An Investigation and Correction Method of Stationary Fan CFD MRF Simulations

Abstract

A common fan model to use in automotive underhood simulations is the Multiple Reference Frame (MRF) model and within the industry, for this specific application, this model is well known to under-predict performance. In a former paper, referenced 2009-01-0178, a simple 'speed correction' of the MRF model was proposed by the authors. The correction was shown to apply across different operating speeds for a specific fan. In this paper the generality and limitation of this correction across fans of different type, design and dimensions are investigated. Investigated in this paper is as well the sensitivity of the MRF model to specific methodology of use. In this paper it is shown that the speed correction of 14% proposed in the former paper applies widely, hence, although the MRF model is erroneous the error is consistent. A 680 mm fan of the similar design to the default (750 mm) choice was simulated and tested, in addition a different 750 mm fan with a more mixed flow design was simulated and tested, and the correction of 14% was shown to apply for these two fans as well. Furthermore is it also shown that the consistency of the MRF model applies with a consistent methodology of use; in its design, the MRF model needs a dedicated rotational volume around the rotating part, this needs to be chosen consistently by the end-user of the CFD software to achieve a consistent behavior of the MRF model. In this paper we show the sensitivity to the results done by the specific choice of rotational domain. In this paper a wiser choice of MRF region resulted in a significant drop in error and for this methodology a speed correction of only 8% was necessary.