Turbulent Convective Heat Transfer Research Articles

Turbulent thermal convection of nanofluids in cubical enclosure using two-phase mixture model

In this paper, numerical simulation is performed to investigate the turbulent thermal convection flow and heat transfer characteristics of nanofluids inside a cubical enclosure with partially mounted heat and cold source. A transient, three-dimensional, two-phase mixture model with LamBremhorst k−ϵ turbulence model is developed, validated and solved using finite difference method. The heat transfer performance of different water based nanofluids such as aluminum oxide (Al2O3), copper (Cu) and silver (Ag) are investigated for a wide range of Grashof numbers (Gr) varying between 106 ≤ Gr ≤ 1010. The nanoparticle diameter (dp) and volume fractions (ϕ) are varied between 20nm ≤ dp ≤ 80nm and 2% ≤ ϕ ≤ 4%. The results indicate that the random Brownian motion of nanoparticles increases the thermal convection and enhances the rate of energy exchange between the fluid and particle phase. The average heat transfer rate increases with increase in Grashof number and volume fraction. It is found that the effect of volume fraction on average heat transfer rate is more effective in transitional flows than fully turbulent flows. The average heat transfer rate increases with decrease in particle size and the influence of nanoparticle size on heat transfer enhancement are significant in turbulent flows than transitional flows.

Heat transfer of hybrid nanofluid in a shell and tube heat exchanger equipped with blade-shape turbulators

In the current study, the turbulent forced convection heat transfer of Fe3O4/-CNT/water hybrid nanofluid (HNF) in a heat exchanger (HE) equipped with blade-shape turbulators is studied. The 3D governing equations have been solved numerically with SIMPLEC algorithm and solution domain by employing the control volume method (CVM). The impact of Reynolds number (Re), volume concentration of Fe3O4 nanoparticles (φM) and CNTs (φCNT) and blade-rod diameter (dr) on the performance features of HE are evaluated. The results showed that the PEC of HNF augments with the augmentation of Re and φM, while with intensifying dr, the PEC first rises and then declines. The outcomes showed that the highest PEC occurs at Re = 9000, φM = 0.9%, φCNT = 1.35% and dr = 15mm.

Heat transfer in turbulent nanofluids: Separation flow studies and development of novel correlations

Convective heat transfer plays a significant role in numerous industrial cooling and heating applications. This method of heat transfer can be passively improved by reconfiguring flow passage, fluid thermophysical properties, or boundary conditions. The broader scope of nanotechnology introduced several studies of thermal engineering and heat transfer. Nano-fluids are one of such technology which can be thought of engineered colloidal fluids with nano-sized particles. In the present study, turbulent forced convection heat transfer to nanofluids in an axisymmetric abrupt expansion heat exchanger was investigated experimentally. During heat transfer investigation, the functionalized multiwalled carbon nanotubes (MWCNT-COOH), polycarboxylate functionalized graphene nanoplatelets (F-GNP), SiO2 and ZnO water-based nanofluids were used. The convective heat transfer coefficient of fully developed turbulent flow of nanofluids flowing through an abrupt enlargement with the expansion ratio (ER) of 2 was experimentally determined at a constant wall heat flux of 12,128.56 W/m2. The experiments were conducted at the Re ranges of 4000–16,000. The observed Nusselt numbers were higher than in the case of fully developed pipe flow indicating the level of the turbulent transport is high even though the recirculating velocities were a few percentages of the bulk mean velocity. The effect of Reynolds number and nanofluid’s volume concentration on heat transfer and friction losses were studied, where all the results reveal that with the increase of weight concentration and Reynolds number, the local Nusselt number enhanced at the increment of axial ratios in all the cases showing greater heat transfer rates than those of the base fluids. Comparison between the examined four types of nanofluids, show that the carbon-based nanofluids have a greater effect on enhancing heat transfer (33.7% and 16.7% heat transfer performance improvement for F-GNP and MWCNT nanofluids respectively at 0.1 wt% concentration) at the downstream of the sudden expansion pipe. There is no reported work dealing with the prediction of the local Nusselt number at the distance equivalent to the axial ratio and flow through sudden expansion. So far, two excellent correlations for the Local Nusselt number are proposed with reasonably good accuracy. Furthermore, a new correlation is developed for the average Nusselt number.

Numerical investigation of heat transfer augmentation in curved channel using hybrid nanofluids

Convective heat transfer can be enhanced by changing flow geometry and/or by enhancing thermal conductivity of the fluid. In this work, CFD modelling of horizontal straight and curved channel with square cross section were presented to investigate the effect of hybrid nanofluids on turbulent forced convective heat transfer. This study proposes simultaneous passive heat transfer enhancement by combining the geometry effect using 0.1% graphene nanoplatelets-silver hybrid nanofluids (GNP–Ag) inflow in straight and curved channel. The results showed that the average Nusselt number is generally higher for curved channel with hybrid nanofluid when compared with straight square channel. Moreover, for 0.1% of GNP–Ag hybrid nanofluid improvement is 22.61% and 34.78% for straight channel and 27.43% and 39.52 for curved channel at the Reynolds number of 5,000 and 17,500, respectively.

Numerical Study of the Turbulent Natural Convection in a Trapezoidal Cavity: Influence of the Inclination of the Lateral Walls

In this paper, we study the heat transfer in turbulent natural convection in a two- dimensional cavity with a trapezoidal section and isoscales filled out of air with as height H =2.5 m. In these conditions, the side walls are differentially heated while the horizontal walls are adiabatic. The k-ε turbulence model with a small Reynolds number was integrated in our calculation code. The governing equations of the problem were solved numerically by the commercial CFD code Fluent; which is based on the finite volume method and the Boussinesq approximation. The elaborated model is validated from the experimental results in the case of the turbulent flow in a square cavity. Then, the study was related primarily to the influence of the slope of the side walls of the cavity on the dynamic behavior and the heat transfer within the cavity.

Augmented heat transfer in tubular heat exchanger fitted with V-baffled tapes

Effects of insertion of a straight tape mounted with double-sided V-baffles used as a longitudinal vortex generator (LVG) on turbulent convection heat transfer behaviors in a constant heat-fluxed tube in the Reynolds number (Re) range of 4120 to 25,800 are experimentally and numerically investigated. In the present experiment, the V-baffles at a fixed attack angle (α = 30°) were placed on the tape with two arrangements: V-apex pointing downstream and upstream (hereinafter called “V-down” and “V-up”). The V-down and up tapes with three baffle height ratios, (RB = b/D = 0.20, 0.15, 0.10) and pitch ratios, (RP = P/D = 1.5, 1.0, 0.5) were tested. Data of the current smooth or plain tube were also examined for comparison. The experimental results have shown that the employ of the V-down tape generally yields higher heat transfer, friction loss and thermal performance than that of the V-up one. A new derived thermal-performance enhancement factor (TEF) is proposed. The maximum TEF of the V-down tape is around 2.07 at RB = 0.15, RP = 1.0 and is, in average, about 1.4% higher than the V-up one having its peak of 2.04. A 3D numerical V-baffled-tube flow model was also simulated to examine the fluid flow and heat transfer patterns. The numerical validation was seen in close agreement with available experimental data. Numerical heat transfer contours and flow structures were also reported.

Semiempirical Model of Convective Heat Transfer of Turbulent Gases

On the basis of the Prandtl semiempirical wall-turbulence hypothesis, the author has substantiated theoretically the possibility of setting boundary conditions to the equations of a mathematical model of turbulent motion and convective heat transfer of a gaseous medium on a coarse grid. It has been shown that the results of numerical simulation are in agreement with experimental data on the heat transfer of air and high-temperature gases in tubes.

Influence of nanofluid properties on turbulent forced convection heat transfer in different base liquids

This paper analyses the influence of nanofluid properties on turbulent convective heat transfer characteristics for various base fluids. The turbulent characteristics of heat transfer and flow have been determined by applying the Van Driest model of the eddy diffusivity for water and ethylene glycol‐based nanofluids. The properties of CuO, Al2O3 and SiO2 nanofluids in two base liquids (water and EG‐water mixture with the ratio of 60:40) have been considered for various concentrations, as well as large temperature ranges. Based on the observations, it is concluded that numerical outcomes are validated with experimental measurements for heat transfer properties. It is also established that SiO2 reaches a higher temperature gradient in comparison with CuO at a similar temperature and concentration in EG‐water, with the mixture of 60:40. The gradients are greater for the EGW mixture compared to water‐based nanofluids. However, the water‐based nanofluids have higher heat transfer coefficients compared to EG‐water nanofluid at identical flow velocities. It is also notable that the key findings demonstrate that for the heat transfer coefficient, especially in practical situations, the use of nanoparticles and the base fluids tends to enhance the parameter of the coefficient of heat transfer. Also, this study is deemed contributory to the literature and extends the works of some of the previous scholarly contributors by affirming that the factor or parameter concerning the coefficient of heat transfer has its values or behavior dependent on the ratio between the water base liquid and the EG. Overall, it is shown that for higher values of the coefficient of heat transfer to be obtained, there is a need for the introduction of base liquids and nanoparticles, especially because these materials come with or exhibit superior thermal conductivity.

Convective heat transfer performance of solar salt in an inclined circular tube

Solar salt is the most common heat storage medium in solar power system, In order to understand convective heat transfer performance of solar salt, and testify the validity of the well-known classical and four literature convective heat transfer correlations, an experimental study on turbulent convective heat transfer of solar salt in an inclined tube was conducted in this paper. Various experiments were respectively carried out within the experimental data of solar salt in the range of Reynolds between 10,000 and 36,000 and Prandtl between 4.75 and 8.0. Based on the experimental data, its local heat transfer coefficient, overall heat transfer coefficient had been studied in detail. Besides, good agreement was observed between the experimental data of solar salt and part classical and literature correlations. Conversely, other correlations over predicted the heat transfer performance of solar salt, Finally, the fitted forced convective heat transfer correlation of solar salt was obtained by using multiple linear regressions within a deviation of ±10%.

A comparative assessment of various single tray core catcher options for future sodium cooled fast reactors

Various single tray in-vessel core catcher options to handle whole core decay heat for a 1500 MWt capacity sodium cooled Indian fast reactor (SFR) have been analyzed towards their efficiency in rendering acceptable core catcher temperatures. For this purpose, a one dimensional transient multiphase heat transfer model that considers sodium boiling, fuel particles re-melting and slumping has been adopted. The boundary conditions for the one dimensional model are obtained from computational fluid dynamics (CFD) simulations of turbulent natural convective heat transfer within the lower sodium plenum of the SFR. It is found that the top surface of the debris bed dissipates more heat than the bottom surface of the core catcher. A single plate core catcher is seen to experience melting of the core catcher plate (CCP) for a short duration in case of CCP thickness greater than 80 mm. The heat shield plate (HSP) of the double plate core catcher is observed to undergo complete melting due to vapor locking between the HSP and CCP. The phenomena of melting and slumping of debris particles is seen in double plate core catchers with a refractory lining. However, safe temperature levels of HSP and CCP were observed in the case of double plate core catchers with refractory lining. This study suggests that a double plate core catcher with a delay bed is the potential option for future sodium cooled fast reactors, which are to be designed to handle whole core meltdown scenarios.

Enhancement of Turbulent Convective Heat Transfer using a Microparticle Multiphase Flow

The turbulent heat transfer enhancement of microfluid as a heat transfer medium in a tube was investigated. Within the Reynolds number ranging from 7000 to 23,000, heat transfer, friction loss and thermal performance characteristics of graphite, Al2O3 and CuO microfluid with the particle volume fraction of 0.25%–1.0% and particle size of 5 μm have been respectively tested. The results showed that the thermal performance of microfluids was better than water. In addition, the graphite microfluid had the best turbulent convective heat transfer effect among several microfluids. To further investigate the effect of graphite particle size on thermal performance, the heat transfer characteristics of the graphite microfluid with the size of 1 μm was also tested. The results showed that the thermal performance of the particle size of 1 μm was better than that of 5 μm. Within the investigated range, the maximum value of the thermal performance of graphite microfluid was found at a 1.0% volume fraction, a Reynolds number around 7500 and a size of 1 μm. In addition, the simulation results showed that the increase of equivalent thermal conductivity of the microfluid and the turbulent kinetic energy near the tube wall, by adding the microparticles, caused the enhancement of heat transfer; therefore, the microfluid can be potentially used to enhance turbulent convective heat transfer.

The arrangement orientation effect on turbulent convection heat transfer of supercritical LNG in submerged combustion vaporizer

A numerical model is established to simulate the thermal performance of supercritical LNG (SLNG) at different arrangement orientations in submerged combustion vaporizer (SCV). The SST k-ω turbulent model is employed after validating against classic experimental data. The arrangement orientation effect of serpentine tube on turbulent convection heat transfer is explored based on the comparison of that at horizontal orientation tube and vertical one, the integrated effect and dominance relation between buoyancy force and centrifugal force are discussed. The influential factors such as heat flux and pressure on the turbulent convection heat transfer are analyzed. The simulation results indicate that: (1) The heat transfer of SLNG in serpentine tube is different significantly at variable arrangement orientations, and the gasification rate related to the fluid temperature is higher in the horizontal orientation serpentine tube than that in the vertical one. (2) Both for the horizontal and vertical orientations, the centrifugal force dominates the heat transfer in the bend region, even affects the heat transfer in straight segments, and the influence of buoyancy force is exhibited obviously near the pseudo critical point owing to the thermophysical properties. (3) The heat flux and pressure always play an important role on the flow and heat transfer of SLNG in serpentine tube at different arrangement orientations.

A mushy Earth's mantle for more than 500 Myr after the magma ocean solidification

SUMMARYIn its early evolution, the Earth mantle likely experienced several episodes of complete melting enhanced by giant impact heating, short-lived radionuclides heating and viscous dissipation during the metal/silicate separation. After a first stage of rapid and significant crystallization (Magma Ocean stage), the mantle cooling is slowed down due to the rheological transition, which occurs at a critical melt fraction of 40–50%. This transition first occurs in the lowermost mantle, before the mushy zone migrates toward the Earth's surface with further mantle cooling. Thick thermal boundary layers form above and below this reservoir. We have developed numerical models to monitor the thermal evolution of a cooling and crystallizing deep mushy mantle. For this purpose, we use a 1-D approach in spherical geometry accounting for turbulent convective heat transfer and integrating recent and solid experimental constraints from mineral physics. Our results show that the last stages of the mushy mantle solidification occur in two separate mantle layers. The lifetime and depth of each layer are strongly dependent on the considered viscosity model and in particular on the viscosity contrast between the solid upper and lower mantle. In any case, the full solidification should occur at the Hadean–Eoarchean boundary 500–800 Myr after Earth's formation. The persistence of molten reservoirs during the Hadean may favor the absence of early reliefs at that time and maintain isolation of the early crust from the underlying mantle dynamics.

Very Large Eddy Simulation of turbulent flow and heat transfer for single cylinder and cylindrical pin matrix

The study of turbulent flow and heat transfer around a cylinder and cylindrical pin matrix is important for turbine blade cooling. A newly developed VLES method is applied to investigate the unsteady flow and heat transfer mechanisms. DDES and unsteady RANS methods are also used in the simulations for comparisons. Two typical cases are selected, i.e. the flow around a single cylinder, and cylindrical pin matrix of wall-bounded 8 × 8 round pins, with Re = 10000. The results show that the VLES predictions agree well with the experiments for both the flow velocities and heat transfer Nusselt numbers for the two cases, which are even better than the DDES and RANS results. The VLES method gives satisfactory predictions of mean endwall Nusselt number of the complex pin matrix, which is within 15% difference from the experiments. The unsteady three-dimensional vortex characteristics play an important role for the heat transfer, especially in the pin matrix due to complex pin disturbance and wall confinement. Accurate representation of the vortex structures is the key for good heat transfer prediction. The results demonstrate the reliability and accuracy of the present VLES method for complex turbulent convection heat transfer of industrial relevance.

MODELING HEAT EXCHANGE DEPENDING ON THE PRANDTL NUMBER FOR VARIOUS GEOMETRIC AND REGIME PARAMETERS

Objectives. The aim is to study the dependency of the distribution of integral heat transfer during turbulent convective heat transfer in a pipe with a sequence of periodic protrusions of semicircular geometry on the Prandtl number using the calculation method based on a numerical solution of the system of Reynolds equations closed using the Menter’s shear stress transport model and the energy equation on different-sized intersecting structured grids.Method. A calculation was carried out on the basis of a theoretical method based on the solution of the Reynolds equations by factored finite-volume method closed with the help of the Menter shear stress transport model, as well as the energy equation on different-scaled intersecting structured grids (fast composite mesh method (FCOM)).Results. The calculations performed in the work showed that with an increase in the Prandtl number at small Reynolds numbers, there is an initial noticeable increase in the relative heat transfer. With additional increase in the Prandtl number, the relative heat transfer changes less: for small steps, it increases; for median steps it is almost stabilised, while for large steps it declines insignificantly. At large Reynolds numbers, the relative heat transfer decreases with an increase in the Prandtl number followed by its further stabilisation.Conclusion. The study analyses the calculated dependencies of the relative heat transfer on the Pr Prandtl number for various values of the relative h/D height of the turbulator, the relative t/D pitch between the turbulators and for various values of the Re Reynolds number. Qualitative and quantitative changes in calculated parameters are described all other things being equal. The analytical substantiation of the obtained calculation laws is that the height of the turbuliser is less for small Reynolds numbers, while for large Reynolds numbers, it is less than the height of the wall layer. Consequently, only the core of the flow is turbulised, which results in an increase in hydroresistance and a decrease in heat transfer. In the work on the basis of limited calculation material, a tangible decrease in the level of heat transfer intensification for small Prandtl numbers is theoretically confirmed. The obtained results of intensified heat transfer in the region of low Prandtl numbers substantiate the promising development of research in this direction. The theoretical data obtained in the work have determined the laws of relative heat transfer across a wide range of Prandtl numbers, including in those areas where experimental material does not currently exist.

Modifying a meshless method to solving κ−ε turbulent natural convection heat transfer

The conventional meshless local Petrov–Galerkin method is modified to enable the method to solve turbulent convection heat transfer problems. The modifications include developing a new computer code which empowers the method to adopt nonlinear equations. A source term expressed in terms of turbulent viscosity gradients is appended to the code to optimize the accuracy for turbulent flow domains. The standard [Formula: see text] transport equations, one of the most applicable two equation turbulent viscosity models, is incorporated, appropriately, into the developed code to bring about both versibility and stability for turbulent natural heat transfer applications. The amenability of the new developed technique is tested by applying the modified method to two conventional turbulent fluid flow test cases. Upon the obtained acceptable results, the modified technique is, next, applied to two conventional natural heat transfer test cases for their turbulent domain. Based on comparing the results of the new technique with those of the available experimental or conventional numerical methods, the proposed method shows good adaptability and accuracy for both the fluid flow and convection heat transfer applications in turbulent domains. The new technique, now, furthers the applicability of the mesh-free local Petrov-Galerkin (MLPG) method to turbulent flow and heat transfer problems and provides much closer results to those of the available experimental or conventional numerical methods.

Turbulent natural convection heat transfer from a vertical hollow cylinder suspended in air: A numerical approach

Numerical simulations have been conducted to predict turbulent natural convection flow field and temperature distribution around an isothermal vertical solid or hollow cylinder. Continuity, momentum and energy equations have been solved along with kinetic energy and specific dissipation rate equations in the turbulent range of Rayleigh number (Ra) spanning from 1010 to 1012 and length to diameter ratio of the cylinder (L/D) from 0.125 to 20. Average surface Nusselt number and non-dimensional mass flow rate of air through the hollow cylinder have been calculated and plotted with respect to the non-dimensional input parameters such as Ra and L/D. Nu for the inner surface of the hollow cylinder is marginally lesser than that of the outer surface of the hollow cylinder for all L/D and Ra. Nu for the outer surface increases monotonically with respect to L/D however, for the inner surface, it decreases up to L/D of 10 and beyond that, it starts rising. The temperature contours and velocity vector plots have been sketched to understand the heat transfer phenomenon in a better way. Correlations for Nu and mass flow rate for hollow cylinder have been proposed in the turbulent range which could be referred in steel industries, electronics industries and academic practices. Exploiting the developed correlation, the cooling curve (temp vs time) of a hollow cylinder at various Ra and L/D has been drawn which shows a cylinder with a higher Ra with high L/D requires more time to cool down than that of a lower Ra with low L/D. The cooling curve is very much helpful in reducing the idle time in loading and unloading operations of the hot products in steel industries.

Geometry and orientation effects in non-uniformly heated microchannel heat exchangers using supercritical carbon dioxide

There is growing interest to use supercritical carbon dioxide (sCO2) as a working fluid in thermal management applications. This study investigates the thermal-hydraulic performance of microchannel heat sinks as a function of flow channel geometry and orientation at operating conditions representative of electronics cooling applications. Three different experimental test sections, subject to non-uniform heat flux boundary conditions, were investigated. Two of the test sections contained parallel arrays of rectangular microchannels with a hydraulic diameter of 750 μm and aspect ratios of 1 and 2, respectively. The third test section had a staggered array of diamond shaped micro-pins with a hydraulic diameter, based on the minimum flow area, of 525.2 μm. Data were collected for varying inlet temperature (16⩽Tin⩽50 °C), mass flux (315⩽G⩽1000 kg m−2 s−1), and heat flux (20⩽q″⩽50 W cm−2) at a fixed reduced pressure (PR) of 1.1. A data analysis method using 2-D and 3-D heat transfer models of the test sections was used to calculate the average heat transfer coefficients for each experimental condition. Additionally, a pressure drop model was developed to resolve the total measured pressure drop into its components. The results of this study indicate that the turbulent convective heat transfer was independent of orientation (top versus bottom heating) for square microchannel (aspect ratio = 1) for the conditions investigated. Increasing the channel aspect ratio from 1 to 2 led to an enhancement in thermal transport. Finally, the heat transfer performance of the staggered pin array flow geometry was superior to the rectangular channels, but this enhancement in heat transfer was commensurate with the increase in pressure drop. Based on these results, this paper concludes with general design recommendations for those considering the early adoption of supercritical carbon dioxide for thermal management applications.

Numerical investigations of turbulent natural convection heat transfer within a wind turbine nacelle operating in hot climate

The Algerian Sahara is characterized by severe climate conditions where temperature reaches high levels with large variations during day and season. Nacelles of wind turbines operating in this region are subjected to the overheating problem, in particular the electromechanical components becoming less effective as they heat up during use. In order to maintain an appropriate temperature of the air within the nacelle, an active cooling system has to be set up to reject the generated heat towards the external environment and the resulting heat transfer must be properly controlled. The present work investigates numerically the impact of turbulent natural convection heat transfer on the nacelle thermal behavior. Reynolds-Averaged Navier–Stokes and energy equations have been considered. ANSYS FLUENT code has been employed to solve the resulting mathematical model. Temperature and velocity profiles within the 2D and 3D-configurations of the nacelle have been presented and discussed. Variations of the average temperature inside the nacelle and the required cooling capacity are determined and thoroughly discussed.

Numerical Investigation of Heat Transfer to Supercritical Water in Vertical Tube under Semicircular Heating Condition

In-depth understanding and analysis of turbulent convection heat transfer of supercritical water under semicircular heating conditions play a major role in system design and security. The inaccurate numerical results on simulating the buoyancy effect under deterioration heat transfer cases are partly attributed to the invalidity of the turbulent model. An improved turbulence model, which is validated suitable to three-dimensional model, is adopted in the present paper to numerical simulated flow and heat transfer in a vertical tube under semicircular heating condition. Heat transfer deterioration phenomenon occurs under semicircular heating condition, while the degree of deterioration is weakened due to the influence of variable physical properties and buoyancy effect. The velocity profile is distorted into “M-shape” in the heating side and present parabolic distribution in the adiabatic side, leading to different deterioration mechanisms under semicircular heating condition compared with uniform heating. The larger density difference between the heating side and the adiabatic side increases the shear stress production of turbulent kinetic energy; turbulent development is much faster recovery than the phenomenon in uniform heating condition. The results show that the semicircular heating condition can effectively alleviate the degree of heat transfer deterioration in a vertical tube.