Natural Convection Airflow Research Articles

Numerical investigation on the effects of non-uniform azimuthal heating in natural convection airflow within a vertical tube

Airflows established by buoyancy forces within vertical tubes with circular cross-section are analyzed. Obtained (finite-volume) numerical results cover the complete laminar-turbulent range (modified Rayleigh number from 10−3 to 109), through a low-Reynolds turbulence model. Both uniform and non-uniform heating conditions are regarded. Three different types of non-uniform heating along the azimuthal direction are considered, which constitutes an advance in the knowledge of the flow behavior, since it has not sufficiently addressed in the literature. Specifically, two modes of continuous non-uniform heating are proposed (temperature fixed at wall, and heat flux fixed at wall), and another mode through discrete heaters (for heat flux condition). The circumstances under which non-uniform azimuthal heating can be beneficial or detrimental with respect to thermal and dynamic performance are investigated. It is encountered that disrupting effect of discrete heat sources can induce, under given conditions, an improvement in the thermohydraulic performance.

Performance Characteristics of a Free-Breathing Polymer Electrolyte Fuel Cell with Various Channel Shapes

This study utilized a small free-breathing fuel cell, with an active area of 4 cm2 (2 cm x 2cm), and channel cathodes with various channel shapes, to experimentally investigate the effect of channel structure on cell performance, to numerically analyze the natural convection air flow in channels, and to compare with the results of performance characterisitics. The results indicated that, compared with a conventional cathode separator with four straight channels (with a width of 3 mm, a depth of 3 mm, and a rib width of 2 mm), cell performance was improved by eliminating some of the channel ribs and increasing the opening ratio, up to a point. A separator with 2 mm square ribs rotated by 45° further improved cell performance. However, when the opening ratio exceeded 72% with a straight center rib of 2 mm width, cell performance leveled off. The analysis results revealed that separators with modified channels promoted natural convection.

Microdischarge dynamics of volume DBD under the natural convection airflow

The dielectric barrier discharge (DBD) at ambient air conditions exhibits discrete structure and contains thin microdischarge plasma filaments. Understanding the formation, self-interaction, and dynamics of such filaments is crucial towards the generation of uniform diffuse-like DBD in air, and also for the study of the memory effects and self-organized complex patterns. In this paper, the impact of natural convective flow, driven by the temperature gradient between self-heated discharge cell electrodes and ambient air, on the collective dynamics of microdischarges was studied in parallel-plate volume dielectric barrier discharge for the different geometrical arrangements. The horizontal arrangement corresponds to the parallel direction of discharge propagation and convective flow, while in the vertical arrangement buoyancy flow transverse to the discharge propagation column. For the horizontal arrangement, the randomly directed motion of microdischarges was observed. While at the vertical arrangement of the discharge cell, the buoyancy flow initiates the directed motion of the microdischarges following the gas flow. The continuous self-heating of the barrier electrodes during the DBD operation leads to a larger thermal gradient and increasing in the microdischarge channels. The larger thermal gradient results in a more pronounced directed motion of microdischarges with a higher velocity. The velocity of convective flow in the discharge gap was estimated by the simulation and compared with the mean velocity of MD channels obtained by the particle image velocimetry method.

Investigation the influence of inclination angle for flat plate solar air collector through experiment and simulation

The heat behaviour of a flat plate solar collector (FPSC) has been analyzed experimentally and numerically under natural convection air flow. The thermal energy transferred through the air by heat convection and heat radiation. The data collections for the solar collector includes various of the inclination angles (15°, 30°, 45°, and 60°). The conventional glazed solar collector has a very low efficiency. Hence, the collector is unglazed in this study. It also covered with the insulation on both sides and at the bottom plate to prevent heat loss. The analysis concluded that rising solar radiation causes temperature to rise. Besides that, it is found that the studies conducted simultaneously on the experimental and numerical models on the different inclination angles are still lacking. Hence, the numerical model was simulated through the utilization of Computational Fluid Dynamics (CFD) method. A comparison between the numerical model and experiment has revealed that the simulation results of FPSC are closer to the experiment result with less than 5% error. Thus, the numerical model that incorporates design parameters can be utilized for forecasting the way FPSC will operate properly. The results concluded that the FPSC with a 5 cm height of air gap has the highest heat output when inclined at 15 degrees.

Transient thermal behavior of a passive heat sink integrated with phase change material: A numerical simulation

This paper presents a numerical investigation of the thermal performance of a heat sink augmented with phase change material (PCM) for passive electronic cooling applications. The PCM-based heat sink employed N-eicosane as the PCM, which has a suitable melting temperature and high latent heat capacity for electronic cooling applications. A numerical simulation based on the enthalpy-porosity method was developed to analyze the impact of both PCM inclusion and induced natural convection airflow on the base temperature of the heat sink. The operational conditions were selected as three constant heat fluxes of 2, 3, and, 4 kW/m2 and four critical base temperatures of 40, 45, 50, and 55 °C. The influence of fin numbers on the thermal behavior of both PCM-based and conventional (without PCM) heat sinks was also investigated by considering the three fin numbers of 4, 5 and, 6. The PCM integration into the heat sink effectively delayed the rise of base temperature and extended the safe operation time compared to a conventional one. The thermal performance enhancement ratio of the PCM-based heat sink over the conventional heat sink ranged from 1.53 to 2.81, depending on the selected heat flux and critical temperature. The effect of fin numbers on thermal performance was more significant at lower heat fluxes. It was observed that the PCM-based heat sinks achieved a higher time-averaged heat transfer coefficient than the conventional heat sinks, and this coefficient increased with the heat flux and decreased with the fin number. For both PCM-based and conventional heat sinks, it was observed that increasing the fin number reduces the heat transfer coefficient value due to the suppressing effect of fins on natural convection current. The results demonstrate the potential of using passive PCM-based heat sinks for improving the reliability and efficiency of electronic devices compared to conventional heat sinks.

Determining the appropriate fin spacing for high performance and low footprint of direct contact ambient air-based vaporization systems

The continuous growth of frost over the tubes during the operation of ambient air vaporizers (AAV), which are used to re-gasify the liquefied permanent gases, eventually creates ice bridges. This phenomenon not only increases the thermal resistance but also restricts the natural convective airflow. In the present paper, the effect of fin spacing on the performance of the natural and forced draft AAV is analysed under different weather conditions. Appropriate fin spacing at different operating conditions is calculated by the three-dimensional model solved in ANSYS Fluent. A dynamic numerical model is developed to predict the frost properties in a forced draft vaporizer. A higher fin spacing not only increases the footprint but also impairs the rate of heat transfer in a forced draft AAV. At 298 K air temperature and 0.7 RH, the rate of heat transfer through a forced draft AAV can be enhanced by 8% with the appropriate fin spacing. The forced draft vaporizers are less sensitive to weather conditions compared to the natural draft vaporizers. For the identical fin specification, the number of tubes needed for vaporizing 240 Nm3hr−1 of LN2 is 16 for the forced draft, while it is 36 for a natural draft AAV. With reduced fin spacing, the footprint of the vaporizer block can be reduced by as much as 65% in a forced draft vaporizer for the identical vaporization capacity and ambient conditions.

Turbulent natural convection of a confined flow in a cubic enclosure: Effect of transient boundary conditions

This paper presents the experimental study of a turbulent natural convection air flow occurring inside an enclosure with an inner block partially heated. Unsteady boundary conditions are applied on the heated face of the block. This allows to achieve boundary conditions and flow regimes that can be relevant for industrial unsteady heating or cooling purposes. The maximum Rayleigh number reached is equal to 1.98×109 which corresponds, for instance, to the flow regime encountered in the underhood of cars. For Rayleigh numbers from 0.25×109 to 1.98×109, a dynamical analysis of the flow is conducted and similarity properties of the ascendant boundary layer are highlighted.

CFD‐based analysis of entropy generation due to mean and fluctuating fields during turbulent natural convection in a square enclosure

AbstractThe current study aims to numerically investigate the entropy generation during the natural convection flow of air in a square cavity. The governing equations for the conservation of mass, momentum, energy, and turbulence are solved using a control volume‐based technique employing the commercial code Fluent. Runs have been performed for both laminar and turbulent flow regimes by varying the Rayleigh number (Ra) from 103 to 1010. On the other hand, various viscous distribution coefficients (ϕ = 10−4, 10−3, and 10−2) and constant Prandtl number (Pr = 0.71) were considered. Given the conflicting perspectives in the literature regarding the entropy generation under turbulent regimes, more research is needed to better understand the impact that the fluctuating flow has on entropy production. The four terms of entropy generation inherent to turbulent natural convection (entropy generation due to dissipation in the mean and the fluctuating velocity fields in addition to the heat flux due to the mean and the fluctuating temperature) are computed in the present work and compared to calculations based on only mean values of temperature and velocity gradients. It was found that taking into account the fluctuating terms of temperatures and velocities augment the total entropy generation by 10.10%, 14.43%, and, 17.70%, up to 32.60%, respectively, for Ra = 5 × 108, Ra = 109, Ra = 1.58 × 109, and Ra = 1010. The gain shows the tendency to increase with the Rayleigh number. Thus, the fluctuating terms cannot be neglected particularly for high Rayleigh numbers. Furthermore, unlike entropy production due to the mean flow field, numerical outcomes reveal that the generated irreversibilities due to fluctuating flow are located around the upper hot and the lower cold corners of the heated walls. In addition, a numerical relationship between the first and the second laws of thermodynamics has been derived. A promising result that emerged from this study has shown that the Nusselt number and therefore the first law of thermodynamics is sufficient to estimate the heat part of entropy generation without the necessity of using the second law.

Novel design of natural solar air heat for higher thermal performance utilizing porous vortex generator

In this study, the thermal performance of a 2-D turbulent natural convection airflow inside a solar air heater is improved by using a flexible porous vortex generator, which is a self-sustained passive oscillator. The inclined thin elastic porous winglet, used as vortex generator, is attached on the heated wall for flow mixing and convection enhancement. Ongoing vortices, due to the resultant fluid–structure interaction, disrupts the thermal boundary layer, which then leads to the mixing process and improving the heater’s performance. Using a porous vortex generator, instead of its main competitor, the solid winglet, not only decreases the blockage effect and causes a higher airflow rate, but also results in a higher mean outlet-temperature by more mixing rate. To demonstrate the benefits of this idea, a numerical simulation considering a two-way strongly-coupled FSI approach in transient condition is done. The present numerical results are validated against the experimental and numerical data. It’s found that employing porous vortex generator confers considerable performance enhancement up to 500% on air temperature increase respect to the conventional clear solar air heater (from 8 to 40.5 °C) while providing a higher airflow rate 8.3% and outlet-temperature (135%) in comparison with solid winglet. This study aims to make the renewable thermal-solar system more efficient and affordable so that the industrial society will become more interested toward sustainable infrastructure.

Benchmarked simulations of natural convection airflow in a square cavity

An open-source code, OpenFOAM, and two commercial finite volume packages are benchmarked for accuracy and convergence speed in solving steady natural convection airflows in a square cavity with adiabatic horizontal boundaries and with high temperature difference on the two vertical side walls. The compressible two-dimensional Navier–Stokes equations are solved at Rayleigh numbers 106 and 107 (steady flow for Rayleigh numbers less than 2 × 108) using seven different CFD algorithms, including both transient and steady flow solvers. Velocity fields, pressures, temperatures and Nusselt numbers are compared against well-established results for three benchmark test cases from the literature. Relative accuracy and CPU time are reported for each algorithm and for each of the three test cases, yielding benchmarked comparisons of OpenFOAM, STAR-CCM+, and ANSYS-Fluent solvers for low Mach number compressible airflows.

Theoretical and Experimental Analysis on Influence of Natural Airflow on Spent Fuel Heat Removal in Dry Cask Storage

A key issue contributing to the success of NPP technology is the safe handling of radioactive waste, particularly spent nuclear fuel. According to the IAEA safety standard, the spent fuel must be stored in interim wet storage for several years so the radiation and the decay heat of the spent fuel will decrease to the safe limit values, after which the spent fuel can be moved to dry storage. In this study, we performed a theoretical analysis of heat removal by natural convection airflow in spent nuclear fuel dry storage. The temperature difference between the air inside and outside dry storage produces an air density difference. The air density difference causes a pressure difference, which then generates natural airflow. The result of the theoretical analysis was validated with simulation software and experimental investigation using a reduced-scale dry storage prototype. The dry storage prototype consisted of a dry cask body and two canisters stacked to store materials testing reactor (MTR) spent fuel, which generates decay heat. The cask body had four air inlet vents on the bottom and four air outlet vents at the top. To simulate the decay heat from the spent fuel in the two canisters, the canisters were wrapped with an electric wire heater that was connected to a voltage regulator to adjust the heat power. The theoretical analysis results of this study are relatively consistent with the experimental results, with the mean relative deviation (MRD) values for the prediction of air velocity, the heat rate using natural airflow, and the heat rate using the thermal resistance network equation are +0.76, −23.69, and −29.54%, respectively.

Thermal Resistance Model of a Flat Plate Solar Air Collector for Energy Efficiency Prediction

The temperature of a PV panel rises during operation which affects its power output. A PV panel is similar to a flat plate solar collector. This paper presents a simple theoretical heat transfer resistance model and a solution procedure to predict the absorber plate surface temperature of the solar collector. The model consisted of a rectangular cross-section steel duct placed inclined at an angle to the horizontal and exposed to solar radiation. The heat absorbed on the top surface of the plate is transmitted by conduction through the plate and heats the air in the duct. This creates a natural buoyancy effect which induces a natural convection air flow rate. A simple one-dimensional theoretical model of the solar collector with the thermal resistances of the various components is proposed. Simulated results of plate temperature and induced air flow velocity are presented.

Effect of some parameters on natural convection heat transfer in finned enclosures- a case study

In this study, we were interested in free convective heat transfer into cavities with finned surface. The hot horizontal bottom wall with attached fins was under variable (sinusoidal) surface temperature while the cold one, on the upper wall side, was kept at constant temperature. The vertical walls were adiabatic. The developed model is related to laminar natural convection airflow in a finned closed cavity. The considered Rayleigh numbers are in the interval 103 to 106 while the Prandtl number is 0.7. The finite volume method is used in solving the resulting equations. The developed numerical code is validated owing to benchmark solutions of De Vahl Davis and results reported by other authors. The results presented in this work are linked to the velocity distribution around the surfaces of the fins, the isothermal lines, the streamlines and local and average Nusselt numbers for various geometrical parameters. Special consideration is given to the effects of Rayleigh number, the variable surface hot temperature, the fins number and their length on the fluid flow patterns and heat transfer in square cavities.

Analysis of temperature rapid rise phenomenon during damper-off cycle in side-by-side frost-free refrigerator

The temperature rapid rise phenomenon often happens in the side-by-side frost-free refrigerators during the damper-off cycle, which leads to a decrease in damper-off duration and an increase in energy consumption. A three-dimensional transient model of natural convection during the damper-off cycle is established and experimentally validated to analyze the refrigerating compartment (RC) temperature rapid rise phenomenon. The results show that the violent natural convection airflow driven by the non-uniform temperature distribution will lead to the significant variation of RC temperature. The hot air rises from bottom into top layer along the three passages, which include the front gap (FG) between shelves and door boxes, the side gap (SG) between the right-side wall and door boxes and the rear gap (RG) between shelves and the rear wall. The air enthalpy along FG into the first layer is the dominant factor of the temperature rapid rise, accounting for almost 64.4% of the total inflow air enthalpy. The net air enthalpy flow along FG is larger than other passages in the first 15 s and decreases rapidly as the temperature distribution becomes uniform.

Experimental study on the effect of airflow conditions on the combustion characteristics of methane hydrate

Due to its large reserves and cleanliness, methane hydrate is widely regarded as a promising energy source. The study of the effect of airflow conditions on the combustion characteristics of methane hydrate can provide a theoretical basis for various engineering applications, such as realizing the method of in-situ combustion mining, improving the safety of mining and predicting the development of fire during the storage and transportation of methane hydrates. In this paper, the combustion process of methane hydrate under the conditions of natural convection, external horizontal airflow and internal vertical airflow was analyzed. This way, the effect of the form of airflow on combustion was studied, and the effect of airflow velocity on the characteristics of flame was documented. The results demonstrated that, under the conditions of natural convection and horizontal airflow, there was an “eruption phenomenon”, during which the ice layer broke and a large amount of methane gas instantly escaped, forming a high and bright flame. The vertical airflow reduced the mass transfer resistance of methane gas, whereas the combustion process was more stable, and the height of flame was more uniform. Under the condition of horizontal airflow, the tilted flame increased the heat transfer to the surface of hydrate. The flame tilt angles were different in different combustion stages. The flame tilt angle in the intensive combustion stage (61.26°) was smaller than that in the steady combustion stage (70.85°), whereas a higher combustion rate had a stronger capability to oppose the horizontal wind.

Desalination of seawater by spray freezing in a natural draft tower

The freeze-melting process can be a viable method for the purposes of desalination because of its low energy consumption, ignorable corrosion issues, and without huge pressure or membrane replacement work. Large contact area for heat and mass transfer per unit mass of water between the water and air and low heat resistance results in higher energy efficiency during spray freezing desalination process compared to other freezing desalination methods. A 200 m high desalination tower was proposed in this paper that could generate 27.7 kg/s fresh water in the form of water droplets with 2 mm diameter at an atmospheric temperature of −26°C. This research has founded that the natural convective airflow induced by the heat released by the warm water in the freezing process could generate through the wind turbine mounted in this system approximately one-third of the energy consumed by the water pump of the system. This free energy has never been studied in previous research. The power consumption required to produce 1 m3 fresh water in this system is approximately 1.07 kWh. Compared to traditional desalination methods, the power consumption of our new spray freezing desalination system is much lower than previous systems with the same mass flow rate of fresh water. Only 375.4 kJ cold energy to produce one-kilogram fresh water. Thus, this spray freezing desalination system could be employed in desalination industry if free cold energy (e.g. from the cold atmosphere or the regasification process of LNG) and seawater resources are available.

Investigation on the stability of natural convection in an annular cavity with non-isothermal walls

The onset and nature of unsteady natural convection in a tall, closed annular cavity with non-isothermal walls is investigated experimentally. In the first of a sequence of experiments, thermal boundary conditions imposed on the inner and outer cylinders promote steady bi-cellular natural convection airflow inside the cavity. As the buoyant potential of the upper convection cell is increased, steady flow inside the cavity first bifurcates into mono-periodic flow and then follows a quasi-periodic route to chaotic flow. Similarly, when the buoyant potential of the lower convection cell is increased, steady flow bifurcates into mono-periodic flow and then follows an intermittent route to chaos. All of the observed unsteady airflows inside the annular cavity were three-dimensional, with all of the periodic flows exhibiting a gentle swirling motion in the azimuthal direction.

Experimental Analysis of Natural Convection and Flow Visualization in an Asymmetrically Heated Open Vertical Channel

An experimental device was designed to perform the thermal and dynamic study of natural convection airflow in an open vertical channel. The two side walls of the vertical channel are made of Plexiglas allowing the visualization of the flow via the particle image velocimetry (PIV) method. For the two other vertical walls, one is heated at a constant temperature, and the other is insulated with a 9-cm thick polystyrene insulation. The dynamic characterization of convection is carried out by nonintrusive measurements (PIV), and thermal phenomena are analyzed using nonintrusive heat flux instrumentation (simultaneous temperature and velocity measurements have been carried out across the channel at different elevations). Moreover, this study deals with the influence of the Rayleigh number on the measured vertical velocity profiles as well as the thermal flux densities recorded along the heated wall. To do this, different values of the modified Rayleigh numbers were considered in the interval with the channel aspect ratio of A = 5 and A = 12.5. The obtained Nusselt number values have been compared successfully with those of the literature. The impacts of the Rayleigh number and the aspect ratio on the velocity profiles and the convective and radiative heat transfer have been examined.

Torrefaction and combustion behaviour of beech wood pellets

Use of renewable sources is still a crucial option for energy supply in view of environmental consequences of fossil fuel-dependent industry. Today, only 10% of world’s energy needs have been met by biomass sources. This is lower than expectations considering vast interest on biomass energy and necessitates more practical approaches to substitute fossil fuel usage. Torrefaction is a promising technique to obtain coal-like solids which can be incorporated into today’s power systems. In this study, torrefaction of beech wood sawdust pellets and their combustion behaviour were investigated using a vertical furnace with a wire mesh basket and necessary equipments. Torrefaction experiments were made as a poly-particles bed in the basket under nitrogen flow and at 493, 533 and 573 K temperatures corresponding to light, mild and severe conditions, respectively. Combustion experiments, however, were carried out by recording mass of single raw or torrefied burning pellet under natural convection airflow at 873, 973 and 1073 K temperatures. It was observed that mild torrefaction was appropriate to obtain a torrefied product retaining over 75% mass of raw pellets but having calorific values higher than most lignite. Volatiles ignition times of torrefied pellets were affected by their moisture contents. This effect lessened with increasing temperatures due to faster heating of pellets. Although torrefied pellets became more porous as severity of torrefaction increased, their carbon combustion rates were generally lower than those of raw pellets and decreased with combustion temperature as well as torrefaction severity.

A Simulation Study of Natural Convection Airflow Pattern for a Phase Change Material Chamber

South Africa is the highest consumer of commercial energy per capita in Africa, ranking 16th in the world for primary energy consumption. It is also ranked among the bottom 50 of the 150 countries regarding energy efficiency, the cold chain is a large contributor. Refrigerated transport vehicles have played a major role in preserving goods over the years. With the current climate change, new refrigeration systems have been put in place to sustain this industry while complying with the changing climate regulations. This paper presents the use of Eutectic plates inside the refrigerated transport vehicle compartment. This study numerically investigates the characteristics of phase change material (PCM) Eutectic plates applied at low-temperature ranges. To obtain a uniform heat transfer and airflow condition inside a refrigerated compartment, using the Reynolds stress model (RSM). A physical model and a mathematical model for three-dimensional (3D) transient natural flow were developed. Using the governing equation of mass, momentum and energy conservation, three Eutectic plate configurations were modeled and simulated in ANSYS Fluent to predict the temperature distribution and the velocity of the air flowing for 5 hours. The configuration with eutectic plates placed at the top and side showed great potential for the system. It had a high-temperature distribution across the compartment and promoted high air circulations compared to the other configurations.