Natural Convection Regime Research Articles

Convective heat transfer efficacy of an experimentally observed non-Newtonian MWCNT-Fe[formula omitted]O[formula omitted]-EG hybrid nanofluid in a driven enclosure with a heated cylinder

This study provides an in-depth analysis of the heat transfer (HT) characteristics of a hybrid nanofluid (HNF) composed of Multi-walled carbon nanotubes (MWCNT), Iron oxide (Fe3O4), and Ethylene glycol (EG), which is loaded in an enclosure with a heated cylinder. As per the exploratory report, this HNF exhibits Newtonian and non-Newtonian behavior depending on the percentage of nano-constituents suspended in the base fluid EG. The governing equations describing the convective HNF flow associated with the boundary conditions are numerically solved using the finite volume method. The study explores the HT performance of a loaded HNF by varying the volume fractions (ϕ), the driving condition of the vertical sides, and the aspect ratio (W∗) of the enclosure. The study investigates the variation of the Richardson number (Ri=0.01∼100) concerning the interactions between shear and buoyancy forces executed on the HNF. The study finds that the combination of the lower aspect ratio W∗=2.1 filled with non-Newtonian HNF with ϕ=1.8% in the forced convection regime (Ri=0.01) and the condition of both aiding and opposing forces applied to the side walls shows superior HT performance. The non-Newtonian HNF with ϕ=1.8% shows HT enhancement of 9.27% for aiding forces and 39.63% for opposing forces when Ri=0.01, revealing ultimate enhancement in the average Nusselt number (Nuavg). The study also investigates entropy production (Savg) from a thermodynamics perspective to assess the system’s thermal performance. The value of Savg is about 11.54 for ϕ=1.8% and Ri=0.01, and that value is comparable for both aiding and opposing force conditions. Results conclude that HT consistently enhances with nanoparticle addition in the forced convection regime. However, HT enhancement is only observed for non-Newtonian cases of HNF in the natural convection regime, suggesting the limitation of the HNF usage in the HT study. Additionally, the study discusses and compares different simulation results to comprehensively evaluate the HT characteristics and the system’s thermal performance, guiding the best benefit of the studied hybrid nanofluids in a thermal engineering system.

A Novel Plate Fin Heat Sink Design With Rectangular Slots and Interruptions: A Computational Approach

Abstract In the present work, we have studied the performance of vertical plate finned heat sinks that protrude from a vertical base. The difference between the heat sink base temperature and the ambient, i.e., ΔT, has been varied in the range of 10 °C to 60 °C, and the flow undergoes a natural convection regime. To enhance the thermal performance, we have explored different configurations of the heat sink by providing rectangular slots, varying the neck thickness, changing the neck location from the fin base, and providing interruptions along the fin height. The pertinent quantities, i.e., heat dissipation rate, Nusselt number, effectiveness, mass of heat sink, and heat dissipation per unit mass, have been obtained by performing 3D computational simulations. The results obtained are compared to assess the thermal performance of heat sinks. We found that among various designs of heat sinks proposed, the heat sink with two slots, with the location of neck closer to the fin base (xm = 9 mm), and with interrupted fins dissipates maximum heat (12.86% more compared to the commonly used rectangular plate finned heat sink). In addition to the heat transfer improvement, 19.82% mass reduction has also been achieved. Based on the simulation data, we have proposed a correlation for the mean Nusselt number as a function of relevant non-dimensional parameters.

Mixed Convection in a Lid-Driven Cavity Filled with Nanofluids Using Single- and Two-Phase Eulerian Modeling Methods

The numerical simulation of the classical lid-driven cavity problem has been carried out to investigate the suitability of two-phase flow modeling techniques for nanofluids in computational fluid dynamics. The nanofluid investigated comprises water as base fluid and Al2O3 nanoparticles. Three types of the Eulerian-multiphase models, including the Eulerian, mixture, and volume of fluid (VOF) were compared with the single-phase model. The model equations were solved using ANSYS Fluent software for the nanoparticle volume fraction, the Richardson and Reynolds numbers in the range 0 ≤ ø ≤ 0.10, 10−4 ≤ Ri ≤ 102, and 1 ≤ Re ≤ 1000, respectively at a fixed Grashof number, Gr = 100. The results were compared with that of single-phase nanofluid modeling. There were similarities in the flow structure and temperature distribution for the single-phase and multi-phase methods when the convection is natural and mixed. However, the Nusselt number computed by the mixture and Eulerian models is higher than that of the single-phase and VOF models under the forced convection regime, with the percentage deviation from that of the single-phase as high as 10%. So, the three multiphase models are suitable for nanofluid convection problems and give results comparable to the single-phase model, especially under the natural and mixed convection regimes.

Transition of natural convection on a cooled circular surface with heated annular extension

Transition of natural convection on a circular cooled surface with heated annular extension is investigated in the present study by numerical simulation. Flow features and regimes of natural convection on the surface are studied for which the cooling and heating Rayleigh numbers (Rac and Rah) vary from 1 to 106 with air as the working fluid (Prandtl number of 0.71) at a fixed geometrical aspect ratio of 1 (heating characteristic length/cooling characteristic length). Three steady states revealed by numerical results include steady plume flow, steady dome flow and steady crown flow, which are corresponding to different dynamic regimes: a positive buoyancy dominated regime, a transitional regime and a negative buoyancy dominated regime. Besides, the dynamics behaviors in those steady regimes are scaled and validated by numerical results. The transition route of natural convection on the cooled circular surface with heated annular extension to the unsteady flow is also characterized in the case of Rac/Rah = 1. It has been demonstrated that the steady regime is followed by the unsteady regime. The periodic puffing flow, quasi-periodic flow, and chaotic flow are present in the unsteady regime. In addition, heat transfer in different regime flows is quantified.

Influence of smooth heater size on critical heat flux and heat transfer coefficient of saturated pool boiling heat transfer

In order to study the influence of smooth heater size on the performance of pool boiling heat transfer, saturated boiling experiments of FC-72 were conducted on eight sized smooth silicon chips (the test surfaces are denoted as S5 ∼ S30, and their dimensionless dimensions are in the range of Lh/Lc = 7 ∼ 42). The bubble dynamics during boiling were observed with a high-speed camera. The results indicated that in natural convection regime, the heat transfer coefficient decreases with increasing heater size at a given heat flux. Besides, the larger surfaces enter into the nucleate boiling regime earlier. In a fully developed nucleate boiling regime, S5 surface displays the optimal heat transfer performance. The reality is that no vapor columns are produced even in the high heat flux range due to its small bubble departure diameter and high bubble departure frequency. For S8 ∼ S15 surfaces, the wall temperatures increase continuously with the increase of heat flux, while S20 ∼ S30 surfaces present a trend of first keeping constant and then increasing. The active nucleation site densities on S15 ∼ S30 surfaces follow the law of Na = 0.45q2 in the whole nucleate boiling regime. By contrast, for S5 ∼ S12 surfaces, the nucleation site densities abide by this law only in a fully developed nucleate boiling regime. In addition, the departure diameter and departure frequency of bubbles increase with the increase of heat flux for all surfaces. It is observed that with the increase of smooth heater size, CHF presents a changing trend of first increasing (in range of Lh ≤ λD, λD is the most dangerous Taylor wavelength), then decreasing (in range of λD < Lh ≤ 3λD) and finally leveling off (in range of Lh > 3λD). Accordingly, the CHF value maximizes at the heater size Lh = λD. In the segment of decreased CHF, the decline rate of CHF in range of λD < Lh ≤ 2λD is larger than that in range of 2λD < Lh ≤ 3λD. Meanwhile, the influencing mechanisms of heater size on CHF were explained by consideration into bubble patterns and behavior.

Heat transfer characteristics for multi-silicon ingots irradiation in a typical research reactor

A computational fluid dynamics (CFD) study is conducted for silicon ingot irradiation facility in a typical material testing reactor (MTR). The ingots are placed inside the irradiation device in staggered and in-line bundles. The configurations are varied according to the variations of longitudinal and transverse pitches Sl and St, respectively. For in-line bundles, Sl and St are changed in the range of 1.1, 1.2, 1.3, 1.4, and 1.5 of the ingot diameter. For staggered bundles, Sl is changed in the range of 1.1, 1.2, 1.3, 1.4, and 1.5 of the ingot diameter while St is changed in the range of 0.95, 1.0, 1.05, 1.1, and 1.15 of the ingot diameter. The silicon ingot volumetric heat generation Qv is allowed to change around its nominal value in the range of 0.25, 0.5, 0.75, 1.0, and 1.25 W/g. The effects of rotation of ingots during irradiation at velocities of 15, 30, and 60 rpm are investigated. The computational fluid dynamics code ANSYS FLUENT is employed to carry out the calculations. The temperature profile inside the silicon ingot during irradiation is of significant importance for uniform doping. Therefore, the impact of heat generation and bundle pitches on the temperature profile through the ingot is calculated and reported for all cases of this study. The results show that the maximum ingot temperature does not exceed the limits for each bundle at maximum heat generation. Two correlations for average Nusselt number for both aligned and staggered configurations are generated to describe heat transfer rates during natural convection regime. The rotation of ingots during irradiation showed considerable enhancement of heat transfer rates.

Natural convection heat transfer from upward‐facing plates in saline water

AbstractSaltwater or brackish water is used as a coolant in most industries. Therefore, understanding the heat transfer processes and hydrodynamics during the natural convection in saline water is crucial for enhancing the efficiency of a heat exchanger. This study elaborates on the natural convection heat transfer in saline water under atmospheric conditions. A DC power supply is used to regulate the power given to the heater in a liquid pool for thermal analysis. The pool liquid comprises solutions with varying salinity from 0%, 0.2%, 0.5%, and 2%. The effect of varying salinity on the heat transfer coefficient and the thermal aspects encountered during the desalination process is analyzed. The temperature distribution across the surface of the heater is monitored using an infrared camera. It is studied for the solution of different salinities. The heat transfer coefficient and Nusselt number are investigated during natural convection for normal water and salt solution of different concentrations. It is inferred from the study that in the regime of natural convection, there is no significant difference in the Nusselt number for normal water and saltwater for the lower value of temperature difference between the plate and pool. The heat transfer coefficient in 0.2% saline water is higher as compared to the other solutions.

On the Role of Gravity in Dissolving Horizontal Fractures

AbstractFractures are ubiquitous in geological systems. As reactive fluid flow through a fracture, dissolution of the fracture walls may occur, thus altering the fracture aperture and increasing permeability. It has been recognized that gravity plays an important role in dissolving vertical fractures due to buoyancy‐driven convection. However, the role of gravity in dissolving horizontal fractures is not well understood. Here, we combine microfluidics/Hele‐Shaw experiments and a numerical method to study how the interplay of buoyancy‐driven convection and forced convection controls dissolution dynamics and permeability increase in horizontal fractures. We first develop the micro‐continuum approach by incorporating the gravitational effects, and then, we perform experiments to validate our method, confirming that the method can well capture gravitational effects on dissolution. Through 3D simulations, we find that buoyancy‐driven convection breaks the symmetry of dissolution on the upper and lower surfaces. We employ a symmetry index to identify three dissolution regimes. As the importance of gravity increases (Ri increases), the dissolution regime shifts from forced convection to mixed convection and to natural (or buoyancy‐driven) convection. We establish a link between these dissolution regimes and permeability evolution. In the forced convection or natural convection regimes, the permeability nearly remains unchanged for various Ri. However, in the mixed convection regime, permeability increase is suppressed by the gravitational effects; the underlying mechanism is that the solid phase tends to dissolve near the fracture inlet due to gravitational instability. This work improves our understanding of the gravitational effects on dissolution regimes and permeability evolution in horizontal fractures.

Numerical Study of the Effects of a Hot Obstacle on Natural Convection Flow Regimes

A numerical study was conducted for natural turbulent and laminar convection induced by a circular hot obstacle in two different positions, the heat obstacle is introduced at the inlet of the cylinder and inside the cylinder. To determine the effects of a circular hot obstacle on the regime of the thermosiphon flow generated by a heated cylindrical channel, we suggest a theoretical and numerical model using the Navier–Stokes equations and the finite volume method. An experimental apparatus was used for the validation of the numerical model. A comparison of the numerical and experimental results showed a good agreement in the global flow behavior. The temperature and vertical velocity profiles demonstrated the presence of a boundary-layer regime along the heated channel's wall in the absence of a hot obstacle and the presence of the hot obstacle at different positions. The introduction of the hot obstacle generates a significant modification of the evolution of the flow above the hot obstacle. The numerical simulation was conducted for a specific radius of the hot obstacle that ensures the existence in the laminar flow regime, and different Rayleigh numbers (Ra=105; 107; 108; 109; and1010). The flow rate, velocity, temperature and Nusselt number profiles were presented and discussed. The effects of the hot obstacle are presented in our results by two maxima of the vertical component of the velocity profile. The temperature profile at the entrance of the vertical cylinder is presented by the appearance of a maximum above the hot obstacle. As the cylinder height increases, the air temperature decreases. The vertical displacement of the obstacle upward induces a modification in the behavior of the dynamical flow. A CFD code was used to study the different natural convection regimes presented by three different form factors, including the critical regime with a form factor of A=0.1.

Numerical analysis on heat transfer and melting characteristics of a solid-liquid phase change process in a rectangular cavity inserted with bifurcated fractal fins

To improve the thermal transmission performance of latent heat thermal energy storage systems, bifurcated fractal fins inspired by leaf venation structures in nature are inserted into a typical vertical rectangular cavity. Melting thermal behaviors of lauric acid in the models with bifurcated fractal fins having different fin location ratios εd, fin length ratios εl, and fin numbers N are numerically studied. Increasing εd, εl, and N yields the enhancement of liquid fractions, and a maximum melting time reduction of 63.6% is achieved by the case with εl = 0.75 and N = 3. Raising εd is conducive to melting convection flows during the natural convection regime. Whereas, the effect of εl is primarily driven through thermal conduction area enhancement. In addition, the results imply that as εl increases, the impact of increasing N leads to a shift in flow intensity from enhancement to suppression. As against the flat fin, the bifurcated fractal fin has a superior thermal transmission efficiency thanks to the larger heat transfer surface area, and the benefit is more pronounced as the branching level changes from k = 2 to k = 3.

A complete CFD study on natural convection in the TRIGA Mark II reactor

This work describes the most complete and up-to-date thermal-hydraulics CFD model for studying the natural circulation regime within the entire TRIGA Mark II reactor at the University of Pavia. Building on preliminary works, this paper tackles the treatment and investigation of natural circulation in the reactor core and pool, highlighting the coupling between the two components. Despite the assumptions behind the model – such as the use of an unsteady RANS turbulence model and the treatment of the supporting grids as a porous media – the validation compared to experimental data shows quite a good agreement. The model is thus accurate enough to describe the natural convection regime within the core and the pool and highlight local phenomena, such as localised sub-cooled boiling and recirculation.

Experimental investigation of mixed mode ultraviolet tent house solar dryer under natural convection regime

A mixed mode natural convection ultraviolet (UV) tent house solar dryer is designed and installed at University Teaching Department, Chhattisgarh Swami Vivekanand Technical University, Bhilai, Chhattisgarh (21°27′N, 81°43′E) India for drying potato slices. This solar dryer is equipped with a solar flat plate collector (area 1.12 m2), covered with 5.0 mm thick glass and consists of two trays of the same size (0.57 m X 1.11 m) in a drying chamber on which equal quantities of potato slices were placed. The local atmospheric conditions including relative humidity, atmospheric temperatures, solar intensity, and wind velocity have been recorded for analyzing various parameters. The solar intensity and atmospheric temperature ranges from 134 to 993 W/m2 and 36.2 °C to 44.9 °C respectively. The initial moisture content value of 85.25% reaches 14.75% for 2.5 mm thick potato slices and for 5.0 mm thickness, the initial & final moisture content values are 85.10% & 14.90% respectively. The upper and lower tray temperature varies from 34.2 °C to 59.7 °C and 33.6 °C to 57.1 °C respectively. The open sun drying took 6 h longer than the solar natural convection drying. Moisture ratio for 2.5 mm thickness and 5.0 mm thickness lies 0.475–0.001 and 0.650–0.003 respectively. The maximum dryer efficiency is 26.62% and 21.61% for 2.5 mm and 5.0 mm thick potato slices respectively. All the drying kinetics is superior in the 2.5 mm thickness of the potato in process of drying using natural convection.

Modelling of pool boiling on the structured surfaces using Lattice Boltzmann method

The process of boiling on spatially structured surfaces is simulated by a hybrid model based on Lattice Boltzmann Method and heat transfer equation. The model permits to study heat transfer at boiling in a wide range of surface superheats for different surface structural characteristics. The regimes of natural convection, nucleate boiling, and transition to film boiling are studied. The boiling curves for the surfaces with different structural and wetting properties are obtained. It was shown that the onset of nucleate boiling occurs at lower wall superheat on the structured surfaces than that on the smooth surface. However, at high wall superheats the heat flux and the critical heat flux at modified surfaces are lower. It was obtained that special modification of both structural and wetting properties of heat exchange surface permits to obtain higher removal heat flux as well as higher critical heat flux.

Magnetohydrodynamic Natural Convection of Water in an L-Shaped Container Filled With an Aluminum Metal Foam

Abstract The current research employed computational simulation to assess magnetohydrodynamic (MHD) natural convection in an L-shaped container with a metal foam, which has a variety of engineering applications. The originality of this study is the use of numerical simulation combined with multi-objective optimization in terms of cooling rate enhancement and entropy generation reduction to investigate the optimal natural convection regimes. The influence of varied aspect ratios (AR), tilted angles of the container (θ), Hartmann number (Ha), and porosities (Φ) on the cooling rate with regard to the average Nusselt number (Nuave), Nusselt number enhancement (NNE), dimensionless surface temperature ([Ts]) and entropy generation (S) were evaluated. According to the findings of this study, little effect was found to be exerted by the MHD and θ on the Nuave, Ts, and S except for a porosity of 0.9. Furthermore, the Nuave and NNE enhance while the Ts and S reduce as the aspect ratio AR increases. In this case, the maximum increase in NNE was 8.78 times, with the greatest reductions in Ts and S as AR = 0.8, Φ = 0.1, θ = 0 deg, and Ha = 0 compared to the least favorable design. Thus, this optimum investigation of the CFD yields a novel approach and valuable recommendations for the optimum design of cooling in an L-shaped container.

Direct numerical simulation of turbulent heat transfer in liquid metals in buoyancy-affected vertical channel

This article investigates the impact of buoyancy force in a vertical channel with combined natural and forced convection liquid metal flow using direct numerical simulation (DNS). In particular, a liquid metal with Pr = 0.025 and Re = 4667 is considered in the combined natural and forced convection regime (Ri = 0, 0.25, 0.50 and 1.00). The liquid metal is driven upward while buoyancy aids the flow close to the hot wall and opposes the flow close to the cold wall. The influence of buoyancy on turbulent heat transfer and mean flow are analyzed. Turbulent transport is enhanced in aiding flow and weakened in opposing flow with increasing Richardson number for low-Prandtl-number fluids. The enhancement of normalized Reynolds stresses is more significant in the opposing flow compared to the aiding flow. Turbulent heat flux also increases and, consequently, enhances heat transfer. Besides, the budgets of streamwise and wall-normal turbulent heat flux, temperature variance, Reynolds shear stress and turbulent kinetic energy are investigated and related to the variations of the quantities in mean fields. The budgets of temperature variance and turbulent kinetic energy are nearly balanced by shear production and dissipation. As for the budget of Reynolds shear stress, streamwise and wall-normal turbulent heat flux, temperature pressure-gradient correlation contributes to the budget significantly in addition to production and dissipation. The DNS results can not only shed light on the heat transfer mechanism of turbulence in liquid metal flows, but also be used for validating and improving turbulent models for low-Prandtl-number fluids, which could be useful for developing Gen IV liquid-metal-cooled fast reactors.

Effects of nonlinear optical parameters on the thermal performance of an indirect solar dryer under natural convection regime

Effects of nonlinear optical parameters on the thermal performance of an indirect solar dryer under natural convection regime

The Succession of Heat and Mass Driven Natural Convection Regimes Along a Vertical Impermeable Wall

Abstract This paper presents the analysis of the natural convection process that takes place near a vertical plane wall embedded in a constant temperature and linearly mass stratified fluid (the Prandtl number and the Smith number are smaller than 1.0, while the Lewis number is greater than 1.0). The wall has a constant temperature, while the flux of a certain constituent is constant at this boundary. The scale analysis and the finite differences method are used as techniques of work. The scale analysis proves the existence, at equilibrium, of heat and/or mass driven convection regimes along the wall. The finite differences method is used solve the governing equations and to verify the scale analysis results using two particular parameters sets.

Preliminary measurements of the thermal field in a rectangular channel by the Planar Laser-Induced Fluorescence method

This paper describes the preliminary measurements of the temperature field in water channel by the Planar Laser-Induced Fluorescence (pLIF) method in the one-colour mode using commercial LaVision system. The aim of the work is to find suitable parameters of the experiment (concentration of fluorescent dye, laser power, arrangement of the experiment, etc.) for measuring the temperature field in a closed glass channel of rectangular cross-section with a heated bottom-wall and to verify an applicability of this method in general. The intention is to carry out measurements in the transverse plane of the duct such that the specific thermos-convective structures of the Rayleigh-Bénard-Poiseuille convection, i.e. mixed convection, are detected. For simplicity, the initial measurements were performed in the natural convection regime, i.e. the water in the channel was not driven by the circulation pump. In this case, a larger temperature gradient was created near the wall compared to the force convection case when a cold liquid flows continuously into the experimental area. The presented work serves as a methodological guide for future measurements and provides valuable experience for the design of a new channel for investigation of the thermal flow instabilities.

MHD mixed convection in a partitioned rectangular enclosure

A numerical study is carried out to explore the influence of external magnetic field on mixed convective heat transfer in a partitioned rectangular cavity with on side moving wall. The vertical walls are isothermally heated while the horizontal walls are thermally insulated. The left vertical wall is moving in + y direction and remaining walls are maintained no-slip condition. A magnetic field of uniform strength is imposed transverse to the temperature gradient. The governing equations are solved utilizing the finite element method for several physical parameters including Richardson number, Hartmann number and Prandtl number. The numerical results are presented graphically using streamlines, isotherms, local and average Nusselt numbers. It is observed that the flow filed is affected significantly for moving wall and the variation in Hartmann and Richardson numbers. The velocity field is found more effective in natural convection regime than forced convection. The results demonstrated that maximum amount of heat transfer is obtained in natural convection domination and higher values of Prandtl number. The enhancement of heat transfer rate is found 14.25% more at higher Prandtl number (Pr =2.56) than lower (Pr =0.71) and it reduction is found 3.03% more at Ha = 50 compared to Ha = 0.

Investigation on the thermal performance of a multi-tube finned latent heat thermal storage pool

• Innovative multi-tube finned latent heat thermal storage pool (LHTSP) is designed. • Charing/discharging duration of innovative LHTSP shortens by up to 29.4% and 22.8%. • Thermal performance is mainly determined by later charging/discharging stages. • Vertical installation facilitates minimizing the heat charging duration. • Inclination affects the heat charging process more than discharging process. The major challenge of multi-energy complementary systems consists of improving the thermal efficiency for latent heat thermal storage pools (LHTSPs). To address this, a new multi-tube LHTSP with tree-shaped fins is designed herein. The enthalpy-porosity approach is employed to model the charging/discharging process in LHTSPs, focusing on the role of inclinations in melting/solidification heat transfer. Moreover, experimental validation is conducted to ensure numerical reliability. The results show that tree-shaped fins effectively improve thermal efficiency and temperature uniformity. Compared with traditional LHTSPs, the innovative LHTSP shortens the total melting/solidification time by up to 29.4% and 22.8%, respectively, and improves the temperature uniformity by 12.3%∼19.2%. The difference in the influence regime of natural convection during melting and solidification lies in its onset and duration. Moreover, the thermal performance of LHTSPs is mainly related to the later charging/discharging stage. The inclination significantly affects the thermal charging performance, while it has less influence on the discharging processes. Compared to the horizontal arrangements, the innovative vertical LHTSP has a 46.3% reduction in the melting duration. Interestingly, there is a transition point in the heat storage efficiency of horizontal and vertical LHTSPs, and the LHTSP arrangement with a higher efficiency differs before and after this point.