The wide applications of indirect heating and cooling processes have opened scope for various researchers to explore in-depth analysis and applications of systems involving heat exchange processes. This paper targets the analysis and visualization of heat transfer in entrapped triangular cavities within adjacent square tubes forming a system of practical application especially in pollution control with hot fluid flowing through the stack and entrapped cold fluid confined within the triangular cavities. Also, efficient heat recovery has been examined for the entrapped fluid in the system. The parameters for this study are the Prandtl number (Pr), Rayleigh number (Ra) and Nusselt number (Nu). Complete details of heating patterns in both triangular cavities have been analyzed with heatline approach for visualization of heat flow. At low Rayleigh number, it is found that the heatlines are smooth and perfectly normal to the isotherms indicating the dominance of conduction for both the triangles. But as Ra increases, flow slowly becomes convection dominant. Multiple secondary circulations within the upper triangle are formed for fluids with low Pr, whereas this is absent in higher Pr fluids. Multiple circulation cells for smaller Pr also correspond to multiple cells of heatlines which illustrate less thermal energy transport from hot wall. On the other hand, the dense heatlines at bottom wall display enhanced heat transport for larger Pr. But interestingly for lower triangle there is hardly any variation of patterns with the increase in Prandtl number in the system. Analysis is concluded with the average Nusselt number plots. It is found that fluid with higher Pr may be recommended for upper triangle, but fluid with all ranges of Pr may be used for lower triangle. © 2009 Elsevier Ltd. All rights reserved.