Large volume tube bundles are usually used in heat transfer equipments. There is always thermal non-uniformity in large volume tube bundles, which will cause large temperature difference. In extreme cases, this will destroy the structures. It needs to understand the mixing process in the tube bundles to correctly predict of the temperature non-uniformity. A method for getting the thermal mixing coefficient, Peclet number, is proposed. Previous experimental data are reanalyzed using this method. Then the thermal mixing process in HTGR steam generator tube bundles is numerically simulated. The thermal mixing coefficient of the HTGR steam generator tube bundle is analyzed based on these numerical data. The results show that thermal mixing in the fully developed region is better than in the inlet region. Thermal mixing effect increases with increasing Reynolds number. The Peclet numbers of HTGR steam generator tube bundle are between 7 and 13 for Re = 10,000-40,000. (C) 2013 Elsevier Ltd. All rights reserved.