Tham khảo tài liệu 'developments in heat transfer part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 270 Developments in Heat Transfer expression for the minimum attainable value for the product aP2E under the given Reynolds number. Concluding remarks As exemplified by the water-water counter-flow heat exchanger the present work shows that there exists an optimal duct aspect ratio for heat exchangers under the fixed Reynolds number and mass velocity when the entransy dissipation number is taken as the performance evaluation criterion. Furthermore the formula for the optimal duct aspect ratio was obtained analytically. Under constraints of the fixed heat transfer area or duct volume and Reynolds number it was shown that there is an optimal dimensionless mass velocity for which an analytical expression was also given. The results indicated that to reduce irreversible dissipations in heat exchangers largest-possible heat transfer areas and lowest-possible mass velocities should be adopted. This conclusion is in agreement with numerical results obtained by design optimization of the shell-and-tube heat exchanger based on the entransy dissipation number as the objective function Guo et al. 2010 . From the results obtained in this study it can be seen that the traditional heat exchanger design optimizations based on total cost as an objective function usually sacrifice heat exchanger performance. This issue has been demonstrated by numerical results Guo et al. 2009 . Guo et al. 2009 found that a little improvement in heat exchanger performance can lead to large gains in terms of energy saving and environmental protection. Hence in heat exchanger design reduction in total cost and improvement in heat exchanger performance should be treated equally. The present work will be useful to drive new research in this direction. 5. Acknowledgements The support of our research by National Basic Research Program of China Project No. 2007CB206900 is greatly appreciated. 6. References Bejan A. 1982 . Entropy Generation through Eieat and Fluid Flow. New York Wiley Bejan A. 1988 .
