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Because of the complexity involved, most researchers avoid studying the entropy generation due to both convection and radiation. In this study, the heat transfers from the horizontal heated cylinder by natural convection and radiation and the entropy generation is the case study and the ability to combine of these two heat transfer methods depending on the thermodynamic principles. Three heating horizontal cylinders were used during this study. The results indicated that the natural convection is effective parameter to produce the entropy generation when the ratio of Ra/Ra, opt lower than 1 and the Ns increased slightly to 3 at Ra/Ra, opt=0.001, but when the radiation is the effective parameter to produce the entropy generation that is done when Ra/Ra, opt higher than 1 and the Ns increases to 200 at Ra/Ra, opt=1000.

During severe accidents in nuclear reactors, the core and internal structures can melt down and relocate into the reactor pressure vessel (RPV) lower head (LH) forming there a stratified molten corium pool. The pool generally consists of superheated oxidic and metallic liquid layers imposing thermo-mechanical loads on the RPV wall. The in-vessel retention (IVR) strategy employs external cooling with water to maintain RPV integrity. Investigating the thermo-fluid behaviour of corium and predicting heat flux distribution on the vessel wall are crucial. The molten pool exhibits natural convection, which can typically consist of two stratified layers. There exists internally heated (IH) natural convection in the oxidic layer and Rayleigh-Bénard (RB) convection in the surface metallic layer. This study starts by illustrating the mathematical models that involve the numerical study of natural convection flow in molten corium. A verification work of the model has been done using a previous direct numerical simulation (DNS) study, and the results show good agreement. In addition, a scaling theory of the natural convection flow is demonstrated to facilitate the pre-estimation based on the Rayleigh number (Ra) and Prandtl number (Pr). After that, the numerical approaches involved in the numerical simulation of the corium are illustrated, especially focusing on the DNS method. A DNS mesh strategy is proposed in the form of a pipeline from the pre-estimation to the post-check. A scalability study of Nek5000 is performed on four different HPC clusters based on a DNS case of the IH molten convection in a hemispherical geometry with Ra=1.6×1011. The results show a super-liner speedup property of Nek5000 on each cluster within a certain range. Then, three numerical studies focusing on turbulent natural convection flow within both the oxidic and metallic layers of corium are demonstrated and discussed. Through these simulations, the thermos-fluid behaviour of the system is examined, Vid svåra kärnkraftsreaktorhaverier kan härdar och interna strukturer smälta och bilda en skiktad pool av härdsmälta i reaktortankens (RPV) nedre plenum (LH). Denna härdsmälta består vanligtvis av överhettade oxidiska och metalliska vätskeskikt, vilket skapar termomekaniska påfrestningar på reaktortankens väggar. För att bevara integriteten hos reaktortanken används extern kylning med vatten, en teknik som kallas IVR-strategin (In-vessel Retention). En avgörande aspekt av säkerhetsstudier är att förstå och förutsäga hur värmeflödet beter sig inom härdsmältpoolen. Naturlig konvektion är en viktig process som inträffar i poolen, och den delas upp i två skikt: internuppvärmd (IH) naturlig konvektion i oxidskiktet och Rayleigh-Bénard (RB) konvektion i det ytliga metallskiktet. Studien börjar med att beskriva de matematiska modellerna som används för att studera naturlig konvektion i härdsmältan. Dessa modeller har validerats genom att jämföra deras resultat med tidigare utförda direkt numerisk simulering (DNS), och det finns en god överensstämmelse mellan dem. Dessutom presenteras en skalningsteori som gör det möjligt att uppskatta naturlig konvektion baserat på Rayleigh-talet (Ra) och Prandtl-talet (Pr). I nästa steg beskrivs de numeriska metoderna som används för att simulera härdsmältan, särskilt DNS-metoden. En DNS-nätstrategi föreslås, som omfattar allt från förhandsuppskattning till efterhandskontroll. En skalbarhetsstudie utförs med hjälp av Nek5000 på fyra olika högpresterande datorkluster, baserat på ett DNS-fall av IH-smältkonvektion i en hemisfärisk geometri med Ra=1,6×1011 och resultaten visar att Nek5000 har en imponerande hastighetsökningsegenskap på varje kluster inom ett specifikt intervall. Slutligen genomförs tre numeriska studier som fokuserar på turbulent naturlig konvektion i både oxidiska och metalliska skikt av härdsmältan. Dessa simuleringar ger detaljerad information om systemets värmeflödesbeteende, inklusive flödeskonfiguration, temperaturförd, QC 2023-09-29

A new energy-consistent discretization of the viscous dissipation function in incompressible flows is proposed. It is implied by choosing a discretization of the diffusive terms and a discretization of the local kinetic energy equation and by requiring that continuous identities like the product rule are mimicked discretely. The proposed viscous dissipation function has a quadratic, strictly dissipative form, for both simplified (constant viscosity) stress tensors and general stress tensors. The proposed expression is not only useful in evaluating energy budgets in turbulent flows, but also in natural convection flows, where it appears in the internal energy equation and is responsible for viscous heating. The viscous dissipation function is such that a consistent total energy balance is obtained: the 'implied' presence as sink in the kinetic energy equation is exactly balanced by explicitly adding it as source term in the internal energy equation. Numerical experiments of Rayleigh-Bénard convection (RBC) and Rayleigh-Taylor instabilities confirm that with the proposed dissipation function, the energy exchange between kinetic and internal energy is exactly preserved. The experiments show furthermore that viscous dissipation does not affect the critical Rayleigh number at which instabilities form, but it does significantly impact the development of instabilities once they occur. Consequently, the value of the Nusselt number on the cold plate becomes larger than on the hot plate, with the difference increasing with increasing Gebhart number. Finally, 3D simulations of turbulent RBC show that energy balances are exactly satisfied even for very coarse grids; therefore, we consider that the proposed discretization forms an excellent starting point for testing sub-grid scale models.

The dynamics of a closed loop thermosyphon are considered. The model assumes a prescribed heat flux along the loop wall and the contribution of axial diffusion. The well-posedness of the model which consists of a coupled ODE and PDE is shown for both the case with diffusion and without diffusion. Boundedness of solutions, the existence of an attractor, and an inertial manifold is proven, and an exact reduction to a low-dimensional model is obtained for the diffusion case. The reduced systems may have far fewer degrees of freedom than the reduction to the inertial manifold. For the three mode models, equivalence with the classical Lorenz equations is shown. Numerical results are presented for five mode models., NATO, CICYT (Spain), EEC, NSF, Depto. de Análisis Matemático y Matemática Aplicada, Fac. de Ciencias Matemáticas, TRUE, pub

Because of the complexity involved, most researchers avoid studying the entropy generation due to both convection and radiation. In this study, the heat transfers from the horizontal heated cylinder by natural convection and radiation and the entropy generation is the case study and the ability to combine of these two heat transfer methods depending on the thermodynamic principles. Three heating horizontal cylinders were used during this study. The results indicated that the natural convection is effective parameter to produce the entropy generation when the ratio of Ra/Ra, opt lower than 1 and the Ns increased slightly to 3 at Ra/Ra, opt=0.001, but when the radiation is the effective parameter to produce the entropy generation that is done when Ra/Ra, opt higher than 1 and the Ns increases to 200 at Ra/Ra, opt=1000.

Understanding multiphase flow close to the electrode surface is crucial to the design of electrolyzers, such as alkaline water electrolyzers for the production of green hydrogen. Vertical electrodes develop a narrow gas plume near their surface. We apply the integral method to the mixture model. Considering both exponentially varying and step-function gas fraction profiles, we derive analytical relations for plume thickness, velocity profile, and gas fraction near the electrode as a function of height and current density. We verify these analytical relations with the numerical solutions obtained using two-dimensional mixture model simulations. We find that for low gas fractions, the plume thickness decreases with an increase in current density for an exponentially varying gas fraction profile. In contrast, the plume thickness increases with increasing current density at high gas fractions for an approximately step-function-shaped gas fraction profile, in agreement with experiments from the literature., Energy Technology

We present a flexible scalable open-source computational framework, named SECUReFoam, based on the finite-volume library OpenFOAM®, for flow and transport problems in highly heterogeneous geological media and other porous materials. The framework combines geostatistical pre- and post-processing tools with specialised partial differential equations solvers. Random fields, for permeability and other physical properties, are generated by means of continuous or thresholded Gaussian random fields with various covariance/variogram functions. The generation process is based on an explicit spectral Fourier decomposition of the field which, although more computationally intensive than Fast Fourier Transform methods, allows a more flexible choice of statistical parameters and can be used for general geometries and grids. Flow and transport equations are solved for single-phase and variable density problems, with and without the Boussinesq approximation, and for a wide range of density, viscosity, and dispersion models, including dual-continuum (dual permeability or dual porosity) formulations. The mathematical models are here presented in details and the numerical strategies to deal with heterogeneities, equation coupling, and boundary conditions are discussed and benchmarked for the heterogeneous Henry and Horton–Rogers–Lapwood problems, and other test cases. We show that our framework is capable of dealing with large permeability variances, viscous instabilities, and large-scale three-dimensional transport problems.

The Fukushima nuclear accident in 2011 became the basis for consideration of the use of gas as a coolant in nuclear reactors. This is because the convection rate of gas flow in the cooling channel can occur naturally due to differences in density and does not require the help of a pumps for the circulation of the coolant. This study aims to analyze how the flow pattern of an inert gas on a vertical-axial reference by natural convection in a thermal system. The focus of this research is to study the flow parameters of the coolant with a gas phase. This research is an experimental study. The analysis was carried out using a descriptive approach and computer simulation-assisted numerical analysis methods. The results showed that the distribution and variation of heat was radially dominant in the middle so that the coolant channel wall received less heat load. The magnitude of the pressure drop along the vertical-axial channel shows a homogeneous pattern and decreases radially from center to edge. These results indicate the use of inert gas as a coolant can be considered as an alternative coolant in heat systems that do not depend on pumps in operating conditions.

For estimating thermal flow in a nuclear reactor during an accident accurately, it is important to improve the accuracy of computational fluid dynamics simulations. The temperature and flow velocity are not homogeneous and have large variations in a reactor containment vessel because of its very large volume. In addition, Kelm’s work pointed out that the influence of variations of initial and boundary conditions was important. Therefore, it is necessary to set the initial and boundary conditions taking into account the variations of these physical quantities. However, it is a difficult subject to set such complicated initial and boundary conditions. Then, we can obtain realistic initial and boundary conditions and an accurate flow field by data assimilation, and we can improve the accuracy of the simulation result. In this study, we applied data assimilation by a local ensemble transform Kalman filter to a simulation of natural convection behavior in density stratification, and we performed a twin model experiment. We succeeded in estimating the flow fields and improving the simulation accuracy by the data assimilation, even if we applied the boundary condition with error for the true condition., source:https://www.mdpi.com/2311-5521/7/7/237

This paper describes an experimental investigation of natural convection driven by outer surface cooling in the presence of density stratification consisting of an air and helium (as mimic gas of hydrogen) gas mixture in an enclosed vessel. The unique cooling system of the Containment InteGral effects Measurement Apparatus (CIGMA) is used, and findings reveal that the cooling location relative to the stratification plays an important role in determining the interaction behavior of the heat and mass transfer in the enclosed vessel. When the cooling region is narrower than the stratification thickness, the density-stratified region expands to the lower part while decreasing in concentration (stratification dissolution). When the cooling region is wider than the stratification thickness, the stratification is gradually eroded from the bottom with decreasing layer thickness (stratification breakup). This knowledge is useful for understanding the interaction behavior of heat and mass transfer during severe accidents in nuclear power plants., source:https://www.sciencedirect.com/science/article/pii/S030645492100668X?via%3Dihub

It is essential to improve computational fluid dynamics (CFD) analysis accuracy to estimate thermal flow in a containment vessel during a severe accident. Previous studies pointed out the importance of the initial and boundary conditions on the analysis. The purpose of this study is to evaluate the influence of initial and boundary conditions by numerical analysis of natural convection experiments. A density stratification layer was initially formed, and natural convection was induced by the external cooling of the vessel. Mixing by natural convection eroded the density stratification. We applied the RANS model with dynamic turbulent Schmidt number model. We used the measured temperature and gas concentration as the boundary and initial conditions. The erosion velocity did not change much with the initial gas distribution. The temperature boundary condition of small internal structures significantly influenced the fluid temperature distribution, and we evaluated this influence of the small internal structure quantitatively., source:https://www.sciencedirect.com/science/article/pii/S0306454921007441?via%3Dihub

A comparative study of nanofluid (Cu–H2O) and pure fluid (water) is investigated over a moving upright plate surrounded by a porous surface. The novelty of the study includes the unsteady laminar MHD natural transmission flow of an incompressible fluid, to get thermal conductivity of nanofluid is more than pure fluid. The chemical reaction of this nanofluid with respect to radiation absorption is observed by considering the nanoparticles to attain thermal equilibrium. The present work is validated with the previously published work. The upright plate travels with a constant velocity u0, and the temperature and concentration are considered to be period harmonically independent with a constant mean at the plate. The most excellent appropriate solution to the oscillatory pattern of boundary layer equations for the governing flow is computed utilizing the Perturbation Technique. The impacts of factors on velocity, temperature, and concentration are visually depicted and thoroughly elucidated. The fluid features in the boundary layer regime are explored visually and qualitatively. This enhancement is notably significant for copper nanoparticles.

It is essential to improve computational fluid dynamics (CFD) analysis accuracy to estimate thermal flow in a containment vessel during a severe accident. Previous studies pointed out the importance of the initial and boundary conditions on the analysis. The purpose of this study is to evaluate the influence of initial and boundary conditions by numerical analysis of natural convection experiments. A density stratification layer was initially formed, and natural convection was induced by the external cooling of the vessel. Mixing by natural convection eroded the density stratification. We applied the RANS model with dynamic turbulent Schmidt number model. We used the measured temperature and gas concentration as the boundary and initial conditions. The erosion velocity did not change much with the initial gas distribution. The temperature boundary condition of small internal structures significantly influenced the fluid temperature distribution, and we evaluated this influence of the small internal structure quantitatively., source:https://www.sciencedirect.com/science/article/pii/S0306454921007441?via%3Dihub

For estimating thermal flow in a nuclear reactor during an accident accurately, it is important to improve the accuracy of computational fluid dynamics simulations. The temperature and flow velocity are not homogeneous and have large variations in a reactor containment vessel because of its very large volume. In addition, Kelm’s work pointed out that the influence of variations of initial and boundary conditions was important. Therefore, it is necessary to set the initial and boundary conditions taking into account the variations of these physical quantities. However, it is a difficult subject to set such complicated initial and boundary conditions. Then, we can obtain realistic initial and boundary conditions and an accurate flow field by data assimilation, and we can improve the accuracy of the simulation result. In this study, we applied data assimilation by a local ensemble transform Kalman filter to a simulation of natural convection behavior in density stratification, and we performed a twin model experiment. We succeeded in estimating the flow fields and improving the simulation accuracy by the data assimilation, even if we applied the boundary condition with error for the true condition., source:https://www.mdpi.com/2311-5521/7/7/237

This paper describes an experimental investigation of natural convection driven by outer surface cooling in the presence of density stratification consisting of an air and helium (as mimic gas of hydrogen) gas mixture in an enclosed vessel. The unique cooling system of the Containment InteGral effects Measurement Apparatus (CIGMA) is used, and findings reveal that the cooling location relative to the stratification plays an important role in determining the interaction behavior of the heat and mass transfer in the enclosed vessel. When the cooling region is narrower than the stratification thickness, the density-stratified region expands to the lower part while decreasing in concentration (stratification dissolution). When the cooling region is wider than the stratification thickness, the stratification is gradually eroded from the bottom with decreasing layer thickness (stratification breakup). This knowledge is useful for understanding the interaction behavior of heat and mass transfer during severe accidents in nuclear power plants., source:https://www.sciencedirect.com/science/article/pii/S030645492100668X?via%3Dihub

This study x-rays the analysis of convection heat transport of hydromagnetic Forchheimer nanofluid flow in the presence of an ideal internal heat source/sink. A similarity tactic is followed by translating the partial differential models into coupled nonlinear ordinary differential equations. The application of the improved series scheme made it possible for the solution to the translated problems to be found. Pictorial developments such as graphs with legends have been gotten by adopting the Wolfram Mathematica package for the sake of understanding the behavioral pattern of flow, temperature, and specie (nanoparticle) concentrations due to the influence of fluid parameters on them. Results demonstrated that an increase in flow and temperature occurs as buoyancy effect, and optimal heat generation number rises. Intensification of Prandtl number leads to a drop in the flow rate and temperature just as the concentration distribution declines for . The impact of fluid parameters on skin friction was also considered.

Natural convection is a commonly occurring heat-transfer problem in many industrial flows and its prediction with conventional large eddy simulations (LES) at higher Rayleigh numbers using progressively coarser grids leads to increasingly inaccurate estimates of important performance indicators, such as Nusselt number (Nu). Thus, to improve the heat transfer predictions, we utilize Gene Expression Programming (GEP) to develop sub-grid scale (SGS) stress and SGS heat-flux models simultaneously for LES. With that as the focus, in the present study, two geometrically distinct natural convection cases are considered to develop and generalize turbulence models. The Rayleigh-Benard Convection (RBC) is used to develop models, while the Concentric Horizontal Annulus (CHA) is used to test the model generalization. An in-house compressible solver, HiPSTAR, for simulating natural convection flows for low Mach number problems is benchmarked against the experiments and Direct Numerical Simulations (DNS) results. Subsequently, HiPSTAR is used to run simulations for the RBC and CHA configurations and the generated DNS database is then used to train and assess LES models. The models’ development starts with RBC, where the fluid is in a cubic box with the bottom wall as the hot wall and the top wall as the cold wall. The alignment between different basis functions and the Gaussian-filtered SGS stress and SGS heat flux is used to determine the most suitable training framework. The trained models in isotropic form, by utilizing the norm of the grid cell as the length scales demonstrate good performance in the bulk region, but less improved performance in the near wall region. It is shown, that for LES of wall-bounded flow, the GEP models in anisotropic form, i.e. using different grid length scales for the different spatial directions, are required to obtain generalized models suitable for different regions. Consequently, the a-priori results demonstrate a significant improvement in th

In a postulated severe accident, a molten pool with decay heat can form in the lower head of a reactor pressure vessel, threatening the vessel’s structural integrity. Natural convection in molten pools with extremely high Rayleigh (Ra) number is not yet fully understood as accurate simulation of the intense turbulence remains an outstanding challenge. Various models have been implemented in many studies, such as RANS (Reynolds-averaged Navier–Stokes), LES (large-eddy simulation), and DNS (direct numerical simulation). DNS can provide the most accurate results but at the expense of large computational resources. As the significant development of the HPC (high-performance computing) technology emerges, DNS becomes a more feasible method in molten pool simulations. Nek5000 is an open-source code for the simulation of incompressible flows, which is based on a high-order SEM (spectral element method) discretization strategy. Nek5000 has been performed on many supercomputing clusters, and the parallel performance of benchmarks can be useful for the estimation of computation budgets. In this work, we conducted scalability tests of Nek5000 on four different HPC clusters, namely, JUWELS (Atos Bullsquana X1000), Hawk (HPE Apollo 9000), ARCHER2 (HPE Cray EX), and Beskow (Cray XC40). The reference case is a DNS of molten pool convection in a hemispherical configuration with Ra = 1011, where the computational domain consisted of 391 million grid points. The objectives are (i) to determine if there is strong scalability of Nek5000 for the specific problem on the currently available systems and (ii) to explore the feasibility of obtaining DNS data for much higher Ra. We found super-linear speed-up up to 65536 MPI-rank on Hawk and ARCHER2 systems and around 8000 MPI-rank on JUWELS and Beskow systems. We achieved the best performance with the Hawk system with reasonably good results up to 131072 MPI-rank, which is attributed to the hypercube technique on its interconnection. Given t, QC 20230213

A liquid metal layer heated from bottom and cooled from both side and top can be encountered in indus-trial applications. A special interest is from safety design of advanced pressurized water reactors that adopt the so-called in-vessel melt retention (IVR) to mitigate severe accident risk. Quantification of heat transfer in a stratified melt pool in the lower head of a reactor pressure vessel (RPV) is of great impor-tance to the qualification of the IVR strategy. The upper liquid metal layer of the stratified melt pool is heated by the lower molten oxide layer (with decay heat) underneath, and cooled by water outside the reactor vessel and by radiation or flooded water at the top. This is essentially a problem of natural convection and heat transfer in a liquid metal layer heated from bottom and cooled from both side and top. The present study is conducted to numerically investigate the heat transfer characteristics of such layer with an emphasis on the influence of the aspect ratio (ratio of radius to height; R/H) of the liq-uid metal layer. Based on the numerical outcomes, three correlations of heat transfer coefficients (for downward, upward and sideward flows) are also developed to account for the impact the aspect ratio. The numerical simulation results show that, under the same Rayleigh number, the bulk temperature and the upward and sideward heat fluxes all increase with R/H, but the downward heat flux decreases with R/H. The Nusselt numbers in all directions decrease with increasing R/H, as a reduced cooling -heating area ratio due to increasing R/H shall suppress the cooling efficiency and the convection. When R/H is larger than a threshold (-8), the heat transfer characteristics are no longer sensitive to R/H. Each correlation of heat transfer coefficient is developed as the product of two terms: a base correlation of heat transfer coefficient that is Ra dependent only, and an aspect ratio factor that considers the effect of aspect ratio R/H. The develope, QC 20220818

Widespread commercialization of PCM-based latent heat storage systems is limited by the low melting and solidification rates during the phase transition process. In this study, fins are used to enhance the phase change process. A parametric study is conducted to understand the effect of fins on the thermal performance of vertically- and horizontally-oriented rectangular storage tanks. Throughout simulations, the fin volume, and thereby the mass of PCM in the tank, were kept constant. It was observed that the horizontal enclosures can take advantage of the development of strong natural convection flow until near the end of the melting process, whereas with vertical counterparts the strength of the convection currents was diminished during the shrinkage stage. The results suggest that longer and thinner fins are more beneficial for enhancing the melting rate than shorter and thicker fins. It was concluded that for horizontal enclosures with fin lengths of 25 and 35 mm, increasing the number of fins does not necessarily shorten the melting time. The maximum melting time reduction compared to the 3-fin vertical enclosure with a fin length of 25 mm (chosen as our benchmark case) was 75.1% when nine 45-mm-long fins are used in a horizontal enclosure., Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Process and Energy

In the present study, we analyse individual and combined effects of conductive horizontal walls and conductive fins on the natural convection of air in side heated cavities (SHC). The flow and heat transfer are studied for Rayleigh numbers in the range of 104−−109: Direct Numerical Simulation (DNS) is conducted for the lower and Large Eddy Simulation (LES) for the higher Rayleigh numbers (>108). Thermally conductive walls destabilize the flow yielding an earlier transition to turbulence and expedite the decay in boundary layer thickness with increase in Rayleigh number. The preheating/precooling along the conductive walls reduces the actual heat transfer at the vertical walls. Above the fin, instabilities are only marginally enhanced for adiabatic horizontal walls, whereas for conductive horizontal walls, plumes erupt from the fin. This localized Rayleigh-Bénard-like effect triggers 3D instabilities in the entire flow field and yields a steeper slope in Nusselt-Rayleigh diagram. The presence of a fin increases the integral heat transfer by 18% for adiabatic and 21% for conductive horizontal walls. We show that 2D and 3D simulations are similar for the smooth cases (i.e., without fin), but differ by 4% and 13% for the adiabatic and conductive fin cases respectively. The local heat transfer characteristics even deviates up to 50%, therefore a 2D simplification should be avoided., ChemE/Transport Phenomena, J.M. Burgers Center for Fluid Mechanics

Circulation of working fluid in closed geothermal loops is an alternative environmentally friendly approach to harvest subsurface energy compared to open hole geothermal doublet systems. However, the rapid decline of production temperature, low generated thermal power, and difficulties in deepening the system are major limitations. Herein, synthetic studies are presented to investigate the system's performance and improve its longevity for better use of this clean baseload power. The investigations are conducted by implementing appropriate equations of state to model state-of-the-art thermal and hydraulics processes in wellbores and considering various geometrical configurations to adopt proper design strategies. They provide insight for maximizing the generated thermal power, decreasing pumping energy, and avoiding production temperature drawdown. The results indicate that a stable thermal condition could be reached in which not only the temperature breakthrough is avoidable, but also the generated thermal power and production temperature continuously enhance over the project lifetime of one century. Analysis of the thermosiphon effect in the designed systems revealed that even with the pressure loss of 900 kPa at surface installations, the triggered natural flow rate is larger than 11 L/s. This thermosiphon flow rate yields the thermal power production of 2 MW and Cumulative extracted energy of 15 PJ over the project lifetime of 100 years. Restriction of this flow rate to 5 L/s leads to an average extraction temperature of 80 °C. It is also found that a change in the subsurface temperature gradient does not affect the optimal 2 km isolation length of the production well.

In nature, many fluid-like materials exhibit a yield stress below which they behave like a solid. The Bingham model aims to describe such materials. This paper draws some mathematical considerations on the flow of a Bingham fluid in a vertical channel. The situation due to the presence of an external magnetic field and natural convection is analyzed: the external magnetic field, which is orthogonal to the walls of the channel, generates the Lorentz forces that influence the motion through the Hartmann number. The behavior of the velocity, the induced magnetic field and the thickness of the plug regions are discussed and presented graphically. We find that the velocity is a decreasing function of the Bingham and Hartmann numbers. In particular, the presence of the external magnetic field increases the thickness of the plug region. The modulus of the induced magnetic field is not monotone when the Hartmann number changes, but it is a decreasing function of the Bingham number.

In the present study, we analyse individual and combined effects of conductive horizontal walls and conductive fins on the natural convection of air in side heated cavities (SHC). The flow and heat transfer are studied for Rayleigh numbers in the range of 104−−109: Direct Numerical Simulation (DNS) is conducted for the lower and Large Eddy Simulation (LES) for the higher Rayleigh numbers (>108). Thermally conductive walls destabilize the flow yielding an earlier transition to turbulence and expedite the decay in boundary layer thickness with increase in Rayleigh number. The preheating/precooling along the conductive walls reduces the actual heat transfer at the vertical walls. Above the fin, instabilities are only marginally enhanced for adiabatic horizontal walls, whereas for conductive horizontal walls, plumes erupt from the fin. This localized Rayleigh-Bénard-like effect triggers 3D instabilities in the entire flow field and yields a steeper slope in Nusselt-Rayleigh diagram. The presence of a fin increases the integral heat transfer by 18% for adiabatic and 21% for conductive horizontal walls. We show that 2D and 3D simulations are similar for the smooth cases (i.e., without fin), but differ by 4% and 13% for the adiabatic and conductive fin cases respectively. The local heat transfer characteristics even deviates up to 50%, therefore a 2D simplification should be avoided., ChemE/Transport Phenomena, J.M. Burgers Center for Fluid Mechanics

In many applications finned tube bundles are commonly used for heating or cooling purpose. Hence, the natural convection heat transfer from finned heat exchanger configurations with novel design in a chimney was experimentally studied. These novel fin designs use integrated pins to enhance the heat conduction from the fin base to the fin tip as well as the convective heat transfer along the fin surface. Oval tubes with conventional circular plain fins (CPF) as well as novel circular integrated pin fins (CIPF) and serrated integrated pin fins (SIPF) were additively generated by a Selective Laser Melting (SLM) process and installed at the bottom of a 6.5 m long chimney. All heat exchanger designs were tested in a 2-row and 3-row configuration with Rayleigh numbers between 25,000 and 120,000. We found the average Nusselt number of SIPF to be higher and the Nusselt number of the CIPF to be lower compared to the CPF. Furthermore, the 2-row configuration achieved greater Nusselt number compared to the 3-row configuration for all heat exchanger designs. The analysis of the individual tube rows showed highest Nusselt numbers at the first tube row and the lowest at the last tube row for both configurations. However, for the SIPF the difference between the first and second tube row is smaller compared to the CPF and CIPF. In order to evaluate the compactness of the heat exchanger, the volumetric heat flux density was applied. Similar to Nusselt number the volumetric heat flux density enhanced for the SIPF and reduced for the CIPF compared to the conventional design. Also the 2-row configuration reaches greater thermal performance compared to the 3-row configuration. Additionally, the surface area and the volume of the heat exchanger material are 30.7 % and 6.9 % lower for the SIPF compared to the CPF. The experimental outcome was used to develop an empirical heat transfer correlation between Nusselt number, Rayleigh number, fin design and tube row number.

Fins are the extended surfaces used for increase heat transfer rate by convection method. In method of natural convection generally fins used should be of rectangular or circular shape, for intension to dissipate more heat to surrounding and maintain valuable components, parts safe and cool. From study of research paper it should be clear that the fins used for the heat dissipation should not arrange in large number in regulated space. In natural convection the air move along the fins as per buoyancy concept so that the air move upward when it becomes comes in contact with fins and lighter, but logical thing is that the natural air movement carried out in all direction there is no particular direction of air to strike the fin wall that means , air inlet direction is not fixed but the outlet becomes fixed. In case of horizontal based fins array when air strike to fins and heat transfer with these air but hot air trapped in between two fins and transfer rate of heat decreases, also in case of vertical based fins there is problem introduced of boundary layer development, so that fins with large fin height should not be used. To overcome above difficulties in natural convection to transfer heat effectively from plate to introduced winglet type fins array, because of winglet shape of fins air strike to fin wall in any condition of movement of air because the fins are inclined to both horizontal and vertical direction so that the heat transfer from fns is logically more than two cases but it interesting to investigate winglet angle of fins for more heat transfer by natural convection method.

The application of a drying kinetic evaluation methodology applied to a set of measurements generated by drying regional products in a natural flow solar dryer that uses a chimney to help drive the air is presented. The experiments were carried out under solar radiation evaluated in the Caribbean Coast of Colombia, air flow driven by density difference provided by the temperature difference caused by the heating of the dryer. Four groups of measurements were taken with different products: pork in thin fillets, cassava in slices, green mango and ripe mango in slices. This work shows the drying behavior of regional products under an analysis methodology: the so-called Dryer Performance Index (IPS). The results of this methodology applied to the analysis of the data obtained confirm that it is appropriate to evaluate under this methodology., Se presenta la aplicación de una metodología de evaluación de la cinética de secado aplicada a un conjunto de medidas generadas secando productos regionales en un secador solar de flujo natural, que utiliza una chimenea para ayudar a impulsar el aire.  Las experiencias se realizaron bajo radiación solar evaluada en la Costa Caribe de Colombia, flujo de aire impulsado por diferencia de densidad proporcionada para la diferencia de temperatura ocasionada por el calentamiento del secador. Se tomaron cuatro grupos de medidas con distintos productos: cerdo en filetes delgados, yuca en rodajas, mango verde y maduro en rodajas. En el presente trabajo se muestra el comportamiento del secado de productos regionales bajo una metodología de análisis: el denominado Índice de Performance del Secador (IPS). Los resultados de esta metodología aplicadas al análisis de los datos arrojados confirman que es apropiado evaluar bajo esta metodología.

The application of a drying kinetic evaluation methodology applied to a set of measurements generated by drying regional products in a natural flow solar dryer that uses a chimney to help drive the air is presented. The experiments were carried out under solar radiation evaluated in the Caribbean Coast of Colombia, air flow driven by density difference provided by the temperature difference caused by the heating of the dryer. Four groups of measurements were taken with different products: pork in thin fillets, cassava in slices, green mango and ripe mango in slices. This work shows the drying behavior of regional products under an analysis methodology: the so-called Dryer Performance Index (IPS). The results of this methodology applied to the analysis of the data obtained confirm that it is appropriate to evaluate under this methodology., Se presenta la aplicación de una metodología de evaluación de la cinética de secado aplicada a un conjunto de medidas generadas secando productos regionales en un secador solar de flujo natural, que utiliza una chimenea para ayudar a impulsar el aire.  Las experiencias se realizaron bajo radiación solar evaluada en la Costa Caribe de Colombia, flujo de aire impulsado por diferencia de densidad proporcionada para la diferencia de temperatura ocasionada por el calentamiento del secador. Se tomaron cuatro grupos de medidas con distintos productos: cerdo en filetes delgados, yuca en rodajas, mango verde y maduro en rodajas. En el presente trabajo se muestra el comportamiento del secado de productos regionales bajo una metodología de análisis: el denominado Índice de Performance del Secador (IPS). Los resultados de esta metodología aplicadas al análisis de los datos arrojados confirman que es apropiado evaluar bajo esta metodología.

Free convective heat transfer in the closed gap between concentric semi-hemispheres is quantified by means of a numerical approach based on the volume control method using the SIMPLE algorithm. The average Nusselt number is determined for several configurations obtained by varying the cavity’s aspect ratio between 0.15 and 1.5, while the Rayleigh number varies within the 5.33 × 103–4.50 × 108 range. The results show that the correlations available in the literature dealing with concentric whole spheres cannot be used for the configuration treated here. The new correlation between the Nusselt and Rayleigh numbers proposed in this work can be applied in various engineering sectors, such as in the electronic packaging considered in this present work, buildings, and architecture.

The application of a drying kinetic evaluation methodology applied to a set of measurements generated by drying regional products in a natural flow solar dryer that uses a chimney to help drive the air is presented. The experiments were carried out under solar radiation evaluated in the Caribbean Coast of Colombia, air flow driven by density difference provided by the temperature difference caused by the heating of the dryer. Four groups of measurements were taken with different products: pork in thin fillets, cassava in slices, green mango and ripe mango in slices. This work shows the drying behavior of regional products under an analysis methodology: the so-called Dryer Performance Index (IPS). The results of this methodology applied to the analysis of the data obtained confirm that it is appropriate to evaluate under this methodology., Se presenta la aplicación de una metodología de evaluación de la cinética de secado aplicada a un conjunto de medidas generadas secando productos regionales en un secador solar de flujo natural, que utiliza una chimenea para ayudar a impulsar el aire.  Las experiencias se realizaron bajo radiación solar evaluada en la Costa Caribe de Colombia, flujo de aire impulsado por diferencia de densidad proporcionada para la diferencia de temperatura ocasionada por el calentamiento del secador. Se tomaron cuatro grupos de medidas con distintos productos: cerdo en filetes delgados, yuca en rodajas, mango verde y maduro en rodajas. En el presente trabajo se muestra el comportamiento del secado de productos regionales bajo una metodología de análisis: el denominado Índice de Performance del Secador (IPS). Los resultados de esta metodología aplicadas al análisis de los datos arrojados confirman que es apropiado evaluar bajo esta metodología.

Free convective heat transfer in the closed gap between concentric semi-hemispheres is quantified by means of a numerical approach based on the volume control method using the SIMPLE algorithm. The average Nusselt number is determined for several configurations obtained by varying the cavity’s aspect ratio between 0.15 and 1.5, while the Rayleigh number varies within the 5.33 × 103–4.50 × 108 range. The results show that the correlations available in the literature dealing with concentric whole spheres cannot be used for the configuration treated here. The new correlation between the Nusselt and Rayleigh numbers proposed in this work can be applied in various engineering sectors, such as in the electronic packaging considered in this present work, buildings, and architecture.

Seeking proper and reasonable constitutive relations to study the characteristics of flow and heat transfer of viscoelastic fluid is a hot topic in recent years. In this work, a new distributed order time fractional constitution model is put forward to analyse the unsteady natural convection boundary layer flow and heat transfer, in which the magnetic field effect is considered. The mid-point quadrature method is applied to approach the distributed order integrals and the L1-formula is employed to discrete the time fractional derivatives. The influences of different involved parameters on the velocity and temperature distributions and the comparisons of the distributions with different constitution models are presented by graphical illustrations and discussed in detail.

The application of a drying kinetic evaluation methodology applied to a set of measurements generated by drying regional products in a natural flow solar dryer that uses a chimney to help drive the air is presented. The experiments were carried out under solar radiation evaluated in the Caribbean Coast of Colombia, air flow driven by density difference provided by the temperature difference caused by the heating of the dryer. Four groups of measurements were taken with different products: pork in thin fillets, cassava in slices, green mango and ripe mango in slices. This work shows the drying behavior of regional products under an analysis methodology: the so-called Dryer Performance Index (IPS). The results of this methodology applied to the analysis of the data obtained confirm that it is appropriate to evaluate under this methodology., Se presenta la aplicación de una metodología de evaluación de la cinética de secado aplicada a un conjunto de medidas generadas secando productos regionales en un secador solar de flujo natural, que utiliza una chimenea para ayudar a impulsar el aire.  Las experiencias se realizaron bajo radiación solar evaluada en la Costa Caribe de Colombia, flujo de aire impulsado por diferencia de densidad proporcionada para la diferencia de temperatura ocasionada por el calentamiento del secador. Se tomaron cuatro grupos de medidas con distintos productos: cerdo en filetes delgados, yuca en rodajas, mango verde y maduro en rodajas. En el presente trabajo se muestra el comportamiento del secado de productos regionales bajo una metodología de análisis: el denominado Índice de Performance del Secador (IPS). Los resultados de esta metodología aplicadas al análisis de los datos arrojados confirman que es apropiado evaluar bajo esta metodología.

The application of a drying kinetic evaluation methodology applied to a set of measurements generated by drying regional products in a natural flow solar dryer that uses a chimney to help drive the air is presented. The experiments were carried out under solar radiation evaluated in the Caribbean Coast of Colombia, air flow driven by density difference provided by the temperature difference caused by the heating of the dryer. Four groups of measurements were taken with different products: pork in thin fillets, cassava in slices, green mango and ripe mango in slices. This work shows the drying behavior of regional products under an analysis methodology: the so-called Dryer Performance Index (IPS). The results of this methodology applied to the analysis of the data obtained confirm that it is appropriate to evaluate under this methodology., Se presenta la aplicación de una metodología de evaluación de la cinética de secado aplicada a un conjunto de medidas generadas secando productos regionales en un secador solar de flujo natural, que utiliza una chimenea para ayudar a impulsar el aire.  Las experiencias se realizaron bajo radiación solar evaluada en la Costa Caribe de Colombia, flujo de aire impulsado por diferencia de densidad proporcionada para la diferencia de temperatura ocasionada por el calentamiento del secador. Se tomaron cuatro grupos de medidas con distintos productos: cerdo en filetes delgados, yuca en rodajas, mango verde y maduro en rodajas. En el presente trabajo se muestra el comportamiento del secado de productos regionales bajo una metodología de análisis: el denominado Índice de Performance del Secador (IPS). Los resultados de esta metodología aplicadas al análisis de los datos arrojados confirman que es apropiado evaluar bajo esta metodología.

The application of a drying kinetic evaluation methodology applied to a set of measurements generated by drying regional products in a natural flow solar dryer that uses a chimney to help drive the air is presented. The experiments were carried out under solar radiation evaluated in the Caribbean Coast of Colombia, air flow driven by density difference provided by the temperature difference caused by the heating of the dryer. Four groups of measurements were taken with different products: pork in thin fillets, cassava in slices, green mango and ripe mango in slices. This work shows the drying behavior of regional products under an analysis methodology: the so-called Dryer Performance Index (IPS). The results of this methodology applied to the analysis of the data obtained confirm that it is appropriate to evaluate under this methodology., Se presenta la aplicación de una metodología de evaluación de la cinética de secado aplicada a un conjunto de medidas generadas secando productos regionales en un secador solar de flujo natural, que utiliza una chimenea para ayudar a impulsar el aire.  Las experiencias se realizaron bajo radiación solar evaluada en la Costa Caribe de Colombia, flujo de aire impulsado por diferencia de densidad proporcionada para la diferencia de temperatura ocasionada por el calentamiento del secador. Se tomaron cuatro grupos de medidas con distintos productos: cerdo en filetes delgados, yuca en rodajas, mango verde y maduro en rodajas. En el presente trabajo se muestra el comportamiento del secado de productos regionales bajo una metodología de análisis: el denominado Índice de Performance del Secador (IPS). Los resultados de esta metodología aplicadas al análisis de los datos arrojados confirman que es apropiado evaluar bajo esta metodología.

[EN] Architectural heritage, building materials and interior space are highly susceptible to temperature and relative humidity. A better knowledge of the hygrothermal dynamics inside buildings allows an adequate conservation of heritage. This work compares three non-destructive techniques (NDT), such as temperature and relative humidity sensors, finite element simulations (CFD) and thermographic pictures (IRT). The work has made it possible to carry out an assessment of the risk of condensation over a year and to identify affected periods and areas of the building. Sensors and IRT pictures provide real data to validate CFD simulations, facilitating a global analysis of the building. The results provided reflect a great concordance between the NDTs used.

Three-dimensional (3D) natural convection (NC) processes in heterogeneous porous media and associated energy losses and mixing processes are still poorly understood. Studies are limited to two-dimensional domains because of computational burden, worsened by heterogeneity, which may demand grid refinement at high permeability zones for accurate evaluation of buoyancy forces. We develop a meshless Fourier series (FS) solution of the natural convection problem in a porous enclosure driven by thermal or compositional variations. We derive the vector potential formulation of the governing equations for vertical and horizontal heterogeneity of hydraulic conductivity and implement an efficient method to solve the spectral system with an optimized number of Fourier modes. 3D effects are induced either by heterogeneity or variable boundary conditions. The developed FS solution is verified against a finite element solution obtained using COMSOL Multiphysics. We evaluate entropy generation (viscous dissipation and mixing) indicators using FS expansions and assess how they are affected by heterogeneity. We define a large-scale Rayleigh number to account for heterogeneity by adopting an arithmetic average effective permeability. The FS solution is used to investigate the effect of the large-scale Rayleigh number and level of heterogeneity on NC processes and energy losses. Results show that increasing the Rayleigh number intensifies fluid flow, thus enhancing convective transfer, which causes a dramatic increase in total entropy generation. Both viscous dissipation and mixing (and thus chemical reactions in the solute transport case) increase. The third dimension effect, which also enhances flow and entropy indicators, is more pronounced at high Rayleigh numbers. Surprisingly, entropy variation indicators remain virtually unchanged in response to changes in heterogeneity, for fixed Rayleigh number, which we attribute to the arithmetic average permeability being indeed appropriat

Fins are the extended surfaces used for increase heat transfer rate by convection method. In method of natural convection generally fins used should be of rectangular or circular shape, for intension to dissipate more heat to surrounding and maintain valuable components, parts safe and cool. From study of research paper it should be clear that the fins used for the heat dissipation should not arrange in large number in regulated space. In natural convection the air move along the fins as per buoyancy concept so that the air move upward when it becomes comes in contact with fins and lighter, but logical thing is that the natural air movement carried out in all direction there is no particular direction of air to strike the fin wall that means , air inlet direction is not fixed but the outlet becomes fixed. In case of horizontal based fins array when air strike to fins and heat transfer with these air but hot air trapped in between two fins and transfer rate of heat decreases, also in case of vertical based fins there is problem introduced of boundary layer development, so that fins with large fin height should not be used. To overcome above difficulties in natural convection to transfer heat effectively from plate to introduced winglet type fins array, because of winglet shape of fins air strike to fin wall in any condition of movement of air because the fins are inclined to both horizontal and vertical direction so that the heat transfer from fns is logically more than two cases but it interesting to investigate winglet angle of fins for more heat transfer by natural convection method.

Fins are the extended surfaces used for increase heat transfer rate by convection method. In method of natural convection generally fins used should be of rectangular or circular shape, for intension to dissipate more heat to surrounding and maintain valuable components, parts safe and cool. From study of research paper it should be clear that the fins used for the heat dissipation should not arrange in large number in regulated space. In natural convection the air move along the fins as per buoyancy concept so that the air move upward when it becomes comes in contact with fins and lighter, but logical thing is that the natural air movement carried out in all direction there is no particular direction of air to strike the fin wall that means , air inlet direction is not fixed but the outlet becomes fixed. In case of horizontal based fins array when air strike to fins and heat transfer with these air but hot air trapped in between two fins and transfer rate of heat decreases, also in case of vertical based fins there is problem introduced of boundary layer development, so that fins with large fin height should not be used. To overcome above difficulties in natural convection to transfer heat effectively from plate to introduced winglet type fins array, because of winglet shape of fins air strike to fin wall in any condition of movement of air because the fins are inclined to both horizontal and vertical direction so that the heat transfer from fns is logically more than two cases but it interesting to investigate winglet angle of fins for more heat transfer by natural convection method.

Fins are the extended surfaces used for increase heat transfer rate by convection method. In method of natural convection generally fins used should be of rectangular or circular shape, for intension to dissipate more heat to surrounding and maintain valuable components, parts safe and cool. From study of research paper it should be clear that the fins used for the heat dissipation should not arrange in large number in regulated space. In natural convection the air move along the fins as per buoyancy concept so that the air move upward when it becomes comes in contact with fins and lighter, but logical thing is that the natural air movement carried out in all direction there is no particular direction of air to strike the fin wall that means , air inlet direction is not fixed but the outlet becomes fixed. In case of horizontal based fins array when air strike to fins and heat transfer with these air but hot air trapped in between two fins and transfer rate of heat decreases, also in case of vertical based fins there is problem introduced of boundary layer development, so that fins with large fin height should not be used. To overcome above difficulties in natural convection to transfer heat effectively from plate to introduced winglet type fins array, because of winglet shape of fins air strike to fin wall in any condition of movement of air because the fins are inclined to both horizontal and vertical direction so that the heat transfer from fns is logically more than two cases but it interesting to investigate winglet angle of fins for more heat transfer by natural convection method.

Fins are the extended surfaces used for increase heat transfer rate by convection method. In method of natural convection generally fins used should be of rectangular or circular shape, for intension to dissipate more heat to surrounding and maintain valuable components, parts safe and cool. From study of research paper it should be clear that the fins used for the heat dissipation should not arrange in large number in regulated space. In natural convection the air move along the fins as per buoyancy concept so that the air move upward when it becomes comes in contact with fins and lighter, but logical thing is that the natural air movement carried out in all direction there is no particular direction of air to strike the fin wall that means , air inlet direction is not fixed but the outlet becomes fixed. In case of horizontal based fins array when air strike to fins and heat transfer with these air but hot air trapped in between two fins and transfer rate of heat decreases, also in case of vertical based fins there is problem introduced of boundary layer development, so that fins with large fin height should not be used. To overcome above difficulties in natural convection to transfer heat effectively from plate to introduced winglet type fins array, because of winglet shape of fins air strike to fin wall in any condition of movement of air because the fins are inclined to both horizontal and vertical direction so that the heat transfer from fns is logically more than two cases but it interesting to investigate winglet angle of fins for more heat transfer by natural convection method.

Fins are the extended surfaces used for increase heat transfer rate by convection method. In method of natural convection generally fins used should be of rectangular or circular shape, for intension to dissipate more heat to surrounding and maintain valuable components, parts safe and cool. From study of research paper it should be clear that the fins used for the heat dissipation should not arrange in large number in regulated space. In natural convection the air move along the fins as per buoyancy concept so that the air move upward when it becomes comes in contact with fins and lighter, but logical thing is that the natural air movement carried out in all direction there is no particular direction of air to strike the fin wall that means , air inlet direction is not fixed but the outlet becomes fixed. In case of horizontal based fins array when air strike to fins and heat transfer with these air but hot air trapped in between two fins and transfer rate of heat decreases, also in case of vertical based fins there is problem introduced of boundary layer development, so that fins with large fin height should not be used. To overcome above difficulties in natural convection to transfer heat effectively from plate to introduced winglet type fins array, because of winglet shape of fins air strike to fin wall in any condition of movement of air because the fins are inclined to both horizontal and vertical direction so that the heat transfer from fns is logically more than two cases but it interesting to investigate winglet angle of fins for more heat transfer by natural convection method.

Fins are the extended surfaces used for increase heat transfer rate by convection method. In method of natural convection generally fins used should be of rectangular or circular shape, for intension to dissipate more heat to surrounding and maintain valuable components, parts safe and cool. From study of research paper it should be clear that the fins used for the heat dissipation should not arrange in large number in regulated space. In natural convection the air move along the fins as per buoyancy concept so that the air move upward when it becomes comes in contact with fins and lighter, but logical thing is that the natural air movement carried out in all direction there is no particular direction of air to strike the fin wall that means , air inlet direction is not fixed but the outlet becomes fixed. In case of horizontal based fins array when air strike to fins and heat transfer with these air but hot air trapped in between two fins and transfer rate of heat decreases, also in case of vertical based fins there is problem introduced of boundary layer development, so that fins with large fin height should not be used. To overcome above difficulties in natural convection to transfer heat effectively from plate to introduced winglet type fins array, because of winglet shape of fins air strike to fin wall in any condition of movement of air because the fins are inclined to both horizontal and vertical direction so that the heat transfer from fns is logically more than two cases but it interesting to investigate winglet angle of fins for more heat transfer by natural convection method.

Дисертаційна робота присвячена вирішенню актуального завдання розробки наукових засад для створення ефективних теплообмінних пристроїв на базі плоскоовальних труб з неповним оребренням, що працюють в умовах природної конвекції та природної тяги. У роботі виконані комплексні експериментальні та CFD-дослідження теплообміну, аеродинамічного опору, характеристик і структури течії при роботі плоскоовальних труб з неповним оребренням в умовах природної конвекції та природної тяги. Отримані узагальнюючі залежності для розрахунку інтенсивності конвективного теплообміну поодиноких плоскоовальних труб з неповним оребренням в умовах вільної конвекції та для розрахунку інтенсивності конвективного теплообміну та аеродинамічного опору шахових пакетів плоскоовальних труб з неповним оребренням при малих числах Рейнольдса.

peer-reviewed, Aircraft confined compartments are subject to a wide range of boundary conditions during operation which leads to the setting up of complex internal thermal environments. These compartments require strict thermal management to ensure safe and reliable operation of installed systems. This work investigates the thermal and fluid flow fields in one such compartment the crown area in a fuselage of a commercial aircraft which contains thermally dissipating equipment. Experimental heat transfer and Ply measurements are compared to 3D numerical simulations and are shown to be in very good agreement. There was found to be significant thermal stratification present due to the ventilation not penetrating into the bulk of the fluid. Convective heat transfer coefficients on the surfaces of the dissipating equipment varied as a function of their location, with the highest values occurring when they are placed close to the ventilation inlet. An enthalpic correction was applied to 2D simulations leading to significantly reduced solution times, and results which give a good approximation of the 3D model results.This type of detailed study of aircraft confined compartments is necessary to improve understanding of the flow regimes present in these areas, and leads to optimal positioning of installed systems in terms of thermal management, as well improving global thermal aircraft model predictions., ACCEPTED, peer-reviewed

peer-reviewed, Natural convection heat transfer from a heat generating horizontal cylinder enclosed in a square cavity where a temperature difference exists across its vertical walls has been experimentally investigated for the range 2×104 < Racyl < 8×104 and a Pr of 0.71. Temperature and cylinder Nusselt number measurements were taken for a range of θ∗. θ∗ is defined as a ratio of cylinder and cavity Grashof numbers. It has been found that at the lower values of Racyl, the heat transfer from the cylinder compares well with correlations available in literature. As Racyl increases however, it deviates away and the overall heat transfer from the cylinder is increased when compared to these correlations due to the interaction from the cavity. 2D-PIV measurements of the flow structures inside the compartment were conducted. They show an increased interaction between the flow structures generated by the cylinder and by the cavity with increasing θ∗, corresponding to the increase in the heat transfer from the cylinder. It is observed that the recirculation generated by the temperature gradient imposed on the cavity is broken down as the plume from the cylinder becomes stronger and a transition process is observed, whereby the flow transitions from being dominated by the temperature difference across the cavity to that dominated by the temperature difference due to the cylinder, ACCEPTED, peer-reviewed

Les phénomènes de transport de matière ainsi que la cinétique des réactions chimiques sont des processus importants en électrochimie car ceux-ci contrôlent le courant mesuré. Dans ce contexte, nous nous intéressons à de simples réactions électrochimiques et à la classe des réactions de transfert couplés électrons-protons (PCET), jouant un rôle important dans les phénomènes biologiques et la conversion d’énergie. Ces réactions impliquent le transfert d’électron(s) et de proton(s) et sont représentées par un schéma carré. Alors que la cinétique de transfert d’électrons est largement étudiée, la cinétique de transfert de protons l’est plus rarement. Ces réactions sont en effet supposées être très rapides alors qu’il existe des situations où les réactions de protonation constituent l’étape limitante. La première partie de la thèse consiste à étudier la cinétique des réactions de protonation en tenant compte de la catalyse de Brönsted. Par le biais de simulations numériques, nous montrons que la catalyse augmente la réversibilité des voltampérogrammes cycliques, à des pH où le transfert couplé s’opère. Les prédictions numériques ont été comparées aux données expérimentales et les résultats sont encourageants car une même tendance est observée. L’accord quantitatif n’est cependant pas satisfaisant à ce stade. Les phénomènes de transport étant connus pour affecter les processus à l’électrode, la seconde partie de la thèse est consacrée à l’étude de l’influence de la convection. Nous commençons par présenter les différentes raisons qui peuvent expliquer les déviations expérimentales par rapport à la diffusion seule, comme la convection naturelle induite par des gradients de densité ou de tension superficielle. Nous présentons le concept de convection spontanée associé aux mouvements microscopiques de la solution. Bien que les fondements théoriques de la convection spontanée soient discutables, la théorie permet de reproduire les résultats d’un certain nombre d’expériences, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

This paper deals with the estimation of the discretization error and the definition of an optimum h-adaptive process in the finite element analysis of natural frequencies and modes. Consistent and lumped mass matrices are considered. In the first case, the discretization error essentially proceeds from the stiffness modelization, so it is possible to apply the same error estimators than those considered in static problems. On the other hand, the error associated with the modelization of the inertial properties must be taken into account if lumped mass matrices are used. As far as h-adaptivity is concerned, it is usually interesting to obtain meshes with a specified error for each mode. However, traditional criteria for static problems consider only one load case. Defining the optimum mesh as the one that gets the desired error with the minimum number of elements, a method is proposed for the h-adaptive process taking into account a set of natural modes simultaneously. The proposed methods have been validated by applying them to bi-dimensional test problems. © 2001 Elsevier Science B.V. All rights reserved.

Дисертаційна робота присвячена вирішенню актуального завдання розробки наукових засад для створення ефективних теплообмінних пристроїв на базі плоскоовальних труб з неповним оребренням, що працюють в умовах природної конвекції та природної тяги. У роботі виконані комплексні експериментальні та CFD-дослідження теплообміну, аеродинамічного опору, характеристик і структури течії при роботі плоскоовальних труб з неповним оребренням в умовах природної конвекції та природної тяги. Отримані узагальнюючі залежності для розрахунку інтенсивності конвективного теплообміну поодиноких плоскоовальних труб з неповним оребренням в умовах вільної конвекції та для розрахунку інтенсивності конвективного теплообміну та аеродинамічного опору шахових пакетів плоскоовальних труб з неповним оребренням при малих числах Рейнольдса., The dissertation is devoted to solving the urgent problem of developing scientific foundations for creating efficient heat exchange devices based on flat oval tubes with incomplete finning, operating under conditions of natural convection and natural draft. In this work, complex experimental and CFD studies of heat transfer, aerodynamic drag, characteristics and flow structure during the operation of flat oval tubes with incomplete finning under conditions of natural convection and natural thrust have been carried out. The results of experimental and computational studies of a single flat - oval tube with incomplete finning under free convection are presented. According to the results of CFD modeling of a number of sizes of flat-oval tubes with incomplete finning, generalized calculation dependences are obtained, which describe with sufficient accuracy the heat 28 transfer intensity of single flat-oval tube with incomplete finning under natural convection conditions in a wide range of characteristics of their basic geometries. Experimental studies of heat transfer of single-row and double-row staggered and corridor bundles of flat-oval tubes with incomplete finning in the conditions of free convection and natural draft in the range of Rayleigh numbers 3000 < Ra < 30000 are performed. The difference of heat transfer in the conditions of free convection and natural draft is shown. Heat transfer of tube bundles in the conditions of natural draft is in 1,8 ... 2 times more than in the conditions of free convection. The results of the comparative analysis of heat exchange of staggered and corridor bundles showed that the intensity of heat exchange of staggered and corridor bundles in the conditions of free convection and natural draft at Rayleigh numbers Ra < 7000 is almost the same. At Rayleigh numbers 7000 < Ra < 20000 in the mode of natural draft the intensity of heat exchange of a staggered bundle is 10 ... 13% higher, than corridor. To obtain a complete picture of t, Диссертация посвящена решению актуальной задачи разработки научных основ для создания эффективных теплообменных устройств на базе плоскоовальных труб с неполным оребрением, работающих в условиях естественной конвекции и естественной тяги. В работе выполнены комплексные экспериментальные и CFD-исследования теплообмена, аэродинамического сопротивления, характеристик и структуры течения при работе плоскоовальных труб с неполным оребрением в условиях естественной конвекции и естественной тяги. Получены обобщающие зависимости для расчета интенсивности конвективного теплообмена одиночных плоскоовальных труб с неполным оребрением в условиях естественной конвекции и для расчета интенсивности конвективного теплообмена и аэродинамического сопротивления шахматных пакетов плоскоовальных труб с неполным оребрением при малых числах Рейнольдса.

Internal report, Bénard-Marangoni convection and its significance with regard to film boiling heat transfer from micro heat sinks is discussed. In recent works cooling performance of micro heat sinks has been studied showing that two-phase cooling could be more efficient than single-phase cooling. However, in those previous works was also found that the critical heat flux (CHF) i.e., the transition from a nucleate boiling regime to an almost insulating film boiling regime was the main limitation of two-phase cooling. Here, it is shown that owing to the induced thermal gradient along the fin and the very small fin-spacing, Bénard-Marangoni convection (which was neither considered nor mentioned so far) can play an important role and in fact driven the entire film boiling heat and mass transfer process. The reason behind this lies in the fact that in film boiling the interfacial vapor-liquid velocity is the capital factor rather than the bulk velocity, and this can be at least one order of magnitude higher from Bénard-Marangoni convection. Utilizing a simplified physical model, an analytical expression for the film boiling heat transfer coefficient driven by Bénard-Marangoni was derived. Computational Fluid Dynamics (CFD) simulations were performed and comparison between natural and Bénard-Marangoni convective heat transfer presented, This research was supported by the Spanish Ministry 281 of Economy and Competitiveness under fellowship grant 282 Ramon y Cajal: RYC-2013-13459, Preprint