Abstract

The work presented concerns the modelling of a fan used for forced convection cooling of a subrack of printed board assemblies (PBAs) as a separate component for accurate CFD simulations. Detailed measurements of the fan outlet velocity profile (axial, tangential nd radial components) have been made for one fan operating point in a measurement set-up apart from the PBAs. CFD simulations have been made for a simplified version of a subrack, comparing a few elementary fan models. Results how that incorporation of the hub into a model of the axial fan is necessary. The tangential velocity at the outlet is not negligible. The value of the radial velocity is small throughout the fan outlet plane and may be ignored in a CFD model. 1.]~ntroduction Detailed measurements of the fan induced velocity pattern which occurs in a printed board assembly (PBA) showed that this pattern is highly non- uniform, resulting in inadequate cooling of parts of the structure [1]. Due to the compacmess of the assembly the lack of uniformity of the velocity profile at the fan outlet has important repercussions on the flow distribution over the different card 'channels'. CFD simulation of the flow can only be successful if it takes this non-uniformity into account. A simulation which used detailed measurements of the velocity pattern upstream of the cards as input conditions, howed important progress in predicting the velocity patterns in the PBA assembly [2]. It is felt however that this approach can't be generalised, as every change of the configuration would require detailed