The second law analysis in fundamental convective heat transfer problems

Second law characteristics of heat transfer and fluid flow due to forced convection of steady-laminar flow of incompressible fluid inside channel with circular cross-section and channel made of two parallel plates is analyzed. Different problems are discussed with their entropy generation profiles and heat transfer irreversibility characteristics. In each case, analytical expression for entropy generation number (N_S) and Bejan number (Be) are derived in dimensionless form using velocity and temperature profiles.     

Basic chemically recuperated gas turbines—power plant optimization and thermodynamics second law analysis

One of the proposals to increase the performance of the gas turbines is to improve chemical recuperated cycle. In this cycle, the heat in the turbine exhaust gases is used to heat and modify the chemical characteristics of the fuel. One mixture of natural gas and steam receives heat from the exhaust turbine gases; the mixture components react among themselves producing hot synthesis gas. In this work, an analysis and nonlinear optimization of the cycle were made in order to investigate the temperature and pressure influence on the global cycle performance. The chemical composition in the reformer was assumed according to chemical equilibrium equations, which presents good agreement with data from literature. The mixture of hot gases was treated like ideal gases. The maximum net profit was achieved and a thermodynamic second law analysis was made in order to detect the greatest sources of irreversibility.     

The second law of thermodynamics as applied to energy conversion processes

The definition of the ‘second law efficiency’ presents certain ambiguities, which are reflected in its applications. It is attempted here to define another figure of merit for energy conversion processes. This incorporates the second law limitations, is based on a state function of the systems considered, exergy, and may be universally applied without ambiguities. A general thermodynamic system is adopted here, which is applicable to all processes, and the utilization factor is used in terms of exergy balances.     

Continuum mechanics versus violations of the second law of thermodynamics

ABSTRACT

Spontaneous, random violations of the second law become relevant as the length and/or timescales become very small. Modern statistical physics tells us that the second law then needs to be replaced by the fluctuation theorem and, mathematically, the irreversible entropy evolves as a submartingale. This is illustrated on the Couette flow of a molecular fluid. On the continuum level, such phenomena lead to a framework of thermomechanics relying on stochastic (rather than deterministic) functionals of energy and entropy, which are applied in heat diffusion and thermoelasticity settings. Counterintuitive thermomechanical behaviors are also discussed in (1) the evolution of acceleration wavefronts of nanoscale thickness and (2) the fluid mechanics which has to enter a permeability model of a poroelastic medium with nanoscale pores.     

Second law optimization of convective heat transfer through a duct with constant heat flux

A second law analysis is carried out on convective heat transfer from a fluid flowing in a duct with constant heat flux. The entropy generated is expressed as a function of the initial temperature difference and the frictional pressure drop. Since the loss in available energy is directly proportional to the entropy generated, an optimum value of the initial temperature difference is found where the entropy generated is the minimum. A similar optimum is found for the ratio of heat transfer to pumping power. An optimum fluid velocity which corresponds to the minimum loss of available power is recommended.     

管内对流换热过程的热优化分析

对管内对流换热过程的温差换热和粘性摩擦引起的熵产进行了分析和优化计算 ,对换热设备和传热技术的设计和优化组织具有一定的指导意义。     

Sustainable product development based on second law of thermodynamics

The necessary reduction of the human footprint demanded by Sustainable Development can be measured by the entropy flow of the society to the environment. The classic tools of process evaluation as e.g. exergetic or life cycle analysis are able to evaluate existing solutions. But their use during the design phase is quite limited because of lacking information about the system and its components. They may be helpful for evolutionary development strategies on a long term only. But reversible process structures can be used as benchmarks already in the conceptual design phase to introduce the demands of the second law. It can be shown that basic human demands on housing, mobility, communication, and infrastructure and industry can be principally supplied by reversible process structures. Because the process logic is a virtual one, real processes can be engineered with a reversible structure however its components produce irreversible entropy flows. Hybrid cars are the most common examples here. The use of exergetic efficiencies allows a transfer of the results of reversible structures to real technology easily. The here presented methodology of sustainable engineering can be summarized by three design rules as using reversible structures, considering technology by exergetic efficiencies, and minimizing components’ entropy export. The still increasing utilization of electricity in upcoming technologies is very helpful for introducing reversible structures within hybrid technologies.     

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.     

Analytical solution of adiabatic heat and mass transfer process in packed-type liquid desiccant equipment and its application

This paper presents an integrated analytical solution of adiabatic heat and mass transfer in packed-type liquid desiccant equipment based on proposed mathematical models in both parallel-flow and counter-flow configurations. In the derivation process, the desiccant concentration at the inlet and outlet of the absorber is assumed to be constant. That means this solution is suitable for high desiccant flow rate conditions. This analytical solution has good accuracy when compared with reliable experimental data available in the literature. The moisture removal rate is derived from this solution and every factor that influences performance can be easily analyzed. Another benefit is that it can be used to discover the optimum air-to-solution flow rate ratio with which the driving force may be kept constant throughout the column. The optimum flow rate ratio may be used for optimal system design and to realize a reversible heat and mass transfer process.     

A review of thermodynamics and heat transfer in solar refrigeration system

Solar energy is one of the most efficient, clean and affordable energy alternatives available today. With the current concerns about global warming and ever increasing energy rates, countries are seriously looking for domestic and industrial usage of solar energy. In the present study, a detail review of the application of solar energy for refrigeration systems has been carried out. The utilization of solar energy for refrigeration systems would help in improvement of energy economics, energy consumption and energy efficiency. The review focuses especially on solar panel, desiccant fluid for icemaker and its components. The study also includes thermodynamic equation and material for making component of refrigeration to improve the coefficient of performance. Study around the economic evaluation and solar performance coefficient in the type of refrigerator, modeling and simulation, mathematical equation of heat transfer and type of absorption used are other topics that could be considered.     

A variational optimization of a finite-time thermal cycle with a nonlinear heat transfer law

In this paper we use a variational approach to study an endoreversible Curzon–Ahlborn–Novikov (CAN) heat engine under both maximum power and maximum ecological function conditions. By means of this procedure we analyze the performance of a CANheat engine with a nonlinear heat transfer law (the Dulong–Petit law) to describe the heat exchanges between the working substance and its thermal reservoirs. Our results are consistent with previous ones obtained by means of other procedures. In addition, we obtain expressions for the temperatures of the isothermal branches of the working fluid under maximum power conditions. Finally, we present an expression for a kind of nonendoreversible Carnot efficiency.     

Convective heat transfer and second law analysis of non-Newtonian fluid flows with variable thermophysical properties in circular channels

This study presents an analytical solution, for fully developed non-Newtonian fluid flows in circular channels under isoflux thermal boundary conditions based on perturbation techniques. Since the physical properties are generally a function of temperature and may not be assumed constant under certain circumstances, the change in viscosity and thermal conductivity with temperature was taken into account. Viscous dissipation term was also included in the performed analysis. In this study, first closed form expressions for velocity, temperature distributions, and Nusselt numbers corresponding to constant thermophysical properties were given in terms of governing parameters. Then, numerical calculation was performed to obtain the values of Nusselt number and global entropy generation for variable thermophysical properties. The results revealed that neglecting the property variation significantly affects heat transfer characteristics and entropy generation, in which the deviation from the constant physical property assumption may reach up to about 32.6%.     

First and second law analyses to an energetic valorization process of biogas

The limitations in the world sources of energy are being mitigated by the exploitation of renewable forms and by increases in the efficiency of energy utilization. Exergy analysis is a useful method for the design, evaluation, and improvement of energy systems, that uses conservation of mass and conservation of energy principles, together with the second law of thermodynamics.This study covers first and second law analyses of a cogeneration system run with the biogas produced in a landfill. Such plant produces useful electrical and thermal energies, while protecting the environment from greenhouse emissions. The objectives were to identify locations where major irreversibilities occur, to evaluate their magnitudes, and to assess the energy and exergy efficiencies of the global system and of its constituent units.The results show that the overall-plant first law efficiency is 37.9% and the exergy efficiency is 36.2%, which is far from the thermodynamic ideal limit. The internal combustion engine and one of the radiators are the most inefficient units, as judged by the parameters degree of thermodynamic perfection and exergy destruction quotient. The main potential for improvement in the plant is the harnessing of the energy in the exhaust gases.     

Field synergy equation for turbulent heat transfer and its application

A field synergy equation with a set of specified constraints for turbulent heat transfer developed based on the extremum entransy dissipation principle can be used to increase the field synergy between the time-averaged velocity and time-averaged temperature gradient fields over the entire fluid flow domain to optimize the heat transfer in turbulent flow. The solution of the field synergy equation gives the optimal flow field having the best field synergy for a given decrement of the mean kinetic energy, which maximizes the heat transfer. As an example, the field synergy analysis for turbulent heat transfer between parallel plates is presented. The analysis shows that a velocity field with small eddies near the boundary effectively enhances the heat transfer in turbulent flow especially when the eddy height which are perpendicular to the primary flow direction, are about half of the turbulent flow transition layer thickness. With the guide of this optimal velocity field, appropriate internal fins can be attached to the parallel plates to produce a velocity field close to the optimal one, so as to increase the field synergy and optimize the turbulent heat transfer.     

The generalized thermodynamic temperature and the new expressions of the first and the second law of thermodynamics

The classical thermodynamics reflects the significant relationship between the heat and the temperature. On the basis of the relationships, according to the mathematical derivation, this paper structures the conceptions of generalized heat, generalized thermodynamic temperature, generalized entropy and so on. The series of conceptions in the classical thermodynamics is merely a special case of the generalized thermodynamics. Based on these conceptions of generalized thermodynamics, this paper presents the new expressions of the first law and the second law of thermodynamics. In other words, these expressions are endued with new explanations. The Eq. LZ = kTS given by this paper provides theoretical basis for these new expressions.     

First and second law analyses of laser cutting process in relation to the end product quality

Thermal analysis of laser cutting process is carried out, and the first and second law efficiencies of the cutting process are formulated. Thermal efficiencies are predicted for various laser scanning speeds and laser output power levels. The experiment is conducted to examine the resulting cutting sections. The F-test is conducted to assess the end product (cutting section) quality and, later, the thermal efficiencies are related to the end product quality. It is found that increasing laser output power lowers the first and second law efficiencies of cutting, which is more pronounced with reducing the laser scanning speeds. The end product quality improves for low laser output power levels and high laser scanning speeds similar to the thermal efficiencies. Copyright © 2007 John Wiley & Sons, Ltd.     

Design optimization of rib-roughened channel to enhance turbulent heat transfer

This work presents a numerical procedure to optimize the shape of two-dimensional channel with periodic ribs mounted on both of the walls to enhance turbulent heat transfer. The response surface method is used as an optimization technique with Reynolds-averaged Navier-Stokes analysis of flow and heat transfer. Standard k-ε turbulence model is used as a turbulence closure. Computational results for overall heat transfer rate show good agreements with experimental data. The width-to-height ratio of the rib, rib height-to-channel height ratio, pitch-to-rib height ratio and distance between opposite ribs to rib pitch ratio are chosen as design variables. The objective function is defined as a linear combination of heat-transfer and friction-loss related terms with weighting factor. D-optimal design is used to reduce the data points, and, with only 36 points, reliable response surface is obtained. Optimum shapes of the channel have been obtained in the range from 0.0 to 0.1 of weighting factor. In the weighting factor range where designer’s goal is shifted to reduction of pressure loss, both of pitch-to-rib height ratio and distance between opposite ribs to rib pitch ratio reach almost constant values.     

线性唯象传热规律下广义不可逆卡诺热机生态学性能系数优化

以反映热机循环输出功率和火用损失率之比的生态学性能系数(ECOP)为目标,用有限时间热力学理论和方法研究广义不可逆卡诺热机的循环性能。导出了线性唯象传热规律(Q∝Δ(T-1))下ECOP的解析式,通过数值计算分析了各种目标极值条件下ECOP与循环功率、效率、熵产率、生态学函数E之间的关系,主要研究了热源温比对最优性能的影响。     

Shape optimization of rib-roughened surface to enhance turbulent heat transfer

This work presents an investigation on numerical optimization technique coupled with Reynolds-averaged Navier-Stokes analysis of flow and heat transfer for design of rib-roughened surface in case of single surface roughened in two-dimensional channel. Standard k-ε model is used as a turbulence closure. The objective function is defined as a function of heat transfer coefficient and friction drag coefficient with weighting factor. And, the ratio of width-to-height of the rib and the ratio of pitch-to-height are selected as design variables. Three different weighting factors and two sets of initial values of the design variables in each case are tested. In case of double design variable, the histories of design variables and objective function are complicated, and the number of iterations is very sensitive to the initial values of design variables. However, overall performance of the optimization process is proved quite reliable.