Numerical modeling of turbulent heat transfer of a nanofluid at supercritical pressure

A numerical study has been conducted to examine the turbulent heat transfer of a nanofluid, methane-CuO, in a circular cooling tube at a supercritical pressure of 8 MPa, a phenomenon relevant to the rocket engine cooling application. Results reveal that at a surface heat flux of 3 MW/m2 and an inlet flow velocity of 25 m/s, the addition of nanoparticles decreases the heat transfer rate, dictated by significant increase of the nanofluid viscosity, which leads to the decreased turbulent viscosity in the near-wall buffer zone. As the surface heat flux is increased to 7 MW/m2 or the inlet velocity is decreased to 10 m/s, however, two physical phenomena of heat transfer improvement are observed in the nanofluid. The first phenomenon, which starts almost immediately from the beginning of the heated section, is controlled by strong increase of the nanofluid density, which results in the increased turbulent viscosity in the near-wall buffer zone. The second phenomenon is dictated by thermophysical property variations in the near-wall turbulent flow region as fluid temperature transits from the subcritical to supercritical state (the transcritical process). Results indicate potential applications of nanofluids in enhancing heat transfer at supercritical pressures.