Effect of Duct Aspect Ratio on Heat Trasfer of S-Co2 in Rectangular Duct With Angled Ribs

Power systems that use supercritical carbon dioxide as a working fluid are being actively studied. One of the researching power cycle is s-CO2 Brayton cycle. Same with basic Brayton cycle, the cycle consists of a compressor, combustor, and turbine part. The turbine inlet temperature of the cycle is about 1200 ˚C and for high thermal efficiency, the high turbine inlet temperature is inevitable. Therefore, design for the cooling system of a supercritical turbine is essential and our group studied heat transfer characteristics of s-CO2 by using numerical methods. Commercial CFD code ANSYS CFX v17.2 is used. The turbulence model is selected as k-w model and the selection is based on two validation processes one is to check the validity of using properties of s-CO2 and the other is to check the physical phenomenon in the ribbed duct. Mass flow rate and static pressure were selected as boundary conditions for the numerical simulation domain inlet and outlet, respectively. Mass flow rate is based on duct Reynolds number which is 30,000 based on hydraulic diameter of the duct and outlet pressure is 300 bar which is a real condition for cooling fluid of developing s-CO2 Brayton cycle named Allam cycle. Basically, three different duct aspect ratio (0.5, 1, 2) are studied and Nusselt number of each case were compared. For all three cases, heat transfer and flow characteristics are not symmetric. Heat transfer is shown asymmetrically between the bottom and top wall due to the buoyancy effect and we can check it by calculation of Richardson number. Richardson number (Ri=Gr/Re²) allows us to know which one is more effective in the duct between natural convection and forced convection. Furthermore, the trend of a difference of area-averaged Nusselt number for the bottom and top wall are different. Different from the air case, the bottom wall shows the highest Nusselt number for aspect ratio 0.5 case and the top wall shows the highest Nusselt number for aspect ratio 2 case. This is due to the difference of duct width and height between three cases. For longer height case, flow cannot reach the top wall of the duct but reattached stronger to the bottom wall than other cases. Furthermore, for longer width case, length of the flow development reduces heat transfer value on the heated wall.