Construction of a Centrifugal Fan Test Bench for Validation Data at Off-Design Conditions
1.6 Test Rig & Instrumentation
Nowadays with eco directives becoming more strictly the efficiency improvement of turbomachinery is getting more important than ever. Relieving experimental investigations computational fluid dynamics (CFD) could be the major tool to optimize the geometry of a turbomachinery, as it can provide a detailed insight into the flow within the impeller. In order to have reliable results from CFD simulations a validation of the acquired simulation tool and the composed numerical model is crucial. Especially in off-design conditions it is very challenging for CFD to predict strongly whirling, highly unsteady flow fields correctly.
In the present paper the construction of a test bench for a centrifugal fan with spiral casing is described and the first results are presented. The main goal in constructing the test case is to provide extensive validation data not only in the design operation point but for off-design conditions also. The geometry was designed based on classical design guidelines. It is close to industrial centrifugal fans however an additional aim was, to provide a geometry which allows an acceptable mesh generation effort and high mesh quality. Due to the rotational periodicity of axial and radial fans without casing, those types can be calculated by regarding only a single blade passage, which decreases the number of required cells and hence the necessary computational power. For centrifugal fans with spiral casing this simplification is not suitable and this construct induces additional issues as pressure pulsation, acoustics, pulsating radial force, periodic load of vanes etc. The present test case should provide availability to investigate such phenomena also.
The characteristic and the efficiency curve of the fan are measured with standardized methods. Beyond this “Particle Image Velocimetry” (PIV) is chosen to measure the flow field within the blade channels and the spiral casing, as well, in several different positions relative to the corner (“tongue”). For the optical accessibility the hub of the impeller and the main parts of the casing are made out of acrylic glass. In order to keep the seeding particles within the system a closed loop test rig is constructed. The inlet section of the fan was designed to get well defined inlet conditions for the numerical simulation.
The first investigations were carried out at a rotational speed of 600 rpm, and compared with results from CFD. These results are presented and discussed in the present paper. Further investigations will be performed at higher rotational speed.