Buoyancy induced flow and heat transfer are important phenomena in a wide range of engineering systems including electronics and photovoltaics cooling, thermosiphon and hydrosiphon heat exchangers, solar-thermal heat absorbers, passive decay heat removal systems, etc. Such systems are vulnerable to thermal stratification, which can significantly compromise performance. The objective of the present work is to study different passive heat transfer systems, characterized by an absence of active components such as pumps. We have investigated two phase (boiling) natural convection, in order to find the ways to modify the extent of thermal stratification. We have considered a base case of a cylindrical tank, fitted with a central heating tube constituting the heat transfer surface. In addition, a large number of design modifications have been considered. The flow field and the corresponding stratification have been compared with the base case. We have considered the following design modifications to this system: (a) changing the ratio of heat transfer length (of the central heating tube) to liquid height (L-T/H), (b) provision of different draft tube designs which act as a chimney, and (c) attachment of circular horizontal baffles (both non-conducting and conducting). The investigations were made using Computational Fluid Dynamic (CFD) simulations implemented via the commercial software FLUENT-6.3. The CFD predictions have been compared with the experimental measurement of flow field obtained using Particle Image Velocimetry (PIV) and temperature measurements using thermocouples. The extents of thermal stratification and mixing have been investigated for a wide range of Rayleigh numbers (9.37 x 10(10)<= Ra <= 5.57 x 10(13)). A good agreement has been obtained between the CFD simulations and the experimental measurements. The results show that the use of conducting baffles with appropriate size, number, and location can significantly reduce the stratification and concomitantly enhance the extent of mixing through modifications in the flow fields and heat flux. (c) 2013 Elsevier Ltd. All rights reserved.