Numerical simulations of the effect of an isotropic heat field on the entropy generation due to natural convection in a square cavity (original) (raw)
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A comprehensive numerical study on entropy generation during natural convection is studied in a square cavity subjected to a wide variety of thermal boundary conditions. Entropy generation terms involving thermal and velocity gradients are evaluated accurately based on the elemental basis set via the Galerkin finite element method. The thermal and fluid irreversibilities during the conduction and convection dominant regimes are analyzed in detail for various fluids (Pr ¼ 0.026,988.24) within Ra ¼ 10 3 -10 5 . Further, the effect of Ra on the total entropy generation and average Bejan number is discussed. It is observed that thermal boundary conditions significantly affect the thermal mixing, temperature uniformity, and the entropy generation in the cavity. A case where the bottom wall is hot isothermal with linearly cooled side walls and adiabatic top wall is found to result in high thermal mixing and a higher degree of temperature uniformity with minimum total entropy generation.
E3S Web of Conferences, 2021
The idea to carry out an exercise to compare the calculation of entropy generation for unsteady natural convection in a square cavity with vertical sides that are maintained at different temperatures was motived by the observation, in the literature, of inaccurate or often erroneous results concerning the values of this significant physical entity. It then appeared necessary to reconsider this problem in order to ensure its consistent assessment. The new approach that we propose allows a direct access to the value of the entropy generation by considering the exact values of the thermophysical properties of the working fluid, which depends on the Prandtl and the Rayleigh numbers.
Article Entropy Generation at Natural Convection in an Inclined Rectangular Cavity
2011
Natural convection in an inclined rectangular cavity filled with air is numerically investigated. The cavity is heated and cooled along the active walls whereas the two other walls of the cavity are adiabatic. Entropy generation due to heat transfer and fluid friction has been determined in transient state for laminar natural convection by solving numerically: the continuity, momentum and energy equations, using a Control Volume Finite Element Method. The structure of the studied flows depends on four dimensionless parameters which are: the thermal Grashof number, the inclination angle, the irreversibility distribution ratio and the aspect ratio of the cavity. The obtained results show that entropy generation tends towards asymptotic values for lower thermal Grashof number values, whereas it takes an oscillative behavior for higher values of thermal Grashof number. Transient entropy generation increases towards a maximum value, then decreases asymptotically to a constant value that depends on aspect ratio of the enclosure. Entropy generation increases with the increase of thermal Grashof number, irreversibility distribution ratio and aspect ratio of the cavity. Bejan number is used to measure the predominance of either thermal or viscous irreversibility. At local level, irreversibility charts show that entropy generation is mainly localized on bottom corner of the left heated wall and upper corner of the right cooled wall.
Heat Transfer Engineering, 2014
This article numerically studies entropy generation due to natural convection in a rectangular cavity with circular corners. In this work, in order to solve the governing equations, an explicit finite-volume procedure and a time-marching method are utilized. Also, instead of the conventional algorithms of SIMPLE, SIMPLEM, and SIMPLEC, an artificial compressibility technique is applied for coupling the continuity to the momentum equations. Entropy generation, as a representation of irreversibility and efficiency loss in engineering heat transfer processes, is analyzed in detail. In this work, effects of the radius of walls corner, Rayleigh number, and distribution ratio on total entropy generation, Nusselt number, and Bejan number are also evaluated. The results show that entropy generation decreases with the increase of the radius of the walls' corner and increases with the increase of Rayleigh number, aspect ratio, and irreversibility ratio.
Natural convection and entropy generation in a square cavity
International Journal of Energy Research, 1998
A natural convection in a square cavity finds considerable interest in thermal engineering applications. However, the use of entropy generation concept enables to identify the optimum conditions for its practical application. Consequently, in the present study, natural convection in a square cavity with differential top and bottom wall temperatures is investigated. A numerical scheme using the control volume approach is introduced when discretizing the governing flow and energy equations. The study is extended to include the analysis of the entropy in the cavity. It is found that the local rise of temperature occurs at the right bottom of the cavity due to vertical circulation developed in the cavity. The entropy generation amplifies when circulation along the x-axis increases and, the entropy generation becomes minimum for a particular Rayleigh number.
Thermal Science, 2018
Numerical simulation of the natural convection and entropy generation in an air-filled cubical cavity with active lateral walls is performed in this work. Both the lateral front and right sidewalls are maintained at an isothermal cold temperature. While an isothermal hot temperature is applied for both the lateral back and left sidewalls. The upper and lower walls are kept adiabatic. Entropy generation rates due to the fluid friction and the heat transfer are simulated by using the Second law of thermodynamics. Results are illustrated for Rayleigh numbers varied from (103 ? Ra ? 106). It was shown that the increase in the Rayleigh number leads to increase the average Nusselt number and to decrease the Bejan number. Also, it was found that both, Sth, and Stot, increase slightly with the increase in Rayleigh number until they reach (Ra = 105) and then begin to jump after this value. After (Ra = 105), the increase in both, Stot, and Sfr, is greater than Sth. Moreover, it was observed t...
Effects of Thermal Boundary Conditions on Entropy Generation during Natural Convection
Numerical Heat Transfer, Part A: Applications, 2011
A comprehensive numerical study on entropy generation during natural convection is studied in a square cavity subjected to a wide variety of thermal boundary conditions. Entropy generation terms involving thermal and velocity gradients are evaluated accurately based on the elemental basis set via the Galerkin finite element method. The thermal and fluid irreversibilities during the conduction and convection dominant regimes are analyzed in detail for various fluids (Pr ¼ 0.026,988.24) within Ra ¼ 10 3 -10 5 . Further, the effect of Ra on the total entropy generation and average Bejan number is discussed. It is observed that thermal boundary conditions significantly affect the thermal mixing, temperature uniformity, and the entropy generation in the cavity. A case where the bottom wall is hot isothermal with linearly cooled side walls and adiabatic top wall is found to result in high thermal mixing and a higher degree of temperature uniformity with minimum total entropy generation.
Entropy generation and natural convection in rectangular cavities
Applied Thermal Engineering, 2009
This work presents a numerical analysis of entropy generation in rectangular cavities that were submitted to the natural convection process. This natural convection process was caused by temperature differences between the vertical walls of the cavities. Momentum and energy equations were used to solve this problem. These equations were coupled by the Boussinesq approximation. Initially the cavities were submitted to uniform temperature and velocity fields. The hypothesis of perfect insulation was considered for the top and bottom walls of the cavity. Impermeability and non-slip condition in the boundary were assumed for every wall of the cavity. The numerical analysis is performed through a two-dimensional model with the Finite Volume method. The results of the entropy generation obtained to a square cavity were used to validate the numerical model and it presented good concordance with results from other authors. Additionally, an analysis of the entropy generation in rectangular cavities was performed with five aspect ratios, five Rayleigh numbers and four irreversibility coefficients. The results of this work indicate that: (a) the total entropy generation in steady state increases linearly in both cases, the aspect ratio and the irreversibility coefficient, and exponentially with the Rayleigh number; (b) the influence of the aspect ratio on Bejan number is proportional to Rayleigh number and inversely proportional to the irreversibility coefficient; (c) for the same aspect ratio, the entropy generation due to the viscous effects increases with the Rayleigh number and, for a certain Rayleigh number, the entropy generation due to the viscous effects also increases with the aspect ratio.
Entropy generation due to natural convection in a partially heated cavity by local RBF-DQ method
Meccanica, 2011
Entropy generation due to natural convection in isosceles triangular enclosures with inclination angles (u) filled with a fluid-saturated porous medium has been studied numerically. The enclosure has different inclination angles and it is non-uniformly heated from one side. The finite difference technique was adopted to solve the governing equations of this natural convection problem. Then, entropy generation due to heat transfer irreversibility (HTI) and fluid friction irreversibility (FFI) was calculated from its definition using dependent variables of velocities and temperature fields. Calculations were performed for different Rayleigh numbers Ra in the range of 100 6 Ra 6 1000 and inclination angle, 0 6 u 6 180 . It is found that both inclination angles and Rayleigh numbers make important effect on natural convection heat transfer, fluid flow and entropy generation. The highest entropy generation due to HTI and FFI and stream function are observed at u ¼ 90 . Multiple cells were formed at this angle. Streamlines, isotherms and entropy contours are symmetric inside the enclosure for both u ¼ 0 and u ¼ 180 .
International Communications in Heat and Mass Transfer, 2020
Entropy generation and heat transfer caused by the natural convection in a square cavity, which is linearly heated from one of the vertical walls and uniformly cooled from the other vertical wall, is studied numerically. The upper and lower horizontal walls are insulated perfectly. According to the five different locations of the center of linear heating (0, 0.25, 0.5, 0.75, and 1) is carried out for Rayleigh numbers between 10 3 and 10 6. Discrete equations are solved by the Boussinesq approach, according to the SIMPLE algorithm. The numerical results are demonstrated as streamline, isotherm, heatline, Bejan number, and entropy generation (heat transfer induced, fluid friction-induced and total) contours, local and average values of Nusselt number, Bejan number, and entropy generation as a function of the center of the linear heating position and Rayleigh number. As a result of the study, it is observed that the position of the center of linear heating has a significant effect on entropy generation and heat transfer caused by the natural convection. It is shown that as the center of linear heating shifted upwards, the fluid flow moved to the right, and its intensity decreased, but the heat transfer increased substantially.