Transient cooling and gelation of a waxy‐oil pipeline during shutdown: Effects of insulation and external convection

Abstract

Wax solidification during pipeline shutdown was investigated in this study using a 3D transient computational fluid dynamics (CFD) model incorporating conjugate heat transfer, temperature‐dependent thermophysical properties, and an enthalpy‐porosity phase‐change formulation. The numerical model was verified through grid refinement and validated against published data, demonstrating good agreement. External cooling was examined using heat transfer coefficients of 10, 20, and 30 W/m ² K. Increasing the convection coefficient ( h ) from 10 to 30 W/m ² K reduced the shutdown time to reach a liquid fraction () of 0.5 from 372 to 80 min. The corresponding effective Biot number () increased from 0.546 to 1.636, indicating stronger boundary heat removal relative to internal thermal diffusion. The influence of insulation thickness ranging from 0 to 6 mm was also evaluated. Increasing the insulation thickness from 0 to 6 mm extended from 149 to 272 min. The increases from 2 to 4 mm and from 4 to 6 mm provided comparable absolute extensions of 55 and 56 min, respectively. A dimensionless –Fo analysis demonstrated the relationship between boundary heat removal, internal thermal diffusion, and solidification time. A marginal material‐effectiveness index was also introduced to quantify the extension of per unit volume of added insulation. The 2–4 mm increment exhibited the highest marginal material effectiveness, while the effectiveness of the 4–6 mm increment was 13.6% lower because it required 17.9% more material. These findings provide an engineering basis for comparing thermal protection and insulation‐material utilization during pipeline shutdown.