Effect of wall roughness on heat transfer in the supercritical water flow

TL;DR

This study numerically investigates how wall roughness affects heat transfer deterioration in supercritical water flow, revealing that increased roughness can mitigate HTD and delay its onset through turbulence enhancement.

Contribution

It introduces a detailed numerical analysis of wall roughness effects on HTD in supercritical water, including an equivalent thermal resistance model and new insights into heat transfer impairment mechanisms.

Findings

Roughness increases can recover HTD gradually.

Higher roughness delays HTD onset by reducing dominant forces.

Both entropy generation and thermal performance factor indicate benefits of rough pipe.

Abstract

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.