Thermodynamic investigation and multi-objective optimization for jet impingement cooling system with Al2O3/water nanofluid
In the present study, a multi-objective optimization procedure combining finite element modeling of impingement cooling system, Response Surface Approximation (RSA) of objective functions and optimization based on Multi-Objective Genetic Algorithm (MOGA), to achieve maximum heat transfer and minimum entropy generation is demonstrated. For the purpose, numerical simulations are performed for impingement cooling system with Al2O3/water nanofluid, to investigate the influence of Reynolds number (Re), non-dimensional channel height (H/L) and nanoparticle volume fraction (Pdbl) on fluid flow, heat transfer and entropy generation. The simulated results illustrate that, a secondary recirculation bubble observed on upper surfaces of fourth (at Re ≥ 500) and fifth (for Re ≥ 800) heat sources, lead to an accumulation of heat. The magnitude of local Nusselt number (Nu) is found to be maximum along stagnation region whereas in the regions of secondary recirculation a minimum value is observed. Further, an increase in overall surface averaged Nusselt number (Nuov) and global total entropy generation (Stot,Ω) is observed with increasing Re,Pdbl and decreasing H/L. Subsequently, Nuov and Stot,Ω are selected as objective functions and are modeled using RSA. Furthermore, MOGA has been implemented to obtain optimum configurations of impingement cooling system encapsulating in the functional space lying on the Pareto-optimal frontier where a trade-off between two performance parameters, Nuov and Stot,Ω are obtained.
A numerical investigation of heat transfer and entropy generation during jet impingement cooling of protruding heat sources without and with porous medium
In the present study, fluid flow and thermal characteristics associated with forced convection cooling of an array of discrete protruding heat sources mounted on impingement plate of channel by an impinging laminar jet is investigated for various Reynolds number (Re) and channel height (H/L). It is observed that, the magnitude of average Nusselt number for all heat sources increases with increasing Re and decreasing H/L, except the regions of heat sources covered by recirculation bubbles which may be due to accumulation of heat resulting in hot spots. In order to eliminate these hot spots, a porous layer is attached to the impingement plate. A parametric study is conducted to predict the performance of porous layer on fluid flow pattern, heat transfer and entropy generation for various values of Darcy number (Da), Reynolds number (Re), channel height (H/L), porosity (∈) and porous layer thickness (h/H). For the purpose, equations governing two-dimensional, time-dependent, incompressible and laminar flow are solved in a Cartesian framework by using Streamline Upwind Petrov-Galerkin (SUPG) Finite Element (FE) method. The generalized Darcy-Forchheimer-Brinkman model is adopted to model the flow in porous medium. The recirculation bubbles on heat sources are completely eliminated with the inclusion of porous layer at Darcy number, Da=10-2. The magnitude of overall Nusselt number and global entropy generation due to heat transfer (Sθ,Ω¯) and fluid friction (Sψ,Ω¯) increases with increasing Da,Re,h/H and decreasing ∈ and H/L. The optimum configuration for maximum heat transfer and minimum entropy generation is observed at Da=10-2,Re=1000,H/L=1.0,h/H=0.75 and ∈=0.5.
A numerical investigation and design optimization of impingement cooling system with an array of air jets
In the present study, fluid flow, heat transfer and entropy generation in impingement cooling system with an array of air jets for different values of Reynolds number (Re), Velocity Ratio (VR) and Channel Height (H/L) are investigated. The magnitude of overall Nusselt number (Nuov‾) and global total entropy generation (Stot,Ω) is found to increase with increasing Re,VR and decreasing H/L. Further, spectral and proper orthogonal decomposition analyses are performed to analyze spatio-temporal dynamics of vortex structures for unsteady configurations of impingement cooling system. It is observed that, along the interface of jet (both primary and secondary) and ambient fluid, the destabilizing effect of shear forces overcome the stabilizing effect of momentum diffusion. This results in evolution of counter-rotating vortex rings along the interfaces of jet and ambient fluid due to shear layer instability. Finally, Multi-Objective Genetic Algorithm (MOGA) has been implemented to obtain optimum configurations of impingement cooling system where a trade-off between two performance parameters, Nuov‾ and Stot,Ωis obtained.
A numerical study on natural convection and entropy generation in a porous enclosure with heat sources
In this paper, fluid flow and thermal characteristics associated with natural convection heat transfer in a porous enclosure containing high temperature heat sources placed on top and bottom walls are studied. For this purpose, two-dimensional, time-dependent Navier-Stokes equations with Darcy-Brinkman-Forchheimer terms are solved in a Cartesian framework by using Streamline Upwind Petrov-Galerkin (SUPG) based finite element method. The effect of heat sources on flow pattern, entropy generation and temperature distribution are studied for different Darcy numbers, porosities and Rayleigh numbers. The results show that maximum entropy generation due to heat transfer irreversibility is observed in the vicinity of heat sources due to the presence of high thermal gradient. The global entropy generation due to fluid friction is found to increase in convection dominated regime. It is also observed that with increasing Darcy number, porosity and Rayleigh number the surface averaged Nusselt number for both top and bottom heat sources is increased. © 2013 Elsevier Ltd. All rights reserved.