Large Eddy Simulation of Hydrodynamic Instability and Optimal Pulsating Frequency in a Ribbed Channel

This paper presents a numerical investigation on the flow and heat transfer in a ribbed channel under the steady inflow and pulsating inflow, at Re=8500 and 12,000. Large eddy simulation, based on OpenFOAM 8, is used to simulate the properties of hydrodynamic instability and thermal enhancement with the variation of forcing frequency (StF=0–0.1). The flow characteristics for steady inflow are firstly discussed, revealing that a row of spanwise vortices is found after the first rib, with a natural frequency induced by the hydrodynamic instability (StN=0.097, 0.104 respectively at Re=8500, 12,000). Hairpin vortices gradually appear and play a main role in turbulence and heat transfer. The flow and thermal performance under the pulsating inflow are then investigated, finding that the shear layer after the first rib is damaged and the spanwise vortex is generated at different rates, leading to higher TKE and higher local Nusselt number. An optimal Strouhal number (StF=0.05) is identified both at Re=8500 and 12,000, which produces the strongest flow ejection and results in superior thermal enhancement. Ultimately, with the increase of StF, the Nusselt number reaches the maximum both at StF=0.05, which can be enhanced by 37.69% at Re=8500 and 44.18% at Re=12,000 respectively. The friction factor is also increased with minimal change as the forcing frequency changes. The thermal performance factor η also reaches the peak at StF=0.05, with the highest increase by 40.91% and 38.71% at Re=8500, 12,000 respectively. It indicates that an optimal frequency of pulsating flow exists in the ribbed channel and it is strongly correlated with flow characteristics.