Investigation On The Optimal Performance Of The Brayton Cycle And Brownian Motor

Finite-time thermodynamics is a new important branch of the modern thermodynamics. It is mainly used to investigate the laws of energy and entropy flows of non-equilibrium systems in finite time. It is of very important significance for exploiting new energy resources, developing new technologies, protecting natural resources and so on. Finite-time thermodynamics has been applied in many research fields. Especially, a lot of important achievements in the optimal design of thermodynamic cycles have been obtained. The Brayton cycle is the backbone of power cycle systems in the present energy conversion fields, and consequently, has important value in the practical applications.This thesis is composed by two parts. In the first part, the performance of the several models of the Brayton cycle is investigated by using Finite-time thermodynamics and the results obtained here may provide some theoretical basis for the optimal design of the Brayton cycle. In the second part, the two typical models of the Brownian motor are studied and some significant results are obtained.In the first part, the performance characteristics of the Brayton refrigerator and heat engine using the ideal gas as the working substance are investigated, in which the irreversible effects in the adiabatic and other heat-transfer processes are considered. The performance of the Brayton cycle with regeneration and without regeneration is compared. The advantages of using the regenerator are expounded. The reasonable ranges of the parameters in the regenerator are determined. An irreversible model of the Brayton refrigeration cycle working with an ideal Fermi gas 3He is established. The characteristics of regeneration and performance of the cycle are revealed. The influence of quantum degeneracy of the gas and the irreversibility in the adiabatic processes on its performance is analyzed comprehensively. The minimum pressure ratio of the cycle is determined. Some special cases are discussed. A cycle model of two-stage magnetization Brayton refrigerators using a paramagnetic material as the working substance is established. On the basis of the thermodynamic properties of a paramagnetic material, the expressions of some important parameters such as the work input, the refrigeration load and the coefficient of performance (COP) are derived. The influence of the inter-magnetization processes, regeneration processes and the irreversibility in the adiabatic processes on the performance of the cycle is discussed. The refrigeration load and the COP of the cycle are optimized, and the optimally operating region of the cycle and the optimal working parameters are determined. The unified cycle model of a class of solar-driven heat engines is presented. This model may include Carnot cycle, Brayton cycle, Braysson cycle,and so on. Several irreversible effects on the performance of the cycle system are taken into account, which include finite time heat transfer, different heat transfer laws, the heat loss, the internal irreversibilities of the heat engine, and so on. When the heat-supplying rate of the system is given, the performance of the cycle model is optimized by using the overall efficiency of the system as the object function, and consequently, the optimally working states of the solar-driven heat engine system are determined.In second part, the background and research status of Brownian motors are introduced and the operating mechanisms of two typical Brownian motor models are investigated. In the first model, a thermal Brownian micro heat engine is set up, in which the periodic potential is spatially asymmetric and contacted with the alternately changed hot and cold reservoirs. The results obtained show that the heat flow via the kinetic energy of the particles is always irreversible and its efficiency cannot approach the Carnot efficiency. In the second model, the temperature of periodic potential is periodically oscillating and the time structure of oscillation will affect the directed transport of Brownian particles. It is found that the temporal symmetric temperature oscillation may not be the best choice for the directed transport.