Exergy, An International Journal最新文献

Certain thermodynamic aspects of cryogenic turbines are investigated based on operational data provided by a cryogenic test facility. The cryogenic turbine is intended to produce power by replacing the throttling valve used in natural gas liquefaction plants. The turbine operates at very low temperatures; it admits liquefied natural gas (LNG) at a high pressure and discharges it at a low pressure. The temperature change of LNG in the expansion process through the cryogenic turbine is studied and compared with the temperature change through the throttling valve. It is found that the temperature will be about 2 ^∘C smaller at the turbine exit than that at the throttling valve exit for the same inlet state and exit pressure. To establish a suitable model for the assessment of cryogenic turbine performance, the isentropic efficiency, the hydraulic efficiency, and the exergetic efficiency are studied and compared. The hydraulic efficiency is determined to be the only feasible method to assess the performance of cryogenic turbines.

An exergy analysis has been carried out for an irreversible Braysson cycle. The analytical formulae of power output and exergy efficiency are derived. The influences of various parameters on the exergy performance are analyzed by numerical calculation, and the results obtained have been compared with those of Brayton cycle under the same conditions. It is shown that the exergy loss in the combustion is the largest in the Braysson cycle, and both specific work and exergy efficiency of the cycle are larger than those of Brayton cycle.

A procedure to establish the optimal performance parameters for the minimum entropy generation during the collection of solar energy, is presented. The Entropy Generation Number, N_s, and the criterion for the optimal thermodynamic operation of a collector under nonisothermally, finite-time conditions, are reviewed. The Mass Flow Number, M, corresponding to the optimum flow of working fluid as a function of the solar collection area, is also considered. A general method for the preliminary solar collector design, based on N_s, M and the “Sun–Air” or stagnation temperature, is developed. This last concept is defined as the maximum temperature that the collector reaches at nonflow conditions for given geographic location, geometry and construction materials. The thermodynamic optimization procedure was used to determine the optimal performance parameters of an experimental solar collector.

The effects of conduction and heat generation due to viscous dissipation in annuli is a topic of interest to researchers, since the applications include aerospace, food processing, electric machines, etc. In the present study, temperature rise and entropy generation in a cylindrical annuli due to conduction and viscous dissipation are considered. In the mathematical formulation, the flow developed in the annuli is assumed as laminar, since Re is selected as $\text{≤50}\text{000}$. The wall temperature of the rotating cylinder is taken as higher than the wall temperature of the stationary inner cylinder. The temperature and entropy profiles are predicted for different Brinkman numbers and temperature difference across the annuli. It is found that as Br increases, the temperature in the fluid close to the rotating cylinder wall becomes higher than the wall temperature. This results in zero temperature gradient and the entropy generation reduces to zero in this region. The point of minimum entropy generation in the fluid moves away from the outer cylinder wall as Br increases. Moreover, the efficient operation and design of bearing systems can be possible with the analysis of entropy generation.

This second part is the continuation of Wall and Gong [Exergy Internat. J. 1 (3) (2001), in press]. This part is an overview of a number of different methods based on concepts presented in the first part and applies these to real systems. A number of ecological indicators will be presented and the concept of sustainable development will be further clarified. The method of Life Cycle Exergy Analysis will be presented. Exergy will be applied to emissions into the environment by case studies in order to describe and evaluate its values and limitation as an ecological indicator. Exergy is concluded to be a suitable ecological indicator and future research in this area is strongly recommended.