Pure Natural Convection Research Articles

Analyzing non-isothermal phase transition problems with natural convection using peridynamic differential operator

In this study, a developed model for non-isothermal phase transition with natural convection is proposed by using peridynamic differential operator (PDDO). The dimensionless governing equations of heat source approach and vorticity-stream function approach are reconstructed into the non-local integral form. The Euler forward difference is used for time integration. The application of the developed PDDO model extends to simulations involving pure phase transition, pure natural convection, and non-isothermal phase transition coupled with natural convection. The validity and accuracy of the developed non-isothermal phase transition model are verified by comparing the PDDO simulation results with results in published literature. This research provides a new alternative approach for simulating phase transition and convection problems.

Accelerating melting time in a triplex tube heat exchanger thermal energy storage unit with micro-channels

This article studies optimal conditions to maximize heat transfer efficiency within a triplex tube heat exchanger (TTHX) filled with phase-change material. In this context, micro-channels (MCs) incorporated into TTHX for the first time for acceleration melting time. A two-dimensional numerical model has been developed, and pure conduction and natural convection are both simulated. The Reynolds numbers range from 1 to 100. The types of temporal velocity profiles into micro-channels were uniform, step, elliptic, sinusoidal, and power. The effect of gravity states, namely no gravity, with gravity, and (MCHE), has been studied with different numbers of microchannels (4, 6, 8, 10, 12, and 14). The results showed that there was no variation in melting duration observed when increasing the Re number from 50 to 100. Thus, the maximum reduction in melting period occurred at Re = 50. Moreover, sinusoidal velocity profiles were more effective and could help the process finish sooner. In addition, micro channel tube exchanger (MCTE) fitted or equipped with 10 micro-channels has an optimum condition considering pressure drop and complete melting time. Using the sinusoidal profile reduced the total fusion time by 325 s compared to uniform flow (8.47 % reduction). This indicates that the sinusoidal profile is an effective way to reduce the melting period.

Theme: Thermal and Dynamic Study of Convection-Radiation Coupling in a Closed Enclosure

In this work, we present a two-dimensional study of natural convection in a square air-filled cavity whose two vertical sides are subjected to a temperature difference, while the other two horizontal ones are adiabatic. The numerical method used is that of finite elements used in the COMSOL calculation code. We first studied pure natural convection, by varying the Rayleigh number from to , then its coupling with surface radiation, by varying the emissivity of the cavity surfaces from 0 to 0.6. The results are presented in the form of isotherms, pressure, current etc. It appears from this study that for a Rayleigh number less than 100, the fluid presents a vertical thermal stratification due to heat transfer only by conduction which does not generate any convective flow. The presence of surface radiation does not modify the equations governing fluid movements but only alternates the thermal boundary conditions. The coupling of natural convection and surface radiation occurs only through thermal boundary conditions. The variation in the emissivity of the walls does not influence the temperature isotherms.

Effects of Geometric Parameters and Heat-Transfer Fluid Injection Direction on Enhanced Phase-Change Energy Storage in Vertical Shell-and-Tube System

Internationally, energy-storage technologies have facilitated the large-scale utilization of renewable energy, reducing reliance on conventional power generation and enhancing energy efficiency. In the pursuit of strengthening the efficiency of phase-change energy-storage systems, the focus lies on further enhancing the efficiency of vertical shell-and-tube energy-storage systems. This study investigates the influence of two different heat-transfer fluid (HTF) injection directions on the melting of phase-change materials (PCM) in a vertical shell-and-tube latent heat storage (LHS) system. The melting behavior of PCM is analyzed under both pure conduction and natural convection conditions. The research findings reveal that during the initial melting stage, both HTF injection methods primarily rely on thermal conduction, resulting in no significant changes in PCM melting. However, in the later stages of natural convection, bottom HTF injection exhibits superior heat-transfer efficiency compared to top injection. Under a constant volume of phase-change material, both pipe length and pipe thickness affect the PCM melting process. As the pipe length increases within the range of 1.6 m to 0.2 m, the PCM melting time also increases. The results show that the melting time of the PCM is reduced by almost 15,000 s when the tube length H exceeds 800 mm, regardless of whether the heat-transfer fluid is injected at the top or bottom. In this paper, we also obtained results that the three composites containing 10% expanded graphite save 5.3%, 10.2%, and 14.3% of melting time, respectively, compared to pure paraffin when H = 200 mm and top injection are considered. For bottom injection, the three composites saved 7.7%, 12.5%, and 17.2% of melting time, respectively. This further emphasizes the more significant effect of priming in improving melting time.

Heat transfer by natural convection and radiation in three dimensional differentially heated tall cavities

Natural convection coupled with radiation is numerically studied in a tall rectangular parallelepiped cavity filled with air and constant differentially heated opposite vertical walls. This type of tall cavity is found in walls built with hollow blocks. Three dimensional steady state numerical simulations are performed. Numerical models for pure natural convection and natural convection coupled with radiation are validated with experimental results. In this work, the tall cavity studied has a 0.06 m width, 0.155 m depth and a 2.56 m height, giving a vertical aspect ratio of A=42.7 and a horizontal aspect ratio of A′=2.6. The cold wall temperature, Tc, is fixed. The hot wall temperature, Th, is varied, obtaining the Rayleigh numbers: RaW, 1.01×104, 1.07×105 and 7.20×105. The emissivity, ϵ, varies from 0.0 to 1.0. The results show that with a given Th, the convective Nusselt number, Nuc, depends on ϵ. For the emissivity value of hollow block materials (ϵ=0.8) the radiative heat transfer represents 83% for the lowest Th. For pure natural convection, a Nuc correlation as a function of RaW is obtained. A simplified calculation of the total heat transfer, Nut, is proposed.

Low-energy activation of large convective heat transfer via flow resonance triggered by impinging jet

Cooling performance by natural convection is generally enhanced by increasing the surface area using fin arrays, but their implementation is problematic in large surfaces such as a building facade. We recently reported a phenomenon whereby flow resonance triggered by an impinging jet can enhance convective heat transfer without the need of increased surface area. Here, we propose and investigate the concept of a low-energy activation method (i.e. using moderate jets) based on flow resonance for convective heat transfer enhancement over extensive surfaces. We focus on varying the impinging location yimp near the leading edge of the heated plate (local Rayleigh number of Ray≤5×107 or yimp≤300 mm), the momentum of the impinging planar jet (Reynolds number of Re≤400), and the net buoyancy of the thermal boundary layer (global Rayleigh number of Ra≤1.12×1011, i.e. ≤4m vertical walls heated at 316 K). Instability waves were observed downstream over the heated surface via simulations (computational fluid dynamics) and experiments (large-aperture interferometry), resulting in a downstream heat transfer enhancement compared to that of pure natural convection of about 40% to 60% depending on the impinging position for plates shorter than 1 m. However, turbulent convective heat transfer was reduced from the natural convection case at high Ray. As a means of introducing flow resonance in the thermal boundary layer, we demonstrate that the frequency of the oscillating flow in the mixed convection region can be tuned by selecting appropriate Reynolds number and impinging position of the jet. A combination of flow resonance generation (downstream) and boundary layer thinning around the impinging region (upstream) yields a low-energy activation method for enhancing convective heating and cooling performance.

Visualization experiment and numerical study of latent heat storage unit using micro-heat pipe arrays: Melting process

This study conducts experimental visualization and numerical study on the melting of phase change material (PCM) in a rectangular enclosure of a latent heat storage unit based on micro-heat pipe arrays. The solid–liquid interface and temperature distribution in the melting of PCM are recorded by photographic observation and data measurement. Numerical results based on the enthalpy–porosity model are validated by using experimental results. The effects of enclosure geometry on the melting process of PCM are numerically explored. The transition point of a pure conduction mode into the natural convection mode is analyzed by comparing the results obtained in the numerical simulation with and without considering thermal convection. The correlations of the transient heat transfer and the melting rate in the pure conduction and natural convection modes are determined through the analysis of the different heat transfer characteristics in both stages. The results are expected to provide guidance for the design of latent heat storage units using micro-heat pipe arrays in engineering applications.

Numerical investigation of the electro-thermo-convection of dielectric liquid in a square cavity with vertical walls composed of alternately arranged electrodes

Research on the improvement of the heat transfer performance by forming a voltage difference in the dielectric fluid has been actively conducted in the recent years. This study investigates the electro-thermo-convective flow in a two-dimensional (2D) square cavity composed of cold right and hot left walls. We introduce herein the results of the heat transfer enhancement when positive and grounded electrodes are alternately arranged on each vertical wall instead of installing the same kind of electrode (positive or grounded electrode) on the entire surface of a vertical wall. The case of alternately arranged electrodes demonstrates that the heat transfer coefficient increases more than twice compared to that of the single kind of electrode. The heat transfer enhancement by the electric field is maximized in the low Rayleigh condition. In the Ra = 2000 and T = 1000 condition, the alternately placed electrodes accomplished a 10 times higher heat transfer coefficient when compared to the pure natural convection. The heat transfer results of using a silicon oil have been summarized for the Rayleigh number range of 2000 to 106 and electrical Rayleigh number of 0 to 1000.

Numerical Computation on Natural Convection Heat Transfer From an Isothermal Sphere With Semicircular Ribs

Abstract For the very first time, this study attempts to address the heat dissipation from an isothermal ribbed sphere under the action of pure natural convection. Semicircular ribs of different radii are superimposed azimuthally on the outer surface of a sphere. The addition of ribs on the sphere serves a dual purpose in its practical applications: beautification of electronic devices such as spherical light sources as well as an increase in heat dissipation from the hot surface, which prevents the electronic component from getting overheated. Finite volume method-based axisymmetric numerical simulations are performed in the laminar flow regime for the following ranges of nondimensional parameters: Rayleigh number (102 ≤ Ra ≤ 108), inter-rib-spacing to sphere diameter (0.191 ≤ P/D ≤ 0.785), and rib-radius to sphere diameter (0.03 ≤ R/D ≤ 0.083). The main target of this study is to identify the critical parameters for heat transfer enhancement from the ribbed sphere compared to a conventional plane sphere. The results obtained from this work show that the average Nusselt number increases with an increase in Ra and P/D, whereas it decreases as R/D increases. Effectiveness of the ribs (εrib) and critical Rayleigh numbers (Racr), corresponding to εrib = 1, are also calculated. Ribs are more effective in heat dissipation at low Ra and P/D and high R/D. A correlation for the average Nusselt number is also developed in this work, which would help design a better thermal management system.

Experimental, analytical, and computational study of natural convection in asymmetrically-heated vertical shafts

During building fires, buoyant smoke is rapidly transported through elevator and ventilation shafts, posing a threat to the safety of occupants on higher floors. Therefore, smoke dynamics in vertical shafts are of interest to fire and building safety engineers. Herein, analytical solutions describing pure natural-convection laminar flows in asymmetrically-heated vertical shafts are introduced and discussed as a model situation. While the analytical solution is limited to laminar flow at relatively low Rayleigh numbers, a practically-relevant turbulent flow is explored experimentally, with a lab-scale 1-m-high shaft, and numerically, with Fire Dynamics Simulator (FDS) software in the same scale. With supplied heating powers of 100–400 W at the left-hand side wall or/and bottom, a circulatory 2D or/and 3D heat-induced convection flow (natural convection) sets in within the shaft. Air near the heated left wall rises due to buoyancy, while air near the cooler right wall descends, yielding a sine-wave-shaped velocity profile with no-slip boundary conditions imposed at both walls. While this sine-wave velocity profile is clearly observed in laminar flow, the buoyancy-driven layers are relatively thin in the corresponding turbulent flow. The temperature profile practically linearly decreases from the left to the right wall for both laminar and turbulent flows. The overall physical insights provided by the laminar analytical solution are consistent with observations in the turbulent flow. The differences between laminar and turbulent flows are investigated and discussed by qualitatively comparing the analytical solutions to the experimental and numerical data.

Anisotropic heat transfer of ferro-nanofluid in partially heated rectangular enclosures under magnetic field

The natural convection and anisotropic heat transfer of ferro-nanofluids in partially heated enclosures is studied for revealing the influence of the anisotropy of thermal conductivity, which is generated by the existence of external magnetic field. The viscosity of the ferro-nanofluid is assumed to vary with the nanoparticle concentration. The constitutive model of the anisotropic thermal conductivity is derived based on the principle of material frame indifference of Continuum Mechanics, and the numerical solver is built based on the library of OpenFOAM. Both problems of pure heat conduction and natural convection are investigated, and a series of numerical simulations are conducted for different relevant parameters such as the types of the magnetic field, nanoparticle concentration, Hartmann number and Rayleigh number. The numerical results show that the heat transfer along the magnetic field direction is apparently enhanced with Hartmann number of 0.1 and nanoparticle concentration of 0.05, which implies the feasibility of controlling the heat transfer of the ferro-nanofluids by adjusting the external magnetic field. Furthermore, for pure heat conduction, the increasing of the Hartmann number and nanoparticle concentration raises the anisotropic thermal conductivity; and in the natural convection case, increasing the intensity of magnetic field could also raises the Lorentz force resistance. In addition, the Nusselt number is larger with higher Rayleigh number and lower Hartmann number, as the Rayleigh number increases from 1×103 to 1×105, the average Nusselt number on the heater increases from 0.888 to 3.139, and it decreases from 3.789 to 2.866 when Hartmann number is 0 and 10.

Non-linear radiation effect on dusty fluid flow near a rotating blunt-nosed body

A two-phase, dusty boundary-layer flow over an isothermally heated rotating hemisphere is studied for the laminar, incompressible fluid. The contribution of non-linear thermal radiation is analyzed by using the Rosseland diffusion approximation model. The difficulty of having a unified mathematical treatment of the above problem is due to the complex nature of the governing partial differential equations, which arises from the transverse curvature of the round-nosed body, buoyant force, and thermal radiation. Therefore, this study presents a theoretical analysis of a dusty flow and heat transfer over a rotating axisymmetric round-nosed body (for example, hemisphere). In industries, for instance, vehicle industry, blunt-nosed shapes (or round-nosed body) are preferred because it provides better thermal management. The governing equations, along with their appropriate boundary conditions, are numerically solved using the iterative, implicit finite difference method. Numerical solutions are illustrated in the form of streamwise velocity distribution, spanwise velocity distribution, temperature distribution, and skin friction and heat transfer coefficients. The numerical scheme is validated through comparison with the benchmark solutions available in the literature. Solutions obtained here are discussed for i) pure and dusty air and ii) pure and dusty water. A wide range of thermal radiation parameter is tested for the above two cases, and it is found that skin friction coefficient achieves asymptotic behavior for [Formula: see text]; however, heat transfer coefficient increases considerably as much as we strengthen the parameter Rd. Further, momentum and thermal boundary layer thicknesses also increase for [Formula: see text]. Computations are also performed for the buoyancy parameter, λ, ranging from 0 to [Formula: see text], which respectively covers the solution of pure forced convection ( λ = 0) and pure natural convection ([Formula: see text]) boundary layer on a rotating hemisphere.

Conjugate fluid, heat and species transports inside an enclosure containing miscellaneous solid arrays: General models of electronic cooling and pollutant removals

Double-diffusive natural convection in a vertical enclosure containing various solid block arrays is numerically and analytically investigated in present work, where heat and species source and sink are simultaneously attached on the walls. Rayleigh number, buoyancy force ratio, number of inner solid blocks, solid thermal conductivity and mass diffusion coefficient have been varied to observe inherent flow structures and mechanism of thermal and mass transports, regarding of different levels of solid-to-fluid volume ratios. Correlations have been presented for different flow regimes, namely thermal buoyancy driven flow and solutal buoyancy driven flow. Simulation and correlation results demonstrate that influences of thermal conductivity and mass diffusion coefficient of the solid block arrays on average Nusselt and Sherwood numbers are not evident compared with that of Rayleigh number. Furthermore, there is a positive correlation between average Nusselt number and Sherwood number. In addition, under certain values of Rayleigh number, absolute values of buoyancy force ratio close to unity will cause multiple steady solutions in pure natural convection. This research could benefit the future thermal and species management for electronics and built environment.

Effect of Radiation on Natural Convection Heat Transfer from Heated Horizontal Cylinder in Vented Enclosure

The natural convection heat transfer from a heated circular cylinder placed in a square vented enclosure was investigated considering the effect of the thermal radiation from cylinder using finite volume method with simple algorithm that used pressure-velocity coupling to solve Navier-Stockes and energy equations using Computational Fluid Dynamics CFD (Fluent software). The flow domain is filled with air. The enclosure width is 12.5 cm, two symmetrical openings at its lower and upper walls with size O = 2.5 cm and the cylinder diameter is 5 cm. Different rang of cylinder’s surface emissivity with Rayleigh number range between 103 to 106 was applied in this study as operating condition. The effect of the Ra, cylinder radiation for vented square enclosure on the convection, radiation and total Nusselt numbers Nu and flow and thermal patterns were investigated. The numerical results for pure natural convection were compared with available experimental results, good agreement was obtained. The results revealed that the natural convection heat transfer from the cylinder in the enclosure is increased as compared with that of the unbounded cylinder without radiation effect. The results also revealed that there is a linear relationship between the heat transfer enhancement and the emissivity, there is a significant effect of the cylinder surface radiation which gives high augmentation compared with that of the free cylinder and natural convection heat transfer without radiation effect. A maximum total Nusselt number is obtained at high emissivity compared with that of natural convection heat transfer, the maximum Nu enhancement ranged between 70 to 350 % for low Ra and 25 to 125% for high Ra.

Numerical investigation of the stability of mixed convection in a differentially heated vertical porous slab

The stability of mixed convection in a vertical porous slab whose vertical walls are rigid and maintained at constant but different temperatures is investigated in the presence of gravity. The Brinkman-extended Darcy model is used as the momentum equation. The disturbance stability equations are solved numerically using the Chebyshev collocation method. The neutral stability curves and the critical values of the Darcy–Rayleigh number, the corresponding wave number, and the wave speed are obtained for different values of the governing parameters. Contrary to the result observed in the case of pure natural convection, it is found that stationary instability disappears in the presence of a pressure gradient. The results for the Darcy case are delineated as a particular case from the present study and it is shown that the system is unconditionally stable.

Modeling mixed columnar-equiaxed solidification of Sn-10wt%Pb alloy under forced convection driven by travelling magnetic stirring

A mixed columnar-equiaxed solidification model was used to investigate the solidification benchmark experiments, as performed at SIMAP Laboratory. The first experiment case of the benchmark made under natural convection condition was successfully simulated previously. The current work is to simulate the second experiment case, i.e. the casting under travelling magnetic stirring (TMS) which is applied in the direction of natural convection to enhance the natural convection. Crystal fragmentation of columnar dendrites is assumed as the sole origin of equiaxed crystals. Through the analysis of the simulation results, deep understanding to the experimentally reported phenomena was achieved. In comparison to the first experiment case of pure natural convection condition, the TMS-enhanced convection provides a favorable condition for columnar-to-equiaxed transition (CET) by homogenizing the temperature distribution in the bulk liquid region, enhancing the fragmentation of the columnar secondary arms and the transport of the fragments to bulk liquid region. Contradicting with the previous knowledge, the TMS-enhanced convection in this experiment case does not enhance the global macrosegregation significantly. However, it strengthens the channel segregates.

Optimal Cooling Channel Layout in a Hot Enclosure Subject to Natural Convection

Abstract We analyzed the optimal cooling channel layouts emerged from minimizing the total entropy generation ST and mean enclosure temperature T¯ in a hot enclosure subject to natural convection. The conservation of mass, momentum, and energy were solved numerically in two-dimensional (2D) space using the finite element method, and an ant colony optimization algorithm was employed to determine the optimal layouts with respect to the number of cooling channels (N) and Rayleigh number (Ra). The total allocatable cooling channel area was fixed in all cases and thus each channel area decreased with increasing N. Subsequently, we examined the heat transfer and flow characteristics based on temperature field, streamlines, and local Bejan number of each optimum and deduced the following: (1) All optimal channel layouts were symmetric about the vertical enclosure centerline; however, the optima resulting in ST,min and T¯min did not always coincide and depended heavily on the flow configuration dictated by N and Ra; (2) the competing nature between heat transfer and its irreversibility was pronounced at low Ra and N when convection and fluid friction were ineffective; and (3) the first and second law performance improved as N increased in most cases. Furthermore, results verified the global convergence and robustness of the ant colony optimization approach in solving a layout optimization problem with pure natural convection.

Significance of non-Oberbeck-Boussinesq effects augmented by power-law rheology in natural convection studies around fins

The augmentation and diminution of non-Oberbeck-Boussinesq (NOB) effects due to power-law rheology cause significant changes in the results and associated implications of natural convection studies. This study focuses on the combined effect of spatial arrangement with NOB and power-law effects. Non-intuitive changes in heat transfer trends are caused by the additional effect on the shear rate distribution due to spatial arrangement of objects, represented here by an array of fins. An order of magnitude analysis was used to derive Oberbeck-Boussinesq type equations for a class of power-law fluids with all properties considered as linear functions of temperature and pressure. Significant temperature dependent properties were identified, and an explicit criterion to neglect viscous dissipation for power-law fluids in pure natural convection was derived. The identified temperature dependencies were incorporated into NOB equations and solved numerically to investigate their effect on flow field and heat transfer trends. Shear thinning significantly augmented (more than doubled) the accelerating NOB effect, while shear thickening diminished (nearly halved) it. The tendency of power-law rheology to augment or diminish NOB effects was demonstrated to considerably increase the sensitivity of results to temperature dependent properties, over and above that for the Newtonian case. Investigations to note their practical implications revealed that optimization results without NOB effects could be quite misleading for the fin array problem, due to the differing cumulative extents of augmentation. Additionally, correlation studies may be inaccurate as the nature of trends was changed fundamentally due to NOB augmentation.

Large Eddy Simulations on a natural convection boundary layer at Pr = 0.71 and 0.025

The development of Gen IV nuclear reactors entails research oh heavy liquid metals, which are considered as appropriate coolant agents due to their high thermal conductivity and favorable nuclear safety characteristics. These fluids are characterized by a very low Prandtl number (between 0.006 and 0.025). During the design of such reactors, the scenarios of maintenance or loss-of-flow accidents consider a pure natural convection loop inside the reactor. The CFD modeling of the momentum and heat transfer must contemplate natural convection in low Prandtl number fluids, however in literature these flow configurations are not usual. This is why in the present article, Large Eddy Simulations over a natural convection boundary layer at Pr=0.025 are presented and described. Turbulence is triggered by adding a perturbation in the boundary layer. The adopted numerical approach is validated through a Large Eddy Simulation at Pr=0.71, where heat transfer, friction coefficient and mean and turbulent flow fields are compared with measurements in literature. Once turbulence is reached in the Pr=0.025 boundary layer, flow characteristics as Nusselt number, skin friction coefficient and turbulent profiles are calculated and compared with the Pr=0.71 simulation to assess the effect of Prandtl number on heat and momentum transfer.

A multi-objective optimization problem in mixed and natural convection for a vertical channel asymmetrically heated

This paper deals with a multi-objective topology optimization problem in an asymmetrically heated channel, based on both pressure drop minimization and heat transfer maximization. The problem is modeled by assuming steady-state laminar natural convection f low. The incompressible Navier-Stokes equations coupled with the convection-diffusion equation, under the Boussinesq approximation, are employed and are solved with the finite volume method. In this paper, we discuss some limits of classical pressure drop cost function for buoyancy driven flow and, we then propose two new expressions of objective functions: the first one takes into account work of pressure forces and contributes to the loss of mechanical power while the second one is related to thermal power and is linked to the maximization of heat exchanges. We use the adjoint method to compute the gradient of the cost functions. The topology optimization problem is first solved for a Richardson (Ri) number and Reynolds number (Re) set respectively to Ri ∈ {100,200,400} and Re = 400. All these configuration are investigated next in order to demonstrate the efficiency of the new expressions of cost functions. We compare two types of interpolation functions for both the design variable field and the effective diffusivity. Both interpolation techniques have pros and cons and give slightly the same results. We notice that we obtain less isolated solid elements with the sigmoid type interpolation functions.Then, we choose to work with the sigmoid and solve the topology optimization problem in case of pure natural convection, by setting Rayleigh number to {3×103,4×104,5×105}. In all considered cases, our algorithm succeeds to enhance one of the phenomenon modeled by the proposed cost functions without deteriorating the other one. The optimized design obtained suppresses any reversal flow at the exit of the channel. We also show that the thermal exchanges are improved by computing the Nusselt numbers and bulk temperature. We conclude that the new expressions of objective functions are well suited to deal with natural convection optimization problem in a vertical channel.