LAUSR

The combined radiation–convection heat transfer occurs in many industrial devices; in many occasions, the significant role of radiation is neglected in engineering modeling. The current study is an attempt to address the importance of radiation heat transfer involvement in numerical computations. The investigation takes place in two heat transfer modes of with radiation (case_rad) and without radiation consideration (case_norad) for different duct geometries of base cylinder, small cylinder, divergent frustum cone, and convergent frustum cone. To account the radiation effect in the ducts, discrete transfer radiation method is applied, while a weighted sum of gray gases model for absorption coefficient modeling in non-homogenous participating media is appropriate for combustion of n-octane gas in ducts. The convergent duct structure shows the most radiative flux of − 1514.4 W m−2, whereas small cylinder demonstrates the lowest radiative flux of − 1033.14 W m−2. The overall heat flux of convergent configuration outperforms due to homogenous distribution of O2 and high oxygen mass fraction for combustion, while increasing the surface area in the flow direction aids the heat transfer improvement. The small cylinder is characterized with better flow dynamics (Vmax = 25.72 m s−1, TKEmax = 9.45 m2 s−2); however, its deficiency in heat transfer leads to high temperature along the duct (Tmax = 710 K).