  This paper discusses the numerical investigation of the wall roughness effect
on the supercritical water flow susceptible to heat transfer deterioration
(HTD). The simulation was carried in the vertical circular pipe using the SST
k-omega turbulence model for different sets of heat flux (220kW/m2 & 1810kW/m2)
and mass flow rate (0.0106kg/s &0.022kg/s) at a maximum pressure of 25.3MPa.
The presence of roughness was incorporated as a uniform sand-grain roughness on
the heated wall. As a result, HTD recuperated gradually as the roughness height
(Ks) was increased. The mitigation of HTD is a direct consequence of the
increase in turbulent kinetic energy (TKE). In contrast, the delay in the onset
of HTD is due to the decrease in the dominant forces (buoyancy or acceleration
effects) responsible for HTD occurrence. An equivalent thermal resistance model
was proposed to elucidate the same. Additionally, the heat transfer coefficient
reduces (HTDn) near the outlet at higher roughness values where HTD has
completely recovered. This is attributed to the high specific heat value around
the pseudocritical temperature. In the end, the efficacy of the roughness was
analyzed using two methods: entropy generation and thermal performance factor.
Both techniques suggest that the use of rough pipe is beneficial. However, the
first method recommends using a Ks value greater than the critical roughness
height (Ksc): as the total entropy generation was found to peak at Ksc because
of the thermal conductivity variation with temperature. The study revealed high
sensitivity of maximum wall temperature and HTD onset to roughness presence. In
addition, contrary to usual HTD, a new type of heat transfer impairment (HTDn)
can occur without any loss of TKE.
