Optimization of Diffuser Parameters for Mixing Fans in Agricultural Buildings

Abstract

Abstract. Mixing fans (MFFs) are widely used in ventilation of agricultural buildings to improve the uniformity of the air supply, thereby improving the ventilation efficiency. In order to improve the ventilation performance of MFFs, a new visor-shaped diffuser was designed and installed on a MFF. The angle and the length of the diffuser were crucial parameters that affected the performance of the MFFs with the diffusers. Thereby, in this study numerical simulation with 42 diffusers of different angles (ranged from 90-270°) and different lengths (ranged from 150-650 mm) with the MFF were studied with Computational Fluid Dynamics (CFD) simulation to acquire the optimal design of diffusers. The numerical simulation results show that the diffusers of 90°/450 mm, 120°/350 mm, and 150°/250 mm with jet lengths of up to 5.85, 5.90, and 5.85 m, respectively, had better performances among all the diffusers. The optimal prototype diffusers of 90°/450 mm, 120°/350 mm, and 150°/250 mm of MFFs were tested by laboratory study and field test. The test was conducted in wind speed distributions at distances of 0.5 to 1.0 m from the axial of MFFs. During the test, we evaluated the MFFs performance such as maximum flow flux, maximum energy efficiency, and non-uniformity coefficient. The diffuser of 150°/250 mm showed the best performance, increasing the flow flux and energy efficiency by 3.8% and 11%, respectively, and obtain higher axial wind speeds and larger non-uniformity coefficients. Finally, the diffusers of 150°/250 mm were tested in a free-stall dairy barn. The field test result shows that the diffusers of 150°/250 mm increased overall average wind speeds by 7.4% and local average wind speeds at bedding 1 and bedding 2 by 31.0% and 27.7%, respectively, which agreed with our numerical simulation and laboratory test. This optimal design of mixing fans could be applied to improve the air mixing in agricultural buildings. Keywords: Agricultural buildings, Diffuser, Mixing fan, Numerical simulation, Optimization.