Optimal Design For Multi-blade Centrifugal Fan Based On The Controlled Velocity Law

Multi—blade centrifugal fan is a kind of fluid machinery and widely used in industry and civil. Multi-blade centrifugal fan has the features of small overall volume, large relative width and impeller radius ratio in structure, with high pressure coefficient, large flow coefficient, low noise and low efficiency in performance. A new optimal design method of impeller has developed in this paper to improve efficiency, the three dimensional numerical simulation has been used to analysis.The main idea of design method is control the growth of boundary layer and flow separation in order to reduce flow loss. Flow control parameters, such as maximum acceleration coefficient and its best location, is selected to represent the control of internal flow in impeller based on the achievement of previous flow field research, on the other hand, these parameters is directly relate to velocity distribution. Thus, to study optimal value of these flow control parameters can realize optimal design of impeller. Firstly, the two-dimensional turbulent boundary layer integral equation considering effect of rotation and curvature is used to calculate the boundary layer in different control parameters then the best value is determined by use of momentum thickness. A mathematical model of the optimal velocity distribution based on these control parameters is set up, the optimum velocity distribution is gotten up after the best control parameters used in this model. The blade shape can be determined according to the optimum velocity distribution, then examining whether the pressure and flow rate satisfy the requirement or not, if it meet demand, the optimal design of impeller is finished.The three-dimensional time averaged N-S equation and RNG k-ε turbulent model are used to analyze the original model. The simulating results are well accord with the experiment, which proves the reliability of the simulation model. The comparison of the performance and flow characteristics between the optimal models and the original model is done, the simulation result shows that efficiency of the optimum model has been increased about5.21%in design point, and averagely increased about3.24%in whole working condition, the analysis of flow characteristics shows that for original model there is a large vortex in the entrance of the blade and the velocity distribution is not very well resulting in a great loss of efficiency. Flow separation is largely reduced at the entrance of the blade and the velocity distribution of the blade is better than the original model for the optimum model, so the efficiency is improved.