Influence of blade inlet depth on the aerodynamic performance of multi-blade centrifugal fans

Centrifugal fans, as critical components of the cleaning device in combine harvesters, significantly influence the aerodynamic performance and operational efficiency of the device. Traditional agricultural cleaning centrifugal fans predominantly feature flat and straight blades, which exhibit suboptimal performance and low efficiency. These conventional designs often suffer from issues such as poor airflow control, excessive turbulence, and significant energy losses. Multi-blade centrifugal fans represent a promising direction for future advancements in agricultural cleaning; however, research on their application in agricultural machinery remains limited, with insufficient reference data available. This study takes the multi-blade centrifugal fan of a large foreign grain combine harvester as the prototype, and reduces its size according to the similarity principle to serve as the benchmark model fan in this paper. A combination of computational fluid dynamics (CFD) and bench experiments was employed to investigate the impact of blade inlet depth on fan performance and flow field characteristics. Firstly, a multi-blade centrifugal fan model is designed based on the similarity theory, the impeller contains 16 fan blades. The depth of the blade inlet is dimensionless to the depth-to-width ratio. Eight sets of impeller models were designed with varying blade inlet depth ranging from 39 mm to 74 mm (corresponding to the depth-to-width ratio of 0.26 to 0.54), focusing on the influence of suction commutating process and inlet efficiency by blade inlet depth of a multi-blade cleaning centrifugal fan. The performance test bench of the fan is set up to regulate the working conditions through the frequency converter, and the photoelectric speed sensor, hot film wind speed sensor and other equipment are used to collect the flow rate, full pressure and other data, so as to verify the reliability of the simulation results. The results demonstrate that optimizing the inlet depth significantly enhances fan performance. At a rated speed of 1000 rpm, increasing the inlet depth can effectively inhibit vortex generation and flow separation, and improve the suction reversal capability and air intake efficiency. The enhanced inlet depth provides a longer acceleration path for the airflow, enabling more efficient conversion of static pressure energy into kinetic energy. This results in a more stable and uniform flow field, reducing the occurrence of turbulent eddies and flow instabilities. The optimal performance is achieved when the blade depth-to-width ratio is increased to 0.46, the total pressure is increased by 11.8% to 194.42 Pa, and the full-pressure efficiency reaches 73.76%. At this time, the peak radial velocity of the airflow is increased by 21.26% compared with L_Abs=39 mm, and the area of the high velocity zone at the outlet is enlarged by 24.3%, which significantly improves the uniformity of the flow field. The pressure pulsation analysis shows that the main frequency domain pressure amplitude is minimized at this depth, and the energy loss is reduced. Velocity streamline and vortex cloud analysis reveals that optimizing the inlet depth can delay the boundary layer separation and reduce the return flow at the tongue. 32% of the impeller curvature is increased at L_Abs =69 mm, and the airflow completes the momentum conversion at 12 mm from the center disk, which realizes the high-efficiency commutation in advance compared with that of the model. The flow field visualization shows that the flow adhesion on the suction surface is enhanced and the intensity of secondary flow in the blade channel is reduced. Bench tests based on the optimal impeller model are carried out to verify the reliability of the simulation analysis. This study reveals the regulation mechanism of the blade geometric parameters on the scavenging flow field, which provides a theoretical basis and parameterization method for the design of the impeller of the combine harvester. The airflow stability and energy conversion efficiency can be improved by adjusting the inlet depth, which promotes the efficient development of agricultural centrifugal fans.