NUMERICAL STUDY OF THE ERYTHRITOL MELTING PROCESS IN A TRIDIMENSIONAL RECTANGULAR CAVITY

Many areas of engineering, such as the petrochemical and food industries, use thermal energy storage to achieve better performance. Thermal energy storage can be used as a heat source or a heat sink. The thermal storage/release technology, based on the use of phase change materials (PCMs), which possess a great capacity of heat accumulation, has raised an important practical interest. Indeed, the improved storage density and the constant temperature release of energy allow a more compact heat exchange design and simplify system management. Phase change phenomena occurring during PCM melting and PCM solidification need to be carefully controlled. The aim of the present numerical study is to investigate the heat transfer and hydrodynamic characteristics of a phase change material (PCM) in a rectangular cavity and the melting process dependence on the cavity height. The geometry consists of a mini rectangular cavity which contains PCM. Water or steam flows in the longitudinal direction of the cavity, thus heating or cooling the PCM in the cavity. The computational domain is made of three materials: air, aluminum and PCM. The grid mesh is structured, hexaedrical and refined near the walls. The study is developed with different operation conditions of the mass flow and water temperature. The numerical simulation is developed through CFD (Computational Fluid Dynamics) with the Fluent code. The mathematical model was validated using results available in the literature. Results of temperature, velocity and volume fraction fields indicate that the cavity height has no impact on the melting process

NUMERICAL STUDY OF THE GEOMETRIC INFLUENCE OF A FIN IN A CYLINDRICAL HEAT EXCHANGER FOR MELTING OF PCM

The thermal heat storage it’s an effective way to suit the energy availability with the demand schedule. It can be stored in the means of sensible or latent heat, the latter applying a material denominated Phase Change Material (PCM), which is provided as organic compounds, hydrated salts, paraffins, among others. The latent heat storage systems offer several advantages, like the practically isothermal process of loading and unloading and the high energy density. However, the low thermal conductivity makes the cycle prolonged on these systems, restricting its applicability. Applying computational fluid dynamics, the behavior of the PCM melting process was studied in cylindrical cavities with horizontal and vertical fins, aiming the optimization of the fin geometry. In this way the fin area was kept constant, varying its aspect ratio. The numerical model was validated with results from the literature and it’s composed of the continuity, momentum and energy equations increased by the phase change model. Qualitative and quantitative results are presented, referring to mesh independence, contours of velocity, net fraction and temperature at different moments of the process. The results of the study indicate that the position of the fin in the heat exchanger influences the melting process, although the vertical fins have a faster total melting process, horizontal fins can reach larger partial liquid fractions in less time in the heat exchanger. Such as the position of the fin, the increase of its length propitiates the reduction of the melting time, evidencing the optimal aspect ratio

Effect of extended surfaces on lauric acid melting process in annular cavities

The objective of the present work is to parametrically analyze the effect of extended surfaces’ proportion and positioning on the lauric acid melting process, in an annular cavity. In total 46 geometric configurations were studied, varying between 5 area ratios, 5 proportions, and 2 positions of the extended surface. Numerical simulations performed with the finite volume method were used to conduct the study. The numerical model, composed of the continuity, momentum conservation, and energy conservation equations, plus the enthalpy-porosity phase change model, was validated with experimental data from the literature. The Grid Convergence Index (GCI) was used to evaluate the computational mesh, resulting in an average index of 0.0026%. The results are presented in terms of liquid fraction vs Fourier, and Nusselt number vs. Fourier. Besides, velocity vectors, and streamlines, liquid fraction, and temperature fields, were presented, comparing different instants and geometric arrangements. For the analysis of the results, the melting time was considered as a performance indicator. The results revealed that: while there is solid PCM in the cavity's upper section, the melting rate in systems with horizontal extensions is 15% higher than systems with vertical extensions; when the extended surface thinness is increased, the overall melting time is reduced by more than 10% in vertical arrangements and less than 1.5% in horizontal arrangements; the total melting time is nearly 45% faster in systems with vertical extensions than in systems with horizontal extensions

CFD NUMERICAL ANALYSIS OF A PROPOSED CHANGE IN THE CHANNEL GEOMETRY DOWNSTREAM OF A REVERSED TAINTER GATE

Reversed Tainter gates are often used to control flow in the filling and emptying locks navigation systems of high fall. High speeds and pressure fluctuations may occur in the flow downstream of these gates, the flow cavitation may occur, damaging the structure. One way proposed to mitigate this problem is through geometric changes in the channel downstream of the gate. In this study was analyzed using CFD the effect of an expansion in the roof and the base of the conduit, followed by a straight section and a subsequent contraction until returning to the original geometry. It was observed that the pressure at the base of the conduit increases with the modification, however peaks of positive and negative pressures occur due to the shape change of the geometry of the corners not be smooth. The size of recirculation downstream of the gate increases with the geometrical alteration, increasing the distance required for pressure recovery

NUMERICAL ANALYSIS OF LIQUID FRACTION AND HEAT FLUX IN THE SOLIDIFICATION PROCESS OF ERYTHRITOL IN SPHERES

The demand for renewable energy resources and the need for the development of components which increase how it collects, transforms, stores and distributes this energy, emphasizes the importance of improving current technological systems to meet these demands. Phase change materials (PCM) offer great potential in this area. as they can increase energy efficiency in thermal systems as well as save energy by storing solar energy or waste heat from industrial processes, which is made possible by the high amount of energy stored per mass and volume unit, with low temperature variation. Therefore, it is of high importance that the suggested mathematical and numerical models are capable of analyzing its energy performance. The present work uses a mathematical and numerical model of Computational Fluid Dynamics (CFD), capable of reproducing the solidification process of erythritol in spheres of 10, 20, 30 and 40 mm diameters, with temperature differences of 10, 15, 20, 25, 30 and 40 K between the sphere wall and the phase change temperature of the material. The problem is considered two- dimensional and transient. The model consists of mass, energy, momentum and volume fraction equations. The mathematical and numerical model is validated with experimental results from the literature, presenting good agreement between them. After space and time discretization tests, we analyze liquid fraction over time and heat flux at the sphere wall. The results show that liquid fraction suffers a strong reduction in the beginning of the solidification process due to the high heat flux in the early stages. As the solid layer near the wall increases, it causes an increase in thermal resistance, causing a significant reduction in heat flux

Numerical investigation on phase change materials (PCM): The melting process of erythritol in spheres under different thermal conditions

In this paper, a numerical study on the melting process of the PCM erythritol in 12 and 7 mm diameter spheres subjected to external flow has been carried out. This configuration is analyzed varying the temperature difference between the PCM melting point and the external flow, the Reynolds number as well as the sphere position in the array. The problem is considered two-dimensional in geometry and transient in time. The numerical model here developed consists of the continuity, momentum and energy equations. The results have been initially validated using numerical and experimental data from literature. Afterwards, results of liquid fraction, heat flux and total melting time have been proposed and illustrated. Based on pure observation, a slight difference in the phase change phenomena when comparing different sphere positions in the array has emerged. These phenomena proved to be much more influenced by the external flow temperature and by the Reynolds number. In all cases, at 30% of the total melting time, 50% of the total energy had been absorbed by the PCM. The liquid PCM layer above the solid has a great influence on the heat flux, precisely the more extensive the PCM layer, the lower the heat exchange. The local heat flux value decreases significantly in regions in contact with air and liquid PCM. Contrarily, at the sphere base, where there is solid PCM during the whole process, the local heat flux value is almost constant during the whole melting. Finally, significant differences have emerged when comparing the results referred to the hypothesis here contemplated of uniform heat transfer coefficient and local heat transfer coefficient function of the sphere angle

Theoretical and numerical analysis on phase change materials (PCM): A case study of the solidification process of erythritol in spheres

Phase change materials (PCM) present great potential for energy efficiency gains in thermal systems by storing solar energy or waste heat in industrial processes. This is due to the great amount of energy stored per mass unit within a small temperature range. In this paper we focus, by means of the numerical investigation, on the solidification process of the PCM erythritol in spheres, having 10, 20, 30 and 40 mm diameter, under wall temperatures of 10, 15, 20, 25, 30 and 40 K below the phase change temperature of the material. The problem is considered two-dimensional in geometry and transient in time. The numerical model here adopted consists of mass, momentum, energy and volume fraction equations. The results have been initially validated by comparison with data found in literature. Afterwards, analysis of the convective streams on the liquid PCM, liquid fraction, heat flux in the sphere wall and total solidification times have been widely illustrated. The liquid fraction suffers a sharp reduction at the beginning of the solidification process due to the high heat flux at the initial times. As the solid layer adjacent to the shell increases, it causes an augmentation of thermal resistance, significantly reducing the heat flux. The shape of the curve representing the solid fraction shows similarity with the S-curve pattern of solidification. The total solidification time proved to be dependent on both the diameter length and the temperature difference DT (between phase change material and wall temperature), being its influence reduced for lower temperature values. Finally, the liquid fraction results, as a function of Fourier and Stefan numbers, have been employed to amend a dimensionless correlation found in literature

Combining grey relation analysis and entropy model for evaluating the operational performance: an empirical study

[[abstract]]Decision-making on operational performance evaluation is a complex multi-objective problem. Through the combination of grey relation analysis and information entropy, the evaluation results are more objective and reasonable. This paper would introduce entropy into the weighting calculation of the grey relational analysis method for improving the precision. The improved decision model was applied in four notebook computer original design manufacturer companies. The result presented the proposed method is practical and useful. Significantly, the proposed method provides more flexible and objective information in determine the weights vector of the criteria. Also the study result represented that the combined method had certain scientific and rationality. The evaluation model indicates that this method be more reasonable and easier to grasp than other methods. As a result, it is easier to popularize this evaluation method in enterprises.[[journaltype]]國外[[incitationindex]]SSCI[[ispeerreviewed]]Y[[booktype]]電子版[[countrycodes]]NL