Thermal-economic Performance Investigation Of Finned Tube Including Phase Change Heat Transfer For Waste Heat Recovery Power Utilization System

The finned tube is widely used for waste heat recovery power utilization system,such as the Organic Rankine Cycle system (ORCs) for the low temperature exhaust heatpower generation. The thermodynamic and exergo-economic performances of finnedtube have been studied by some scholars, based on the first law of thermodynamics orthe entropy generation (available energy loss). And most literatures focus on the singlephase convective heat transfer of finned tube. It should be pointed out that, for thefinned tube used in waste heat recovery power utilization system, on the one hand, onemay be more concerned about the maximal work ability gained by the working fluidand the available energy loss of cold source as little as possible. On the other hand, theworking fluid undergoes phase changes in the evaporator and the condenser for wasteheat recovery power utilization system. Therefore, the existing studies are not suitablefor the finned tube used in waste heat recovery power utilization system.Based on the first and second laws of thermodynamics, this thesis takes the singlephase and phase change heat transfer processes through a finned tube and the wholeheat transfer process including tube inside and outside simultaneously intoconsideration for the first time. Selecting the pure fluid (R245fa and R134a) and themixed fluid (R407C and R22/R142b) as the working fluid, exergo-economic analysis ofthe finned tube phase change heat transfer for waste heat recovery power utilizationsystem is gained. The effects of relevant parameter on the exergo-economicperformances of finned tube are discussed, and comparative studies with the smoothtube have been carried out.The calculation results show that: regardless of the pure or the mixed working fluid,there exists an optimal value of internal Reynolds number Reiwhere the net profit perunit transferred heat load η peaks and the annual total cost per unit transferred heat loadΓ minimizes for the finned tube and the smooth tube. The larger the external Reynoldsnumber Reoand the unit cost of thermal exergy εqare, the larger the η and Γ are. Withincreasing Reo, the η and Γ grow up at a rapid growth rate. The η value of finned tubeis obviously greater than that of smooth tube and the Γ value of finned tube isobviously less than that of smooth tube, that is to say, the exergo-economic performanceof finned tube phase change heat transfer is superior to that of smooth tube. The finnedtube is more suitable for the waste heat recovery power utilization system.Besides, the optimal Reivalue of R134a is greater than that of R245fa, while the η value of R134a is obviously less than that of R245fa. Comparing with the R22/R142b,the optimal Reivalue of R407C is larger, while the η value of R407C is obviouslysmaller. When the working fluid is pure work fluid, the η values of finned tube andsmooth tube increase linearly with the increment of the boiling temperature Tv. Andthere exists a critical phase change temperature, where the net profit per unit transferredheat load is equal to zero. For the finned tube, under the larger Reoand the smaller ηoβconditions, there exits a critical internal Reynolds number Rei,criwhere the Γ value ofR245fa is greater than that of R134a when the Reiis less than the critical Rei,cri. While itis contrary when the Reiis greater than the critical Rei,cri. Furthermore, when theworking fluid is mixed fluid, there also exits a critical Rei,crifor the smooth tube. Whenthe Reiis less than Rei,cri, the Γ value of R407C is obviously greater than that ofR22/R142b. However, when the Reiis greater than Rei,cri, the tendency is contrary. Forthe finned tube, the larger the geometric parameter of finned tube ηoβ is, the larger the ηvalue of finned tube is, but the smaller the Γ value of finned tube is. The η value ofmixed fluid is greater than that of pure fluid, and the Γ value of mixed fluid is less thanthat of pure fluid. Therefore, the exergo-economic performance of mixed fluid issuperior to the pure fluid, and the mixed fluid is more suitable for the waste heatrecovery power utilization system.