  The design of advanced nuclear reactors (Gen IV) involves an array of
challenging fluid-flow issues that affect safety and performance. Currently,
these problems are addressed in an ad-hoc manner at varying scales which are
time-consuming and expensive. The creation of a high-resolution heat transfer
numerical database has the potential to help develop to accurate and
inexpensively reduced resolution heat transfer models. Such models can help
address industrial-driven issues associated with the heat transfer behavior of
advanced reactors. The models can be developed using the multiscale hierarchy
developed as part of the recently DOE-funded center of excellence for
thermal-fluids applications in nuclear energy. Ultimately this can lead to
fast-running reliable models, thus accelerating the deployment of advanced
reactors. In this paper, we performed a series of Direct Numerical Simulation
using the spectral element codes Nek5000 and NekRS to investigate heat transfer
in mixed convection conditions. First, we investigate the heat transfer of the
flow in heated parallel plates for high Prandtl number fluids. The calculated
database will eventually be used to evaluate existing heat transfer
correlations and some modifications will be proposed for cases where no
satisfactory choice is available. We have also investigated the heated transfer
alteration phenomena in a straight heated tube for supercritical carbon dioxide
(sCO2). The low-Mach-number approximation is used to decouple thermal and
dynamic pressure, as pressure drop is negligible in this problem. The
properties of sCO2 are calculated using multi-region polynomials. We observed
that the heat transfer deterioration occurred in combination with the property
changes of sCO2 and the depreciation of turbulence kinetic energy (TKE) for
upward flow. Whereas, in downward flow, the heat transfer is enhanced thanks to
the increase of TKE.
