Entropy Generation Minimization Theory Research Articles

Improving performance of air conditioning system by using variable-circuit heat exchanger: Based on the Chinese APF standard

For finned-tube heat exchangers, owing to the extremely different refrigerant pressure drop characteristics between condensation and evaporation processes, the optimum refrigerant circuits of these two conditions are different. However, for a typical heat pump type air conditioner, refrigerant circuit of the conventional heat exchanger (CHX) is fixed no matter what operation condition is conducted. To solve this problem, a modified variable-circuit heat exchanger (VCHX) is developed for air conditioner outdoor unit applications. The VCHX, for which the refrigerant circuit can be easily switched by using a globe valve, could provide better matching characteristics between refrigerant circuit and operation condition variations. With a developed simulation model, refrigerant circuit of the VCHX is optimized by using the entropy generation minimization theory. Then, comparative experimental works were conducted to investigate the performance of VCHX and CHX systems in detail. Experimental results show that the presented VCHX is more suitable for high heating capacity and low cooling capacity applications. Based on the Chinese APF (annual performance factor) Standard, the VCHX system yields 6.5% higher APF value over CHX system. Significantly, economic analysis indicates that the investment payback period of VCHX system is 1.46 year, which is much lower than the designed lifetime of a residential air conditioner. In general, the VCHX technique has a great application potential for improving the overall performance of a heat pump type air conditioner.

Progress in theoretical study on constructal optimization of marine steam power plants

<p indent="0mm">Theoretical research of the constructal optimization of marine steam power plants is introduced in this study from two aspects: ship steam power plants and ocean thermal energy conversion plants. First, reviews are conducted on the main heat exchange component models of ship steam power plants, including the evaporator, superheater, economizer, whole boiler and condenser, main heat exchange, and energy conversion components, and system models of ocean thermal energy conversion plants, including the evaporator, condenser, dual-pressure turbine, and whole system. Then, constructal optimization of the main components of the two power plants and constructal thermodynamic optimization of the ocean thermal energy conversion plant based on modern thermal theories such as entropy generation minimization theory, finite time thermodynamics theory, entransy theory, and constructal theory are introduced. The similarities and differences in the results obtained before and after optimization, as well as the different optimization objectives and system structures, are extensively compared. Finally, the application prospects of constructal thermodynamic optimization to the optimal designs of energy transfer and conversion processes are made considering theoretical research on reduced energy consumption of power plants.

Simulation study on the performance of a vapor-bypassed evaporator for heat pump applications

• A mathematical model for the vapor-bypassed evaporator is developed. • Corresponding experiments were conducted to validate the model accuracy. • Entropy generation is applied to evaluate the evaporator overall performance. • Phase separation position and channel number of the evaporator are optimized. This paper presents a simulation study on the vapor-bypassed evaporator for heat pump applications. The vapor-bypassed evaporator, in which a phase separator is set in the middle of the refrigerant circuit to bypass vapor refrigerant, can reduce the refrigerant pressure drop for enhancing the evaporator overall performance. A mathematical model for the vapor-bypassed evaporator is developed and the experiments were conducted to validate its accuracy. Based on the entropy generation minimization theory, the optimum phase separation positions and refrigerant channel numbers of the evaporator for various operation conditions are selected out. According to the simulation results, as the required heat transfer capacity ranges 4~10 kW, the vapor-bypassed (VB) mode obtains 25.6~84.6 kPa lower refrigerant pressure drop and 3.1~25.3% lower entropy generation than the conventional operation (CO) mode. Moreover, when the ambient temperature ranges 5~15°C, the entropy generation of VB mode is decrease by 22.3~11.5% than CO mode. In general, applying the vapor-bypassed technique is a promising way for improving the evaporator performance especially under high heat transfer capacity and low ambient temperature conditions.

Constructal design of a non-uniform heat generating disc based on entropy generation minimization

Non-uniform heat generating phenomenon is ubiquitous in real electronic devices. Based on this point, this paper researches the entropy generation rate (EGR) performance of a non-uniform heat generating (NUHG) disc. In this NUHG model, constructal design of the radial-pattern disc is performed with the conditions of constant- and variable-cross-sectional highly conductive routes (HCRs), respectively. The overall generation of heat over the entire disc area stays invariable, while the geometry of the disc is free to morph. The influence of heat generation non-uniformity on the optimized geometry of the disc is studied. The results manifest that increasing the thermal conductivity ratio and area ratio of HCRs both can reduce the EGR. Increasing the number of elements involved in the disc will compel the optimal HCRs to stretch towards the centre. In the areas with more heat generation and severer heat conduction requirement, more high conductivity material should be arranged to converge more heat flow and reduce the EGR aroused during the heat transfer process. The dimensionless EGR slumps by 11.5% on account of the employment of variable-cross-sectional HCR stratagem. Henceforth, the variable-cross-sectional HCR structure can reduce the EGR and improve its thermal performance. Additionally, the results obtained by minimizing EGR are compared with those obtained by minimizing maximum temperature difference. The primary novelty of this paper is introducing entropy generation minimization theory into the constructal design of radial-pattern disc with both non-uniform heat generation and constant- and variable-cross-sectional HCRs, which can provide benefits to the designs of practical electronic devices and the improvement of heat transfer performance.

Progresses in generalized thermodynamic dynamic-optimization of irreversible processes

Generalized thermodynamic optimization theory is one of the research hotspots in the field of thermal science and technology. Firstly, the emergences and developments of finite time thermodynamics, entropy generation minimization, entransy theory and generalized thermodynamic optimization are reviewed in this paper, respectively. Besides, the physical connotations of the above thermal optimization theories are analyzed in detail, and their similarities and differences are also indicated. Secondly, the research progress of the generalized thermodynamic optimization theory is introduced from the aspects of dynamic optimizations of several irreversible processes including heat-transfer, mass-transfer process, capacitor charging circuit, and economical trade processes. The similarities and differences between the entropy generation minimization theory and the entransy theory in the optimization of heat and mass transfer processes are compared in detail. Finally, in addition to the expansion of the research subjects and the refinement of the research contents, the pursuit of the universal of the physical models, the unity of the optimization methods and the generalization of the optimization results is another important development trend of the generalized thermodynamicoptimization theory.

Constructal design of nonuniform heat generating area based on triangular elements: A case of entropy generation minimization

This paper reports a constructal investigation of heat transfer in a nonuniform heat generating triangular area, and numerically studies the entropy generation performance over the area inserted with constant and discrete variable cross-sectional high conductive channels (HCCs), respectively. The effects of the nonuniform heat generating (NUHG) coefficient and width coefficient on the overall thermal performance are also analyzed with several conditions of heat distributions. This model has two constraints: the area ratio of HCCs to the whole heat generating area and the global heat generation (the heat is nonuniformly distributed over the area). The structure of the HCCs is free to morph in order to present a better thermal performance. The numerical results show that for an elemental triangle, the optimal structure tends to elongate as the NUHG coefficient increases. It is also indicated that the dimensionless entropy generation rate for the first-order construct constituting of six elements is decreased by 20.0% in comparison with the counterpart constituting of four elements, which means that increasing the number of incorporated elements makes it favorable to improve the overall thermal performance. Additionally, the quartic minimized dimensionless entropy generation rate of the first-order construct with discrete cross-sectional HCCs performs 14.2% better than the counterpart with constant highly conductive routes. In this context, the thermal performance can be improved reasonably by intruding the variable cross-sectional architecture into the heat generating area. In general, the effects of the nonuniform heat generating coefficient p, width coefficient m and the number of triangular elements n on the overall thermal performance are studied. The pivotal innovation of this paper is introducing entropy generation minimization theory into the constructal design of triangular assemblies with both nonuniform heat generation and discrete variable cross-sectional HCCs, which can contribute to the design of practical electronic devices to a promising heat transfer performance.

Entropy­based methods applied to the evaluation of a real refrigeration machine

The aim of the research is the thermodynamic analysis of a real refrigerating machine with solid fouling on the heat­exchange surface of an air­cooled condenser with the help of entropy methods such as: the entropy­cycle method, the entropy­statistical method and the entropy generation minimization method. The experimental data of the refrigerating machine with solid fouling in the form of dust on the external surface of the air­cooled condenser was used for the thermodynamic analysis. The influence of irreversible losses in separate elements of the refrigerating machine caused by the presence of external solid fouling on the external heat exchange surface of the air­cooled condenser was determined according to the entropy­cycle and entropy­statistical methods. The entropy­cycle method has estimated the absolute value of energy losses in each element. The entropy­statistical method was used to determine the excessive consumption of work in the real cycle in comparison with the theoretical compression process in the compressor. The irreversible losses associated with aerodynamics and heat transfer during air motion through a finned surface in an air­cooled heat exchanger were estimated by using the entropy generation minimization theory. The influence of growing fouling on the thermal and mechanical components of the total entropy generation in the heat exchanger was determined. In addition, the fouling mass that determines the time of its cleaning was defined. Application of entropy methods for analyzing the real refrigerating machine at the design stage allows minimizing irreversible losses associated with operating conditions. Moreover, it allows predicting a maintenance schedule which contributes to energy saving

Entropy Generation Minimization for Reverse Water Gas Shift (RWGS) Reactors.

Thermal design and optimization for reverse water gas shift (RWGS) reactors is particularly important to fuel synthesis in naval or commercial scenarios. The RWGS reactor with irreversibilities of heat transfer, chemical reaction and viscous flow is studied based on finite time thermodynamics or entropy generation minimization theory in this paper. The total entropy generation rate (EGR) in the RWGS reactor with different boundary conditions is minimized subject to specific feed compositions and chemical conversion using optimal control theory, and the optimal configurations obtained are compared with three reference reactors with linear, constant reservoir temperature and constant heat flux operations, which are commonly used in engineering. The results show that a drastic EGR reduction of up to 23% can be achieved by optimizing the reservoir temperature profile, the inlet temperature of feed gas and the reactor length simultaneously, compared to that of the reference reactor with the linear reservoir temperature. These optimization efforts are mainly achieved by reducing the irreversibility of heat transfer. Optimal paths have subsections of relatively constant thermal force, chemical force and local EGR. A conceptual optimal design of sandwich structure for the compact modular reactor is proposed, without elaborate control tools or excessive interstage equipment. The results can provide guidelines for designing industrial RWGS reactors in naval or commercial scenarios.

Thermodynamic analyses and optimizations of extraction process of CO2 from acidic seawater by using hollow fiber membrane contactor

Extraction process of CO2 from acidic seawater by using hollow fiber membrane contactor is studied in this paper by using finite time thermodynamics or entropy generation minimization theory. Analytical formulae of the extraction rate of CO2 and entropy generation rate (EGR) caused by mass transfer (MT) and the flow of the process are obtained when the acidic seawater (liquid phase) flows in the lumen side and the H2 (gaseous phase) flows in the shell side conversely. Relations between the concentration of CO2 and contactor length in both liquid phase and gaseous phase, between the MT flux and contactor length, between the pressure of the two phases and the contactor length, as well as between the local EGR and contactor length are obtained by numerical examples. The influences of the volume flow rates of the acidic seawater and the H2 at the inlet, the number and the length of hollow fibers on the extraction rate of CO2 and EGR per MT rate are also analyzed. The results show that the extraction rate of CO2 can reach up to more than 98%. The EGRs caused by MT and fluid flow account for 85.99% and 14.01% of the total EGR, respectively. Furthermore, the total EGR caused by MT and fluid flow is minimized by using nonlinear programming method with fixed extraction rate of CO2 when the gas mixture of H2 and CO2 flows in the shell side and the concentration of CO2 in the gaseous phase is completely controllable. The EGR of the minimum EGR MT strategy can be decreased by 36.57% than that of the MT strategy that H2 flows in shell side, in which the EGR caused by MT is decreased by 42.52% and the EGR caused by the flow remain basically unchanged. The EGR caused by MT is minimized by using optimal control theory and the analytical solution of the optimal concentration configuration of CO2 in the gaseous phase is obtained.

Entropy generation minimization for CO2 hydrogenation to light olefins

An optimization model is established for the reaction process of CO2 hydrogenation to light olefins in a fixed-bed tubular reactor based on finite time thermodynamics or entropy generation minimization theory. In the present study, the specific generation rate (entropy generation rate averaged by the production rate of the target product) is proposed as an optimization objective function and the optimal design parameters which minimize the objective function have been investigated. The model is developed based on the reversible kenetic models and their cooresponding kinetic parameters, which are obtained by fitting the experimental data. The irreversibilities due to heat transfer, chemical reactions and viscous flow are considered and the local entropy generation rate of each term is calculated according to the irreversible thermodynamics. The analyses of the performance characteristics are conducted as well. The results show that the CO2 hydrogenation to light olefins accords with a two-step reaction mechanism, and Fischer-Tropsch reaction is the rate-controlling step. The irreversibility mainly located in the front of the reactor, which most contributions are caused by chemical reactions. The reductions of the specific entropy generation up to 24.78% and 10.04% can be achieved for optimal reactor inner diameter and optimal catalyst bed density, respectively.

Optimization of peripheral finned-tube evaporators using entropy generation minimization

The peripheral finned-tube (PFT) is a new geometry for enhanced air-side heat transfer under moisture condensate blockage (evaporators). It consists of individual hexagonal (peripheral) fin arrangements with radial fins whose bases are attached to the tubes and tips are interconnected with the peripheral fins. In this paper, experimentally validated semi-empirical models for the air-side heat transfer and pressure drop are combined with the entropy generation minimization theory to determine the optimal characteristics of PFT heat exchangers. The analysis is based on three independent parameters, i.e., porosity, equivalent particle diameter and particle-based Reynolds number. The total heat transfer rate is a fixed constraint. The optimal heat exchanger configurations, i.e., those in which the entropy generation number reaches a minimum, are calculated for constant heat flux and constant tube wall temperature boundary conditions. Performance evaluation criteria of fixed geometry, fixed face area and variable geometry were implemented. In all cases, it was possible to determine a combination of independent parameters that provided a minimum entropy generation rate.