Presence Of Natural Convection Research Articles

Thermal interaction between a group of geothermal piles in the presence of natural convection

Adequate estimation of thermal efficiency for a group of simultaneously acting geothermal piles is an important factor that may affect sustainability and technical feasibility of pile-anchored energy harvesting technology. This paper presents a series of coupled finite element analyses (FEAs) of pile-soil heat transfer to explore the impact of natural convection on group thermal performance of geothermal piles. FEA results demonstrated that, in comparison to a purely conductive heat flow condition in the ground, the presence of natural convection may reduce pile thermal interaction within a large pile group up to 90 % and elevate average heat rejection capacity per unit pile length by around 25 %. Such positive influences were enhanced with increase in intrinsic soil permeability, inlet fluid temperature, and number of closely spaced geothermal piles in a group. Moreover, the mobilization of natural convection in soil surrounding geothermal piles resulted in reduction in average pile surface temperature as much as 9 °C in a 3 × 3 pile group and a steep temperature gradient along the length of a pile with a maximum difference between temperature near head and base of a pile reaching up to 14 °C. Such findings may further influence thermo-mechanical and settlement behaviour of a group of geothermal piles.

Numerical Investigation of phase transition in different latent heat storage systems in the presence of natural convection and porous media

Securing a reliable supply of energy is critical given the ever-increasing demand for energy and the challenges posed by a growing population. Latent energy storage is being introduced as one of the most efficient solutions to harness renewable and waste energy and ensure a constant supply. The phase change material (PCM) in this type of energy storage suffers from low responsiveness, which slows down the industrialization of latent storage. This study succinctly identifies the influencing factors that affect the efficiency of PCM-based latent energy storage, including fluid temperature, the role of natural convection and the benefits of porous media. It also provides new insights through comprehensive configuration analysis, ultimately contributing to the understanding of the field and addressing sustainable energy demands. For the numerical computations, enthalpy-porosity methodology via ANSYS Fluent 18.2 is employed a rigid grid for precise dual-phase simulation. The working fluid temperature is of similar importance for three types of units, as an improvement of 22% has been obtained by increasing the inlet temperature by 5 degrees, while the pipe model has benefited from an improvement of 26% under the same condition. The Triplex Tube Heat Exchanger (TTHX) outperforms the other units in all scenarios, as the heat transfer surface of this type is greater than that of its counterparts. Natural convection phenomenon is most effective in the pipe model, as its absence slows the melting rate by 236%. The inclusion of porous media produced on average 93% faster systems due to the additional heat transfer surface, although it suppressed the gravitational movement.

Determination of mutual diffusivity of CO2/C3H8-heavy oil systems with consideration of natural convection under reservoir conditions

By performing dynamic volume analysis, we developed and applied an experimental and theoretical technique to determine mutual diffusivity (i.e., diffusivity of a gas component in an oil phase and that of the extracted oil components in the gas phase) of CO2/C3H8-heavy oil systems in the presence of natural convection under reservoir conditions. Experimentally, diffusion tests are conducted for CO2/C3H8-heavy oil systems at high pressures and elevated temperatures with a PVT setup, while swelling factors of the oil phase are measured and recorded continuously. We have collected gas and oil samples at end of the test for gas chromatography (GC) and oil compositional analysis. Theoretically, mutual diffusivity between a solvent and heavy oil can be determined once the discrepancies between the measured and calculated parameters, including swelling factors and gas compositions, have been minimized. The diffusivities of both solvents (i.e., CO2 and C3H8) and oil components are found to increase with pressure and temperature. Except for the dissolution of solvent(s) into heavy oil due to concentration difference, there exists an obvious extraction process from the oil to gas phases during each experiment at elevated temperatures as light-medium components (i.e., C1-C4 and C6-C8) have been detected in the collected gas samples at the end of the test, during which the oil swelling effect is diminished at its late period when the solvent-heavy oil system approaches its equilibrium state. Compared to the CO2-heavy oil systems, more light components have been detected for the C3H8-heavy oil systems, indicating that stronger extraction occurs when adding C3H8 to the heavy oil at elevated temperatures. Since natural convection has an opposite direction from molecular diffusion, separating the former from the latter results in larger diffusion coefficients of CO2 and C3H8. In addition to new experimental data, both diffusivity of a solvent/CO2 in heavy oil and that of the extracted oil components in the gas phase are quantified in a more complex solvent(s)/CO2-heavy oil system at high pressures and elevated temperatures, not only serving as the benchmark for future studies to consider the contribution of temperature and porous media, but also allowing us to evaluate and optimize the hybrid solvent-steam method in a heavy oil reservoir within a unified and consistent framework.

Pathways to Boost Solar-to-Hydrogen Efficiencies for Z-Scheme Photocatalytic Reactors: Learnings from Equivalent Circuit and Continuum Multiphysics Models

Z-scheme semiconductor particles present in an aqueous solution with soluble redox shuttles provides a promising two-step approach to split water to produce hydrogen and oxygen using sunglight. This approach facilitates efficient utilization of the incident solar spectrum with tandem light-absorbers and promotes safe reactor operation with hydrogen and oxygen produced in physically distinct locations. Thermodynamically, these systems are projected to achieve solar-to-hydrogen (STH) efficiencies that are comparable to the electrode architectures, which are as large as 34% with idealized light absorbers and perfect reaction selectivity1. Up to 4% STH efficiencies were theoretically projected for state-of-the-art light absorbers and cocatalysts with diffusion-driven redox shuttle transport between reaction sites2. However, practical demonstrations are limited to efficiencies that are significantly smaller (less than 1%)3. Two main factors impeding actual performance in these systems include: (a) undesired reactions of any redox species, including hydrogen, oxygen, or the redox shuttle species, on the photocatalyst site; and (b) mass-transfer limitations of reactant/product species between oxygen and hydrogen evolution reaction sites. In this work, I will present materials- and device-level insights obtained from physics-based models of varying complexities. An expanded, zero-dimensional (0-D), equivalent-circuit model is developed to probe the effectiveness of semipermeable oxide coatings to achieve reaction selectivity. This model introduces powerful innovations to model parallel/competing redox reactions at the valence and conduction bands, while also accounting for mass-transfer limitations. Moreover, our framework provides a tractable approach to quantify the multidimensional dependencies of coating transport and kinetic parameters, concentration of light absorbers, and species mass-transfer rates on the STH efficiencies. At the reactor-scale, we assess the viability of natural convective currents to enhance species mixing. Temperature and concentration profiles are predicted from a two-dimensional (2-D), coupled flow, heat, and mass-transfer model of two reaction compartments separated by a membrane. Modeling predictions are compared to and validated against experimental measurements on dual-compartment reactor prototypes. Enhanced rate of species transport is predicted in the presence of thermal convection in addition to diffusion. Improved species mixing is observed for the horizontal, as compared to the vertically stacked, arrangement of reaction compartments, and is attributed to attenuation of incident sunlight by the membrane present along the optical path. With 2-cm tall reaction compartments, the minimum redox shuttle concentration needed to sustain a 10% STH efficiency decreases by 80% in the presence of natural convection as compared to diffusion. Overall, these results provide new insights and strategies to realize highly effective materials-to-device scale designs for Z-scheme photocatalytic solar fuel reactors with soluble redox shuttles. References (1) Keene, S.; Bala Chandran, R.; Ardo, S. Calculations of Theoretical Efficiencies for Electrochemically-Mediated Tandem Solar Water Splitting as a Function of Bandgap Energies and Redox Shuttle Potential. Energy Environ. Sci. 2019, 12 (1), 261–272. https://doi.org/10.1039/C8EE01828F.(2) Bala Chandran, R.; Breen, S.; Shao, Y.; Ardo, S.; Weber, A. Z. Evaluating Particle-Suspension Reactor Designs for Z-Scheme Solar Water Splitting via Transport and Kinetic Modeling. Energy Environ. Sci. 2018, 11 (1), 115–135. https://doi.org/10.1039/C7EE01360D.(3) Wang, Z.; Hisatomi, T.; Li, R.; Sayama, K.; Liu, G.; Domen, K.; Li, C.; Wang, L. Efficiency Accreditation and Testing Protocols for Particulate Photocatalysts toward Solar Fuel Production. Joule 2021, 5 (2), 344–359. https://doi.org/10.1016/j.joule.2021.01.001.

Using Micropolar Nanofluid under a Magnetic Field to Enhance Natural Convective Heat Transfer around a Spherical Body

An analysis is discussed of the heat and mass transfer for micropolar nanofluid in presence of natural convection from a spherical body with magneto-hydrodynamic (MHD) effects. The constant wall temperature boundary condition is also studied. By employing proper similarity transformations, the governing equations are converted into a set of partial differential equations (PDEs) with the used boundary conditions, which can then be solved numerically via the efficient Keller-box implicit numerical finite difference method. The numerical results of impacts of the controlling parameters on heat transfer physical quantities have been presented, tabular and graphically, by MATLAB symbolic software. Comparisons of the current study results to previously published results show good agreement, indicating that our numerical computations are legitimate and accurate. Increasing nanoparticle volume fraction is observed to depress local skin friction, Nusselt number, and angular velocity while the reverse effects are observed for velocity and temperature.

Time evolution of composite fuel/water droplet radii before the start of puffing/micro-explosion

Puffing and micro-explosion processes in composite water/fuel (kerosene, Diesel fuel and rapeseed oil) droplets are investigated experimentally and numerically, using the recently developed model of the phenomena. The experiments were performed in quiescent air at atmospheric pressure in the presence of natural convection. The model uses the assumption that a spherical water subdroplet is located exactly in the centre of a spherical fuel droplet; the start of the puffing/micro-explosion process is identified as the time instant when the temperature at the water/fuel interface reaches the water nucleation temperature. Both experimentally observed and predicted times to puffing/micro-explosion τp are shown to decrease with increasing ambient temperature and increase with increasing initial droplet radii for all three fuels. The longer observed τp compared with those predicted for rapeseed oil droplets is attributed to the time required for bubble formation. This time is not considered in the model used in the analysis. The observed and predicted time evolutions of droplet radii before the start of puffing/micro-explosion are shown to be close.

Measurement and Scaling of Lake Surface Skin Temperatures

AbstractWater temperature microprofiles at the air‐water interface of a stratified lake were measured concurrently with moderate wind speeds of 0.8–8.2 ms−1 above the lake surface. The day‐time measurements revealed the cool skin effect of water surface temperatures that triggers the cooling of the lake surface and the occurrence of natural convection. Natural convection velocity scale mediates the thickness of the diffusive thermal sublayer and the corresponding net heat flux at the interface. The diffusive thermal sublayer thickness ranged from 0.7 to 3.6 mm. The temperature microprofiles and proposed parameterization of skin‐to‐bulk temperatures enable similarity scaling of temperature microprofiles over the diffusive thermal sublayer thickness. The proposed parameterizations allow for the estimation of diffusive thermal sublayer thickness and skin‐to‐bulk temperature difference in the presence of natural convection. Our results have the potential to facilitate the validation and integration of satellite‐derived lake surface temperatures.

Study on Efficiency of Fully Wet Porous Trapezoidal Fin Structures in the Presence of Convection and Radiation

The thermal behaviour of fully wet porous trapezoidal profiled longitudinal fin structures in the presence of natural convection and radiation has been scrutinized in the present analysis. The rectangular and trapezoidal profiles have been comparatively analysed. The Darcy’s law has been incorporated to study the solid-fluid interactions. Further, the internal heat generation has been assumed to be a linear function of temperature. The obtained non-linear second order ordinary differential equation has been reduced and evaluated numerically. The impact of fully wet condition, porous nature, internal heat generation and other relevant parameters on the thermal profile and efficiency of trapezoidal and rectangular fin profiles has been interpreted graphically and discussed. It has been derived that the rectangular fin profile is more efficient than the trapezoidal profile.

Computational study for natural convection effects on temperature during batch and continuous industrial scale radio frequency tempering/thawing processes

Radio frequency (RF) processing have been used for tempering and thawing. There have been significant experimental and mathematical model based studies for this process especially in the last two decades. The modeling studies applied the convective boundary condition with effective heat transfer coefficient and constant medium temperature in through field flat plate RF systems. The objective of this study was first to develop a mathematical model for conjugate heat transfer (natural convection) in a staggered through field electrode RF system and to use this model for natural convection effects in industrial scale processes. For this purpose, the developed comprehensive mathematical model was experimentally validated using the data from frozen tuna samples. The air temperature variation was also used for validation. Then, the developed model was used in industrial scale process simulations with the presence of natural convection. The industrially preferred flat plate systems were used in this part to determine the temperature and electric field evolutions. The results demonstrated the significant effect of the natural convection on sample surface temperature distribution and temperature change of air within the cavity. The evolved natural convection resulted in the variation of the heat transfer coefficient along the sample surfaces with significant changes in temperature. Based on these results, it might be concluded that the effect of air temperature change within the cavity during an RF processes should also be controlled and considered for industrial system design and process optimization.

Discharging process and thermal evaluation in the thermal energy storage system with fractal tree-like fins

Under microgravity environment for spacecraft operation, the melting rate of phase change material is restricted by its low heat transfer performance and disappearance of natural convection. To accelerate the melting under microgravity, the insertion of fractal tree-like fins into a latent heat thermal energy storage unit is proposed. The dynamic melting behavior and heat transfer performance in cavities with fractal tree-like fins having bifurcation angle of 30°, 60° and 90° are investigated through a transient model based on the enthalpy-porosity method. The relationship between the effects of tree-like fins and thermocapillary convection on melting is disclosed. The results show that during melting driven by combined heat conduction and thermocapillary convection, the presence of tree-like fins significantly accelerates melting, and the increment degree of melting rate increases with the increase of bifurcation angle. Compared with cavity without fin, 30°, 60° and 90° fins reduce melting time by 47.7%, 57.2% and 64.3%, respectively. The fractal tree-like fins also enhance the increment speed of energy storage capacity, while enhancing and slightly deteriorating heat transfer at the beginning and end of melting, respectively. The thermocapillary convection accelerates the melting under microgravity, and the acceleration effect degree increases with the increase of bifurcation angle. With the presence of natural convection, compared with the cavity without fin, 30°, 60° and 90° fins reduce the melting time by 57.8%, 60.6% and 65.3%, respectively; the acceleration effect of fins on melting is enhanced, and the effect of bifurcation angle on melting becomes weaker. The present design innovatively introduces tree-like fins into thermal energy storage under microgravity condition, and can be used for temperature control of spacecraft electronic devices.

Interpretation of TRT data in the presence of natural convection and groundwater flow in saturated ground

Ground thermal properties play important roles in the design of geothermal heat exchangers (GHEs). Constant ground thermal properties, as often considered in analysis and design of GHEs, often do not account for the coupled effect of thermal and pore fluid flow in saturated ground. This paper presents a series of advanced finite element analyses (FEAs) of thermal response test (TRT) on a geothermal borehole heat exchanger (BHE) installed in uniform and multi-layered saturated soil deposits. Considering coupled thermo-hydro flow in saturated ground, FEAs explore the effects of ground hydraulic characteristics (e.g., groundwater flow, soil permeability), power injection rate, and duration of thermal response test on the value of effective ground thermal conductivity keff. Results show that the value of keff, in the presence of coupled thermo-hydro flow condition, can be multiple times of the thermal conductivity value estimated considering only conductive heat transport in coarse grained soil. Example calculations following two available BHE design standards confirm that consideration of soil permeability and selection of appropriate duration of TRT may lead to significant reduction in the design length of BHEs.

Intermittent nucleation and periodic growth of grains under thermo-solutal convection during directional solidification of Al-Cu alloy

The presence of natural convection during solidification of alloys significantly influences the heat and mass transfer processes in the liquid phase, and thus affecting the nucleation and growth behaviors of grains. It is crucial to understand the impacts of thermo-solutal convection on the solidification process to control the final properties of casting ingots. In this study, the directional solidification of Al-15 wt.% Cu alloy under different temperature gradients has been in-situ observed using synchrotron radiation X-ray radiography. The solute distribution in the field of view (FOV) and three dimensional solid microstructure are obtained by our proposed image processing technique, which aims to determine the permeability and solutal Rayleigh number in the mushy zone over time. Then the interplay between the thermo-solutal flow and microstructure is quantitatively studied. Results indicate the solute elements are periodically discharged out of and confined to the mushy zone owing to the solutal instability in the mushy zone, which results in the periodic oscillation of solute concentration. Therefore, the oscillatory constitutional undercooling gives rise to the intermittent nucleation and periodic growth of grains, which in turn cause the periodic oscillation of permeability and the associated solutal Rayleigh number, thereby in turn resulting in the oscillation of solute concentration. Moreover, owing to the superposed solute flow and convective flow, the oscillation amplitudes of solute concentration and nucleation number vary in a sinusoidal manner in general.

ИНЖЕНЕРНЫЕ ФОРМУЛЫ ДЛЯ ОЦЕНКИ ВЛИЯНИЯ ЕСТЕСТВЕННОЙ КОНВЕКЦИИ У ПОВЕРХНОСТИ ТВС ВВЭР-1000 НА РЕЗУЛЬТАТЫ ИЗМЕРЕНИЯ ЕЕ РАЗМЕРОВ УЛЬТРАЗВУКОВЫМ МЕТОДОМ В БАССЕЙНЕ ВЫДЕРЖКИ АЭС

Engineering formulas are presented for calculating the value of methodological error in measuring the distances from the ultrasonic sensor to the surface of the irradiated fuel assembly of WWER-1000 caused by decay heat from the fuel assembly. The methodical error arises due to the water temperature gradient along ultrasonic wave path. This is due to the presence of natural convection at the surface of the fuel assembly discharged from the reactor into cooling pool at nuclear power plant. The paper presents a methodology for calculating the methodical error. It is assumed that the water temperature between the sensor and the surface of the fuel assembly is determined by convective heat transfer between the fuel assembly and the water in cooling pool at nuclear power plant. The surface of VVER-1000 fuel assemblies is modeled by a flat vertical plate with a uniform surface heat flux. The propagation of an ultrasonic wave in a medium between the surface of a fuel assembly and an ultrasonic sensor is described in the approximation of geometric acoustics. The results of numerical calculations according to the presented method are executed in the form of nomograms. Engineering formulas for calculating the value of methodological error are obtained by processing an array of calculated data. The obtained formulas are used in the development of measuring systems for monitoring the deformation of fuel assemblies of WWER-1000 in cooling pond of nuclear power plant.

Performance analysis of natural convection in presence of internal heating, strong turbulence and phase change

Experimental study and numerical simulation work were carried out to investigate the natural convection performance in presence of internal heating, strong turbulence and phase change. The experiments were performed in large-scale facility with two simulants for comparison: water without solidification and molten salt with solidification. Three heating power levels were conducted for each simulant, all with high Rayleigh numbers up to 1016. In addition, the simulation work with large eddy simulation was performed to obtain detail information inside melt pool. The distributions of temperature and velocity, as well as heat flux and crust characteristics were presented and compared from experiments and simulation. The distributions of temperature and heat flux presented increasing distribution and the crust showed the contrary trend. The non-uniform solidification will strengthen natural convection turbulence and heat transfer in the pool top region, which will finally increase the risk of vessel wall failure. This work could help to better understand the thermal and flow performance related to complicated natural convection behavior.

Analysis of thermal behavior of a porous fin fully wetted with nanofluids: convection and radiation

Abstract The analysis of heat movement through longitudinal permeable fin fully wetted by nanoliquid has been modeled in the presence of natural convection and radiation condition. Three kinds of water based nanoliquids having titanium dioxide, aluminium oxide and copper nanoparticles are used for the study. The derived energy equation has been resolved numerically by Runge Kutta Fehlberg method. The influence of geometric and flow factors on the temperature distribution of the fin are discussed in detail by means of plotted graphs. It is found that, Cu-water nanoliquid enhances more heat transport from the fin surface.

Scrutinization of different shaped nanoparticle of molybdenum disulfide suspended nanofluid flow over a radial porous fin

Purpose The purpose of this paper is to study the thermal behaviour of radial porous fin wetted with nanofluid containing different shaped nanoparticles in the presence of natural convection and radiation. Here, the nanofluid suspended with molybdenum disulfide nanoparticle with base fluid as water is considered. The influence of non-spherical nanoparticles such as platelet, cylinder, brick and blade shapes is also investigated. Design/methodology/approach The modeled equations are non-dimensionalized and solved numerically via Runge–Kutta–Fehlberg method combined with shooting scheme. Findings The flow natures of the pertinent parameter are represented graphically and discussed their physical significance. From the validation of obtained outcome, it is found that the use nanofluid has significant influence on heat transfer rate. Among platelet, cylinder, brick and blade shapes, brick-shaped nanoparticle shows better heat transfer rate. Originality/value The present paper deals with an analysis of the flow of molybdenum disulfide nanoparticles suspended in water over a porous fin of a radial profile. The effect of differently shaped nanoparticles on the heat transfer enhancement through the radial porous fin is investigated for the first time. The natural convection and radiation effects are also considered. The modeled equations are non-dimensionalized and solved numerically via Runge–Kutta–Fehlberg method combined with shooting scheme. The effect of pertinent parameters on temperature field is examined. From the validation of obtained outcome it is found that the use nanofluid has significant influence on heat transfer rate. Among platelet, cylinder, brick and blade shapes, brick-shaped nanoparticle shows better heat transfer rate.

Numerical simulation of paraffin wax melting in a rectangular cavity using CFD

ABSTRACT The study presents the numerical simulation of the melting process of phase change material, paraffin wax in a rectangular cavity using ANSYS fluent. The simulation is done for three different Stefan numbers 0.13, 0.18 and 0.28. Enthalpy porosity model and volume of fluid model is used to solve the governing equations. The presence of natural convection and sinking of the solid phase is observed in the density contours. The presence of air content in the cavity is taken into account during estimation of un-melted solid fraction in the cavity which is the novelty of this work. An empirical equation is also derived to calculate solid fraction at different times with different Stefan numbers. This type of empirical equation is proposed for the spherical cavity but not very common in a rectangular cavity. This empirical equation can be useful for designing of latent heat energy storage unit, heat exchanger using phase change material and for the study of metal casting processes. The melting process is also simulated with three different initial shapes of solid paraffin wax in the rectangular cavity. The study reveals that the melting time is not linearly proportional to the volume of paraffin wax.

A two-dimensional adaptive remeshing method for solving melting and solidification problems with convection

Melting and solidification problems in presence of natural convection are known to require high computational resources. Very fine meshes are essential to accurately determine the flow structure and interface position between the different phases. In this work, a time-dependent adaptive remeshing method based on an efficient error estimator is presented. The proposed method greatly reduces the number of mesh elements while maintaining and even enhancing the efficiency of the simulations. A variant of the enthalpy-porosity formulation is used where the different thermo-physical properties between the solid–liquid phases are easily taken into account. A second order mixed finite element formulation for both space and time is employed for solving the momentum and energy equations. The efficiency of the proposed methodology is established by comparing solutions on very fine meshes with those obtained on adapted meshes and with existing experimental and numerical results found in the literature.

Characteristics of Heat Transfer During Boiling of Liquid Metal in the Fuel Assemblies of Fast Reactors in Emergency Regimes

The results of experimental studies of heat transfer during boiling of sodium-potassium alloy in the presence of natural convection in model FA of a fast reactor with seven fuel-rod simulators are reported. The method of investigation is described. Data are obtained for three regimes of boiling of the liquid metal in FA: stable (nucleate), unstable (slug), and secondary stable (disperse-annular). The unstable boiling regime developed in two directions depending on the state of the surface of the simulators: in the presence of manufacturing roughness – in the direction of damping of pulsations, stabilization of boiling, and subsequent stable heat removal in the presence of a fi lm of liquid on the surface of the simulators (disperse-annular regime of boiling) or in the presence of small roughness – in the direction of increasing amplitude of flow and temperature pulsations. Data on an emergency regime with pressure reduction in the circulation loop, initial superheating upon boiling-up of the liquid metal, heat transfer, and a cartogram of the two-phase flow regimes of liquid metals are presented. The boundaries of the regimes are described by analytical expressions.

Thermal Radiation on Transient Laminar Gray Gas Flow Past an Oscillating Vertical Plate with Variable Temperature

Authors have presented an analytical approach on transient laminar gray gas flow past an oscillating vertical flat plate in the presence of natural convection and radiation. The fluid is considered to be a gray, absorbing-emitting radiation but non- scattering medium. The Rosseland flux approximation is employed to simulate radiation heat transfer contribution. We have also studied Stoke’s first and second problem to justify the physical significance on this problem. This problem is solved by employing Laplace Transform method. Numerical results of velocity and temperature distributions are depicted graphically. Also, numerical results of frictional shearing stress and critical Grashof number are presented in tables.