• Energy Research

The entropy generation minimization method is applied to the optimization of a buoyancy-driven laminar magnetohydrodynamic flow in a long vertical rectangular duct with thin conducting or insulating walls. The flow takes place under a strong uniform magnetic field applied transversally to one pair of walls and is driven by a known constant temperature gradient aligned with the field. Numerical solutions for the velocity and electric current density in both fluid and walls are calculated using a spectral collocation method. It is shown that an optimum value of the wall conductance ratio (i.e. the ratio of the electrical conductance of the wall to that of the fluid) that minimizes the global entropy generation rate can be found. The analysis of the irreversibilities caused by heat conduction, viscosity and Joule dissipation allows to explain the existence of the optimum value.

Abstract Combined effects of hydrodynamic slip, magnetic field, suction/injection and convective boundary conditions on the global entropy generation in steady flow of an incompressible electrically conducting fluid through a channel with permeable plates are studied. Analytical solutions of the governing equations are obtained in closed form. Particularly, using thermal boundary conditions of the third kind, the temperature field is derived analytically. Also, the influences of the governing parameters on global entropy generation are discussed in detail and depicted graphically. The analysis of our results indicates that entropy generation minimization can be achieved by appropriate combination of the geometrical and physical parameters of the system. It is possible to determine optimum values of Hartmann number, Biot number and Prandtl numbers which lead to a minimum global entropy generation rate. The effects of slip flow on the optimum values of some other parameters are also explored.

The production of electric power through the oscillatory motion of an electrically conducting fluid in a continuous electrode Faraday generator is considered. The performance of this alternate magnetohydrodynamic (MHD) generator is analyzed using the conventional isotropic electrical efficiency and an overall second law efficiency, based on the global entropy generation rate. The velocity, electric current density and temperature fields for the oscillatory Hartmann flow are calculated in order to assess, in terms of the entropy generation rate, the dissipative phenomena caused by fluid friction, Joule heating and heat transfer in the MHD generator. The overall second law efficiency is used to determine optimum operation conditions that minimize process irreversibilities.

A methodology for the optimization of renewable hybrid low power generation systems (RHLPS) is presented, analyzing its performance under different control strategies and thus reducing the costs of power generation using the existing equipment, and varying only the configuration of the factory settings. The above is achieved through the use of software tools for simulations and sensitivity analysis. In the first instance, a description of the different control strategies that have been applied to the RHLPSs is made. Secondly, a RHLPS optimization methodology is developed by means of control strategies. As a third and last point, the methodology is applied to a system in operation, where, through simulations, the optimal values are obtained and those allow to analyze the operation of the system under different control strategies. The results show that an appropriate control strategy allows a better performance and operation of the systems, and therefore it is important to perform an optimization and operational analysis to the existing systems, to make a better use of the equipment, as well as the available renewable resources.

Abstract Combined effects of hydrodynamic slip, magnetic field, suction/injection, thermal radiation, nanoparticle volume fraction and convective boundary conditions on the heat transfer and global entropy generation in a viscous electrically conducting nanofluid flow through a microchannel with permeable plates are studied. Analytical solutions of the momentum and the energy equations are obtained in closed form. Particularly, considering the radiative term, joule heating and viscous dissipation in the energy equation, the temperature field of the nanofluid is derived analytically. Influences of pertinent parameters on global entropy generation are discussed in detail and depicted graphically. Analysis of our results indicates that entropy generation minimization can be achieved by appropriate combination of the geometrical and physical parameters of the system. It is possible to determine optimum values of radiation parameter, nanoparticle volume fraction, Hartmann and Biot numbers which lead to a minimum global entropy generation rate. The Nusselt number is also calculated and explored for different conditions. Optimum values of nanoparticle volume fraction and magnetic field strength that maximize heat transfer are derived.

The effects of slip flow on heat transfer and entropy generation by considering the conjugate heat transfer problem in microchannels are studied, analytically. The heat transfer equations in the fluid and the finite thickness walls of the microchannel are solved analytically using thermal boundary conditions of the third kind at the outer surfaces of the walls and continuity of temperature and heat flux across the fluid–wall interfaces. After the analytic solutions for the velocity and temperature fields in the fluid and walls of microchannel are obtained, the entropy generation rate is discussed in detail and investigated considering slip flow and convective effects, simultaneously. The results show that the global entropy generation rate is minimized for certain suitable combination of the geometrical and physical parameters of the system. It is possible to find an optimum slip velocity which leads to a minimum global entropy generation rate. The Nusselt number is also calculated and explored for different conditions. An optimum value of the slip length that maximizes the heat transfer is derived.

Abstract The heat transfer and entropy generation in a magnetohydrodynamic flow of Al 2 O 3 –water nanofluid through a porous vertical microchannel with nonlinear radiative heat flux were investigated numerically. Then, combined effects of nanoparticle volume fraction, hydrodynamic slip, magnetic field, suction/injection and thermal radiation on heat transfer and entropy generation were studied. The dimensionless governing equations were solved numerically by applying Runge–Kutta integration method together with shooting technique. In this study, the accuracy of the numerical results was verified by comparing its predictions with exact solutions of model without both radiation effects and buoyancy force. Here, different from previous literature, heat transfer subject to nonlinear thermal radiation, Joule heating and viscous dissipation was solved and analyzed using conjugate convective-radiative heat transfer on the boundary surfaces. Moreover, influences of pertinent parameters on nanofluid velocity, temperature, local and global entropy generation and Nusselt number were discussed in detail and illustrated graphically. Based on the numerical results, it was proved that the global entropy generation decreased with both nanoparticle volume fraction and suction/injection Reynolds number while it increased with Grashof number (Buoyancy force intensity), radiation parameter and conduction-radiation parameter. In addition, it was possible to determine optimum values of slip flow with minimum values of global entropy generation rate. The Nusselt number was also calculated and explored for different conditions. In this way, optimum values of Grashof number with maximum heat transfer on the heated left plate were derived.

The Analysis and Optimization in Micro-hydro-generation is the best way to improve new renewable systems. As is known, the Micro-hydroelectric generation is an alternative in areas that has zero access to electrical energy, or isolated areas. The objective of this paper, is to show how a methodology could improve the behavior in the hydroelectric generation. In this case, is based on design considerations by Bilal Abdullah. This paper is about an specific test bench installed in Chiapas, with flow conditions from 4 to 16 l/min, the flow considerations are presented in a new scheme, that is proposed in a previous work made by us. In this specific case, includes the evaluation of the flow due to a tank that is installed in the evaluation area, and the mathematical models are adapted to this case.

The entropy generation minimization method is applied to the optimization of a magnetohydrodynamic flow between two infinite parallel walls of finite electrical conductivity. The aim is to minimize the total energy loss due to irreversibilities produced by heat conduction, viscosity and Joule dissipation. The analytic solutions for the velocity, temperature and electric current density fields are used to calculate the global entropy generation rate explicitly in terms of the dimensionless parameters of the problem. It is shown that the entropy generation reaches a minimum when the walls that contain the flow are cooled convectively in an asymmetric way. In order to get a deeper insight of the physical process, a detailed analysis of the different dissipation sources is carried out.