Buoyancy effect on entropy generation and heat transfer performances of transcritical liquefied natural gas in a printed circuit heat exchanger

In this work, we conduct 3-dimensional large eddy simulations to shed lights on the turbulent pulsation and thermal transport characteristics of transcritical liquefied natural gas (LNG) within a printed circuit heat exchanger (PCHE). Emphasis is being placed on comparing the effect of buoyancy on subcritical and supercritical LNG and its effect on the heat transfer rate. The average streamwise velocity of supercritical LNG in the viscous bottom layer no longer satisfies a linear distribution, but still follows a logarithmic distribution in the log-law layer. The density of near-wall heated LNG is varied dramatically, inducing Kelvin-Helmholtz instability at the interfaces. Buoyancy is shown to weaken the generation and evolution of high-velocity strips and streamwise vortices near the wall, slowing down the upthrow and downsweep flows. The buoyancy early breeds the coherent structures such as hairpin vortices, streamwise vortices, and spanwise vortices within the PCHE. The spanwise and streamwise pulsation velocities near the wall are observed to be higher than those in the turbulent region, while the normal pulsation velocity is reversed. Buoyancy effect is observed on enhancing the normal velocity pulsation of subcritical LNG but weakening the normal oscillation of supercritical LNG. The fundamental frequency ω_f of LNG during transcritical flow is observed to be 400 Hz, and the harmonic frequencies 2 ω_f, 3 ω_f and 4 ω_f within the energy-containing region correspond to large-scale vortex motions. Turbulence is shown to be located in the inertial subregion around frequencies above 2000 Hz. The spanwise and normal flatness factors in the turbulence region are close to a Gaussian distribution, while streamwise skewness and flatness factors deviate from a Gaussian distribution. We confirm that buoyancy considerably suppresses the energy loss due to irreversible viscous dissipation, and neglecting the buoyancy will lead to an overestimation of the heat transfer rate of supercritical LNG.