) and turbulent flow (108). The model predictions were validated against previous numerical studies and good agreement was observed. The simulated results indicate that for laminar and turbulent motion states, computing the radiation heat transfer significantly enhanced the Nusselt number (Nu) as well as the heat transfer coefficient. Higher Richardson numbers did not noticeably affect the average Nusselt number and corresponding heat transfer rate. Besides, as expected, the heat transfer rate for the turbulent flow regime surpassed that in the laminar regime. The simulations additionally demonstrated that for a constant Richardson number, computing the radiation heat transfer majorly affected the heat transfer structure in the enclosure; however, its impact on the fluid flow structure was negligible.

Investigation of nanofluid mixed convection in a shallow cavity using a two-phase mixture model

Laminar and turbulent mixed convection heat transfer of water/Cu nanofluids in a rectangular shallow cavity was studied utilizing a two-phase mixture model. The upper movable lid of the cavity was at a lower temperature compared to the bottom wall. Simulations were performed for Grashof numbers of 105 (laminar flow) and 1010 (turbulent flow) for Richardson numbers from 0.03 to 30, and nanoparticle volume fractions of 0.00–0.04. The two-dimensional governing equations were discretized using a finite volume method. The effects of nanoparticle concentration, shear and buoyancy forces, and turbulence on flow and thermal behavior of nanofluid flow were studied. The model predictions for very low solid volume fraction (φ ≈ 0) were found to be in good agreement with earlier numerical studies for a base fluid. It is shown that for specific Grashof (Gr) and Richardson (Ri) numbers, increasing the volume fraction of nanoparticles enhances the convective heat transfer coefficient and consequently the Nusselt number (Nu) while having a negligible effect on the wall shear stress and the corresponding skin friction factor.

Effect of mass transfer on the free convective flow through a porous medium with variable permeability in slip flow regime with couple stresses

This paper examines the problem of mass transfer on MHD unsteady free convective flow of a polar fluid through a porous medium of variable permeability bounded by an infinite horizontal porous plate in slip flow regime. The permeability of the porous medium decreases exponentially with time about a constant mean. Using perturbation technique the expressions for the velocity distribution, mean angular velocity of rotation of particles, concentration distribution and skin friction are obtained. The effects of permeability parameter K0, Magnetic parameter M, Prandtl number Pr , Schimdt number Sc ,and Grashof number Gr ,Modified Grashof number Gm entering into the problem on velocity, temperature distribution and concentration distribution are shown graphically and discussed numerically. It can be observed that this velocity decreases with the increase in M, Pr, Sc and increases with the increase in K0, Gr, Gm. Temperature and Concentration decreases with the increase in the value of Pr and Sc respectively. INTRODUCTION The convection problem in a porous medium has important application in geothermal reservoirs and geothermal energy extractions. It is obvious that in order to utilize the geothermal energy to maximum, one should have a complete and precise knowledge about the amount of perturbations needed to generate convection current in geothermal fluids. Also the knowledge of the quantity of perturbation is essential to entreat convection current in mineral fluids that is found in the earth’s crust helps one to utilize minimal energy to extract the minerals. Cheng and Lau[1] and Cheng and Teckchandani [2] obtained numerical solutions for the convective flow in a porous medium bounded by two isothermal parallel plates in the presence of withdrawl of the fluid. All the above mentioned studies treat the permeability and the conductivity or thermal resistance of the medium as constant or neglect the effect of porosity. Lai and Kulacki [6] studied Coupled heat and mass transfer by natural convection from vertical surfaces in porous media. Lin and Wu[7] discussed Coupled heat and mass transfer by laminar natural convection from vertical plate. Malasetty and Gaikwad[8] investigated effect of cross diffusion on double diffusion convection in the presence of horizontal gradients. Yan et. al [11] have discussed Simultaneous heat and mass transfer in laminar mixed convection flows between vertical parallel plate with asymmetric heating. Yan[12] studied Turbulent mixed convection heat and mass transfer past a vertical porous plate. Mazumdar and Deka[9] investigated MHD flow past an impulsively started infinite vertical plate in presence of thermal radiation. In geothermal region situation may arise when the flow becomes unsteady and sliping at the boundary may take place as well. In such situation of slip flow, ordinary continuum approach fails to yield satisfactory results. Many authors have solved problems taking slip conditions at the boundary. Jain and Teneja [3] solved the problem of magnetopolar flow through a porous medium in slip flow regime. Khandelwal et al [5] have studied effect of couple stresses on the flow through a porous medium with variable permeability in slip flow regime. Varshney et. al.[10] have discussed effect of heat transfer on the flow through a porous medium with variable permeability in slip flow regime with couple stress. Our problem under study is an extension of the problem of Varshney et. al.[10] with mass transfer. The effects of different parameter entering into the problem viz. Permeability parameter Ko, Magnetic Am. J. Sci. Ind. Res., 2011, 2(3): 469-477 470 parameter M, Prandtl number Pr , Schimdt number Sc ,and Grashof number Gr ,Modified Grashof number Gm entering into the problem on velocity, temperature distribution and concentration distribution are shown graphically and discussed numerically FORMULATION AND SOLUTION OF THE PROBLEM Let us consider the unsteady flow of a polar fluid through a porous medium of variable permeability in slip flow regime past an infinite horizontal porous plate in presence of a transverse magnetic field. The external velocity of the fluid is assumed as nt 0 U [1 e ] − + e . The flow is chosen to be at small magnetic Reynolds number which enables us to neglect the induced magnetic field. The x-axis is taken along the plate and y-axis normal to it. Under these conditions the equations which govern the flow are: v 0 y ∂ = ∂ (1)

Turbulent mixed convection of a nanofluid in a horizontal curved tube using a two-phase approach

Turbulent mixed convection heat transfer of a nanofluid consisting of water and Al 2O 3 ( d p = 28 nm) throughout a horizontal curved tube has been investigated numerically. Two-phase mixture model has been implemented to study such a flow field. Elliptical governing equations have been solved to investigate the flow behaviors. Simultaneous effects of the buoyancy force, centrifugal force, and nanoparticles concentration have been presented and discussed. The computed results are compared with previously published experimental and numerical data for a base fluid (very low volume fraction, Φ ≈ 0, in the present simulation) and good agreement between the results is observed. It is seen that the nanoparticle volume fraction does not have a direct effect on the secondary flow and the skin friction coefficient. However its effects on the thermal parameters and flow turbulent intensity are significant.