Thermal pollution level reduction by sweeping jet-based enhanced heat dissipation: A numerical study with calibrated Generalized k-ω (GEKO) model

Abstract

Thermal pollution is commonly the damage to natural ecosystems caused by industrial water discharge into the environment above ambient temperature. The highly efficiently dissipative sweeping jet actuator is thus considered as the solution to the reduced thermal pollution level. The thermal dissipation capacities of free steady, pulsed, and sweeping jets with a Reynolds number of 49,761 and a Strouhal number of 0.0152 are compared by numerical simulation. The Reynolds stress model (RSM) is used as the turbulence model, with the Generalized k-ω (GEKO) model innovatively used to calibrate jet dissipation rates. The temperature fields are first compared to confirm the overwhelming temperature dilution effect of the sweeping jet in both streamwise and normalwise directions. Then the reasons for the enhanced heat dissipation by sweeping jet are explained by numerous time-averaged and time-resolved contours of the distributions of flow-related physical quantities. The time-averaged flow fields directly show that the sweeping jet has the fastest streamwise velocity decay and the most uniform spanwise momentum distribution due to the continuous transverse oscillation, and hence has the moderate turbulence kinetic energy (TKE) over a wide spanwise range due to the destruction of the potential core commonly seen in a steady jet. The relative turbulence kinetic energy fields, excluding the contribution to turbulence generation from velocity magnitudes, show that the alternating formation and dissipation of shear layer-induced vortices most contribute to the inherent unsteady patterns. In the planes perpendicular to the streamwise direction, the spacings between adjacent velocity contour lines of the sweeping jet increase at the fastest rate with the streamwise distance, indicating the excellent entrainment capacity. The difference among the three primary Reynolds stresses of the sweeping jet is smaller than that of the other two jets, indicating that the sweeping jet converts more streamwise momentum into spanwise and normalwise velocity and turbulence and thus has higher spatial homogeneity. Pathlines of massless particles dispersed in the near-field show that the sweeping jet has the most substantial entrainment capability due to the longest motion distance of the traces originating outside the jet shear layer, although it is difficult for the ambient fluids to penetrate the central jet impact region. Finally, three mixing potential metrics, i.e., jet decay rate, jet spreading rate, and entrainment factor, are used to evaluate the three jets' more general mixing characteristics qualitatively. To extract the sweeping jet's centerline velocity and spreading width, the Lagrangian transformation, which invariably aligns the jet ejection direction with x-axis, is performed on the phase-averaged velocity flow fields. The near-field results show that compared with the steady jet, the sweeping jet boasts a 176% increase in the jet decay rate, a 241% increase in the jet spreading rate, and a 369% increase in the entrainment factor. In the far-field, the entrainment capacity of the sweeping jet is not as good as that of the pulsed jet but is still 68% higher than that of the steady jet. In all, this paper establishes a theoretical basis for the application of fluid oscillators to heat pollution level reduction and more general scenarios involving requirements for enhanced heat and mass transfer by proposing the standard mixing indicators of sweeping jets similar to those of conventional jets, and also serves as a pioneering example for the future possible extension of the GEKO model to a broader range of industrial CFD simulations.