Numerical Simulation Of Heat Transfer Research Articles

Direct numerical simulation of turbulent forced and mixed convection of LBE in a bundle of heated rods with [formula omitted

In this work, reference data obtained by means of Direct Numerical Simulations of fully-developed flow and heat transfer around a vertical rod bundle are presented. Finite-Volume computations are performed by an original discretization technique based on the representation of arbitrarily-shaped cylindrical boundaries on a non-uniform Cartesian grid. A periodic domain consisting of four subchannels of a triangular lattice of rods with a pitch-to-diameter ratio P/D=1.4 is considered as the reference geometry. A Prandtl number Pr = 0.031 is chosen to represent Liquid Lead-Bismuth Eutectic (LBE) as the working fluid. A single friction Reynolds number value is simulated, namely Reτ=550. Both forced and mixed convection regimes are investigated, buoyancy effects being introduced by imposing a Rayleigh number Ra = 5×105, corresponding to a Richardson number Ri = 0.22. Instantaneous snapshots and relevant statistics of the velocity and thermal fields are reported and discussed for the considered cases, focusing on the effect of aiding buoyancy on turbulent flow and heat transfer. Integral results are also compared against available literature data.

Numerical simulation and analysis of flow and heat transfer in a 5 × 5 vertical rod bundle with buoyancy effects

A numerical investigation of mixed and forced turbulent convection in a 5 × 5 rod bundle with spacer grids has been performed in this paper. Wall temperatures and Nusselt numbers calculated by a Realizable k~ε model and a Reynolds stress model are both validated with the experimental data and the latter model shows a better performance. From the CFD results, it is found that under conditions of high Bo* numbers, the higher values of axial velocity appear in the vicinity regions of the heated rod rather than in the center of sub-channels. The buoyancy force also influences the turbulence intensity, and it seems that the fluid flow is harder to get fully developed under conditions of high Bo* numbers. In addition, based on the comparison of calculation results for different buoyancy parameters, the heat transfer behavior downstream of a spacer grid is considered to be related to the turbulence kinetic energy affected by the buoyancy force.

Numerical Simulation of Heat and Moisture Transfer of Wall with Insulation

This paper improves the heat-moisture coupling transfer model with temperature and relative humidity as the driving force, and regards the effective thermal conductivity and vapor permeability of the wall material as a function of temperature and relative humidity. The latent heat generated by steam diffusion was introduced into the model and the model was numerically calculated using the finite element software COMSOL Multiphysics. The calculation results show that as the thickness of the insulation increases, the heat transfer and moisture transfer inside the wall slow down. When the temperature gradient or the water vapor concentration gradient on both sides of the wall increases, the heat transfer rate will also increase. But they will gradually stabilize. After reaching a steady state, there will still be moisture and temperature transfer due to the existence of the gradient.

Numerical simulation of heat and moisture transfer in fibrous porous media dehydration process with ultrasound assisted

In recent decades, the enhancement of ultrasound on heat and mass transfer in the processes of food processing, waste sludge dehydration and silica gel regeneration has been confirmed by experiments and simulations. The effect of ultrasound in the dehydration process of fibrous porous media has been experimentally studied, however, the theoretical simulation is still lacking, which will be presented in this paper. In the first place, the effects of ultrasound frequency and intensity on ultrasound propagation in fibrous porous media are investigated. For the pressure and deformation produced by ultrasound propagation in porous media, ultrasound intensity has an important influence on their magnitude, and their distribution trend is also affected by ultrasound frequency. Then, the fundamental heat and mass transfer process in fibrous porous media with ultrasound assisted dehydration is studied and the mathematical model is validated by experiments in terms of the drying curves under different ultrasound intensity. By numerical simulation, the distribution of liquid transmission velocity, temperature and dry basis moisture content are obtained which is useful to further analyse the mechanism of ultrasound enhancement in fibrous porous media dehydration.

Comparison approximate analytical solution of the nonlinear differential equation of heating with numerical

To improve the efficiency of existing networks, special mathematical models for assessing the losses and temperature of conductors in real time can be used, with the climatic factors being taken into account. The approximate analytical solution of the nonlinear differential equation of heating and cooling of the insulated conductor with numerical method simulation of heat transfer is proposed comparison in this work. The solution is based on lowering the degree of temperature of the conductor using the least squares method in the integral form. A positive feature of the proposed solution is its universality. It allows the analysis of overhead conductors both with and without insulation. The developed method is almost as accurate as the calculation of the conductor temperature by numerical methods. The reliability of the heat balance equation of overhead power lines at non-stationary thermal mode developed by this method is confirmed by comparison with the results obtained by the finite elements method.

Forced convection around horizontal tubes bundles of a heat exchanger using a two-phase mixture model: Effects of nanofluid and tubes Configuration

In this paper, numerical simulation of laminar flow and heat transfer of nanofluid on a group of heat exchanger tubes is described. For better prediction of the behavior of the nanofluid flow on the tube arrays, two-phase mixture model was used. To achieve this aim, heat transfer and laminar flow of two-phase nanofluid as cooling fluid at volume fraction of 0, 2, 4, and 6% solid nanoparticles of silver and Reynolds numbers of 100 to1800 were investigated for different Configurations of tube arrays. The results indicated when the nanofluid collides with the tube arrays, the growth of heat boundary layer and gradients increase. The increase in the growth of boundary layer in the area behind the tubes was very remarkable, such that at the Reynolds number of 100, due to diffusion of the effect of wall temperature in the cooling fluid close to the wall, it had a considerable growth. Further, from the second row onwards, the slope of pressure drop coefficient diagrams was descending. Among the different Configuration s of tubes and across all the investigated Reynolds numbers, square Configuration had the maximum pressure drop coefficient as well as the highest extent of fluid momentum depreciation.

Conception and numerical simulation of heat and mass transfer in a solid state hydrogen storage reactor

Nowadays energy storage seems to be a vital point in any new energy paradigm. It has become an important and strategic issue, to ensure the energetic sufficiency of humanity. Indeed, hydrogen storage in solids has been proved and revealed as clean and efficient energy storage. Moreover, it can be thought as a seriously considered solution to enable renewable energy to be a part of our quotidian life. To achieve storing hydrogen in solid form, the present study aimed to concepts and simulates a solid-state hydrogen storage reactor (tank). An investigation of the parameters influencing the hydrogen storage performance is carried out. Meanwhile, to understand the physical phenomenon taking place during the storage of hydrogen, a 2D numerical modelling for a metal hydrides-based in hydrogen reactor is presented. A strong coupling between energy balance, kinetic law, as well as a mass momentum balance at sorbent bed temperature under a non-uniform pressure was resolved based on finite element method. The temporal evolutions of pressure, the raising temperature in the bed during the hydriding process as well as the impact of the hydrogen supply pressure within the tank are analysed and validated by comparison with the experimental work in literature, a good agreement is obtained. From an industrial point of view, this study can be used to design and manufacture an optimal solid-state hydrogen storage reactor.

Numerical Simulations of Flow and Heat Transfer in a Wall-Bounded Pin Matrix

A detailed computational study was performed for the case of a wall-bounded pin-fin-array in a staggered arrangement, representative of industrial configurations designed to enhance heat transfer. In order to evaluate the level of turbulence modelling necessary to accurately reproduce the flow physics at three (3) different Reynolds numbers (3,000, 10,000 and 30,000), four models were selected: two eddy-viscosity URANS models (k-ω-SST and ϕ-model), an Elliptic Blending Reynolds-stress model (EB-RSM) and Large Eddy Simulation (LES). Global comparisons for the pressure loss coefficients and average Nusselt numbers were performed with available experimental data which are relevant for industrial applications. Further detailed comparisons of the velocity fields, turbulence quantities and local Nusselt numbers revealed that the correct prediction of the characteristics of the flow is closely related to the ability of the turbulence model to reproduce the large-scale unsteadiness in the wake of the pins, which is at the origin of the intense mixing of momentum and heat. Eddy-viscosity-based turbulence models have difficulties to develop such an unsteadiness, in particular around the first few rows of pins, which leads to a severe underestimation of the Nusselt number. In contrast, LES and EB-RSM are able to predict the unsteady motion of the flow and heat transfer in a satisfactory manner.

АНАЛИЗ ВОЗМОЖНОГО МЕТОДА ИСПОЛЬЗОВАНИЯ ГЕОТЕРМАЛЬНОЙ ЭНЕРГИИ

Проведено численное моделирование процессов теплопереноса в каскаде термосифонов, представляющих собой систему извлечения геотермальной энергии с больших глубин. Предложена математическая модель теплопереноса в слое теплоносителя на нижней крышке термосифона и паровом канале, отличающихся от известных упрощенным описанием комплекса теплофизических процессов, протекающих в зонах испарения, транспорта и конденсации термосифона. Целью исследования является разработка упрощенного метода расчета температурных полей в каскаде термосифонов, обеспечивающего возможность проведения опытно-конструкторских работ по созданию систем извлечения геотермальной энергии на основе каскада термосифонов. Краевая задача математической физики решалась методом конечных разностей. Показана возможность анализа основных характеристик – температур – в рамках модели «эффективной» теплопроводности, коэффициенты переноса которой могут быть определены экспериментально. Установлена возможность переноса теплоты с больших глубин с «эффективностью», достаточной для достижения в системе теплоснабжения температур около 330 К в условиях полной теплоизоляции внешнего контура (поверхностей термосифона). Полученные результаты являются базой для дальнейшего развития моделей и методов анализа процессов извлечения геотермальной энергии с больших глубин с использованием каскада последовательно работающих термосифонов. По результатам полученных теоретических следствий сформулированы основные направления экспериментальных исследований с целью обоснования сделанных по результатам численного анализа выводов. Результаты численного моделирования дают основания для вывода о перспективности дальнейшей (экспериментальной и теоретической) разработки метода извлечения геотермальной энергии с больших глубин залегания грунтовых вод с использованием каскада термосифонов большой высоты.

Three dimensional numerical simulation of combustion and heat transfer in porous radiant burners

In the present study, combustion phenomenon and heat transfer in a three-dimensional rectangular porous radiant burner are numerically studied. Methane-air mixture with five-step reaction mechanism is used to model the combustion process inside the porous matrix. Because of the non-local thermal equilibrium between solid and gas phases, separate energy equations for each of them are numerically solved. Besides, the discrete ordinates method is used to solve the radiative transfer equation for solid phase, where the gas phase is considered to be transparent. The set of governing equations including, gas and porous energy equations, the chemical species transport equations and the radiative transfer equation are simultaneously solved to calculate temperature profiles, radiative heat fluxes and species concentrations inside the porous medium. By this technique, thermal characteristics of porous radiant burners operating under different steady conditions are carried out. Results show that by increasing the excess air ratio, porous burner operates under low maximum temperature and consequently less emission of pollutants in the combustion product. Also, numerical results reveal that thermal behavior of porous burner is much affected by the optical thickness of radiating medium. Good agreement is found between numerical results and theoretical data obtained by other investigators.

Simulation of CO2 under supercritical pressure in solar collectors by variable Lattice Boltzmann method

Supercritical CO2 is widely used in the HVAC facilities, such as gas coolers and solar collectors. The solar collector is a fluid-filled cavity, and the heat and mass transfer characteristics in the cavity is the key factor that affects the performance of solar collector. Considering that the thermophysical properties of supercritical CO2 change dramatically, this article employs the Lattice Boltzmann Method (LBM) to implement the numerical simulation of supercritical heat and mass transfer. Based on analysis of the simulate results, it is found that the heat transfer in the solar collector is mainly related to Rayleigh number. Consequently, several correlations for the average Nusselt number are derived using Rayleigh number. Furthermore, a new concept named the critical Rayleigh number is proposed for the bifurcation phenomenon that occurs during the simulation. The numerical simulation results and the newly proposed correlations in this article could be used directly to improve and evaluate the performances of solar collectors.

Liquid cooling of a microprocessor: experimentation and simulation of a sub-millimeter channel heat exchanger

A heat exchanger dedicated to the cooling of a microprocessor has been designed and realized. It consists of a bottom wall in contact with the processor and a cover that has been dug to a depth of 200 μm on one side and 1 mm on the other. Thus, by turning the cover, the hydraulic diameter of the channel can be changed. Both hydraulic and thermal performances of this heat exchanger have been experimentally tested. Three-dimensional numerical simulations were simultaneously carried out and good agreement was obtained. The influence of the distributor and the collector on the distribution of fluid flow and heat fluxes is emphasized. A new concept of micro-heat exchanger is proposed for the cooling of electronics devices for which wall to fluid heat exchange quality and pumping effect are critical. The ability of a liquid heat exchanger involving a dynamic deformation of one of its walls to cool a microprocessor is investigated. Three-dimensional transient numerical simulations of fluid flow and conjugate heat transfer were performed using commercial software. Effect of geometrical and actuation parameters has been explored, demonstrating the ability of such heat exchanger to simultaneously pump the fluid and enhance the heat transfer.

Numerical simulation of mixed convective heat transfer of a molten salt in a square tube with a thermally conductive wall

Numerical simulation of mixed convective heat transfer of a molten salt in a square tube with a thermally conductive wall

Numerical Simulation of Natural Convection Heat Transfer of Nanofluid With Cu, MWCNT, and Al2O3 Nanoparticles in a Cavity With Different Aspect Ratios

Effect of the size of a closed cavity and different nanoparticles on natural convection heat transfer is investigated using the finite volume method. In the current numerical study, free convection of nanofluid with Cu, multi-walled carbon nanotubes, and Al2O3 nanoparticles is considered at Rayleigh numbers (Ra) of 10–100,000 inside a two-dimensional rectangular cavity with different aspect ratios. Results of this study show that in the presence of cooling fluid with low temperature and hot zone in the cavity, due to the temperature difference between the fluid and hot zone, heat transfer occurs. Heat transfer in the cavity is influenced by fluid circulation caused by natural convection heat transfer and conductive heat transfer mechanism.

A numerical investigation on the heat transfer characteristics of nanofluid flow in a three-dimensional microchannel with harmonic rotating vortex generators

Three-dimensional numerical simulation of conjugate heat transfer in a microchannel involving harmonic rotating longitudinal vortex generators is performed. The Al2O3–water nanofluid with concentration of 0.5% of volume fraction is used. The mean diameter of particles is 29 nm. The Nusselt number and Fanning friction factor of the microchannel changes are investigated for different Reynolds numbers, rotation frequency, and rotation’s direction. Results show that time variations of Nusselt number and friction factor are periodic and their frequencies are the same as vortex generator’s rotation frequencies. It was observed that the amplitude of Nusselt number increases with increasing Reynolds number. Furthermore, when all the vortex generators are rotating in the same direction, the amplitudes of Nusselt number and friction factor are higher than when they are rotating in opposite directions.

Application of four-equation closure for turbulent heat flux in prediction of convective heat transfer to hydrocarbon fuels at supercritical pressures

Accurate modelling of the turbulent thermal fields is of great importance for simulations of convective heat transfer to fluids at supercritical pressures. The widely adopted Reynolds analogy between turbulent momentum transfer and turbulent heat transfer may fail for variable property flows under certain conditions, especially as the fluid approaches the pseudo-critical temperature (Tpc). In order to accurately predict the flow and heat transfer behaviors, the two-equation turbulence model for the thermal field by Hattori, Nagano, and Tagawa (HNT kt−εt) in association with the low-Reynolds number model for the velocity field by Myong and Kasagi (MK k−ε) were incorporated in an in-house code developed within the framework of OpenFOAM. Numerical simulations of convective heat transfer to supercritical pressure hydrocarbon fuels (RP-3 aviation kerosene and n-decane) in vertical heated tubes were performed using the MK-HNT model, MK k−ε model with Prt set as 1.0, and SST k−ω model with Prt set as 1.0. The validity of the models was examined through comparisons with the available experimental data under a variety of operating conditions. All three models performed reasonably well for the cases of normal heat transfer regime. Meanwhile, improvements in the prediction of inner wall temperature could be obtained by the use of the MK-HNT model for cases of heat transfer in the thermal entrance region, heat transfer enhancement (HTE) under forced convection conditions, and heat transfer deterioration (HTD) under mixed convection conditions. Moreover, the different heat transfer mechanisms were investigated by analyzing the predicted turbulent flow and thermal fields. The results indicated that the development of the thermal boundary layer in the entrance region and the drastic variation of specific heat and density in the trans-pseudo-critical region would have considerable effects on the distributions of the Reynolds stress and turbulent heat flux. In addition, it was found that the computational cost was not severely increased by solving two additional turbulence equations.

Heat transfer enhancement in MHD free surface flow by controlling the electromagnetic force with fin structure

A partly insulated fin (PIF) structure has been proposed to mitigate the temperature stratification of flowing-type liquid metal divertors. This study conducted a three-dimensional magnetohydrodynamics (MHD) flow and heat transfer numerical simulation to investigate the influence of the PIF on the velocity and the temperature field inside the film flow. An analytical model simulated a liquid metal film flow of 25 mm thickness under a transverse magnetic field up to 1.0 T. The simulation was conducted with a PIF, a metal fin, and without a fin. The results showed the generation of a high velocity region near the surface and the velocity was 2.68 times higher than the case without a fin due to the MHD effect. In addition, an upward flow was observed in front of the PIF. The contribution of the flow field generated by the fin to the temperature field was confirmed through numerical analysis, and the highest temperature of the liquid metal surface decreased by 0.96% at its maximum. A deformation of the surface due to the fin was estimated using the pressure distribution obtained from the simulation. The result indicated expected deformation is 21.8 μm at the magnetic field of 1.0 T.

Numerical simulation of natural convection heat transfer of copper-water nanofluid in a vertical cylindrical annulus with heat sources

In this work, natural convection of Cu—water nanofluid in a vertical cylindrical annulus enclosure with two discrete heat sources of different lengths is numerically investigated using the finite volume method with SIMPLER algorithm. The adiabatic unheated portions and the discrete heat sources are mounted at the inner wall. The top and bottom walls are thermally isolated, while the outer wall is maintained at a lower temperature. The effects of nanofluid solid volume fraction on hydrodynamic and thermal characteristics such as average and local Nusselt numbers, streamlines, and isotherm patterns for the Rayleigh number ranges from 103 to 106 and solid volume fraction ranges from 0 to 0.1 are presented. The heat transfer and temperature of heaters depend on the Rayleigh number, the solid volume fraction of nanoparticles, and the length of heaters.

Numerical simulation of forced convective heat transfer of molten salt in tubes

Numerical simulation of forced convective heat transfer of molten salt in tubes

Heat storage and release behavior in a thin vessel heat storage unit using salt hydrate latent heat storage material

Recently, there has been considerable interest in heat storage materials. Among them, sodium acetate trihydrate (melting point: 58 °C; latent heat: 264 kg/kg) is a suitable latent heat storage material for recovering and using exhaust heat in a temperature range lower than 100 °C, which is discharged in homes and factories. The purpose of this study was to investigate the heat storage and release behavior of sodium acetate trihydrate in a thin-vessel heat storage unit (160 mm × 130 mm × 4.8 mm) made of glass and polycarbonate. The sodium acetate trihydrate in the unit was heated or cooled by flowing water in water channels (width: 1 mm) disposed on both sides of the heat storage unit. From the experimental results, performance deterioration was evident in the heat storage and release process of sodium acetate trihydrate due to phase separation during heat storage and supercooling during heat release. Therefore, a thickener and a nucleating agent were added to sodium acetate trihydrate to suppress phase separation and supercooling effects. The results revealed that the thickener and nucleating agent were unevenly distributed in the case of repeated heat storage and release. The heat transfer numerical simulation adopting the lumped capacitance model was confirmed to be useful for investigating the melting behavior of sodium acetate trihydrate.