Fluid flow and heat transfer when cold and hot fluids mix in a T-junction packed with spherical particles have been numerically investigated using large eddy simulations (LES) with the sub-grid scale Smagorinsky-Lilly model. The numerical results of both normalized time-averaged temperature and normalized root mean square temperature were found to be in good agreement with experimental data. Numerical simulations of three porous structures with different particle diameters and porosity were completed, in order to evaluate the effect of particle diameter and porosity on fluctuations in temperature and velocity. The results indicate that the velocity and temperature fluctuations decrease gradually with increasing particle diameter and decreasing porosity of the porous medium. The pressure drop increases sharply (by a factor of similar to 8) as the particle diameter increases from 26 mm to 30 mm and the porosity decreases from 0.4 to 0.2. That are caused stronger inertial resistance and viscous resistance, the effective thermal conductivity increase and the turbulent mixing weakened as the particle diameter increases and the porosity decreases. (C) 2015 Elsevier Ltd. All rights reserved.