Design And Numerical Investigation Of A Centrifugal Fan

As a kind of common mechanical equipment,centrifugal fan is widely used in various fields of national economy.In the process of design,it is very important to improve the head and efficiency of the fan and enlarge the flow range of the high-efficiency operation.Generally,the interior of the fan is a complex three-dimensional unsteady turbulent flow,and the flow field has a significant influence on the characteristics of the centrifugal fan.It is an important and effective way to conduct in-depth analysis of the flow patterns and explore the influence of design parameters on the performance of the fan,especially with the help of CFD technology.It would greatly benefit the understanding of the internal flow mechanism and the design of fans with better performance.In this paper,a centrifugal fan is designed using three-dimensional-blade design theory,and compared with the two-dimensional-blade fan which is designed with traditional design method.The results show that the fan with 3D blades has a significant advantage in both head and efficiency compared to the 2D-blade fan.Under the design flow rate,the 3D-blade shows a 204Pa higher head,a 4.3%higher efficiency and a wider overall high-efficiency flow range.Besides,the 3D-blade fan has a better flow pattern at the volute outlet diffuser.In the 2d-blade fan the radial velocity at the impeller outlet is larger,which leads to flow separation at the side wall near the tongue region and thus lowers the of fan performance.Then the unsteady numerical simulation of the flow inside the fan with 3D blades is conducted,under three different working conditions,i.e.Q₀,0.68Q₀ and 1.31Q₀.The velocity and pressure distribution in the interior flow field is monitored and studied by means of visualization analysis and FFT method.More attention is focused on the volute tongue region to discuss the flow characteristics.The results show that the flow pattern at design flow rate changes little with time,and the isobar distribution is more uniform.At 0.68Q₀ the circulation flow rate increases,and vortices appear at the inlet of some blade channels,which block the flow passage and lead to large pressure fluctuation as well as low fan performance.At 1.31Q₀ the flow direction at tongue region is from the worm section to the diffuser.The radial velocity at the impeller outlet is larger,resulting in flow separation at the position downstream the tongue in the volute.In addition,the effects of the width of the volute,the shape of the flow channel and the radial gap between the impeller and the volute on the aerodynamic performance of the centrifugal fan are investigated.It is found that the optimal design of the volute width should guarantee a good matching between the flow rate and the channel section area.The traditional design of volute section shape is improved with a baffle to separate the flow passage where vortices appear from the volute.The results show that the fan head curve hardly changes,while the efficiency at design and large flow rates are markedly increased,and the high-efficiency range is broadened.The radial gap between the impeller and the tongue is changed and its influence is investigated.The fan with larger gap is more efficient under off-design conditions,and its time-dependent head curve is smoother with weaker fluctuation.The phase difference of the head curve between three fans with different gap ratios is in accordance with the angle difference of the tongues,indicating that the interaction between the tongue and the impeller is the main cause of the periodic fluctuation of the fan head.