  Additive manufacturing creates surfaces with random roughness, impacting heat
transfer and pressure loss differently than traditional sand-grain roughness.
We conducted high-fidelity heat transfer simulations over three-dimensional
additive manufactured surfaces with varying roughness heights and skewness.
Based on an additive manufactured Inconel 939 sample from Siemens Energy AB, we
created six surfaces with different normalized roughness heights, $R_a/D =
0.001, 0.006, 0.012, 0.015, 0.020,$ and $0.028$, and a fixed skewness, ${s_k} =
0.424$. Each surface was also flipped to obtain negatively skewed counterparts
(${s_k} = -0.424)$. Simulations were conducted at a constant Reynolds number of
8000 and with temperature treated as a passive scalar. We analyzed temperature,
velocity profiles and heat fluxes to understand the impact of roughness height
and skewness on heat and momentum transfer. The inner-scaled mean temperature
profiles are of larger magnitude than the mean velocity profiles both inside
and outside the roughness layer. This means the temperature wall roughness
function differs from the momentum wall roughness function. Surfaces with
positive and negative skewness yielded different estimates of equivalent
sand-grain roughness for the same $R_a/D$ values, suggesting a strong influence
of slope and skewness on the relationship between roughness function and
equivalent sand-grain roughness. Analysis of the heat and momentum transfer
mechanisms indicated an increased effective Prandtl number within the rough
surface in which the momentum diffusivity is larger than the corresponding
thermal diffusivity due to the combined effects of turbulence and dispersion.
Results consistently indicated improved heat transfer with increasing roughness
height and positively skewed surfaces performing better beyond a certain
roughness threshold than negatively skewed ones.
