THE EFFECT OF TURBULENT MIXING MODELS ON THE PREDICTIONS OF SUBCHANNEL CODES

In this paper, the predictions of the COBRA-IV and ASSERT-4 subchannel codes have been compared with experimental data on void fraction, mas s flow rate, and pressure drop obtained for two interconnected subchan nels. COBRA-IV is based on a one-dimensional separated flow model with the turbulent intersubchannel mixing formulated as an extension of th e single-phase mixing model, i.e. fluctuating equal mass exchange. ASS ERT-4 is based on a drift flux model with the turbulent mixing modelle d by assuming an exchange of equal volumes with different densities th us allowing a net fluctuating transverse mass flux from one subchannel to the other. This feature is implemented in the constitutive relatio nship for the relative velocity required by the conservation equations . It is observed that the predictions of ASSERT-4 follow the experimen tal trends better than COBRA-IV; therefore the approach of equal volum e exchange constitutes an improvement over that of the equal mass exch ange.