Session: 15-09 Impingement and Internal Cooling

Paper Number: 120999

120999 - Numerical Analysis of Heat Transfer Inhomogeneity of Supercritical Pressure Carbon Dioxide in Horizontal Minichannels 

To investigate the heat transfer mechanism of supercritical carbon dioxide (SCO₂) in horizontal channels (circular, semicircular and rectangular), a solver based on OpenFOAM was developed in this paper to numerically investigate the turbulent mixed heat transfer of SCO₂ in the heat exchanger at the low-pressure side of Brayton cycle system (p=8 MPa, T_in=303 K, G=600-1200 kg·m^-2·s^-1, q=25-300 kW·m^-2, q/G=41.67-250 J·kg^-1). Due to the dramatic change of the thermophysical properties, the density gradient in radial direction formed a buoyancy force under the effect of gravity. A secondary flow was formed in the direction perpendicular to the main flow. Under the double role of the buoyancy force and the special structure at the corners, the circumferential heat transfer of the horizontal minichannles was inhomogeneous. The wall temperature at the corners was much higher than that at other locations, while the temperature at the top wall was higher than the bottom wall. The influence of heat-to-mass ratio (q/G) and mass flux on heat transfer inhomogeneity were analyzed using the circumferential wall temperature inhomogeneity δ_T and the maximum temperature difference ΔT_max. With the increase of q/G, the heat transfer intensified from 41.67 J·kg^-1 to normal heat transfer at 83.33 J·kg^-1, and finally deteriorated when q/G≥125 J·kg^-1. In the heat transfer intensified state, δ_T and ΔT_max in the circular channel were below 0.00048 and 0.42 K, respectively, and the heat transfer inhomogeneity can be ignored. During normal heat transfer, δ_T and ΔT_max can reach 0.0048 and 4.8 K, and the heat transfer inhomogeneity started to be noticeable. When heat transfer deterioration occured, the average Nusselt number Nu_ave decreased rapidly at T_b/T_pc=0.997, where δ_T and ΔT_max also peaked at 0.024 and 25 K. As q/G increased, δ_T and ΔT_max can reach to 0.2 and 300 K at q/G=250 J·kg^-1 . Under the same q/G condition, the ratio of secondary flow velocity to main flow velocity was higher when the mass flux was smaller, so the smaller the mass flux the stronger the heat transfer inhomogeneity. The semicircular and rectangular channels have much higher δ_T and ΔT_max than the circular channels due to the high temperature regions at the corner locations, and the heat transfer inhomogeneity increased with higher mass flux.

Presenting Author: Ni Li Dalian university of technology

Presenting Author Biography: Li Ni, female, born in Shandong in 1996, is currently pursuing a PhD in heat and mass transfer and heat transfer enhancement in supercritical fluids.

Authors:

Ni Li Dalian university of technology
Zheng Fang Dalian university of technology
Hang Pu Dalian university of technology
Lin Mu Dalian university of technology
Yan Shang Dalian university of technology
Ming Dong Dalian university of technology