• The increase in forced ventilation velocity reduces plume longitudinal propagation. . • Vent discharge rate increases with increase in Grashof number. . • Under forced ventilation, mass flow rate is higher through downstream roof vent. . • Increase in volume blockage reduces the plume buoyancy force . • Mass flow rate through roof vents decreases with increase in volume blockage ratio.. © 2020 Elsevier Ltd The objective of this study is to numerically investigate the effects of volumetric blockage ratio on the thermal plume behavior in a mixed ventilated tunnel with heat source and vehicular blockage. The tunnel model consists of three naturally ventilated roof openings and longitudinal ventilation is provided by supplying forced air through the tunnel inlet port. The non-Boussinesq variable density approach is used to model the high thermo-buoyant flows with large eddy simulations (LES) turbulence model discretized by finite difference method (FDM). The investigation is performed by varying the Grashof number (Gr) in the range 108 ≤ Gr ≤ 1010 and the forced ventilation velocity (Uc) and volumetric blockage ratio (ξ) are varied between 0.05 ≤ Uc ≤ 0.5 and 0.012 ≤ ξ ≤ 0.05. The outcome of this study emphasizes that the longitudinal ventilation and volume blockage ratio significantly affects the thermal plume behavior and venting efficiency of the roof openings. It is found that inertial force from forced air increases the mass flow rate through downstream vent in comparison with the upstream vent. It is observed that the increase in Grashof number enhances the bidirectional exchange rate through roof vents and an increase in volume blockage ratio decreases the discharge rate through the roof openings. © 2020 Elsevier Ltd