Research On The Optimal Design And Matching Performance Under Variable Speed Of A Contra-rotating Axial Flow Fan

First this dissertation summarizes the current situation, development trend and existing problems of contra-rotating axial flow fan and analyzes its features of aerodynamic performances and structures. Then by comparing the effects of hub/tip radius ratio, blade hub/tip chord ratio and variable circulation flow pattern on the fan performance, the design scheme of a two-stage contra-rotating axial flow fan with high efficiency is obtained. And for each study case, the equal loads between two stage rotors at design point are sustained. Further the matching performance of this two-stage contra-rotating axial flow fan under variable speed is investigated by numerical simulation. The main tasks and results are as following:1. The constant circulation flow pattern method is adopted. First the effects of hub/tip radius ratio on the performance of the contra-rotating axial flow fan at design mass flow point are discussed and the optimal hub/tip radius ratio is selected. Then the role of solidity in adjusting the distribution of loads between the two stages is studied by changing the distribution of the blade chord along blade height. The preliminary optimum design scheme of the contra-rotating axial flow fan with high pressure rise and high efficiency is obtained. Finally the performance of the contra-rotating axial flow fan at off design points is also anyalyzed for this optimization design.2. Based on the constant circulation flow pattern design scheme, the variable circulation flow pattern method is adopted to further improve the performance of a contra-rotating axial flow fan. For each stage rotor, the effects of different radial load distributions on the performances of the contra-rotating axial flow fan are compared. For each study case, the equal loads between two stage rotors at design point are sustained. Through comparison, the best radial load distributions for the two stage rotors are obtained. Then the fitting formulas that can approximately describe these radial load distributions are given.3. The matching performance of this two stage contra-rotating axial flow fan under variable speed is investigated through full annulus numerical simulation, which consists of the inlet bell mouth, two stage rotors and outlet diffuser. With four matching schemes of speed, the characteristics of total pressure rise, efficiency and power of the whole machine and single stage are analyzed and compared. Further the features of inner flow field are elucidated, and the results are compared with those of single passage simulation model only including the two stage rotors. For the single stage rotor, the most difference between single passage and full annulus simulation models is the characteristic of the second stage rotor. As for the total pressure rise and efficiency, the results of single passage model are obviously higher than those of the full annulus model. All the results under different speed matching schemes show that the second stage rotor is more sensitive. Except for the well design of stage rotor, the inlet bell mouth and outlet diffuser are also important for the performance improvement of contra-rotating axial flow fan.