Theoretically, the grid spacing for Direct Numerical Simulations of heat transfer at Prandtl numbers higher than one should be inversely proportional to the square root of Prandtl number (Tennekes, H., Lumley, J.L ). The grid refinement study of Na and Hanratty (2000)  at Pr = 10 showed that finer grid is not required in streamwise and spanwise directions, but is necessary in the wall-normal direction.
In the present work three different DNS studies were performed with different resolutions at Pr = 5.4 and friction Reynolds number Re = 171.
The first DNS was performed using the resolution, which is known to be sufficient for the velocity field simulation, the second simulation was performed with the refined grid in the wall-normal direction and the third DNS was using finer grid in all three directions.
Results have shown that there are no significant differences in the first-order statistics and spectra for all three cases.