Experimental investigation of an annular diffuser for axial fans at different inflow profiles

Axial fans are used in power plants for fresh air supply and flue gas transport. A typical configuration consists of an axial fan and annular diffuser which connects the fan to the following piping. In order to achieve a high efficiency of the configuration not only the components have to be optimized but also their interaction. The present study focuses on the diffuser of the configuration. Experiments are performed on a diffuser-piping configuration in order to investigate the influence of the velocity profile of the fan outlet on the pressure recovery of the configuration. Two different diffuser inlet profiles are generated, a homogeneous profile and a profile with the typical outlet characteristics of a fan. The latter is generated by the superposition of screens in the inlet zone. The tests are conducted at a high Reynolds number (Re≈4∙10^5). Mean velocity profiles and wall  shear stresses are measured with hydraulic methods (Prandtl and Preston tubes). The results show that there is a lack of momentum at the outer wall of the diffuser and high shear stresses at the inner wall in case of the homogeneous inflow profile. For the typical fan outlet profile it can be shown that there is an opposite effect with high wall shear stresses at the outer wall while the boundary layer of  the inner wall lacks momentum. The pressure recovery of the configuration is in good agreement with previous studies.