Study of Turbulence and Pressure Recovery in the Heat Pipe Vapor Flow Using the Spectral-Element Method

Heat pipes can efficiently and passively remove heat in nuclear microreactors. Nevertheless, the flow dynamics within heat pipes present a significant challenge for their design and optimization in nuclear energy applications. This work investigates vapor-core behavior with particular emphasis on pressure recovery in the condenser section, with the ultimate goal of developing improved correlations for one-dimensional heat pipe models. To this end, two complementary modeling approaches were employed. Two-dimensional axisymmetric Reynolds-averaged Navier–Stokes (RANS) simulations were performed using the k–τ turbulence model implemented in the open-source spectral element code Nek5000. Three-dimensional large eddy simulations (LES), conducted using NekRS, were employed to resolve turbulence and laminar–turbulent transition effects, which significantly influence condenser pressure recovery. The LES approach, in particular, captured the detailed interplay of radial mass injection and extraction, axial acceleration and deceleration, and the onset of turbulence. The simulation framework was verified and validated against experimental data from the literature as well as established analytic (Cotter) and 1D (Sockeye) models, demonstrating good agreement across cases. The study was then extended to eight aspect ratios ((Formula presented.)) and five radial Reynolds numbers based on the vapor core radius ((Formula presented.)). Analysis of the resulting dataset revealed that while Re_r plays a secondary role, both the axial Reynolds number (Re_a) and, more significantly, the heat pipe aspect ratio govern pressure-gradient behavior. Building on these insights, a new mechanistic correlation was developed to predict condenser pressure recovery in incompressible vapor flows. This correlation provides a foundation for optimizing heat pipe performance and supports the validation of reduced-order models for nuclear applications.