Convective Behavior Research Articles

Numerical analysis of natural convection behavior in density stratification induced by external cooling of a containment vessel

It is essential to improve computational fluid dynamics (CFD) analysis accuracy to estimate thermal flow in a containment vessel during a severe accident. Previous studies pointed out the importance of the initial and boundary conditions on the analysis. The purpose of this study is to evaluate the influence of initial and boundary conditions by numerical analysis of natural convection experiments. A density stratification layer was initially formed, and natural convection was induced by the external cooling of the vessel. Mixing by natural convection eroded the density stratification. We applied the RANS model with dynamic turbulent Schmidt number model. We used the measured temperature and gas concentration as the boundary and initial conditions. The erosion velocity did not change much with the initial gas distribution. The temperature boundary condition of small internal structures significantly influenced the fluid temperature distribution, and we evaluated this influence of the small internal structure quantitatively.

A Hybrid High-Order method for incompressible flows of non-Newtonian fluids with power-like convective behaviour

Abstract In this work, we design and analyse a Hybrid High-Order (HHO) discretization method for incompressible flows of non-Newtonian fluids with power-like convective behaviour. We work under general assumptions on the viscosity and convection laws, which are associated with possibly different Sobolev exponents $r\in (1,\infty )$ and $s\in (1,\infty )$. After providing a novel weak formulation of the continuous problem, we study its well-posedness highlighting how a subtle interplay between the exponents $r$ and $s$ determines the existence and uniqueness of a solution. We next design an HHO scheme based on this weak formulation, and perform a comprehensive stability and convergence analysis, including convergence for general data and error estimates for shear-thinning fluids and small data. The HHO scheme is validated on a complete panel of model problems.

Study on Convective Heat Transfer of Supercritical Nitrogen in a Vertical Tube for Liquid Air Energy Storage

The convective heat transfer behavior of supercritical nitrogen (S-N2) has played a significant role in optimizing the design of recently emerging cryogenic cold storage and recovery systems. However, studies on S-N2 heat transfer have been relatively scarce, not to mention that there is a legitimate urge for a robust numerical model to accurately predict and explain S-N2 heat transfer under various working conditions. In this paper, both experimental and numerical studies were conducted for convective heat transfer of S-N2 in a small vertical tube. The results demonstrated that the standard k-ε model performed better for predicting the key heat transfer characteristics of S-N2 than the SST k-ω model. The effects of heat flux and inlet pressure on the heat transfer characteristics under a large mass flux were evaluated. The variation mechanisms of local heat transfer performance were revealed by illustrating radial profiles of thermophysical properties and turbulent parameters of N2. It was found that the local performance variation along the flow direction was mainly determined by the radial profile of specific heat while the variation of the best local performance with the ratio of heat flux to mass flux was mainly determined by the radial profile of turbulent viscosity.

Experimental effect of inclination on the process of melting paraffin in a square cavity

Phase change material PCM is a promising strategy for reducing energy consumption in various applications. Among the large number of PCMs studied in the literature, paraffin are considered to be promising for latent heat thermal energy storage LHTES due to its appropriate thermal properties and their chemical stability. The interest of this work is to carry out an experimental procedure to visualize the phase change process of paraffin in a square cavity at different inclination angles. This article reveals how the melting rate could be affected by changing three orientations 90° (vertical heating), 0° (bottom horizontal heating) and 45° (inclined heating). The enclosure is heated on one side while the other walls are thermally insulated. The numerical photos, infrared thermal image and thermocouples-temperatures recorded during the melting processing are used to calculate the melting fractions and to estimate the intensity of heat transfer in different angles. The results show that the inclination angle has a great influence on the behavior of natural convection, affecting the front melting propagation and heat transfer rate. When the inclination angle decreases from 90° to 0° the convection currents in the cavity progressively evolve from a dominant single-cell movement to an unstable Rayleigh-Benard movement. The total melting time for the bottom and inclined heating cavities were, on average, 56% and 45% less than that of the vertical heating, respectively.

Experimental investigation of natural convection and gas mixing behaviors driven by outer surface cooling with and without density stratification consisting of an air-helium gas mixture in a large-scale enclosed vessel

This paper describes an experimental investigation of natural convection driven by outer surface cooling in the presence of density stratification consisting of an air and helium (as mimic gas of hydrogen) gas mixture in an enclosed vessel. The unique cooling system of the Containment InteGral effects Measurement Apparatus (CIGMA) is used, and findings reveal that the cooling location relative to the stratification plays an important role in determining the interaction behavior of the heat and mass transfer in the enclosed vessel. When the cooling region is narrower than the stratification thickness, the density-stratified region expands to the lower part while decreasing in concentration (stratification dissolution). When the cooling region is wider than the stratification thickness, the stratification is gradually eroded from the bottom with decreasing layer thickness (stratification breakup). This knowledge is useful for understanding the interaction behavior of heat and mass transfer during severe accidents in nuclear power plants.

Evaluation of heat transfer augmentation and pressure drop by water/ethylene glycol nanofluid

The turbulent convective heat transfer behavior of CaCO3/TiO2/CuO nanocomposite dispersions in water/ethylene glycol is investigated experimentally in horizontal pipe-lines at constant heat flux at various in the Reynolds number range of 2900–18200 at 30 °C, heat flux of 800 kW/m2, and nanocomposite concentration of 0.01–0.1 Vol. %. The results showed considerable enhancement of convective heat transfer using the nanofluids. The enhancement was particularly significant in the entrance region, and was much higher than that solely due to the enhancement on thermal conduction. The Nusselt number and friction factor at 0.1 vol. % is greater than that values of water/ethylene glycol about 34.5 % and 18.9 %, respectively. The pressure drop increased with the concentration more than 3.6 % and decreased thereafter. Also, heat transfer coefficient in rough pipe-lines was higher than that in smooth pipe-lines at the same Reynolds number and it enhanced as the diameter decreased. Eventually, the greatest thermal performance factor of the nanofluid was 1.28 at 0.1 vol. % and Reynolds number of 4800.

Moist Air Flow Analysis in an Open Enclosure. Part A: Parametric Study

Heat and mass transfer in many systems are widely accomplished applying natural convection process due to their low cost, reliability, and easy support. Typical applications include different mechanisms in various fields such as (solar energy, solar distiller, stream cooling, etc…). Numerical results of turbulent natural convection and mass transfer in an open enclosure for different aspect ratios (AR = 0.5, 1, and 2) with a humid-air are carried out. Mass fraction and local Nusselt number were proposed to investigate the heat and mass transfer. A heat flux boundary conditions were subjected to the lateral walls and the bottom one make as an adiabatic wall, while the top area was proposed as a free surface. Effect of Rayleigh numbers (106≤????????≤108) on natural convection and mass flow behavior are analyzed. The governing equations are solved using CFD Fluent code based on the SIMPLE algorithm. The results showed that the cavity with an aspect ratio of AR = 2 has a significant enhancement to raise the rates of both heat and mass transfer. When the Rayleigh number increases, maximum heat transfer rates were observed due to the fluid flow becomes more vigorous. However, mass transfer improves as the Rayleigh number decreases.

The effects of roughness levels on the instability of the boundary-layer flow over a rotating disk with an enforced axial flow

This paper investigates the effects of surface roughness on the convective stability behavior of boundary-layer flow over a rotating disk. An enforced axial flow and the Miklavčič and Wang (MW) model of roughness are applied to this flow. The effects of both anisotropic and isotropic surface roughness on the distinct instability properties of the boundary-layer flow over a rotating disk will also be examined for this model. It is possible to implement these types of roughness on this geometric shape while considering an axial flow. This approach requires a modification for the no-slip condition and that the current boundary conditions are partial-slip conditions. The Navier–Stokes equations are used to obtain the steady mean-flow system, and linear stability equations are then formulated to obtain neutral stability curves while investigating the convective instability behavior for stationary modes. The stability analysis results are then confirmed by the linear convective growth rates for stationary disturbances and the energy analysis. The stability characteristics of the inviscid type I (or cross-flow) instability and the viscous type II instability are examined over a rough, rotating disk within the boundary layer at all axial flow rates considered. Our findings indicate that the radial grooves have a strong destabilizing effect on the type II mode as the axial flow is increased, whereas the concentric grooves and isotropic surface roughness stabilize the boundary-layer flow for the type I mode. It is worth noting that the flows over a concentrically grooved disk with increasing enforced axial flow strength are the most stable for the inviscid type I instability.

Impacts of terrain on convective surface layer turbulence over central Himalaya based on Monin–Obukhov similarity theory

In the context of non-uniform Monin–Obukhov similarity scaling of unstable surface layer turbulence over complex terrain, this study attempts to identify the range of slope angles (as a proxy for terrain complexity) over which convective turbulence behaviour is similar to that over flat terrain. Attempts are also made to optimize constants of numerical relationships between integral turbulence parameters (ϕu,v,w) and the stability parameter (z/L) for observations having a significantly strong signature of terrain undulation. High frequency wind observations during the Indian winter season obtained from two field stations (on-slope and ridge-top) in the central Himalaya near Kosi-Katarmal, Uttarakhand, India, are analysed. Convective day-time period observations from rain-free days at these two sites are used with measures of terrain heterogeneity, represented by the vertical wind-derived slope angle (θw), vertical wind-derived slope angle normalized with respect to maximum θw (θwˆ), and the terrain-derived slope angle (θt). When the relationship of wind-derived slope angle and terrain derived slope angle was assessed, we found that wind-derived slope angle(θw) sufficiently represents the terrain undulation with a correlation coefficient of 0.86 at a p-value < 0.001. Optimization was undertaken of the numerical relationships between the integral turbulence parameters (ϕu,v,w) and z/L for observations having a significantly strong signature of terrain undulation characterized by |θˆw|> 0.2. Results indicated that for selected runs with increasing terrain complexity, c1 decreases and d1 increases in ϕu,v,w=c1(1−d1zL)1/3 having ranges of c1 and d1 between 0.85–2.39, and 0.98–13.85, respectively.

Analysis of Marangoni boundary layer flow and heat transfer with novel constitution relationships

Based on traditional constitutive relationships of Maxwell fluid flow and Cattaneo heat transfer, a modified model of unsteady Marangoni convective boundary layer flow and heat transfer behavior is constructed, which introduces the distributed fractional-order derivative. The governing equations and the corresponding initial and boundary conditions are derived, in which the finite difference method with L1 scheme is used to solve the obtained equations and the solvability is analysed. Afterwards, a pair of source terms are added into the momentum equation and energy equation respectively to facilitate obtaining the exact solution, and the validity of the numerical scheme is verified by comparing with the exact solution. The influences of the physical parameters on the thickness of the velocity and temperature boundary layer and rheological characteristics with the flow and heat transfer mechanism are discussed and analysed in detail.

The interaction of deep convection with the general circulation in Titan’s atmosphere. Part 2: Impacts on the climate

The impact of methane convection on the circulation of Titan is investigated in the Titan Atmospheric Model (TAM), using a simplified Betts–Miller (SBM) moist convection parameterization scheme. We vary the reference relative humidity (RHSBM) and relaxation timescale of convection (τ) parameters of the SBM scheme. Titan’s atmosphere is mostly insensitive to changes in τ, but convective instability and precipitation are highly impacted by changes in RHSBM. Convection behavior changes from infrequent (<1 per Titan year), intense events at summer solstice that quickly encompass the entire globe at low RHSBM to near-continuous precipitation at the poles during summer at high RHSBM (85%). The intermediate regime (RHSBM=70%–80%) consists of frequent events (∼10 per Titan year) of moderate intensity that are limited in meridional extent to their respective hemisphere. Using results from the Titan Regional Atmospheric Modeling System (TRAMS) and observations, we tune the parameters of the SBM parameterization with optimum values of RH=80% and τ=28800 s. We present a simulated decadal climatology that qualitatively matches observations of Titan’s humidity and cloud activity and generally resembles previous results with TAM. Comparing this simulation to one without moist convection demonstrates that convection strengthens the meridional circulation, warms the mid-levels and cools the surface at the poles, and magnifies zonal-mean global moisture anomalies.

The onset of natural convection in a horizontal nanofluid layer heated from below

AbstractA linear stability analysis is performed for the onset of natural convection in a horizontal nanofluid layer heated from below. The motion of nanoparticles is characterized by both the thermophoresis and Brownian diffusion effects. Different from previous studies in the literature, both the dependences of thermophoresis on nanoparticle volume fraction and Brownian motion on temperature are taken into consideration in the theoretical model. The result reveals that the base flow is mainly dominated by the effect of thermophoresis and the Brownian diffusion coefficient can be treated as a constant reasonably when a finite temperature difference is imposed across the nanofluid layer. Accordingly, a novel base solution of nanoparticle volume fraction is derived. It is found that the profile of nanoparticle concentration depends heavily on the magnitude of thermophoretic diffusion, which may exhibit a nonlinear distribution across the nanofluid layer once the effect of thermophoresis is significant. The suspended nanoparticles produce a strong destabilizing effect and a tiny volume fraction of nanoparticles is sufficient to trigger the onset of convection and make the nanofluid layer become unconditionally unstable. The dispersion spectra of unstable modes are demonstrated and the most unstable mode with the maximum growth rate is explored. The growth rate of the most unstable mode is found to increase significantly with increasing nanoparticle concentration, while the influence of heat capacity ratio of nanoparticle to base fluid on the behavior of thermal convection is negligible.

Numerical study of fluid flow and heat transfer for flow of Cu-Al2O3-water hybrid nanofluid in a microchannel heat sink

The use of micro-electronic devices in many applications such as aerospace, telecommunications, micro-electronics, robotics is drastically increasing and they need efficient cooling system. Micro-channel heat sinks (MCHS) are getting significant importance due to their very high surface to volume ratio and high heat dissipation rate. The hybrid nanofluids playing a vital role as cooling fluids in various thermal transportation applications due to their extraordinary heat transport characteristics. A numerical study is taken up for laminar flow of hybrid nanofluid as a heat transport medium in a rectangular MCHS to realize thermal and hydrodynamic behaviour in forced convection. The influence of Reynolds number (Re) and volume concentration of nanoparticles in Cu-Al2O3/water hybrid nanofluid on MCHS performance is investigated in the present paper. The correlations presented in the literature are used to calculate the properties of hybrid nanofluids. For various mixture ratios of 25–75%, 50–50% and 75–25% hybrid nanofluids the heat transport rate and pressure drop are calculated at different Reynolds numbers and compared with mono nanofluids of Cu-water (H2O) and Al2O3-water. An enhancement in heat transfer rate is observed with raise in volume fraction and marginal increase in pressure drop is observed for hybrid nanofluids over mono nanofluids of Cu-water and Al2O3-water. The results indicate an increase of 13.2% and 23.07% in Nusselt number for Cu-Al2O3/water hybrid nanofluid compared with Cu/water and Al2O3/water mono nanofluids respectively at 2.5% volume fraction. It is found from the numerical results that the pumping power requirement is less for hybrid Cu-Al2O3/water nanofluid compared to Cu/water nanofluids and an increase of 430 Pa compared to pure water. The data existing for Al2O3-water nanofluid in the literature is used to validate the present model.

Convective self–aggregation in a mean flow

Abstract. Convective self-aggregation is an atmospheric phenomenon seen in numerical simulations in a radiative convective equilibrium framework thought to be informative of some aspects of the behavior of real-world convection in the deep tropics. We impose a background mean wind flow on convection-permitting simulations through the surface flux calculation in an effort to understand how the asymmetry imposed by a mean wind influences the propagation of aggregated structures in convection. The simulations show that, with imposing mean flow, the organized convective system propagates in the direction of the flow but slows down compared to what pure advection would suggest, and it eventually becomes stationary relative to the surface after 15 simulation days. The termination of the propagation arises from momentum flux, which acts as a drag on the near-surface horizontal wind. In contrast, the thermodynamic response through the wind-induced surface heat exchange feedback is a relatively small effect, which slightly retards the propagation of the convection relative to the mean wind.

Evaluating the heat and mass transfer effective coefficients during the convective drying process of paddy (Oryza sativa L.)

AbstractThis work investigates important heat and mass transfer parameters of a variety of paddy named Fajr, during a thin layer convective drying. The experiments on drying are conducted at air temperatures in the domain of 30–80°C with air velocities between 0.54 and 3.27 m s−1. Mathematical equations are developed to adjust the experimental data obtained for the thin layer drying. The drying results show that the Henderson and Pabis' model satisfactorily fit all the experimental data for the convective drying behavior of paddy. The effective diffusivity of moisture transfer, calculated by the Fick's second law of diffusion, varies from 1.109 × 10−11 to 5.858 × 10−10 m2 s−1 and the highest values of the Dincer, Biot, Reynolds, and Nusselt numbers obtained are 2.383 × 105; 0.2026; 1.833 × 107; and 8,434, respectively. The obtained results demonstrate that the coefficient of mass transfer (hm) for paddy lies between 5.118 × 10−8 and 4.807 × 10−6 m s−1, while the heat transfer coefficient (hc) ranges from 811 to 2,474 W m−2 K−1. Furthermore, the values of the energies of activation obtained for moisture diffusion (Ed) and convective mass transfer (Ec) are in the range of 50.538–61.825 kJ mol−1 and 72.325–87.386 kJ mol−1, respectively. Therefore, knowledge regarding to the determination process of heat and mass transfer characteristics can help to distinguish the suitable operating conditions for saving the maximum value of energy.Practical ApplicationsSome grains have a short shelf life that limits their commercialization as fresh products and increase postharvest losses. Drying is an interesting alternative to the development of new products of grains like paddy with added value. This research work involved the estimation of heat and mass transfer properties during convective drying by using two calculation methodologies, allowing estimating the diffusivity and the mass and heat transfer coefficients for paddy. Also, the activation energy for moisture diffusion and for convective mass transfer was determined, assuming that the temperature dependence of the diffusion coefficient and the mass transfer coefficient follows an Arrhenius type relationship. Therefore, also the study presents a simple method to greatly enhance the shelf life of wet paddy by convective drying and it can be applied for the better preservation of this product.

Convective heat transfer and fluid flow characteristics in fin and oval-tube heat exchanger

The forced convective heat transfer behavior of a turbulent air flow, steady and Newtonian over a fin and oval-tube heat exchanger has been examined numerically. Where, the effect of the tube tilt angle (α) on the heat transfer coefficient and the friction factor was tested. The inclination angle of the oval-tubes going from 0° (Baseline case) to 90° with a step of 10°. The fluid flows and heat transfer characteristics are presented for Reynolds numbers ranging from 3.000 to 12.000. All investigations are carried out with the help of the CFD ANSYS Fluent. Heat transfer coefficient results in the term of the Nusselt number are validated with the available experimental data and a maximum deviation of 9 % is observed. Reasonable agreement is found. The obtained results show that the tube's inclination angle of 20° is the best design which significantly removes the hot spots behind the tubes, thus giving an increase in the heat transfer coefficient of 13 % compared to the baseline case. In addition, useful correlations are developed to predict Nusselt number and friction factor in the fin and oval-tube heat exchanger.

Study on Convective Flow Behaviors of Phosphor Particles During Curing Process of Silicone and the Influences on the Optical Performance of White LEDs

The correlated color temperature (CCT) variation of the phosphor-converted light-emitting diodes (LEDs) after curing is a common phenomenon. In the past, it was believed that the phenomenon was caused by the sedimentation of phosphor particles. However, not only sedimentation, but these particles can also flow with the natural convection of silicone because of nonuniform heating during curing. The convective flow of the phosphor particles during curing was studied in detail in this work. The CCT variation and phosphor particles flow in the LED device cup with different depths and wall inclination angles were studied. The results show that the CCT increases after curing when the depth of the device cup is shallow. The case is the opposite when the depth of the device cup is deep. The convective flow behaviors of the silicone were revealed in the simulation. The large-angle inclined wall can increase the silicone flow path length and inhibit the particles flow downward caused by the flow to a certain extent. It is envisioned that the findings will strengthen the understanding of the CCT variation phenomenon and guide the encapsulation structure design of LEDs for better color quality.

Simulation of the onset of convection in a porous medium layer saturated by a couple-stress nanofluid

Linear and nonlinear stability analyses for the onset of time-dependent convection in a horizontal layer of a porous medium saturated by a couple-stress non-Newtonian nanofluid, intercalated between two thermally insulated plates, are presented. Brinkman and Maxwell–Garnett formulations are adopted for nanoscale effects. A modified Darcy formulation that includes the time derivative term is used for the momentum equation. The nanofluid is assumed to be dilute and this enables the porous medium to be treated as a weakly heterogeneous medium with variation of thermal conductivity and viscosity, in the vertical direction. The general transport equations are solved with a Galerkin-type weighted residuals method. A perturbation method is deployed for the linear stability analysis and a Runge–Kutta–Gill quadrature scheme for the nonlinear analysis. The critical Rayleigh number, wave numbers for the stationary and oscillatory modes, and frequency of oscillations are obtained analytically using linear theory, and the nonlinear analysis is executed with minimal representation of the truncated Fourier series involving only two terms. The effect of various parameters on the stationary and oscillatory convection behavior is visualized. The effect of couple-stress parameter on the stationary and oscillatory convections is also shown graphically. It is found that the couple-stress parameter has a stabilizing effect on both the stationary and oscillatory convections. Transient Nusselt number and Sherwood number exhibit an oscillatory nature when time is small. However, at very large values of time, both Nusselt number and Sherwood number values approach their steady-state values. The study is relevant to the dynamics of biopolymers in solution in microfluidic devices and rheological nanoparticle methods in petroleum recovery.

Thermo-economic and entropy generation analyses of magnetic natural convective flow in a nanofluid-filled annular enclosure fitted with fins

A numerical analysis on the thermo-natural convection as well as entropy generation of Al2O3-H2O nanofluid enclosed by two circular cylinders in the presence of magnetic fields was performed. The internal hot cylinder is fitted with rectangular fins of different lengths. Irreversibilities related to the thermal effects, friction effects, magnetic effects were considered. FEM was selected as solving method. Results described the impact of active parameters on the thermo-natural convective flow and heat transfer behaviour as well as entropy generation characteristics. In addition, a basic economic analysis has been proposed to consider the cost of nanofluids in comparison to their contribution in enhancing heat transfer rate. Results show that significant effects of pertinent parameters e.g. Rayleigh, Hartmann, and fins' size on flow, heat transfer, and irreversibilities within annulus. It is also found that for low Rayleigh, irreversibility related to thermo-effects is predominant within the annulus, and by augmenting Rayleigh values, the irreversibility due to the thermo-effects is no longer the key contributor to overall entropy production. The magnetic forces help to increase thermo-effects irreversibility contribution to overall irreversibilities. Finally, thermal transfer enhancement in case of the presence of magnetic forces is found to be more costly in terms of nanofluid utilization.

Low-melting-point liquid metal convective heat transfer: A review

Low-melting-point liquid metal convection is rapidly emerging as a high-performance heat transfer technology in electronics thermal management and energy fields. The advantages of gallium-based and bismuth-based liquid metals, such as low melting point, high thermal conductivity, nonflammability, and nontoxic characteristic, make liquid metals highly attractive for high heat flux density applications at high temperatures. Their convective heat transfer behavior, driving techniques, and application systems are different from those of conventional liquids, such as water, oil, sodium-potassium, and lead-bismuth. Although their significance in both academia and industry has gradually increased, to the best of our knowledge, a systematic description of gallium-based and bismuth-based liquid metal convection and its applications has not been reported thus far. Therefore, in this paper, we present a thorough review of low-melting-point liquid metal convective heat transfer technologies. Specifically, liquid metal fluids and their convection mechanisms are introduced first, followed by the description of typical driving techniques of liquid metals based on electromagnetic, thermal, electrical and magnetic methods. Subsequently, applications of typical liquid metal convection methods in industrial heat transfer and energy fields are presented. Finally, extended liquid metal convection enhancement techniques based on microchannel and jet impingement are interpreted. Both the fundamental mechanisms and recent application research are elaborated, and critical issues are discussed. The scientific and technical challenges, along with future developments in these areas, are also highlighted in the paper.