scholarly journals Time Evolution Features of Entropy Generation Rate in Turbulent Rayleigh-Bénard Convection with Mixed Insulating and Conducting Boundary Conditions

Entropy ◽  
2020 ◽  
Vol 22 (6) ◽  
pp. 672
Author(s):  
Yikun Wei ◽  
Pingping Shen ◽  
Zhengdao Wang ◽  
Hong Liang ◽  
Yuehong Qian
Keyword(s):  
Energy Transfer ◽  
Kinetic Energy ◽  
Boundary Conditions ◽  
Time Evolution ◽  
Entropy Generation ◽  
Thermal Convection ◽  
Entropy Generation Rate ◽  
Total Entropy ◽  
Wide Range ◽  
Rayleigh Benard

Time evolution features of kinetic and thermal entropy generation rates in turbulent Rayleigh-Bénard (RB) convection with mixed insulating and conducting boundary conditions at Ra = 109 are numerically investigated using the lattice Boltzmann method. The state of flow gradually develops from laminar flow to full turbulent thermal convection motion, and further evolves from full turbulent thermal convection to dissipation flow in the process of turbulent energy transfer. It was seen that the viscous, thermal, and total entropy generation rates gradually increase in wide range of t/τ < 32 with temporal evolution. However, the viscous, thermal, and total entropy generation rates evidently decrease at time t/τ = 64 compared to that of early time. The probability density function distributions, spatial-temporal features of the viscous, thermal, and total entropy generation rates in the closed system provide significant physical insight into the process of the energy injection, the kinetic energy, the kinetic energy transfer, the thermal energy transfer, the viscous dissipated flow and thermal dissipation.

Theoretical and experimental investigation of an organic Rankine cycle for a waste heat recovery system

2009 ◽  
Vol 223 (5) ◽  
pp. 523-533 ◽  
Author(s):  
W Gu ◽  
Y Weng ◽  
Y Wang ◽  
B Zheng
Keyword(s):  
Heat Source ◽  
Entropy Generation ◽  
Waste Heat ◽  
Organic Rankine Cycle ◽  
Rankine Cycle ◽  
Entropy Generation Rate ◽  
Working Fluid ◽  
Total Entropy ◽  
Waste Heat Recovery System ◽  
Cycle Efficiency

This article describes and evaluates an organic Rankine cycle (ORC) for a waste heat recovery system by both theoretical and experimental studies. Theoretical analysis of several working fluids shows that cycle efficiency is very sensitive to evaporating pressure, but insensitive to expander inlet temperature. Second law analysis was carried out using R600a as a working fluid and a flow of hot air as a heat source, which is not isothermal, along the evaporator. The result discloses that the evaporator's internal and external entropy generation is the main source of total entropy generation. The effect of the heat source temperature, evaporating pressure, and evaporator size on the entropy generation rate is also presented. The obtained useful power is directly linked to the total entropy generation rate according to the Gouy—Stodola theorem. The ORC testing system was established and operated using R600a as a working fluid and hot water as a heat source. The maximum cycle efficiency of the testing system is 5.2 per cent, and the testing result also proves that cycle efficiency is insensitive to heat source temperature, but sensitive to evaporating pressure. The entropy result also shows that internal and external entropy of the evaporator is the main source of total entropy generation.

scholarly journals Entropy Generation Rates in Two-Dimensional Rayleigh–Taylor Turbulence Mixing

Entropy ◽  
2018 ◽  
Vol 20 (10) ◽  
pp. 738 ◽  
Author(s):  
Xinyu Yang ◽  
Haijiang He ◽  
Jun Xu ◽  
Yikun Wei ◽  
Hua Zhang
Keyword(s):  
Time Evolution ◽  
Entropy Generation ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Two Dimensional ◽  
Spatial Averaging ◽  
Mixing Process ◽  
Thermal Entropy ◽  
Large Gradient ◽  
Turbulence Mixing

Entropy generation rates in two-dimensional Rayleigh–Taylor (RT) turbulence mixing are investigated by numerical calculation. We mainly focus on the behavior of thermal entropy generation and viscous entropy generation of global quantities with time evolution in Rayleigh–Taylor turbulence mixing. Our results mainly indicate that, with time evolution, the intense viscous entropy generation rate s u and the intense thermal entropy generation rate S θ occur in the large gradient of velocity and interfaces between hot and cold fluids in the RT mixing process. Furthermore, it is also noted that the mixed changing gradient of two quantities from the center of the region to both sides decrease as time evolves, and that the viscous entropy generation rate ⟨ S u ⟩ V and thermal entropy generation rate ⟨ S θ ⟩ V constantly increase with time evolution; the thermal entropy generation rate ⟨ S θ ⟩ V with time evolution always dominates in the entropy generation of the RT mixing region. It is further found that a “smooth” function ⟨ S u ⟩ V ∼ t 1 / 2 and a linear function ⟨ S θ ⟩ V ∼ t are achieved in the spatial averaging entropy generation of RT mixing process, respectively.

Entropy Analysis of the Nonlinear Convective Flow of a Jeffrey Fluid over an Inclined Sheet with Variable Electrical Conductivity and Thermal Conductivity

2021 ◽  
Vol 52 ◽  
pp. 73-91
Author(s):  
Jacob A. Gbadeyan ◽  
Joseph O. Akinremi
Keyword(s):  
Thermal Conductivity ◽  
Electrical Conductivity ◽  
Boundary Conditions ◽  
Entropy Generation ◽  
Convective Flow ◽  
Transverse Magnetic ◽  
Entropy Generation Rate ◽  
Jeffrey Fluid ◽  
Convective Boundary ◽  
Fourth Order Method

A steady two-dimensional nonlinear convective flow of a viscous, incompressible, electrically conducting, and non-Newtonian Jeffrey fluid over an inclined stretching sheet with convective boundary conditions and entropy generation is studied under the influence of transverse magnetic field, electrical conductivity and thermal conductivity. The thermal conductivity and electrical conductivity are temperature dependent functions. The governing continuity, momentum and energy equations are transformed to ordinary differential equations (ODEs) using appropriate similarity variables. The resulting coupled ODEs and the corresponding boundary conditions, are solved numerically using Runge-Kutta fourth order method and shooting technique. The velocity, entropy generation rate, temperature and Bejan distributions are presented graphically and discussed. The numerical values of the skin-friction and Nusselt number are obtained and also discussed for various thermophysical parameters through a Table. Furthermore, a comparison with earlier work done with limiting case was carried out and found to be in excellent agreement.

scholarly journals Analytical Analysis of Heat Transfer and Entropy Generation in a Tube Filled with Double-Layer Porous Media

Entropy ◽  
2020 ◽  
Vol 22 (11) ◽  
pp. 1214 ◽  
Author(s):  
Kun Yang ◽  
Wei Huang ◽  
Xin Li ◽  
Jiabing Wang
Keyword(s):  
Heat Transfer ◽  
Porous Medium ◽  
Porous Media ◽  
Nusselt Number ◽  
Double Layer ◽  
Entropy Generation ◽  
Single Layer ◽  
Entropy Generation Rate ◽  
Total Entropy ◽  
Interfacial Radius

The heat transfer and entropy generation in a tube filled with double-layer porous media are analytically investigated. The wall of the tube is subjected to a constant heat flux. The Darcy-Brinkman model is utilized to describe the fluid flow, and the local thermal non-equilibrium model is employed to establish the energy equations. The solutions of the temperature and velocity distributions are analytically derived and validated in limiting case. The analytical solutions of the local and total entropy generation, as well as the Nusselt number, are further derived to analyze the performance of heat transfer and irreversibility of the tube. The influences of the Darcy number, the Biot number, the dimensionless interfacial radius, and the thermal conductivity ratio, on flow and heat transfer are discussed. The results indicate, for the first time, that the Nusselt number for the tube filled with double-layer porous media can be larger than that for the tube filled with single layer porous medium, while the total entropy generation rate for the tube filled with double-layer porous media can be less than that for the tube filled with single layer porous medium. And the dimensionless interfacial radius corresponding to the maximum value of the Nusselt number is different from that corresponding to the minimum value of the total entropy generation rate.

Entropy generation in MHD free convection of nanoliquid within a square open chamber with a solid body

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Sivaraj Chinnasamy ◽  
Vignesh E. ◽  
Mikhail Sheremet
Keyword(s):  
Entropy Production ◽  
Entropy Generation ◽  
Solid Body ◽  
Thermal Convection ◽  
Energy Transport ◽  
Entropy Generation Rate ◽  
Liquid Motion ◽  
Content Type ◽  
Aspect Ratios ◽  
Solid Block

Purpose The study aims to investigate magnetohydrodynamics thermal convection energy transference and entropy production in an open chamber saturated with ferrofluid having an isothermal solid block. Design/methodology/approach Analysis of thermal convection phenomenon was performed for an open chamber saturated with a nanofluid having an isothermal solid unit placed inside the cavity with various aspect ratios. The left border temperature is kept at Tc. An external cooled nanofluid of fixed temperature Tc penetrates into the domain from the right open border. The nanofluid circulation is Newtonian, incompressible, and laminar. The uniform magnetic field of strength B at the tilted angle of γ is applied. The finite volume technique is used to work out the non-linear equations of liquid motion and energy transport. For Rayleigh number (Ra=1e+7), numerical simulations were executed for varying the solid volume fractions of the nanofluid (ϕ = 0.01–0.04), the aspect ratios of a solid body (As = 0.25–4), the Hartmann number (Ha = 0–100), the magnetic influence inclination angle (γ = 0–π/2) and the non-dimensional temperature drop (Ω = 0.001–0.1) on the liquid motion, heat transference and entropy production. Findings Numerical outcomes are demonstrated by using isolines of temperature and stream function, profiles of mean Nusselt number and entropy generations. The results indicate that the entropy generation rate and mean Nu can be decreased with an increase in Ha. The inner solid block of As = 0.25 reflects the maximum heat transfer rate in comparison with other considered blocks. The addition of nano-sized particles results in a growth of energy transport and mean entropy generations. Originality/value An efficient computational technique has been developed to solve natural convection problem for an open chamber. The originality of this research is to scrutinize the convective transport and entropy production in an open domain with inner body. The outcomes would benefit scientists and engineers to become familiar with the investigation of convective energy transference and entropy generation in open chambers with inner bodies, and the way to predict the energy transference strength in the advanced engineering systems.

The Equivalence of Maximum Power and Minimum Entropy Generation Rate in the Optimization of Power Plants

1996 ◽  
Vol 118 (2) ◽  
pp. 98-101 ◽  
Author(s):  
Adrian Bejan
Keyword(s):  
Heat Transfer ◽  
Power Plant ◽  
Entropy Generation ◽  
Heat Input ◽  
Power Plants ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Total Entropy ◽  
Minimum Entropy ◽  
Minimum Entropy Generation

It is shown that to maximize the power output of a power plant is equivalent to minimizing the total entropy generation rate associated with the power plant. This equivalence is illustrated by using two of the oldest and simplest models of power plants with heat transfer irreversibilities. To calculate the total entropy generation rate correctly, one must recognize that the optimization process (e.g., the variability of the heat input) requires “room to move,” i.e., an additional, usually overlooked, contribution to the total entropy generation rate.

scholarly journals Effect of variable thermal conductivity on entropy generation in a plate with internal energy generation

2018 ◽  
Vol 144 ◽  
pp. 04001
Author(s):  
T. K. Favas ◽  
G. Jilani
Keyword(s):  
Thermal Conductivity ◽  
Mathematical Model ◽  
Entropy Generation ◽  
Energy Generation ◽  
Variable Thermal Conductivity ◽  
Entropy Generation Rate ◽  
Fluid Temperature ◽  
Numerical Predictions ◽  
Wide Range ◽  
Generation Parameter

The current numerical investigation aims at analyzing the effect of variable thermal conductivity on local and global entropy generation rates in an energy generating plate dissipating heat by conjugate conduction-forced convection heat transfer. In order to fulfill this objective, the physical model of the plate dissipating heat into surrounding coolant is transformed into a mathematical model governing the temperature field in the plate as well as flow and thermal fields in the fluid. The resulting mathematical model, being a set of coupled and non linear partial differential equations, is solved by adopting stream function-vorticity formulation and by employing Alternating direction implicit scheme. Keeping Prandtl number of the fluid, temperature of the free stream coolant and maximum permissible plate temperature as fixed, numerical predictions are obtained for wide range of values of aspect ratio, conduction-convection parameter, energy generation parameter and flow Reynolds number. It is concluded that unrealistic constant thermal conductivity assumption leads to underestimation of entropy generation rates. It is also found that an increase in energy generation parameter results in significant increase in underestimation of global entropy generation rate.

scholarly journals Multi-Objective Optimization of Braun-Type Exothermic Reactor for Ammonia Synthesis

Entropy ◽  
2021 ◽  
Vol 24 (1) ◽  
pp. 52
Author(s):  
Tianchao Xie ◽  
Shaojun Xia ◽  
Chao Wang
Keyword(s):  
Entropy Generation ◽  
Flow Rate ◽  
Ammonia Synthesis ◽  
Storage System ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Inlet Temperature ◽  
Total Entropy ◽  
Nsga Ii ◽  
Molar Flow Rate

The exothermic reactor for ammonia synthesis is a primary device determining the performance of the energy storage system. The Braun-type ammonia synthesis reactor is used as the exothermic reactor to improve the heat release rate. Due to the entirely different usage scenarios and design objectives, its parameters need to be redesigned and optimized. Based on finite-time thermodynamics, a one-dimensional model is established to analyze the effects of inlet gas molar flow rate, hydrogen–nitrogen ratio, reactor length and inlet temperature on the total entropy generation rate and the total exothermic rate of the reactor. It’s found that the total exothermic rate mainly depends on the inlet molar flow rate. Furthermore, considering the minimum total entropy generation rate and maximum total exothermic rate, the NSGA-II algorithm is applied to optimize seven reactor parameters including the inlet molar flow rate, lengths and temperatures of the three reactors. Lastly, the optimized reactor is obtained from the Pareto front using three fuzzy decision methods and deviation index. Compared with the reference reactor, the total exothermic rate of the optimized reactor is improved by 12.6% while the total entropy generation rate is reduced by 3.4%. The results in this paper can provide some guidance for the optimal design and application of exothermic reactors in practical engineering.

scholarly journals Entropy Generation and Exergy Analysis of Premixed Fuel-Air Combustion in Micro Porous Media Burner

Entropy ◽  
2020 ◽  
Vol 22 (10) ◽  
pp. 1104
Author(s):  
N. C. Ismail ◽  
M. Z. Abdullah ◽  
N. M. Mazlan ◽  
K. F. Mustafa
Keyword(s):  
Porous Media ◽  
Double Layer ◽  
Entropy Generation ◽  
Equivalence Ratio ◽  
Single Layer ◽  
Exergy Efficiency ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Total Entropy ◽  
Porous Media Burner

The performance of porous media micro-burners plays an important role in determining thermal efficiency and improving our daily life. Nowadays, a lot of scholars are actively involved in this research area and ongoing studies are still being carried out due to the burners’ excellent performance. The exergy efficiency and entropy generation of a porous media burner are strongly dependent on the characteristics of the flame and its thermal behavior. In this study, a single-layer and double-layer porous media form were constructed to investigate the effects of various types of porous foam arrangement in a cylindrical burner. The burner was operated using premixed butane-air combustion with an inner diameter of 23 mm and a length of 100 mm. The experiments were carried out in rich fuel conditions with an equivalence ratio, φ ranging from 1.3 to 2.0. The results showed significant improvement in the thermal and exergy efficiency with an increase in the equivalence ratio in a double-layer compared with a single-layer. The peak temperature recorded was 945.21 °C at φ = 1.3 for a porcelain single-layer, and the highest exergy efficiency was 83.47% at φ = 2.0 for an alumina-porcelain double-layer burner. It was also found that the average temperature of the burner wall decreased with an increase in the equivalence ratios for PMB2 and PMB4, whereas the average wall temperature for PMB3 was largely unaffected by the equivalence ratios. The total entropy generation rate reached the highest value at φ = 2.0 for all PMB configurations, and the highest percentage increase for total entropy generation rate was 46.09% for PMB1. The exergy efficiency for all burners was approximately similar with the highest exergy efficiency achieved by PMB4 (17.65%). In addition, the length and location of the flame with thermal distribution was significantly affected by the equivalence ratio between the single-layer and double-layer porous material. Overall, a double-layer porous media burner showed the best performance calculated based on the second law of thermodynamics when compared with other configurations, and it is ideal for domestic application.

Single-phase natural circulation loop using oils and ternary hybrid nanofluids: Steady-state and transient thermo-hydraulics

2020 ◽  
pp. 1-32
Author(s):  
Mayaram Sahu ◽  
Jahar Sarkar ◽  
Laltu Chandra
Keyword(s):  
Steady State ◽  
Mass Flow Rate ◽  
Entropy Generation ◽  
Mass Flow ◽  
Flow Rate ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Width Ratio ◽  
Total Entropy ◽  
Hybrid Nanofluids

Abstract Steady-state and transient behaviours of single-phase natural circulation loop (SPNCL) are investigated using four thermal oils (Therminol VP1, Paratherm CR, Dowtherm A and Dowtherm Q) and water-based ternary hybrid (various combinations of different nature and shaped nanoparticles: Al2O3, Cu, CNT and Graphene) nanofluids as loop fluid. The influences of nanoparticle volume concentration and loop height to width ratio on the mass flow rate and total entropy generation rate of SPNCL are investigated. Results disclose that ternary hybrid nanofluids enhance flow initiation, reduce fluctuation and are expected to attain a steady-state faster than water. Steady-state mass flow rate increases/decreases for ternary hybrid nanofluid depending on the shape of the nanoparticle and total entropy generation rate decreases as compared to water. Thermal oil shows a higher mass flow rate and total entropy generation rate as compared to water. Al2O3-Cu-CNT-water and paratherm CR show the best result among all ternary hybrid nanofluids and thermal oils, respectively. The nanoparticle shape decides the optimum nanoparticle volume fraction. Increasing the height to width ratio decreases the total entropy generation and upsurges the mass flow rate at specified input power. The optimum height to width ratio depends on fluid.

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