Entropy generation analysis of two-dimensional high-temperature confined jet

In high-temperature systems, thermal radiation becomes the dominant mode of heat transfer. The analysis of entropy generation mechanism is very important to optimize the second-law performance of these energy conversion devices. In this paper, the entropy generation in a two-dimensional high-temperature confined jet flow is analyzed. The computation of combined radiation and convection heat transfer is carried out with the help of a CFD code, and the entropy generation due to heat transfer and fluid friction is calculated as post-processed quantities with the computed data of velocity, temperature and radiative intensity. Numerical results show the entropy generation due to radiative transfer cannot be omitted in high-temperature systems such as boilers and furnaces, in which thermal radiation is one of the main modes of heat transfer. In the case that the temperatures of the inlet gas and the top and bottom are not changed, the total entropy generation number decreases with the increase of jet Reynolds number and Boltzmann number, respectively. For enhancing heat transfer and advancing energy conversion efficiency, large jet Reynolds number and Boltzmann number should be selected.     

A miniaturized power generation system cascade utilizing thermal energy of a micro-combustor: Design and modelling

Currently, combustion-based micro power devices encounter the problem of low conversion efficiency. A miniaturized power generation system cascade utilizing thermal energy of a micro-combustor is proposed, because thermophotovoltaic (TPV) cells and thermoelectric (TE) modules work at different temperature levels. The system consists of a planar micro-combustor with a bended extension at the exit, two GaSb TPV modules to convert high temperature thermal radiation and two Bi–Te based TE modules attached to the bended extension to harness medium temperature thermal energy. The mathematical modelling approach to quantify the power output and conversion efficiency is systematically presented. The modelling results show that the integration of the TE modules could significantly improve the system efficiency. When burning the H₂/air mixture, the overall system efficiency could reach 2.5% under the flow condition of U₀ = 3 m/s and Φ = 1.0. Finally, measures for better thermal management to further enhance the conversion efficiency are discussed.     

Entropy generation in a hollow cylinder with temperature dependent thermal conductivity and internal heat generation with convective–radiative surface cooling

This article investigates entropy generation in an asymmetrically cooled hollow cylinder with temperature dependent thermal conductivity and internal heat generation. The inside surface of the cylinder is cooled by convection on its inside surface while the outside surface experiences simultaneous convective–radiative cooling. The thermal conductivity of the cylinder as well as the internal heat generation within the cylinder are linear functions of temperature, introducing two nonlinearities in the one-dimensional steady state heat conduction equation. A third nonlinearity arises due to radiative heat loss from the outside surface of the cylinder. The nonlinear system is solved analytically using the differential transformation method (DTM) to obtain the temperature distribution which is then used to compute local and total entropy generation rates in the cylinder. The accuracy of DTM is verified by comparing its predictions with the analytical solution for the case of constant thermal conductivity and constant internal heat generation. The local and total entropy generations depend on six dimensionless parameters: heat generation parameter Q, thermal conductivity parameter β, conduction–convection parameters Nc₁ and Nc₂, conduction–radiation parameter Nr, convection sink temperature δ and radiation sink temperature η.     

Modeling and performance of microscale thermophotovoltaic energyconversion devices

We analyze the feasibility of energy conversion devices that exploit microscale radiative transfer of thermal energy in thermophotovoltaic devices. By bringing a hot source of thermal energy very close to a receiver fashioned as a pn-junction, the near-field effect of radiation tunneling can enhance the net power flux. We use the fluctuational electrodynamic approach to microscale radiative transfer to account for the spacing effect, which provides the net transfer of photons to the receiver as a function of the separation between the emitter and receiver. We calculate the power output from the microscale device using standard thermophotovoltaic device relations. The results for the performance of a device based on indium gallium arsenide indicate that a ten-fold increase in power throughput may be realized with little loss in efficiency. Furthermore, we develop a model of the microscale device itself that indicates the influence of semiconductor band-gap, energy, carrier lifetime and doping     

Entropy flow,entropy generation,exergy flux,and optimal absorbing temperature in radiative transfer between parallel plates

Taking nonequilibrium radiative heat transfer between two surfaces as an example, the nonequilibrium thermodynamics of radiation is studied and discussed. The formulas of entropy flow, entropy generation, exergy flux, and optimal temperature of absorbing surface for maximum exergy output are derived. The result is a contribution to the thermodynamic analysis and optimization of solar energy utilization and can be applied in more complex radiative heat transfer cases.     

Entropy flow, entropy generation, exergy flux, and optimal absorbing temperature in radiative transfer between parallel plates

Taking nonequilibrium radiative heat transfer between two surfaces as an example, the nonequilibrium thermodynamics of radiation is studied and discussed. The formulas of entropy flow, entropy generation, exergy flux, and optimal temperature of absorbing surface for maximum exergy output are derived. The result is a contribution to the thermodynamic analysis and optimization of solar energy utilization and can be applied in more complex radiative heat transfer cases.     

Entropy generation analysis in a uniformly heated microchannel heat sink

Present investigation analyzes the issue of entropy generation in a uniformly heated microchannel heat sink (MCHS). Analytical approach used to solve forced convection problem across MCHS, is a porous medium model based on extended Darcy equation for fluid flow and two-equation model for heat transfer. Simultaneously, closed form velocity solution in a rectangular channel is employed to capture z-directional viscous effect diffusion and its pronounced effect on entropy generation through fluid flow. Subsequently, governing equations are cast into dimensionless form and solved analytically. Second law analysis of problem is then conducted on the basis of obtained velocity and temperature fields and expressions for local and average entropy generation rate are derived in dimensionless form. Average entropy generation rate is then utilized as a criterion for assessing the system performance. Finally, the effect of influential parameters such as, channel aspect ratio (α_S), group parameter (Br/Ω), thermal conductivity ratio (C) and porosity (ε) on thermal and total entropy generation is investigated. In order to examine the accuracy of the analysis, the results of thermal evaluation are compared to one of the previous investigations conducted for thermal optimization of MCHS.     

Second-law analysis of flow and heat transfer inside a microannulus

The purpose of this work is to investigate the entropy generation in a microannulus flow. Fully developed laminar flow is considered with uniform heat flux at the walls. The viscous dissipation effect, the velocity slip and the temperature jump at the wall are taken into consideration. The velocity and temperature profiles are obtained analytically and used to compute the entropy generation rate. The effects of Kn, Br, Br/Ω and r? on velocity, temperature profiles, entropy generation rate and average entropy generation are discussed. The present analytical results for the case with and without the viscous dissipation effect are compared with those available in the literature and an excellent agreement is observed. Entropy generation is shown to decrease with an increase in Kn while increasing Br, Br/Ω and r? results in increasing entropy generation.     

Entransy flux of thermal radiation and its application to enclosures with opaque surfaces

Entransy is a new concept developed in recent years to measure the transport ability of heat at a temperature in conduction and convection. This paper develops the concept of entransy flux for thermal radiation in enclosures with opaque surfaces. The entransy balance equation and entransy dissipation function are derived. The minimum principle of radiative entransy loss is developed. The potentials and the heat fluxes distribution which meet the Stefan–Boltzmann’s law and the energy balance equation would make the radiative entransy loss minimum if the net heat flux of each surface or the thermal potentials of the surfaces are given. The extremum entransy dissipation principles (EEDP) for thermal radiation are developed. The minimum radiative entransy dissipation leads to the minimum average radiative thermal potential difference for prescribed total heat exchange and the maximum radiative entransy dissipation leads to the maximum heat exchange for prescribed average radiative thermal potential difference. The minimum and maximum principle can be concluded into the minimum thermal resistance principle (MTRP) for thermal radiation by defining the thermal resistance with the entransy dissipation. The EEDP or MTRP is proved to be reliable when they are used to optimize some radiative heat transfer problems, and a comparison is made between the minimum principle of entropy generation and the EEDP.     

Entropy generation due to natural convection in discretely heated porous square cavities

Optimization of industrial processes for higher energy efficiency may be effectively carried out based on the thermodynamic approach of entropy generation minimization (EGM). This approach provides the key insights on how the available energy (exergy) is being destroyed during the process and the ways to minimize its destruction. In this study, EGM approach is implemented for the analysis of optimal thermal mixing and temperature uniformity due to natural convection in square cavities filled with porous medium for the material processing applications. Effect of the permeability of the porous medium and the role of distributed heating in enhancing the thermal mixing, temperature uniformity and minimization of entropy generation is analyzed. It is found that at lower Darcy number (Da), the thermal mixing is low and the heat transfer irreversibility dominates the total entropy generation. In contrast, thermal mixing is improved due to enhanced convection at higher Da. Friction irreversibility is found to dominate the total entropy generation for higher Prandtl number (Pr) fluids at higher Da, whereas the heat transfer irreversibility dominates the total entropy generation for lower Pr fluids. Based on EGM analysis, it is established that larger thermal mixing at high Darcy number may not be always recommended as the total entropy production is quite large at high Darcy number. Overall, it is found that the distributed heating methodology with multiple heat sources may be an efficient strategy for the optimal thermal processing of materials.     

Entropy generation and thermodynamic optimization of fully developed laminar convection in a helical coil

The present paper analyses the entropy generation of the fully developed laminar convection in a helical coil with constant wall heat flux and presents the optimal design based on the minimum entropy generation principal. The important design parameters, including Reynolds number (Re), coil-to-tube radius ratio (δ) and nondimensional coil pitch (λ) are varied to investigate their influences on the entropy generation. The results presented in this paper cover Re range of 100–10,000, δ and λ range from 0.01 to 0.3. Compared with Re and δ, the coil pitch λ is found to have minor influence on the entropy generation. For a demonstrated case, the minimum entropy generation occurs in the range bounded by Re from 2271 to 4277 and δ from 0.17 to 0.3, within which the irreversibility of the system is lowest and the system performance would be optimum. The details show that there is an optimal Re for a helical coil with a fixed δ; meanwhile for a helical coil flow with a specified Re, the smaller δ should be selected when the Re is larger than 5000, and the larger δ should be selected when the Re is less than 5000. These results provide worthwhile information for heat exchanger designers to find the optimal helical coil design from the viewpoint of the thermodynamic second law.     

Entropy generation through combined non-grey gas radiation and forced convection between two parallel plates

The current study presents a numerical computation of combined gas radiation and forced convection through two parallel plates. A laminar flow of a temperature-dependent and non-grey gas in the entrance region of the channel was investigated. Over-heated water vapor was chosen as a gas because of its large absorption bands. Some special attention was given to entropy generation and its dependence on geometrical and thermodynamic parameters. The radiative part of the study was solved using the “Ray Tracing” method through S₄ directions, associated with the “statistical narrow band correlated-k” (SNBCK) model. The temperature fields were used to calculate the distributions of local and global entropy generation.     

Methanol solution promoting cotton fiber chemical looping gasification for high H2/CO ratio syngas

Chemical looping gasification (CLG) is expected to act as an efficient novel technique realizing energy conversion, but solid fuel CLG suffers from inadequate fixed carbon conversion. Sufficient oxidant would be the grand cure for the torment that ever besets solid fuel CLG. We here reported a strategy to promote cotton fiber CLG to produce syngas with high H₂/CO ratio by introducing methanol solution for supplying adequate H₂O and CO. Experiments were performed under various oxygen excess number (Ω) at 850 °C. Ω around 0.3 favors the generation of syngas with high H₂/CO ratio and high carbon conversion (η). Comparatively experiments concerning effect of methanol solution on cotton fiber CLG verified its significant promotion on syngas generation. Methanol solution can even play a more significant role than water to increase the H₂/CO ratio and η from 5.64 to 71% to 6.82 and 87%, respectively, while H₂/CO ratio and η for the direct cotton fiber CLG without water and methanol solution is only 1.28 and 29%. Stoichiometry and thermodynamics analysis further verified the possibility of cotton fiber CLG using methanol solution, which shows a double advantage of waste solution treatment and energy conversion.     

Spectral emittance measurements of micro/nanostructures in energy conversion: a review

Micro/nanostructures play a key role in tuning the radiative properties of materials and have been applied to high-temperature energy conversion systems for improved performance. Among the various radiative properties, spectral emittance is of integral importance for the design and analysis of materials that function as radiative absorbers or emitters. This paper presents an overview of the spectral emittance measurement techniques using both the direct and indirect methods. Besides, several micro/nanostructures are also introduced, and a special emphasis is placed on the emissometers developed for characterizing engineered micro/nanostructures in high-temperature applications (e.g., solar energy conversion and thermophotovoltaic devices). In addition, both experimental facilities and measured results for different materials are summarized. Furthermore, future prospects in developing instrumentation and micro/nanostructured surfaces for practical applications are also outlined. This paper provides a comprehensive source of information for the application of micro/nanostructures in high-temperature energy conversion engineering.     

Analysis of entropy generation using nanofluid flow through the circular microchannel and minichannel heat sink

In this paper, different types of entropy generations in the circular shaped microchannel and minichannel are discussed analytically using different types of nanoparticles and base fluids. In this analysis, Copper (Cu), alumina (Al₂O₃) as the nanoparticle and H₂O, ethylene glycol (EG) as the base fluids were used. The volume fractions of the nanoparticles were varied from 2% to 6%. In this paper, the irreversibility or entropy generation analysis as the function of entropy generation ratio, thermal entropy generation rate and fluid friction entropy generation rate for these types of nanofluids in turbulent flow condition have been analyzed using available correlations. Cu–H₂O nanofluid showed the highest decreasing entropy generation rate ratio (36%) compared to these nanofluids flow through the microchannel at 6 vol.%. The higher thermal conductivity of H₂O causes to generate much lower thermal entropy generation rate compared to the EG base fluid. The fluid friction entropy generation rate decreases fruitfully by the increasing of volume fraction of the nanoparticles. Cu–H₂O and Cu–EG nanofluid gave the maximum decreasing rates of the fluid friction entropy generation rate are 38% and 35% respectively at 6% volume fraction of the nanoparticles. Smaller diameter showed less entropy generation in case of all nanofluids.     

Entropy generation in Casson nanofluid flow over a surface with nonlinear thermal radiation and binary chemical reaction

We introduce a model that precisely accounts the flow of fluid of Casson nanofluid over a stretched surface with activation energy and analyze entropy generation. The model is an attempt to investigate heat transfer and entropy generation in the laminar boundary layer near a stagnation point. The modified Arrhenius function for activation energy is used. Here, the flow of the fluid is subjected to nonlinear thermal radiation, viscous disipation, binary chemical reaction, and external magnetic field. The coupled nonlinear system is further validated using the spectral lineralization method. The method is found to be accurate and convergent. The results show that the Reynolds number and Casson parameter have a significant effect in entropy generation.     

The effect of surface thermal radiation on entropy generation in an open cavity with natural convection

In this work, a numerical study on entropy generation in a square open cavity with natural convection and surface thermal radiation is presented. The overall continuity, momentum, and energy equations for the gas phase in the open cavity were solved numerically by means of the finite-volume method. Temperature-dependent fluid properties were considered. During the calculations, the values of the Rayleigh number (Ra) were set in the range of 10⁴–10⁶. The temperature difference between the hot wall and the bulk fluid (ΔT) was varied between 50 and 400 K, and was represented by a dimensionless temperature difference (φ) for the purpose of generalization of the present study. The results of this investigation indicate that surface thermal radiation increases the overall entropy generation rate between 33.52% and 560.87%, and thus cannot be neglected in the analysis of this type of system.     

Analysis of first and second laws of thermodynamics between two isothermal cylinders with relative rotation in the presence of MHD flow

In this paper, an analysis of the first and second laws of thermodynamics is presented to show the effects of MHD flow on the distributions of velocity, temperature and entropy generation between two concentric rotating cylinders. The flow inside the gap is assumed to be steady state and laminar for an incompressible, viscous, and Newtonian fluid. The walls of the cylinders are kept at different constant temperatures. Governing equations in cylindrical coordinates are simplified and analytically solved to obtain the local and average (overall) entropy generation rate. Due to the nature of the problem, the velocity distribution in the annulus becomes as the modified Bessel functions I₁(MR) and K₁(MR). Therefore, to obtain the temperature field, the expansions of the modified Bessel functions I₁(MR) and K₁(MR), with 3 terms, are employed in the energy equation. The results are presented for various values of Hartmann number (M), radius ratio (Π), group parameter (Ω/Br), and Brinkman number (Br).     

Entropy Generation due to Combined Natural Convection and Thermal Radiation within a Rectangular Enclosure

ABSTRACT

The present work investigates entropy production due to coupled natural convection/radiation heat transfer phenomenon in an inclined rectangular enclosure, isothermally heated from the bottom side and isothermally cooled from the other sides. The discrete-ordinate method is used in modeling the radiative transport equation while the statistical narrow band correlated-k model is adopted to deduce the radiative properties of the medium. The influence of pertinent parameters such as aspect ratio, inclination angle and walls emissivities on entropy generation is studied. It is found that the volumetric entropy generation is reduced when increasing the inclination angle of the enclosure. Moreover, it is shown that the minimum entropy production due to radiation heat transfer in participating media occurs at aspect ratio equal to unity.     

Experimental evaluations of the building shell radiant exchange in clear sky conditions

The results of an experimental investigation on the exchange of radiant energy in the infrared band (λ = 4–100 μm) between the walls of a building and the surrounding environment, constituted in part by the sky and in part by the ground, are presented in this paper. The measures were obtained on a purpose built test module for studies and research on building energy. Through measurements of infrared radiation on a South facing vertical wall and on a horizontal roof, and of the relative surface temperatures, the radiant field between these surfaces and the outdoor environment was resolved in conditions of clear hourly diurnal and nocturnal sky. The investigation allowed for the determination of the hourly values of the radiative heat transfer coefficients between the vertical wall and the sky, between the vertical wall and the ground and between the horizontal roof and the sky. Furthermore, EN ISO 13790:2008, which is used for the evaluation of energy requirements of building air-conditioning, was considered and the various contributions used in order to evaluate the radiative exchange with the experimentally obtained values were compared.