AN INVESTIGATION OF THE CONSTITUTIVE RELATIONS FOR INTERSUBCHANNEL TRANSFER MECHANISMS IN HORIZONTAL FLOWS AS APPLIED IN THE ASSERT-4 SUBCHANNEL CODE

In this paper, the influence that the constitutive relations used to r epresent some of the intersubchannel transfer mechanisms has on the pr edictions of the ASSERT-4 subchannel code for horizontal flows is exam ined. In particular the choices made in the representation of the grav ity driven phase separation phenomena are analyzed. This is done by co mparing the predictions of the ASSERT subchannel code with experimenta l data on void fraction and mass flow rate, obtained for two horizonta l interconnected subchannels. ASSERT uses a drift flux model which all ows the two phases to have different velocities. In particular ASSERT contains models for the buoyancy effects which cause phase separation between adjacent subchannels in horizontal flows. This feature, which is of great importance in the subchannel analysis of CANDU reactors, i s implemented in the constitutive relationship for the relative veloci ty. In order to isolate different intersubchannel transfer mechanisms, three different subchannel orientations are analyzed. These are the t wo subchannels at the same elevation, the high void subchannel below t he low void subchannel, and the high void subchannel above the low voi d subchannel. It is observed that for all three subchannel orientation s ASSERT does a reasonably good job of predicting the experimental tre nds. However, certain modifications to the representation of the gravi tational phase separation effects which seem to improve the overall pr edictions are suggested.