In the present study, the simulation of mixed convection in a square cavity with protruding body having different aspect ratios is carried out. The governing flow and energy equations are solved numerically using a control volume approach. Air is used as fluid in the cavity while steel is considered as protruding body. To investigate the heat transfer characteristics due to different aspect ratios of the protruding body, Stanton number (St) variation with the aspect ratio is considered. The entropy analysis is carried out to determine the irreversibility generated in the cavity for different aspect ratios. The heat transfer to irreversibility ratio is determined for each aspect ratio. It is found that the aspect ratio influences the heat transfer characteristics and the irreversibility generated in the cavity, in which case, the heat transfer increases at high aspect ratio while the irreversibility reduces. Moreover, heat transfer to irreversibility ratio improves considerably at an aspect ratio of 3. Copyright (C) 2002 John Wiley Sons, Ltd.