Empirical model for the quantitative prediction of losses of radial fans based on CFD calculations

Abstract

In a classical layout process of a fan the quantity of losses is estimated as a sum and expressed in the overall efficiency rate η. However the characteristic of the pressure rise, the losses and the efficiency rate beside the design point is not known. Against this background a numerical model was developed to calculate quantitative values of occurring losses at radial fan impellers at an early stage in the design process. It allows to estimate the pressure rise and efficiency rate of a given fan geometry at and beside the design point. The physics of losses are described in literature, but obtaining quantitative values is still a challenge. As common in hydraulic theory the losses are calculated with analytic formulas supported by coefficients and efficiency rates, which have to be determined empirically. This paper shows the method how to determine the coefficients for a given radial fan. Therefore a representative radial fan with backward curved blades was designed in reference to classical design guidelines. Performance measuring was done conform to ISO 5801. The flow was calculated at 8 different operation points using CFD methods. The RANS equations are solved by using the SST-k-omega turbulence model. The flow domain consists of one blade section including inlet channel and outflow chamber. Spatial discretization is done by a block-structured mesh of approx. 1.8 million cells. Performance data show a very good agreement between measurement and calculation.