Natural Convection Conditions Research Articles

Modelling Air Cooling in a Module of Prismatic Li-Ion Cells

The control of temperature in lithium-ion stacks is of increasing industrial importance. In fact, an efficient thermal conditioning system ensures a homogeneous temperature distribution among the cells, preventing conditions which could hinder performance or led to temperature runaway and so a disruptive failure of the module. CFD models for thermal analysis can be useful in the design phase to optimize the module layout and to size the cooling system, as well as to support experimental campaigns in the definition of reduced-order thermal models to be implemented into the Battery Management System (BMS) for in-operation temperature prediction. The present work presents the development of a computational model of a 48V-20Ah 20-cell Li-ion module and its air-cooling system based on COMSOL Multiphysics software. Unlike most papers available in the literature, the individual cells are modelled by considering an internal ohmic heat generation term as a function of both state of charge, cell temperature and sign of the applied current (charge or discharge), exploiting experimentally validated data provided by the manufacturer in temperature-controlled conditions. In addition, the reversible thermal generation term due to entropic phenomena, which is often ignored in the literature, was computed alongside an irreversible term accounting for cell overpotentials. The entropic contribution, which was estimated experimentally and optimised computationally, resulted non-negligible when the module is cycled at ≤ 1C. To take into account the effect of the fan cooling system, the motion field was solved in COMSOL using a turbulent k-ε model. To ensure model accuracy, only minor geometrical simplifications were introduced to the fluid volume. The resulting geometrical complexity, made of manifold and narrow channels, called for special attention in the computational resolution. Assuming a decoupled behaviour, namely that the thermal evolution does not affect the flow field, allowed to use the stationary fluid-dynamic solution as the starting point for the time-dependent thermal simulation. The model was run under different experimental current profiles and the validation was performed on the entire module under effective operating conditions and not on a single cell under natural convection condition with the external environment, as is usually found in the literature. The computational data were found to be in good agreement with experimental measurements for all the cells even after charge-discharge cycles lasting more than two hours. The simulations provided insights to improve the positions of thermal sensors inside the module and to optimize the efficiency of the cooling system, that could not be done based on thermal measurements only. Figure 1

Investigation of free convection heat transfer from vertical cylinders with semicircular fins

ABSTRACT In this paper, the effects of three parameters on the thermal performance of the vertical cylinder with semicircular fins was investigated experimentally. The fin shape, fin number and input heat flow on the thermal performance are studied effected. Three different heat sink models (SC-0, SC-40, and SC-50) were contrasted in order to choose the best reference model. The result show that the heat transfer coefficient increased up to 30% with SC-0 and SC-40, 27% with SC-0 and SC-50, while the thermal resistance of the radial heat sink was demonstrated to be less than that of SC by 17% with SC-40 and less than 19% with SC-50. Additionally, it was found that, the larger number of fins was more effective at dissipating heat than the smaller number of fins, this can be attributed to the heat transfer occurred under natural convection conditions. Finally, to developed Nusselt number correlation with experimental data, the correlation shows the good agreement with experimental data with error less than 5%.

Exploring the Impact of Concentration and Temperature Variations on Transient Natural Convection in Metal Electrodeposition: A Finite Volume Method Analysis

A mathematical model was developed to accurately determine the current density, concentration, and velocity profiles in electrochemical reactors operating under natural convection conditions. The model considers the interplay of key variables, such as potential, concentration, and temperature linked to the hydrodynamics through the density field using the Boussinesq approximation. Thus, a comprehensive analysis under both laminar and turbulent flow conditions is provided. The computational algorithm that represents the model is implemented in OpenFOAM and it offers the ability to operate the reactor at a fixed cell potential difference or under potentiostatic and galvanostatic control. The model’s accuracy was verified in different parallel-plate reactors using copper deposition as a test reaction from dilute and concentrated solutions, showing a strong agreement with the experimental results. This tool can be leveraged to optimize cell design and operating conditions in electrochemical systems that involve significant natural convection, such as in the electrowinning and electrorefining industries.

Disruption and microexplosion mechanisms of burning alcohol droplets with the addition of nanoparticles

Microexplosions of burning liquid fuels have been actively investigated and considered due to the advantage for substantially increasing the overall burning rate. This study concerns possible control of microexplosion occurrence by adding nanoparticles in alcohol fuel without effects of surfactant and ignitable particles as usually seen in the literature. In addition, the essential characteristics and differences between various particles on combustion are discussed. Specifically, we have added three types of ceramic nanoparticles into butanol and isopropanol droplets under different convection environments, i.e., forced convection and natural convection in normal gravity, and free convection in microgravity. A suspended droplet method was performed under convection-free and natural convective conditions while a free-falling droplet method provided a forced convection environment. It was observed that the combustion characteristics of droplets changed from extinction to burnout or from burnout to microexplosion by using different base fuels, by increasing the intensity of flow convection, or by using different kinds of nanoparticles. During combustion, droplets would deform due to the presence of gelation in the outer layer when Al2O3 or SiO2 nanoparticles were added, but not for TiO2. This yielded one of the key factors leading to microexplosions, in addition to the amount of heat absorption during the burning. For the latter, specifically, microexplosions happened when butanol/SiO2 droplets were tested, but not for butanol/Al2O3 droplets. By comparing the infrared absorptivity of Al2O3 and SiO2, distinct amount of absorbed infrared radiation was demonstrated. Such a difference in heat absorption was also key to the creation of microexplosion. As a consequence, microexplosions could be generated when nanoparticles were added into the fuel and formed gelation on the droplet surface, which trapped bubbles in the droplet, while sufficient heat was absorbed by the particles during the burning. The resulted consumption rate had increased by 46% compared to that of pure fuel.

Study on aerosol thermophoretic force and thermophoresis deposition model in reactor building

During nuclear reactor severe accident, the input of external passive containment cooling system will result in significant aerosol thermophoresis deposition in reactor building, which plays important factor affecting the safety assessment of nuclear reactor. Ratio of aerosol particles radius and gas molecule mean free path (Kn number) in reactor building is not greater than 2, corresponding to the near-continuous medium zone and transition zone. Firstly the typical thermophoretic force equations for these zones are evaluated. Combined with key influencing factors analysis, the correction factor connected to Kn number is introduced to address the deviation in the prediction for the transition zone of the current equations. The modified thermophoretic force equation obviously improves the underestimation of the original equation and is then used to obtain the thermophoresis deposition coefficient. According to experimental validation, the obtained coefficient is more accurate than the model that is widely employed in analysis codes in predicting the thermophoresis deposition of aerosol particles. Finally, fluid continuity, momentum, energy and particle continuity equations are established with deduced thermophoresis deposition coefficient and solved numerically. The thermophoresis deposition model considering natural convection (CT model) is established by theoretical derivation and fitting with the numerical solutions. By introducing the effects of natural convection on temperature distribution and aerosol concentration distribution the CT model could accurately describe the aerosol thermophoresis deposition behavior under natural convection conditions for Kn number is not greater than 2, which can be applied to aerosol thermophoresis deposition prediction in reactor building.

Effects of streamlined container design on control melting time of Phase Change Materials

ABSTRACT Control of melting time for any chemical is important on behalf of the energy efficiency. In this study, two types of container with stepped geometry was designed to melt phase change materials (PCMs) under natural convection conditions. Volume of two containers are the same but one of geometry has streamlined corners. Thus, this two containers are compared to see the melting time under the same boundary conditions. The Paraffin was used as PCM and its melting temperature and latent heat of fusion are 53°C and 243 kj/kg, respectively. In the first case, stepped geometry was used and then, this geometry was by streamlined corners for different aspect ratios and temperature differences. It is observed that the streamlined geometry can be used as passive control method for melting time. For the chosen geometry in the present study, the melting time is reduced at least 5% and energy efficiency can be improved.

Modeling analysis of non-azeotropic and immiscible binary mixed vapors condensation under natural convection

A calculation model describing the condensation heat and mass transfer process of non-azeotropic immiscible binary mixed vapors on the vertical wall under natural convection conditions was developed in this study. The model includes the two possible condensation modes of mixed vapors on the vertical wall as well as the conversion conditions between them. Cyclohexane and water were used as non-azeotropic immiscible mixtures. The reliability of the model was verified first, then the influence of vapor mass concentration and wall subcooling on the condensation mode and heat and mass transfer of the mixed vapors were analyzed. The results showed that the maximum deviation between the model results and the experimental results was less than 21%, confirming that the model was reliable. When the mixed vapors mass concentration was nearly eutectic or the wall subcooling was large, the condensation Mode B (Co-condensation) was more likely to occur than Mode A (Single condensation) due to the decrease in vapor-liquid interface temperature. The condensation mode may change from B to A as the wall length increased along the gravity direction of the condensation wall, and the condensation heat transfer coefficient and mass flux decreased. The thermal resistance of the vapor and liquid phases during condensation was also calculated. In Mode A, regulation of the thermal resistance of the vapor-liquid two phases can be achieved by adjusting the vertical length of the condensation wall and the vapor mass concentration. Condensation Mode B can only be realized by adjusting the wall subcooling. The results of this work may provide a workable reference for condensation heat transfer and enhancement of non-azeotropic immiscible mixed vapors in industrial environments.

Using entropy generation as evaluating indicator of charging completion in a latent thermal energy storage system

To further promote the melting rate of latent thermal storage (LTES) system, the gradient porosity has been proved to be a feasible method. Entropy, as a long developed characteristic parameter to evaluate the irreversibility of a system, can be a good measurement to estimate the completion of charge process of latent thermal energy storage system. In this paper, a representative elementary volume-scale (REV-scale) solid-liquid phase change lattice Boltzmann model is established to study the melting of PCM in a gradient porosity enhanced LTES unit. The entropy generation is realized and verified, and applied in LTES unit analysis. The result shows that, a bigger porosity gap of 0.82–0.98 and more addition of porous matrix below the system can better promote the melting and entropy generation. The stepped and linear porosity distribution reduce the melting time by 17.4% and 13.3% respectively. By researching the entropy generation, it is found that, the friction-caused entropy dominates the entropy generation during charge process. By studying the melting in different Rayleigh number, it is found that, the gradient porosity can only accelerate the melting in high natural convection conditions of approximately Ra > 2 × 106.

Evolution of intermetallics between solid Fe-Cr/Fe-Ni alloys and molten aluminium

Investigations of the evolution of intermetallic compounds (IMCs) at the interface between iron-based alloys and molten aluminium have important technical and scientific significance in many key areas. Most previous studies of IMCs formed at molten aluminium/solid steel interfaces have been focused on relatively long reaction times, but less research has been performed on the evolution of IMCs at the aluminizing interfaces of Fe-Cr and Fe-Ni alloys within a few seconds. In this study, hot-dip aluminizing experiments under natural convection conditions were employed to study the evolution of two interfacial IMCs (Fe-15Cr alloy and Fe-35Ni alloy with molten aluminium) at 700–900°C within 30 s. The IMCs for Fe-15Cr consisted of an Fe4Al13 layer next to the aluminium and an Fe2Al5 layer adjacent to the steel. In contrast, only one layer of Fe4Al13 was detected at the interface between the Fe-35Ni alloy and molten aluminium. The growth of Fe2Al5 was described by a parabolic rate law. An activation energy of 7.53 kJ/mol was determined for the temperature range of 700–900°C. The evolution of Fe4Al13 for the two alloys was similar: their thicknesses decreased anomalously with increasing dipping temperature, and their kinetics were controlled by both diffusion and dissolution. In this study, based on reaction-diffusion theory, growth and dissolution equations of the IMCs were established, and the diffusion coefficients of Al atoms in Fe2Al5 and Fe4Al13 were obtained for the first time. The similarities and differences in the IMCs for the two alloys were explained mainly by the different diffusion coefficients.

Three-dimensional numerical evaluation of skin surface thermal contrast by application of hypothermia at different depths and sizes of the breast tumor

Background and Objective: Thermal procedures can provide improvements in the thermal contrast of thermographic images in an attempt to diagnose early cases of breast cancer. This work aims to analyze the thermal contrast of different stages and depths of breast tumors from hypothermia treatment using an active thermography analysis. The influence of variation in metabolic heat generation and adipose tissue composition on thermal contrasts is also analyzed.Methods: The proposed methodology was based on the solution of the Pennes equation for a three-dimensional model similar to the real anatomy of the breast by commercial software COMSOL Multiphysics. The thermal procedure consists of three steps: Stationary, hypothermia and thermal recovery. During hypothermia, the boundary condition of the external surface was replaced by a constant temperature of 0, 5, 10, and 15 ∘C, simulating a gel pack, for cooling times of up to 20 min. In the thermal recovery, after the removal of the cooling, the breast was submitted again to the condition of natural convection on the external surface.Results: Thermal contrasts in superficial tumors, for all hypothermia resulted in improvements in thermographs. For smallest tumor, the use of high resolution and sensitive thermal imaging cameras to acquire this thermal change may be necessary. For tumor of diameter of 10 cm, cooling from 0 ∘C can increase the thermal contrast by up to 136% compared to the passive thermography. Analyzes with deeper tumors showed very small temperature variations. Even so, the thermal contrast gain in cooling at 0 ∘C for the tumor with a diameter of 1 cm reached 37% in relation to passive thermography.Conclusions: Thus, this work contributes as an important tool in the analysis of the appropriate use of hypothermia for different cases in early stages of breast cancer, considering that long times are needed to obtain the best thermal contrast.

Alternative Internal Configurations for Enhancing Heat Transfer in Telecommunication Cabinets

Telecommunication systems have become a critical part of society which enables connectivity to many essential and trivial services. Consequently, telecommunication equipment is housed in cabinets to protect the electronics from a variety of hazards; one of which is temperature-related failure. Current practices use a notable amount of power for the thermal management of telecommunication cabinets which can be reduced by considering alternative methods of cooling. In this paper, experiments were carried out to investigate the effectiveness of different internal mounting configurations of electronic components on the thermal performance of a telecommunication cabinet. The investigation tested inclinations (0–90°), different staggered offsets (0–50 mm), changing stream-wise spacing (29–108 mm), and fan speed (with a Reynolds number in the range of 1604 to 5539). The experimental study revealed that heat transfer was enhanced by 9.99% by altering component inclination to 90°, 25.90% by increasing stream-wise spacing from 29 mm to 108 mm, and 36.02% by increasing the Reynolds number from 1604 to 5539. However, the staggered arrangement of internal components decreased Nu by 3.26% for the natural convection condition but increased by 5.69% for the forced convection condition over the tested range and increasing the centre offset of the staggered components with respect to the cabinet did not influence Nu in any significant manner. Natural convection and forced convection also had notable influence on the heat transfer rate. Hence it was seen that alternative internal configurations positively influence heat transfer in telecommunication cabinets for the cases studied.

Heat transfer augmentation of natural convection heat sink through notched fin design

A novel notched fin heat sink with a central opening is proposed in the present study to augment the heat transfer performance of the natural convection heat sink under horizontal configuration. Thermal and airflow behavior in the heat sinks were studied using both experimental and numerical methods. The results showed that the performance of the conventional heat sink can be enhanced by optimizing its fin design. The basic heat sink model provided a 17% reduction in thermal resistance when its fin height and fin spacing were doubled, and it was accompanied by a 21% increase in surface area. The heat sink with a notched fin design with a less surface area offered a 13% lower thermal resistance compared to the basic model by entraining more airflow into the interior fin region. Providing a central opening in the notched fin heat sink can further reduce the thermal resistance by about 13%. This study also analyzed the influence of emissivity of the heat sink on natural convection conditions. The findings demonstrated that the heat sink with an emissivity of 0.9 can reduce the thermal resistance by nearly 23% compared to one with an emissivity of 0.15.

PCM-based ceiling panels for passive cooling in buildings: A CFD modelling

To relieve the upsurge of the energy demand associated to building indoor cooling, passive cooling employing phase change materials (PCMs) has been proposed as a promising technique to stabilise indoor air temperature fluctuation and reduce building energy demand. PCMs can exchange a large amount of latent heat during the melting and solidification processes, which can be utilised in line with the discharging and charging processes of a cooling system. The use of encapsulated PCM ceiling panels in buildings has immense application potential for passive cooling, which has attracted investigations in recent years. The geometry selection of the PCM ceiling panel is vital as it determines the exposed surface area to the surrounding air and the natural convection, and therefore the heat transfer rate to/from the PCM, which generally suffers from low thermal conductivity. This paper builds on the geometric optimisation of enclosed PCM capsules driven by natural convection, aiming to investigate the phase change behaviour and thermal performance of encapsulated PCM ceiling panels of different geometries. Here, the presented study focuses on the conjugate heat transfer phenomena through the surrounding air, panel shell, and PCM. A mixture of lauric, capric, and oleic acids was selected as PCM encapsulated in a thin-shell enclosure made of acrylonitrile butadiene styrene plastic, which forms the aesthetic panels attached to the ceiling. A computational fluid dynamics model was validated using full-scale experimental data and further employed to analyse the discharging process and design considerations of the PCM ceiling panels. The effects of various influencing factors were explored, such as panel volume, the initial temperature difference between PCMs and surrounding air, shell thickness, and geometrical features. The results indicate that the panel comprised of a pyramid array is more advantageous than the tetrahedral-based panel in terms of thermal performance, where the average melting rate is 20.8 % higher when the panel volume is 250 mL. In addition, tweaking the geometric features of the panel will largely impact its thermal performance as the effective heat transfer area and natural convection conditions are varied. Finally, it was found that hanging the panel to the ceiling will effectively enhance the heat transfer compared to directly attaching it to the ceiling. The results complement the knowledge body of macro-encapsulated PCM ceiling panels in terms of thermal performance and geometric optimisation. The effects of geometric features on the thermal performance of the PCM ceiling panels are studied in a more holistic manner.

Experimental Study of Frost Crystals Dendrite Growth on Two Neighboring Separate Frozen Water Drops on a Cryogenic Cold Surface under Natural Convection Conditions

The effects of cold surface temperature, wet air state (temperature and humidity) and original drop size on frost dendrites growth of two neighboring separate frozen water drops of same size under natural convection conditions were investigated by quantitative measurement. It was determined that for different cold plate surface temperature conditions, i.e., the ordinary-low temperature and the cryogenic temperature range, the frost formation mechanism is different. Under the conditions that the air temperature is not too high and absolute humidity is not too excessive, the influence of frozen water drop size on the longest dendrite of frost crystals becomes more and more obvious with the decrease in cold plate temperature. The changes in air temperature and relative humidity both change air absolute humidity, so they have similar effects on the growth of dendrites. However, the effect of wet air state on the growth of frost dendrites is not monotonous, which needs to be considered comprehensively in combination with heat and mass transfer and the existence of heavy phase layer. The thickness of ‘the initial continuous frost layer’ was measured and it was disclosed that the initial frost layer thickness is 1.7–3.0 times that of the height of the frozen water drop diameter. This value may be possibly used as initial frost layer thickness in heat and mass transfer-based frost layer growth prediction models, at least for ordinary-low temperature conditions.

Enhanced behaviour of a passive thermoelectric generator with phase change heat exchangers and radiative cooling

Heat exchangers are essential to optimize the efficiency of Thermoelectric Generators (TEGs), and heat pipes without fans have proven to be an advantageous design as it maintains the characteristic robustness of thermoelectricity, low maintenance and lack of moving parts. However, the efficiency of these heat exchangers decreases under natural convection conditions, reducing their heat transfer capacity and thus thermoelectric power production. This work reports on a novel heat exchanger that combines for the first time, phase change and radiative cooling in a thermoelectric generator to improve its efficiency and increase the production of electrical energy, specially under natural convection. For this, two thermoelectric generators with heat-pipes on their cold sides have been tested: one with the radiative coating and the other without it. Their thermal resistances have been determined and the electric power output was compared under different working conditions, namely, natural convection and forced convection indoors and outdoors. The experimental tests show a clear reduction of the heat exchanger thermal resistance thanks to the radiative coating and consequently, an increase of electric production 8.3 % with outdoor wind velocities of 1 m/s, and up to 54.8 % under free convection conditions. The application of the radiative surface treatment is shown to result in a more stable electrical energy production, suppressing the drastic decrease in the generated electric power that occurs in thermoelectric generators when they work under free convection.

Water-air aerosol cooling of rows of cylindrical elements under conditions of natural convection

Water-air aerosol cooling of rows of cylindrical elements under conditions of natural convection

Study of the temperature and humidity regime in solar drying of agricultural products

This article presents the results of experiments on the temperature and humidity regimes in the drying chamber when drying apples and apricots in a refrigerator with a combined solar collector and heat pump. The combined dryer is designed for simultaneous drying and long-term storage of the product. Experiments on the study of temperature and humidity regimes were carried out under natural convection conditions. Experimental studies of temperature and humidity regimes showed that the maximum temperature inside the chamber reached 46 oC and the maximum humidity reached 62%, and in these temperature and humidity regimes, apples were dried in 30 hours and apricots in 48 hours. From a product with a starting mass of 5 kg, 0.65-0.7 kg of dried product was obtained. Experimental results on temperature and humidity during drying of apples and apricots t=f(h) and w=f(h) and presented in the form of a dependency graph.

Анализ длительно допустимых токов и потерь активной мощности в воздушных линиях электропередачи с учетом климатических факторов

Formulas for calculating continuous admissible currents and active power losses for overhead power transmission lines under conditions of natural and forced convection for insulated and non-insulated conductors are presented. The correct determination of maximum current loads plays an important role in the efficient use of the load capacity of power transmission lines, and the refinement of the calculation of active power losses is the basis for increasing the efficiency of energy transmission. The results obtained can be used in the selection of measures aimed at increasing the capacity of overhead lines and reducing energy losses.

Research on Anti-Icing Performance of Graphene Photothermal Superhydrophobic Surface for Wind Turbine Blades

In this study, graphene is used as a photothermal material, which is added to the SiO2 superhydrophobic solution treated with fluorine silane, and then sprayed on the copper plate surface to prepare a new type of photothermal superhydrophobic surface with contact angles up to 160.5° and 159.8°. Under the conditions of natural convection, the effects of photothermal superhydrophobic surfaces on droplet condensation, freezing, and frost growth are investigated in different environments. The results show that the photothermal superhydrophobic surface can not only delay the freezing of surface droplets, prolong the freezing time of droplets, and reduce the thickness of the frost layer, but also allow for the rapid removal of droplets under near-infrared (NIR) irradiation. If the droplet is irradiated by an infrared laser emitter while the cooling system is still turned on, the internal temperature of the droplet will always be higher than the crystallization temperature under the illumination intensity of 2 W/cm2, and the droplets will not freeze. With the extension of irradiation time, the droplet will evaporate, and the volume of the droplet will decrease. On the basis of summarizing and evaluating the study on the anti-icing performance of superhydrophobic surfaces and the properties of photothermal materials, a new research direction regarding the anti-icing of fan blade surfaces was established. This kind of surface combines the photothermal capabilities of light absorption materials with the micro- and nanostructure of the superhydrophobic surface to improve the anti-icing capability of wind turbine blade surfaces in difficult conditions.

Influence of natural convection on the electrodeposition of copper nanowires in anodic aluminium oxide templates

Templated electrodeposition is a versatile method for preparing metal nanostructures. Among the various parameters of this process, the effect of convection on the deposition process is less explored. The present work is devoted to the experimental and theoretical study of the influence of natural convection near nanoporous anodic aluminium oxide templates on the deposition of nanowires. Electrochemical deposition of copper nanowires was carried out with vertical templates parallel to the gravitational acceleration at various overpotentials. A model of nanowire electrodeposition was developed based on the Navier‒Stokes and diffusion equations. The length profiles of the nanowires, experimentally measured using scanning electron microscopy, were compared with the results of the numerical simulation. The simulation results were also compared with current-time transients recorded during metal electrodeposition into nanopores. In addition, an analytical expression has been proposed to describe the evolution of the deposition current density into nanopores under natural convection conditions.