جستجوی مقالات مرتبط با کلیدواژه « تلفات اگزرژی » در نشریات گروه « مکانیک »

The present study aimed to introduce two novel power-refrigeration combined cycles into optimal usage of LNG cryogenic energy and reducing exergy losses due to high-temperature difference in the heat transfer process. The combined cycles include a compression-ejector refrigeration cycle and two low and high-pressure Rankine cycles in which the power required to operate the compression-ejector refrigeration cycle’s compressor is provided by the power generated by the two-cycle Rankin turbines. Increasing the cooling energy compared to direct LNG evaporation is considered as the benefits of the novel combined cycles. A thermodynamic analysis, along with optimizing both novel combined power-refrigeration cycles, was performed through the first and second thermodynamics laws and the fixed surface model assumption for the ejector. Analyzing the design parameters demonstrated that the maximum thermal and exergy efficiency and maximum refrigeration increasing ratio in both novel combined power-refrigeration cycles increase as the pump discharge pressure increases and the output pressure of Rankine cycle turbine reduces. The maximum thermal and exergy efficiency in cycle (І) were 77.31% and 23.69%, and 87.49% and 23.95% in cycle (ІІ), respectively through performing optimization in the boundaries set for the design parameters. Finally, the highest refrigeration increasing ratio to direct evaporation of LNG for cycle І and ІІ was 63.37% and 73.9%, respectively.

In the present study, effect of the using corrugated tube (modified tube) as inner tube of a double tube heat exchanger on the heat transfer rate and exergy loss is numerically investigated. Each corrugation is made of a semi-circle with various diameters of 2.5, 5 and 7.5 mm, and a diagonal line. The inner tube of the heat exchanger is made of copper with inner diameter of 27 mm. The outer tube with inner diameter of 54.5 mm is considered isolated. The obtained results indicated that the use of corrugated tube as inner tube of heat exchanger had a significant effect on the heat transfer rate and exergy loss. Heat transfer in double tube heat exchangers with corrugated tubes was 1.07 to 3.08 times more than the transferred heat in a simple double tube heat exchanger. Furthermore, the effects of inlet hot water temperature and inlet cold water temperature on the exergy loss are studied. Based on acquired data, by increasing of the inlet hot water temperature or decreasing of the inlet cold water temperatures, exergy loss increases.

A signi cant part of the total annual energy consumption is used in buildings. The most part of the energy consumption in buildings is due to the HVAC systems. As a result, the eciency of air conditioning systems plays an important role in energy saving. In the recent years, variable refrigerant ow (VRF) systems have become popular, especially for commercial buildings, due to their high eciency and their ability in simultaneous heating and cooling. In this research, VRF systems are investigated from the second law of thermodynamic point of view; the eect of design parameters and the refrigerant type on the system performance is studied. At rst, the thermodynamic cycle of VRF systems is modeled. Comparison of the numerical results with the available experimental data shows that the developed model can predict the system behavior with reasonable good accuracy. Based on the results, the coecient of performance (COP), the exergy destruction, and the exergy eciency of the VRF system are evaluated to investigate the system eectiveness. The results indicate that the compressor and the condenser are responsible for the most exergy destruction in the system, while the exergy destruction in the sub-cooling heat exchanger and the expansion valve is the least. In addition, the refrigerant type is an important parameter which may aect the system performance. Considering the importance of the sub-cooling heat exchanger, the eect of the heat exchanger on system performance has also been investigated. It has been found that the eect of the subcooling heat exchanger depends on the refrigerant type. In the systems using R-134a and R-407c, the heat exchanger improves the system eectiveness; if R-22 and R-410a are used as a refrigerant, the system performance decreases. The results of this paper are useful in getting a better understanding of the VRF systems and in designing more ecient ones.